There are many different kinds of screws and fasteners, each of which serve a specific purpose. Shoulder screws have threading that does not continue all the way from the tip to the head, so there is a smooth portion of the shank at the base of the head. These screws are used for several different applications, including those that involve rotation or require a locking type of screw. They are also frequently called “shoulder bolts.”
The difference between these screws and other types lies in how far up the threading — the ridges that spiral up the shank of the screw and draw it into the hole as it turns — goes. On many screws, the threading goes all the way from the tip to the head. On a shoulder screw, however, the threading stops some distance from the head, leaving a smooth-sided round or square shank between the threading and the head. This area is usually slightly larger than the threaded portion of the screw.
Because of their design, these screws are able to serve many specialized purposes. For instance, they are often used in automotive or mechanical applications, because the unthreaded shoulder makes a good fastener for pulleys, linkages, and other moving parts. It allows for rotation or other movement, like a pin linkage.
Shoulder screws can also be made with square shoulders, such as in a round head wood bolt. Square shoulders are useful because they resist rotation, allowing them to act as a strong permanent fastener. For example, the round head wood bolt is used in wood because the square portion of the shank resists movement, allowing it to lock into place in the wood. This bolt has a smooth, round head with no slots for a screwdriver or wrench; the bolt is fastened with a nut that is screwed onto the threaded end.
Different types of metal, including steel alloys and stainless steel, can be used to make shoulder screws. They may also have different types of heads, such as slotted, Phillips, square, or hexagon. The length and diameter of the shoulder is often as important as the size of the threaded part, as the smooth part of the shaft may serve the most important function of the bolt.