A fluid coupling is a hydrodynamic device that uses two couplings and fluid to transmit kinetic energy. Fluid couplings use a primary mover, which is usually some type of motor, and a driven machine. The term "fluid coupling" can be applied to a number of different such devices, including fans, pumps and conveyors.
All fluid couplings follow the same basic operating principles, regardless of specific model type and manufacturer. An impeller, or rotor that can transmit motion, is attached to the primary motion motor. A runner is connected to the driven machine. These two devices are set up so that they face one another, and are then surrounded with a hydraulic fluid. This entire apparatus forms the working circuit, according to the website Fluid Coupling Engineering.
The hydraulic fluid inside of the coupling is used to transmit fluid kinetic energy when it is thrown by the rotating motor. This causes the runner to rotate, which activates the driven machine at the end of the process. Even though the two components of the working circuit are not physically connected, the driver is still able to provide energy to the driven machine. Because of the flexibility of this design, fluid couplings are used in industries as diverse as steel production, mining, quarrying and power generation.