Fluid Friction Coupling
A fluid coupling exhibits the take-up characteristics, which ideally suit the motor vehicle. However, it has two inherent limitations. Firstly, idling drag tries to make the vehicle creep forwards unless the parking brake is fully applied, and secondly there always exists a small amount of slip, which is less than 2% under part load torque transmission but becomes greater near full load transmission. These drawbacks have been overcome for large commercial vehicle applications by incorporating a shoe and drum centrifugal operated clutch. This arrangement provides a positive lock up at higher output speeds with a smallar coreless fluid coupling than would be normally necessar}' for the drive only through a fluid coupling. This reduction of fluid circulation in the coupling eliminates residual idling drag (Fig. 24.29).
Fig. 24.28. Relationship between engine speed, torque and slip for fluid coupling.
Fig. 24.29. Fluid friction coupling.
The fluid coupling automatically provides the maximum possible smooth drive during acceleration of the engine. Towards maximum engine torque transmission, the friction clutches shoes are thrown outwards by the centrifugal effect until they come into contact with the flywheel drum. The input and output drives are now locked together due to frictional grip. Subsequently, the fluid vortex circulation stops so that the fluid coupling ceases to function (Fig. 24.30). Due to the automatic engagement of friction clutch relative slip between input and output member in low gear is considerably reduced, and engine braking is effectively retained down to idling speeds.
Fig. 24.30. Relationship between torque carrying capacity, efficiency and output
speed for a fluid coupling.