A brake chamber is a circular container divided in the middle by a flexible diaphragm. Air pressure pushing against the diaphragm causes it to move away from the pressure, forcing the push rod outward against the slack adjuster. The force exerted by this motion depends on air pressure and diaphragm size. If a leak occurs in the diaphragm, air is allowed to escape, reducing the effectiveness of the brake chamber. If the diaphragm is completely ruptured, brakes become ineffective.
Front brake chambers are usually smaller than rear brake chambers because the front axles carry less weight than the rear axles. A brake chamber is usually mounted on the axle, near the wheel that is to be equipped for braking. Air pressure is fed through an inlet port. The air pushes against the diaphragm and the push rod. The push rod is connected by a clevis and pin to a crank arm type lever called a ‘slack adjuster’. This converts the pushing motion of the push rod from the brake chamber to a twisting motion of the brake camshaft and ‘S’ cams. When the air is exhausted, the spring in the brake chamber returns the diaphragm and push rod to the released position.
As indicated by its name, the slack adjuster is also the means of adjusting the ‘slack’ or free play in the linkage between the push rod and the brake shoes. This slack occurs as the brake linings wear. If the slack adjusters are not adjusted within the limitations, effective braking is reduced and brake lag time is increased. If too much slack develops, the diaphragm will eventually ‘bottom’ in the brake chamber. If this condition is allowed to occur, the brakes will not be effective at all.
The diagram illustrates a common type of slack adjuster, showing the worm adjusting gear. When the brakes are fully applied, the angle between the push rod and the arm of the slack adjuster should be no more than 90 degrees (a right angle) On this type of slack adjuster, the adjusting worm bolt is turned until the brake linings touch the drums and then backed off, normally 1/4 to 1/2 a turn. A locking device, which may be a spring loaded collar over the head of the adjusting bolt, must be depressed when the wrench is slipped over the bolt head. Some slack adjusters use a spring loaded internal check ball to lock the adjustment. The more often the driver checks the ‘slack’, the less the probability of brake failure Vehicles rarely ‘lose’ their brakes because of loss of air; it is usually a loss of adjustment.
Some systems have automatic slack adjusters which adjust automatically to compensate for brake lining wear, thus maintaining the correct clearance between the brake lining and drum. Automatic slack adjusters must be checked regularly to ensure that correct adjustment is being maintained.
NOTE: Detailed brake adjustment procedures are outlined in Section VI.
The diagram below illustrates a common type of brake assembly used on truck rear axles and trailer axles. A front axle assembly has the brake chamber and slack adjuster mounted on the backing-plate because of the steering action.
Brake lining material is attached to the shoes. The material used depends on the braking requirements of the vehicle. Brake lining must give uniform output of brake effort with minimum fade at high temperature.
Fading or a reduction in braking effort occurs when the heated drums expand away from the brake linings. The brake linings also lose their effectiveness with overheating.
The twisting action of the brake cam shaft and ‘S’ cam forces the brake shoes and linings against the drums. The brake linings generate heat from friction with the brake drum surface.
The thickness of the drums determines the amount of heat they are able to absorb and dissipate to the atmosphere. Drums worn thin will build up heat too quickly. Dangerously undependable brake performance will result from distorted drums, weak return springs, improper lining, poor adjustment, or grease or dirt on the lining.
Drums must never be turned or worn beyond the manufacturer’s specification.