Specifying and Servicing Wedge Brakes on Vehicles
In our preceding discussions of service brake systems on fire apparatus, we have shown in our illustrations the conventional means of converting pressure energy in hydraulic or air systems to mechanical energy to activate or apply the wheel brakes. A newer and more effective design of wheel brake is rapidly replacing the conventional types on new fire apparatus. This is the wedge brake.
The wedge brake has been briefly discussed and its advantages outlined in this column in the September 1967 issue of FIRE ENGINEERING (pages 190-193) and in “Preventive Apparatus Maintenance” (pages 50-53), but some incorrect applications of the wedge brake have been delivered to the fire service that warrant a review for the protection of fire departments buying apparatus.
The proving of the wedge brake and proper application has been done mostly in commercial truck service during the past several years, though a number of fire departments have apparatus in service equipped with the wedge brake.
Road experience: The wedge brake acts faster than the conventional hydraulic or air-operated brake and is more effective with lower hydraulic or air pressure. This difference in brake action has produced some problems in vehicle operation when all the brakes are not of the same type. In the application to tractors equipped with the wedge brake, commercial truck operators found that when the trailer had conventional cam-type brakes, the tractor was doing all the braking, and jackknifing was a potential hazard. Drivers soon learned to brake the vehicle by using the hand control for the trailer brakes, thus eliminating the jackknifing potential and using mainly the trailer brakes with some assist from the tractor brakes.
As a result of this experience, it was established that all the wheel brakes on a vehicle or combination of tractor and trailer should be of the same design to achieve proper braking balance. This is the recommendation of the brake manufacturers.
It is regrettable that some commercial truck chassis and fire apparatus manufacturers have not taken advantage of this operating experience and choose to ignore the brake manufacturers’ recommendation. It is important for the fire service to understand this situation and make sure any new apparatus delivered and their specifications for new fire apparatus include brakes to be of matched design. The brakes should be of the same type, front and rear, and not split types, as is the current practice of some manufacturers.
For example, the truck chassis specifications of the Big 3 commercial chassis manufacturers, when furnishing air brakes, provide wheel brakes as follows: Manufacturer A—wedge-type front brake and “S” cam-type rear are standard, with wedge-type rear as optional; Manufacturer B— “S” cam-type brakes on the rear as with wedge-type brakes on the rear as standard; Manufacturer C—wedge-type brakes front and rear or “S” cam-type brakes front and rear at the customer’s option. Manufacturer C clearly states that a split in types will not be furnished. It is reassuring to find a manufacturer who takes a definite stand on furnishing proper brake combinations for balanced braking and safety on the highway.
Adjustment problem: A midwestern fire department placed in service a custom-built fire truck about 18 months ago. The fire department mechanic recently advised he despairs of ever being able to keep the brakes on this apparatus in operating balance. This apparatus has “S” cam-type brakes on the front and wedge-type brakes on the rear. Initially the brakes seemed to work well, but as the lining wore, the drivers were complaining of poor, or lack of, braking on the front. The rear brakes were doing all the work. This was to be expected as the rear brakes (wedge-type) have built-in automatic adjustment and for a given air pressure exert more braking effort. The front brakes have a manual adjustment through the slack adjuster which means frequent maintenance of the front brakes in a losing effort to keep the adjustment equalized between front and rear.
The rear brakes are doing the major braking. This extra demand on the wedge brake can cause overheating, brake fade, drums out of round and heat-checking. A consequence will also be short lining life from using the split-type combination of brakes.
Experience has shown that with the wedge-type brake only on the front axle and the cam-type brake on the rear, the driver is soon operating the apparatus with the front brake limiting valve in use in an effort to limit the front braking action and get more braking at the rear for a better balance.
Courtesy Rockwell-Standard Corp., Brake Division
The wedge brake is sealed against water and dirt, and all operating parts are packed in grease. The adjustment is automatic and will operate for the life of the lining, which would normally be at least 10 years in fire service.
Brake inspection: Good preventive maintenance does call for an annual checkup. This need only be a check on the lining clearance with the drum, which should not be over .060 of an inch. Lining clearance within this tolerance indicates the automatic adjustors are working properly.
After five years or 25,000 miles, the brakes should be carefully inspected. If spring brakes are used, be sure the spring in each brake is “caged” by turning the manual release screw to hold the spring in the compressed position before removing the rear wheels and brake drums. The brake operating mechanism should then be checked as follows:
- Inspect plunger seals.
- If seals are cut, torn or leaking—disassemble, clean and lubricate brake-actuating components and replace seals.
- If seals are in good condition, remove upper adjusting plunger and its seal and check internal condition.
- If grease is contaminated or hardened, or if parts are dry, disassemble and clean parts and repack with grease.
- If internal condition is satisfactory, reassemble adjusting plunger and replace seal.
Vehicle operation can now be continued to next scheduled inspection.
Life of linings: The several parts referred to above (1-5) are identified in the sectional line drawings, Figure 1 (hydraulic) and Figure 2 (air).
Though the brake relining period in commercial truck service is 50,000 to 100,000 miles for wedge brakes, the heavy braking demands with limited mileage in the fire service require a time schedule rather than a mileage schedule for checking lining wear. Normal lining life in fire service should be 10 years or more. (That is, if the drivers do not ride the brake pedal!)
The lubricant recommended for the wedge brakes is NLGI No. 1 (National Lubricating Grease Institute), possessing high heat resistance and free of fillers and abrasives. It should retain its softness at all times for smooth brake application and release. The following brands and specifications are approved for the wedge brake:
Texaco Thermotex EP No. 1.
Marathon 528 H.D.
American Amdex No. 1, EP.
Shell Darina No. 1.
Sunaplex No. 1, EP.
Philube B, No. 1.
When there is sustained operation or exposure to low atmospheric temperature, —40°F and lower, a grease conforming to military specification MIL-G-25013C should be used to insure free brake action.