Introduction To Fluid Coupling:

It is a device for transmitting rotation between shafts by means of the acceleration and deceleration of a hydraulic fluid (such as oil). Also known as hydraulic coupling. Structurally, a fluid coupling consists of an impeller on the input or driving shaft and a runner on the output or driven shaft. The two contain the fluid. Impeller and runner are bladed rotors, the impeller acting as a pump and the runner reacting as a turbine. Basically, the impeller accelerates the fluid from near its axis, at which the tangential component of absolute velocity is low, to near its periphery, at which the tangential component of absolute velocity is high. This increase in velocity represents an increase in kinetic energy. The fluid mass emerges at high velocity from the impeller, impinges on the runner blades, gives up its energy, and leaves the runner at low velocity.

Parts Of Fluid Coupling:

Torque Convertor
Torque Convertor

A fluid coupling consists of three components, plus the hydraulic fluid:

  • The housing, also known as the shell (which must have an oil-tight seal around the drive shafts), contains the fluid and turbines.

Two turbines (fan like components):

  • One connected to the input shaft; known as the pump or impeller, primary wheel input turbine
  • The other connected to the output shaft, known as the turbine, output turbine, secondary wheel or runner
Working Of Fluid Coupling:
Construction and Working Of Fluid Coupling
Construction and Working Of Fluid Coupling

The driving turbine, known as the ‘pump, is rotated by the prime mover, which is typically an internal combustion engine or electric motor. The impeller’s motion imparts both outwards linear and rotational motion to the fluid.
The hydraulic fluid is directed by the ‘pump’ whose shape forces the flow in the direction of the ‘output turbine’ . Here, any difference in the angular velocities of ‘input stage’ and ‘output stage’ result in a net force on the ‘output turbine’ causing a torque; thus causing it to rotate in the same direction as the pump.
The motion of the fluid is effectively toroidal – travelling in one direction on paths that can be visualized as being on the surface of a torus:
  1. If there is a difference between input and output angular velocities the motion has a component which is circular (i.e. round the rings formed by sections of the torus). 
  2. If the input and output stages have identical angular velocities there is no net centripetal force – and the motion of the fluid is circular and co-axial with the axis of rotation (i.e. round the edges of a torus), there is no flow of fluid from one turbine to the other.
Application of Fluid Coupling:
  1. Used For industrial application where heavy starting torque or ineria is needed under constant cyclic Loading.
  2. Automobile: Mainly used in automobile sector in semi Automatic or Fully Automatic Transmission system:-In automotive applications, the pump typically is connected to the flywheel of the engine—in fact, the coupling’s enclosure may be part of the flywheel proper, and thus is turned by the engine’s crankshaft. The turbine is connected to the input shaft of the transmission. While the transmission is in gear, as engine speed increases torque is transferred from the engine to the input shaft by the motion of the fluid, propelling the vehicle. In this regard, the behavior of the fluid coupling strongly resembles that of a mechanical clutch driving a manual transmission.
  3. Aeronautical applications 
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