Servo Steering
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for Large Vehicles
The Bosch Vacuum servo System Embodies an Ingenious Electro-magnetically Controlled Double Distribution Valve for the Servo Cylinder
IN the case of certain modern vehicles intended for very heavy duty the .driver has to exert considerable physical effort in the control of the steering. This can, of course, be reduced by the simple expedient of providing ti.' low-geared steering set ; this has the disadvantage of giving slow action, which is undesirable -in the case of vehicles driven at high speeds, such as large buses. Where road speeds are low this disadvantage does not arise, but there are many cases in which easy mancouvring and high road speeds are essential.
To meet these circumstances the wellknown concern of Robert Bosch, ot Stuttgart, has introduced a vacuumservo system for aiding the driver in his control of the steering. Those who attended the Commercial Motor Show at Olympia in November last will recollect its use on the Henschel 12-cylindered model.
The ordinary steering system is unaffected in principle and the ratio of the box is normal. Power for the servo system is provided by a vacuum cylinder of the type to which vehicle users have become accustomed in connection with braking systems. The vacuum is, of course, derived from the induction manifold on petrol vehicles, or from an engine-driven exhauster in the case of an oil-engined vehicle.
Obviously there must be a steady supply of power to operate the steering, . otherwise there would be highly undesirable fluctuations; to this end there is provided a vacuum reservoir, coupled to the source of vacuum and to the double distributor. .
The piston of the vacuum cylinder is connected up to the steering drop-arm in a simple and direct manner, whilst, as this piston is double-acting, it is able
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to assist the movement of the drop-arm, either forwards or backwards.
The vacuum motor being double-acting, it follows that the cylinder must be closed at both ends. As the movement of the piston has to be transmitted directly to a pivoted lever, the ordinary type of rigid piston rod, with gland and stuffing-box, cannot be used, and an articulated connecting rod has to be employed. On emerging from the cylinder this rod is, therefore, provided with a bellows-type leather gaiter. As will be seen from a partly sectioned view above, the piston is _guided by a tubular member, within which is the articulated rod connected to the steering system.
We now come to the most interesting portion of the Bosch device. This is the automatic control of piston movement in the vacuum motor. A double distributor valve, electro-magnetically operated, allows the vacuum to act either on one or other side of the piston.
The distributor comprises a small aluminium box divided into two separate chambers, in eaCh of which are located a vacuum valve, an air valve and an electro-magnet for their operation. The chambers are connected to opposite ends of the servo-motor cylinder by pipes, whilst the two vacuum valves open into a common passage; connecting up with the engine induction pipe. The air valves also open into a common passage, communicating with the atmosphere and provided with' an air filter.
The action of the magnets is controlled by an ingenious electric commutator on the steering column. The springs of the vacuum valves are stronger than those of the air valves, and it follows that when the magnets are not in action, the vacuum valves are held closed, whilst the air valves remain open.
When the driver turns the steering wheel to the right or left, one or other of the magnets starts to function, and the servo motor immediately supplements the effort of the driver. The control device on the steering column is so designed that, should a breakdown of any kind occur either in the electrical or pneumatic portion of the system, the whole effort of steering is instantly taken up by the ordinary worm and sector or other type of mechanism Under existing British regulations there is only a limited demand for power-assisted steering. In the future conditions may change, whilst there is in other lands an existing opportunity to construct larger vehicles which will inevitably require some such device as
this'
For military machines there lean undoubted need for servo steering ; high speeds over rough terrain impose conditions which may well exceed physical limits.