Introduction

Belt Drive Design Calculator is a free online mechanical engineering calculator for analyzing an open (uncrossed) flat-belt or V-belt drive between two pulleys. It can calculate the driver pulley diameter (D₁), driven pulley diameter (D₂), driver speed (N₁), or driven speed (N₂) using the belt-drive speed relationship.

The calculator also provides the velocity ratio, and optionally calculates center distance, belt length, wrap angle, belt speed, belt tension ratio, slack-side tension, and transmitted power.

It is useful for mechanical design, machine design, power transmission, maintenance, manufacturing, CAD work, and engineering students for preliminary belt-drive calculations.

How to Use the Belt Drive Design Calculator

1. Select what you want to calculate

Under “I want to find”, select one parameter:

• Driver Pulley Diameter (D₁)
• Driven Pulley Diameter (D₂)
• Driver Speed (N₁)
• Driven Speed (N₂)

The calculator automatically asks you for the other three required values.

2. Enter the known values

For example:

• To calculate D₁ → enter D₂, N₁ and N₂
• To calculate D₂ → enter D₁, N₁ and N₂
• To calculate N₁ → enter D₁, D₂ and N₂
• To calculate N₂ → enter D₁, D₂ and N₁

Pulley diameters can be entered in mm, cm, m or inches, while rotational speed is entered in rpm.

3. View the velocity ratio

The calculator automatically displays:

Velocity ratio N₁:N₂ = D₂:D₁

This allows you to quickly understand the speed relationship between the driver and driven pulley.

4. Calculate belt length and wrap angle

Enable “Calculate center distance, belt length & angle of wrap” and enter the center distance (C).

The calculator then provides:

• Belt Length (L)
• Wrap Angle — Smaller Pulley
• Wrap Angle — Larger Pulley

The center distance must be greater than the difference between the pulley radii for the calculation to be valid.

5. Calculate power transmission capacity

Enable “Also calculate power transmission capacity”.

Enter:

• Coefficient of Friction (μ)
• Tight-Side Tension (T₁)

The calculator then determines:

• Belt Speed (v)
• Tension Ratio (T₁/T₂)
• Slack-Side Tension (T₂)
• Power Transmitted (P)

Formula Used

Speed Relationship

For an open belt drive with no slip:

D₁ × N₁ = D₂ × N₂

Therefore:

D₁ = D₂ × N₂ / N₁

D₂ = D₁ × N₁ / N₂

N₁ = D₂ × N₂ / D₁

N₂ = D₁ × N₁ / D₂

Belt Length

For the open-belt arrangement:

L = 2C + π(D₁ + D₂)/2 + (D₂ − D₁)²/(4C)

where:

• L = Belt length
• C = Center distance
• D₁ = Driver pulley diameter
• D₂ = Driven pulley diameter

Power Transmission

The calculator uses Eytelwein’s equation:

T₁/T₂ = e^(μθ)

where:

• T₁ = Tight-side tension
• T₂ = Slack-side tension
• μ = Coefficient of friction
• θ = Wrap angle in radians

The transmitted power is calculated from the effective belt tension and belt velocity.