Fluid Couplings work on the hydrodynamic principle. It consists of a pump-generally known as impeller and a turbine generally known as rotor, both enclosed suitably in a casing.

The impeller and the rotor are bowl-shaped and have large number of radial vanes. They face each other with an air gap. The impeller is suitably connected to the prime mover while the rotor has a shaft bolted to it. This shaft is further connected to the driven machine through a suitable arrangement.

Oil is filled in the fluid coupling from the filling plug provided on its body.

A fusible plug is provided on the fluid coupling which blows off and drains out oil from the coupling in case of sustained overloading.

There is no mechanical interconnection between the impeller and the rotor (i.e. the driving and driven units) and the power is transmitted by virtue of the fluid filled in the coupling.

The impeller when rotated by the prime mover imparts velocity and energy to the fluid, which is converted into mechanical energy in the rotor thus rotating it.

The fluid follows a closed circuit of flow from impeller to rotor through the air gap at the outer periphery and from rotor to impeller again through the air gap at the inner periphery.

To enable the fluid to flow from impeller to rotor it is essential that there is difference in the "head" between the two and thus it is essential that there is difference in R.P.M., known as slip between the two.

Slip is an important and inherent characteristic of a fluid coupling resulting in several desired advantages. As the slip increases more and more fluid can be transferred from the impeller to the rotor and more torque is transmitted. However when the rotor is at standstill, maximum fluid is transmitted from the coupling.

The maximum torque is limiting torque. The fluid coupling also acts as a torque limiter.