4-20
Published 01-29-2014, Control # 496-00
BOOM TMS800E SERVICE MANUAL
LIFT CIRCUIT
Description
The boom lift circuit consists of the lift electric controller, lift
directional control valve, holding valve, and the lift cylinder.
These components enable the boom to be raised or lowered
to various degrees of elevation ranging from -3 to +78
degrees from horizontal.
The lift directional control valve is the closed spool type and
is described under Valves, page 2-22.
Refer to Valves, page 2-22 for a complete description of the
hydraulic remote controller.
The lift cylinder has a 30.48 cm (12.0 in) bore. The cylinder is
a double acting type. Dirt and other foreign material is
prevented from entering the cylinder and causing internal
damage by a wiper seal during rod retraction. Oil seals on
both the piston and cylinder head prevent internal and
external hydraulic oil leakage. Refer to Cylinders, page 2-54
for a complete description of the lift cylinder.
The holding valve is a balanced poppet type hydraulic valve.
It is threaded into the port block which is an integral portion of
the lift cylinder barrel. The holding valve functions when
booming up (cylinder rod extended), booming down (cylinder
rod retracted), or holding (cylinder rod stationary).
Theory of Operation
The directional control valve bank housing the lift control
valve is supplied by flow from the hydraulic pump.
When booming up, oil unseats the poppet (check) valve in
the holding valve, letting oil flow to the piston side of the
cylinder. Pressure is applied to the piston, forcing the rod to
extend, raising the boom.
When booming down, oil enters the retract port of the port
block and flows to the cylinder rod side. When pilot pressure
reaches a pre-determined value, the main poppet unseats
and oil flows from the piston side of the cylinder to the
reservoir.
All return flow from the control valve goes to the reservoir.
Maintenance
Troubleshooting
Symptom Probable Cause Solution
1. Boom raises
erratically.
a. Low hydraulic oil. a. Check for leaks. Repair any found.
Replenish hydraulic oil to proper level.
b. Low engine RPM. b. Increase engine RPM to recommended
setting.
c. Main relief valve damaged. c. Replace relief valve.
d. Air in cylinder rod. d. Bleed cylinder rod.
e. Bent boom pivot shaft. e. Replace pivot shaft.
2. Boom lowers
erratically.
a. Low hydraulic oil. a. Check for leaks. Repair any found.
Replenish hydraulic oil to proper oil
level.
b. Low engine RPM. b. Increase engine RPM to recommended
level.
c. Circuit and/or relief valve inoperative. c. Repair or replace relief valve.
d. Air in hydraulic cylinder. d. Bleed air from cylinder.
e. Damaged hydraulic pump section. e. Repair or replace pump section.
Reference Only
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