Automatic Clutches

An automatic clutch installed in a motor vehicle makes for much more comfortable driving in comparison with an ordinary pedal-operated clutch.

The clutch pedal inside the vehicle is obviated, since the clutch automatically disengages when the engine is idling and also when the motorist takes hold of the gear-shift lever.

In principle, an automatic clutch comprises - besides the gear selecting clutch, which may be mechanically, pneumatically, hydraulically or electrically actuated - an additional starting clutch which may be engaged, for example, by centrifugal action or may be in the form of a fluid flywheel clutch or a magnetic powder clutch.

In the Saxomat automatic clutch the starting clutch, which in this case is of the centrifugal type, is a plate clutch which transmits power as soon as the centrifugal weights are flung outwards when the speed of the engine exceeds the idling speed. These weights press the pressure plate against the drive plate and thrust the latter against the flywheel.

At low speeds of rotation this clutch disengages automatically. In order nevertheless to take advantage of the braking action of the engine when travelling downhill, a freewheel is installed, which establishes the mechanical connection between the crankshaft and the gearbox drive shaft as soon as the latter tends to rotate faster than the crankshaft.

The gear selecting clutch is likewise a plate clutch, which is always engaged during starting and which is actuated by means of a vacuum-controlled servo mechanism. When the motorist takes hold of the change-speed lever, a contact is closed, with the result that an electromagnet is energised.

It moves the valve to the right, so that the intake pipe of the engine is connected to the servo mechanism. Its diaphragm is now subjected on one side to a vacuum, and on the other side it is subjected to atmospheric pressure, with the result that it pulls the clutch actuating lever to the ‘disengaged’ position. When the change-speed lever is released, the flow of current to the electromagnet is interrupted, the valve returns to its initial position and diaconnects the servo system from the induction pipe. Atmospheric air now slowly flows into the vacuum chamber of the servo through a nozzle with a very fine orifice (which is so narrow that it does not disturb the sequence of events described above).

As the pressure on both sides of the diaphragm has now been equalised, the latter returns the clutch actuating lever to the ‘engaged’ position. Engaging the clutch is expedited by depressing the accelerator pedal, as the vacuum which is developed in the choke tube of the carburettor is applied, through a special pipe, to an auxiliary diaphragm, which lifts the valve ‘b’, so that the atmospheric air can flow much more quickly into the servo.

The tension of the spring which holds the valve ‘b’ closed, and thus determines the vacuum pressure at which this valve opens, can be varied by means of an adjusting screw.