What is a Split Pin?

A split pin is a simple closure generally made of soft, malleable metal used to lock the lower parts of torque in place. The cotter pin typically consists of a TV, round or round pin half folded in two with a pronounced loop at the closed end. After a part has been put in place, the pin is passed through a hole in the stem and one or both of its “legs” bent backwards to prevent loosening while the ring at the closed end stops the fall through the hole. If the part is to be removed, the pin legs bent are simply straightened and cotter pin removed.

Split pins are among the cheapest and most used fasteners for locking low speed, low stress parts. They can be found in a myriad of applications ranging from brake disc locks to heavy vehicles to paper puppets and plush toy joints. Also known as cotter pins, these fasteners are available in a range of sizes and designs including pins, spring pins, and hammer lock pins. There are also several cross section profiles available, such as round, flat or half. The cotter pin is also available with different end profiles including tapered ends, miter ends, and extended pole pins which are all suitable for specific uses too.

Part of the beauty of using cotter pins as fasteners is their sheer simplicity. Once the part to be locked is in position, the pin is pushed through an adjacent locking hole to the end ring or firm eye going forward. One or both legs are then bent back to stop the loosening passage, thus effectively preventing the part from coming out of its case or coming out of the tree. There are some cotter pin designs, such as the press stud, which are a bit more sophisticated though. These pins characterize the spring steel construction and a “belly” profile in one of the legs that locks around the outer surface of round trees.

One of the known uses of the cotter pin is the locking mechanism for the honeycomb nuts commonly used on automobile hubs. These lightweight nuts lock the hubs in place and exert an adequate amount of torque on the wheel bearings. They are equipped with a series of cards placed along their upper edges reminiscent of the castle’s bastions. The stub axle has a hole through it, and once the nut is tightened to the correct torque, the cotter pin is inserted through the hole between the tabs of the nut. The ends are then bent backwards to lock the pin and prevent the crown nut from loosening.

The materials used in the split pin manufacture tend to be soft, malleable metals such as aluminum, bronze, brass, mild steel and. This makes it easy to flex the legs when the locking pin is in place. Unfortunately this also means the legs surviving only to be bent once or twice before they break. For this reason, split pins must be used only once and then discarded; this is quite acceptable considering the low cost of these small versatile fasteners.

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