Controlled Power Braking for Buses
Page 31
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AS reported in last week's .issue of "The Commercial Motor," the new Laycock-Neale bus brake is notable in its use of a two-cylindered single-stage pump and of self-adjustment for the brake shoes.
Delivering 8 cubic ft. per min. at 850 r.p.m., the compressor -weighs 48 lb. and is simple and compact. Width is reduced by the use of a roller-bearing crankshaft. Lubrication is fully automatic, and •the system requires no priming after draining the crankcase. No topping up is needed between fortnightly inspections.
Serving several important purposes in this braking system is the controller unit. Primarily, it is a form of automatic cut-in for the compressor. It comes into use when the pressure of air in a reservoir reaches a pre-set maximum or falls below a minimum. In other words, it retains compressed air in the reservoir for use as required. In addition, it controls the flow of air to the brakes, according to pedal movement, in such a way that the driver still has the feel," of the brakes.
Important Safety Precaution An important emergency duty of the controller is to isolate either the front or the rear braking if a pipe breaks. Thus one set is always operative.
The reservoir is a welded-steel vessel measuring 81 ins, by 30 ins. It has a hand-operated blow-off valve for drain: ing. Even during the winter of 1947 it was not necessary to use this on a bus in daily service, the maker states. It is recommended, however, that the reservoir be blown every day in sub-zero (F.) weather.
. As only a small volume of air is required for each brake application, the reservoir gives a supply adequate for up to 50 normal stops without need for regeneration.
Much interest is attached to the method of arranging automatic adjustment between the brake facing and the drum, and between the shoe-operating lever and the shoe, so that no manual action is necessary up to the time when the facing is worn out.
Each shoe carries two adjusters. One operates against a stop on the brake carrier, and controls clearance between the retracted facing and the drum, in conjunction with a lost-motion device. The other abuts against the lever which applies the shoe, taking up any clearance caused by the retracted shoe approaching the drum after facing wear. 'fhis permits the use of 'a short piston stroke and keeps to the minimum the air used in applying the facing to the drum.
These adjusters, four to each brake, are neat, self-contained units. Their design was set out in a patent specification described in "The Commercial Motor" dated September 5, 1947. Braking force, it was then stated, is
transmitted via a rocking lever, which conveys it through the adjuster unit. This is a cylindrical sliding ratchet, consisting of a central member with internal saw-toothed grooves, in which pawls are held by a coil spring round their exterior.
The like unit for shoe control has a lost-motion collar to govern the permissible clearance. Each unit lengthens itself as permitted, and is unable to return by more than the set clearance. Each adjuster is fully enclosed except where the ratchet shaft protrudes.
The primary actuating mechanism of the shoes is simple. Application is by two metallic pistons acting through specially shaped levers. In the case of the rear brakes, the shoes can also be operated through the rotation of a floating cam worked by a lever connected to the 'hand-brake system.
The cnanufacturer of the system is the Laycock Engineering Co., Ltd., Victoria Works, Millhouses, Sheffield,