Air Brake Systems Require Proper Care
Apparatus MAINTENANCE/HUBERT WALKER
The air brake system is noted for effective braking of heavy vehicles and when properly maintained is a reliable braking system. The basic system, in successful service for many years and millions of vehicle miles (see Fire Engineering, June 1969, page 64), had a minimum of line fittings and components where loss of air pressure could occur in the system. But lack of proper maintenance by some vehicle operators gave the system a reputation for questionable reliability.
The news media in reporting accidents gave the cause as “brake failure” or “loss of air,” failing to mention the real cause as lack of maintenance. Broken hose lines (flexible hose or tubing connecting the solid tubing on the chassis frame to the brake chambers) and ruptured brake chamber diaphragms account for 95 percent of all air brake failures. Brake fading can also be a cause for lack of braking ability, but that is another subject to be discussed at a future time.
The spring brake was introduced as a safety device for emergency mechanical braking when loss of air pressure occurred. The spring brake has proven very effective. But it has introduced additional maintenance requirements without compensating for the neglect of maintenance that caused nearly all of the component failures.
A study of the air brake system currently being used (see Fire Engineering, May 1971, page 44, for a diagram) discloses that the original quick buildup system, which required a fast pressure buildup in only one reservoir to make the system operational (60 psi), may now require building up pressure in the emergency reservoir and lines as well as in the quick buildup reservoir. This is important to the fire service, particularly for apparatus housed in stations with a response average of once a day to once a week. An air leak at any fitting or in any component will reduce air pressure in the emergency brake system with a corresponding reduction in air pressure in the quick buildup reservoir and lines, increasing the air pressure buildup time for a response.
Some loss is normal: Without the air compressor in operation for extended periods, some loss in system pressure is normal. A system in good condition will not have a loss of more than 2 psi per hour. A loss of 5 psi per hour is the permissible maximum. And the answer is not starting up the engine every day to recharge the air system. This practice will only bring more problems than it will solve, such as crankcase oil contamination.
The answer is periodic check of components for leakage. Information on these tests was given in this column in the July, August and September 1969 issues of Fire Engineering. The brake chamber diaphragms and the air hose are rubber compounds which are affected by the sulfur used in compounding the rubber and by ozone. These should be replaced every two years. If this practice is followed, there will be no failure of the brakes when responding to an alarm.
The parking brake system is also used to meet the requirements of NFPA Standard No. 19 for a brake lock to function when the apparatus is parked. While primarily for aerial ladder and aerial platforms, it is also specified by many fire departments for pumpers. The system uses the service brakes at all wheels for effective braking. Effective, that is, if there are no leaks in the system to bleed off pressure. Again, maintenance is most important if reliability is to be retained.
Monthly maintenance: The following monthly preventive maintenance should be scheduled:
- Lubricate pedal roller and roller pin. Also, hinge pin. Use engine oil for this lubrication.
- Lift boot away from plunger and put a few drops of engine oil between plunger and mounting plate. Avoid using excess oil because it could get on the rubber spring and cause it to deteriorate. Check boot to be sure it is replaced in its proper place.
The R-5 relay valve and quick release is mounted near the rear of the chassis. Its function is to serve as a relay station to speed up the application and release of the rear brakes. The relay valve is actuated by the foot brake valve and air from the foot brake valve serves only to actuate the relay valve, not to supply air for brake application. Air, at system pressure, is supplied to the relay valve through large tubing (⅝-inch) to ensure adequate pressure and volume to activate the brakes without the supply needing to come through the foot brake valve.
The use of the relay valve is an important contribution to more effective braking.
It will be noted by reference to Figure 2 of the system diagram (May 1971 Fire Engineering), that the parking brake operates on all wheels. This is also an improvement in system design.
Synthetic tubing: Another change that is of great interest to maintenance men is the growing use of synthetic tubing (a special type of nylon) to replace copper tubing. This is now standard with some of the commercial truck and fire apparatus manufacturers.
The trade name for this tubing is Synflex. It is manufactured by Samuel Moore and Company at Mantua, Ohio. The Bendix-Westinghouse Automotive Air Brake Company is, however, exclusive distributor for this tubing for automotive service. It is available at all distributors for Bendix-Westinghouse, so it is quickly available for service replacement.
Synflex tubing has several advantages for use in brake systems over metal tubing, but has some limitations which must be observed. First, a review of the points that make it better for service on fire apparatus. It is chemical and corrosion-resistant, not subject to work-hardening, lightweight (only 25 percent of metal tubing), makes shorter bends without tools or kinking, flexible, heat-stabilized and ultraviolet-resistant, and it retains its original dimensions.
The following table of specifications may be useful when making service replacement of tubing.
Heat limitation: The temperature range is -65° to 180°F. Synflex tubing, because of the maximum temperature limit (180°F) should not be used between the air compressor and the quick-buildup reservoir or for any other line subject to high temperature from external sources.
Synflex tubing is not recommended for frame to brake chamber connections. Only the flexible hose currently supplied should be used as a replacement. Some truckers are violating this Bendix-Westinghouse recommendation, but the fire service should observe this recommendation for safety.
Standard metal tubing fittings are used with Synflex but with the addition of a tube insert. See Figure 1 for the use of the insert.
Technical data was provided through the courtesy of the BendixWestinghouse Automotive Air Brake Company.