Belt drives for power transmissions are classified as frictional drives since the belt transmits power via friction between the belt and the driving and driven pulleys.

Flat belts and V-belts transmit power by their grip on the pulley. The three major factors that determine the potential of grip are the area of contact, belt tension, and the coefficient of friction.


Area of Contact – The area of contact in a belt drive system refers to the amount of contact area that the belt has with the pulley. It is determined by the width and arc of contact. In systems that use pulleys with equal diameters, the arc of contact is 180 degrees for each pulley, but equal sized pulleys are not always used. When different sized pulleys are used, the area of contact on the smaller pulley is less than 180 degrees, and in most conditions the smaller pulley will be the driver.

Belt Drives

Belt Tension The best tension for a V-belt is the lowest tension at which the belts will not slip under full load. Belt tension should be checked frequently during run-in and the first 24-48 hours of operation. Over-tensioning will lead to reduced belt and bearing life, while too much slack will cause slippage and excessive belt wear.


Coefficient of Friction The coefficient of friction is the ratio of force between two bodies and the force pressing them together. If a body of weight, or “W”, rests on a flat horizontal surface and a force, or “P”, parallel to the surface is enough to cause the body to be at the point of slipping, then the ratio of “P” to “W” is the coefficient of friction, or “F”, between the two surfaces.

When selecting a belt, several considerations must be taken into account to ensure proper operation, long belt life, and low maintenance requirements. Belt drives may seem simple in design but many factors are considered when selecting the proper belt for all applications.


Environmental Factors Belt selection must take into account the environmental conditions, such as:

  • Oil and grease exposure
  • Operating temperatures
  • Abrasive dust and dirt contamination
  • Sunlight and other weather conditions


System Requirements – System requirements are also crucial to belt selection, the following are some considerations that are taken when selecting a drive belt:

  • Type of drive required
  • Driver/Driven RPM
  • Horsepower requirements
  • Pulley diameters and center distance
  • Take-up allowances and take-up design
  • Space limitation for operation
  • Pulsating or shock load conditions
  • Static dissipation problems
  • Belt availability
  • Belt construction and service life