How do air brakes on semi's work?

Answer 1

This is going to be long but, heres how it goes. The air governor controls the air compressor to pull in the air from outside, the air compressor than compresses it into very small amounts, the air than goes though the air lines to the air dryer. Here it is basically cleaned of any moisture and debri, after that it continues to the air tank where it is stored for use. When you push on the treadle valve or foot valve, or in normal terms the brake pedal a valve is opened in the air tank which allows the air to escape though the air lines, when it gets to the brake mechanism the force of the air actually pushes against a slack adjuster which in turn, turns an S-Cam which pushes the brake shoes against the brake brum, which in turn creates friction and brings your commercial motor vehicle to a stop.

SIMPLE AS THAT

Answer 2

Let's give a more accurate account of it, shall we? Air Compressor: The air compressor is run off the engine's accessory drive. The air inlet can either come off the air cleaner assembly, or off the engine manifold. While compressed air is being supplied to the system, the compressor is in what's known as a loaded state. Compressed air leaves the compressor outlet, then goes through a discharge line made of steel, copper, or braided steel to dissipate heat. That discharge line goes to the air dryer.

Air dryer and air governor: As air goes into the air dryer, it goes through a cartridge which contains an oil absorbent material at the bottom, and a desiccant material at the top. After going through this cartridge, some 95% of the moisture is removed from the air. From the air dryer, the air goes through an outlet port where it gets routed to the tank - it might go to a wet tank, then to the primary and secondary tanks from there, or it might go straight to the primary and secondary tanks. From one of those tanks, an air line connects to the governor. This is a signal line. When it reaches the cutout pressure ranging from 115 - 135 psi, a plunger in the governor opens up, and releases air through signal lines to the air compressor and the air dryer. This signal air closes the compressor outlet, and opens up the piston inlets, putting the compressor in an unloaded state. The air signal to the air dryer closes the outlet port, and opens up the purge valve. Air in the dryer reverses direction, pulls the moisture and oil off of the absorbent material in the air dryer, and expels it out of the open purge valve. The purge cycle lasts approximately 25 seconds.

Parking/emergency brakes: On the drive axles, at least one axle will have what are called spring brakes - the brake chamber has two chambers, ones of which contains a very power spring that extends the pushrod out, causing the brakes to be engaged. In an s-cam foundation brake system, a slack adjuster will be attached to the pushrod, and the s-cam end will run through the slack adjuster. On a disc brake system, the spring will hold the calipers closed. On a North American FMVSS121 system, there is a valve with a yellow knob on the dash. On modern tractor-trailers, this is typically an MV3 valve. On older tractor-trailers and straight trucks not set up to pull a trailer, the brake control valve for the truck will typically be a PP1 valve and the valve for the trailer will be a PP7 valve. Anyhow, once that valve is pushed in, air flows through it, then to the metering valve for the spring brakes to release them. This may be one of two types of valve.

With a quick release valve (such as a QR-1 or QR-1C valve), air supplied from the brake control valve will push down a diaphragm which covers the exhaust port, and will open up outlet ports to the lines which supply the spring brake chamber section of the brake chamber. When the brake control valve is pulled out, air is no longer supplied. The diaphragm opens up, allowing the air from the brake chambers to evacuate the brake chamber and flow out the exhaust port of the quick release valve, thus setting the parking brakes.

With a relay valve (such as an R14), a line coming from the primary air tank will connect to the inlet of the relay valve. When air flows from the PP1 or MV3 valve, it travels through a signal line which pushes down a plunger in the relay valve. This plunger being pushed down both covers the exhaust port and opens up the inlet and outlet ports, allowing the air from the tank to flow through the inlet port and valve and be supplied to the spring brake chamber through the outlet ports.

When air is supplied to the spring brake chamber, it inflates a diaphragm which pushes against that spring. Once 60 psi of hold off pressure is achieved, the spring collapses, and the pushrod is pulled back, either rotating the s-cam or releasing a caliper in order to release the brakes.

Service brake: How it works on the drive axles and steer axles is a little different. The foot pedal is attached to a treadle valve, which has two chambers. There's a chamber which feeds off the primary air system and a chamber which feeds off the secondary air system. The top chamber is the primary, the lower chamber is the secondary. The brake pedal actuates a rod in the primary chamber, and a cushion of air is established between the rod for the primary chamber and the rod for the secondary chamber. This cushion of air effectively forces the rod in the secondary chamber down when the rod for the primary chamber is pushed down. We'll start from the secondary chamber. As the rod is pushed down against a spring, air flows in from the inlet port and out through the outlet port. From the outlet port, the air goes to a quick release valve which works exactly the same way as described above for the parking brake.

In the primary chamber, inlet and outlet ports are opened, and air from the outlet port goes to a relay valve to actuate the service brake, again, which works exactly how the relay valve as described in the parking brake section works. This inflates a diaphragm in the service side of the brake chamber which forces the pushrod out to engage the brakes.

And THAT is the simplified version of how it works.