Belt Sheave Speed Calculator - Calculator Academy

Enter the driver sheave diameter, driven sheave diameter, driver speed, and driven speed into the calculator to determine the missing variable.

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Belt Sheave Speed Formula

The following formula is used to calculate the missing variable in a belt sheave speed system.

D_1 \cdot N_1 = D_2 \cdot N_2

Variables:

D₁ is the driver sheave diameter
D₂ is the driven sheave diameter
N₁ is the driver speed
N₂ is the driven speed

To calculate the missing variable, rearrange the formula to solve for the desired variable and input the known values.

What is a Belt Sheave?

A belt sheave, also known as a pulley, is a wheel on an axle or shaft that is designed to support the movement and change of direction of a belt or cable. In mechanical systems, belt sheaves are used to transfer power between shafts, change the speed of rotation, or alter the direction of motion. The diameter of the sheave and the speed at which it rotates are critical factors in determining the performance of the belt drive system. Belt sheaves are commonly used in various applications, including automotive engines, industrial machinery, and HVAC systems.

How to Calculate Belt Sheave Speed?

The following steps outline how to calculate the Belt Sheave Speed.

First, determine the driver sheave diameter (D₁).
Next, determine the driven sheave diameter (D₂).
Next, determine the driver speed (N₁).
Next, determine the driven speed (N₂).
Finally, use the formula D₁ * N₁ = D₂ * N₂ to calculate the missing variable.
After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem :

Use the following variables as an example problem to test your knowledge.

Driver Sheave Diameter (D₁) = 10 cm

Driven Sheave Diameter (D₂) = 20 cm

Driver Speed (N₁) = 1500 RPM

Driven Speed (N₂) = ?

Follow-Up Calculations

A 6-inch crank pulley driving a 2-inch alternator pulley at 3,000 engine rpm spins the alternator at 9,000 rpm, which is a common overdrive setup for better charging at idle. Once you know that speed, you can check alternator torque needs to see whether the belt can deliver enough force for the electrical load without slipping.

If your belt sheave ratio is 2:1, a 4-inch drive pulley can still transmit a lot of power into the shaft, and the belt tension shows up as side force on the bearings. It’s worth using a pulley bearing load check before you choose a tiny idler or high-speed alternator, because bearing life often depends more on side load than on rpm alone.

A 5-inch crank pulley driving a 3-inch accessory pulley gives a 1.67:1 speed increase, so the driven shaft sees more rpm even though the belt path looks simple. If you want to estimate shaft radial load, that same pulley geometry matters because belt force acts sideways on the shaft as speed and torque rise.