Gib and cotter joint

The essentials

A gib and cotter joint is used to connect rods of square (or rectangular) section, such as those in the strap end of a connecting rod or in square tie rods. The end of one rod is held in a strap or socket, and a slot passes through both the strap and the rod. A gib and a cotter are inserted together through this slot. The gib is a piece with hook-shaped heads (noses) at its ends, and its inner face matches the cotter's taper, so that gib and cotter together form a parallel-sided block.

Key points

• Parts: two square rods (or a rod and a strap), a gib and a cotter.
• The gib is inserted first, then the cotter is driven in alongside it, their tapered faces in contact.
• Function of the gib: its heads hold the two sides of the strap together and stop them from spreading when the cotter is driven in.
• Gib and cotter together give parallel faces bearing on the slot, so the bearing pressure is even instead of acting along a sloping face.
• The cotter has a small taper on one side; the gib's matching face has the same taper.
• Like all cotter joints, it carries axial load, not torque.

Exam tips

• Draw the gib and cotter together in the sectional view and show their contact along the tapered faces.
• Remember the order of assembly: gib first, cotter second.

Common mistakes

• Confusing the gib with a second cotter; the gib has heads and a parallel outer face.
• Showing the gib and cotter both with tapers on their outer faces.