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Air brakes, 10

28th August 1982, Page 32
28th August 1982
Page 32
Page 32, 28th August 1982 — Air brakes, 10
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Which of the following most accurately describes the problem?

IN THE first of these articles on Air Brakes it was said that the components used could be divided into four groups, (1) Compression and Storage, (2) Control, (3) Actuation and (4) Indication. Having dealt with the first three we now turn to Indication.

It is a legal requirement for a visible means (usually a pressure gauge) to be fitted in the cab of vehicles equipped with air brakes to "indicate any impending failure or deficiency in the .... pressure system" (C and U Regs 1978, regulation 14).

Figure 1 shows the gauges fitted to an ERF lorry. A dual gauge in the top left-hand corner deals with two parts of the system while the one on its immediate right covers the third.

Should the pressure fall below the safe minimum advisable for normal vehicle operation buzzers are also fitted to give automatic warning to the driver. Figure 2 illustrates a low pressure indicator, a pressure Sensitive electric switch, which makes contact when the air pressure to which it is subject falls below a predetermined level. (This illustration, and those in figures 3 and 4 are reproduced by permission of Bendix Ltd.) The low pressure indicator consists of a nylon cover and an aluminium alloy body which contains a springloaded contact disc, contact plate, nylon piston and a rubber operating diaphragm.

When the air pressure rises above the setting pressure (usually 4.1 bar, 60 p.s.i.) the pressure behind the diaphragm raises the piston and the contact disc against the pressure of the control spring thereby breaking the electrical contact.

When the pressure falls below the setting pressure the diaphragm can no longer keep the spring compressed and the contact disc comes into contact with the contact plate making electrical contact and causing the buzzer to sound.

Figure 3 shows how a stop light switch is installed on a vehicle.

The limiting factor on braking efficiency is road-tyre adhesion and one of the factors on which this depends is vehicle weight. On goods vehicles, particularly at the rear axle, the difference in the weight between the laden and unladen weight is considerable. If a brake is designed to be fully effective when the vehicle is laden it is likely to cause the wheels to lock when the brakes are applied when the lorry is empty. To deal with this problem the variable load valve, illustrated in Figure 4, is fitted to regulate the braking force in proportion to the load the axle is carrying. The valve is mounted on the vehicle chassis and the operating lever is connected to the axle by a simple linkage.

As the load on the axle is increased, the distance between the axle and the chassis decreases. The linkage operates the valve mechanism which varies the output of air pressure to the brake actuators according to the load on the axle.

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