A cotter is a flat wedge-shaped piece of steel as shown in figure-126.96.36.199. This is used to connect rigidly two rods which transmit motion in the axial direction, without rotation. These joints may be subjected to tensile or compressive forces along the axes of the rods. Examples of cotter joint connections are: connection of piston rod to the crosshead of a steam engine, valve rod and its stem etc.
188.8.131.52F- A typical cotter with a taper on one side only.
A typical cotter joint is as shown in figure-184.108.40.206. One of the rods has a socket end into which the other rod is inserted and the cotter is driven into a slot, made in both the socket and the rod. The cotter tapers in width (usually 1:24) on one
side only and when this is driven in, the rod is forced into the socket. However, if the taper is provided on both the edges it must be less than the sum of the friction angles for both the edges to make it self locking i.e α1 +α2 <φ1 +φ2 where α1 , α2 are the angles of taper on the rod edge and socket edge of the cotter respectively and φ1, φ2 are the corresponding angles of friction. This also means that if taper is given on one side only then α <φ1 +φ2 for self locking. Clearances between the cotter and slots in the rod end and socket allows the driven cotter to draw together the two parts of the joint until the socket end comes in contact with the cotter on the rod end.
220.127.116.11F- An isometric view of a typical cotter joint .