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Air Brake Basics Part 5

All air entering the tractor brake system must first pass through the tractor protection valve located at the rear of the tractor cab. The supply line air then passes through the trailer spring brake relay valve which is located near the trailer air tanks. With the trailer hooked up to the tractor, the service and supply lines connected and the trailer air supply knob pressed in, the trailer spring brake valve directs the trailer air tank to be fully charged with pressurized air from the tractor.

The trailer air tank is sometimes referred to as the trailer air reservoir and there may be more than one on a trailer. Only after the tank is filled to 75 psi of air will the trailer spring valve allow the parking brakes on the trailer to be released.

This is a built in safety mechanism so that if there is an insufficient amount of air in the air tank, the spring brakes remain engaged and the vehicle will not be going anywhere. Now, with the trailer air tank full and the parking brakes released, the trailer can be moved. When the driver needs to stop, the driver will step on the treadle valve, which will allow air to flow from the cab and into the trailer service line.

This small quantity of air will enter the relay valve located by the trailer tandems. The relay valve will then open, allowing for the passage of air from the trailer storage tank to the trailer service brake chambers and thus apply the trailer brakes. When the treadle valve is released, the air pressure in the service line drops to zero which closes the relay valve and prevents air from flowing from the air tank to the service brake chambers. A vent is then opened in the relay valve and air from the brake chambers is vented to the atmosphere, releasing the trailer service brakes.

Air Brake Chambers

The basic function of an air brake chamber is to convert air pressure to mechanical movement. When the relay valve on a trailer allows air to enter a brake chamber, the increase in air pressure displaces a rubber diaphragm which extends a steel push rod which ultimately is responsible for activating the brakes. This is called the service brake chamber.

The brake chambers on a tractor trailer are referenced by their dimensions. For instance, a steering axle may be equipped with Type 16 or Type 20 brake chambers, whereas the remaining brake chambers are usually Type 30.

The numerical value associated with the type simply refers to the area in square inches of the diaphragm located inside the chamber housing. The larger the brake chamber and diaphragm area, the more braking force that can be applied at a given air pressure, creating more work and stopping power.

For example, if 10 psi of air is applied to a Type 30 brake chamber, that means that the 10 psi of air will push on the 30 square inch diaphragm and create a force on the push rod of 10 x 30 or 300 pounds.

An 18-wheeler is usually equipped with 10 brake chambers, 5 axles with a brake chamber on each side. The steering axle will almost always be equipped with smaller brake chambers compared to the other axles due to the fact that the brake shoes are smaller and only one tire is being braked.

Figure 2- 11 depicts the anatomy of a typical brake chamber. The single brake chamber, clamp type, is comprised of two concave metal halves. Inside the chamber is a rubber diaphragm that is sandwiched between the two metal halves which are held together by a clamp. There is a round metal plate inside the rubber diaphragm, with a threaded metal pushrod attached. This pushrod extends through a hole in one end of the brake chamber. The brake line which is a single rubber hose, feeds pressurized air into the chamber on one side of the diaphragm, which pushes it and the pushrod and in turn applies the brakes. When the pressurized air leaves the brake chamber, the return spring pushes the diaphragm and pushrod back to its original position.

Air Chamber Push Rod

As just described above, the threaded pushrod extends outside of the brake chamber housing. The interior end of the pushrod rests against the rubber diaphragm. The exterior end of the pushrod is threaded onto a clevis which is secured with a lock nut. When the brake chamber is built at the factory, this pushrod is a standard length.

When the brake chamber is installed, the installer measures and cuts the pushrod to fit the particular vehicle. This is important to note, since the pushrod is such an integral part of the brake system and a pushrod of improper length reduces braking efficiency.

Slack Adjuster

The clevis at the end of the pushrod is pinned to a slack adjuster, shown In Figure 2-12. The slack adjuster has two main functions. It first converts the straight line pushing action of the pushrod into a rotation movement that ultimately applies the brakes. Secondly, the slack adjuster adjusts the space or slack between the brake shoes and the brake drum, which rotates around the shoes and linings. Over time and with normal brake usage, the brake linings and drums will wear, increasing the space between them. It is essential that this space be checked regularly and periodic adjustment must be performed not only to retain the safe stopping ability of the vehicle, but also to comply with the law.

There is no way to measure the amount of slack in the brake mechanism from the drivers seat. The driver must inspect the pushrod travel on every brake by crawling under the truck.

In order to adjust the space between brake linings and the brake drum, the mechanic must first depress a spring loaded metal ring which surrounds the adjustment nut. In its normally extended position, the ring prevents the adjustment nut from rotating.

With the ring depressed, the mechanic can apply a wrench to it and rotate it which in turn adjusts the brake lining spacing.

The most widely used method for brake adjustment is to turn the adjustment nut until it stops, putting the brake linings against the drum. Then turn the nut in the opposite direction 1/2 to 3/4 turns. The adjustment is then checked by measuring the pushrod travel distance.

Automatic slack adjusters are also shown in Figure 2-12. They have an internal rachet mechanism that automatically adjusts the travel of the pushrod as the brake components wear. The driver still needs to inspect the pushrod travel to ensure that the brakes are in proper adjustment. If not, the automatic slack may require manual adjustment, reconditioning, or replacement.

Brake adjustment indicators are required on air brake-equipped vehicles with auto slack adjusters manufactured on or after October 20, 1994.


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