Fluid Coupling
Fluid couplings are devices that transmit rotation between shafts by acceleration and deceleration of hydraulic fluid by the hydrodynamic principle. The structure consists of a housing containing an impeller on the input or driving shaft and a runner on the output shaft. Both of these contain the fluid, which is usually oil and is filled into the coupling through a filling plug on the housing. The impeller, which acts as a pump, and the runner, which acts as a turbine, are both bladed rotors. Fluid couplings are used in the automotive, railroad, aerospace, marine and mining industries. Forklifts, cranes, pumps of all kinds, mining machinery, diesel trains, aircrafts and rotationally-powered industrial machinery all use fluid couplings.
The components of fluid couplings are generally made out of metallic materials-aluminum, steel or stainless steel. The impeller and rotor are both bowl-shaped and have many radial vanes. They face each other but have no mechanical interconnection and never touch. The velocity and energy is transferred to the fluid when the impeller rotates. It is then converted into mechanical energy in the rotor. Every fluid coupling has differing stall speeds, which is the highest speed that the pump can turn when the runner is locked and maximum input power is applied. Slipping always occurs because the input and output angular velocities are identical, and therefore the coupling cannot reach full power efficiency-some of it will always be lost in the fluid friction and turbulence.