Mechanical

Belt & Pulley Calculator

Calculate drive system pulley diameters, belt length, and speed ratios.

Input Parameters

Units:
in
in
RPM

Results

Enter pulley diameters and click Calculate

What is Belt & Pulley Drive?

Belt and pulley systems transmit power between shafts using friction (V-belts) or positive engagement (timing belts). By using different pulley diameters, you can change speed and torque ratios.

Unlike gears, belt drives are quieter, absorb shock loads, and can span longer distances. They're commonly used in HVAC, machine tools, automotive engines, and industrial equipment.

How to Use

  1. Select your unit system (Imperial or Metric)
  2. Choose calculation mode (ratio or belt length)
  3. Enter driver and driven pulley diameters
  4. Add input RPM for speed calculations
  5. For belt length, enter center distance
  6. Click Calculate to see results

FAQs

Pulley ratio = Driven pulley diameter ÷ Driver pulley diameter. For example, a 4" driver with an 8" driven pulley gives a 2:1 ratio. This halves the speed and approximately doubles the torque (minus belt slip losses).

Standard V-belts should not exceed 6,500 ft/min (33 m/s). High-performance belts may allow higher speeds. Exceeding the maximum speed causes excessive centrifugal force, reduces belt life, and can cause the belt to jump off the pulleys.

Ideal center distance is 1.5 to 2 times the sum of pulley diameters. Minimum is typically the larger pulley diameter. Too short reduces wrap angle and belt life; too long causes belt flutter and requires tensioning systems.

V-belts are quieter, absorb shock, and allow slip (acts as overload protection). Timing belts provide positive engagement with no slip - required for synchronous applications like CNC machines, 3D printers, and camshaft drives where exact timing matters.

V-belts typically slip 1-3% under normal load, more under starting loads. This means actual output speed is slightly lower than calculated. Timing belts have zero slip. For critical speed applications, use timing belts or measure actual output speed.

Limitations

  • • Does not account for belt slip (typically 1-3% for V-belts)
  • • Belt length formula assumes parallel shafts
  • • Does not calculate belt tension or power capacity
  • • Use pitch diameter for timing belts, not OD
  • • Verify belt selection with manufacturer specs