Air Brake Mechanical Linkage

This topic describes the mechanical air brake parts which move when the brakes are applied. Most of the line drawings in this topic are the courtesy of AlliedSignal Bendix Truck Brake Systems, and were taken from their Air Brake Handbook.

Air Chamber Forces

The air chambers of the brake system contain a piston which pushes on a push rod. The pounds per square inch (PSI) of air pressure felt on the piston is multiplied by the number of square inches of the piston. The drawing below shows a typical front wheel brake chamber and the table shows the force generated by the air chamber push rod for various air pressure values.

Air Pressure Control

By controlling the air chamber pressure, we can control the braking force applied to the brakes. The truck brake pedal (sometimes referred to as a brake tredle) operates a dual valve which outputs air pressure which is proportional to the angle of motion of the pedal.

The dual pedal valve has separate valving for the front brakes and the rear brakes. As you step harder on the pedal, the air pressure outputs from this valve increases. As you let up on the brake pedal, the air pressure outputs decrease accordingly.

The pedal angle determines the amount of air pressure delivered to the brake circuit. For simplicity sake, we will assume that both the front brake circuit and the rear brake circuit consists of the air chambers only. If you refer to our tractor brake system topic, you will see that it is much more complicated in real truck systems. The trailer control (hand control valve) works in the same way as the foot brake valve. The angle of trailer control valve motion determines the air pressure from the trailer control valve to the service brake chambers on the trailer.

Brake Foundation Components

The drawing to the right has the air chamber behind the brake drum, and you can see the air chamber push rod connected to the slack adjuster. As air pressure is applied to the air chamber, the slack adjuster is moved by the push rod.

The slack adjuster rotates the brake s-cam, and the scam spreads apart the cam rollers which spread the brake shoes apart and forces them to rub against the brake drum. This collection of parts is called the brake foundation. When the air pressure to the air chamber is removed, the air chamber push rod withdraws backwards, moving the slack adjuster backwards, which moves the scam, which allows the brake shoes to be pulled back to the release position by the return springs.

Manual Slack Adjusters

As the foundation brake shoe linings wear, the brake shoes require further travel to effect the same braking forces. The slack adjusters allow for taking up this slack as the linings wear. If the slack was not adjusted, then the air chamber would have to travel further to effect the same braking force as the brake linings wear. This would cause an additional time delay before the air chambers could fill with enough air to make the brake shoes start rubbing the brake drum. There are many different brands of slack adjuster, but only two types of slack adjuster. Manual slack adjusters require periodic manual adjustments, and the automatic slack adjusters maintain proper adjustment as the brakes are used. The picture above is a new manual slack adjuster. A cutaway view of this slack adjuster is shown to the left.

Notice the ring around the adjustment screw in the picture. This is called the lock sleeve in the cutaway view. To move the adjusting screw, you first push in the lock sleeve, and then use a wrench to adjust the slack adjusting screw. The worm shaft rotation changes the relationship between the spline and the slack adjuster arm.

As the brake shoe lining wears, the angle between the slack adjuster arm and the spline has to be adjusted. When you have finished with the adjustment, always make sure that the spring loaded lock sleeve pops back out to keep the adjusting screw from turning.

Notice the grease fitting in the picture above, and the grease hole in the cutaway view above. Frequent and responsible greasing of the slack adjuster keeps moisture out of the worm gear. This allows the slack adjuster to be adjusted as the brake linings wear down. When you obtained your CDL license, you probably had to demonstrate knowledge about how to determine when the slack adjusters need adjustment.

Automatic Slack Adjusters

The automatic slack adjusters reduce the amount of attention which is needed with regard to slack adjuster slack. But even automatic slack adjusters can fail to adjust the slack properly, so they still need to be checked on occasion. Automatic slack adjusters need to be greased on a regular basis to keep moisture out, and to lubricate the auto-adjust mechanism so that it keeps working properly.

Refer to the drawing below for the following explaination. As the brakes are applied, the motion of the slack adjuster causes a lifting force on the link which rotates the pinion gear clockwise. This rotation tightens the slack adjuster until the force of brake shoe contact increases the resistance on the worm gear. This increased resistance of the worm gear makes the clutch spring slip, so that the slack tightening ceases at this point. When the brakes are released, the anti-reverse spring prevents the worm shaft from turning backwards, so the clutch spring slips as the slack adjuster rotates back to the rest position. The clutch spring and the anti-reverse spring are wound in opposite directions. This is why the two springs grip and slip in opposing directions. Both springs can be overpowered by the manual adjuster hex.

Sometimes, automatic slack adjusters can over adjust, which results in brake shoes which are to tight, which causes excessive dragging of the brake shoes, which causes heat damage to the brake foundation parts. Most of the automatic slack adjusters have a manual override capability similar to the one shown above. Consult with the manufacturer for the procedures for manual adjustment if the slack adjuster sets the brake shoes to tight.

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