Couplings

Couplings are used to make a semi-permanent joint to transmit torque. These may be used for any of the following functions:

• a. Shafts are available in a maximum length of 7 meters due to transportation limitations. Hence, to increase the length of a shaft, couplings are used to join two shafts (Plate 20.1).

• b. The shafts may be aligned co-axially or could be misaligned also. Typical couplings are used for misaligned shafts.

• c. Transmission of angular shocks can be reduced by using rubber bushings in the couplings.

• d. These can be used as an overload protection device also. If the power transmitted exceeds beyond a certain limit, the key breaks to stop power.

In the design of couplings, generally the sizes are fixed by empirical relations and then these sizes are checked for strength in torsion, shear, crushing, etc.

Types of Couplings

Couplings can be classified as under:

1. Rigid couplings These are used to connect the shafts, which are in exact axial alignment. Following are the rigid type of couplings:

• a. Marine coupling

• b. Sleeve or Muff coupling

• c. Split muff coupling

• d. Half lap muff coupling

• e. Rigid flange coupling:

• • Unprotected rigid coupling (See Plate 20.1)

• • Protected rigid coupling

2. Flexible couplings These couplings can tolerate some amount of misalignment or offset without hampering power transmission. Following are the flexible types of couplings:

• a. Rubber bush coupling can tolerate some misalignment).

• b. Oldham's coupling can tolerate some amount of offset between the axes of the shafts.

• c. Universal coupling can transmit power at any angle.