crankshaft. The pistons are connected to an angled
swash plate using ball joints. Swash plate compressors
are of three typesfive-cylinder, six-cylinder, and
The five- and six-cylinder swash compressor
has, in effect, three cylinders at each end of its
inner assembly. A swash plate of diagonal design
is mounted on the compressor shaft. It actuates the
pistons, forcing them to move back and forth in the
cylinders as the shaft is rotated. Reed valves
control suction and discharge; crossover passages
feed refrigerant to both high- and low-service
fittings at the rear end of the compressor. A gear
type of oil pump in the rear head provides for
The five-cylinder variable swash plate compressor
is different from the other swash plate compressors. It
uses a plate connected to a hinge pin that permits the
swash plate to change its angle. The angle of the swash
plate is controlled by a bellows valve that senses
suction pressure. During high load conditions the
swash plate angle is large, and during low load
conditions, the swash plate is smaller. The
displacement of the compressor is high at a large angle
and low at a small angle.
A scotch-yoke compressor changes rotary motion
into reciprocating motion. The basic mechanism of the
scotch yoke contains four pistons mounted 90 degrees
from each other. Opposed pistons are pressed into a
yoke that rides on a slide block located on the shaft
eccentric (fig. 7-38). Rotation of the shaft provides a
reciprocating motion with no connecting rods.
Refrigerant flows into the crankcase through the rear
and is drained through the reeds attached to the piston
tops during the suction stroke. Refrigerant is then
discharged through the valve plate out the connector
block at the rear. These compressors are shorter in
length and larger in diameter than other compressors.
Compressor Service Valves
Compressor service valves are built into some
systems. They serve as a point of attachment for test
gauges or servicing hoses. The service valves have
three position controlsfront seated, back seated, and
midposition (fig. 7-39).
The position of this double-faced valve is
controlled by rotating the valve stem with a service
valve wrench. Clockwise rotation seats the front face
of the valve and shuts off all refrigerant flow in the
system. This position isolates the compressor from the
rest of the system.
Figure 7-39.Three-way service valve positions.
Figure 7-38.Four-cylinder scotch-yoke mechanism.