Set screw

A set screw is a type of screw generally used to secure an object within or against another object, normally not using a nut (see bolts compared with screws). The most common examples are securing a pulley or gear to a shaft. Set screws are usually headless (also called blind), meaning that the screw is fully threaded and has no head projecting past the major diameter of the screw thread. If a set screw has a head, the thread will extend all the way to the head (whereas a bolt might have an unthreaded shank between the head and thread). A blind set screw (known in the UK as a grub screw, quite possibly from its figurative resemblance to a soil-dwelling grub) is almost always driven with an internal-wrenching drive, such as a hex socket (Allen), star (Torx), square socket (Robertson), or slot. 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 compressional or clamping force through the bottom tip that projects through the hole.

An example application is when a set screw is screwed into a pulley hub so that its end-point bears firmly against the shaft. The fastening action is by friction between the screw and the shaft, often (but not always) with some amount of elastic or plastic deformation of one or both.

Set screws are not always the best way to resist the torque of driven shafts. To reduce the chance of slipping and to increase load capacity, a detent (often called a "flat") may be milled or ground at the part of the shaft where the set screw's point contacts. The detent, however, must be closely aligned with the threaded hole prior to fastening. The operator can often feel the screw push the flat into final alignment as he or she makes the last half- or quarter-turn that tightens the screw. Set screws can successfully hold against heavy-torque applications if the details are right. For example, endmill holders of the solid type typically use large set screws holding against flats to fasten the endmill, and heavy loads are resisted. However, small set screws without flats are prone to spinning and galling if overloaded. The use of keys and keyways instead of, or in combination with, set screws is common for applications requiring high torque resistance or transmission. Splines offer yet more strength.

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