Gradall LOED 534A Service Manual Download

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MAINTENANCE PROCEDURES - RELIEF VALVES

The Cartridge relief valve shown it, Figure -1 is of the differential area type. When fluid enters the relief
valve through the hole in the piston, an unbalanced condition in this area keeps main poppet and piston
tightly sealed.

As system pressure exceeds the relief valve setting, the pilot poppet is unseated, allowing oil to flow around
the pilot poppet stem through the holes in the pilot poppet plug and into the low pressure area. This oil
flow reduces the pressure in area (A) allowing the piston to seat against the pilot poppet. The seating of
the piston shuts off the oil flow through the sensing hole in the piston. Oil pressure in area (A) is greatly re-
duced thus allowing the main poppet to unseat. System fluid is then dumped instantaneously to low
p r e s s u r e . ( F o r d e t a i l e d t h e o r y o f o p e r a t i o n s e e p a g e 8 . ) .

DISASSEMBLY

Remove pilot poppet plug assembly and plug from housing. Remove piston spring and piston from sleeve.
Using brass rod, carefully drive sleeve out through valve plug end of housing. Main poppet can then be
removed by tapping sleeve into palm of hand. Pilot poppet in the pilot poppet plug assembly is preset at
the factory; further disassembly will alter this setting and is not recommended. If, however, it is necessary
to disassemble, remove acorn nut, lock nut and the two washers from the adjustment screw. Remove
a l l O - r i n g s a n d s p i r a l b a c k u p r i n g s .

INSPECTION AND REPAIR

NOTE. THE MOST COMMON CAUSE OF RELIEF VALVE FAILURE IS DAMAGED OR FATIGUED O-RINGS. IT IS,
THEREFORE, RECOMMENDED THAT NEW O-RINGS BE INSTALLED.

Clean all parts in solvent and dry with compressed air. Inspect seating surfaces of poppets and piston for
nicks, grooves and excessive wear. The seating edge of the sleeve must be sharp and free from nicks. Check
the sensing hole in the piston, hole in the pilot poppet plug assembly, and holes in the sleeve for obstruc-
tions. Assemble piston to the main poppet. It is essential that the piston move freely through the hole in
t h e m a i n p o p p e t . A n y p a r t s f o u n d t o b e f a u l t y m u s t b e r e p l a c e d .

ASSEMBLY

Assemble andl position all O-rings and spiral back up rings as shown in Fig. 34. Stretch O-rings into position
(do not roll). Insert pilot poppet and pilot poppet spring into valve plug and install the adjustment screw.
Lubricate main poppet with hydraulic oil and insert it with piston and piston spring into sleeve. Lubricate
this assembly and mate with sleeve. Install this assembly in the valve housing. Because the sleeve is a honed
fit, gentle tapping may be required to install. Care must be exercised to avoid damaging the contrtol valves
internal lands. Install plug to, valve housing. Acorn nut, lock nut and washers can be assembled after relief
cartridge is adjusted.

Relief Valve - Pilot Operated (External Adjustment)

FIGURE 34