Some people will struggle to accept the test figures for the Fluid Coupling application as a highly efficient hydraulic fan drive for the cooling commercial vehicles, (see download on left of page), until they can fully understand how the fluid coupling actually works and/or see it for themselves. 

The Fluid Coupling cannot be viewed as a pump because the majority of energy transfer is through pressurized oil forcing the ring and output shaft to rotate. Yes, some oil can be bleed off to reduce torque and/or output speed but the majority of power is from compressed oil on the ring contour forcing the ring and output shaft assembly to rotate.

In hydraulics there are 2 inefficiencies, Volumetric and Mechanical.

Volumetric is how much internal leakage there is from the pressure areas to lower pressure areas. We have reduced this leakage by improving tolerances and using thicker oil. It needs to be noted that when pumping of oil in these couplings occurs it is at the differential speed of input and output, for example 1400 rpm in and 900 rpm out can be viewed as a pump operating at 500 rpm. Because the relative speeds are low, but centrifugal forces remain high, the manufacturing tolerances can be reduced. As the relative speed of the pump is low, inlet issues with high viscosity oils is not a problem.

Mechanical inefficiencies are the frictional forces between the rotor, vanes, ring and pressure plates (bearing efficiency has been ignored as a vane pump has balance pressure quadrants producing no side loads and uses low friction ball bearings). In a standard vane pump this is a loss of power as the ring and pressure plates are stationary, however in a coupling where power is being transferred through to the rotating ring and pressure plates from the rotor and vanes, it actually improves our efficiency by assisting drive. IE: Any frictional forces help to drive the ring and output shaft.

Pressure Controlled Coupling

The Pressure Controlled Coupling speed is controlled by bleeding some oil off at a controlled pressure. This bleed off oil is a loss of power, however, in a fan drive, the power required is the cube of the speed so as speed reduces the pressure greatly reduces and the frictional gains spoken about above are a larger proportion of the transferred energy further reducing the required pressure. The higher the volume of bleed off volume the lower the output speed, the lower power the fan draws and pressure becomes even less with friction supplying a larger portion of the power, hence the high efficiency.