They are generally endless and their general cross section shape is trapezoidal. The 'V' shape of the belt tracks in a mating groove in the pulley (or sheave) with the result that the belt cannot slip off.

 

The belt also tends to wedge into the groove as the load increases. The greater the load, the greater the wedging action - improving torque transmission and making the V-belts trump flat belts with their small centre distances and high reduction ration.

 

The preferred centre distance is larger than the largest pulley diameter, but less than three times the sum of both pulleys. Optimal speed range is 1000 - 7000ft/min