V-Belt & Rope Drives | Mechanical Engineering SSC JE (Technical) PDF Download

V- BELT AND ROPE DRIVES

• The V-belts are made of fabric and cords moulded in rubber and covered with fabric and rubber. These belts are moulded to a trapezoidal shape and are made endless. These are particularly suitable for short drives. The included angle for the V-belt is usually 30°-40°. The power is transmitted by the wedging action between the belt and the V-groove in the pulley or sheave. 
• A clearance must be provided at the bottom of the groove in order to prevent touching of the bottom as it become narrower from wear.

• The wedging action of the V-belt in the groove angle will require more force to pull the belt out of the groove which will result in loss of power and excessive belt wear due to friction and heat. Hence the selected groove angle is a compromise between two. 
• When one of the set of belts break, the entire set should be replaced at the same time. If only one belt is replaced, the new unworn and unstretched belt will be more tightly stretched and will move with different velocity. 
• Length is defined as the circumferential length of the belt at the pitch width of the belt.
• Advantages 
  • The V-belt drive gives compactness due to the small distance between centers of pulleys. 
  • The drive is positive, because the slip between the belt and the pulley groove is negligible. 
  • Since the V-belts are made endless and there is no joint trouble., therefore the drive is smooth. 
  • It provides longer life.
  • It can be easily installed and removed.  
  • The operation of the belt and pulley is quiet. 
  • The belts have the ability to cushion the shock when machines are started. 
  • The high velocity ratio may be obtained. 
  • The wedging action of the belts in the groove gives high value of limiting ratio of tension. Therefore the power transmitted by V-belts is more than flat belts for the same coefficient of friction, arc of contact and allowable tension in the belts. 
  • The V-belt may be operated in either direction, with tight-side of the belt at the top or bottom. The centre line may be horizontal, vertical or inclined.
•  Ratio of Driving Tensions for V-belt

Let, R₁ = Normal reactions between belts and sides of the groove.
 R = Total reaction in the plane of the groove.
 µ = Coefficient of friction between the belt and sides of the groove.

Frictional force = µ.R. cosec b

• V-flat Drives 
  • In many cases, particularly when a flat belt is replaced by V-belt. It is economical to use flat faced pulley, instead of large grooved pulley. The cost of cutting grooves is thereby eliminated. Such a drive is known as V-flat drive.

• Rope Drives 
  • The rope drives are widely used where a large amount of power is to be transmitted from one pulley to another, over a considerable distance. If large amounts of power to be transmitted; by the flat-belt then it would result in excessive belt cross-section.
• Fibre Rope. The ropes for transmitting power are usually made from fibrous materials such as hemp, manila and cotton. When the hemp and manila ropes are bent over the sheave, there is some sliding of the fibres, causing the rope to wear and chafe internally. In order to minimize this defect, the rope fibres are lubricated with a tar, tallow or graphite. The lubrication also makes the rope moisture proof.
  • The manila ropes are more durable and stronger than cotton ropes. The cotton ropes are costlier than manila ropes.
  • The number of grooves should not be more than 24.

• Wire Ropes : When a large amount of power is to be transmitted over long distance from one pulley to another, then wire ropes are used.
  • The wire ropes are widely used in elevators, mine hoists, cranes, conveyors, housing devices and suspension bridges.
  • The wire ropes run on the grooved pulleys but they rest on the bottom of the grooves and are not wedged between the sides of the grooves.
• Advantages.
  • The wire ropes have the following advantages as compared to fibre ropes
  • These are lighter in weight.
  • These offer silent operation.
  • These can withstand shock loads.
  • These are more reliable.
  • These are more durable.
  • They do not fail suddenly.
  • The efficiency is high.
  • The cost is low.
• Construction of wire ropes 
  • The wires are first given special heat treatment and then cold drawn in order to have high strength and durability of the rope.
  • First, 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, asbestoses or a wire of softer steel. The core must be continuously saturated with lubricant for the long life of the core as well as the entire rope.

• 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 strands.

(i) Right handed (ii) Left handed 

• Parallel or long lay ropes. In these types of ropes, the direction of twist of the wires in the strands is same as that of strains in the rope. These ropes are more flexible and resists wear more effectively.

(i) Right handed (ii) Left handed 

• Composite or reverse lay ropes. In these type of ropes the wires in the two adjacent strands are twisted in the opposite direction.

• The wire ropes are designated by the number of strands and the number of wires in each strands. For example, a wire rope having six strands and seven wires in each strands is designated by 6 × 7 rope.