Bendix Commercial Vehicle Systems BA-921 SMC COMPRESSOR - Manual (Page 3)

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Compressed air acts on the interior surfaces of the unloader
piston and, along with the unloader piston spring, holds the
unloader piston in the down position, against its seat on
the valve plate. See Figure 5.
NON-COMPRESSION OF AIR (UNLOADED)
When air pressure in the supply reservoir reaches the cut-
out setting of the governor, the governor delivers system air
to the compressor unloader port. Air entering the unloader
port acts on the unloader piston causing the piston to move
away from its seat on the valve plate assembly. When the
unloader piston is unseated, an air passageway is opened
between the cylinder bore and a secondary compartment
or “closed room” in the interior of the cylinder head.
As the piston moves from bottom dead center (BDC) to top
dead center (TDC) air in the cylinder bore fl ows past the
unseated unloader piston, into the “closed room”. The size
of the closed room is suffi cient to accept the compressed
air provided by the compressor piston without creating
excessive air pressure in the “closed room”. On the piston
down stroke (TDC to BDC) air fl ows in the reverse direction,
from the “closed room” past the unseated unloader
piston and inlet reed valve, and into the cylinder bore as
shown in Figure 6. Note: For optimum performance, it is
recommended that the air dryer is equipped with a “turbo
cut-off” feature.
LUBRICATION
The vehicle’s engine provides a continuous supply of
oil to the compressor. Oil is routed from the engine to
the compressor’s oil inlet. Note: There is no external oil
supply line; the oil delivery is located at the engine to
compressor mounting face. This pressurized oil fl ows to
the precision front sleeve main bearing, via an oil passage
in the crankshaft, routes pressurized oil to the connecting
rod bearings and the rear journal associated with the end
cover. Spray lubrication of the cylinder bore and connecting
rod wrist pin bushing is obtained as oil is forced out around
the crankshaft journals by engine oil pressure. Oil then falls
to the bottom of the compressor crankcase and is returned
to the engine through the opening at the compressor
mounting fl ange.
FIGURE 5 - OPERATION - LOADED (COMPRESSION)
Piston Moving Up
Air Inlet
Port
Inlet
Valve
Closed
Air
Discharge
Port
Discharge
Valve
Open
Unloader
Piston
Down &
Seated
Valve
Plate
Unloader
Port
Cooling
Plate
FIGURE 6 - OPERATION - UNLOADED
Air From
Governor
Unloader
Port
Air in Pistons Shuttles Back and Forth from the
Piston to the Closed Room
Unloader
Piston Up &
Unseated
Closed
Room