Ur brakes, 8
Page 41
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HE previous article described a 3ring brake actuator for perating shoes through
nkage. It is also designed for tting directly to twin-wedge )undation brakes. Figure 1, )produced by courtesy of endix, illustrates this type. Figure 1 (a) shows the brake in ie normal driving position. Air ressure against the spring rake piston is keeping the owerful brake spring mpressed and the brake is in le "off" position.
Figure 1 (b) shows the service rake, of the conventional iaphragm type, being applied. is controlled by the footperated brake valve which is nder the control of the driver. he valve supplies a gradual air ressure to the service brake lamber while the spring brake held off by air pressure from le hand valve.
In Figure 1 (c) the hand control alve has been moved towards le secondary park position radually releasing the air ressure allowing the spring to (tend and apply the brake. At aark" the air pressure has been Khausted and the vehicle brake held on by the force of the 3ring. It will be seen that any iss of air pressure from the raking system will cause the
spring brakes to be automatically applied.
Figure 1 (d) shows how it is possible to move the vehicle in the absence of air pressure by unscrewing the release bolt in the end of the long piston boss allowing the spring brake piston to retract into the piston, thus relieving the spring pressure on the push rod and releasing the brake.
A word of warning. Care must be taken to ensure the wheels of the vehicle are firmly chocked before the brakes are released in the manner described in the previous paragraph.
Care must also be taken when dismantling brake spring actuators. Bendix issue the following warning: Never overhaul a spring brake actuator without using a genuine Bendix field maintenance kit. Full service instructions are contained in each kit and must be strictly observed when dismantling because of the physical danger from the potential energy of the powerful spring.
When brake shoes are operated by a cam, this is often "S" shaped, providing a greater expansion of the shoes and a more constant leverage than is possible with the simple flat cam. A brake shoe assembly, as fitted to an ERF, with an "S" cam is shown in the photograph below.