Air Brake Pressure Components
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
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.
Compressor and Governor
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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
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.
valve B closes because the pressure inside the canister is no
longer greater than the pressure in the reservoir tank.
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.
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.
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.
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.
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.
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.
to top of page , Driver
Knowledge Page , Truck Brakes page