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:

  1. The oil becomes oxidized.
  2. The system slowly accumulates water.
  3. Oil viscosity slowly becomes less stable.
  4. Particles or contaminates steadily build up.
  5. Oil additives are slowly depleted.

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
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Thompson and Johnson Equipment Company
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
Hydraulic Filter
Counterbalance, reach, turret and stock picker 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.