V Belt Rated Horsepower Equations

V and Flat Belt Design and Engineering Data

The following equations are defined on this webpage

  • V Belt Rated Horsepower Equations
  • Design Pulley Speed Ratio
  • Power Calculations V-Belt
  • Dynamic or Two-Plane Balancing Equation

V Belt Rated Horsepower Equations

Description Equation
Rated Horsepower V Belt Rated Horsepower Equations
V belt Design HP

V Belt Rated Horsepower Equations

Variables Declarations

Nsf = Service life factor

a = Belt constant (obtain from mfg)
c = Belt constant (obtain from mfg)
e = Belt constant (obtain from mfg)
D1 = Small sheave pitch diemeter
Kd = Small diameter factor (estimate) or manufacture tables for given velocity ratios
Vm (fpm) = Belt speed


Adding a Service Factor to the required power to be transmitted, called nominal horsepower, ensures the efficiency of
a drive. This nominal horsepower refers to the motor horsepower or required horsepower for the application.

A Service Factor takes into account such situations as power losses due to vibration, shocks, heat and other related
factors caused by the motor and the application. The Design Power is calculated based on these considerations thus
producing more accurate results to ensure that the drive will function more efficiently.

Design Pulley Speed Ratio

In a belt drive system, a ratio is used to determine the speed relation between two v-belt pulleys. The speed ratio would be stable if slippage did not occur; however as belt slip is inevitable, the ratio varies and is therefore only theoretical. If the speed ratio is < 1 (ex. 1:4), we refer to a speed up system; if the ratio is > 1 (ex. 4:1 ), it's a speed reduction system. In both cases, the ratio is obtained using the dimensions of the input drive (driver) pulley and the output (driven) pulley. In the following ratio, RS is the speed ratio, D1 the diameter of the driver pulley, D2 the diameter of the driven pulley:

Rs = RPM1/RPM2 = D2/D1

V Belt Rated Horsepower Equations

Power Calculations V-Belt

Power is a measure of performance or capacity and is defined as the amount of work performed in a given time. The most work accomplished in the least amount of time, equals greater power. This formula also shows the relation between torque and HP. Power in hp (HP) can be calculated using the following formulas:

HP = [T x RPM] / 63025 , When T is given in lb-in

or HP = [T x RPM] / / 5252, When T is given in lb-ft

Hp to Kilowatts, HP = Kw x 1.341

Dynamic or Two-Plane Balancing Equation

Dynamic balancing is strongly recommended in applications running at high speeds. To determine whether dynamic balancing is advisable, perform the following calculation.

RPM = 15,500 / (DF)1/2


D is Diameter in inches
F is Face Width in inches

RPM = 25.54 x 15,500/ (DF)1/2

D is Diameter in millimeters
F is Face Width in millimeters

The resultant RPM is the maximum recommended operating RPM for a sheave or pulley with a single plane balance
(static balancing).

Note: If the sheave or pulley is to be operated at a higher speed, a two plane balance is recommended.