Belt and Pulley Drive Glossary

July 2, 2026

Use this glossary alongside the Pulley RPM Calculator when a term in a formula or a manufacturer catalog isn’t clear.

Arc of Contact

The portion of a pulley’s circumference actually wrapped by the belt, measured in degrees. Smaller pulleys in a reduction drive have a smaller arc of contact than larger pulleys. Below roughly 120°, torque capacity drops and slip risk rises. See the belt drive formulas doc for the calculation.

Belt Slip

The percentage difference between theoretical belt speed (calculated from pulley diameter and RPM) and actual belt speed, caused by elastic creep in the belt material under load. Classical and narrow V-belts slip 1–3%. Synchronous (timing) belts have 0% slip because their teeth engage the pulley directly rather than relying on friction.

Belt Speed

The linear velocity of the belt as it travels around the pulleys, expressed in feet per minute (ft/min) or meters per minute (m/min). Calculated as V = π × D1 × N1 / 12 (imperial) using the driver pulley’s diameter and RPM. Every belt type has a maximum rated belt speed; exceeding it causes heat, vibration, and early failure.

Center Distance (C)

The straight-line distance between the centers (shaft axes) of two pulleys in a belt drive. Used in the belt length formula and in arc-of-contact calculations. Larger center distances generally improve arc of contact on high-ratio drives.

Compound Drive

A belt drive using more than one belt stage (each stage having its own driver/driven pulley pair) to achieve a total speed ratio too large for a single stage to handle efficiently. See multi-stage pulley drive calculations.

Driven Pulley

The pulley connected to the output shaft — the equipment being powered (fan, pump, spindle, drum). Its speed is the result being solved for in most pulley RPM calculations.

Driver Pulley

The pulley mounted on the power source shaft — typically an electric motor. Its RPM is usually a known, fixed value (the motor’s nameplate speed).

Nameplate RPM

The actual operating speed of an AC induction motor under normal load, stamped on the motor’s data plate. Always slightly below the motor’s synchronous RPM (e.g., a 4-pole 60 Hz motor has a synchronous speed of 1,800 RPM but a nameplate speed of roughly 1,725–1,760 RPM). Use nameplate RPM, not synchronous RPM, in pulley calculations.

Pitch Diameter

The effective diameter of a pulley measured at the point where the belt’s tensile cord rides — not the pulley’s outside diameter. Pitch diameter is smaller than outside diameter by an amount that depends on belt cross-section (roughly 0.15–0.35 inches per side for classical V-belts). Using outside diameter instead of pitch diameter is one of the most common pulley sizing errors — see common pulley sizing mistakes.

Sheave

Another name for a grooved pulley, most often used for V-belt and multi-groove pulleys in industrial contexts. “Sheave” and “pulley” are used interchangeably in most catalogs.

Speed Ratio

The relationship between driver and driven pulley diameters: Speed Ratio = D1 / D2. A ratio below 1 means speed reduction (driven shaft slower than driver); above 1 means speed increase. The inverse of the speed ratio is the torque ratio.

Step Pulley

A pulley with multiple integrated diameters (“steps”) on a single hub, allowing a belt to be moved between steps to select different speeds without swapping pulleys. Common on drill presses, wood lathes, and some milling machines.

Synchronous Belt (Timing Belt)

A toothed belt that engages matching teeth on the pulley (properly called a “sprocket” for toothed drives), producing an exact, slip-free speed ratio. Used where precise speed or position control matters — CNC machines, conveyors requiring synchronized motion.

Torque Ratio

The inverse of the speed ratio (D2 / D1). In an ideal belt drive, power is conserved, so reducing speed at the driven pulley proportionally increases torque, and vice versa.

V-Belt

A belt with a trapezoidal (V-shaped) cross-section that wedges into a matching groove in the pulley, using friction on the angled sides to transmit power. Common sections include A, B, C, D, E (classical) and 3V, 5V, 8V (narrow, higher power density per belt width).

For the full set of formulas that use these terms, see the belt drive formulas reference. To run the calculations directly, use the Pulley RPM Calculator.