Cylinder Leakage
Hydraulic cylinders may retract due to the cooling of the oil in cylinder. Oil shrinks
approximately 1% per 100°F of cooling, or as an example, if a cylinder is extended 100”
and it cools 100°F, it would shorten approximately 1”.
TELESCOPE CYLINDER
If excessive leak-down is encountered, check items in the following sequence:
1. With boom offside and horizontal, extend the boom approximately 6 ft. per section.
Mark the first telescoping section at the end of the base section.
2. Elevate the boom to maximum angle and suspend a load on the hook. (7 tons on a 2-
part line would approximate manufacturer’s inspection procedures.)
3. With engine shut off, hold or tie the telescope foot pedal in the full “extend” position
for approximately 15 minutes.
4. Return the telescope foot pedal to neutral, start engine, ground the load, and return
the boom to horizontal. Re-mark the boom section as in (1). Measure the distance
between marks to determine leak-down of the cylinder.
The manufacturer’s allowable drift specification for production machines is as follows:
With 14,300 lb. hook load, 2-part hoist line, boom extended about 6 ft. per section at
maximum boom angle, and 160 degree F. hydraulic oil temperature, the leak-down per
cylinder is not to exceed 3/4 inch in a 15-minute period.
IDENTIFY A DEFECTIVE HOLD VALVE in the cylinder which drifts excessively by
interchanging the hold valve cartridge with one removed from a cylinder that is not drifting,
or by replacement with a new cartridge. Before installing the cartridge, visually inspect the
external “O” rings and backup washers. Retest per the procedure above to determine if
hold was defective.
An alternate method to test the hold valve would be to disconnect the two hoses coming
from the valve bank and then elevate the boom . If oil continues to flow slowly from the
extend line then it is a hold problem. If oil continues to flow from retract, then it is faulty or
leaking by piston in cylinder.
NOTE: With hollow rod telescope cylinders you can drain in excess of 40 gallons of
oil from the rod. If the boom comes in while doing this test, then the cylinder is
faulty or leaking by the piston in the cylinder.
IF THE HOLD VALVE IS NOT FOUND DEFECTIVE, the cylinder must be removed from the
boom assembly for repacking and checking. Prior to reassembly of the cylinder, conduct
an air test on the piston rod by blocking the retract ports on the rod near the piston end.
Slip a plastic bag over piston end of rod and retain and seal with rubber band. Apply and
hold a slight amount of air pressure at the retract port of the rod. (Port stamped with “R”.)
Expansion of the plastic bag indicates a defective rod weldment or seals on the port tube in
the rod.
T300-1_T500-1_T780
Troubleshooting
420 P/N 12261-683
REVISED: January 2015
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