A coupling is a mechanical device or hub used to connect two shafts to transmit power from drive end, for example: motor, to driven end, for example: pump or fan. Other purposes are to protect overloads and to provide mechanical flexibility due to misalignment. They can be the source of vibration if they are not installed properly.
Couplings in general can be divided into four categories:
Solid couplingsSolid couplings are fundamentally rigid hubs that do not compensate for misalignment, but allow two shafts to be connected for the purpose of transmitting torque. These couplings are fixed in between two shafts by mechanical fasteners such as bolts. Bolted hubs keyed on the shafts are a common example of a machine with solid couplings. Due to coupling arrangement, the vibration can be transmitted from one end to the other.
Magnetic couplingsThese couplings use a powerful permanent magnet to transfer torque from one shaft to the other; the two shafts are not in contact. Non-contact couplings eliminate wear and tear from vibration and protects equipment from overloads. A magnetic drive pump is a common example.
Flexible couplingsMany flexible couplings use flexible metallic, rubber or plastic elements, such as discs or bushings that rotate with the shafts and allow misalignment. These couplings do not require lubrication for proper performance and longevity.
Fluid couplings
The main parts of fluid coupling consists of two components: impeller which is connected to the drive end (motor) and runner which is connected to the driven machine. These two components are positioned face to face inside a casing, which is partially with an proper fluid. When the motor rotates, the fluid is picked up by the vanes of the impeller and thrown outwards, towards the vanes of the runner. As a result, the runner begins to rotate together with the driven machine.
Left picture: rubber coupling. Right picture: helical coupling.