When we want to transfer the rotation energy or
torque of one shaft to another, we have to connect
those two shafts with such a arrangements which
can transfer the power with maximum efficiency
and as per need.
In above kind of situations, ”The couplings” is
The coupling is device which connects the driving
and driven shaft and also transfer the power.
4. Fluid Coupling
A Fluid coupling is used for transmitting power or
torque from one to another shaft with the help of oil
Without MECHANICAL connection of two shafts.
It consist of two rotating
elements as a radial pump
impeller and radial flow turbine
The pump impeller is mounted
on driving shaft and turbine
runner is mounted on a driven
shaft. Both the impeller and
runner are identical in shape and
they are enclosed in single
housing filled with oil.
There is no direct contact
between the shafts.
When the driving shaft with pump impeller is
rotated, the oil starts moving from the inner radius
to outer radius of the pump impeller by centrifugal
Due to centrifugal action and the speed of
impeller, the pressure and kinetic energy of oil at
outer radius increases. The oil then enters the
turbine runner at the outer radios of the runner
and flows inwardly to the inner radios of the
runner. It will exert force and make it run.
The magnitude of the torque increases with an
increases in the speed of driving shaft.
And cycle will continue.
In actual prectice, the speed of driven shaft is
always less then the driving shaft by 2% to 4%
due to friction and turbulence loss in the impeller
and runner passage, which is known as SLIP.
The efficiency of the power transmitted by fluid
coupling is about 98%.
In fluid coupling the driven shaft is free from
11. Need… … …
Fluid couplings are found in some Diesel locomotives as part
of the power transmission system. Self-Changing Gears
made semi-automatic transmissions for British Rail.
semi-automatic transmissions and automatic transmissions.
Since the late 1940s.
The most prominent use of fluid couplings in aeronautical
applications was in the DB 601, DB 603 and DB 605 engines
where it was used as a barometrically controlled hydraulic
clutch for the centrifugal compressor and the Wright turbo-
compound reciprocating engine
The fluid torque convertor is used for transmitting
variable (increased or decreased) torque from one
shaft to another shaft.
A fluid torque convertor is nothing but a improved
The different torque( +/-) is achieved by adding
third member (reaction member) between pump
impeller and turbine runner.
15. How it works
When oil flowing from the pump impeller to turbine
Runner exerts the torque on the stationary guide
vanes which change the direction of oil. As result of
this, the oil reacts upon the turbine runner and
reduces the speed of turbine runner.
As we know, P NT.
Hence, if the T at the driven shaft is to be increased,
the corresponding value of the speed at the same
shaft should be decreased.
The speed of driven shaft is decreased by decreasing
the velocity of oil, which is flow from pump impeller to
the turbine runner and then through the STATOR
which change the direction of oil, therefore the
convergent is possible.
Automatic transmissions on automobiles, such as
cars, buses, and on/off highway trucks.
Forwarders and other heavy duty vehicles.
Marine propulsion systems.
Industrial power transmission such as conveyor
drives, almost all modern forklifts, winches, drilling
rigs, construction equipment, and railway