Lull 944E-42 Service Manual User Manual download pdf (Page 259)

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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).

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