Sentence:  A <WSD>sprag</WSD> (fig. 349) is a steel block so shaped as to act as a wedge in the complete assembly. In the <WSD>sprag</WSD> unit under discussion, there are 42 <WSD>sprags</WSD> assembled into an outer race and held in place by two energizing springs (fig. 350). 
Meaning of the word marked with <WSD> tags:  A billet of wood; a piece of timber, a similar solid object or constructed unit used as a prop.