Fluid Coupling: Definition, Types, and Application

What is Fluid Coupling?

A fluid coupling or hydraulic coupling is a hydrodynamic or ‘hydrokinetic’ device used to transmit rotating mechanical power. It has been used in automobile transmissions as an alternative to a mechanical clutch.

It also has widespread application in marine and industrial machine drives, where variable speed operation and controlled start-up without shock loading of the power transmission system is essential.

Hydrokinetic drives, such as this, should be distinguished from hydrostatic drives, such as hydraulic pump and motor combinations.

Fluid couplings are torque-transmitting couplings that use hydraulic oil or water to transmit power. They differ from torque converters in that the input torque is equal to the output torque (no multiplication of torque).

Fluid couplings are much more efficient than torque converters and have typically only 2–4% losses. Fluid couplings are used to provide gentle acceleration control, torque limiting control, load sharing control, and variable torque/speed control.

The constant power law still applies, but the power in the driven load is reduced with speed. The difference between the input power and the output power is the loss of power dissipated in the coupling.

Fluid Coupling


Fluid couplings consist of two-bladed wheels that face each other, but that is not in physical contact. The pump wheel, or driving turbine, is connected to the driving machine. The turbine wheel is connected to the driven machine.

Power is transmitted hydraulically in a wear-free manner. The higher the input speed, the greater the amount of mechanical energy transmitted from the blades on the pump wheel to the blades on the turbine wheel.

With fluid couplings, the only connecting element between the two-bladed wheels is the fluid in the working circuit.

During machine startup, the amount of fluid in the coupling can be varied to control the machine startup behavior and the amount of power transmitted. By dampening torsional vibration and driveline shock, hydrodynamic devices can help extend equipment life.

Changing the fill level in fluid couplings also allows the speed of the driven machine to be controlled. Additionally, hydrodynamic couplings protect the drive and the machine against damaging torque spikes. If necessary, the slip can reach 100%, meaning that the motor can continue to run in a stable machine range.

What are the types of fluid couplings?

In practice there are two types of fluid couplings; Constant Filling, where the oil filling is fixed and variable filling also known as variable speed where the quantity of oil in the working circuit can be varied whilst running to give control over the amount of slip between the impeller and runner and so give speed regulation.

The variable filling fluid coupling also gives a means of declutching a machine from its drive and is used extensively in conveyor drives to control accurately the torque applied during acceleration and give easy load balancing using external controls.

Applications of Fluid couplings

Fluid couplings are used in automotive, mining, power generation, material handling, oil and gas, marine, mobile equipment, and other transportation-related applications.

In terms of fill, there are two types of devices: fluid couplings with constant fill and fill-controlled turbo couplings. Generally, fill-controlled turbo couplings are used to provide a controlled start.

They are also used when application requirements include an adaptation of machine speed, and starting and stopping the driven machine with the drive motor running. With turbo couplings, the type of operating fluid is important to consider.

In general fluid, couplings are used in Belt conveyors, Extruders, Centrifuges, Bucket elevators, Ball mills, Crushers, Mixers, Slurry pumps, Compressors, Industrial fans, Feeders, Diesel locomotives, Wagon tipplers, Boiler feed pumps, Reciprocating pumps, Process Pumps, Dryers, etc.

This fluid affects the fluid coupling’s transmission behavior and must be compatible with the coupling components.