Analyze an ideal two-pulley belt drive. Enter any three of high-speed pulley diameter, low-speed pulley diameter, high shaft rpm, and low shaft rpm to calculate the fourth.
US + metric
Formula shown
Shareable results
Belt-Drive Pulleys
in
in
rpm
rpm
Enter any three variables
The highlighted fourth field is calculated automatically from the ideal two-pulley speed ratio.
Low pulley speed600rpm(10 rev/s)
Use effective or pitch diameters
Outside diameter may not equal the belt pitch-line diameter. This ideal relation also excludes belt slip, creep, and elastic speed loss.
Ideal belt speed is V = πDN/60 in coherent units. The same value must result at both pulley pitch lines when there is no slip.
For preliminary analysis and educational use. Verify pitch diameters, actual loaded speeds, slip, belt type, speed rating, wrap, center distance, tension, power capacity, and manufacturer selection requirements.
This calculator analyzes the speed and effective diameters of an ideal two-pulley belt drive. Choose the variable to calculate, then enter the other three: high-speed pulley diameter DH, low-speed pulley diameter DL, high-speed shaft speed NH, or low-speed shaft speed NL.
The belt-drive relationship
With no slip, the belt has the same linear speed at both pulleys. Therefore NH × DH = NL × DL, and the speed ratio is R = NH/NL = DL/DH. A larger driven pulley turns more slowly; a smaller driven pulley turns faster.
Solve any fourth variable
High-speed diameter (DH) — enter DL, NH, and NL.
Low-speed diameter (DL) — enter DH, NH, and NL.
High pulley speed (NH) — enter DH, DL, and NL.
Low pulley speed (NL) — enter DH, DL, and NH.
Worked example
A 4-inch high-speed pulley turning at 1,800 rpm drives a 12-inch low-speed pulley. The diameter ratio is 12/4 = 3, so the low-speed shaft turns at 600 rpm. The ideal belt speed is approximately 1,884.96 ft/min or 9.57557 m/s.
Use pitch or effective diameter
Use the pulley diameter at the belt pitch line—the pitch diameter or effective diameter specified by the pulley manufacturer. Outside diameter is not always the same value, especially for V-belt sheaves, and substituting it can shift the predicted speed ratio. For synchronous belts, pulley tooth-count ratio is normally the preferred exact ratio.
Assumptions and limits
The drive has two pulleys connected by one belt with no intermediate or idler ratio effect.
The relation is ideal and assumes no belt slip, creep, or elastic speed loss.
Diameters are compatible effective or pitch diameters measured in the same units.
The calculator does not size belt section, pulley grooves, center distance, wrap angle, belt length, tension, service factor, power capacity, or shaft loading.
Confirm allowable pulley and belt speeds with the component manufacturer.
Variables
Symbol
Meaning
US unit
SI unit
DH
Effective or pitch diameter of the high-speed pulley
in
mm
DL
Effective or pitch diameter of the low-speed pulley
