Power Transmission

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POWER TRANSMISSION

POWER TRANSMISSSION:

We know that the electrical energy is converted into rotational energy by the

electric motor. The motor is generally used to drive machines and machine tools .The

former is termed as “driver ” and the latter as “driven “.the systems which transmit power

from the driver shafts to the driven shafts are known as “transmission systems or drives”.

A “shaft” is a rotating machine member usually having a circular cross section much

smaller in diameter than its length .Power transmitting element, namely, pulley belt

,ropes, chain ,gear etc, are on the shafts.

POWER TRANSMITTED BY SHAFTING

POWER IS THE RATE OF DOING WORK

=FORCE×.VELOCITY

If n is the speed of the shaft in pm, T is the torque transmitted by the shaft in N-m ,then

the power transmitted is given by the equation:

P=(2πNT) ⁄ (60×1000)

TYPES OF DRIVES

1.Belt drives

2.Rope drives

3.Chain drives

4. Gear drives

The belts , ropes and chains are for the transmission of power over comparatively long

distance and are knowas“flexible machine elements”

1 Belt drives :Belts are used to transmit power between two parallel shafts . A belt drive

consists of two pulleys on which a belt is passed .One of the pulleys called the “driver is

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mounted on the driving shaft, while the other, which is mounted on the shaft to which

power is transmitted is called the “driven”.

The diameters of the driving and driven pulleys depend on the “speed ratio ” of

the drive . The power is transmitted from the driver to the driven by the frictional grip

between the belt and the surfaces of pulleys For the belt to more to move over the

pulleys, the tension on one side of the belt should be certainly more than the other side .

TYPES OF BELTS

Based on the belt cross section, the drives are divided as follow:

FLAT BELT

It is used in factories , workshops and flour mills for transmitting a moderate amount of

power ,when the centre distance between the driver and driven is not more than 8m.

V-BELT

It is used for transmitting a moderate amount of power, when the centre distance between

the driver and driven is very less.

Belt materials

The properties of belt materials are high co-efficentof ,flexibility, strength and durability.

Leather:The best leather belt is made from 1to 1.5 m long strips cut from either side of

back bone of ox .The hair side of the leather is smoother than the flush side and is in

contact with pulley surface to give a more intimate contact between the belt and pulley

In order to increase the thickness of the belt, the layer or strips of leather are connected

together . Belts are specified according to the number of layer ,e.g single–ply ,double –

ply or triple –ply belts .These are used in machine tools, compressor ,generators,etc.

Cotton or fabric belts:Fabric belts are made of folding canvass or cotton duck to 2 to 4

layers and impregnated with some filler like linseed oil in order to make the belts water

proof and to prevent injury to the fibers’ .cotton belts are used in agricultural machines ,

belts conveyors , etc.

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Rubber belts: These are made of layer of fabric impregnated with rubber layer on the

faces. These belts are very flexible and are used in saw mills, where they are exposed to

moisture.

Balata belts: These are similar to the rubber belts ,but ,balata gum is used instead of

rubber .These belts are acid and water proof. Also they should not work at the

temperature above 40ºC ,because balata begins to soften and becomes sticky.

TYPES OF BELT DRIVES

a) Open belt drives

b) Crossed belt drives

a)Open belt drive:

The open belt drive is used when two parallel shafts of the driver and the driven rotate in

the same direction.

Fig: Open belt drive

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TERMINOLOGY

VELOCITY RATIO:The velocity ratio (i) is defined as the ratio of the speed of the driving

pulley (N1 in rpm) to the speed of the driven pulley (N2 in rpm) Assuming no slip

between the belt and pulleys , the speed at every point in the belt is same .Therefore

the linear speed of the belt and the circumferential speeds of the driving and driven

pulleys are equal.

Tight Side: Assume clockwise rotation of driven. The driver pulls the belt from lower

side and drivers it to the upper side .Thus the tension pulls the belt from side and

delivers it to the upper side .Thus the tension in the lower side belt will be more than

that of the upper side belt .Because of higher tension,the lower side belt is known as

“tight side”.

T1= TENSION IN THE TIGHT SIDE

SIACK SIDE:

Because of less tension, the upper side is known as “slack side”

T2=tension in the slack side

NOTE: 1 The tight and slack sides of the belt depend on the direction of rotation .

2. The tight side of the belt should be at the bottom, so that the sag that is

present on the slack side will increase the arc of contact and hence the power

transmitted.

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Crossed beltdrive:

Fig.Crossed belt drive

It is used when two parallel shafts of the and driven rotate in the opposite directions .At

the point when the belt crosses , it runs against itself causing excessive wear .To avoid

this , the shaft should be placed at a maximum distance of 20w , where w is the width of

the belt and operated at velocities less than 15m/sec.

STEPPEDCONEPULLEY:

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FIG Stepped cone pulley

It is used the speed of driven shaft is to be changed often while the driving shaft

runs at constant speed .(e.g.) machine tools such as lathe , drilling machine ,planer etc .

The stepped cone pulley or stepped cone is an integral casting of three or more number

of pulleys of different sizes .one stepped pulleys is mounted on the driving shaft while

another similar stepped pulley is mounted in the “reverse position ” on the driven shaft.

An endless belt is passing over one of pulleys .The speed of the driven shaft can

be changed by shifting the belt from one pair of pulleys to another.

Belt drive with idler pulley: In an open belt drive, if the centre distance is less or if one of the

pulley is very small, then the angle of contact will be very small. The tensions in the belt will

also be less. Hence the belt drive cannot be used.

Fig. BELT DRIVE WITH IDLER PULLEY

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In this case ,an”idler pulley”or “jockey pulley” is provide on the slack side of the belt and near

the the smaller of the two pulleys. The introduction of idler pulley will increase the angle of contact

hence the value of tensions.

Advantages of belt drive:

They are simple. They are economical.

Parallel shafts are not required.

Overload and jam protection are provided.

Noise and vibration are damped out. Machinery life is prolonged because load

fluctuations are cushioned (shock-absorbed).

They are lubrication-free. They require only low maintenance.

They are highly efficient (90–98%, usually 95%). Some misalignment is tolerable.

They are very economical when shafts are separated by large distances

Disadvantages of belt drive:

They require routine belt replacement

Speed ratio is not constant (slip & stretch)

Heat accumulation

Speed limited

Power limited

Endless belts needs special attention to install

Less efficiency compared to gear drives

Life is less

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GEAR DRIVE:

Gears are defined as toothed wheels which transmit power from one shaft to another by means of

successive engagement of teeth.

Gear drives are used to transmit moderate or large amount of power positively over a short distance

with a constant velocity ratio. It is possible to drive shaft that are parallel,intersecting or neither parallel

nor intersecting by the use of tooth gears.

Working of gears:Gears work in pair as “driver “and “driven”. Teeth of one gear fitted on to a

shaft,mesh into the corresponding recess on the other gear,fitted on to another shaft.in a gear pair,the

smaller gear called “pinion”and the larger is called “gear”.Meshing of teeth of a pair of gears makes it a

“positive”drive.

TYPES OF GEARS:

Gears are classified according to the relation of axes as follows:

*Spur gears-for parallel axes shafts:

*Helical gears-For parallel,non-parallel and non-intersecting axes shafts;

*Bevel gears-For intersecting axes shafts;

*Worm gears-For non-parallel and non-coplaner axes shafts.

Spur Gears

Fig. SPUR GEARS:

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Gears whose axes are parallel and teeth are straight and parallel to centre line of the gear

are called “spur gears”. In spur gears ,teeth are cut on cylindrical surface and parallel to the gear

axis.

Pitch circle: it is an imaginary circle; which by; pure rolling action, would give the same

motion as the actual gear. Gears represented by their pitch circles.

USES:

Spur gears are used over a wide range –from small watches, precision measuring instruments to

machine tools such as lathe, drilling machines, milling machine,etc,automobile gear boxes,

rolling mills,etc.

HELICAL GEARS:

Helical gears are similar to spur gears except that the teeth are cut in the form of helical around the gear

Fig.Helical gear

The outstanding advantage of helical gears, as compare with corresponding spur gears,

is that helical gears run more smoothly and more quietly at high speeds and heavy loads,

because of gradual engagement of teeth.

Also a smaller helical gear can transmit the same load as a large spur gear.

Bevel Gears:

When the axes of two shafts are inclined to one another and intersect, bevel gears are

used. In a bevel gear, teeth are cut on a conical surface, such as would be represented by a

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truncated cone. In the great majority of bevel gear drives, the shaft are at right angle (Fig. a),

but the angle between the shaft may also be either greater or less than 90º(Fig.b).

Worm Gears:

Worm gear drive consists of a worm having one or more helical threads and a worm

wheel being a gear wheel. Worm is the driver and worm wheel is the driven. Worm gears are

used when the axes of the driving and driven shafts are at right angles and do not intersect.

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Fig.Worm Gears

Worm gearing is employed to obtain very high velocity ratios upto 100:1. It is very

compact compared with equivalent spur or helical gear drive for the same speed reduction.

Efficiency is, however, very low compared with other type.

Uses: Machine tools like lathe, drilling machine , milling machine etc, and materials handling

equipment are equipped with worm gear drives.

Velocity Ratio : It is inversely proposional to the number of teeth of the gears.

Velocity Ratio or Speed Ratio of Gear drive is given by the ratio.

= speed of driving gear/speed of driven gear = N1/N2 =T2/T1

Where N1 and N2 : Speed of driving and driven gears,rpm;

T1 and T2 : Number of teeth on driving and driven gears.

For worm and worm wheel drive,

Velocity Ratio = RPM of worm/RPM of worm wheel =no. of teeth in worm wheel/no. of

threads in worm

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Advantage of Gear Drives

• Velocity ratio is constant and is hence positive drive;

• Used to transmit large power, which is beyond the range of belt or chain drives;

• Compact construction, since centre distance is small;

• Efficiency is as high as 99% in case of spur gears;

• Reliability in operation;

Provision can be made in the gear box for gear shifting, thus changing the velocity ratio over a

wide range,(e.g) Machine tools like lathe, etc.

Disadvantages

• Gear manufacturing requires special tools;

• Error in cutting teeth profile causes vibration and noise;

• Maintenance cost is high; lubrication is required;

• Require precise alignment of shaft.

Rack and pinion gears:

A rack and pinion is a type of linear actuator that comprises a pair of gears which

convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on

a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to

move, thereby translating the rotational motion of the pinion into the linear motion of the rack.

For example, in a rack railway, the rotation of a pinion mounted on a locomotive or a railcar

engages a rack between the rails and pulls a train along a steep slope. Rack and pinion

arrangement is used in feeding mechanisms, and reciprocating drives. It is used in various

industries like sag mills, sugar industries, gold mines, sponge iron plants, ball mills, cement

plants, coal mills, grinding mills and tube mills, etc.

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A perfect example of a rack and pinion gear system is the steering system on many cars.

The driver turns the steering wheel which rotates the gear which then engages the rack so as the

gear turns it slides the rack to the right or the left depending on which way the steering wheel is

turned.

Gear Trains:

Any combination of gear wheel by means of which power is transmitted from one shaft to another shaft is called a “Gear Train”.Usually, the “train” is applied only to those combinations, in which there are more than one pair.

Gear trains are not confined to spur gears only, but may include helical, bevel, etc. The

nature of gear train used depends upon the required velocity ratio and the relative position of

the axes of the shafts.

Types of Gear trains

• Simple gear Trains

• Compound Gear Trains

• Reverted Gear Trains

• Epicyclic Gear Trains.

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Simple Gear Trains :

A simple gear train is one in which each shaft carries one gear only.Simple gear trains

are employed where a small velocity ratio is required.

Fig.Simple gear trains

When the distance between two shafts is small, gears 1 and 2 are mesh with each other

to transmit motion from one shaft to the other.Since gear 1 drives 2, the former is called the

“driver” and the latter is called the “driven”.

Note :(i) The direction of rotation of the driven gear is opposite to that of the driving gear.

(ii) Velocity Ratio is inversely proportional to number of teeth of gear

And velocity ratio =N1/N2 =T2/T1

Compond Gear Train:In a gear train, when the intermediate shaft carries two gears, it is known

as a Compound Gear Train.

For high velocity ratios above 7:1, simple gear train cannot be used because of the

diffculty of connecting the driver and driven for a given centre distance.In such cases,

compound gear train is used.In other words the compound gear train is used when the velocity

ratio is so high that one of the gears of the simple gear train would be unpracticably small.

The intermediate shaft has two gears rigidly fixed to it so that they have the same

speed. The driving gear 1 is mounted on shaft A. Gears 2 and 3 are compound gear mounted on

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shaft B. Driving gear 1 is in mesh with gear 2. Compound gear 2 and 3 rotate at the same speed.

Gear 3 is meshing with the driven gear 4, mounted on shaft c.

Let N1,N2 ,N3 and N4 be the speeds and T1, T2,T3 and T4 be the number of teeth of the gears

1,2,3, and 4 respectively.

N1/N2 =T2/T1 ; N3/N4 = T4/T3

Velocity Ratio = N1/N2*N3/N4 = T2/T1*T4/T3

Since gears 2 and 3 are mounted on the same shaft B, N2 = N3.

Hence, velocity Ratio =N1/N4 = T2*T4/T1*T3

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Epicyclic gear train:

Epicyclic gearing or planetary gearing is a gear system consisting of one or more outer

gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are

mounted on a movable arm or carrier which itself may rotate relative to the sun gear. Epicyclic

gearing systems also incorporate the use of an outer ring gear or annulus, which meshes with the

planet gears. Planetary gears (or epicyclic gears) are typically classified as simple and compound

planetary gears. Simple planetary gears have one sun, one ring, one carrier, and one planet set.

Compound planetary gears involve one or more of the following three types of structures:

meshed-planet (there are at least two more more planets in mesh with each other in each planet

train), stepped-planet (there exists a shaft connection between two planets in each planet train),

and multi-stage structures (the system contains two or more planet sets). Compared to simple

planetary gears, compound planetary gears have the advantages of larger reduction ratio, higher

torque-to-weight ratio, and more flexible configurations.

The axes of all gears are usually parallel, but for special cases like pencil sharpeners they

can be placed at an angle, introducing elements of bevel gear (see below). Further, the sun,

planet carrier and annulus axes are usually concentric.

The three basic components of the epicyclic gear are:

Sun: The central gear

Planet carrier: Holds one or more peripheral planet gears, all of the same size, meshed

with the sun gear

Annulus: An outer ring with inward-facing teeth that mesh with the planet gear or gears

In many epicyclic gearing systems, one of these three basic components is held

stationary; one of the two remaining components is an input, providing power to the system,

while the last component is an output, receiving power from the system. The ratio of input

rotation to output rotation is dependent upon the number of teeth in each gear, and upon which

component is held stationary.

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Chain drive:

Slip occurs in both belt and rope drives. Chains are made up of rigid links hinged

together. Chain drives are positive drives with no slipping. Hence ,velocity ratio remains

constant. Power transmitted is as high as 100 KW.

Uses; Chain drive is used when the centre distance is less as in bicycles,motor cycles,road

rollers,agricultural machinery,etc.

Types of chain drive:

Roller chain drive:

Fig .Roller chain drive

Roller chain is the type of chain drive most commonly used for transmission of mechanical

power on many kinds of domestic, industrial and agricultural machinery, including conveyors,

wire and tube drawing machines, printing presses, cars, motorcycles, and bicycles. It consists of

a series of short cylindrical rollers held together by side links. It is driven by a toothed wheel

called a sprocket. It is a simple, reliable, and efficient means of power transmission.

Advantages of Chain drive:

1. Relatively inexpensive.

2. Virtually any length chain can be obtained (splicing).

3. Large selection of chain and sprockets, especially for #80 and smaller chain.

4. Positive drive provides synchronization of two shafts

5. Bearing loads are generally lower than for belts (no slack side tension).

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6. Chain drives are 95-99% efficient (Poly Chain is 98-99% efficient).

7. Tends to be self-cleaning.

8. Simplicity of design and selection of components.

9. Versatile - large variety of attachments can be adapted

10. Breakable - splice capability allows for varying length and installation on drives where

endless chain cannot be installed.

11. Due to chain's symmetric design characteristics, serpentine drives are possible

12. Fixed center drives can be "accommodated" by removing links to take up chain slack

13. Chain tends to be fairly forgiving when misapplied and users are willing to live with poor

performance.

14. Chain drives seem to give the appearance that they will do the job - i.e., steel is tough.

15. Chain offers higher HP capacities on smaller diameters.

Disadvantages of Chain drive:

1. Lubrication is critical - unlubricated drives can wear 300 times faster than lubricated drives ).

2. The lubrication attracts dirt which leads to wear problems.

3. Life is usually low since an estimated 90-95% of chain drives are improperly lubricated.

4. Frequent maintenance is required due to wear and stretch.

5. Necessary lubrication is messy (may be a problem in food/beverage industry).

6. Alignment is important as it affects life and stability.

7. Chain drives are noisy (proportional to speed) due to metal-to-metal contact.

8. Linear speed is limited to 3000 ft./min. for roller chain.

9. Vertical drives may present problems since less slack can be permitted than in a horizontal

drive in order

to insure proper chain/sprocket engagement.

10. Vertical "shaft" drives are generally discouraged.

11. Equipment damage can result upon chain failure due to steel construction.

12. Available only in full box length increments except in rare cases.

Reference Books

1. K.Venu gopal andV.Praburaja,(1996)., Basic mechanical Engineering,Anuradha

publications,Kumbakonam.

2. O.P.Khanna,(1988) Welding Technology,Dhanpat raj&sons,Delhi

3. G.Shanmugam, Basic mechanical Engineering, Tata McGraw-Hill publishing company

Limited.

4. O.P.Khanna,M.Lal,Production technology, Dhanpat raj&sons,Delhi

5. G.R.Nagpal,Metal forming processes