JLG 680S, Service And Maintenance Manual

Page: 43 / 530

SECTION 2 - GENERAL
3121234 2-5
2.4 CYLINDER DRIFT
Theory
When a hydraulic cylinder is supporting a load, cylinder drift
may occur as a result of any of the circumstances below:
• Normal leakage of load holding valves or malfunction of
load holding valves. See Cylinder Leakage Test and Table 2-
2, Cylinder Drift below for evaluation.
• Damaged or worn piston seal.
• Normal thermal expansion and contraction of the hydrau-
lic oil within cylinders (See Cylinder Thermal Drift below).
The first two circumstances may result in cylinder movement
due to oil leakage out of the cylinder externally or by leaking
back to tank or due to oil leaking internally from one cylinder
chamber to the other.
Thermal expansion or contraction of oil in hydraulic cylinders
is a normal occurrence and does not result in oil leaking out of
the cylinder or leaking internally from one cylinder chamber
to the other. Thermal expansion or contraction is the tendency
for materials to change size in response to a change in tem-
perature.
Cylinder Leakage Test
Cylinder oil must be at stabilized ambient temperature
before beginning this test.
Measure drift at cylinder rod with a calibrated dial indicator.
In an area free of obstructions, cylinder must have load
applied and appropriately positioned to detect drift.
Cylinder leakage is acceptable if it passes this test.
Cylinder Thermal Drift
The oil in all hydraulic cylinders will expand or contract due to
thermal effects over time and may result in changes to the
boom and/or platform position while the machine is station-
ary. These effects occur as the cylinder oil changes tempera-
ture, usually from a higher oil temperature as it cools and
approaches the ambient air temperature. Results of these
effects are related to several factors including cylinder length
and change in temperature over the time the cylinder remains
stationary.
Table 2-2. Cylinder Drift
Cylinder Bore Diameter
Max. Acceptable Drift
in 10 Minutes
inches mm inches mm
3 76.2 0.026 0.66
3.5 89 0.019 0.48
4 101.6 0.015 0.38
5 127 0.009 0.22
6 152.4 0.006 0.15
7 177.8 0.005 0.13
8 203.2 0.004 0.10
9 228.6 0.003 0.08
NOTE: This information is based on 6 drops per minute cylinder
leakage.