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Hydraulic
Oil Maintenance
The training tip for this month is hydraulic oil maintenance
and what you need to know about the hydraulic oil used
in your Material Handling Equipment. This information
was written by the Crown Equipment Corporation, but
it very much applies to all brand forklifts.
Hydraulic Oil
The hydraulic oil in Crown Lift Trucks not only transmits
force for hydraulic functions such as lift and lower,
it also lubricates, cools and prevents corrosion. If
the truck is low on hydraulic oil, the system will operate
hotter because there is less oil to absorb and dissipate
heat. Heat causes the oil to break down and not lubricate
as well as it should so parts wear out prematurely,
seals leak, and the system may even operate slower.
Not having enough oil in the system could also starve
the pump for oil, causing the oil to become aerated.
The resulting cavitation sometimes creates a loud banging
noise and occurs when air bubbles collapse due to system
pressure. Because air is mixed with the oil, lubrication
is reduced, wear is increased, and the system runs hotter.
Cavitation may even cause components to become pitted.
The combination of air and heat also causes the oil
to oxidize. It is important to change the hydraulic
oil at the recommended interval because, over time,
as the oil is cycled during normal truck operation,
chemical reactions, heat, etc. slowly cause the characteristics
of the oil to change. These changes increasingly get
worse and, if the oil is left in the truck long enough,
will damage the system. Changes include:
- The oil becomes oxidized.
- The system slowly accumulates water.
- Oil viscosity slowly becomes less stable.
- Particles or contaminates steadily build up.
- Oil additives are slowly depleted.
Oxidation
During normal operation, oil becomes oxidized due to
heat and exposure to air. Gum and varnish are a byproduct
of the oxidation process and will cause valves to stick
and be less responsive.
 Water
Accumulation
Every time a cylinder is extended and retracted, air
within the reservoir changes. Humidity and slight differences
in air temperature cause condensation. Over time, water
accumulates within the system and this leads to corrosion,
a decrease in lubricity, and an increase in component
wear. Water can also enter the system through worn seals
and will attack certain seal materials such as polyurethane
(polyurethane seals are primarily used in cylinders
on Crown trucks because they perform very well at high
pressures).
Viscosity Change
Viscosity is the ability of a fluid to flow over a period
of time. When syrup is removed from the refrigerator
it flows slowly, but as it is allowed to warm, it flows
faster. This is because as the syrup heats, its viscosity
changes (it gets thinner). Unlike syrup, viscosity stabilizers
are added to hydraulic oil so it flows at a more even
rate as the temperature fluctuates. However, viscosity
stabilizers in hydraulic oil wear out over time and
this results in the oil becoming thinner at high temperatures
and thicker at low temperatures (just like syrup). Thinner
oil provides less lubricity so parts wear out quicker
and may cause internal leaks, thus reducing system efficiency.
Thicker oil increases power consumption, slows hydraulic
operation, and creates more opportunity for air to be
introduced in the oil, thus increasing the oxidation
rate.
 Contamination
Build-Up
During normal truck operation, pumps, valves, bearings,
seals, etc. slowly wear and release metal, rubber, etc.
particles in the oil. The filter cannot remove all of
these particles because some are extremely small. These
particles (called silt) build up in the oil causing
an increase in wear, may cause valves to stick, and
can cause severe damage to components.
 Oil
Additives Depletion
Additives give oil its anti-wear, anti-foam, rust and
oxidation protection. Chemical reactions with oil, water
and air deplete these additives, and heat accelerates
the chemical reactions. As additives are depleted or
decreased, wear and corrosion increase.
Hydraulic Filter
Counterbalance, reach, turret and stockpicker trucks
are equipped with a 10 micron filter element. A micron
is a unit of measure equal to one millionth of a meter.
A grain of salt is approximately 100 microns thick.
So the filter removes particles from oil that are approximately
10 times smaller than a grain of salt. If left in the
oil, particles of this size could severely damage components.
Oil can't be too clean. It's important to change the
filter at the recommended interval because if the filter
is dirty, oil will bypass the filter. A low pressure
relief valve is contained within the filter. As the
filter fills with contaminants and becomes plugged,
oil will intermittently flow through the valve without
being filtered, risking severe component damage throughout
the system. If the filter is left in the truck long
enough and becomes completely plugged, oil will completely
bypass the filter and not be filtered at all.
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