Air Brake Pressure Components

This topic explains the fundamental components which build, store, use, and control the brake system air pressure. We will explain what each component does and how it fits in with the other air brake system components.

For specialized valves and other unique components such as anti-compounding valves, please refer to the brakes topic which brought you here. We have a lot to talk about, so lets get started.

The picture above is a new cat air compressor. Notice the silver part at the top and front of the compressor head. That is the air pressure governor. The compressor builds air pressure and the governor controls the system air pressure limits. When 120 PSI is reached, the governor bypasses the compressor so that it stops pumping air. When the system pressure falls below 100 PSI, then the governor enables the compressor and the compressor rebuilds the air pressure back up to 120 PSI.

This compressor is gear driven from the timing gear of the engine. Many truck compressors are belt driven with two or more belts. Most compressors are water cooled and oil pressure lubricated from the engine. The air supply can be from it's own air filter or tapped off of the engine's air filter. A normal compressor will build the truck's air pressure from 85 PSI to to 100 PSI within 45 seconds.

Refer to the drawing on the right for the following explanation. The compressor has two valves. The discharge valve is spring loaded, and the valve spring lifts when the piston pressure exceeds the discharge line pressure. This pumps additional air supply into the reservoir. Any time that the piston pressure is less than the discharge pressure, the discharge valve spring forces the discharge valve shut and holds the reservoir pressure.

The other valve is the compressor inlet valve. This valve is spring controlled during normal compressor operation. Whenever the compressor piston travels down, the suction overcomes the spring and allows air to enter into the piston area from the compressor inlet adapter. When the compressor piston travels upwards, the compressor inlet valve is closed by the spring.

The compressor also has an unloader piston shown in the diagram, which lifts the inlet valve off its normal seat and keeps it open. When the reservoir pressure reaches 120 PSI, the governor ports this pressure out the unloader port of the governor, over to the unloader piston in the compressor, and lifts the inlet valve off its seat. This pressure is sufficient to hold the compressor inlet valve open, which prevents the compressor from pumping any more air. Each time the compressor piston travels down, it draws air from the inlet adapter. Each time the compressor piston goes up, it exhausts air out the inlet adapter.

The governor works on a two spring piston concept. The first spring resists the motion of the piston which sets the point that a second spring force starts coming into play. The second spring force seats and unseats an inlet/exhaust valve (opens/closes the valve). As the reservoir pressure increases, the piston is forced up against this inlet/exhaust pressure adjustment point. All this time the inlet/exhaust valve within the piston is in its exhaust position which vents the governor casing unloader port to the governor casing exhaust port. This prevents unloader port pressure from building. At some point, further upwards motion of the piston starts to unseat the inlet/exhaust valve spring, which now closes the exhaust port and allows pressure to build up in the unloader port via the inlet valve. The upper portion of this piston has a larger surface area than the lower portion of the piston, and once the inlet valve ports the reservoir pressure into the second part of the piston, the piston snaps upwards a bit farther. This ensures that the inlet valve is fully open and ensures that the exhaust valve is fully closed.

We now have reservoir pressure on the unloader port of the governor, which is piped over to the compressor, which raises the compressor unloader piston, and holds open the compressor inlet valve; which unloads the compressor and stops it from pumping.

With the compressor disabled, the reservoir pressure will slowly fall back down to 100 PSI. When the pressure reaches around 100 PSI, the governor piston has slowly moved down, and at this pressure, the inlet/exhaust valve within the piston closes the inlet port and opens the exhaust port. This allows the air pressure from the upper part of the piston to escape out the governor exhaust port. At the same time, the reservoir pressure at the governor unloader port is also vented away.

The bottom portion of the piston still has less surface area than the top portion which is now vented, so the piston drops a little lower and snap closes the inlet valve and snap opens the exhaust valve. This ensures sudden and complete venting of the governor unloader port, which is ported over to the compressor, where it closes the compressor inlet valve and restores compressor pressure building. The entire cycle repeats when the reservoir pressure reaches 120 PSI.

Air Dryer

Have you ever blown compressed air onto your hand? If so, you fond that it left your hand slightly moist. This moisture can cause rusting and freezing of air brake components, and must therefore be removed. The air dryer actually performs two purposes. It removes moisture from the compressed air, and it removes oil particles which leaked past the compressor piston rings. The air dryer ties in with the air compressor, first reservoir tank (referred to as the wet tank from the old days), and the compressor governor which we discussed above.

The drawing above shows the piping associated with the air dryer. The dryer has three pipes. The intake pipe comes directly from the compressor head pressure which was referred to above as the discharge line. The second port of the dryer samples the unloader port signal from the governor back to the compressor. The third pipe passes the dry air from the dryer on over and into the first reservoir tank, which is now a dry tank. Let's take a look inside the dryer and see just how it manages to dry the compressor air. The dryer also has an exhaust at the bottom called the purge valve. This is where the moisture and oil contaminates are ejected out of the dryer.

The drawing on the right is the courtesy of AlliedSignal Bendix and depicts charge cycle of their dryers. The charge cycle is when the dryer extracts the moisture and oil contamination from the compressor air supply, and pushes clean and dry air into the reservoir.

Port A is the intake from the compressor discharge port. This is where the compressed air enters into the dryer. Notice the check valve E, which has governor unloader port pressure under it shown in green color. We know from the above governor discussion that there is no unloader pressure while the compressor is working. Therefore the compressor head pressure goes into port A, pushes right past the E valve and into the orange chamber. The orange chamber ports around the blue chamber, and the contaminated compressor air forces up through the screen B and up through the sides of canister C. These screens and desiccant strips the oil and moisture from the air as it flows upwards around the canister.

When the stripped air gets to the top of the canister, it passes back down the middle of the canister, pushes past the check valve at port D, and enters into the reservoir tank. As long as the compressor pressure at port A is greater than the reservoir pressure at port D, the check valve is held open and stripped air flows into the reservoir. Meanwhile, all the moisture and oil is trapped within the outside of the canister and screen.

Eventually the governor unloads and shuts off the compressor. This unloader pressure signal starts a dryer sequence of events which will purge the dryer screen and desiccant of the oil and moisture which was collected.

The drawing on the right is the courtesy of AlliedSignal Bendix and depicts the discharge cycle of their dryers. Notice that the unloader port at the bottom of the dryer is now blue, which depicts that the governor unloader port has now placed reservoir pressure on this passage. The unloader pressure on the bottom of check valve C slams it shut to hold the compressor discharge pressure. This same pressure also pushes valve A to the right, which exhausts the contents of the orange chamber.

Meanwhile, valve B closes because the pressure inside the canister is no longer greater than the pressure in the reservoir tank.

Because we still have compressed air inside the canister, the canister now becomes the pressure source and the blue air inside the canister flows upwards and then down the sides of the canister to get to the orange chamber which is vented to the outside air pressure by valve A. As this pressure rushes down the sides of the canister, it collects and purges all moisture and contaminates which were trapped in the screens and desiccant, and forces it down through the orange cavity and outside the dryer through valve A. When all of the pressure within the canister has vented, then the purge cycle is completed.

Eventually the reservoir pressure will fall to the point in which the governor will exhaust the unloader air pressure, and the dryer will go back into it's charge cycle.

Just above valve A you will see a silver tube. This is an electric heater which prevents collected moisture in the orange passage from freezing in cold weather. The heater only requires power and cycles by itself to keep the dryer above freezing temperature.

Just above the heater and on the right side of the orange passage is a safety release valve. At a given pressure it will pop open to prevent blowout of the purge canister, when the reservoir pressure exceeds safe limits.

Reservoir Tanks

Most tractors have three reservoir tanks. The first tank is connected to the compressor and/or air dryer, and is called the supply reservoir. The second tank is the rear axle service reservoir, and the third tank is the front axle service reservoir. Each tank is feed through a check valve to keep the tank pressure if the source pressure drops. Remember, the check valve for the supply reservoir is within the air dryer.

Each of these tanks also has a drain spigot at the bottom. These should be purged on a daily basis to remove any moisture which may have gotten into the tank. These tanks normally have a pressure valve at the cab dash so the driver can see the pressure within each tank. Warning indicators are also present to alert the driver if the tank pressure drops below a minimum of 60 PSI.

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