Snub Pulley

A snub pulley is a small-diameter, non-driven pulley positioned close to the drive pulley to increase the belt wrap angle around the drive pulley. The natural wrap angle of a single-drive arrangement is often limited to around 180–190°. By deflecting the belt path with a snub pulley, the wrap angle can be increased to 210–240°, which significantly raises the maximum transmittable drive force without belt slip. Snub pulleys are also used on the return strand to improve belt tracking and tension distribution.

Wrap angle is the arc of belt contact around the drive pulley surface. The maximum belt tension ratio (T1/T2) that can be maintained without slip is determined by the Euler-Eytelwein equation: T1/T2 = e^(μα), where μ is the friction coefficient and α is the wrap angle in radians. Doubling the wrap angle effectively squares the tension ratio capacity. A snub pulley is the most practical way to increase wrap angle in installations where a dual-drive arrangement is not justified.

Snub pulleys carry only the belt tension loads — no drive torque — and are positioned where space near the drive station is limited. A smaller diameter reduces the installation footprint. However, minimum diameter must still comply with the belt carcass requirements per ISO/DIN standards, since the belt bends around the snub pulley at every revolution. The reverse-bend loading imposed by a snub pulley positioned on the return strand is more damaging to the belt carcass than a normal bend, and this must be factored into minimum diameter selection.

Snub pulleys contact the belt on the carrying side (top cover) rather than the carcass or bottom cover. Adding lagging to a snub pulley would increase the friction against the belt's top cover, causing accelerated surface wear without providing any useful traction benefit. The snub pulley's function is purely geometric — to redirect the belt path — and a smooth, accurately machined steel surface is sufficient and preferred for this role.

The snub pulley is positioned on the slack-side of the drive pulley — between the drive pulley and the return strand. Its angular position determines the additional wrap angle achieved. The centre distance between the snub and drive pulley, and the snub pulley diameter, together determine the achievable wrap angle. Standard arrangements typically position the snub pulley to achieve a total wrap angle of 200–240°. The snub pulley must be aligned precisely parallel to the drive pulley to avoid introducing lateral belt forces.

Snub pulley bearings carry radial loads from the belt tension on both the incoming and outgoing belt strands. Because the snub pulley deflects the belt through a significant angle, the resultant radial load on the snub pulley shaft can be substantial — often comparable to the belt tension itself at high deflection angles. Bearing selection must account for this combined resultant load, the pulley rotation speed, and the required service life. Despite the smaller pulley diameter, snub pulley bearings should not be under-specified relative to the belt tension.