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 loss power dissipated in the coupling. 

Fluid couplings consist of two bladed wheels that face each other, but which are 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 by 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. 

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

Special features for fluid couplings include delay chambers, dynamic refill, adjustable drain nozzles, centrifugal valves, and multiple operating circuits. Delay chambers are used to provide greater relief of the drive motor during run-up. These devices also provide smoother starts and better efficiency during normal operation. Turbo couplings with both delay chambers and dynamic refill offer excellent relief of the drive motor as the motor speed is reduced. Typically, mineral oil or water are used as operating fluids. Fluid couplings with multiple circuits are also available.