Origin of the Terms

• A cotter was originally any wedge or cone-shaped part used to affix two other pieces together. This definition is still used in England today, but the word pin is more generally accepted elsewhere.
  
  

Cotter Pin

• In general use, a cotter pin is a retaining piece made of metal (usually steel). Cotter pins come in various sizes and are sometimes called straight pins. The pin is inserted and both sides one end are bent out to hold the pin in place, retaining the two parts.
  
  

Hairpin

• The term hairpin in engineering refers to a specific type of retainer that is also called a "R clip." These fasteners are made of springy treated steel and resemble the uppercase R.
  
  

Difference

• Where a cotter pin goes entirely through a shaft to secure parts, only one side of the hair pin goes through, while the bent side goes around the outside. The pressure provided by the steel holds the parts in place.
  
  

Usage

• The R-pin or hairpin is generally a heavier-duty part, excellent in applications involving clevis pins and axles, while the split pin may be structurally weaker. Never use a hairpin intended for beautification in any mechanical application.