Fluid Coupling / hydraulic Coupling

A fluid coupling (FC) or hydraulic coupling is a device used for transmitting power from one shaft to another shaft by means of acceleration and deceleration of a hydraulic fluid. It is based on hydro-kinetic principles and requires that the output speed be less than the input. This difference in speed is called slip.

Essential Parts:

  • The driving (input) section known as impeller which acting as a pump.
  • The driven (output) section known as runner or rotor which acting as a turbine.
  • The casing which bolts to the impeller enclosing the runner providing an oil tight reservoir.

Fluid Coupling Working Principle

fluid coupling

As the electric motor accelerates to the running speed, the impeller begins to centrifugally pump oil to the stationary runner. Transmission of oil is diffused by the conical impeller baffle, producing a gradual increase in torque, allowing the motor to accelerate rapidly to full running speed.

fluid coupling workingWhen all oil is pumped into working circuit, continuous circulation of oil will occur between the impeller and runner forming a flow path like a helical spring formed in a ring.

As soon as transmitted torque reaches the value of resisting torque, the runner starts rotating and accelerates the driven load. The time require to reach full speed is dependent on the inertia of the driven load, resisting torque and the torque being transmitted by the fluid coupling.

Types Of Fluid coupling

  1. Constant fill / fixed speed Type
  2. Variable speed / Scoop Type


  • The power transmission is free from vibration and noises.
  • Power transmission is smooth even in extreme condition.
  • Motor or engine starts unloaded.
  • Overload protection.
  • Controlled start up speed without shock loading of power transmission system.
  • The maximum torque can be adjusted by varying the amount of oil filled in the casing.
  • It can be used in both vertical and horizontal application.


  • There is always slip. There is always slight difference in speed of impeller and runner.
  • It cannot develop torque when the driving shaft and driven shaft are rotating in same angular velocity.
  • Under stalling condition, the coupling dissipates energy as heat it may lead to damage.

Important Questions

  1. Why the output speed of a fluid coupling is always lower than the input speed?

To enable the fluid to flow from impeller to rotor it is essential that there is a difference in ‘Head’ between the two and thus it is essential that there is a difference in speed known as slip, between the two. Slip is an important and inherent characteristic of a fluid coupling resulting in several advantages. As the slip increases, more and more fluid can be transferred from the impeller to the rotor and more torque is transmitted.

  1. What is the importance of the type of operating fluid used in fluid coupling?

Characteristics of operating fluid affect the transmission behavior of a coupling. The higher the density of the operating fluid, the better the transmission capacity. The higher the viscosity of the operating fluid, the more unfavorable the transmission behavior. The viscosity Index and flash points of operating fluid are also important. It must be ensured that the operating fluid is compatible with coupling components and their materials and operating conditions.

  1. What is the purpose of Thermal Protection on constant fill fluid couplings?

Thermal protection on this type of coupling provides safety to the fluid coupling and directly to the driver and driven machine.Fusible plug is the most common thermal protection. If the temperature of fluid in the coupling increases for any reason (which includes overloads) then the fusible metal in plug melts & all the fluid in the coupling drains out thus stopping power transmission and over loading the prime mover.

  1. What kind of oil does a fluid coupling need?

Normally, mineral oil of viscosity class ISO VG 32 is used for constant fill type coupling and ISO VG 46 for variable speed coupling.


Shaft Coupling