Air-brake Components in Trucks and Buses
Foundation brakes are the most common air-brake systems found in trucks and buses and work the same way as in rail cars. Using the triple-valve principle, air builds up inside the brake pipes or air lines, releasing the brakes. Virtually all of the roadgoing vehicles equipped with air brakes have a graduated release system where a partial increase in pressure dictates a proportional release in brakes.
The following components are exclusive to a foundation air-brake system in a truck or a bus:
- Air compressor: Pumps the air into storage tanks to be used in the brake system
- Air compressor governor: Controls the cut-in and cut-out point of the air compressor to maintain a set amount of air in the tank or tanks
- Air reservoir tanks: Hold compressed or pressurized air to be used by the braking system
- Drain valves: Release valves in the air tanks used to drain the air when the vehicle isn't in use
- Foot valve (brake pedal): When depressed, air is released from the reservoir tanks
- Brake chambers: Cylindrical container that houses a slack adjuster that moves a diaphragm or cam mechanism
- Push rod: A steel rod similar to a piston that connects the brake chamber to the slack adjuster. When depressed, the brakes are released. If extended, the brakes are applied.
- Slack adjusters: An arm connects the push rod to the brake s-cam to adjust the distance between the brake shoes
- Brake S-cam: An s-shaped cam that pushes brake shoes apart and against the brake drum
- Brake shoe: Steel mechanism with a lining that causes friction against the brake drum
- Return spring: A stiff spring connected to each of the brake shoes that returns the shoes to the open position when not spread by the s-cam or diaphragm.
At idle (foot off the brake and vehicle's air system charged), air pressure overcomes the diaphragm or the s-cam is in the closed position, resulting in a released brake system. As soon as you depress the brake pedal, the air pressure decreases, turning the s-cam and spreading the brake shoes against the drum. The compressor refills the reservoir tanks and when you allow the pedal to retract, the air pressure increases back to the original state.
Emergency air brakes complement standard air-brake systems and can be activated by pulling a button on the dash (near the one with the light that we saw in the introduction). Before you can drive a vehicle with air brakes, you must push in the emergency brake button to fill the system with air. As long as the emergency system is pressurized, the emergency brake will remain free. If the system has a leak, the pressure can decrease enough to engage the emergency brake. In addition, heavy trucks are often equipped with an exhaust brake that aids the braking process, but this relies on the engine, not the air-brake system.
We've learned how air brakes work. Now let's look at how to maintenance can prevent brake failure in the next section.