Pins, Roll Pins, Cotter Pins and Keys: Selecting the Right Pin for Mechanical Assembly

A selection of pins, from left to right, dowel pin, slotted pin, spiral pin, split pin, R-clip, cotter and grooved pin.

Pins are widely used within manufacturing machinery and other plants, especially to fix collars and pulleys to shafts. There is a confusing array of choices such as dowels, grooved pins, spiral pins, coil pins and cotters. In this article, I’ll look at what each of these pins is, why it exists and its specific advantages. Pins have an unthreaded, approximately cylindrical shaft, which is inserted into a hole. Many types of pins rely purely on the friction of this shaft within the hole to remain securely in place. Some types of pins also have other features, such as a head or bent tines, which provide a positive lock to prevent removal from the whole. This article will consider pins in these two categories. Pins that rely purely on friction within the hole involve an inherent compromise between high forces and secure fastening. On the other hand, pins with some form of positive lock preventing removal can be held securely without requiring high insertion forces and without inducing a high preload stress on the hole.

Pins that Rely on Friction within the Hole to Remain in Position

dowel pin

There are many types of pins that rely on friction to hold them in a hole. They can be considered as ranging from completely solid dowel pins to much more compliant roll pins. The more solid the pin, the more precisely it can locate components and greater the shear force it can withstand. A solid pin may be able to provide a secure connection, provided the components it is fitted into have the correct dimensioned holes and are strong enough to resist the insertion force. Conversely, a more compliant pin will be easier to insert, more tolerant of dimensional variation in the hole size and will reduce fatigue loading when there is relative movement between the components being fastened together.

The most solid type of pin is a dowel pin. This is simply a solid cylinder of material. These are normally used with an interference fit. Elastic deformation of the pin and hole results in a radial surface normal force, leading to friction that holds the pin in place. Most dowel pins are chamfered at each end to aid insertion. However, because they offer little compliance, dowel pins usually require precise reamed holes that are well-aligned. This type of pin provides the most precise location for concentric holes in jigs and precision machinery. It is also able to transmit the highest shear forces.

grooved pin

A grooved pin is also a solid pin, similar to a dowel pin. However, a grooved pin has three grooves swaged along all or part of its length. Because the grooves are swaged, the displaced material increases the diameter of the pin along the length with the grooves. When the pin is forced into a hole, the grooves close, giving grooved pins considerably more elasticity than dowel pins and enabling insertion into holes that aren’t as tightly toleranced. They can often be inserted into simple drilled holes with deviations in diameter and circulatory that would not be suitable for dowel pins. It is also often possible to remove and reuse them. These pins are typically driven into interference fit holes to provide strong semi-permanent connections that are robust and resistant to vibrations. They are almost as strong as dowel pins. Typically, there are three grooves at equal spacing around the circumference of the pin, and there are smooth pilot sections at the ends, before the grooves start, to enable alignment before driving the pin into the hole. The grooves may have parallel or tapered sides, with parallel grooves providing a tighter fit that requires higher insertion forces but is better able to resist vibration and remain in position.

Half-length and third-length grooved pins have grooves along only part of their length. This enables them to grip along part of their length while providing a smooth clearance pin along another part. Components may be drilled through with a single diameter, and the grooved pin inserted through the stack of components. The pin will firmly locate into one component while allowing the other component to freely rotate.

Knurled pins are similar to grooved pins but instead of having grooves swaged longitudinally, they have a knurled pattern swaged into their surface. Grooved pins may have higher pullout forces.

cotter

Cotters are solid pins with a tapered flat face. This enables them to be driven securely into holes with a range of diameters. The flat face may also be used to prevent rotation of the pin, allowing a nut to be attached to a threaded end section. In this case, they are no longer relying entirely on friction within the hole to remain in position. Simple wedges that have no cylindrical faces, but which are also used to secure components by driving into a hole, may also be referred to as cotters.

Spring pins, also known as roll pins, are considerably more compliant than grooved pins. They are produced from a thin sheet of material, usually steel, rolled into a cylindrical shell with the outer diameter of the pin. This enables elastic deformation over a range of hole diameters. They can, therefore, be easily inserted into holes and typically have chamfered ends to make it even easier. There are two types of spring pins, or roll pins:

slotted spring pin

Slotted pins have the sheet material coiled by less than one revolution, leaving a slot along the length into which the pin can compress. They are generally used for light-duty applications or where a slightly more accurate and rigid location is required.
Coiled or spiral roll pins coil the sheet material by more than one complete revolution, typically about two full revolutions, so that the sheet coils toward the center. This enables them to be made from thinner, more flexible material while achieving a greater overall strength. This means that they are able to withstand a greater shear force than a slotted pin while also being more flexible. The increased flexibility helps reduce stress concentrations and the cyclic loading that can lead to fatigue, particularly around the edges of a hole. Coiled pins are, therefore, well-suited to heavy-duty applications. For example, they are used to pin joints on earth-moving equipment.

Pins with Some Form of Positive Locking

Pins that do not solely rely on friction to remain in a hole can be fitted much more loosely into the hole but still provide a secure fastening. This can enable rapid assembly and disassembly by hand, often without any tools. It also minimizes the insertion and removal forces, which might pose a risk of damaging some delicate components.

split pins

A split pin is produced from a malleable material with a half-circular profile. It is bent back on itself so that the two ends together form an approximately circular profile. The bent end is formed into an enlarged circular head. At the other end, the two tines can be initially inserted through a hole and then bent over to prevent removal.

R-clip

An R-clip is a type of pin produced from a sprung material that can be easily inserted by hand. It has two ends. One is straight and intended to be inserted through a hole in a shaft. The other end is curved so that it can deform outward to fit around the shaft as the other end is being inserted into the hole. It then grips around the shaft, remaining in place despite high vibrations. It allows rapid and repeated assembly and disassembly by hand without any tools.

Lynch pin

A lynch pin is used in similar applications to R-clips, but it uses a sprung ring attached to a pin. While an R-clip can be used at any position along a shaft, a lynch pin can only be used at an end, for example, to retain a wheel on an axle.

There are a range of different pins available, but they all have specific advantages and disadvantages in different applications. If you have a good knowledge of the different options, you should always be able to select the right pin for a particular job.