2 New Components Used In Dual Air Brake Systems
With product changes that are legislated in the interest of safety or design changes to improve the product, today’s fire apparatus maintenance mechanic must possess technical knowledge and skill to a far greater degree than required for the maintenance of equipment in any other service. When the Motor Vehicle Bureau in the State of Massachusetts made effective a requirement for hydraulic brake systems to provide separate application of front and rear brakes, it served as a catalyst for change in other brake systems.
The air brake system for fire apparatus has been modified by some fire apparatus manufacturers to provide separate application of front and rear brakes for the same basic reason, greater brake reliability and safety. A study of the dual air brake system discloses not only the use of new components, but the type of brake actuation is changing from cam operation to the more efficient wedge brake operation.
Brake systems—hydraulic, air-overhydraulic, air, Jake brakes, and wedge brakes—have been described and illustrated with component maintenance information in the issues of Fire Engineering for February through November 1969, so this discussion will be confined to the important changes.
A schematic diagram of the dual air brake system is illustrated by Figure 1. A reference to Figure 6, page 72, of Fire Engineering for September 1969 will quickly identify the basic change in components.
Wedge brake: It will be noted that the wedge brake is included. As pressure is applied at each end of the brake shoe, two brake chambers are used at each wheel. A spring-type brake is furnished at the rear axle, as standard, with this type of brake. If a tandem type of rear axle is used, the spring brake is mounted, as shown, on the forward tandem axle, with the rear tandem axle carrying only the standard brake chambers. These chambers are connected to the same relay valve and quick release as used to activate the brakes on the front tandem axle.
Two new components are used in this revised brake system: (a) dual brake valve (Bendix-Westinghouse Type E-4), and (b) relay and quick release valve (Bendix-Westinghouse Type R-5) for faster actuation of the rear wheel brakes.
The E-4 dual brake valve, Figure 2, differs from the other Type E brake valves (see Figure 1, page 62, August 1969 Fire Engineering) in that it is two valves in one housing. Each part of the valve assembly, (upper and lower), has separate inlet and exhaust valves controlling two separate circuits, receiving air supply from one or two separate reservoirs and controlled by the movement of one foot pedal.
How valve operates: A brief explanation of how the valve works will lead to a better understanding of the construction shown by Figure 2.
In normal operation, when the pedal is depressed, force is exerted on top of the piston and rubber graduating spring. The top piston moves down and its stem, which is the exhaust seat, closes the upper exhaust valve. As the upper exhaust valve closes, the upper inlet valve is moved off its seat and air pressure flows out the upper delivery port to the brake chambers, applying the brakes.
Up to this point, the operation is similar to the other Type E brake valves, except that when the upper inlet valve is moved off its seat, air is permitted to pass through the bleed passage and enters the cavity above the outer relay piston. The pressure moves the outer relay piston and inner relay piston down. As the inner and outer relay pistons move down, the seat on the inner relay piston seals off the exhaust valve and at the same time moves the lower inlet valve off its seat, permitting air pressure to flow out the lower delivery port to the brake chambers to actuate the brakes.
Action nearly simultaneous: Because of the small volume of air required to move the outer relay piston, action in the lower portion of the valve is almost simultaneous with action in the upper portion.
Should air be lost in the lower circuit, the top circuit will continue to function as described.
Should air be lost in the top circuit, the operation will be as follows: As the pedal is depressed and no air pressure is present in the upper supply and delivery ports, extended pedal travel and continued mechanical force will move the inner relay piston down, sealing the exhaust seat and moving the inlet valve off its seat, allowing the air pressure to flow out the delivery port to the brake chambers, applying the brakes.
Thus a sudden air hose failure or brake chamber diaphragm failure can only affect braking ability in one circuit, and the apparatus will still have operational brakes, though on only two wheels.
Continued next month