The main difference between keys and cotters are as follows:
(a) Keys are driven parallel to the axis whereas cotters are driven perpendicular to the axis.
(b) Keys are used in parts subjected to torque whereas cotters are used in parts subjected to tensile or compressive force.
(c) Keys resist shear over a longitudinal section whereas cotters resist shear over two transverse sections.
Cotter joints are used to connect together two rods rigidly to transmit power in the length-wise direction. Sometimes when it is required to increase the length of the rod or to connect a rod directly to the machine to transmit power through the rod as in the case of a connecting rod end of a steam engine, a joint is used. Cotter joints can be used for round or square rods. Some of the joints are described below.
Sleeve and Cotter Joint – This type of joint is used for light transmission of axial loads from one rod to another rod. The ends of the two rods are forced into a sleeve. Two slots are provided in the sleeve, and one slot each is provided in the rods to take one cotter each.
The slot considerably decreases the strength of the rod that is why the diameter of the rods is increased for the length which comes inside the sleeve.
Image below shows the full sectional front view and side view of a sleeve and cotter joint when assembled. Note carefully the clearance provided in the sleeve and the rod on the sides of the cotter.
Spigot and Socket Cotter Joint – This type of joint is used for round rods. The end of one rod is forged in the shape of a socket and the other in the shape of a spigot. Slots are provided in the spigot and socket ends to accommodate the cotter. The diameter of the spigot end is increased to overcome the loss of strength due to the slot. To make the joint rigid and perfectly tight, the slots are made slightly out of alignment, so that when the cotters is driven in, it will tend to force the spigot and socket ends towards each other.
Gib and Cotter Joint for Square Rods – This type of joint is used for joining two square rods. One end of the rod is forged in the shape of a fork. The other rod is pushed into the fork. Slots are provided in the fork and the rod to accommodate the gib and cotter while assembling the parts. The gib is inserted first so that the straight surface touches the slot of the fork and then cotter is hammered into the rest of the slot. Care should be taken that the tapered side of the gib and cotter should be face to face with each other.