Set screw

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A set screw, also called a grub screw in British English, is a type of screw generally used to secure an object within another object. The set screw passes through a threaded hole in the outer object and is tightened against the inner object to prevent it from moving relative to the outer object. It exerts its clamping force through the bottom tip that projects through the hole rather than with a larger head that remains outside. They are used to prevent relative motion between two rotating parts, such as the movement of pulley on a shaft. For this, a set screw is screwed into the pulley hub so that its end-point bears firmly against the shaft. The fastening action is by friction between the screw and the shaft. Typically, set screws are not efficient and so are used only for transmitting very light loads. To reduce the chance of slipping and to increase load capacity, a detent may be drilled in the shaft into which the set screw's point enters. The detent, however, must be precisely aligned with the threaded hole prior to fastening. For longer life, they are usually made of steel and case hardened.