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Air Governors

Legend tells that if left running on its own, a compressor will keep pressurizing air until the pressure gets so high that everything explodes!






An explosion is unlikely due to safety valves that prevent excessive pressure. But on mobile equipment in particular, the air compressor shaft is often driven directly by the engine, continuously compressing air whenever the engine is running. This is inefficient and hard on the air compressor.

This is where the air governor comes in.

In this lesson we will be taking a closer look at an air governor. We will examine the internal structure and the operation of the governor.

The Structure

Despite how it looks from the outside, the governor is a surprisingly complex device.

The side of the governor has three external ports.

There is:

  • An inlet port attached to the reservoir.
  • An unloader port which is attached to both the compressor and the air dryer.
  • An exhaust port which is left open to the atmosphere.


The top half of the governor contains the adjusting screw, pressure setting spring, and the lower spring seat/spring guide combo.

Adjusting
Screw
Pressure
Setting
Spring
Lower
Spring
Seat
Inlet
Port
Spring
Guide
Unloader
Port
Unloader
Port
Filter
Exhaust
Port
Piston
Exhaust
Stem Spring
Exhaust
Stem
Inlet/Exhaust
Valve
Inlet/Exhaust
Valve Spring

The bottom of the governor has the piston.

Within the piston there is the exhaust stem with its spring and the inlet/exhaust valve and its spring.

Operation

The governor is usually attached directly to the compressor.

Inlet Port

The inlet port connects the governor to the air reservoir it regulates.

As the air pressure in the reservoir increases, the piston and inlet/exhaust valve shifts up against the resistance of the pressure setting spring.

Inlet/
Exhaust
Valve
Piston
Pressure
Setting
Spring

Exhaust
Stem
Inlet/
Exhaust
Valve
Blocked!

Once the air pressure reaches the pressure setting of the pressure setting spring, also known as the cut-out pressure, the exhaust stem pushes the inlet/exhaust valve open.

This seals the exhaust stem against the inlet/exhaust valve, blocking any air from escaping out the exhaust port.

There is now an open passage through the piston which connects the inlet port to the unloader port.

This allows air entering from the inlet port to exit out the unloader port, unloading the compressor and initiating the air dryer purge cycle.

Inlet Port
Unloader
Port
A Passage!

The piston will remain shifted as long as the reservoir pressure is above the cut-in pressure, which is roughly 25 PSI less than the cut-out pressure.

Once the pressure drops below the cut-in pressure the pressure setting spring forces the piston back to its original position.

The inlet/exhaust valve will once again be closed and the exhaust stem path will be opened.

Exhaust
Port

Air in the unloader line can now pass through the exhaust stem and out the exhaust port.

It is important that the exhaust port not be plugged. If the air from the unloader line is unable to escape through the exhaust port then the compressor will not be able to build pressure.

Now the compressor can build more air pressure and the dryer can begin its charge cycle.

Cut In
Cut Out

The complete cycle looks something like this!

This is just a preview!

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