Fluid couplings are hydrodynamic devices that transmit rotation between shafts by acceleration and deceleration of hydraulic fluid. Shafts are used industrially to provide rotary motion to a wide spectrum of vehicles and equipment and shaft couplings are key to providing secure rigid, flexible or non-linear connection between shafts, wheels and rotary equipment.
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Fluid Coupling Applications
Fluid couplings are used in the automotive, railroad, aerospace, marine and mining industries. They are used in the transmissions of automobiles as an alternative to mechanical clutches. Forklifts, cranes, pumps of all kinds, mining machinery, diesel trains, aircrafts and rotationally-powered industrial machinery all use fluid coupling when an application requires variable speed operation and a startup without shock loading the system. Manufacturers utilize these couplings to connect rotary equipment such as drive shafts, line shafts, generators, wheels, pumps and turbines in a variety of automotive, oil and gas, aerospace, water and waste treatment and construction industries.
Design of Fluid Couplings
Fluid couplings consist of a housing containing an impeller on the input or driving shaft and a runner on the output shaft. Both of these contain a fluid which is usually oil that is added to 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. The components of fluid couplings are generally made out of metallic materials-aluminum, steel or stainless steel.
In a fluid coupling, the impeller and rotor are both bowl-shaped and have many radial vanes. They face each other but unlike gear couplings have no mechanical interconnection and never touch. Fluid is directed by the pump into the impeller. The driving turbine or pump is rotated by an internal combustion engine or electric motor imparting both linear and rotational motion to the fluid. 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. Flexible shaft couplings such as fluid couplings are necessary because during operation, some types of shafts tend to shift, causing misalignment. Flexible couplings provide efficient accommodation for moderate shaft misalignment that occurs when the shafts' axes of rotation become skewed. Shaft movement is caused by bumps or vibration and it results in parallel, angular or skewed shaft misalignment.