Belt Selection Considerations

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Belt Selection Considerations

1.1Belt Drive Advantages

1.2Belt Drive Disadvantages

2Belt Drive Principles

2.1Area of Contact

2.1.1Crossed Belt Drive

2.1.2Pulley Center-to-Center Distance

2.1.3Idler Pulleys

2.2Belt Tension

2.2.1Accurate Belt Tensioning

2.2.2Establishing Belt Tension

2.3Coefficient of Friction

3V-Belts

3.1V-Belt Construction

3.2V-Belt Components

3.2.1Tensile Member3.2.2Undercord

3.2.3Overcord

3.2.4Cover

3.2.5Adhesion Resin

3.3V-Belt Length

3.4V-Belt Performance

3.5V-Belt Types

3.5.1Conventional Belts

3.5.2Narrow V-Belt

3.5.3Molded Notched V-Belts3.5.4Double V-Belts

3.5.5Power Band V-Belts

3.5.6Light Duty V-Belts

3.6Correct V-Belt Selection

3.6.1Poly V-Belt Advantages

3.6.2Poly V-Belt Cross-Section

4Variable Speed Belts

4.1Variable Speed Belt Construction

4.2Variable Speed Cross-Sections

4.3Variable Speed Belt Sheaves4.4Variable Speed Drives

4.4.1Single Variable Sheave

4.4.2Dual Variable Sheaves

4.4.3Countershaft Dual Variable Sheaves

4.4.4Variable Speed Sheave Alignment

4.4.5Variable Sheave Alignment

4.4.6Variable Sheave Maintenance

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4.5

Positive, or timing, belts are used in applications where slippage cannot be tolerated. Input and

output shafts of the drive unit must be synchronized. These belts have a

[[Gear_Drives#Spur_Gear_Tooth_Profiles|tooth profile]] which mates with corresponding grooves in

the [[Simple_Machines#Pulley|pulleys]], thereby providing the same positive engagement as chain or

gear drives.

4.5.1Timing Belt Pulleys

4.5.2Positive Drive Pitch Sizes

4.5.3Positive Drive Belt Pulleys

4.5.4Minimum Pulley Diameters

4.5.5Selecting Positive Drive Belts

4.5.6Positive Drive Idlers

4.5.7Linked V-Belts

4.5.8Flat Belts

4.5.9Flat Belt Pulleys

4.5.10Crowned Pulleys

4.5.11Flat Belt Idler Pulleys

4.5.12Cone Pulleys

4.5.13Flat Belt Joining

4.5.14Vulcanized Advantages

4.5.15Flat Belt Fasteners

4.5.16Plate-Type Fasteners

4.6V-Belt Sheaves

4.6.1Standard Dimensions

4.6.2Routine Sheave Maintenance

4.6.3Checking Belt Alignment

4.6.4Sheave Balancing

4.6.5Sheaves for V-Belt Drives

4.6.6Taper Lock Bushing Installation

4.6.7Taper Lock Bushing Removal

4.6.8Belt Installation

4.6.9Worn Belt Removal

4.6.10Belt Tensioning Motor Bases

4.6.11Troubleshooting Belt Drives

Belt drivesfor power transmission are classed as frictional drives. The belt transmits powerby friction contact between the belt and the driving and driven sheave.

Power transmission belts are available in several types: flat belts, V-belts, synchronous

belts, and multi-ribbed belts.

To obtain the best service from any particular belt application, remember:

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1. Select the correct belt for the job.

2. Ensure that the belt is installed correctly and used properly.

Belt Selection Considerations

Environmental conditions in which the belt will operate, such as: exposure to oil and grease,range of operating temperatures, abrasive dust and chemical conditions, sunlight, and other

weather conditions. Other considerations include:

Type of drive required

Driver/DrivenRevolutions Per Minute(RPM)

Horsepower requirements

Pulley diameters and center distance

Take-up allowances and take-up design

Space limitationfor operation

Pulsating or shock load conditions

Static dissipation problems

Belt availability and inventory considerations

Belt construction and service life

Belt Drive Advantages

Wide range of speeds available.

Belts permit flexibility ranging from high horsepower drives to slow speed and high speed

drives.

Belt drives are less expensive than chain drives for low horsepower and low ratio

applications.

Belts require nolubrication.

Single belt drives will accept more misalignment than chain drives.

Flat beltsare best for extremelyhigh speeddrives.

Belt drives cushion shock loads and load fluctuations.

Belts will slip under overload conditions, preventing mechanical damage to shafts, keys,

and other machine parts.

Belt Drive Disadvantages

Belts cannot be used where exact timing or speed is required because slippage does occur(onlytiming beltscan be used).

Belts are easily damaged by oil, grease, abrasives, some chemicals, andheat.

Belts can be noisy; also loose or worn belts can be a major cause of machinery vibration.

Belt Drive Principles

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Flat beltsandV-beltstransmit power by their grip on thepulleyor sheave.

Three major factors determine the potential of the grip:

1. Area of contact

2. Belt tension

3. Friction between the belt and pulley or sheave surface (coefficient of friction)

Area of Contact

The area of contact is determined by width and the arc of contact. The arc of contact

withpulleysof equal diameters is 180 degrees on each pulley, as shown in Figure 1.

Figure 1: Area of ContactPulleys of equal size are not always used. With pulleys of unequal diameter, the arc of

contact is less than 180 degrees on the smaller pulley. Under most conditions, this small

pulley is the driver. An example is shown in Figure 2.

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Figure 2: Unequal Pulleys

An arc of contact greater than 180 degrees can be obtained three ways:

1. Acrossed belt drive.

2. Moving theinput and outputshafts farther apart.

3. Using an idler or snub pulley.

Crossed Belt Drive

A crossed belt drive, as shown in Figure 3, is not usually recommended forV-belts. In the

crossed position, the center-to- center distance between thepulleysmust be long enough

to limit the internal stress in a belt. Crossed belt drives make the pulleys rotate in opposite

directions to each other.

Figure 3: Crossed Belt Drive

Pulley Center-to-Center DistanceFor maximum power transfer on the belts andpulleys, the pulley ratio should be 3 to 1 or

less as shown in Figure 4 Top. Higher ratios, as in Figure 4 Bottom, lessen the arc of

contact, causing slippage and loss of power.

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Figure 4: Pulley Center to Center Distance

The arc of contact on the critical smaller pulley may be increased if the shafts are moved

farther apart as shown in Figure 5. Where a high ratio is required, a two-step drive (counter-

shaft) can be used to avoid excessive single-step ratios or undersize pulleys.

Figure 5: Increase Arc of Contact

Idler Pulleys

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A properly designed V-belt drive does not require an idler to deliver fully rated horsepower if

properbelt tensionandarea of contactare maintained. Idlers put an additional bending

stress on the belt, which reduces belt life. Also, the smaller the idlerpulley, as shown in

Figure 6, the greater reduction in belt life.

Figure 6: Idler Pulley

The best location for an inside idler is on the slack side of the drive. Figure 7 Top shows a

backside idler that is commonly used to help increase the arc of contact on both pulleys.

This idler forces a backward bend in the belt, which decreases belt life. The idler puts

additional strain on the bottom portion of the belt, which may crack that section. The

diameter of the flat idler pulley should be at least 1.5 times the diameter of the smallest

sheave located as close as possible to the small sheave. Figure 7 Middle shows an inside

idler. An inside idler reduces the arc of contact but the amount of take-up is unlimited. The

smaller arc of contact will decrease the horsepower rating of each belt. Figure 7 Bottom

shows a backside idler, which is located as close as possible to the driven pulley. In this

example, the idler helps to increase the arc of contact on the large diameter pulley, which

reduces belt slippage problems that may be encountered on the driven side.

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stock drive tables of belt manufacturers and the number of belts on the drive conforms to

manufacturer's recommendations.

Accurate Belt Tensioning

Refer to Figure 8 for reference to the following tensioning steps:

1. Measure the span "T".

2. At the center "T" apply force with the tension tester perpendicular to the span,

sufficient to deflect one belt of the drive 1/64th inch per one inch of span length from

its normal position.

Figure 8: Belt Tension Measurement

1. Determine the amount of deflection distance on the lower linear scale of tension

tester, as shown in Figure 9, by sighting straight across the tops of the belts. A

straight edge laid across the belts can provide accurate readings.

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Figure 9: Tension Tester

1. Find the amount of deflection force on the upper scale of the tester. The sliding

rubber O-ring collar slides down scale as the tester compresses. The collar stays

down to give a pressure reading.

2. Compare the deflection force reading with the general range of forces listed in Table

1. If less than normal recommended deflection force exists, belts should be

tightened. If more than maximum deflection force is found, the drive may be tighter

than needed.

Table 1: Recommended deflection Forces

Recommended Deflection Forces (in lbs.)

Belt NormalMaximumNew Belts

A 2 3 4

B 4 6 8

C 8 12 14

D 12 22 26

E 21 35 40

3V 4 7 9

5V 9 12 15

8V 20 30 40

1. A V-belt manufacturers manual will provide a proper deflection force figure to suit

specific belt types.

Example: Find the deflection required for a new C-section V-belt installed on sheaves with

32-inch centers and a required pull of 14 pounds.

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T=32 inches

Deflection= T/64=32"/64"=12 "(12.7 mm)

The drive will be tightened up until the deflection of the belts is 1/2 inch with a 14-pound

push. Figure 10 shows how a spring scale can be used to obtain the required deflection

force for accurately tensioning a belt. A ruler can be used to measure the belt after therequired deflection force is applied.

Figure 10: Spring Scale

Establishing Belt Tension

The drive powers the drivenpulleyby the pull, which results in increased tension and

stretch on the tight side of the unit as it overcomes the load resistance. The slack side has

no tension increase, it simply returns to the driven pulley. As shown in Figure 11, belts

should run with a distinct tight and slack side.

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Figure 11: Establishing Belt Tension

Keep take-up guides, rails and motor base area free of dirt, moisture and grit. Keep the

take-up screws clean and periodically apply a lightlubrication. This makes for easieradjustments when belts have to be tightened or replaced. If one or more belts are too loose

(Figure 12) or too tight (Figure 13), one of the following problems exists:

Worn sheaves

Improper belt tension

Damaged belts

Improperly matched belts

Angular sheave misalignment

Figure 12: Too Loose

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Figure 13: Too Tight

Coefficient of Friction

Figure 14 can be used to help define the coefficient of friction for belt drives. If a body of

weight ("W") rests on a horizontal plane surface and a force ("P") parallel to the surface is

sufficient to cause the body to be at a point of slipping, then the ratio of "P" to "W" is the

coefficient of friction ("F") between the two surfaces.

Coefficient of Friction (F)= P/W

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Figure 14: Determining COF

Friction between sliding surfaces, as in belt andpulleysurfaces, is not influenced by the

area of surface. The friction is solely dependent on the character and condition of the faces,

and the total pressure normal to the faces.

V-BeltsV-belts are designed to operate in V-shaped grooves in the sheaves used for power

transmission. V-belts have a major advantage over other types of belt friction drives; as the

wedging effect of the belt pushing into the sheave results in lower belt take-up tension being

required. For the same horsepower, sheave diameter, and sheave speeds, V-belts will

operate with lower tension and, therefore, lower bearing load than other friction-type belt

drives.

V-Belt ConstructionIndustry standards exist which control sheave groove details forV-belts. Due to

manufacturing differences, mold details and various belt materials, the belt of one

manufacturer may differ slightly in shape, stretch and friction characteristics from belts of

the same cross-section made by another manufacturer. Belt manufacturers meet the

standards and tolerances as set by the Rubber Manufacturers Association (RMA). Each

manufacturers belt must operate at the same speed in the standardized sheave groove.

V-Belt Components

A V-belt consists of five inter-related components. Refer to Figure 15 for reference to

typicalV-belt construction.

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Figure 15: V-Belt Components

Tensile Member

The tensile member orpitchline purpose is to withstand the tension or pull that is imposed

to transmit the desired power. The tensile member materials commonly used are rayon,nylon, polyester, steel, fiberglass, and Kevlar.

Undercord

The undercord materials commonly used are: natural or synthetic rubber compounds, fiber-

loaded rubber compounds, woven natural or synthetic cords, or piles.

Overcord

The overcord locates thetensile membercorrectly in relation to other belt components,

and it also assists in preventing the tensile member from sagging in the center under load.

Cover

The cover protects the internal belt components from weather and environmental

conditions. It also provides the wearing surface for the belt. The cover must remain flexible,

and may be oil andheatresistant. The cover material meets RMA standards for static

conductivity, and most belt covers are flame-resistant; they do not catch fire from heat build-

up if the belt is subjected to severe slippage.

Adhesion ResinThe adhesion resins or gums act as a cushion to preventtensile membersfrom rubbing

together as well as fully bonding all of the belt components together. Continual flexing of the

belt tends to loosen the cords from the surrounding bonding material. To prevent excess

cord separation, the adhesive resins must completely saturate the tensile cords. Through a

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V-Belt Performance

Some exact length V-belts may appear to hang unevenly when installed. It is normal for

belts that are only hundredths of an inch apart in length to create noticeable differences in

deflection (within RMA tolerances). The sag is more noticeable on longer length drives, but

does not affect the drive performance or the belts ability to equally share the load. This

condition may also be referred to as the catenary effect. If an 8V2500 V-belt is in use, theallowable length tolerance is approximately .45 inch. This means that in any set 8V2500

belts, the difference between the shortest and longest belt cannot exceed .45 inch.

Therefore, over the belt length span each belt will hang differently because of the allowable

length tolerance set by the RMA.

V-Belt Types

Conventional Belts

The conventional V-belt is the most common belt in use. These belts are sized as to cross-

section and there are five cross-section sizes (A, B, C, D, E) as shown in Figure 16.

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Figure 16: Conventional V-Belt Sizes

Conventional V-belts are made with either a flat or a concave sidewall. Bend a V-belt as if it

were running around a sheave. One can feel the concave sides fill out and become straight

as in Figure 17. This precise fit ensures full contact with the sides of the sheave and the belt

grips the sheave evenly, distributing the wear uniformly across the side of the belt.

Figure 17: V-Belt on a Sheave

Table 3 identifies conventionalV-Belt lengthranges, angles, maximum cross-section width,

and belt thickness.

Table 3: Conventional V-Belts

Section Length Range (in.)Angle (0)Top Width (in.)Thickness (in.)

A 14-180 42 .500 (1/2) .313 (5/16)

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B 22-330 42 .656 (21/32) .406 (13/32)

C 51-480 42 .875 (7/8) .531 (17/32)

D 120-600 42 1.250 (1 1/4) .750 (3/4)

E 210-660 42 1.250 (1 1/2) .906 (29/32)

Narrow V-Belt

Narrow cross-section V-belts transmit up to three times the horsepower of conventional V-

belts in the same drive space, or the same horsepower. Three cross-sections of narrow V-

belts are available, as shown in Figure 18; again all sizes are nominal.

Figure 18: V-Belt Cross-Section

Narrow V-belts provide savings in drive space with narrower sheaves, shorter centers,

smaller sheave diameters, and reduced sheave weight which may help decrease bearing

loads. Greater speeds can be handled by this type of V-belt; up to 6,500 FPM. Narrow V-

belts have a narrow cross-section, but they sit deeper in the sheave groove than a

conventional V-belt. Concave sides are commonly used which makes for more uniform belt

wear. The radius relief minimizes corner wear and the arched top helps prevent dishing and

distorting of thetensile member. The belt number identifies the belt cross-section and

effective length. The number preceding, such as 3V, indicates the top width of the belt in

1/8ths of an inch. The number following indicates the effective outside circumference.

Example: 3V400 = 3/8 inch (9.5 mm) cross-section and 40 inch (1,016 mm) effective

outside circumference Table 4 identifies narrowV-belt lengthranges, angles, widths, and

thicknesses.

Table 4: Narrow V-Belts

Section Length Range (in.)Angle (0)Top Width (in.)Thickness (in.)

3V 25-150 38 .375 (3/8) .313 (5/16)

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5V 50-355 40 .625 (5/8) .531 (17/32)

8V 100-560 40 1.000 (1) .875 (7/8)

Molded Notched V-BeltsNotched V-belts provide higher horsepower rating than conventional cross-section belts.

They are suited for drives with smaller sheave diameters where conventional cross-section

V-belts would not be practical. Notched V-belts, as shown in Figure 19, can be used on

some heavy duty A, B, C, and D drives. The molded notch in the belts bottom surface helps

to reduce bending stress and provides uniform distribution of load. Notches also help to

dissipate theheatof rapid flexing.

Figure 19: Notched V-Belt

Double V-Belts

Double V-belts are used on serpentine drives, as shown in Figure 20, transmit power to two

or more sheaves through both the top and bottom of the belt.

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thicker than on a normal V-belt, thus the backing rides well above the sheave. The same

wedging action in the sheave groove is obtained as with a set of individual V-belts. The

backing provides for increased transverse rigidity. Figure 23 shows a cross-section view of

a power band narrow V-belt. These belts do not prevent vibration, they merely restrict it to

an up and down motion, and prevent the belts from rolling over in the groove or jumping off

the sheave.

Figure 23: Cross-Section View of Power Band Narrow V-Belt

The cross-section and spacing of a power band is such that standard multiple groove

sheaves can be used. Cross-sections are also available in 3V, 5V, or 8V. Power band belts

are also available in B, C, or D cross-sections. A and E cross-sections are available in

production lots only. Figure 24 shows a conventional cross-section power band belt.

Figure 24: Conventional Cross-Section Power Band Belt

Table 6 identifies the length range for both conventional and narrow cross-section power

band V-belts.

Table 6: Power Band V-Belt Length Range

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Classical Wedge

Section Length Range (in.)SectionLength Range (in.)

B 60-300 3V 60-140

C 120-420 5V 118-355

D 120-660 8V 112-660

Light Duty V-Belts

Light duty V-belts are used in the fractional horsepower range and are often referred to as

fractional horsepower or FHP V-belts. They are commonly used singly on small pumps,

compressors, lawn mowers, garden tractors, home appliances, small fans, and other light

equipment. They generally transmit less than one horsepower. The RMA standard nominal

cross-sections for light duty belts are identified as 2L, 3L, 4L and 5L. Figure 25 identifies thecross-section and nominal size of FHP V-belts.

Figure 25: Cross-Section and Nominal Size of FHP V-Belts

Table 7 indicates the length range, angle, cross-section width, and the belt thickness for

FHP V-belts.

Table 7: Light Duty (FHP) V-Belts

Section length Range (in.)SectionLength Range (in.)

B 60-300 3V 60-140

C 120-420 5V 118-355

D 120-660 8V 112-660

Correct V-Belt Selection

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Figure 27: Poly V-Belt

Poly V-Belt Advantages Reduced belt thickness permits use of smaller sheaves.

Lighter, more compact drives are available.

Speed ratios of up to 40 to 1 are available.

Center distances are reduced; space is saved with no loss in horsepower.

Even distribution of pressure over all parts of the drive surface provides uniform loading.

Smooth running, good response to shock loads.

No belt turnover, smooth tracking.

Poly V-Belt Cross-Section

Table 8 identifies the cross-section of the three common poly V-belts. J, L, and M cross-section sizescovera broad range of applications including appliances, automotive

accessories, agricultural equipment, as well as light- and heavy-duty industrial drives. H and

K cross-sections are available but they are limited to specialized drives. H is intended for

miniature drives and K for automotive accessory drives.

Table 8: Poly V-Belt Cross-Section

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FHP V-Belts

Section Length Range (in.)No. of RibsRib Width (in.)Power Range (hp)

J 18-98 2-145 .094 (3/32) up to 15

L 50-145.5 4-90 .188 (3/16) 5-50

M 90-361 3-40 .375 (3/8) 25-1,700

Figure 28 is a cross-section selection chart for poly V-belts based on design horsepower.

Figure 28: Cross-Section Selection Chart for Poly V-Belts

In reference to Figure 28:

Along the horizontal axis of the chart, find the design horsepower of the drive.

On the left side of the chart, along the vertical axis, find the RPM of the faster shaft. The

proper poly V-Belt cross-section is found where the two lines intersect.

Variable Speed Belts

Variable Speed Belt ConstructionVariable speed belts are molded into an arch construction shown in Figure 29. A strong

compression section gives these belts excellent crosswise rigidity that resists squashing or

distorting. Abrasion resistant compounds assure that the belt grips both faces of the sheave

uniformly.

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Table 9: Normal Variable Speed Belt Cross-SectionRefer to Figure 29 for reference to the dimensions of twelve cross-sections of variable

speed belts on Table 9. The variable speed belts produced by various manufacturers may

differ form the nominal dimension indicated in the tables, but all standard variable speed

belts will operate interchangeably in standard sheave grooves designated by the same

number. The twelve selected cross-sections of variable speed belts, ranging in top width

from 7/8 inches and four sheave groove angles (22, 26, 30, and 36 degrees) will provide the

necessary speed variation and power capacity for many industrialvariable speed drives.

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Table 10: Standard Variable Speed Belt Lengths

Variable Speed Belt SheavesStandard variable speed belt sheave designs conform to the dimensions and tolerance

indicated in Table 11 and Figure 31. The included groove angle of the sheave, top width,

and clearance are also identified.

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Figure 31: Closed and Open Sheaves

Table 11: Variable Sheave Groove Dimensions

The sides of the sheaves grooves should be smooth with a surface finish of 125 micro-

inches or less. The groove surfaces should be free of defects, scratches, and the edges of

the groove should be rounded. Variable speed sheaves should have a maximum TIR (Total

Indicator Reading) of .010 inch eccentricity. Sheaves over 10 inches in diameter can haveallowable eccentricity of .0005 inch per inch of additional diameter. Side wobble and run-out

on the sheave should be held to within .001 inch TIR per inch of outside diameter. Most

variable speed sheaves are designed for maximum rim speeds of 6,500 FPM (Feet Per

Minute). Dynamic balancing is recommended where high speeds and vibration are present.