Sprag

Sprag is a term used in mechanical engineering and construction to refer to a wedge-shaped element that prevents unwanted motion, in different but related ways.

In its primary engineering sense, a sprag is a key component of a sprag clutch (also called a one-way clutch). Sprags are wedge-shaped blocks arranged between inner and outer races. When torque is applied in one direction, the sprags engage and transmit rotation from the input to the output; when carried in the opposite direction, the sprags slip or disengage, allowing free rotation. Sprag clutches come in various designs, with different numbers and shapes of sprags to suit specific torque and speed requirements. They are valued for compactness, smooth engagement, and maintenance simplicity, but can wear under high loads and may require careful alignment and lubrication to manage heat and longevity.

A second, older sense of sprag refers to a brace or wedge used to prevent movement of a component in timber framing, carpentry, or machinery assembly. In these contexts, a sprag is a short block of wood or metal inserted to brace, lock, or stabilize parts during construction, transport, or temporary operation. The term conveys the idea of a obstruction or stopper that fixes position until a more permanent arrangement is put in place.

Etymology generally traces to sprag, meaning a wedge or obstruction. The term appears in both modern mechanical devices and traditional bracing practices.

See also: sprag clutch, one-way bearing, wedge.