COTTER JOINTA cotter joint is used to connect rigidly two co-axial rods or bars which are subjected to axial tensile or compressive forces . It is a temporary fastening. A cotter is a flat wedge shaped piece of rectangular cross section and its width is tapered (either on one side or on both sides) from one end to another for an easy adjustment.

.

Socket Cotter

Spigot

APPLICATIONS OF COTTER

• 1. Connection of the piston rod with the cross heads2. Joining of tail rod with piston rod of a wet air pump3. Foundation bolt 4. Connecting two halves of fly wheel (cotter and dowel arrangement)

Design of Spigot and Socket Cotter

Let P = Load carried by the rods,d = Diameter of the rods,d1 = Outside diameter of socket,d2 = Diameter of spigot or inside diameter of socket,d3 = Outside diameter of spigot collar,t1 = Thickness of spigot collar,

d4 = Diameter of socket collar,c = Thickness of socket collar,b = Mean width of cotter,t = Thickness of cotter,l = Length of cotter,a = Distance from the end of the slot to the end of rod,σt = Permissible tensile stress for the rods material,τ = Permissible shear stress for the cotter material, andσc = Permissible crushing stress for the cotter material.

1. Failure of the rod(Spigot) in tension

P= Area x Tensile stress

2. Failure of spigot in tension across the weakest section (or slot)

Area Resisting=

P

d2

3. Failure of the rod (spigot) or cotter in crushing

• P=d2 x t x σc

d2

9. Failure of spigot collar in crushing

8. Failure of rod(Spigot) end in shear

10. Failure of the spigot collar in shearing

4. Failure of the socket in tension across the slot

Area Resisting=

Load P=

.

6. Failure of the socket collar in crushing

7. Failure of socket end in shearing

5. Failure of cotter in shear

Problem 1Design and draw a cotter joint to support a load varying

from 30 kN in compression to 30 kN in tension. The material used is carbon steel for which the following allowable stresses may be used. The load is applied statically. Tensile stress = compressive stress = 50 Mpa shear stress = 35 MPa crushing stress= 90 MPa.

t = Thickness of cotter. It may be taken as d2/4.

The length of cotter ( l ) is taken as 4 d.

Problem 2Design a cotter joint to transmit a load of 90 kN in

tension or compression. Assume the following stresses for socket, spigot and cotter.

Allowable tensile stress = 90 MpaAllowable crushing stress = 120 MpaAllowable shear stress = 60 Mpa.

Problem 3

Two rod ends of a pump are joined by means of a cotter and spigot and socket at the ends. Design the joint for an axial load of 100 kN which alternately changes from tensile to compressive. The allowable stresses for the material used are 50 MPa in tension, 40 MPa in shear and 100 MPa in crushing