Belt and Pulley Drive Simulator
A driver pulley (blue, left) and driven pulley (red, right) connected by a belt. The tight side is green (top), the slack side is orange (bottom). Both pulleys have visible spokes that rotate at the correct speed ratio. Tension labels, power flow arrow, wrap angle arc, and a formula bar at the bottom show all calculations live.
2 belt types:
Open belt drive — both pulleys rotate in the same direction. Belt runs straight between them. Wrap angle on the smaller pulley is always less than 180° (less grip). This is the default in most industrial drives.
Cross belt drive — belt crosses between pulleys, reversing the driven pulley direction. Wrap angle exceeds 180° on both pulleys (more grip). Used when counter-rotation is needed. Belt wears faster at the cross point.
4 sliders:
D₁ (50 to 300 mm): Driver pulley diameter. Making D₁ smaller while D₂ stays large gives speed reduction and torque multiplication. The ratio pill updates with "speed up" or "reduce" label.
D₂ (50 to 500 mm): Driven pulley diameter. Making D₂ larger than D₁ slows the output but increases torque.
N₁ (100 to 3000 RPM): Motor/input speed. N₂ = N₁ × D₁/D₂. A standard 1440 RPM motor with 2:1 ratio gives 720 RPM output.
μ (0.10 to 0.60): Friction coefficient between belt and pulley. Rubber on steel ≈ 0.3, leather on steel ≈ 0.4. Higher μ means the belt can carry more torque before slipping. Watch T₁/T₂ ratio change as you drag.
Visual details:
Pulley spokes rotate at the correct angular velocity ratio when you hit Play. If D₂ = 2×D₁, the driven pulley rotates at exactly half speed.
Green tight side (top) is drawn thicker than orange slack side (bottom), showing tension difference visually.
Wrap angle arc drawn on the smaller pulley with θ value in degrees.
Speed comparison bars at the bottom show N₁ vs N₂ proportionally.
Purple dashed power arrow between pulleys shows energy flow direction.
Formula bar at bottom shows: i, N₂, T₁/T₂, and power in one line.
Key slider experiments::
Set D₁=150, D₂=300. Ratio = 0.50 (speed reduction). Hit Play and count: driver spins twice for every one driven rotation. That's the 2:1 ratio visible.
Flip it: D₁=300, D₂=150. Ratio = 2.0 (speed up). Driven pulley now spins twice as fast. But torque is halved.
Switch to cross belt. Both pulleys still rotate but the driven one now goes the opposite direction. The wrap angle jumps above 180° because the belt wraps more of each pulley.
Drag μ from 0.30 down to 0.10. T₁/T₂ drops from ~2.5 to ~1.3. The belt can carry much less torque difference before slipping. This is why greasy belts slip.
Make D₁ = D₂ = 150. Ratio = 1.0. Same speed, same torque, wrap angle = 180°. Direct 1:1 drive.