The UNLOADER assembly (fig. 7-43) is mounted
in the compressor head and controlled by the governor.
The unloader valve may be either a poppet-type or a
spring-loaded control valve. Air pressure from the
governor opens the unloader valves to unload or stop
compression in the compressor.
When the reservoir air pressure reaches the
maximum setting of the governor, air under pressure is
allowed by the governor to pass into a cavity below an
unloading diaphragm. This air pressure lifts one end of
the unloading lever, which pivots on its pin and forces
the unloading valves off their seats. With the
unloading valves off their seats, the unloading cavity
forms a passage between the cylinders above the
pistons. Air then passes back and forth through the
cavity between the cylinders and compression is
stopped. A drop in air pressure below the minimum
setting of the governor causes it to release the air
pressure from beneath the unloading diaphragm,
allowing the unloading valves to return to their seats
AIR TANKS (RESERVOIRS)
The two steel air tanks, commonly known as
reservoirs, are used to cool, store, remove moisture
from the air, and give a smooth flow of air to the brake
At the bottom of each tank is a drain valve (fig.
7-44). This valve is used to allow the operator a means
to drain the air from the tanks daily, thereby preventing
Figure 7-43.Unloader assembly.
Figure 7-44.Air reservoir with an air drain valve.