COTTER JOINTS

Temporary fastening and is used to connect rigidly two co-axial rods or bars which are subjected to axial tensile or compressive types of load.

Flat wedge shaped piece of rectangular cross-section and its width is tapered (either on one side or both sides) from one end to another for an easy adjustment.

Taper varies from 1 in 48 to 1 in 24 and may be increased up to 1 in 8, if a locking device is provided.

The locking device: taper pin or a set screw.

Material: Mild Steel or Wrought Iron

Applications: Cross-head of a reciprocating steam engine, piston rod and its extension as a tail or pump rod, strap end of connecting rod etc.

Types of Cotter Joints:1. Socket and Spigot Cotter Joint2. Sleeve and Cotter Joint3. Gib and Cotter Joint

Cotter joint

Design of Socket and Spigot Cotter Joint

1. Failure of the rods in tension:

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

3. Failure of cotter in crushing:

4. Failure of the socket in tension across the slot:

5. Failure of cotter in shear:

6. Failure of socket collar in crushing:

7. Failure of socket end in shearing:

8. Failure of rod end in shear:

9. Failure of spigot collar in crushing:

10. Failure of the spigot collar in shearing:

11. Failure of cotter in bending:

12. Length of cotter (l): 4d13. Taper : 1 in 24, for greater taper locking device must be provided.14. Draw of cotter: 2 to 3 mm

Note: When all the parts of the joint are made of steel, the following proportions in terms of diameter of the rod (d) are generally adopted:

………….Design Data Handbook

Sleeve and cotter joint

The enlarged ends of the rods butt against each other with a common sleeve over them

The rod ends are enlarged to take care of the weakening effect caused by slots

For circular rods

Sleeve and Cotter Joint

Gib and Cotter Joint for strap end of a Connecting rod

Gib and cotter joint for rectangular rods One bar end is made in the form of a strap

A Gib is used along with the cotter.

Gib is like a cotter but with two gib heads at its ends .

The thickness of the gib and cotter are same

Gib and cotter joint or rectangular rods

When the cotter alone (without the gib) is driven, the friction between the ends and the insides of the slots in the strap tends to cause the sides of the strap to spring open as shown.

To prevent this gibs are provided which hold together the ends of the strap.

Gib and Cotter Joint for square rod

Cotter joint to connect Piston rod and Crosshead

Books:

1. Shigley’s Mechanical Engineering Design (In SI Units) – (TMH)

2. Machine Design – An Integrated Approach – Robert L. Norton (Pearson Education)

3. Design of Machine Elements – M. F. Spotts (Pearson Education)

4. Design of Machine Elements – V B Bhandari (TMH)

5. A Textbook of Machine Design – Khurmi & Gupta (S. Chand)

6. Machine Design DataBook – K. Lingaiah (TMH)