5. Flexible Mechanical Elements
• Belts, ropes, chains, and other similar elastic or flexible machine
elements are called as flexible elements.
• It is used transmit mechanical power comparatively long distances and
conveying purpose also.
• Since these elements are elastic and usually quite long, they play an
important part in absorbing shock loads and in damping out and
isolating the effects of vibration.
• Most flexible elements do not have an infinite life. The elements
should be replaced at the first sign of deterioration.
6. Selection of V-belts and pulleys
• The belt drive is used to transmit power from one shaft to another by
means of pulleys which rotate at the same speed or at different speeds.
Selection of a belt drive depends:
• Speed of the driving and driven shafts.
• Speed reduction ratio
• Power to be transmitted
• Centre distance between the shafts
• Space available
• Service conditions
7. Types of Belt Drives
• 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.
• 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.
• Heavy drives. These are used to transmit large powers at belt speeds
above 22 m/s as in compressors and generators.
• Flat belt- is mostly used in the factories and workshops, moderate
amount of power can be transmitted, distance between two pulleys
should not more than 8 metres.
• V- belt- is mostly used in the factories and workshops, large amount of
power can be transmitted, two pulleys should very near to each other.
• Circular belt- used in the factories and workshops, great amount of
power can be transmitted for more than 8 meters distance.
11. Types of Flat Belt Drives
Open Belt Drive
• The open belt drive is used to transmit power between parallel shafts
rotating in the same direction.
12. Cross Belt Drive
• The crossed or twist belt drive is used to transmit power between
parallel shafts rotating in the opposite direction.
13. Quarter Turn Belt Drive
• The quarter turn belt drive is used transmit power between right
14. Variable-Speed Belt Drive
• It is used for changing the speed of the driven shaft while the main or
driving shaft runs at constant speed.
15. Fast and Loose Pulley Drive
• A fast and loose pulley drive is used when the driven or machine shaft
is to be started or stopped whenever desired without interfering with
the driving shaft.
16. V-Belt Drive
• V-belts are made of fabric and cords moulded in rubber and covered
with fabric and rubber.
• Among flexible machine elements, perhaps V-belt drives have widest
• These belts have trapezoidal cross section and do not have any joints.
Therefore, these belts are manufactured only for certain standard
• To accommodate these belts the pulleys have V shaped grooves which
makes them relatively costlier.
17. Nomenclature of V-belt
• The geometrical features of the belt section are indicated in the figure.
• The pitch line, which is also marked as N-A, is the neutral axis of the
• The design calculations for V-belt drives are based on the pitch line or
the neutral axis.
• These belts are available in various sections depending upon power
18. Standard V-belt sections
• The standard V-belt sections are A, B, C, D and E. The table below
contains design parameters for all the sections of V-belt.
19. ROPE DRIVES
• When power is to be transmitted over long distances then belts cannot
be used due to the heavy losses in power. In such cases ropes can be
• Ropes are used in elevators, mine hoists, cranes, oil well drilling,
aerial conveyors, tramways, haulage devices, lifts and suspension
• two types of ropes are commonly used. They are fiber ropes and
metallic ropes. Fiber ropes are made of Manila, hemp, cotton, jute,
nylon, coir etc., and are normally used for transmitting power.
• The fibre ropes operate successfully when the pulleys are about 60
metres apart, while the wire ropes are used when the pulleys are up to
150 metres apart.
• The wire ropes are made from cold drawn wires in order to have
increase in strength and durability. It may be noted that the strength of
the wire rope increases as its size decreases.
• The wires are first given special heat treatment and then cold drawn in
order to have high strength and durability of the rope.
• The steel wire ropes are manufactured by special machines. First of
all, a number of wires such as 7, 19 or 37 are twisted into a strand and
then a number of strands, usually 6 or 8 are twisted about a core or
centre to form the rope.
• The core may be made of hemp, jute, asbsestos or a wire of softer
22. Classification of Wire Ropes
• Cross or regular lay ropes- In these types of ropes, the direction of
twist of wires in the strands is opposite to the direction of twist of the
• Parallel or lang lay ropes- In these type of ropes, the direction of twist
of the wires in the strands is same as that of strands in the rope.
• Composite or reverse laid ropes- In these types of ropes, the wires in
the two adjacent strands are twisted in the opposite direction.
24. Chain Drives
• The chains are mostly used to transmit motion and power from one
shaft to another, when the centre distance between their shafts is short
such as in bicycles, motor cycles, agricultural machinery, conveyors,
rolling mills, road rollers etc.
• The chains may also be used for long centre distance of up to 8 metres.
The chains are used for velocities up to 25 m / s and for power up to
25. Classification of Chain Drives
• Block or bush chain- This type of chain was used in the early stages of
development in the power transmission.
• It produces noise when approaching or leaving the teeth of the
sprocket because of rubbing between the teeth and the links. Such type
of chains are used to some extent as conveyor chain at small speed.
26. Bush Roller Chain
• It 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.
28. Silent Chain
• It is designed to eliminate the evil effects caused by stretching and to
produce noiseless running.
• When the chain stretches and the pitch of the chain increases, the links
ride on the teeth of the sprocket wheel at a slightly increased radius.
• This automatically corrects the small change in the pitch.