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ROAD AND WORKSHOP by HANDYMAN

9th July 1965, Page 46
9th July 1965
Page 46
Page 46, 9th July 1965 — ROAD AND WORKSHOP by HANDYMAN
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Accident Investigation Failure to Stop

FA1LURE to stop before hitting something is not always caused by speed. Although such incidents may not cause injury or serious damage, the fact that the brakes of a large vehicle can fail in heavy traffic and damage a third-party vehicle or property, however lightly, is a serious matter.

With any large fleet there may be two or three such instances each year, where for no apparent reason a vehicle rolled on until brought to a halt by another vehicle, obstacle or the like. The main reason for this is the loss of air or vacuum, because of repeated brake pedal operation in downhill traffic, with engine ticking over.

At peak traffic periods a vehicle may be moving slowly downhill for 10 or 15 minutes—with air or vacuum being steadily depleted and with the compressor at idling speed only—then the supply fails to meet the demand.

Quite often, a vehicle may be inching its way downhill with no opportunity to accelerate for more than a yard or two at a time, when suddenly the driver can find his brake pedal hard down and the vehicle beginning to roll freely. It is true that there are one or more early warnings of pressure loss—recent vehicles have air gauge, semaphore and a lowpressure warning buzzer, with a safe reserve, and there should be no reason for a vehicle running away.

Earlier machines, equipped with gauge and buzzer only, are often the cause of this type of accident, particularly as a weakness existed in certain pressure switches. A driver busily engaged with traffic problems can easily miss a critical gauge reading. Quite experienced drivers are at times caught in this position, and it it not always possible to halt a loaded vehicle with handbrake only in a few feet.

This is no reflection on the handbrake, but where it is necessary to ratchet this on in two or three movements, with a closing gap ahead, it is all too easy to be caught out. Normally, a good compressor can hold its own almost at idling

speed in such circumstances, if the tank is full to start with, but repeated application may beat it in the end.

Many experienced drivers, operating in such conditions, run their engines gently between moves and avoid loss of brakes, but where the pump is down in output, or the system has a slight leak, some quite considerable revving may be needed to maintain a safe pressure.

This is the point where assistance is needed from the engineer's department, not just to clear any leaks or faulty pressure indicators, but to check on another feature, pump output. There is a reliable method to apply at service periods which will show pump condition. I refer to compressor or exhausting pumping times, and the thoughtful engineer may prefer to record pumping times on receipt of any new vehicle.

A single example of many now on record is a 1964 Beaver tractor, with dual system. The pumping times were as follows: vacuum, 0 to 25 in. in 80 sec.; air, 0 to 100 p.s.i. in 1 mm. 50 sec. These figures may improve a little as a new pump is run in, but they are near enough to make a sensible yardstick for later comparison on the same model. Entered in each vehicle's history record, pumping times taken when new are well worth while.

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