Process & Mechanical
Belt & Pulley Speed Ratio
Driven pulley speed and belt surface speed from pulley diameters — exact kinematic relationship (N₁D₁ = N₂D₂), assuming no belt slip.
Standard method — stated scope limits apply
Pulleys & Speed
Enter pulley diameters and input speed.
About This Calculation
A belt-and-pulley drive transmits power between two shafts at a speed ratio set purely by the ratio of pulley diameters — the same conservation-of-power principle as a gear drive applies (smaller output pulley for a given belt speed means higher output shaft speed but lower output torque, and vice versa), but with the practical advantages of some slip tolerance, easier misalignment accommodation, and simpler maintenance than a rigid gear mesh.
Frequently Asked Questions
Why does pulley diameter ratio determine speed ratio, not pulley width or belt type?
Belt speed (linear velocity at the belt-pulley contact point) must be the same on both pulleys for the belt to move as a continuous loop — since that linear speed equals pulley radius times its rotational speed, a smaller pulley radius must correspondingly spin faster (and a larger pulley slower) to maintain that same shared belt speed.
Does belt slip meaningfully affect the theoretical speed ratio?
For a V-belt or flat belt under normal load, slip is typically small (often under 1-2%) and often ignored for a first-pass calculation — but under heavy load or with a worn/glazed belt, slip can increase enough to matter for precision applications, which is why a synchronous (toothed) belt is used instead wherever exact speed ratio must be guaranteed regardless of load.
Why choose a belt drive over a direct gear mesh for the same speed ratio?
Belt drives tolerate shaft misalignment and center-distance variation that a rigid gear mesh cannot, run quieter, need no lubrication, and provide inherent overload protection through slip — trade-offs against a gear drive’s more precise ratio, higher torque capacity, and better efficiency at the same physical size.