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.
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. 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.
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.
