AT THE HEART OF THE ROAD TRANSPORT INDUSTRY.

Call our Sales Team on 0208 912 2120

Brakes, 7

10th July 1982, Page 43
10th July 1982
Page 43
Page 43, 10th July 1982 — Brakes, 7
Close
Noticed an error?
If you've noticed an error in this article please click here to report it so we can fix it.

Which of the following most accurately describes the problem?

THE SIMPLEST device for converting the stored energy of the compressed air in the reservoir into the mechanical effort necessary to open the brake shoes is the single diaphragm brake chamber illustrated in Figure 1, reproduced by courtesy of Bendix. It is fitted externally and operates through a mechanical linkage to the brake shoes.

A two-piece pressed steel casing, held together by a pressed steel clamp ring, contains a reinforced rubber diaphragm. Two air inlet bosses are provided; the one that is not required is plugged, and two mounting studs are welded into the casing on the non-pressure side. When air is not being applied to the diaphragm it is held in the "off" position by a spring.

When the driver applies the brakes, compressed air enters the inlet port and the diaphragm and push rod are pushed outwards with a force proportional to the air pressure. When the brakes are released and the air pressure is reduced, the return spring moves the push rod assembly and the diaphragm back to the "off" position.

If double diaphragm brake chambers are used, the casing is divided into three compartments, two of which can be supplied with air as required. The third compartment contains a return spring. Air pressure applied to either of the diaphragms applies the brake.

In the 1960s the Ministry of • Transport set minimum standards of braking efficiency and it was apparent that because of the mechanical linkage there would be difficulty in reaching that stipulated for a parking brake unless all the wheels could be braked.

Additionally, there was growing concern over brake failures causing serious accidents. The air pressure in the braking system can leak away during the night and cause brake failure in the morning.

The advent of the spring brake actuator, illustrated in Figure 2 (again, by courtesy of Bendix), prevented this, as it is impossible to move a vehicle until the air pressure has built up to a reasonable level. It also gives the vehicle designer an easy way of providing a parking brake on all the wheels. The spring actuator combines the function of a normal service brake with that of an emergency and parking brake.

When the driver's hand brake control is in the "off' position, a constant air pressure is maintained on the spring brake piston, J, keeping the powerful spring, K, compressed. This holds off the vehicle brake. Moving the hand lever towards the secondary/park position gradually releases the air pressure from the spring brake chamber allowing the brake spring to extend and to apply the brake. When the lever is in the "park" position all the air is exhausted and the brake is held on by the force of the brake spring.

The service brake is of the conventional diaphragm type, the diaphragm, F, being controlled by the driver's foot valve in the same way as the single diaphragm type. Mounting studs are shown at A, and the push rod which operates the brake at B. The reinforcement plate, C, at the front of the service brake chamber, D, helps to support the overhang weight of the complete actuator. A return spring E, is fitted; G is the clamp ring end H the aluminium brake chamber.

Tags


comments powered by Disqus