How to Select Cotter Pins and Wire Clips

How to Select Cotter Pins and Wire Clips How to Apply Cotter Pins Usage How to Select Cotters and Wire Pins

Image credit: Thread-Rite; NAXJA; Relex Industries

Cotter pins and wire clips are penetrating and/or coupling mechanical fasteners. They are easy to install and remove. Cotter pins come in several forms, with each designed for a specific kind of assembly. Some cotter pins are suitable for use as shear pins.

Cotter Pin and Wire Clip Operation

How the pin or clip's wire is bent determines its application and operation, though the general principle remains the same: pins and clips are easy to install and remove, and provide a fastening and alignment characteristic. All of the pins and clips covered in this tutorial are made of a deformable wire, be it plastic or elastic in nature. Generally, these clips need to be inserted into a predrilled hole, or applied around a shaft. They also account for some type of locking measure. Individual operation and application is covered more extensively below.

If the pin or clip has at least one straightened leg, a predetermined shear point, and it will not interfere with other parts of the assembly, the pin or clip could be suitable for use as shear pin, which will break at the point of mechanical overload to prevent damage to more expensive equipment.

Cotter Pin and Wire Clips Production

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Pin and Clip Material

While other material options exist, the following are the most common for wire clip manufacturing:

Plastic is inexpensive, corrosive resistant, and easy to mold, but only should be used for light loads.
Hardened steel is a stronger, but more brittle version of steel. It undergoes a heat treatment process.
Unhardened steel is more malleable than other steel types, so is also somewhat weaker.
Stainless steel is chemical and corrosion resistant, and has high stress ratings. It cannot be hardened.
Brass is strong, conductive, corrosion resistant, and has low magnetic permeability.
Monel® is a proprietary metal blend with good corrosion resistance, strength, but is expensive and difficult to form.

Pin and Clip Size

Manufacturers will make pins and clips according to the market's needs. After performance specifications, size of the pin or clip must be considered. Since there are a wide variety of pins made into multiple asymmetrical shapes, the most important predetermined dimensions are wire/pin diameter, and length. In some instances the measurement of cavities or diameters created by the wire bend will be specified, as well as additional lengths that may be relevant.

How to measure hairpin cotter clip

Image credit: Windsor Parts

Cotter Pin and Wire Clip Configurations

Split cotter pins are the most well recognized type of cotter pin. They are made of semi-circular wire that is bent over its flattened side to create a bulbous head. This pin is inserted into predrilled components to fasten them with its deformable tines, which take up any play in the assembly. They are also used as a brake in clevis pins. These pins are meant for one-time application.

How to Select Split Cotter Pin Types Configurations

Image credit: Smew

- Split cotter pins have multiple individual styles, each
  portraying a different kind of wire terminus.

Image credit: Hi-Line Inc.

Hairpins have an asymmetrical bulb and wave on one half of the pin. The other half of the pin is straight. This type of pin assembles components between the cavity created by the waved and straightened ends, with the assembly held tight by the pin's elastic deformation. These are used to quickly assemble or disassemble components.	Image credit: Reilly Supply Co.
Twist pins share similar mechanical properties with hairpin clips. A twist pin's main variance is that the clip legs cross, creating a positive, self-locking mechanism.	Image credit: Speeco
Bowtie pins are similar to hairpin clips and twist pins, except the clip wire is formed into a completed circuit. A deformable wire break allows the clip to be applied to a shaft.	Image credit: Pivot Point Inc.
Hairpin clips, also called retaining pins, provide a positive grip on a grooved shaft by the means of elasticized wire.	Image credit: F.C. Mason Co.
Safety pins resemble their household namesake, and operate similarly. Components are assembled on a straightened side of the clip, with the other side providing a clasp to contain the parts.	Image credit: Rainbow Spring Industries
Circle cotter pins have deformable wire in the shape of a circle, with the pin's entry a wire terminus pointed inward. Application is much like threading a key onto a key ring. This is best for use in parts that can be damaged from an abrasive edge.	Image credit: Drillspot
Knockout rings also have a deformable wire in the shape of a circle, and are applied like circle cotter pins. However, the pin's entry point is pointed upward.	Image credit: Wikimedia
Ring cotters are a hybrid between a twist pin and a circle cotter. One side of the wire clip is kept straight, while the other side is helixed over and under the straightened side, before terminating with a final bend across the straight side. This helix and twist method provides an automatic, secure lock when components are threaded appropriately.	Image credit: Wilson Mfg. Co.

Cotter Pin and Wire Clip Applications

Pins and clips are reliable forms of fasteners for applications where other hardware types may be unacceptable. In whole, pins and clips are relegated to light-duty assignments. Split cotter pins are used in many industrial devices, but also assemble children's teddy bears. Hairpins are suited for vehicle axles, but may also be used on the vehicle hood. Sometimes hairpins are substituted for linch pins, despite specific pins for this application. They are also used as the locking pin for hitch pins. Circle cotters are designed for use near fabrics and insulated electrical wires to prevent damage to these sensitive components. Nearly all of the above clips and rings are suitable for clevis devices, which are prevalent in the farming, sailing, and construction industries.

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