What are overrunning clutches?

An example of a roller ramp overrunning clutch from Altra Industrial Motion.

Overrunning clutches, sometimes called freewheel clutches, are used to freewheel in one direction while driving in another direction of rotation. When the driven shaft is rotating faster than the driveshaft, the clutch mechanically disconnects the driveshaft from the driven shaft.

They are used in heavy-duty applications, such as agriculture, mining, aerospace, industrial metal processing, indexing, and more. They are often used when backstops, multiple-speed, dual- and one-way drives are required.

Four types of overrunning clutches include the sprag, roller ramp, wrap spring, and wedge styles.

One of the longest lasting designs is the roller ramp clutch, which features cylindrical rollers that are positioned in a wedge-shaped area formed by an outer cylindrical surface and inner flat surface. When friction is generated between the rollers and these surfaces, the rollers wedge firmly as the flat inner part rotates in the driving direction. This engages the clutch.

A sprag overrunning clutch from Altra Industrial Motion.

The sprag clutch operates in a similar manner but it has an inner and outer cylindrical shape and instead uses irregular or bean-shaped sprags that operate in one direction only. They offer greater torque but are limited by their bore size due to radial space restrictions.

Wrap spring clutches use a helical spring to transmit torque in one direction by tightening on a cylindrical sleeve. They are used mostly in low-speed operations that don’t require a heavy-duty design.

Finally, the wedge ramp brings together the best of all three designs. They are usually low cost, easy to repair and offer high overrunning speed and torques. Rather than in a cylinder-shaped outer member, it features a flat surface and compact springs, giving it a larger area to boost torque.