LECTURE 4 BELTS DRIVES.pdf

1. BELTS, ROPES & CHAINS
Jonathan Monteiro
Assistant Professor,
Department of Mechanical & Manufacturing Engineering,
MIT, Manipal

2. BELTS, ROPES & CHAINS
Gear Drives
• They are called rigid or non-flexible
drives. There is direct contact between
the driving and driven shafts through
the gears.
• Rotary motion of the driving shaft is
directly converted into rotary motion of
the driven shaft by means of pinion and
gear.
Flexible Drives
• There is an intermediate link such as
belt, or chain between the driving and
driven shafts. Since this link is flexible,
the drives are called '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. Thus, a flexible
element is superimposed between the
driving and driven elements.

3. BELTS, ROPES & CHAINS
Advantages over Rigid drives
• Transmit power over a comparatively long distance due to an
intermediate link
• Since the intermediate link is long and flexible, it absorbs shock loads and
damps vibrations.
• Provide considerable flexibility in the location of the driving and driven
shafts. The tolerances on the center distance are not critical as compared
with a gear drive.
• Cheap compared to gear drives.
• Their initial and maintenance costs are low.

4. BELTS, ROPES & CHAINS
Disadvantages
• They occupy more space
• Velocity ratio is relatively small
• In general, the velocity ratio is not constant
• Belts are used to transmit power between two shafts by means of
friction

5. BELT DRIVES
Advantages compared with other types of drives:
• Can transmit power over considerable distance between the axes of driving and driven shafts
• Operation is smooth and silent
• 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
• Ability to absorb the shocks and damp vibration
• Simple to design
• Low initial cost
Disadvantages of belt drives compared to other types of drives:
• Have large dimensions and occupy more space
• Velocity ratio is not constant due to belt slip
• Belts impose heavy loads on shafts and bearings
• Considerable loss of power resulting in low efficiency
• Comparatively short service

6. • Types
BELT DRIVES

7. BELT DRIVES
Advantages of a V-belt drive over flat belt drive:
• The force of friction between the surfaces of the belt and V-grooved pulley is high due to wedge action.
This wedging action permits a smaller arc of contact, increases the pulling capacity of the belt and
consequently results in increase in the power transmitting capacity.
• V-belts have short center distance, which results in compact construction.
• They permit high speed-reduction even up to seven to one.
• Flat belts are hinged, while V-belts are endless which results in smooth and quiet operation, even at high
operating speeds.
• The drive is positive because the slip is negligible due to wedge action.
• V-belt drive can operate in any position, even when the belt is vertical.
Disadvantages of a V-belt drive over flat belt drive:
• The ratio of the cross-sectional height to the pulley diameter is large in case of the V- belt. This increases
bending stresses the belt cross-section and adversely affects the durability.
• The efficiency of the V-belt is lower than that of the flat-belt and the creep is also higher.
• The construction of V-grooved pull is complicated and costlier compared with pulleys of the flat-belt drive.

8. BELT CONSTRUCTION
Desirable properties of Belt materials:
• Should have high coefficient of friction with the pulleys
• Should have high tensile strength to withstand belt tensions
• Should have high wear resistance
• Should have high flexibility and low rigidity in bending in order to avoid bending
stresses while passing over the pulley
• Belts are made of leather, canvas, rubber or rubberized fabric and synthetic
materials

9. BELT CONSTRUCTION
• Belts are made of leather, canvas, rubber or rubberized fabric and synthetic materials.
• V-belts are made of polyester fabric and cords, with rubber reinforcement.
 The central load carrying layers of polyester cords or
polyester fabric, that are located on horizontal lines near
the centre of gravity of the belt cross-section
 The surrounding layer of rubber to transmit force from
cords to side walls
 Outer polychloroprene impregnated elastic cover
 The polyester cords or fabric, transmit force from driving
pulley to driven pulley, which in turn transmit power
 Since they are located near the neutral axis of the cross-
section of the belt, the stresses due to bending of the
belt around the pulley are almost negligible

10. Next Lecture: Problems on Design of V-Belts
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