• An All-hydraulic Power-servo Brake
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ON the trolleybuses in service in this country, the two most widely employed systems of braking are lhose relying for their actuating power upon compressed air and vacuum, respectively. In the latter case, it is not unusual for the force .exerted by atmospheric pressure to be transmitted hydraulically.
Sunbeam Commercial Vehicles, Ltd., Moorfield Works, Wolverhampton, claims to be the first +ehicle manufacturer in this country to build a trolleybui chassis having an anhydraulic servo braking system, which is dependent on neither air nor vacuum as a source of power. The Sunbeam company has supplied many trolleybuses with compressed-air brakes, the System including a twincylindered reciprocating type of compressor. Many others have had vacuum-hydraulic brakes, vacuum being created by rotary exhausters. In both cases separate motors drive the pumps, and weight has tended to become formidable.
Furthermore, with the vacuum-hydraulic system, pedaloperating pressures need to be considerably greater than those of the air-pressure system on which maximum braking is obtainable with a pressure of not more than about 25 lb. on the pedal.
Having these points in mind, Sunbeam engineers have for some time past been co-operating with the Automotive Products Co., Ltd. (Lockheed), with the view, first, to overcoming the disadvantage of weight and, secondly, to avoiding excessive pedal pressures.
As a result of these efforts, the Lockheed company has now produced an hydraulic servo system, of which the function is to regulate the pressure in the lines in proportion to the pressure on the pedal. The unit, in which fluid is supplied by a high-pressure radial pump, incorporates a master cylinder, connected to the rear brakes, which, in the unlikely event of pump failure, can be operated directly from the pedal independently of the servo apparatus. The front brakes. however, are applied directly by servo pressure, therefore the front and rear systems are independent of one another. Nevertheless, under normal operating conditions, front and rear pressures are kept in correct proportion by the servo valve.
it is of interest to note that, in effect, this new servo system has been made possible by the use of a high-pressure pump developed, primarily, for use on aircraft. It is a small, compact seven-cylindered radial unit mounted on and driven from the front end of the traction motor. Between this pump and an hydraulic accumulator is a cut-out valve, the function of which is to regulate the pressure in the accumulator bq by-passing the fluid to the reservoir when the predetermined maximum pressure is reached, and by reconnecting the pump to the accumulator when the pressure falls to the lower value.
When the hydraulic accumulator is charged to the higher pressure, it has sufficient fluid for 50 or more brake applications before the pressure falls to zero; in practice, of course, the pump cuts in so soon as the lower pressure is reached. The time required to regain pressure from zero to 600 lb. per sq. in. when the vehicle is travelling at 10 m.p.h., is 2 seconds, and from zero to 1,000 lb. per sq. in., 28, seconds.
An interesting test showed that the pump can build up pressure from zero to 1,000 lb. per sq. in., while the brake pedal is being repeatedly depressed as frequently as the driver can operate it, in 75 seconds. It is possible to obtain maximum braking with only 25 lb. pressure on the pedal, and this, it must be remembered, is with a vehicle having a total laden weight of approximately 10i tons.
Road tests of a partly laden double-deck trolleybus show that the vehicle can be brought to rest from 20 m.p.h. in 17 ft., which is equivalent to a deceleration of 24 ft. per second, or an efficiency of 75 per cent. A considerable saving in weight is effected, as "the motorized exhauster system weighs 2 cwt. 3 qrs., whereas the all-hydraulic servo system scales only 3 qrs. This shows a saving of 2 cwt.
By arrangement with Mr. C. Owen Silvers, M.I.E.E., 14.1.Inst.T„ general manager and engineer, Wolverhampton Corporation Transport Department, an experimental set of this equipment has been installed on one of another fleet of Sunbeam trolleybuses, recently delivered to this operator. The vehicle has been inregular service for some months and according to reports the system is giving entire satisfaction.
On the road there is an almost complete absence of noise from the pump unit, which, of course, cuts-in only occasionally, and when this happens a faint buzzing sound is all that can be heard. This compares favourably with the familiar " chug chug " of the reciprocating pump.
A further incidental, but none the less valuable, advantage of this accumulator type of hydraulic servo apparatus is that in addition to the braking system other equipment can be operated hydraulically—for instance, sliding doors.