Pulley Calculator

What Is a Pulley Calculator?

A pulley calculator analyzes a two-pulley belt drive. Given driver and driven pulley diameters, driver RPM, and center distance, it finds driven RPM, belt speed, belt length, and the RPM ratio. Mechanical designers use these values to select belts, check speed limits, and verify torque transmission.

Pulley Drive Formulas

For a belt connecting two pulleys, the speed product is constant:

$$d_1 \times n_1 = d_2 \times n_2$$

Belt linear speed in m/s:

$$v = \frac{\pi \times d_1 \times n_1}{60}$$

Open belt length with center distance $D$:

$$L = \frac{\pi d_1}{2} + \frac{\pi d_2}{2} + 2D + \frac{(d_1 - d_2)^2}{4D}$$

Frequently Asked Questions

How do I find driven pulley RPM?

Multiply driver diameter by driver RPM, then divide by driven diameter: n₂ = d₁ × n₁ / d₂. A smaller driven pulley spins faster.

What units should I use for diameter?

Use consistent length units. This calculator accepts millimeters for diameters and center distance, then converts internally to meters for speed and length results.

Why does belt length depend on center distance?

Longer spans between pulley centers add straight belt sections. The correction term (d₁ − d₂)² / (4D) accounts for the angle wrap on each pulley.

What is the RPM ratio?

RPM ratio is n₂ divided by n₁. Values greater than 1 mean the driven pulley spins faster than the driver.

Does this work for timing belts and V-belts?

The geometry formulas apply to any flexible belt drive with two pulleys. Check manufacturer slip limits for flat belts under heavy load.

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What Is a Pulley Calculator?

Pulley Drive Formulas

Frequently Asked Questions

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