There are many different kinds of screws and fasteners, each of which serve a specific purpose. Shoulder screws are used for a couple of different applications, including those that involve rotation or require a locking type of screw. Shoulder screws are also frequently called “shoulder bolts.”

The difference between shoulder screws and other types of screws lies in how far up the threading – the ridges that spiral up the shaft of the screw and draw it into the hole as it turns – goes. On normal screws, the threading goes all the way up to the head of the screw. On shoulder screws, the threading stops a distance from the head, leaving a smooth-sided round or square shaft between the threading and the head. This shaft is usually slightly larger than the threaded portion of the screw.

Because of their design, shoulder screws are able to serve many specialized purposes. For instance, shoulder screws are often used in automotive or mechanical applications, because the cylindrical shoulders make good fasteners for pulleys, linkages, and other moving parts. The round shoulder makes shoulder screws appropriate for these applications, because 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 shoulder screws are useful because they resist rotation, therefore acting as a strong permanent fastener. For example, the round head wood bolt is used in wood because its square shoulders resist movement, allowing it to lock into place in the wood. A round head wood 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. You may recognize these types of shoulder screws from the wooden play structures on children’s playgrounds or schoolyards.

Shoulder screws may be made from different types of metal, such as steel alloy or stainless steel. They may also have different types of heads, such as slotted, Phillips, square, or hexagon. In shoulder screws, 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.