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.
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)
COMPARISON BETWEEN KEY AND COTTER
1. Key is usually driven parallel to the axis of the shaft which is subjected to torsional or twisting stress. Whereas cotter is normally driven at right angles to the axis of the connected part which is subjected to tensile or compressive stress along its axis.
2. A key resists shear over a longitudinal section whereas a cotter resist shear over two transverse section.
DIFFERENT TYPES OF COTTER JOINTS
1. Socket and spigot cotter joint
2. Sleeve and cotter joint
3. Gib and cotter joint
DESIGN PROCEDURE FOR THE SPIGOT AND COTTER JOINT
To understand the design steps let take a question and solve it step by step.
Q. Design a cotter joint subjected to a tensile load of 35 kN and a compressive load of 40 KN . The allowable stresses are
tensile stress qt = 70N/mm2
compressive stress qc = 110 N/mm2
shear stres t = 50 N/mm2
Failure of rod in tension or compression
tensile stress = tensile load / area
where d is the diameter of the rod
from above equation we will find out the value of d and round it off to the higher integer. Now we apply the Standard shaft rule..i.e the diameter of the rod should be of the given range
diameter increment in steps
24-45 mm 3mm
45-100 mm 5mm
above 100mm 10mm
we will make the value of the d such that it will satisfy the above table.
compressive stress = compressive load /area
again find the value of d ,round it off ,make according to the above table and now compare the two values of d .Take the value which is greater .
Failure of spigot in tension across slot
where t =d1/4
find the value of d1 and then t
again find the value of d1 and choose which one is larger ..
Failure of rod or cotter in crushing
check for the condition
if the above condition meet then the value of d1 and t is correct else increase the value of d1 and t to satisfy the condition .Because if the above condition is not satisfied then cotter will be get crushed due to load .
Tensile failure of socket across slot
find the value of d3 from above equation
Emperically , d3=1.75d
choose the greater one.
Crushing of cotter against collar of socket
find the value of d4
Emperically , d4=2.4d
choose the greater one .
Crushing of collar of spigot to socket
find the value of d2
Emperically , d2 =1.5d
choose the greater one.
Shear failure of spigot end against cotter
where T=shear stress
find the value of a
choose the greater value of a
Shear failure of spigot collar
find the value of h from the above equation
Choose the greater value of h
Double shear failure of collar of socket by cotter
find the value of e from the above equation
Choose the greater value of e.