The shaft collar is a simple, yet important, machine component found in many power transmission applications, most notably motors and gearboxes. The collars are used as mechanical stops, locating components, and bearing faces. The simple design lends itself to easy installation. Many people will be familiar with shaft collars through using Meccano.

The first mass-produced shaft collars were set screw collars and were used primarily on line shafting in early manufacturing mills. These early shaft collars were solid ring types, employing square-head set screws that protruded from the collar. Protruding screws proved to be a problem because they could catch on a worker's clothing while rotating on a shaft, and pull them into the machinery.

Clamp-style shaft collars are designed to solve the problems associated with the set-screw collar. They come in one- and two-piece designs. Instead of protruding into the shaft, the screws act to compress the collar and lock it into place. The ease of use is maintained with this design and there is no shaft damage. Since the screws compress the collar, a uniform distribution of force is imposed on the shaft, leading to a holding power that is nearly twice that of set-screw collars.

Perhaps the most innovative and useful of the collars is the two-piece clamping collar. Two-piece clamp-style shaft collars can be disassembled or installed in position without having to remove other components from the shaft. The two-piece design provides greater clamping force than a single piece clamp because all of the force is transferred directly into clamping the shaft. In single piece designs, the non-tightened side provides negative force as it must hold the collar open to allow it to be placed onto the shaft. The single tightener must work against this force as well as provide clamping force of its own.

Two-screw clamps still provide force on two sides (one dimension) only. Four (or more) screw clamps provide force on four (or more) sides, and thus two dimensions.