# What is Hydraulic Coupling & expiations

Hydraulic coupling:-

Hydraulic coupling is a device that is employed for the transmission of power from one cell to another through a liquid medium. Instead of a mechanical connection, is a liquid connection is established due to whirling motion which transmits power from one shaft to another.

The coupling consists of;
i) a pump impeller attaches driving shaft;
ii) a turbine runner attached to a Ribbon shaft;
iii) a housing enclosing pump impeller and turbine runner which is fully filled with a liquid usually oil.
When the shaft is rotated then the pump pimple starts rotating which throws the outward by centrifugal action. The thrown oil then enters the turbine runner and it exerts a force on the Runner blades. The torque on the turbine runner increased with the speed of the driving shaft and the driven shaft start rotating when exerted torque overcomes the inertia effect of the driven shaft. The oil from the runner return to the pump impeller where it is again thrown outward by centrifugal action. This way a complete Hydraulic circuit please completed between the pump impeller and the turbine Runner.
The efficiency of hydraulic coupling is defined as the ratio of power output two power input.
η  = power output / power input = T0 ω0 Ti ωi
but,                T0 = Ti

η  = ω0 / ωi            where;, ω0 = angular speed of the driven shaft
ωi = angular speed of the driving shaft
The slip of a hydraulic coupling has defined the ratio of difference of angular velocities the driving and driven to the angular velocity of the driving shaft.
Slip = ωi – ω0 / ωi
= 1 – ω0/ ωi

= 1 – η

The efficiency of the coupling is zero at the start, i.e., when the speed ratio is zero and it increases with the increase of speed ratio till the speed ratio is about 0.95. The efficiency reduces after the speed ratio as…..

The variation of the driving shaft torque input with driving shaft speed for different values of slip. These are parabolic curves. The curves indicate that for a given slip, the input torque increases as the cube of the driving shaft speed  (Ti Ni3 ). Hence, the power transmitted will also vary as the cube of the speed for a given slip. In order to reduce the input, the slip must be of a smaller value.
The merits of fluid coupling are:
a) there is no mechanical or rigid connection between the driving and driven shaft.
b) driving shaft start gradually no initial higher torque is required at driving shaft or source.
c) Driving shaft with heavy initial load can be started.
d) transmission is Muth without Shock.
e) slip is preferred for automobiles and Marine engines.
The limitations are:-
i) the speed of the driven shaft various depending upon the input torque and speed.
ii) in case of extremely high load, the driven shaft cannot be started by this method.
iii) oil of the coupling has to be checked and maintained regularly.
Application of the hydraulic coupling:-
i) in automobile and Marine engines where the driven shaft is preferred to be run at a lower speed than the driving shaft.
ii) in a bucket elevator and belt conveyor, where the gradual start of the driven shaft load is preferred.