43Fluid friction coupling Figs 46 and

A fluid coupling has the take-up characteristics which particularly suit the motor vehicle but it suffers from two handicaps that are inherent in the system. Firstly, idling drag tends to make the vehicle creep forwards unless the parking brake is fully applied, and secondly there is always a small amount of slip which is only slight under part load (less than 2%) but becomes greater when transmitting anything near full torque.

These limitations have been overcome for large truck applications by combining a shoe and drum centrifugally operated clutch to provide a positive lock-up at higher output speeds with a smaller coreless fluid coupling than would be necessary if the drive was only to be through a fluid coupling. The reduced size and volume of fluid circulation in the coupling thereby eliminate residual idling drag (Fig. 4.6).

With this construction there is a shoe carrier between the impeller and flywheel attached to the output shaft. Mounted on this carrier are four brake shoes with friction material facings. They are each pivoted (hinged) to the carrier member at one end and a garter spring (coil springs shown on front view to illustrate action) holds the shoes in their retraction position when the output shaft is at rest.

When the engine is accelerated the fluid coupling automatically takes up the drive with maximum smoothness. Towards maximum engine torque speed the friction clutch shoes are thrown outwards by the centrifugal effect until they come into contact with the flywheel drum. The frictional grip will now lock the input and output drives together. Subsequently the fluid vortex circulation stops and the fluid coupling ceases to function (Fig. 4.7).

Relative slip between input and output member in low gear is considerably reduced, due to the automatic friction clutch engagement, and engine braking is effectively retained down to idling speeds.

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