Fluid Coupling/ Fluid Clutch Construction, Working and Power Transmission

fluid coupling oil flow direction diagram

Fluid coupling or hydraulic coupling is a hydrodynamic device used for rotating mechanical power transmission. In fluid coupling, there is no mechanical contact between driving shaft and driven shaft. It transmits power from one shaft to another with the help fluids or oils.

Construction of hydraulic coupling

Hydraulic coupling consists of a pump impeller and turbine runner, which are identical in shape, completely housed in an oil filled casing. Typically mineral oil is used as working fluid. The pump impeller (primary wheel) is connected to the drive shaft (input shaft), and the turbine runner (secondary wheel) is connected to the driven shaft (output shaft). Both shafts are in coaxial. Both pump impeller and turbine runner have vanes to guide fluid flow direction. The wheels are facing each other with very narrow clearance between them.

Read: Application, Advantages and Disadvantages of Fluid Coupling
Working of Torque Converter

Fluid coupling working principle and flow path of fluid

FLUDEX fluid coupling cross section
Cross section of FLUDEX fluid coupling - siemens

The input shaft is connected to a prime mover, usually an engine or an electric motor. When prime mover rotates the driving shaft, the centrifugal force on the fluid is increased. The oil inside the impeller starts to move from inside radius to outer radius of the pump wheel as shown in figure. The movement of oil through impeller convert mechanical energy into centrifugal energy to increase both kinetic energy and pressure energy of oil at the outer radius. The high energized fluid then enters into outer radius of turbine runner. Fluid particle then flows inwardly from outer radius to the inner radius of turbine runner. This flow of fluid exerts a force on vanes of turbine runner and rotates the runner and output shaft. Finally, the oil from runner is flow back to the pump impeller, and the process continues.

  • In a fluid coupling, fluid at outer radius flows faster than the fluid in inner radius.

Power transmitted in Fluid coupling - equation

Fluid drive coupling cannot achieve 100% efficiency, but it has much appreciated efficiency of 98%. The equation for fluid coupling efficiency and slip is discussed below
Efficiency of fluid coupling = Power at output/ power at input

η = (Power transmitted to driven shaft)/(Power available to the driving shaft)

Power at any shaft = 2 πNT/60

Substituting this value in efficiency equation, Here “A” stand for diving shaft, “B” stand for driven shaft
η=  (N_B  T_B)/(N_A  T_A ) fluid coupling efficiency equation

But the torque transmitted is same TA = TB
Efficiency,η=  (N_B  )/N_A fluid coupling efficiency equation

Slip of fluid coupling S

Slip of fluid coupling defined as the ratio of the difference of speed of driving shaft and driven shaft to the speed of the driving shaft.

Slip,S=  (N_A  〖- N〗_B)/N_A fluid coupling equation of slip

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