DESIGN OF COTTER JOINT(BASICS OF DESIGN)

COTTER JOINT

·         Cotter joint is used to connect two co-axial rods, which are subjected to either axial tensile or compressive force.

·         It is used to connect rod on one side with some machine parts like cross head or base plate on the other side.

·         The principle of wedge action is used in cotter joints.

·         A cotter is a wedge shaped piece of made of steel plate.

·         The joint is tightened and adjusted by means of wedge action of cotter.

Notations:

             P    =    tensile force acting on the rod (N)

             d    =    diameter of each rod (mm)

             D₁ =    outside diameter of socket (mm)

             d₁   =    dia. of spigot or inside dia of socket (mm)

             d₂   =    Diameter of spigot collar (mm)

             D   =    Diameter of socket – collar (mm)

             t₁    =    thickness of spigot collar (mm)

             l     =    Axial distance from slot to end of socket collar (mm)

             B    =    Mean width of cotter (mm)

             t     =    thickness of cotter (mm)

             L    =    Length of cotter (mm)

             l₁    =    Distance from end of slot to end of spigot on rod B – (mm)

·         In order to design the cotter joint and find out the above dimensions, failure in different parts and at different cross sections are considered.

1.          Tensile Failure of Rods

             Each rod of diameter d is subjected to tensile force P.

DESIGN OF SQUARE AND FLAT KEYS:

             The design of square and flat keys is based two criteria.

             1. Failure due to either shear stress or compressive stress.

             The forces acting on the key is shown in fig.

             The force p’ acts as a resisting couple preventing the key to roll in the keyway. It is assumed that the force p is tangential to shaft diameter.

              .......... (I)

             The shear failure is given by

              ...... (II)

             From (I) and II 

            

             The compressive stress is given by

                       ....... (III)

             From I and III

            

SPLINES:

             Splines are keys which are made integral with the shaft.  They are used when there is a relative axial motion between the shaft and hub.

The torque transmitting capacity of splines

             M_t = p_m AR_m

             The area A is given by

             A = ½ (D – d) n

             km =

            

REFERENCES

1. Mechanical Engg. Design                               –          Joseph Shigley
2. Machine Design                                              –          Mubeen
3. Machine Design                                              –          Black
4. Machine Design                                              –          R. K. Jain
5. Machine Design an integral approach             –          Norton, Pearson
6. Machine Design data hand book                                –          Lingayah Vol I.
7. Elements of Machine Design                         –          Pandya & Shah