What is a frictional locking device?

By Zak Khan

A locking device is designed to prevent mated shafts and components from loosening out of place when they are subjected to movement, varying temperatures, vibrations, stresses, and other operating conditions. They are critical components, as they often ensure the safety of the system. They appear frequently in systems that require coupling various components together.

Frictional locking devices are devices that perform the above functions using the coefficient of friction between the two contacting surfaces. A primary example occurs when inserting the locking device between the shaft and the hub of a system. The locking device then expands to fill the gap, holding the components in place by friction. These usually take the form of metallic or non-metallic hollow cylinders, often with a slit on one side. Another familiar friction locking device is the nut. These ubiquitous pieces of assembly and mating components work with a combination of friction on the threads of the shaft, slight tension on the bolt and compression of the parts held together.

Frictional locking devices have the advantage that they do not require keying. That is, no need to properly align keys and key-ways, and no need to worry if these will be compatible when designing systems. Indeed, because the locking is completely performed by friction between the locking device and the shaft, the system can even deal with oversized and undersized shafts.

No keys also means no worry over loose keyed components at reduced torque ratings; loose keys can cause vibrations and injuries, and damage equipment. All that engineers need out of the system is the ability to insert the shaft into the locking device, the frictional locking device then exerts radial pressure, locking the components in place. When compared to keyed connections, they can be backlash free with proper fit tolerances, they allow the ability to make adjustments to the axial position and angular timing in a system, and no impact between key and key-way occurs when reversing the system because no keys are present.

Advantages such as these make friction locking devices applicable in many cases. With their compatibility and ease of use, engineers often select them for a variety of situations. But which situations are best suited for frictional locking devices and which are best to avoid? Generally engineers should avoid employing them in situations with high external centrifugal forces. These situations can cause a drop in the pressure between the components and lead to slipping. Because there is often a small slit in frictional locking devices—to accommodate shafts of varying diameters—these can cause imbalances in certain operating conditions, usually at higher speeds. In such applications, engineers can use slit-less friction locking devices, which have stricter machining and application tolerances, or use another type of locking device.

Frictional locking devices come in varying configurations, usually anywhere from one to three pieces, Smaller sizes are usually reserved for lower torque, less demanding operations. Systems that operate at high torques or in especially demanding operations are often available in specialty configurations from various manufacturers. Engineers should consult documentation and have good design calculations to select frictional locking devices for their systems. Manufacturers usually provide the necessary equations to size locking devices. Always consult manufacturers with any questions and concerns.