Lift Circuit; Description; Theory Of Operation; Maintenance - Manitowoc Grove RT770E Service Maintenance Manual

RT770E SERVICE MANUAL

LIFT CIRCUIT

Description

The boom lift circuit consists of the lift hydraulic remote
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 in Valves, page 2-30.
Refer to Valves, page 2-30 for a complete description of the
hydraulic remote controller.
The lift cylinder has a 30.48 cm (12.0 in) bore. Both are the
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 2-61
for a complete description of the lift cylinder.
Symptom
1. Boom raises
erratically.
2. Boom lowers
erratically.
GROVE
Probable Cause
a. Low hydraulic oil.
b. Low engine rpm.
c. Main relief valve damaged.
d. Air in cylinder rod.
e. Bent boom pivot shaft.
a. Low hydraulic oil.
b. Low engine rpm.
c. Circuit and/or relief valve inoperative.
d. Air in hydraulic cylinder.
e. Damaged hydraulic pump section.
Published 11/26/2014, Control # 447-05
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

a. Check system for leaks. Make repairs
as needed. Fill reservoir.
b. Increase engine rpm to recommended
setting.
c. Replace relief valve.
d. Bleed cylinder rod.
e. Replace pivot shaft.
a. Check system for leaks. Make repairs
as needed. Fill reservoir.
b. Increase engine rpm to recommended
level.
c. Repair or replace relief valve.
d. Bleed air from cylinder.
e. Repair or replace pump section.
BOOM
Solution
4-41
4