Changing pulley diameter changes shaft speed and the available torque relationship, but it also changes belt bending, wrap and the belt rating. A ratio should therefore be treated as the first design step, not the whole design.
Speed ratio
For an ideal drive, driven speed is approximately driver speed multiplied by the driver datum diameter divided by the driven datum diameter.
Torque relationship
When power is roughly conserved, reducing speed raises available torque at the driven shaft, minus drive losses. The exact machine torque still depends on efficiency and load.
Small pulley effects
A very small driver can increase belt bending and reduce belt life. Check the minimum recommended diameter for the chosen belt section.
Wrap angle
Large ratios can reduce belt wrap on the small pulley. Poor wrap can limit power transmission even if the arithmetic ratio is correct.
Energy efficiency
Correct tension, alignment, suitable section and a properly rated number of belts matter more than choosing a ratio that looks mathematically neat. Use the belt supplier’s design method for final rating.
For model matching or a quotation, email [email protected] with the pulley marking, shaft diameter and application.