Calculate driven pulley RPM, belt speed, and find the right pulley sizes. Essential for setting up lathes, drill presses, bandsaws, and other shop machinery.
To increase speed, use a larger driver (motor) pulley or smaller driven (machine) pulley. To decrease speed, use a smaller driver pulley or larger driven pulley. The ratio of diameters determines the speed change.
For V-belts, maximum practical reduction is about 6:1, and maximum increase is about 1.5:1 in a single stage. Beyond these limits, belt slip becomes a problem. For higher ratios, use multiple belt stages or consider gear reduction.
Pulley size affects speed and torque, not power (which comes from the motor). When you reduce speed with pulleys, torque increases proportionally. A 2:1 reduction doubles torque at half the speed. Power remains constant minus friction losses.
Belt speed is how fast the belt travels, measured in feet per minute. It's determined by pulley circumference times RPM. Belt speed affects heat buildup and wear. Most V-belts work best between 4,000-5,000 FPM and should not exceed 6,500 FPM.