The Complete Handbook of Belts

Chapter 1: Understanding Belts

What a Belt Is

A belt is a flexible loop of material—typically rubber, rubber-textile composite, or other elastomers—that wraps around two or more pulleys (also called sheaves) to transmit mechanical power and motion from one rotating shaft to another. The simplest example is a single belt stretched around two pulleys: one driven by a motor or engine, one driving a load (a pump, fan, compressor, or generator). As the driving pulley rotates, friction between the belt and the pulley surface grips the belt and drags it forward. The belt, in turn, grips the second pulley and forces it to rotate. No gears, no rigid mechanical connection—just friction and tension doing the work.

This simple principle has been in use for more than a century, and it remains the most common way to transmit power in machines ranging from industrial compressors to household washers to agricultural equipment. A typical car engine drives its water pump, alternator, air conditioning compressor, and power steering pump with belts. A factory floor may have dozens of belt-driven motors and compressors. A single residential HVAC system uses at least one belt to connect the motor to the blower. Belts are everywhere, largely invisible, and almost always taken for granted.

How a Belt Transmits Power

The transmission of power through a belt depends entirely on three factors: the friction between the belt and the pulley surface, the tension of the belt, and the arc of contact—the amount of pulley surface the belt actually