A cotter joint connects two coaxial rods subjected to axial forces. It uses a tapered wedge-shaped cotter inserted into slots in the overlapping ends of the rods. The cotter prevents longitudinal movement but allows for disassembly. Cotter joints are commonly used to connect piston rods, pump parts, flywheels, and foundation bolts. The document provides design considerations and equations for sizing the rods, slots, and cotter to withstand various failure modes from the tensile, compressive, and shear stresses based on the applied load and material properties. It also presents three example problems of designing cotter joints for specified loads.

1. COTTER JOINT
A 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

2. APPLICATIONS OF COTTER
• 1. Connection of the piston rod with the cross
heads
2. Joining of tail rod with piston rod of a wet
air pump
3. Foundation bolt
4. Connecting two halves of fly wheel (cotter
and dowel arrangement)

3. 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, and
σc = Permissible crushing stress for the cotter material.

7. 1. Failure of the rod
(Spigot) in tension
P= Area x Tensile stress

8. 2. Failure of spigot in tension across
the weakest section (or slot)
Area Resisting=
P
d2

9. 3. Failure of the rod (spigot) or cotter
in crushing
• P=d2 x t x σc
d2

10. 9. Failure of spigot collar in crushing

11. 8. Failure of rod(Spigot) end in
shear

12. 10. Failure of the spigot collar in
shearing

13. 4. Failure of the socket in tension
across the slot
Area Resisting=
Load P=
.

14. 6. Failure of the socket collar in
crushing

15. 7. Failure of socket end in
shearing

16. 5. Failure of cotter in shear

17. Problem 1
Design 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.

18. Problem 2
Design 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 Mpa
Allowable crushing stress = 120 Mpa
Allowable shear stress = 60 Mpa.

19. 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