How this calculation works
A smaller driver pulley turning a larger driven pulley reduces rotational speed in the no-slip model. Torque must then be evaluated together with power and losses; a diameter ratio alone does not describe a real output shaft completely.
Driven RPM = driver RPM × driver diameter/driven diameter × (1 − slip); output power = input torque × input angular speed × overall efficiencyA worked example
A 100 mm driver at 1000 rpm driving a 200 mm pulley gives 500 rpm before slip. Ten N m at the driver corresponds to about 1047.20 W of input mechanical power. At ninety percent overall efficiency the output is about 942.48 W. At 500 rpm that produces eighteen N m of modeled torque, demonstrating the speed reduction and power loss together.
Good to know
Steady shaft kinematics and an entered power budget; no belt selection, traction, bearing load, tension, variable slip curve or drive approval. Inputs describe a scenario rather than a manufacturer-rated transmission. Check actual operating limits and component data before treating the calculated torque as available continuous output.
Reference: OpenStax — right-triangle trigonometry (opens in a new tab) ↗
What this model answers
A smaller driver pulley turning a larger driven pulley reduces rotational speed in the no-slip model. Torque must then be evaluated together with power and losses; a diameter ratio alone does not describe a real output shaft completely.
This page keeps speed slip and overall power efficiency explicit. Slip adjusts driven speed. Efficiency sets how much input mechanical power reaches the output, after all losses included in the entered efficiency.
The Belt pitch length and pulley centres calculator determines the geometric belt path and contact angle for the same pitch circles. Correct speed ratio does not by itself establish the required belt length.
Measure the right inputs
Use driver and driven pitch diameters in the same units. The driver is the shaft supplying input power; swapping the diameters reverses the ratio. Input torque belongs to the entered driver speed.
Overall efficiency must already include any losses associated with slip. The calculator does not subtract slip from power a second time. If you have only component-specific efficiency data, determine a consistent overall value before using the torque result.
Read the result in context
Driver pitch-line speed is a geometric belt speed at the driver pulley. Driven speed can be lower after the slip factor. That does not provide belt tension, traction reserve or a way to choose belt width.
Output torque is output power divided by output angular speed. With a fixed power-efficiency input, more entered speed slip can increase the arithmetic output torque because the same modeled output power is delivered at lower RPM. This is an assumption-driven calculation, not a physical slip-performance curve.
Work through the numbers
A 100 mm driver at 1000 rpm driving a 200 mm pulley gives 500 rpm before slip. Ten N m at the driver corresponds to about 1047.20 W of input mechanical power.
At ninety percent overall efficiency the output is about 942.48 W. At 500 rpm that produces eighteen N m of modeled torque, demonstrating the speed reduction and power loss together.
Where the model stops
Steady shaft kinematics and an entered power budget; no belt selection, traction, bearing load, tension, variable slip curve or drive approval.
Inputs describe a scenario rather than a manufacturer-rated transmission. Check actual operating limits and component data before treating the calculated torque as available continuous output.
A few questions, answered
Why enter both slip and efficiency?
They answer different questions: slip reduces speed while overall efficiency reduces transmitted power.
Will torque always follow the diameter ratio?
Only in the ideal no-loss model. Here torque follows the power balance and the calculated output speed.