Bearing Notes

LECTURE NOTESDesign of machine members-IIBearings

The basic purpose of a bearing is to allow relative movement between the components of machines whilst carrying a load as well as providing some type of location between components. This section of the website will focus on describing in more detail the various types of bearings that are available today as well as selection procedures.

Bearing Types

Bearings are broadly categorised into two main groups; Sliding Bearings and Rolling Contact Bearings. The picture below highlights the main types of bearings and the categories that they fall under :-

Bearing types

Sliding Bearings

Sliding bearings refers to bearings where two surfaces move relative to each other without the benefit of rolling contact.

Usually a lubricant is used to allow the two surfaces to slide freely (lubricant is denoted by the black layer in the above diagram) with reduced friction contact and wear. A typical application of a sliding bearing is to allow rotation of a load bearing shaft. The portion of the shaft at the bearing is referred to as the 'journal', and the stationary part which supports the load is called the bearing.

Journal Bearing

This is why sliding bearings are often referred to as journal bearings. There are three main regimes of lubrication for sliding bearings:1. Boundary Lubrication 2. Mixed film lubrication 3. Full film lubrication 1. Boundary LubricationThis state occurs typically at low relative velocities between the journal and bearing surfaces. Even though there is lubrication present between the two surfaces, there is insufficient pressure as to keep the surfaces from coming into contact and so frictional forces need to be considered. As a result, this type of bearings are typically used for low-speed applications such as bushes and linkages where simplicity and compactness are desirable.2. Mixed film LubricationMixed film lubrication occurs when the relative motion between the twp surfaces is sufficient as to create enough pressure to partially separate the surfaces for periods of time. However, contact will still occur at certain places and for certain amounts of time.3. Full film LubricationThis regime occurs at high velocities where the motion creates a high enough pressure in the lubricant such that the two surfaces separate and the gap created is filled with lubricant, thus eliminating frictional forces that would be experienced had there been surface contact.

Rolling Bearings

The term rolling contact bearing encompasses the wide variety of bearings that use spherical balls or some type of roller between the stationary and moving elements.

The most common type of rolling bearings support a rotating shaft whilst resisting a combination of radial and thrust loads. Some bearings however are designed in such a way as that they carry radial or only thrust forces. The animation below lists the most common types of rolling bearings as well as giving a general description.

The remainder of this section will deal with the various equations that are used in selecting a rolling bearing for a particular application.

A bearing is a mechanical device which provides relative motion between two or more parts. Bearings are widely used in various industrial as well as in our day-to-day applications. Bearings permit four common types of motions like linear motion (for eg. drawer), spherical motion (for e.g. ball and socket joint), axial motion (for eg. shaft rotation) and hinge motion (for eg. door, elbow, knee). There are different types of bearings used in mechanical devices. Using a suitable bearing in many applications help in improving accuracy, efficiency, reliability, operation speed, purchasing costs of operating machinery.

Types of BearingsThere are many types of bearings each used for different aplications and different purposes. Bearings can be used either singularly or in combinations. Bearings are classified broadly according to their motions, operating principle, load capacity and speed and size they can handle. Accordingly, we find bearings of different shapes, speed, lubrication, materials and so on. The most popular bearing types are given below. Click on the following links to get a detailed description of different bearings used in various industries and machines.

Tags:- Types Of Bearings, Types Of Industrial Bearings

BearingsNeedle BearingCylindrical BearingSleeve Bearing

Spherical BearingLinear BearingRoller Bearing

The Sommerfeld Number is typically defined by the following equation[1].

Where:S is the Sommerfeld Number or bearing characteristic numberr is the shaft radiusc is the radial clearance is the absolute viscosity of the lubricantN is the speed of the rotating shaft in revs/sP is the load per unit of projected bearing areaThe Sommerfeld Number is typically defined by the following equation[1].

Where:S is the Sommerfeld Number or bearing characteristic numberr is the shaft radiusc is the radial clearance is the absolute viscosity of the lubricantN is the speed of the rotating shaft in revs/sP is the load per unit of projected bearing areaBearing modulus is used in Journal Bearing Design.

Bearing Modulus,C= (Zn/p)

Z=oil viscosityn=speed of rotation (rpm)p=bearing pressure

For any given bearing, there is a value for indicated by C, for which the coefficient of friction is minimum. The bearing should not be operated at this value of bearing modulus, since a slight decrease in speed or a slight increase in pressure will make the journal to operate in partial lubrication state resulting in high friction, heating and wear.

To prevent this, average value ofBearing modulus should be,

Zn/p >= 3C

for large fluctuations and heavy impact loads,

Zn/p = 15C (approx)Normally bearing number is having 4 digits.

EX 1 : Bearing # is 6308

I st digit 6 is indicates type of bearing. 6 means single row deep ball bearing.3 indicates applied loads depends upon the application.The last two digits indicates diameter of the shaft.As per above example 08 means shaft dia is 40 mm.The simple calculation is shaft dia divided by5 is last two digits of bearing number.EX 2: Bearing # is 6207, The shaft dia is 35 mm.Bearing characteristic number=Z*N/p

Z--->viscosity N-s/m^2N--->rotation of the bearing in rpmp--->bearing pressureAntifriction bearing A machine element that permits free motion between moving and fixed parts. Antifrictional bearings are essential to mechanized equipment; they hold or guide moving machine parts and minimize friction and wear.In its simplest form, a bearing consists of a cylindrical shaft, called a journal, and a mating hole, serving as the bearing proper. Ancient bearings were made of such materials as wood, stone, leather, or bone, and later of metal. It soon became apparent for this type of bearing that a lubricant would reduce both friction and wear and prolong the useful life of the bearing. Petroleum oils and greases are generally used for lubricants, sometimes containing soap and solid lubricants such as graphite or molybdenum disulfide, talc, and similar substances.MaterialsThe greatest single advance in the development of improved bearing materials took place in 1839, when I. Babbitt obtained a United States patent for a bearing metal with a special alloy. This alloy, largely tin, contained small amounts of antimony, copper, and lead. This and similar materials have made excellent bearings. They have a silvery appearance and are generally described as white metals or as Babbitt metals.Wooden bearings are still used for limited applications in light-duty machinery and are frequently made of hard maple which has been impregnated with a neutral oil. Wooden bearings made of lignum vitae, the hardest and densest of all woods, are still used.Some of the most successful heavy-duty bearing metals are now made of several distinct compositions combined in one bearing. This approach is based on the widely accepted theory of friction, which is that the best possible bearing material would be one which is fairly hard and resistant but which has an overlay of a soft metal that is easily deformed. Figure 1 shows bearings in which graphite, carbon, plastic, and rubber have been incorporated into a number of designs illustrating some of the material combinations that are presently available.

Bearings with (a) graphite; (b) wood, plastic, and nylon Rubber has proved to be a surprisingly good bearing material, especially under circumstances in which abrasives may be present in the lubricant. The rubber used is a tough resilient compound similar in texture to that in an automobile tire. Cast iron is one of the oldest bearing materials. It is still used where the duty is relatively light.Porous metal bearings are frequently used when plain metal bearings are impractical because of lack of space or inaccessibility for lubrication. These bearings have voids of 1636% of the volume of the bearing. These voids are filled with a lubricant by a vacuum technique. During operation they supply a limited amount of lubricant to the sliding surface between the journal and the bearing. In general, these bearings are satisfactory for light loads and moderate speeds.LubricantsThe method of supplying the lubricant and the quantity of lubricant which is fed to the bearing by the supplying device will often be the greatest factor in establishing performance characteristics of the bearing. For example, if no lubricant is present, the journal and bearing will rub against each other in the dry state. Both friction and wear will be relatively high. The coefficient of friction of a steel shaft rubbing in a bronze bearing, for example, may be about 0.3 for the dry state. If lubricant is present even in small quantities, the surfaces hydrodynamic pressure in film become contaminated by this material whether it be an oil or a fat, and depending upon its chemical composition the coefficient of friction may be reduced to about 0.1. Now if an abundance of lubricant is fed to the bearing so that there is an excess flowing out of the bearing, it is possible to develop a self-generating pressure film in the clearance space as indicated in Fig. 2. These pressures can be sufficient to sustain a considerable load and to keep the rubbing surfaces of the bearing separated.

Hydrodynamic fluid-film pressures in a journal bearingThe types of oiling devices that usually result in insufficient feed to generate a complete fluid film are, for example, oil cans, drop-feed oilers, waste-packed bearings, and wick and felt feeders. Oiling schemes that provide an abundance of lubrication are oil rings, bath lubrication, and forced-feed circulating supply systems. The coefficient of friction for a bearing with a complete fluid film may be as low as 0.001.Fluid-film hydrodynamic bearingsIf the bearing surfaces can be kept separated, the lubricant no longer needs an oiliness agent. As a consequence, many extreme applications are presently found in which fluid-film bearings operate with lubricants consisting of water, highly corrosive acids, molten metals, gasoline, steam, liquid refrigerants, mercury, gases, and so on. The self-generation of pressure in such a bearing takes place no matter what lubricant is used, but the maximum pressure that is generated depends upon the viscosity of the lubricant. Thus, for example, the maximum load-carrying capacity of a gas-lubricated bearing is much lower than that of a liquid-lubricated bearing. The ratio of capacities is in direct proportion to the viscosity. Gas is the only presently known lubricant that can be used for operation at extreme temperatures. Because the viscosity of gas is so low, the friction generated in the bearing is correspondingly of a very low order. Thus gaslubricated machines can be operated at extremely high speeds because there is no serious problem in keeping the bearings cool.The self-generating pressure principle is applied equally as well to thrust bearings as it is to journal bearings. The tiltingpad type of thrust bearing (Fig. 3a) excels in low friction and in reliability. A typical commercial tthrust bearing (Fig. 3b) is made up of many tilting pads located in a circular position. One of the largest is on a hydraulic turbine at the Grand Coulee Dam. There, a bearing 96 in. (2.4 m) in diameter carries a load of 2,150,000 lb (9,560,000 newtons) with a coefficient of friction of about 0.0009.

Fluid-film hydrostatic bearingsSleeve bearings of the self-generating pressure type, after being brought up to speed, operate with a high degree of efficiency and reliability. However, when the rotational speed of the journal is too low to maintain a complete fluid film, or when starting, stopping, or reversing, the oil film is ruptured, friction increases, and wear of the bearing accelerates. This condition can be eliminated by introducing high-pressure oil to the area between the bottom of the journal and the bearing itself, as shown schematically in Fig. 4. If the pressure and quantity of flow are in the correct proportions, the shaft will be raised and supported by an oil-film whether it is rotating or not. Friction drag may drop to one-tenth of its original value or even less, and in certain kinds of heavy rotational equipment in which available torque is low, this may mean the difference between starting and not starting. This type of lubrication is called hydrostatic lubrication and, as applied to a journal bearing in the manner indicated, it is called an oil lift. Hydrostatic lubrication in the form of a step bearing has also been used on various machines to carry thrust.

Fluid-film hydrostatic bearingLarge structures have been floated successfully on hydrostatic-type bearings. For example, the Hale 200-in. (5-m) telescope on Palomar Mountain (California Institute of Technology/Palomar Observatory) weighs about 1,000,000 lb (450,000 kg); yet the coefficient of friction for the entire supporting system, because of the hydrostatic-type bearing, is less than 0.000004. The power required is extremely small and a 1/12-hp (62-W) clock motor rotates the telescope while observations are being made.Rolling-element bearingsEveryday experiences demonstrate that rolling resistance is much less than sliding resistance. This principle is used in the rolling-element bearing which has found wide use. In the development of the automobile, ball and roller bearings were found to be ideal for many applications, and today they are widely used in almost every kind of machinery.These bearings are characterized by balls or cylinders confined between outer and inner rings. The balls or rollers are usually spaced uniformly by a cage or separator. The rolling elements are the most important because they transmit the loads from the moving parts of the machine to the stationary supports. Balls are uniformly spherical, but the rollers may be straight cylinders, or they may be barrel- or cone-shaped or of other forms, depending upon the purpose of the design. The rings, called the races, supply smooth, hard, accurate surfaces for the balls or rollers to roll on. Some types of ball and roller bearings are made without separators. In other types there is only the inner or the outer ring, and the rollers operate directly upon a suitably hardened and ground shaft or housing. Figure 5 shows a typical deep-grooved ball bearing, with the parts that are generally used.

Deep-groove ball bearingThese bearings may be classified by function into three groups: radial, thrust, and angular-contact bearings. Radial bearings are designed principally to carry a load in a direction perpendicular to the axis of rotation. However, some radial bearings, such as the deep-grooved bearings shown in Fig. 5, are also capable of carrying a thrust load, that is, a load parallel to the axis of rotation and tending to push the shaft in the axial direction. Some bearings, however, are designed to carry only thrust loads. Angular-contact bearings are especially designed and manufactured to carry heavy thrust loads and also radial loads.A unique feature of rolling-element bearings is that their useful life is not determined by wear but by fatigue of the operating surfaces under the repeated stresses of normal use. Fatigue failure, which occurs as a progressive flaking or sifting of the surfaces of the races and rolling elements, is accepted as the basic reason for the termination of the useful life of such a bearing

BEARINGSRolling contact bearingsThe terms rolling contact bearing, anti friction bearing and rolling element bearing are used to describe that class of bearing in which the main load is transferred through elements in rolling contact with each other. Friction in a rolling element bearing is present, but it is negligible when compared to the starting friction of a journal type bearing. The load, bearing speed and the viscosity of the lubrication all affect the friction within the bearing. Although it is not technically correct to refer to this type of bearing as anti friction, it is a name that is in constant use.Bearings reduce friction by providing smooth/polished metal balls or rollers, and a smooth/polished inner and outer metal surface for the balls to roll against. These balls or rollers carry the load, allowing the device to rotate smoothly.In general a rolling element bearing is a bearing which carries a load by placing round elements between two surfaces. These surfaces are referred to as the inner race and the outer race. The relative motion of the races causes the bearing elements to roll, with little or no sliding. Bearings are normally selected on the basis of a requirement to carry a given load for a given period of time. Rolling contact bearings are designed to carry pure radial loads, pure axial loads or a combination of the two.The bearing designer is confronted with the problems of designing a group of elements which make up a rolling element bearing. Parameters such as fatigue, loading, heat, corrosion resistance and lubrication, to name but a few, must be considered. There are many types of rolling element bearings, each designed to carry a specific kind of load. Each of the different types of bearing contain either a ball bearing, a roller bearing or a needle type bearing. A needle bearing is an elongated roller bearing.

Rolling-element bearings may rotate at over 100,000 rpm. Maximum rolling element bearing speeds may be specified in DN, which is the product of the diameter (in mm) and the maximum revolutions per minute (rpm).There are also many material issues for bearings. For example, a harder material may be more durable against abrasion but more likely to suffer fatigue fracture. Therefore, the material varies with the application, and whilst steel is the most common for rolling element bearings, plastics and ceramics are also in use.A bearing can last indefinitely; longer than the life of the machine, if it is kept clean, lubricated, and operated within its load rating. Also, every effort needs to be made during manufacture to make sure the bearing materials are sufficiently free of microscopic defects. Note, that cooling, lubrication, and sealing are also important parts of bearing design.The operating environment and servicing needs must also be considered in bearing design. Some bearing assemblies require routine addition of lubricants, while others are factory sealed, requiring no further maintenance for the life of the bearing or assembly. Although seals are appealing, they increase bearing friction, and a permanently sealed bearing may have the lubricant contaminated by hard particles, due to bearing wear, which will abrade the bearing.The meaning of bearing lifeStephen J. MrazHow long will a bearing last? Standardized life equations help to answer.Printer-friendly version

Relative effects of contamination and lubrication condition on bearing life with different load levels.

Experience shows seemingly identical rolling bearings operated under identical conditions may not last the same amount of time. In most cases, it is impractical to test a statistically significant number of bearings, so engineers rely on standardized bearing-life calculations to select and size bearings for a particular application. These calculations continue to evolve and become more accurate over time, reflecting the collective experience of the bearing industry, including recent advances in manufacturing, tribology, materials, end-user condition monitoring, and computation.In February of this year, the International Organization for Standardization (ISO) published a revised ISO 281:2007 Standard for the calculation of bearing ratings and life. It builds on the previous Standard ISO 281:2000 to account for such factors as internal stresses from mounting, residual stresses from hardening and other manufacturing processes, and material cleanness. Also included are the effects of solid contaminants with various lubricating systems, as well as bearing material fatigue stress limits. Before going into further detail, it's probably a good time to review the basics of bearing-life calculations, starting with the common definitions of life.Basic life or L10 as defined in ISO and ABMA standards is the life that 90% of a sufficiently large group of apparently identical bearings can be expected to reach or exceed. The median or average life, sometimes called Mean Time Between Failure (MTBF), is about five times the calculated basic rating life. Service life is the life of a bearing under actual operating conditions before it fails or needs to be replaced for whatever reason. The so-called specification life is generally a requisite L10 basic rating life and reflects a manufacturer's requirement based on experience with similar applications.CALCULATING LOADSEngineers typically employ rolling-contact fatigue models that compare bearing load ratings to applied dynamic and static loads as they impact service life and reliability. The basic dynamic load rating covers dynamically stressed bearings that rotate under load. This rating, defined in ISO 281, is the bearing load that results in a basic rating life or L10 of 1 million revolutions. Dynamic loads should include a representative duty cycle or spectrum of load conditions and any peak loads.The basic static load rating applies to bearings that rotate at speeds less than 10 rpm, slowly oscillate, or remain stationary under load over certain periods. Be sure to include loads of extremely short duration (shock) because they may plastically deform contact surfaces and compromise bearing integrity.Classical mechanics along with known or calculable external forces are used to calculate the loads acting on a bearing. These external forces may include resultants from power transmission, shaft or housing supports, or inertia. When calculating loads on a single bearing, assume the shaft to be a beam resting on rigid, moment-free supports.Basic catalog or simplified calculations typically ignore elastic deformations in the bearing, housing, or machine frame, as well as moments produced in the bearing by shaft deflection. Such calculations may assume loads are constant in magnitude and direction and act radially on a radial bearing, or axially and centrically on a thrust bearing. Oftentimes, bearings in actual service see simultaneous radial and axial loads. When the resultant of radial and axial loads is constant in magnitude and direction, calculate an equivalent dynamic bearing load from:P = XFr + YFa where P = equivalent dynamic bearing load, lb; Fr = actual radial bearing load, lb; Fa = actual axial bearing load, lb; X = radial load factor for the bearing; and Y = axial load factor for the bearing.For single-row radial bearings, axial load influences P only when the ratio Fa Fr exceeds a certain limiting value. Conversely, even light axial loads are significant for double-row radial bearings. The above equation also applies to spherical thrust bearings and other thrust types that handle both axial and radial loads. Be sure to consult manufacturer catalogs for axial-radial thrust bearings because designs can vary widely. For thrust ball bearings and other types that carry pure axial loads, the equation simplifies to P = Fa, provided the load acts centrically.RATING LIFE EQUATIONSThe equation from ISO 281 or the American Bearing Manufacturers Association (ABMA) Standards 9 and 11 figures basic, nonadjusted rating life by:L10 = (C P)p in millions of revolutionswhere C = basic dynamic load rating, lb; P = equivalent dynamic bearing load, lb; p = life-equation exponent ( p = 3 for ball bearings; and p = 10/3 for roller bearings)For bearings run at constant speed, it may be more convenient to express the basic rating life in operating hours:L10h = (1,000,000/60)nL10 where n = rotational speed, rpmPredicted bearing life is a statistical quantity in that it refers to a bearing population and a given degree of reliability. The basic rating life is associated with 90% reliability of bearings built by modern manufacturing methods from high-quality materials and operated under normal conditions. In practice, predicted life may deviate significantly from actual service life, in some documented cases by nearly a factor of five.Service life represents bearing life in real-world conditions, where field failures can result from root causes other than bearing fatigue. Examples of root causes include contamination, wear, misalignment, corrosion, mounting damage, poor lubrication, or faulty sealing systems.Ongoing advances in bearing technology and manufacturing processes continue to extend bearing life and reduce sensitivity to severe operating conditions. Standard ISO 281 has developed in step with these advances to predict service life more accurately. The latest version expands coverage to include bearing material fatigue stress limits, and a factor for solid contamination effects on bearing life when using various lubrication systems such as grease, circulating oil, and oil bath.The equation calculates modified rating life at n% reliability Lnm in millions of revolutions at constant speed by:Lnm= a1aISOL10 where a1 = life-adjustment factor for reliability (1.0 for 90% reliability); and aISO = manufacturer life modification factor according to ISO 281.Finding aISO involves the use of a contamination factor that considers the lubrication system type, cleanliness class, bearing size, and lubrication operating conditions as defined in ISO 4406. This contamination factor, along with the ratio of the bearing fatigue load limit to the bearing equivalent load limit, and the lubrication condition, determine aISO. In general, better lubricant conditions and lower equivalent loads lessen bearing life sensitivity to contamination levels. Conversely, high loads and poor lubricant conditions raise bearing life sensitivity to contaminationCalculating machine reliability from bearing lifePaul DvorakHere's how to calculate machine reliability based on the collective life of several bearings.Printer-friendly version

A Weibull bearing routine in Weibull-Ease software from Applications Research Inc., Golden Valley, Minn., allows considering all bearings in a system along with input rpm, load, and basic dynamic capacity of the bearing. The routine calculates the L10, L 95, L 99, median, and MTBF lives. The full version of Weibull-Ease can be downloaded on a 30-day trial basis from applicationsresearch.com.

Selecting a set of bearings to withstand anticipated loads is a common design task with power-transmission equipment. Another deals with finding a cumulative component reliability that exceeds the standard 90% at a specified number of cycles or hours. A good way to begin a reliability study defines what constitutes failure. Then identify the most significant failure modes for the machine. A next step analyzes test data for each failure mode to establish the reliability of each as a function of time, cycles, or some other quantifiable measurement. In other words, at time t: R1= f1(t), R2= f2(t) and so on. Then total system reliability becomes: Rsystem= R1 R2R3and so on at time t. As a general rule, evaluate test data in terms of its Weibull distribution because it is rare that any set of field or lab data will fall clearly into a particular standard distribution, such as exponential, binomial, or normal. In addition, a particular failure mode may not occur until something else has worn to a certain level. In other words, a failure mode may be, at least partially, a secondary failure. These usually show up as an unreasonably high Weibull slope number. It can generally be corrected by applying Weibull's third parameter, or offset, from the origin. This is a fairly straight-forward calculation for most any reasonably capable Weibull software routine. Bearings present a unique problem in that they are typically specified by the manufacturer in terms of a dynamic load rating. For ball bearings, this is the load at which about 10% of them fail when rotated to 1 million revolutions of the outer race. Roller bearings are specified using a similar system, but the 10% failure corresponds to 3,000 hr at 500 rpm, or 90 million revolutions. The machine's rpm can then be converted to hours. For example, consider an individual ball bearing with a Basic Dynamic Capacity, CBD, rating of 3,000 lb. It is to carry an effective load, Peff, of 400 lb at 2,500 rpm. Since wear damage varies as the cube of the load, the number of revolutions corresponding to 10% (B10) failure will be

At 2,500 rpm, this corresponds to

Where L10h = bearing life, hr , and = bearing speed, rpm

This is the first 10% failure point. This is all well and good for finding one bearing's 10% failure hours (or revolutions). However, designers should be interested in the complete system which probably includes more than one bearing. The reliability of agricultural hardware or equipment is typically specified by the marketing group as something like "No more than 1% failure (99% reliability) through the first year and no more than 10% failure (90% reliability) through the fifth year." Many consumer products are subject to similar requirements, particularly in the first year. To come up with a system that exceeds 99% reliability through the first year and 90% through the fifth, all critical-component reliabilities must exceed those figures by a significant amount. Bearings are typically included in the category of critical components. One reliability indicator comes from Weibull plots. For example, several research projects tend to show a range of variability in the value of Beta, the Weibull shape (slope) factor for bearings. It is also reasonable to assume that the load level on the bearing or perhaps even the ratio of the Basic Dynamic Capacity to the applied load can affect the Weibull shape factor. For instance, a light load might produce a shape factor close to 1.0. A heavy load might increase it to 4.0 or more. So a reasonable starting value of Weibull Beta or shape factor for most agricultural-equipment (including lawn and garden) bearings is around 2.0. There are good reasons for the assumptions but they are involved and beyond the scope of this article. To continue, recall that a straight line is defined either by two points, or one point and the line's slope. For example, Weibull cumulative density function and its linearized version are:

where F(t) = fraction of population failed, = characteristic life, and = slope or shape factor. Taking the natural log of both sides gives:

the standard slope-intercept form for a straight line for which is the slope. Referring back to the earlier example, we know one of the points to be

We also know that = 2.0, so

and

Also, R(t) = 1-F(t) so

Suppose the most significant failure modes for a particular mechanism involve four bearings at critical locations. Using the method above, we've calculated the other three bearing characteristic lives for this application. We have:

We want the reliability at 1,000 hr for all bearings considered as a single system. Therefore:

Similarly, R2(1,000) = 97.30%, R3(1,000) = 98.23%, and R4(1,000) = 99.04% Rsystem(1,000) = R1R2R3R4 =93.41%Another way to accomplish the above is with statistical analysis software with Weibull plot functions that include a Multiple Mode calculation routinePiston

Piston DesignIts hard to believe the reciprocating piston engine has been around for 137 years. Nicholaus August Otto invented the first such engine in 1866, one year after the Civil War ended. Given that much time, youd think the pistons inside todays engines would be radically different from those of their ancestors. Piston materials and designs have evolved over the years and will continue to do so until fuel cells, exotic batteries or something else makes the internal combustion engine obsolete. But until that happens, pistons will continue to power the vehicles we drive.One thing that has not changed over the years is the basic function of a piston. The piston forms the bottom half of the combustion chamber and transmits the force of combustion through the wrist pin and connecting rod to the crankshaft. The basic design of the piston is still pretty much the same, too. Its a round slug of metal that slides up and down in a cylinder. Rings are still used to seal compression, minimize blowby and control oil.So what has changed? The operating environment. Todays engines run cleaner, work harder and run hotter than ever before. At the same time, engines are expected to last longer than ever before, too: up to 150,000 miles or more and with minimal maintenance. Consequently, heat management is the key to survival of the fittest."Piston design used to be a process of trial and error." says Kent Fullerton, an engineer with Zollner Pistons. "Youd make and test a new design three or four times before you got it right. Today, everything is modeled in 3D on a computer, then evaluated with finite element analysis software before anything is made. That speeds up the design and testing process, reduces the lead time to create new piston designs, and produces a better product."According to a book produced by Mahle Inc. called Pistons for Internal Combustion Engines, engineers use two methods to evaluate new piston designs before they are actually produced for engine dyno testing: finite analysis and photoelastic stress analysis. The idea behind finite analysis is to divide a model piston into a fixed (finite) number of elements. The resulting grid forms lines that intersect and connect. Computer software generates equations for each individual element and predicts the overall stiffness of the entire piston. Analyzing the data shows how the piston will behave in a real engine and allow the engineer to see where loads and temperatures will be greatest and how the piston will react.With photoelastic stress analysis, a 3D transparent resin model is cast of a piston. When the model piston is subjected to loads, the refractive properties of the plastic change causing polarized light passing through the piston to change colors. This reveals how the piston deforms under load and the areas where it is experiencing the greatest stress.Hot PistonsThe most critical area for heat management is the top ring area. One of the "tricks" engine designers came up with to reduce emissions was to move the top compression ring up closer to the top of the piston. A decade ago, the land width between the top ring groove and piston crown was typically 7.5 to 8.0 mm. Today that distance has decreased to only 3.0 to 3.5 mm in many engines.The little crevice around the top of the piston between the crown and top ring creates a dead zone for the air/fuel mixture. When ignition occurs, this area often does not burn completely leaving unburned fuel in the combustion chamber. The amount isnt much, but when you multiply the residual fuel in each cylinder by the number of cylinders in the engine times engine speed, it can add up to a significant portion of the engines overall hydrocarbon (HC) emissions.One of the consequences of relocating the top ring closer to the top of the piston is that it exposes the ring and top ring groove to higher operating temperatures. The top rings on many engines today run at close to 600 F, while the second ring sees temperatures of 300 F or less. These extreme temperatures can soften the metal and increase the danger of ring groove distortion, microwelding and pound-out failure. The reduced thickness of the land area between the top of the piston and top ring also increases the risk of cracking and land failure.The evolutionary advances that enable todays pistons to handle this kind of environment include changes in piston geometry, stronger alloys, anodizing the top ring groove and using tougher ring materials. Ordinary cast iron top compression rings that work great in a stock 350 Chevy V8 cant take the kind of heat thats common in many late model engines. Thats why ductile iron or steel top rings are used in some of these engines.Anodizing has become a popular method of improving the durability of the top ring groove and is now used in many late model engines. Anodizing reduces microwelding between the ring and piston to significantly improve durability. But it cant work miracles: an anodized piston can still fail if it gets too hot.Anodizing is done by treating the ring groove with sulfuric acid. The acid reacts with the metal to form a tough layer of aluminum oxide, which is very hard and wear-resistant. Part of the layer is below the surface of the metal and part is above. On average, the layer is about 20 microns (.001) thick so the piston manufacturer compensates for the added thickness when the top ring groove is machined.Another approach some piston manufacturers have used to improve top ring durability is to weld nickel alloy into the top ring groove. This was the approach used for the OEM pistons in Saturn 1.9L engines made from 1991 to 2001. The 2002-03 Saturn engine uses an anodized top ring groove.Low Tension RingsTo further complicate the problem of heat management, rings have been getting smaller. Starting in the 1980s, "low tension" piston rings began to appear in many engines. Typical ring sizes today are 1.2 mm for the top compression ring, 1.5 mm for the second ring, and 3.0 mm for the oil ring. Some are even thinner. A few engines have top compression rings only 1.0 mm thick, and the current Buick 3800 V6 uses a narrow 2.0 mm thick oil ring.The OEMs went to thinner, shallower rings to improve fuel economy because the rings account for up to 40 percent of an engines internal friction losses. Thinner rings produce less drag and friction against the cylinder walls. But the downside is they also reduce heat transfer between the piston and cylinder because of the smaller area of contact between the two. Consequently, pistons with low tension rings run hotter than pistons with larger rings.Low tension rings also present another problem. They are less able to handle bore distortion. To maximize compression and minimize blowby, the cylinder must be as round as possible. This often requires the use of a torque plate when honing to simulate the bore distortion that is produced by the cylinder head.Piston GeometryChanges in piston geometry have also been made to improve their ability to survive at higher temperatures. Russ Hayes, an engineer with Federal Mogul/Sealed Power, said piston manufacturers used to grind most pistons with a straight taper profile. When the piston got too hot, it would contact the cylinder along a narrow area producing a thin "wear strip" pattern on the side of the piston. "Now we use CNC machining to do a barrel profile on our pistons. The diameter of the piston in the upper land area is smaller to allow for more thermal expansion and to spread any wall contact over a larger area."Pistons are getting shorter and lighter. In the 1970s, a typical 350 small block Chevy piston and pin assembly weighed around 750 grams. The same parts in a late model Chevy LS1 engine weigh only about 600 grams.Part of the weight reduction has been achieved by reducing piston height and using shorter skirts. The distance from center of the wrist pin to the top of the piston (called "compression height") used to be 1.5 to 1.7 back in the 1970s, said Hayes. Today, wrist pins are located higher up. On Ford 4.6L engines, the compression height is 1.2, and its 1.3 on small block Chevys.Moving the location of the wrist pin higher up on the piston also allows the use of longer connecting rods, which improve torque and make life easier on the bearings and rings.Some aftermarket pistons are now available with wrist pins that have been relocated upward slightly to compensate for resurfacing on the block and heads. The other alternative is to shave the top of the piston if the block has been resurfaced, but this reduces the depth of the valve reliefs which may increase the risk of detonation and/or valve damage.Pistons used to have long tail skirts (which sometimes cracked or broke off). Now most pistons have mini-skirts. Instead of a 2.5 skirt length, the piston may only have 1.5 skirt. Shorter skirts reduce weight but also require a tighter fit between the piston and cylinder bore to minimize piston rocking and noise. Consequently, todays piston clearances are much less than before (typically .001 to .0005 or less). Some have a zero clearance fit or even a slight interference fit (made possible by special low friction coatings).Piston MaterialsThe alloy from which a piston is made not only determines its strength and wear characteristics, but also its thermal expansion characteristics. Hotter engines require more stable alloys to maintain close tolerances without scuffing.Many pistons used to be made from "hypoeutectic" aluminum alloys like SAE 332 which contains 8-1/2 to 10-1/2 percent silicone. Today we see more "eutectic" alloy pistons which have 11 to 12 percent silicone, and "hypereutectic" alloys that have 12-1/2 to over 16 percent silicone.Silicone improves high heat strength and reduces the coefficient of expansion so tighter tolerances can be held as temperatures change. Hypereutectic pistons have a coefficient of thermal expansion that is about 15 percent less than that for standard F-132 alloy pistons. Because of this, the pistons can be installed with a much tighter fit up to .0005 less clearance may be needed depending on the application.Hypereutectic alloys are also slightly lighter (about 2 percent) than standard alloys. But the castings are often made thinner because the alloy is stronger, resulting in a net reduction of up to 10 percent in the pistons total weight.Hypereutectic alloys are more difficult to cast because the silicon must be kept evenly dispersed throughout the aluminum as the metal cools. Particle size must also be carefully controlled so the piston does not become brittle or develop hard spots making it difficult to machine. Some pistons also receive a special heat treatment to further modify and improve the grain structure for added strength and durability. A "T-6" heat treatment, which is often used on performance pistons, increases strength up to 30 percent.Machining hypereutectic pistons is also more difficult because of the harder alloy. Consequently, hypereutectic pistons typically cost several dollars more than standard alloy pistons. Thats why most OEMs (except Ford) have gone back to eutectic alloy pistons in their late model engines. High copper eutectic alloys offer most of the advantages of hypereutectic alloys without as much cost.Piston CoatingsSurvival of the fittest also requires a high degree of scuff resistance. Cold starts without adequate lubrication can cause piston scuffing. The same thing can happen if the engine overheats. Piston-to-cylinder clearances close up and the piston scuffs against the bore. The initial start-up of a freshly built engine is also a risky time for scuffing and is of special concern to engine builders because thats when many warranty problems occur.Applying a permanent low friction coating to the sides of the pistons provides a layer of protection against scuffing. Many rebuilders have found that using coated pistons has virtually eliminated warranty problems due to scuffing.Many late model OEM engines including Ford 4.6L V8, Chrysler 3.2L, 3.5L, 3.8L and 4.0L, and GM 3.1L use pistons with graphite moly-disulfide coatings on the piston skirt to improve scuff resistance. Most aftermarket piston manufacturers also offer some type of coated replacement pistons to rebuilders who want them. Coatings typically add about a buck to the price of a replacement piston, but the added scuff protection and reduction in warranty claims more than offsets the higher cost say many engine builders who use them."Thermal barrier" ceramic-metallic coatings for the tops of pistons are another type of coating that have been used on some diesel pistons and performance pistons. Improving heat retention in the combustion chamber improves thermal efficiency and makes more power. It also helps the piston run cooler. But too much heat in the combustion chamber also increases the risk of detonation and preignition, which is not a problem with diesels but is with gasoline engines. So when a coating is used, ignition timing must usually be retarded several degrees to reduce the risk of detonation.Piston CrownsThe shape and finish on the tops of pistons has also been changing. Flat top pistons have been replaced by dished pistons, domed pistons and pistons with intricate contours to swirl the fuel mixture and promote better fuel atomization.Some piston crown designs can be very complex because they are designed to produce the lowest possible emissions with the best overall fuel efficiency. The shape of the crown controls the movement of air and fuel as the piston comes up on the compression stroke. This, in turn, affects the burn rate and what happens inside the combustion chamber. Replacement pistons for stock engines with complex piston designs should be the same as the original to maintain the same emissions and performance characteristics.With performance pistons, designs can be even more specialized. Manufacturers have developed special "fast burn" configurations that allow engines to safely handle more compression without detonating.John Erb of United Engine & Machine (Silvolite and KB Pistons) said an "Attenuator-Groove" is used on some KB pistons to enhance the valve reliefs. The groove removes two potential hot spots in the combustion chamber and improves airflow and wet flow atomization.Another unique design feature, said Erb, is the "Mini-Grooves" machined into the top ring land on KB performance pistons. If the piston gets too hot, the top of the piston swells causing the Mini-Grooves to contact the cylinder. This momentary contact helps cool the piston to reduce the danger of detonation and piston destruction.Piston PinsZollners Fullerton says piston pin holes have also been changing. "Rather than being round and straight, pin bores are taking on new shapes. Some are oval and some are trumpet-shaped, flaring out toward the inside edges of the pin bosses. The reason for these shapes is to accommodate wrist pin bending and ovalization. These variances from straight and round are quite small, measured in tenths of a thousandth, but have proven to extend piston life."

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder wall.

Piston piston is a cylindrical engine component that slides back and forth in the cylinder bore by forces produced during the combustion process. The piston acts as a movable end of the combustion chamber. The stationary end of the combustion chamber is the cylinder head. Pistons are commonly made of a cast aluminum alloy for excellent and lightweight thermal conductivity. Thermal conductivity is the ability of a material to conduct and transfer heat. Aluminum expands when heated and proper clearance must be provided to maintain free piston movement in the cylinder bore. Insufficient clearance can cause the piston to seize in the cylinder. Excessive clearance can cause a loss of compression and an increase in piston noise. Piston features include the piston head, piston pin bore, piston pin, skirt, ring grooves, ring lands, and piston rings. The piston head is the top surface (closest to the cylinder head) of the piston which is subjected to tremendous forces and heat during normal engine operation.A piston pin bore is a through hole in the side of the piston perpendicular to piston travel that receives the piston pin. A piston pin is a hollow shaft that connects the small end of the connecting rod to the piston. The skirt of a piston is the portion of the piston closest to the crankshaft that helps align the piston as it moves in the cylinder bore. Some skirts have profiles cut into them to reduce piston mass and to provide clearance for the rotating crankshaft counterweights.A ring groove is a recessed area located around the perimeter of the piston that is used to retain a piston ring. Ring lands are the two parallel surfaces of the ring groove which function as the sealing surface for the piston ring. A piston ring is an expandable split ring used to provide a seal between the piston and the cylinder wall. Piston rings are commonly made from cast iron. Cast iron retains the integrity of its original shape under heat, load, and other dynamic forces. Piston rings seal the combustion chamber, conduct heat from the piston to the cylinder wall, and return oil to the crankcase.Piston rings commonly used include the compression ring, wiper ring, and oil ring. A compression ring is the piston ring located in the ring groove closest to the piston head. The compression ring seals the combustion chamber from any leakage during the combustion process. When the air-fuel mixture is ignited, pressure from combustion gases is applied to the piston head, forcing the piston toward the crankshaft. The pressurized gases travel through the gap between the cylinder wall and the piston and into the piston ring groove. Combustion gas pressure forces the piston ring against the cylinder wall to form a seal. Pressure applied to the piston ring is approximately proportional to the combustion gas pressure.A wiper ring is the piston ring with a tapered face located in the ring groove between the compression ring and the oil ring. The wiper ring is used to further seal the combustion chamber and to wipe the cylinder wall clean of excess oil. Combustion gases that pass by the compression ring are stopped by the wiper ring.An oil ring is the piston ring located in the ring groove closest to the crankcase. The oil ring is used to lubricate the cylinder wall during piston movement. Excess oil is returned through ring openings to the oil reservoir in the engine block.

The connecting link or arm between the Piston and the Crankshaft. It converts the up-and-down (Reciprocating) motion of the piston into the circular (rotary) motion of the spinning Crankshaft. Often called con rod. It is an element which provides connection between the piston and the crankshaft. It is made by drop forging process, from the steel or duralumin. A lighter rod produces less vibration and regulates power efficiently. Its usual length is kept twice the stroke.Its small end which is connected to the piston may be of solid eye, split eye or slotted type. The big end which is connected to the crankcase pin split type and h as a separate cap. The cap is secured to the body of the rod by means of two or four big end bolts. In some of the connecting rods, through hole or holes at the ends are provided for lubricating process.

The detachable metal (Aluminum or iron) plate or cap that is bolted to the top of the Cylinder block. It is used to Cover the tops of the cylinders, in many cases the cylinder head contains the valves, it also forms part of the Combustion chamber. It has water and oil passages for cooling and lubrication. It also holds the Spark plugs. On most engines a Valve cover or Rocker arm cover is located on top of the cylinder head. Some engines have just one cylinder head covering several cylinders, while others have separate heads for each cylinder. In some Motorcycle engines and small engines, the cylinder head is not detachable -- it is Cast with the cylinder which forms a blind hole.

A metal, split ring installed in the Groove on the outside wall of the Piston. The ring contacts the sides of the Ring groove and also rubs against the Cylinder wall thus sealing the space between the piston and the wall. Poor rings can cause poor Compression and severe Blowby. Often seen as blue smoke out the Exhaust pipe.Functions of Piston rings:Following are the main functions of Piston rings:1. To prevent the leakage of the compressed and expanding cases above the piston into the crankcase.2. To control and provide the lubricating oil between the piston skirt and cylinder walls.3. To prevent the entry of the lubricating oil from crank case to the combusion chamber above the piston head.4. To scrap out the unnecessary and excessive lubricating oil from cylinder walls.5. To prevent the deposit of carbon and other matters on the piston head caused by burning of lubricant.6. To provide easy transmission of heat from piston to cylinder walls7. To balance the side tilting of the piston and to save its life to a certain limitPiston ring Material:A ring should have excellent heat, wear resisting and elastic qualities. Therefore fine grained alloy cost iron has proved superior to any other material used for this purpose. Rings of alloy steel with chromium or hard material plating have also been used but they have not given the results upto the mark.Types of piston rings:A piston ring consists of a set of rings. There may be two, three or four numbers of rings in a set. there are(a)Compression or Gas rings(b)Oil Regulated rings(c) Spring expander piston rings

Crankshaft

Crankshaft is a main rotating shaft running the length of the engine. The crankshaft is supported by Main bearings. Portions of the shaft are offset to form throws to which the Connecting rods are attached. As the Pistons move up and down, the Connecting rods move the crankshaft around. The turning motion of the crankshaft is transmitted to the Transmission and eventually to the driving wheels..

The main parts of the crankshaft are crank pins, main journals, balance weights, webs and flywheel flange. It also contains oil passages for lubrication purposes as shown in the figure. It carries a starting pulley at the front and a flywheel at the rear end. The crankshaft may be of single piece as well as built up type. In case of built up type, pins and journals are bolted to the crank arms. The number of journals and the positions of the pins in different planes depend upon the number of engine cylinders and their arrangements. The material used for the crankshaft is generated described as copper chromium with high carbon and some silicon. A thrust collar provided on one of the main bearings so as to sustain the axial loads along the axis of the crankshaft.

Belt &PulleyA belt and pulley system is characterized by two or more pulleys in common to a belt. This allows for mechanical power, torque, and speed to be transmitted across axes and, if the pulleys are of differing diameters, a mechanical advantage to be realized. A belt drive is analogous to that of a chain drive, however a belt sheave may be smooth (devoid of discrete interlocking members as would be found on a chain sprocket, spur gear, or timing belt) so that the mechanical advantage is given by the ratio of the pitch diameter of the sheaves only (one is not able to count 'teeth' to determine gear ratio).

Belt DrivesWe supply belt drives for use as positive-locking or force-locking drive components. An example of a well-known force-locking drive component is a V-belt and an example of a positive-locking one is a timing belt.Main characteristics of belt drives Force-locking belt drives Force-locking belt drives are characterised by the simple construction of the entire belt transmission. Most drives work without the need for further drive components such as couplings or torque limiters; Belt drives offer a wide scope of application in terms of the power that can be transmitted and reliable rotational speeds; Belt drives require minimal maintenance and do not require lubrication; Belt drives are easy to install; A maximum transmission ratio of up to 1:12 is possible. With ribbed belts, ratios of up to 1:35 can be reached; Unlike gear pairs, belt drives with two pulleys have the same sense of rotation; Multiple shaft drives, whether they have the same or opposite directions of movement, can be dismantled cheaply and easily; The power train is highly flexible thanks to the elasticity of the belts and the force-locking transmission of the drive force. Flexible couplings are usually unnecessary!Positive-locking belt drives Positive-locking, non-slip power transmission; Thanks to the lack of slippage, transmission ratios are constant; Generally speaking, belt drives require minimal maintenance and do not require lubrication; Transmission ratios of up to 1:10 are possible; Power transmission without pre-stressing force and without additional load on the bearing; The lowest of circumferential speeds can be achieved with ease; Simple design of entire belt transmission. Most drives work without the need for further drive components such as couplings; Unlike with gear pairs, the sense of rotation remains the same when belt drives with two pulleys are used; Multiple shaft drives, whether they have the same or opposite directions of movement, can be dismantled cheaply and easily; High circumferential speeds of up to 80 m/s as well as rotational speeds of up to 20000 rpm are possible; The power train is flexible thanks to the elasticity of the belts and the spreading of the load over all the engaged teeth. Flexible couplings are usually unnecessary! An efficiency factor of up to 98% is possible. Belts are the cheapest utility for power transmission between shafts that may not be axially aligned. Power transmission is achieved by specially designed belts and pulleys. The demands on a belt drive transmission system are large and this has led to many variations on the theme. They run smoothly and with little noise, and cushion motor and bearings against load changes, albeit with less strength than gears or chains. However, improvements in belt engineering allow use of belts in systems that only formerly allowed chains or gears. Power transmitted between a belt and a pulley is expressed as the product of difference of tension and belt velocity:[1] where, T1 and T2 are tensions in the tight side and slack side of the belt respectively. They are related as: where, is the coefficient of friction, and is the angle subtended by contact surface at the centre of the pulley.Flat belts

The drive belt: used to transfer power from the engine's flywheel. Here shown driving a threshing machine.Flat belts were used early in line shafting to transmit power in factories.[2] They were also used in countless farming, mining, and logging applications, such as bucksaws, sawmills, threshers, silo blowers, conveyors for filling corn cribs or haylofts, balers, water pumps (for wells, mines, or swampy farm fields), and electrical generators. The flat belt is a simple system of power transmission that was well suited for its day. It delivered high power for high speeds (500hp for 10,000ft/min), in cases of wide belts and large pulleys. These drives are bulky, requiring high tension leading to high loads, so vee belts have mainly replaced the flat-belts except when high speed is needed over power. The Industrial Revolution soon demanded more from the system, and flat belt pulleys needed to be carefully aligned to prevent the belt from slipping off. Because flat belts tend to climb towards the higher side of the pulley, pulleys were made with a slightly convex or "crowned" surface (rather than flat) to keep the belts centered. Flat belts also tend to slip on the pulley face when heavy loads are applied and many proprietary dressings were available that could be applied to the belts to increase friction, and so power transmission. Grip was better if the belt was assembled with the hair (i.e. outer) side of the leather against the pulley although belts were also often given a half-twist before joining the ends (forming a Mbius strip), so that wear was evenly distributed on both sides of the belt (DB). Belts were joined by lacing the ends together with leather thonging,[3][4] or later by steel comb fasteners.[5] A good modern use for a flat belt is with smaller pulleys and large central distances. They can connect inside and outside pulleys, and can come in both endless and jointed construction. Round beltsRound belts are a circular cross section belt designed to run in a pulley with a 60 degree V-groove. Round grooves are only suitable for idler pulleys that guide the belt, or when (soft) O-ring type belts are used. The V-groove transmits torque through a wedging action, thus increasing friction. Nevertheless, round belts are for use in relatively low torque situations only and may be purchased in various lengths or cut to length and joined, either by a staple, a metallic connector (in the case of hollow plastic), glueing or welding (in the case of polyurethane). Early sewing machines utilized a leather belt, joined either by a metal staple or glued, to great effect.[edit] Vee belts

Belts on a Yanmar 2GM20 marine diesel engine.

A multiple-V-belt drive on an air compressor.Vee belts (also known as V-belt or wedge rope) solved the slippage and alignment problem. It is now the basic belt for power transmission. They provide the best combination of traction, speed of movement, load of the bearings, and long service life. The V-belt was developed in 1917 by John Gates of the Gates Rubber Company. They are generally endless, and their general cross-section shape is trapezoidal. The "V" shape of the belt tracks in a mating groove in the pulley (or sheave), with the result that the belt cannot slip off. The belt also tends to wedge into the groove as the load increases the greater the load, the greater the wedging action improving torque transmission and making the V-belt an effective solution, needing less width and tension than flat belts. V-belts trump flat belts with their small center distances and high reduction ratios. The preferred center distance is larger than the largest pulley diameter, but less than three times the sum of both pulleys. Optimal speed range is 10007000ft/min. V-belts need larger pulleys for their larger thickness than flat belts. They can be supplied at various fixed lengths or as a segmented section, where the segments are linked (spliced) to form a belt of the required length. For high-power requirements, two or more vee belts can be joined side-by-side in an arrangement called a multi-V, running on matching multi-groove sheaves. The strength of these belts is obtained by reinforcements with fibers like steel, polyester or aramid (e.g. Twaron or Kevlar). This is known as a multiple-V-belt drive (or sometimes a "classical V-belt drive"). When an endless belt does not fit the need, jointed and link V-belts may be employed. However they are weaker and only usable at speeds up to 4000ft/min. A link v-belt is a number of rubberized fabric links held together by metal fasteners. They are length adjustable by disassembling and removing links when needed. Multi-groove beltsA multi-groove or polygroove belt[6] is made up of usually 5 or 6 "V" shapes along side each other. This gives a thinner belt for the same drive surface, thus it is more flexible, although often wider. The added flexibility offers an improved efficiency, as less energy is wasted in the internal friction of continually bending the belt. In practice this gain of efficiency causes a reduced heating effect on the belt and a cooler-running belt lasts longer in service.A further advantage of the polygroove belt, and the reason they have become so popular, stems from the ability for them to be run over pulleys on the ungrooved back of the belt. Although this is sometimes done with Vee belts with a single idler pulley for tensioning, a polygroove belt may be wrapped around a pulley on its back tightly enough to change its direction, or even to provide a light driving force.[7]Any Vee belt's ability to drive pulleys depends on wrapping the belt around a sufficient angle of the pulley to provide grip. Where a single-Vee belt is limited to a simple convex shape, it can adequately wrap at most three or possibly four pulleys, so can drive at most three accessories. Where more must be driven, such as for modern cars with power steering and air conditioning, multiple belts are required. As the polygroove belt can be bent into concave paths by external idlers, it can wrap any number of driven pulleys, limited only by the power capacity of the belt.[7]This ability to bend the belt at the designer's whim allows it to take a complex or "serpentine" path. This can assist the design of a compact engine layout, where the accessories are mounted more closely to the engine block and without the need to provide movable tensioning adjustments. The entire belt may be tensioned by a single idler pulley. Ribbed beltA ribbed belt is a power transmission belt featuring lengthwise grooves. It operates from contact between the ribs of the belt and the grooves in the pulley. Its single-piece structure is reported to offer an even distribution of tension across the width of the pulley where the belt is in contact, a power range up to 600kW, a high speed ratio, serpentine drives (possibility to drive off the back of the belt), long life, stability and homogeneity of the drive tension, and reduced vibration. The ribbed belt may be fitted on various applications: compressors, fitness bikes, agricultural machinery, food mixers, washing machines, lawn mowers, etc.Film beltsThough often grouped with flat belts, they are actually a different kind. They consist of a very thin belt (0.5-15 millimeters or 100-4000 micrometres) strip of plastic and occasionally rubber. They are generally intended for low-power (10hp or 7kW), high-speed uses, allowing high efficiency (up to 98%) and long life. These are seen in business machines, printers, tape recorders, and other light-duty operations. Timing belts

Timing belt

Belt-drive cog on a belt-driven bicycleTiming belts, (also known as toothed, notch, cog, or synchronous belts) are a positive transfer belt and can track relative movement. These belts have teeth that fit into a matching toothed pulley. When correctly tensioned, they have no slippage, run at constant speed, and are often used to transfer direct motion for indexing or timing purposes (hence their name). They are often used in lieu of chains or gears, so there is less noise and a lubrication bath is not necessary. Camshafts of automobiles, miniature timing systems, and stepper motors often utilize these belts. Timing belts need the least tension of all belts, and are among the most efficient. They can bear up to 200hp (150kW) at speeds of 16,000ft/min.Timing belts with a helical offset tooth design are available. The helical offset tooth design forms a chevron pattern and causes the teeth to engage progressively. The chevron pattern design is self-aligning. The chevron pattern design does not make the noise that some timing belts make at certain speeds, and is more efficient at transferring power (up to 98%).Disadvantages include a relatively high purchase cost, the need for specially fabricated toothed pulleys, less protection from overloading and jamming, and the lack of clutch action.Specialty beltsBelts normally transmit power on the tension side of the loop. However, designs for continuously variable transmissions exist that use belts that are a series of solid metal blocks, linked together as in a chain, transmitting power on the compression side of the loop.Belt tensionPower transmission is a function of belt tension. However, also increasing with tension is stress (load) on the belt and bearings. The ideal belt is that of the lowest tension which does not slip in high loads. Belt tensions should also be adjusted to belt type, size, speed, and pulley diameters. Belt tension is determined by measuring the force to deflect the belt a given distance per inch of pulley. Timing belts need only adequate tension to keep the belt in contact with the pulley.Rope Last updated 0 seconds ago

Coils of rope used for long-line fishingA rope is a length of fibres twisted or braided together to improve their neatness and usability. Ropes have tensile strength and so can be used for dragging and lifting, but are far too flexible to provide compressive strength. As a result, they cannot be used for pushing or similar compressive applications. Rope is thicker and stronger than similarly constructed cord, line, string, and twine.

Why use a chain drive?Chain drives have characteristics which, in certain applications, are more favourable than belt drives or gear drives for providing power transmission between two or more shafts. Their major advantage over V-belt drives is that there is no-slip. A V-belt can slip over the pulleys and hence a constant drive speed is not assured. The positive action of the tooth and sprocket in a chain drive means that chain drive assemblies will be more compact than belt drive assemblies.

Chain drives are often preferred to geared drives because they have less rigorous design requirements. For example shaft alignment is less critical for a chain drive. Often the centre distance between shafts is too long for geared drives so a chain drive is used.

Types of Power Transmission Chain:The most common form of chain drive is the roller chain drive found in applications such as the push-bike, motor bike and car timing chain. Another common form is the inverted tooth or silent chain. For specific information on transmission chains refer to the MECHANICAL DESIGN DATA MANUAL.

Design requirements.The most favourable position for a chain drive is with the sprocket centre lines in the same horizontal plane or inclined at an angle up to 60 degrees. Vertical drives should be avoided. Ideally the centre distance between sprockets should be between 30 and 50 times the chain pitch.

30xPitch(P) * Distance between shaft centres (C) * 50xP

Chain drives are designed with centre to centre adjustment.

Wherever possible sprockets should have more than 17 teeth. This is because the chain forms a polygon (not a circle) around the sprocket. If low teeth numbers are used a cyclic drive speed variation will occur which is called cordal speed variation. If this speed variation is not critical to design specification of the equipment then lower teeth numbers may be used at low drive speeds (say less than 3 m/sec). At higher speeds cordal speed variation will cause excessive sprocket wear. Sprockets with 17 to 25 teeth running at speeds greater than 3 m/sec should be heat treated to give a tough, wear-resistant surface with a Rockwell 'C' hardness 35 to 45.DESIGN PROCEDURE

This design procedure complies with BS 228: 1970.

Chain drive design is done by following this procedure and referring to the DESIGN DATA MANUAL page 77.

1. Calculate the drive ratio R (velocity ratio) given the input RPM and output RPM.

2. Select sprocket tooth numbers. In order to reduce costs standard sprockets are used - (Chart 1 p77).

3. Determine the Service (selection) factor. The service factor takes into consideration the conditions under which the chain drive will be working. Shock loads created by the driving machinery and driven load require more robust design. Using Chart 2 in the DESIGN DATA MANUAL determine the type (class) of the driven machinery, then choose the correct column for the driving machinery and determine the selection factor for the number of teeth on the smaller sprocket.

4. Calculate the Design (selection) power.

5. Select chain size from the power rating chart. The design power rating for simplex, duplex and triplex chains are shown in the three columns on the left hand side of the chart. Wherever possible simplex chains are used. The design power on the vertical axis is referenced with the speed of the smaller sprocket on the horizontal axis to obtain a chain size and lubrication requirements.

6. Check the maximum sprocket bores against the required shaft diameters if known. List or tabulate details of stock numbers for chain, sprockets and bushes.

7. Determine a suitable centre distance if not given. As mentioned a centre distance 30 to 50 times the chain pitch is recommended.

8. Determine the length of chain in number of pitches.Round off answer to an even number of pitches.

As the chain must be made up of an even number of pitches, the actual centre distance must be redetermined so that it corresponds to the chain length calculated above.

9. Calculate the actual centre distance CA.

10. Determine the sprocket pitch diameters and other dimensions as required.

PCDsprocket = number of teeth in sprocket * PCD factor

or

WORKED EXAMPLESelect a suitable chain drive to transmit 2.5kW from a geared electric motor running at 200RPM to a rotary kiln running at 80RPM. Assume moderate (medium impulsive) shock loads. Using a centre distance which is twice the pitch diameter of the wheel sprocket, determine:

(a) the length of the chain in pitches,(b) the actual centre distance.

Data:Power transmitted (P) = 2.5kWPrime mover - geared electric motor; RPM=200Driven machine - rotary kiln (medium impulsive load); RPM=80'Anticipated' centre distance (C) = 2*PCDwheel sprocket

Solution:1. Calculate the drive ratio.2. Select sprocket tooth numbers.

n=number of teeth in pinionN=number of teeth in wheel (driven)N=R*nn1719202123

N42.547.55052.557.5

The first available combination is:

n=23,N=57

Exact Drive Ratio (R) =57/23= 2.478 which is within 1% of desired ratio

3. Determine the service factor.Using the table on Chart 2; n=23

Medium impulsive load (class 2) Steady electric motor input

Service factor=1.03

4. Calculate the design power.

P=Theoretical power*service factor P=2.5kW*1.03 P=2.575kW

5. Select chain size.Refer chart 19T pinions Chain: simplex;RPM=200;P=2.575kW.

Chain can be:

15.875mm pitch, manual lubrication 19.05mm pitch, drip lubrication

6. List catalogue information. Chain;Choose 5/8" (15.875mm) lubrication: Class 1 (manual) Reynold chain No 110056.

Pinion;n=23 Number with Plain bore: 213015 (14mm stock, 55mm max) Number with taper bore 213015/9 (Bush No TB1610) PCD=116.59mm

Wheel:N=57 Number with Plain bore: 213042 (24mm stock, 50mm max) Number with taper bore: 213043/9 (Bush No TB2012)

7. Determine centre distance.

Given: C = 2*PCDwheel = 2*288.19 = 576mm

8. Length of chain in pitches.

9. Actual centre distance (CA).

Gear Design

Gears have been around for hundreds of years and are as old as almost any machinery ever invented by mankind. Gears were first used in various construction jobs, water raising devices and for weapons like catapults.Nowadays gears are used on a daily basis and can be found in most peoples everyday life from clocks to cars rolling mills to marine engines. Gears are the most common means of transmitting power in mechanical engineering.Gears are used in almost all mechanical devices and they do several important jobs, but most important, they provide a gear reduction. This is vital to ensure that even though there is enough power there is also enough torque(is a movement of force).

Spur GearsSpur gears are the most common type of gear they have straight teeth and are mounted on parallel shafts. The main reason for the popularity of spur gears is their simplicity in design, easy manufacturer and maintenance. However due to their design spur gears create large stress on the gear teeth. Spur gears are known as slow speed gears. Spur gears are seen as noisy due to their design so if noise is not a problem spur gears can be used at almost any speed. Spur gears are noisy because every time a gear tooth engages a tooth on the other gear, the teeth collide, and this impact makes a noise. Spur gears can be found in applications like washing machines and electric screwdrivers but due to the noise you will never find them in your car.

The notes below relate to spur gears. Notes specific to helical gears are included on a separate pageHelical GearsIntroductionGears are machine elements used to transmit rotary motion between two shafts, normally with a constant ratio. The pinion is the smallest gear and the larger gear is called the gear wheel.. A rack is a rectangular prism with gear teeth machined along one side- it is in effect a gear wheel with an infinite pitch circle diameter. In practice the action of gears in transmitting motion is a cam action each pair of mating teeth acting as cams.Gear design has evolved to such a level that throughout the motion of each contacting pair of teeth the velocity ratio of the gears is maintained fixed and the velocity ratio is still fixed as each subsequent pair of teeth come into contact. When the teeth action is such that the driving tooth moving at constant angular velocity produces a proportional constant velocity of the driven tooth the action is termed a conjugate action. The teeth shape universally selected for the gear teeth is the involute profile.

Consider one end of a piece of string is fastened to the OD of one cylinder and the other end of the string is fastened to the OD of another cylinder parallel to the first and both cylinders are rotated in the opposite directions to tension the string(see figure below). The point on the string midway between the cylinder P is marked. As the left hand cylinder rotates CCW the point moves towards this cylinder as it wraps on . The point moves away from the right hand cylinder as the string unwraps.The point traces the involute form of the gear teeth.

The lines normal to the point of contact of the gears always intersects the centre line joining the gear centres at one point called the pitch point.For each gear the circle passing through the pitch point is called the pitch circle.The gear ratio is proportional to the diameters of the two pitch circles.For metric gears (as adopted by most of the worlds nations) the gear proportions are based on the module.m = (Pitch Circle Diameter(mm)) / (Number of teeth on gear).In the USA the module is not used and instead the Diametric Pitch d pis used d p = (Number of Teeth) / Diametrical Pitch (inches)

Profile of a standard 1mm module gear teeth for a gear with Infinite radius (Rack ).Other module teeth profiles are directly proportion . e.g. 2mm module teeth are 2 x this profile

Many gears trains are very low power applications with an object of transmitting motion with minium torque e.g. watch and clock mechanisms, instruments, toys, music boxes etc. These applications do not require detailed strength calculations.

Standards AGMA 2001-C95 or AGMA-2101-C95 Fundamental Rating factors and Calculation Methods for involute Spur Gear and Helical Gear Teeth BS 436-4:1996, ISO 1328-1:1995..Spur and helical gears. Definitions and allowable values of deviations relevant to corresponding flanks of gear teeth BS 436-5:1997, ISO 1328-2:1997..Spur and helical gears. Definitions and allowable values of deviations relevant to radial composite deviations and runout information BS ISO 6336-1:1996 ..Calculation of load capacity of spur and helical gears. Basic principles, introduction and general influence factors BS ISO 6336-2:1996..Calculation of load capacity of spur and helical gears. Calculation of surface durability (pitting) BS ISO 6336-3:1996..Calculation of load capacity of spur and helical gears. Calculation of tooth bending strength BS ISO 6336-5:2003..Calculation of load capacity of spur and helical gears. Strength and quality of materials

If it is necessary to design a gearbox from scratch the design process in selecting the gear size is not complicated - the various design formulea have all been developed over time and are available in the relevant standards.However significant effort, judgement and expertise is required in designing the whole system including the gears, shafts , bearings, gearbox, lubrication.For the same duty many different gear options are available for the type of gear , the materials and the quality.It is always preferable to procure gearboxes from specialised gearbox manufacturers

Terminology - spur gears Diametral pitch (d p )...... The number of teeth per one inch of pitch circle diameter. Module. (m) ...... The length, in mm, of the pitch circle diameter per tooth. Circular pitch (p)...... The distance between adjacent teeth measured along the are at the pitch circle diameter Addendum ( h a )...... The height of the tooth above the pitch circle diameter. Centre distance (a)...... The distance between the axes of two gears in mesh. Circular tooth thickness (ctt)...... The width of a tooth measured along the are at the pitch circle diameter. Dedendum ( h f )...... The depth of the tooth below the pitch circle diameter. Outside diameter ( D o )...... The outside diameter of the gear. Base Circle diameter ( D b ) ...... The diameter on which the involute teeth profile is based. Pitch circle dia ( p ) ...... The diameter of the pitch circle. Pitch point...... The point at which the pitch circle diameters of two gears in mesh coincide. Pitch to back...... The distance on a rack between the pitch circle diameter line and the rear face of the rack. Pressure angle ...... The angle between the tooth profile at the pitch circle diameter and a radial line passing through the same point. Whole depth...... The total depth of the space between adjacent teeth.

Spur Gear DesignThe spur gear is is simplest type of gear manufactured and is generally used for transmission of rotary motion between parallel shafts.The spur gear is the first choice option for gears except when high speeds, loads, and ratios direct towards other options.Other gear types may also be preferred to provide more silent low-vibration operation.A single spur gear is generally selected to have a ratio range of between 1:1 and 1:6 with a pitch line velocity up to 25 m/s.The spur gear has an operating efficiency of 98-99%.The pinion is made from a harder material than the wheel.A gear pair should be selected to have the highest number of teeth consistent with a suitable safety margin in strength and wear. The minimum number of teeth on a gear with a normal pressure angle of 20 desgrees is 18.

The preferred number of teeth are as follows12 13 14 15 16 18 20 22 24 25 28 30 32 34 38 40 45 50 54 6064 70 72 75 80 84 90 96 100 120 140 150 180 200 220 250

Materials used for gearsMild steel is a poor material for gears as as it has poor resistance to surface loading. The carbon content for unhardened gears is generally 0.4%(min) with 0.55%(min) carbon for the pinions.Dissimilar materials should be used for the meshing gears - this particularly applies to alloy steels.Alloy steels have superior fatigue properties compared to carbon steels for comparable strengths.For extremely high gear loading case hardened steels are used the surface hardening method employed should be such to provide sufficient case depth for the final grinding process used.

MaterialNotesapplications

Ferrous metals

Cast IronLow Cost easy to machine with high dampingLarge moderate power, commercial gears

Cast SteelsLow cost, reasonable strengthPower gears with medium rating to commercial quality

Plain-Carbon SteelsGood machining, can be heat treatedPower gears with medium rating to commercial/medium quality

Alloy SteelsHeat Treatable to provide highest strength and durabilityHighest power requirement. For precision and high precisiont

Stainless Steels (Aust)Good corrosion resistance. Non-magneticCorrosion resistance with low power ratings. Up to precision quality

Stainless Steels (Mart)Hardenable, Reasonable corrosion resistance, magneticLow to medium power ratings Up to high precision levels of quality

Non-Ferrous metals

Aluminium alloys Light weight, non-corrosive and good machinability Light duty instrument gears up to high precision quality

Brass alloys Low cost, non-corrosive, excellent machinability low cost commercial quality gears. Quality up to medium precision

Bronze alloys Excellent machinability, low friction and good compatability with steel For use with steel power gears. Quality up to high precision

Magnesium alloys Light weight with poor corrosion resistance Ligh weight low load gears. Quality up to medium precision

Nickel alloys Low coefficient of thermal expansion. Poor machinability Special gears for thermal applications to commercial quality

Titanium alloys High strength, for low weight, good corrosion resistance Special light weight high strength gears to medium precision

Di-cast alloys Low cost with low precision and strength High production, low quality gears to commercial quality

Sintered powder alloys Low cost, low quality, moderate strength High production, low quality to moderate commercial quality

Non metals

Acetal (Delrin Wear resistant, low water absorbtionLong life , low load bearings to commercial quality

Phenolic laminates Low cost, low quality, moderate strength High production, low quality to moderate commercial quality

Nylons No lubrication, no lubricant, absorbs water Long life at low loads to commercial quality

PTFE Low friction and no lubrication Special low friction gears to commercial quality

Equations for basic gear relationshipsIt is acceptable to marginally modify these relationships e.g to modify the addendum /dedendum to allow Centre Distance adjustments. Any changes modifications will affect the gear performance in good and bad ways...

Addendum h a = m = 0.3183 p

Base Circle diameterDb = d.cos

Centre distance a = ( d g + d p) / 2

Circular pitch p = m.

Circular tooth thickness ctt = p/2

Dedendum h f = h - a = 1,25m = 0,3979 p

Module m = d /z

Number of teeth z = d / m

Outside diameter D o = (z + 2) x m

Pitch circle diameter d = z . m ... (d g = gear & d p = pinion )

Whole depth(min) h = 2.25 . m

Top land width(min) t o = 0,25 . m

Module (m) The module is the ratio of the pitch diameter to the number of teeth. The unit of the module is milli-metres.Below is a diagram showing the relative size of teeth machined in a rack with module ranging from module values of 0,5 mm to 6 mm

The preferred module values are 0,50,81 1,25 1,5 2,5 3 4 5 6 8 10 12 16 20 25 32 40 50

Normal Pressure angle An important variable affecting the geometry of the gear teeth is the normal pressure angle.This is generally standardised at 20o.Other pressure angles should be used only for special reasons and using considered judgment. The following changes result from increasing the pressure angle Reduction in the danger of undercutting and interference Reduction of slipping speeds Increased loading capacity in contact, seizure and wear Increased rigidity of the toothing Increased noise and radial forces

Gears required to have low noise levels have pressure angles 15o to17.5o

Contact RatioThe gear design is such that when in mesh the rotating gears have more than one gear in contact and transferring the torque for some of the time. This property is called the contact ratio.This is a ratio of the length of the line-of-action to the base pitch. The higher the contact ratio the more the load is shared between teeth.It is good practice to maintain a contact ratio of 1.2 or greater. Under no circumstances should the ratio drop below 1.1.

A contact ratio between 1 and 2 means that part of the time two pairs of teeth are in contact and during the remaining time one pair is in contact. A ratio between 2 and 3 means 2 or 3 pairs of teeth are always in contact. Such as high contact ratio generally is not obtained with external spur gears, but can be developed in the meshing of an internal and external spur gear pair or specially designed non-standard external spur gea