Slack Adjuster Maintenance On Air Brake Systems

Slack Adjuster Maintenance On Air Brake Systems


Apparatus Maintenance

The slack adjuster is an important part of the air brake system, particularly to provide accurate brake adjustment without disassembly of the brake or levers. When preparing specifications for new fire apparatus to be equipped with air brakes, be sure slack adjusters are specified to be furnished at all wheels. Some fire apparatus manufacturers do not furnish slack adjusters on the front axle brakes unless specified by the purchaser. Brake equalization and beter maintenance requires the slack adjuster for each brake. Figure 1 illustrates the slack adjuster (lever) as assembled with the brake chamber push rod, in the full release position.

Figure 1. Slack adjuster and brake chamber assembly.

Brake actuating cams are usually one of two types on fire apparatus: a flattened cam on the T Series and an S-shaped, constant rise cam on the P Series. The lining on the T Series brake shoe is continuous or one piece up to ⅛ inch in thickness. The lining on the P Series brake shoe is two ¾-inch tapered blocks.

The slack adjuster is designed to permit adjustment for proper working clearance for practically the full service life of the lining. To make sure the slack adjuster is properly set—and even new equipment should be checked as the factory may have missed one brake which will subsequently affect the proper braking of the vehicle—apply full air pressure to the brakes. Check now while the brakes are fully applied to be sure the slack adjuster lever is at a substantially right angle with the push rod, as shown in Figure 2. With the brakes released, the angle of the center of the slack adjuster lever and the push rod should be greater than 90 degrees. The position of the slack adjuster lever in relation to the push rod should be equal on both brakes of an axle, in either released or applied position to have equal application.

Figure 2. Correct positioning of slack adjuster lever in relation to brake chamber push rod for both released and applied positions.

The adjustment of the slack adjuster is best made by turning the adjusting screw until the linings are tight against the brake drum. The wheel having the brake adjusted should be jacked off the ground before making the adjustment. After the linings are adjusted tight to the drum, back off the adjustment until the wheel rotates without drag. Make sure the adjusting screw is locked with the locking mechanism.

Lubrication of the slack adjuster should be on a monthly schedule, using the high-pressure fitting provided.

Limiting valve: Most fire departments require a limiting valve to reduce the applied air pressure to the front brakes. Weather conditions producing icy or slippery roads or streets, make such control desirable for the driver. This load limiting valve is installed in the line from the foot brake valve to the front brakes quick release valve.

Figure 3. Load limit valve, single-unit type.

Two designs of limiting valve are used. One design is shown by Figure 3, which is mounted on the panel in the cab. The control wheel is marked for four positions: No. 1, or low, permits delivery up to 20 psi of air; No. 2 position permits delivery of air pressure to 44 psi; No. 3 position permits delivery of air pressure to 55 psi; and No. 4 or full position delivers full reservoir pressure. The tolerance an the first three pressure settings is plus or minus 6 psi.

The leakage test is made by setting the valve in the No. 3 position for 55-psi outlet pressure. Fully apply the foot brake valve and with the exhaust port coated with soap suds any leakage will be evident. If leakage is shown by this test, the cause could be a worn piston packing cup, dirty or worn valves or valve seats. Replacement of a defective unit with a factory rebuilt unit is recommended.

Figure 4. Cab-mounted unit of load limiting valve, two-unit type.Figure 5. Quick release valve for twounit load limiting valve.

The second type of limiting valve is in two parts. One part is mounted in the cab with a handle for driver use. It has only two positions: dry road (full pressure) and slippery road (one-half of full system pressure). Figure 4 illustrates this part of the design of limiting valve. The second part is a modified quick release valve, as shown in section by Figure 5, and replaces the standard quick release valve for the front brakes.

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To test the valves, two calibrated pressure gages are required. One in the line from the foot brake valve and one gage in one of the lines to one of the front brake chambers. With the handle in the “dry” position and the foot brake valve applying full pressure, both gages should read the same pressure. With the control handle set in the “slippery” position, and with full pressure applied by the foot brake valve, the test gage in the brake chamber line should read one-half the pressure registered by the gage in the supply line. Leakage test is made by coating the exhaust ports with soap suds. Any leakage shown by either valve should be corrected by replacement with a factory rebuilt unit.

Parking brake lock: The NFPA Specification No. 19, paragraphs 8121, 8122, and 9121, requires that a “suitable device shall be provided to lock all four wheels on aerial ladder tractors and all wheels of single chassis aerials.” Paragraph 9121 requires the same locking ability for aerial platform chassis.

It is also quite a common practice for fire departments to require this parking brake lock on pumpers.

To provide this lock, a third reservoir is installed with a two-way valve in the cab to control the air application. The layout of this system is shown on schematic air brake system arrangement, Figure 6. Double check valves are required to be installed in the brake lines ahead of the quick release valves. These double check valves permit brake application and holding, unless leakage occurs at the chambers, regardless of the regular brake system and prevents any feedback from the standard supply line.

Figure 6. Schematic arrangement of air brake basic system plus accessory and additional control equipment.

The double check valve may be one of two types. The disk type is illustrated by Figure 7 and the shuttle type is shown by Figure 8. In either type the disk or shuttle moves under pressure to close the opposite inlet port.

Leakage test: Maintenance requirement is only a leakage test, which requires removal of one inlet connection and applying air pressure. Coating the inlet port with soapsuds will indicate if leakage exists. If leakage is found, the disk or shuttle gaskets, depending on the type, should be replaced. If leaking, it is preferable to replace the unit with a factory rebuilt unit.

Current practice for air brake installations includes several additional uses of air pressure for powering accessories or supplementing the basic equipment for additional control. Figure 6 shows most of the equipment added to the basic system. Not all fire departments use all the items shown, but they are included on the diagram as a matter of information. Details of operation and maintenance of the air-operated windshield wipers and the air horn are omitted as most mechanics have a full knowledge of these items.

Figure 7. Disk type of double check valve.

Some departments specify a spring brake type of control, which is mounted on the rear axle only. This type of brake functions only when the air pressure in the system drops below 35 psi. A separate air reservoir is installed to be sure sufficient air pressure is available to release the spring brake when required by the driver. While these spring brakes have a manual release, the release is time-consuming and some states, notably California, require by law that provision must be made to release the spring brake by a control from the driver’s seat in event of air pressure loss by the system.

Figure 8. Shuttle type of double check valve.

Illustrations and technical data are through the courtesy of The BendixWestinghouse Automotive Air Brake Company.

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