Fluid Coupling – Working, Diagram, parts, Advantages

Written by Sachin Thorat in Thermal Engineering

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.

Fluid Coupling Diagram : 

fluid coupling parts diagram
fluid coupling parts diagram

Parts Of Fluid Coupling:

Fluid coupling or hydraulic coupling is used as clutches in cars employing automatic transmissions. It consists of two members, the driving and driven as shown in fig. The driving member is attached to the engine flywheel and the driven member to the transmission shaft. The two members do not have any direct contact with each other. The driven member is free to slide on splines on the transmission shaft. The two rotors are always filled with oil.

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:

When the crankshaft rotates, the driving member or impeller also rotates. The driving member is filled with oil and the centrifugal force causes the oil to be forced outward radially. As a result of this, the driven member or turbine is forced to rotate. Thus the engine power is transmitted from crankshaft to the transmission shaft.

As the engine speed increases, the thrown out oil from the driving member strikes the driven member with greater force and tends the driven member to rotate at the same speed, becoming one unit by means of oil film which combines both the members. As the engine speed falls down, the oil film between the driving and driven members is broken away and the members are disengaged.

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 the ‘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 – traveling 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.

Difference Between Torque Converter and Fluid Coupling 

Sr . no.Torque converterFluid coupling
1.The main components are the pump, stator, and turbine.The main components are impeller and runner.
2.It is a torque multiplication unit.It is simply a means to connect driving and driven members.
3.The turbine blades are inclined to have pitch.Blades are merely fins.
4.It acts as an automatic clutch and serves the purpose of an automatic gearbox to increase torque.It serves the purpose of an automatic clutch.
5.It is not as efficient as fluid coupling at highway speeds but is slightly more efficient under load.It is efficient at highway speeds.
6.It is usually used in conjunction with automatic clutch (mostly fluid flywheel) to eliminate the slight loss of efficiency at highway speeds.It is not assisted by a friction clutch.
7.It never locks up and the flow of oil never stops but continues.Impeller and runner are locked up and movement of oil stops during engagement when centrifugal force is approximately the same on both members.

Advantages of Fluid Coupling : 

the following are the advantages of fluid coupling

  • Controlled start-up speed without shock loading of the power transmission system
  • There is no mechanical contact between the driving shaft and the driven shaft (or between the pump wheel and turbine wheel). Hence there is no frictional wearing of them.
  • Power transmission is smooth. Motor or engine starts unloaded.
  • Fluid coupling can dampen shock loads. Fluid coupling can run smoothly even in extreme conditions.
  • The power transmission is free from vibration. There is no chance of vibration noises when power transmitted from the vibrating engine to the driven shaft by using a fluid coupling.
  • Fluid coupling can be used in both vertical and horizontal applications.

Disadvantages of Fluid Coupling : 

  • There is always a slip. There is always a slight difference in the speed of the pump wheel and turbine wheel
  • The fluid-filled in casing must be compatible with the coupling component, it directly affects the transmission behavior of the fluid coupling.
  • Fluid coupling cannot develop torque when the driving shaft and driven shaft are rotating in the same angular velocity.
  • Under stalling condition, the coupling dissipates energy as heat it may lead to damage.

Application of Fluid Coupling:

  1. Used For industrial application where heavy starting torque or inertia is needed under constant cyclic Loading.
  2. Automobile: Mainly used in the 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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Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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