Belt & Pulley
How long must the belt be, how fast does it run, and how far does it wrap around the small pulley? For open belts (same direction) and crossed belts (reversed).
Belts Instead of Gears
A belt connects pulleys that are far apart, runs quietly and slips instead of breaking when overloaded. The speed ratio is simply the ratio of the diameters. An open belt keeps both pulleys turning the same way; a crossed belt reverses one of them and wraps further round both.
The wrap angle on the small pulley limits how much force the belt can transmit before it slips, so designers keep it above about 120° — by not putting the pulleys too close together.
Belt Drive
to scaleKey Relations
A very good approximation of the exact length; the error is far below a millimetre for normal drives.
Crossing the belt makes it wrap further round both pulleys.
The angle of contact on the small pulley.
Sources & Verification
- R. G. Budynas & J. K. Nisbett, Shigley’s Mechanical Engineering Design, 10th ed., McGraw-Hill, 2015 — Ch. 17 (Flexible Mechanical Elements).