JLG 740AJ Service Manual - Manual (Page 206)

SECTION 3 - CHASSIS & TURNTABLE
3-154 – JLG Lift – 3121160
3.31 AIR COMPRESSOR
Description
The compressor consists of a heavy duty rotary screw air
compressor with integral inlet valve assembly, oil separa-
tion system, minimum pressure/discharge check valve
and oil filter housing. The complete system incorporates
compressor oil cooling system, hydraulic drive and valv-
ing.
Oil Injection
Lubricant is injected into the compressor air end unit and
mixes directly with the air in the compression chamber,
internal porting also injects oil into the bearings and seal
area. The lubricant has three primary functions:
• Controls the rise of air temperature normally associ-
ated with the heat of compression.
• Seals the leakage paths between the rotors and the
stator, and also between the rotors themselves.
• Acts as a lubricating film between the rotors allowing
one rotor to directly drive the other which is an idler. It
also lubricates the bearings and seal.
The screw compressor assembly is mounted inside the
main casting and consists of a male and female rotor sup-
ported with
anti-friction bearings suitably sized for long
life.
Inlet Valve and Control Valving
The inlet valve and control solenoid valve assembly are
mounted directly on top of the compressor module. On
initial start-up the solenoid is energized and the inlet valve
opens from pilot air being passed through the solenoid
actuated valve. When final pressure is reached a pressure
switch de-activates the solenoid and the inlet valve closes.
At the same time the compressor pressure will relieve
down to a low pressure (typically about 40 psig [2.75
bar]). Only the compressed air within the compressor
module will reduce down to this lower pressure due to the
operation of the discharge minimum pressure/check
valve. This reduction in internal air pressure reduces the
power requirement considerably during this unloaded
state. The pressure switch located in the downstream air
line senses air demand and upon reducing pressure in
discharge line (ie. air being used) will re-activate the inlet
valve and the compressor again starts to load and pro-
duce air.
The discharge air pressure switch will typically be set with
a 30 psi (2.0 bar) differential pressure.
Air Filter Unit
The air filter is dry type replaceable element and is
mounted directly on top of the inlet valve assembly. The
element is easily replaced for service changeout - Refer to
Maintenance Section.
Oil Reservoir and Primary Oil Separation
The main casting which contains the screw compressor is
also the oil reservoir and primary oil separation unit. The
initial (primary) oil separation is caused by both changes
in velocity and direction. The main casting also contains
the oil level/fill plug and oil drain connection. A separate
oil reservoir is not required.
Secondary Spin-On Oil Coalescer/
Separator
This spin-on element screws directly onto the filter support
housing at the rear of the compressor module. The sepa-
rator element (coalescer) recovers the finer particles of
residual oil after pre-separation oil, which is collected in
this element is scavenged back into the compressor unit.
The oil return line passes through the Oil Sight Glass
which indicates the amount of oil being deposited (scav-
enged) in the element. At start-up the sight glass most
likely will be full for a short period which is due to drainage
from the element when it is not in use, this should diminish
fairly quickly and a lesser amount should be observed
which indicates that the element is separating out oil
deposited within the spin-on element.
Spin-On Oil Filter
Located on the filter support housing at the rear of the
compressor. The filter incorporates a by-pass valve which
will open to by-pass the filter during cold start-up when the
oil is very viscous. It will also open if the filter element is
plugged. Filter element rating is 10 Micron.