V-belts rely upon friction to transmit power between pulleys. How much power a belt can transfer is related to the coefficient of friction, which is the value that describes the ratio of the force of friction between the belt and pulleys when pressed together.
Rubber tires also rely upon friction to grab a road’s surface, yet tires will also “slip” when the power of the wheel overcomes the coefficient of friction between the tire and the road surface.
As an example of tire slippage, check out RevZilla’s video, “How To Do A Burnout On A Motorcycle”. You can see some crazy tire damage occurring starting around 1:38.
Not all slippage is bad. Machine designers typically choose v-belts to transfer because they are relatively low-cost and they can slip in the event of machine overloading. That ability to slip can prevent catastrophic damage to the machine’s shafts and bearings.
However, most of the time, v-belt slippage is bad.
The most common causes of belt slippage are worn or glazed belts and worn pulley grooves. These problems cause one another in a vicious cycle of belt and pulley wear. Replacing one without replacing the other won’t solve the slippage problem.
Improper tensioning, poor pulley alignment, and dust or oil contamination on the belt surface are also common causes of belt slippage.
A less common—but more difficult to solve—cause is poor design of the V-belt drive. At EIC, we have worked on about a dozen machines with several under-designed drives. The worst example was a belt drive capable of transmitting 175hp that was advertised as a 300hp drive. Needless to say, the belts on those machines lasted only about 2% of their rated life.