sprags

Sprags are wedge-shaped devices used to prevent or control motion in mechanical systems. The term is applied to both simple immobilization wedges and to the core elements of a specific type of one-way drive mechanism known as the sprag clutch. In its most basic sense, a sprag is a blocker inserted between rotating parts, such as a wheel and its mounting, to keep the part from moving during maintenance or when safety requires it. These sprags can be referred to as wheel sprags or wheel blocks and are commonly used as temporary restraints or permanent safety devices when spoilage or unintended movement must be avoided.

In machinery, the most well-known application of the term is the sprag clutch. A sprag clutch consists of wedge-shaped elements (sprags) that engage between inner and outer races to permit torque transmission in one direction while allowing free motion in the opposite direction. When driven in the allowed direction, the sprags wedge outward and lock the races together; in the reverse direction they disengage, permitting rotation without backdrive. Sprag clutches are valued for their simplicity, robustness, and high torque capacity.

Sprags are typically made from hardened steel or other wear-resistant materials and may be designed as individual inserts or as integral parts of a race assembly. Design considerations include load and speed requirements, wear resistance, lubrication, and compatibility with the surrounding mechanism. Sprags remain a common solution in industrial gear drives, cranes, winches, conveyors, and other equipment requiring controlled one-way motion or reliable immobilization.