Secondary - controls front brakes
Primary -
controls rear brakes and Park/Emergency Brakes
Air must be dried, compressed, regulated, and distributed
SUPPLY SYSTEM – Air Compressor is driven by engine
The Governor controls compressor output by cycling and keeping air in a predetermined range.
It monitors the supply tank and sends a signal to
air compressor.
The SUPPLY TANK is called the wet tank.
The signal tells compressor when to pump and when to rest & keeps pressure in range. Cut in pressure is 105 and cutout is 125.
The Governor is mounted on the compressor or may be mounted remotely
The Compressor
unloaded mechanism controls the loading and unloading of air
during the compressor cycle.
During loading or pumping the piston moves downward creating a vacuum on top of the piston.
This causes the inlet valve to open & draw air into the cylinder.
Next, the piston moves upward compressing the air in the
cylinder.
This closes the inlet valve and opens the compressor inlet
valve building pressure in the system.
When the system cut out pressure is reached - the unloaded port is pressurized by the governor signal.
This moves the sliding leaf valve
and uncovers the inlet cap
During the pistons upstroke, the air returns to the
inlet port and no compression occurs
once the pressure in the system is depleted to cut in level.
The Governors signal to the compressor is exhausted.
The sliding leaf valve returns to the loaded position and compression resumes.
THE AIR DRYER helps collect and expel moisture and other contaminates from the air system. The type of air dryer depends on application and the type of compressor being used.
Compressed air enters air dryer thru inlet port and passes thru drying material called desiccant. Desiccant captures moisture as well as oil and other contaminants.
When the system cutout pressure is reached, a signal from
the governor opens the
air dryer purge valve
to expel moisture and contaminates.
This same signal also unloads the compressor
after the purge cycle.
The regeneration valve on the dryer opens &
allows dried system air to flow back to the dryer.
The back flow air-dries the desiccant preparing it for the next cycle.
Dry air flows to the supply tank from the
dryer outlet.
Typically there are 3 air tanks or reservoirs.
Supply Air – Primary and Secondary air supply
The air is stored in reservoirs until brakes air is applied.
The supply tank is first tank in system and moisture tends to collect in it more than others. Supply tank has pressure relief valve if system becomes over pressurized.
The Relief valve vents excess air to prevent and damage to system.
The Supply tank includes a low pressure indicator which monitors over all system pressure.
If pressure falls below 60 PSI – an alarm will sound and light will come on to notify the driver of a low pressure condition.
Air from the supply tank flows to the primary and secondary tanks. These tanks are protected by check valves to prevent the total loss of air in the event of a failure between the service tank and compressor.
One of the tanks will have a pressure control valve that allows some backflow of air
Tank and Compressor.
One of the tanks will have a
pressure control valve
that allows some backflow of air in the event of a failure
between the services for the dryer purge cycle.
The tanks have a manual drain valve at the
lowest point that expels water that accumulates over time. Some tanks have an
automatic drain valve for convenience.
PRIMARY/SECONDARY
The dual circuit foot valve is a key valve in both systems because it separates and
controls both brake circuits. When the brake pedal is
depressed, air from the secondary tank flows thru the
secondary portion of the foot valve to a
quick release valve and applies pressure to
activated front brakes.
A quick release valve
speeds brake release time by exhausting air near the
activated brake. The air does not need to flow back
through the system. The exhaust function of the
quick
release valve is actuated when foot is lifted off
pedal. Air flows back from the air chambers to the
exhaust port. At the same time air from the
primary tank flows thru the primary portion of the
dual CRT foot valve to a
relay valve
and applies pressure to activate the rear brakes.
Always check pressure rating on a valve before you replace it.
The harder you push on brake pedal, the more air the relay
valve lets thru to activate the brakes more aggressively.
Relay valves help minimize the delays in brake activation. They quickly direct air from the tank to the brakes & uses input from the foot valve as their signal to activate. Relay valves can be ordered with various activation or crack pressure valves. Crack pressure is the amount of pressure needed to open valve.
Foot Valve also delivers pressure to a 2-way check valve that separates front and rear brake pressure and allows the dominant air pressure to open the stop light sw.
PARK and EMERGENCY
Starts with a 2 way check valve that directs the higher pressure of the Primary and Secondary tank to the emergency brake
Parking Brakes are controlled by dash Push/Pull valves. When valve is pushed in – air travels to spring brake overcoming spring force and brakes are released. Control and supply air is passed thru the tractor protection valve in the event of significant pressure loss.
Auto slack Adjusters maintain proper brake adjustment.
A quick release valve with a 2-way check valve provides anti compounding protection preventing park brake and service brake from working at the same time.
An Inversion valve is used to protect system from a Primary system pressure failure. This feature uses the Secondary system in conjunction with the spring brake to insure modulated brake stops without primary air.