Lull 944E-42 Service Manual - Manual (Page 259)

8.7 Model 644E-42/944E-42 Rev. 6/04
Hydraulic System
8.3 SPECIFICATIONS
Refer to Section 2.5.11, “Hydraulic System,” for hydraulic
system specifications.
8.4 HYDRAULIC FLUID
General information and specifications pertaining to
hydraulic fluid are found in Section 2.6, “Fluids,
Lubricants and Capacities.”
Use only clean, filtered fluid in the hydraulic system.
After servicing or replacing a hydraulic system
component, cycle all vehicle functions several times to
remove (purge or “bleed”) air from the hydraulic system.
DO NOT build maximum pressure at the end of stroke by
continuing to hold the joystick function.
Inspect the hydraulic system and fluid frequently, and
look for problems or conditions including the following:
Aeration
Aeration is the presence of air bubbles in hydraulic fluid.
Sudden, violent inward collapse of the air (known as
implosion) can occur when compressed air bubbles are
subjected to system pressure. Aeration can thus lead to
damage and overheating of components.
Pump aeration will produce a loud crackling sound, as
though rocks or marbles were being forced through the
pump. Excessive aeration will give the hydraulic fluid a
cloudy or milky appearance, and cause erratic operation
of hydraulic system components.
Aeration is caused by air being introduced into the
system, possibly through a leaky seal or joint in the pump
or elsewhere in the system. Check for leaky hydraulic
cylinders and threaded fittings by introducing a special
dye, visible under fluorescent light, which may be helpful
in determining the source of the leak.
Cavitation
Cavitation occurs when hydraulic fluid fails to entirely fill
an existing space. Cavitation, like aeration, can produce
a loud crackling sound, as though rocks or marbles were
being forced through the system. Cavitation can be due
to a low hydraulic oil level in the reservoir, a restricted
intake line, or improperly high fluid viscosity.
Contamination
Contamination, or dirt, can be any material other than
hydraulic fluid that causes harm or decreases the
performance of the system. Contaminants include solids,
liquids and gases. Some contaminants are abrasive in
nature, causing rapid wear or damage to the system.
Excessive heat
Excessive heat is a relative thermal condition that effects
fluid viscosity. Extreme operating conditions, such as
excessive operation, outside air temperature and other
factors including aeration, cavitation, contamination and
over-pressurization can cause fluid and component
temperatures to go beyond their specified limits.
Excessive heat causes oxidation in hydraulic fluid to
occur more rapidly, breaking down its viscosity. A chain
reaction then results and can lead to component damage
or failure. The true cause of an excessive heat condition
must be diagnosed and cured, or the problem will recur.
Implosion
An implosion is a sudden, violent inward collapse, and
can often cause a vacuum effect. Imploding air bubbles
within a hydraulic system can cause component damage.
Over-pressurization
Over-pressurization is pressure greater than that for
which a component was engineered to withstand or
operate under. Over-pressurization can cause equipment
damage. DO NOT attempt to adjust or tamper with
pressure cartridges, settings, etc. unless otherwise
specified.
Viscosity
Viscosity is the property of a fluid or semi-fluid that
enables it to develop and maintain shearing stress
dependent upon flow rate (velocity), and to offer
continued resistance to flow. In other words, viscosity is a
measurement of fluid resistance to flow (internal friction).