Introduction to Pin Joints

Joints made by rivets or welding form a rigid joint, while a pin joint permits movement along the axis of the pin. These joints join two parts under tensile loads. Examples of a pin joint are, cross head, valve rod, and eccentric in a steam engine, a cycle chain, etc. The most commonly used pin joint is the knuckle joint, whose design is described in this chapter.

Knuckle Joint

Knuckle joint consists of an eye on one end and a forked end on other side (Figure 13.1). Each end has a hole in it. Both the ends are placed such that the holes are aligned in one line. A circular pin is inserted in the holes to form a pin joint. Then, a collar is placed over the protruding length of the pin. A thin tapered circular pin is inserted in the hole drilled in the collar and the main pin. This prevents the main pin from coming out of the hole. The ends of the knuckle joint are generally made octagonal and then square near the eye and forked ends, as shown in Figure 13.1 and Figure 13.2.

Design Procedure for a Knuckle Joint

Rather than the theoretical analysis, some empirical relations have been established after experience and are used to get the dimensions of the knuckle joint. These relations are marked in Figure 13.2. Thus, the design consists of calculating sizes for the given force P by these relations and then checking for different possible failures.