cotter
A pin or wedge inserted through a slot to hold machine parts together.
cotter: the pin that locks parts so they stay put
A cotter is a wedge-shaped or tapered pin driven through aligned holes in two or more parts to create a mechanical lock. The taper means the pin tightens itself as it is driven home, and cannot easily back out under vibration or load. Cotters hold axles to wheels, secure rod ends to pins, and lock components together where bolts alone might loosen or where a quick-release mechanism is needed.
The most common type is the cotter pin, a split pin made from soft steel wire bent back on itself, with a blunt eye for driving and tapered prongs that spread as they pass through the hole. These come in fractional sizes from 1/16 inch to 1/2 inch diameter, measured at the eye. A cotter key is similar but solid rather than split. Wedge cotters are thicker and more heavily tapered, used where higher shear loads demand it; they may have a head for striking or a hole for a retaining ring. Push cotters, less common, are driven through without spreading.
Installation requires drilling or punching two or more aligned holes through the parts to be locked. The cotter is then driven through by hand or light hammer blow. With split pins, the two prongs must be able to spread inside the hole or beyond it; if they cannot, the pin will not seat and will work loose. This is a frequent failure point. Removing a corroded or seized cotter requires patience, penetrating oil, and careful work to avoid damaging the parts underneath.
Where cotters appear in the field
Cotters are most visible in structural steel, shipbuilding, bridges, and older machinery. They secure clevis pins, shear pins, and castle nuts in place. On modern equipment, they have been partly displaced by safety wire, lock washers, and designed redundancy, but remain standard in many industries because they are cheap, require no electricity or complex tooling, and provide visible assurance that a joint is locked. A missing or loose cotter pin is a red flag that maintenance has been neglected.
The name comes from Old French cote, meaning a small shelter or covering, reflecting the pin's role in housing or securing other parts. The term is used widely across trades, though machinists and mechanical engineers are most familiar with the distinctions between types and the critical sizing rules that prevent cotter pin failure.