Belts and chains

Weightage 16 marks
The belts or ropes are used to transmit power from one shaft
to another by means of pulleys which rotate at the same
speed or at different speeds. The amount of power
transmitted depends upon the following factors :
1. The velocity of the belt.
2. The tension under which the belt is placed on the pulleys.
3. The arc of contact between the belt and the smaller pulley.
4. The conditions under which the belt is used.
Belt drives:
• Belt, chain and rope drives are called ‘flexible’ drives. There are two
types of drives—rigid and flexible. Gear drives are called rigid or non-
flexible drives.
• In flexible drives, there is an intermediate link such as belt, rope or
chain between the driving and driven shafts. Since this link is flexible,
the drives are called ‘flexible’ drives.
• In gear drives, rotary motion of the driving shaft is directly converted
into rotary motion of the driven shaft by means of pinion and gear. In
flexible drives, the rotary motion of the driving shaft is first converted
into translatory motion of the belt or chain and then again converted
into rotary motion of the driven shaft.
Belt drives
Belts are used to transmit power between two shafts by means of friction. A belt
drive consists of three elements—driving and driven pulleys and an endless belt,
which envelopes them.
Belt drives offer the following advantages compared with other types of drives:
1. Belt drives can transmit power over considerable distance between the axes of
driving and driven shafts.
2. The operation of belt drive is smooth and silent.
3. They can transmit only a definite load, which if exceeded, will cause the belt to
slip over the pulley, thus protecting the parts of the drive against overload.
4. They have the ability to absorb the shocks and damp vibration.
5. They are simple to design.
6. They have low initial cost.
Belt drives
The disadvantages of belt drives compared to other types of drives are as follows:
1. Belt drives have large dimensions and occupy more space.
2. The velocity ratio is not constant due to belt slip.
3. They impose heavy loads on shafts and bearings.
4. There is considerable loss of power resulting in low efficiency.
5. Belt drives have comparatively short service life.
Selection of a Belt Drive
Following are the various important factors upon which the selection of a
belt drive depends:
1. Speed of the driving and driven shafts,
2. Speed reduction ratio,
3. Power to be transmitted,
4. Centre distance between the shafts,
5. Positive drive requirements,
6. Shafts layout,
7. Space available, and
8. Service conditions.
Types of Belt Drives
The belt drives are usually classified into the following three groups:
1. Light drives. These are used to transmit small powers at belt speeds up
to about 10 m/s as in agricultural machines and small machine tools.
2. Medium drives. These are used to transmit medium powers at belt
speeds over 10 m/s but up to 22 m/s, as in machine tools.
3. Heavy drives. These are used to transmit large powers at belt speeds
above 22 m/s as in compressors and generators.
Types of Belts
Though there are many types of belts used these days, yet the following are
important from the subject point of view:
1. Flat belt. The flat belt as shown is mostly used in the factories and workshops,
where a moderate amount of power is to be transmitted, from one pulley to
another when the two pulleys are not more than 8 metres apart.
Types of Belts
2. V- belt. The V-belt as shown is mostly used in the factories and workshops,
where a great amount of power is to be transmitted, from one pulley to
another, when the two pulleys are very near to each other.
Types of Belts
• Circular belt or rope. The circular belt or rope
as shown is mostly used in the factories and
workshops, where a great amount of power is
to be transmitted, from one pulley to another,
when the two pulleys are more than 8 metres
apart.
• If a huge amount of power is to be transmitted,
then a single belt may not be sufficient. In such
a case, wide pulleys (for V-belts or circular
belts) with a number of grooves are used. Then
a belt in each groove is provided to transmit
the required amount of power from one pulley
to another.
Material used for Belts
1. Leather belts. The most important material for flat belt is leather. The best
leather belts are made from 1.2 metres to 1.5 metres long strips cut from
either side of the back bone of the top grade steer hides.
• There are two varieties of leather—oak-tanned and mineral or chrome tanned.
• The main advantage of leather belt is the high coefficient of friction and
consequently, high power transmitting capacity.
Material used for Belts
• Cotton or fabric belts. Most of the fabric belts are made by folding canvas or
cotton duck to three or more layers (depending upon the thickness desired) and
stitching together.
• They are impregnated with some filler like linseed oil in order to make the belt
water-proof and to prevent injury to the fibres. The cotton belts are cheaper and
suitable in warm climates, in damp atmospheres and in exposed positions.
• Since the cotton belts require little attention, therefore these belts are mostly
used in farm machinery, belt conveyor etc.
Material used for Belts
3. Rubber belt. The rubber belts are made of layers of fabric impregnated with
rubber composition and have a thin layer of rubber on the faces. These belts are
very flexible but are quickly destroyed if allowed to come into contact with heat, oil
or grease.
One of the principle advantage of these belts is that they may be easily made
endless. These belts are found suitable for saw mills, paper mills where they are
exposed to moisture.
Material used for Belts
4. Balata belts. These belts are similar to rubber belts except that balata gum is
used in place of rubber. These belts are acid proof and water proof and it is not
effected by animal oils or alkalies. The balata belts should not be at temperatures
above 40°C because at this temperature the balata begins to soften and becomes
sticky. The strength of balata belts is 25 per cent higher than rubber belts.
Belt Joints
• When the endless belts are not available, then the belts are cut from big rolls and
the ends are joined together by fasteners. The various types of joints are
• 1. Cemented joint, 2. Laced joint, and 3. Hinged joint.
Cemented Joints: Leather belts are cemented with a tapered lap joint of length of
20 to 25 times the belt thickness. Multiple ply belts are cemented in the form of
stepped joint. Cementing is widely used for rubber and leather belts. The strength
of cemented joint is 80 to 85% of the strength of the belt.
Belt Joints
• Laced Joint Lacing is done with catgut or with
rawhide strips. The strength of a laced joint is 50%
of the strength of the belt. The laced joint is formed
by punching holes in line across the belt, leaving a
margin between the edge and the holes.

•Joints with Metal Fasteners Metal fasteners

require least time to make the joint. However,
they are not suitable for high-speed operations.
The strength of screwed or hinged joint is 25%
of the strength of belt.
Types of Flat Belt Drives
1. Open belt drive. The open belt drive, as shown in Fig is used with shafts
arranged parallel and rotating in the same direction. In this case, the driver
pulls the belt from one side and delivers it to the other side. Thus the tension
in one of the side of belt will be more than that in the other side of belt.
• (because of more tension) is known as tight side ,
• (because of less tension) is known as slack side,
Types of Flat Belt Drives
2. Crossed or twist belt drive. The crossed or twist belt drive, as shown is used
with shafts arranged parallel and rotating in the opposite directions. In this case,
the driver pulls the belt from one side (i.e. RQ) and delivers it to the other side
(i.e. LM).
• Thus, the tension in the belt RQ will be more than that in the belt LM.
• The belt RQ (because of more tension) is known as tight side,
• whereas the belt LM (because of less tension) is known as slack side
Types of Flat Belt Drives
• A little consideration will show that at a point where the belt crosses,
it rubs against each other and there will be excessive wear and tear.
• In order to avoid this, the shafts should be placed at a maximum
distance of 20 b, where b is the width of belt and the speed of the
belt should be less than 15 m/s.
Types of Flat Belt Drives
• 3. Quarter turn belt drive. The quarter turn belt
drive (also known as right angle belt drive) as
shown is used with shafts arranged at right
angles and rotating in one definite direction.
• In order to prevent the belt from leaving the
pulley, the width of the face of the pulley should
be greater or equal to 1.4 b, where b is width of
belt.
• In case the pulleys cannot be arranged as when
the reversible motion is desired, then a quarter
turn belt drive with a guide pulley, may be used.
Types of Flat Belt Drives
4. Belt drive with idler pulleys. A belt drive with an idler pulley (also known as
jockey pulley drive) as shown is used with shafts arranged parallel and when an
open belt drive can not be used due to small angle of contact on the smaller pulley.
This type of drive is provided to obtain high velocity ratio and when the required
belt tension can not be obtained by other means.
• When it is desired to transmit motion from one shaft to several shafts, all
arranged in parallel, a belt drive with many idler pulleys, may be employed.
Types of Flat Belt Drives
5. Compound belt drive. A compound belt drive as shown is
used when power is transmitted from one shaft to another
through a number of pulleys.
Velocity Ratio of a Belt Drive
Velocity Ratio of a Belt Drive
Velocity Ratio of a Belt Drive
Slip of the Belt
• The motion of belts and pulleys assuming a firm frictional grip between the belts
and the pulleys. But sometimes, the frictional grip becomes insufficient.
• This may cause some forward motion of the driver without carrying the belt with
it. This is called slip of the belt and is generally expressed as a percentage.
• s1 % = Slip between the driver and the belt, and
• s2 % = Slip between the belt and follower,
Velocity of the belt passing over the driver per second,
Slip of the Belt
Velocity of the belt passing over the follower per second

If thickness of the belt (t) is considered, then

Creep of Belt
• When the belt passes from the slack side to the tight side, a certain portion of
the belt extends and it contracts again when the belt passes from the tight side
to the slack side.
• Due to these changes of length, there is a relative motion between the belt and
the pulley surfaces. This relative motion is termed as creep.
• The total effect of creep is to reduce slightly the speed of the driven pulley or
follower.
Considering creep, the velocity ratio is given by
Length of an Open Belt Drive

r and r = Radii of the larger and smaller pulleys,

1 2

x = Distance between the centres of two pulleys (i.e. O O ), and

1 2

L = Total length of the belt.

Length of an Open Belt Drive

We know that the length of the belt,

L = Arc GJE + EF + Arc FKH + HG
= 2 (Arc JE + EF + Arc FK)
Length of a Cross Belt Drive
Length of a Cross Belt Drive
In a cross belt drive, both the pulleys rotate in the opposite directions
• r1 and r2 = Radii of the larger and smaller pulleys,
• x = Distance between the centres of two pulleys (i.e. O1O2 ), and
• L = Total length of the belt.
We know that the length of the belt,
L = Arc GJE + EF + Arc FKH + HG
= 2 (Arc JE + FE + Arc FK)
Length of a Cross Belt Drive
In a cross belt drive, both the pulleys rotate in the opposite directions
• r1 and r2 = Radii of the larger and smaller pulleys,
• x = Distance between the centres of two pulleys (i.e. O1O2 ), and
• L = Total length of the belt.
We know that the length of the belt,
L = Arc GJE + EF + Arc FKH + HG
= 2 (Arc JE + FE + Arc FK)
Angle of contact
Hence Angle of contact or lap, for open belt drive

Hence Angle of contact or lap, for Cross belt drive

Ratio of Driving Tensions for Flat Belt Drive
Consider a driven pulley rotating in the clockwise direction
Let
Power Transmitted by a Belt
Maximum Tension in the Belt
Centrifugal Tension
• Since the belt continuously runs over the
pulleys, therefore, some centrifugal force is
caused, whose effect is to increase the tension
on both the tight as well as the slack sides. The
tension caused by centrifugal force is called
centrifugal tension.
• At lower belt speeds (less than 10 m/s), the
centrifugal tension is very small, but at higher
belt speeds (more than 10 m/s), its effect is
considerable and thus should be taken into
account.
Centrifugal Tension
Let

The centrifugal tension acting tangentially at P and Q keeps the belt in

equilibrium. Now resolving the forces
Condition for the Transmission of Maximum Power
• We know that the power transmitted by a belt,
1

ratio of driving tensions is

Condition for the Transmission of Maximum Power
• We know that

For maximum power, differentiate the above expression with respect to v and equate to
zero, i.e.
Condition for the Transmission of Maximum Power
For maximum power, differentiate the above expression with respect to v and equate to
zero, i.e.

It shows that when the power transmitted is maximum, 1/3rd of the maximum
tension is absorbed as centrifugal tension.
Condition for the Transmission of Maximum Power

Where T is max .tension

Advantages
1. As no slip takes place during chain drive, hence perfect velocity ratio is obtained.
2. Since the chains are made of metal, therefore they occupy less space in width
than a belt or rope drive.
3. It may be used for both long as well as short distances.
4. It gives a high transmission efficiency (up to 98 percent).
5. It gives less load on the shafts.
6. It has the ability to transmit motion to several shafts by one chain only.
7. It transmits more power than belts.
8. It permits high speed ratio of 8 to 10 in one step.
9. It can be operated under adverse temperature and atmospheric conditions.
Disadvantages
1. The production cost of chains is relatively high.
2. The chain drive needs accurate mounting and careful maintenance, particularly
lubrication and slack adjustment.
3. The chain drive has velocity fluctuations especially when unduly stretched.
An open chain drive system connecting the two sprockets is shown
• A bush roller chain as shown in Fig, consists of outer plates or pin link plates,
inner plates or roller link plates, pins, bushes and rollers.
• A pin passes through the bush which is secured in the holes of the roller between
the two sides of the chain.
• The rollers are free to rotate on the bush which protect the sprocket wheel teeth
against wear. The pins, bushes and rollers are made of alloy steel.
• A bush roller chain is extremely strong and simple in construction. It gives
good service under severe conditions.
• There is a little noise with this chain which is due to impact of the rollers on
the sprocket wheel teeth.
• This chain may be used where there is a little lubrication.
• When one of these chains elongates slightly due to wear and stretching of
the parts, then the extended chain is of greater pitch than the pitch of the
sprocket wheel teeth.
The roller chains are standardised and manufactured on the basis of pitch.
These chains are
available in single-row or multi-row roller chains such as simple, duplex or
triplex strands
• A silent chain is also known as inverted tooth chain.
There are two important rules in the design of a chain drive. They are as
follows:
(i) The number of pitches or links of the chain should be always ‘even’.
(ii) The number of teeth on the driving sprocket should be always ‘odd’,
such as 17, 19 or 21.
All the best for your mid Exams 1 in 2020