© 1996 Manitowoc Engineering Co.
Rev. 06-10-96 Folio 574-1
UNLOADER PILOT VALVE MAINTENANCE
General
The unloader pilot valve (see back page) automatically controls air system
pressure by controlling when the compressor starts and stops compressing air.
Air pressure from the air tank acts against unloader valve (8) during operation.
As air system pressure increases, unloader valve (8) moves up against the
resistance of unloader spring (6). When air pressure reaches the “cut-out” setting,
the unloader valve seats against unloader cap (4). This action closes the exhaust
port in adjusting screw (2) and opens a flow path from the air tank to the
compressor unloading mechanism. The air compressor then stops compressing air.
When air system pressure decreases to the “cut-in” setting, unloader spring (6)
forces unloader valve (8) down, seating it against unloader body (7). This action
closes the flow path from the air tank and opens the exhaust port in adjusting screw
(2). The air at the compressor unloading mechanism then exhausts and the
compressor starts compressing air.
Adjustment
The unloader pilot valve has a 12 psi (0.83 bar) range between the “cut-out” and
“cut-in” pressures. The range is fixed and can be changed only slightly by
removing or installing shims (5). REMOVE one shim to INCREASE the range or
ADD one shim to DECREASE the range.
To adjust the “cut-out” setting, loosen lock nut (3) and turn adjusting screw (2) IN
to INCREASE the pressure or OUT to DECREASE the pressure. Hold the
adjusting screw and securely tighten the lock nut.
Maintenance
If the unloader pilot valve sticks or flutters, take it apart and clean it thoroughly in
non-flammable solvent. Be sure to clean filter (10) by removing it and washing it
thoroughly in non-flammable solvent. Be sure to reinstall the filter, as it is
important that no foreign matter enters the valve chamber.
In case of unsatisfactory operation, perform the following services:
1. Check the compressor unloading mechanism for damage (see Air Compressor
manual).
2. Disconnect the air line from the air tank at the unloader pilot valve; blow out
all oil, sludge, scale, etc.
3. Disassemble the entire unloader pilot valve. Wash all parts in non-flammable
solvent, and reassemble.
4. In case of major repair work, it is recommended that the unloader pilot valve
be returned to the Gardner-Denver factory in Quincy, Illinois, due to the special
tools and testing equipment required to lap and align the seating surfaces.
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