The traditional set screw has a hexagonal drive on one end and is pointed on the other, though this may vary according to consumer and producer specifications. The set screw is often used together with a shaft collar, and like its functional relatives, eg. the dowel pin and shoulder screw, they are used in assembly.
What are set screws?
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.
What is the difference between bolts and set screws?
The difference between a bolt and a set screw is that there is thread the entire length of the set screw, whereas a bolt is only partly threaded.
Terminology
In the UK we refer to these products as hexagon headed bolts and set screws, however, in the USA they refer to them as cap screws.
Another confusing terminology associated with these is the fact that a grub screw is also referred to as a set screw….confusing isn’t it?
Sizes
As mentioned, a set screw is fully threaded along the length of the body, therefore the length is stated by the measurement from under the head to the end.
The length of a bolt is also stated by the measurement from under the head to the end, but the length of the thread is determined by the diameter, as shown above.
How dose set screws work
Unlike traditional screws that depend solely on threads to hold objects together, the set screw is designed to fasten one object inside the other. A threaded hole is created in the outer object, just like the norm, and the screw is tightened against the inner object. Seems normal, so far.
The difference comes about in terms of inner working. Unlike traditional screws that solely rely on threads to hold the two fastened objects in place, the set screw works by exerting a force through its tip to prevent any relative movement between the two objects, hence the mechanical advantage.
