BRAKE ADJUSTMENT, WITHIN AN INCH OF YOUR LIFE
APPARATUS: THE SHOPS
On May 10, 1990, a 1974 custom pumper responding to an emergency call ran off the road and hit a large tree when the driver lost control on a steep downgrade. The pavement was wet from a previous rain. The engine was carrying five career firefighters and 500 gallons of water. Two fire-fighters were fatally injured, one firefighter sustained moderate injuries, and the driver and remaining firefighter sustained minor injuries.
The fire engine was equipped with an automatic transmission and airmechanical service brakes. A mechanical examination of the vehicle following the accident indicated that the front axle brakes had no defects and that the push-rod adjustments were within operating limits. An accumulation of rust was observed in the left and right rear-axle brakes. Three of the four rear-axle brake-shoes were not making contact with the drum on application. The lower left and upper and lower right hrakeshoes were frozen at the anchor pins.
That fire department’s policy was to perform preventive maintenance on fire apparatus after every 150 hours of operation, as measured by the engine hour meter. The manufacturer recommended service intervals of 50 hours in its maintenance manual, whose purpose is to ensure that a vehicle performs as designed.
Although this accident had extenuating circumstances, apparatus maintenance played an important part. After investigating this and several other accidents involving apparatus, the National Transportation Safety Board recommended that fire departments establish vehicle maintenance and inspection programs that follow all manufacturer service requirements and schedules.
“Because fire apparatus are frequently operated at higher speeds than conventional vehicles, with more frequent need to stop quickly, and are operated under hazardous conditions, it is essential that they be properly maintained,” the Safety Board cautioned.
A U.S. Department of Transportation bulletin entitled “Brake Adjustment, Within an Inch of Your Life” indicates that impaired stopping ability due to poorly adjusted brakes jeopardizes the safety of firefighters and others. The only way to be sure that apparatus brakes are properly adjusted is to physically check each wheel at regular preventive maintenance intervals. It is difficult to detect out-of-adjustment air brakes simply by the feel of the pedal. Out-of-adjustment brakes generally will be adequate to stop the vehicle under normal conditions; but the vehicle may be unable to stop in time under hard or panic stop conditions, especially after the brakes have heated.
CHECKING BRAKE ADJUSTMENT
As with any maintenance procedure, the first step is to thoroughly read and understand the technical publications provided by the manufacturer. Tolerances and inspection procedures are outlined for every vehicle.
Before starting the brake-inspection procedure, make sure the vehicle is parked on a level surface, with both front and back wheels blocked. Build the system air pressure to 100 psi, release the parking brakes, and shut down the engine.
Checking S-cam brakes with manual slack adjusters can be accomplished in one of two ways, depending on the available number of personnel.
One-person approach. Pull the chamber pushrod to its limit by hand or pry it with a short pry bar. Measure the distance from the chamber face to the center of the clevis pin when the pushrod is at rest and also when it is at full extension. The difference between these measurements is the pushrod travel or stroke. On a Type 30 chamber (30 cu. in.), the distance should be one-half inch; the maximum allowable travel is one inch.
Two-people approach. Measure in the same manner as described above, with the service brakes released, and again after the brakes have been fully applied using the foot pedal in the cab. Because of the considerable stretching and bending of various parts when the brakes are applied with system air, the maximum allowable \travel is two inches for Type 30 air chamber brakes. The tolerances given here are for a Type 30 air chamber; measurements vary slightly for other size units.
AUTOMATIC SLACK ADJUSTERS
Automatic slack adjusters are widely used on apparatus equipped with air brakes. As the name implies, these units automatically maintain the brake adjustment as the vehicle is driven. They usually can be identified by a small pushrod attached in the area of the clevis pin and connected to the body of the slack adjuster. This device must be manually adjusted only when it is originally installed or when the brakes are relined.
Check the stroke in the same manner as for manual slack adjusters. Once again, refer to the manufacturer’s reference material for the maximum allowable shaft travel.
S-cam brakes with either type of slack adjuster can be adjusted by turning the adjustment bolt. Due to the critical nature of this adjustment and the close tolerances that must he maintained, only qualified maintenance personnel should attempt to change the adjustment This is not the place to play “backyard mechanic.”
OTHER BRAKE COMPONENT INSPECTIONS
While checking the brake adjustment, inspect the other components of the brake system in the following manner:
- Visually inspect all brake system components for loose, broken, or worn parts.
- Apply the brakes; all components should move smoothly and fully return without any binding or chatter evident.
- Look for signs of wheel-seal failure, indicated by liquid grease in or around the brake drum area.
- Evaluate the thickness of the brake lining through the inspection ports located in the brake-backing plate Both shoes on each wheel, as well as both wheels on each axle, should have approximately the same amount of lining remaining. Most manufacturers recommend that vehicles with remaining lining of onefourth inch or less be removed from service and the brakes relined.
- Check for any signs of air leaks at the brake chambers, lines, and valves in the system.
According to NFPA 1901, Standard for Pumper Fire Apparatus, 1991 edition, air pressure drops should be limited to normal air losses. The presence of the following conditions indicates the need for immediate corrective action:
“ 1. Air brake pressure drop of more than 2 psi in 1 minute for single vehicles or more than 3 psi in 1 minute for vehicle combinations, with engine stopped and the service brakes released.
“2. Air pressure drop of more than 3 psi in 1 minute for single vehicles or more than 4 psi in 1 minute for combinations, with engine stopped and service brakes fully applied.”
SYSTEM AIR PRESSURE
In addition to the mechanical adjustments, the system air pressure is also critical for proper braking action. Air brakes need clean, dry air at an adequate pressure for maximum performance.
NFPA 1901 specifies that the following components be included in air-actuated braking systems:
An automatic moisture ejector to
- remove condensation buildup from air tanks. If tanks are equipped with manual bleeders, they should be drained on a regular basis.
- An air drier to remove liquid and water vapor from the compressor discharge air before it reaches the airbrake reservoirs. If not removed, moisture from the compressed air can cause air-brake malfunctions.
- A quick buildup section in the air reservoir system to allow the apparatus to move within 30 seconds of startup with a completely discharged air system.
- An air pressure protection valve to prevent the use of air horns and other air-operated accessories when the system air pressure drops below 80 psi.
The excessive use of air horns while responding causes the air pressure to drop to a point where the emergency spring brakes begin to apply. When this happens, the brake shoes make contact with the drums and the friction involved rapidly causes heat to build up. A combination of the brake lining’s glazing and the drum’s expanding from the heat causes brake “fade” and possibly the inability to stop.
Most apparatus manufactured before the 1991 edition of NFPA 1901 have this safety device, so it is important that the operator monitor the system air pressure to prevent brake loss. The effects of low air pressure readily can be seen in the following example: A 24-square-inch air chamber (Type 24) exerts 2,400 pounds of braking force at 100 psi air pressure. The same chamber at 60 psi exerts only 1,440 pounds of force.
Remember the following points about air brake systems:
- Follow the manufacturer’s documents provided with the apparatus with regard to maintenance; they contain the specific tolerances for brake measurements and adjustments.
- Only qualified personnel should attempt to make adjustments.
- As is true for all equipment that affects life safety, proper maintenance and adjustments are absolutely essential for apparatus.
