THE PERFORMANCE of lubricating oil systems can be affected by the presence of air in various forms.
Generally, air can be found in four phases in lubricating systems. Free air, or air bubbles, which are trapped in sections, have minimal contact with the fluid, but can contribute to other air-related problems.
Dissolved air can often be caused by rapid changes in temperature and pressure. During system start-ups or overheating, the air becomes small bubbles in the liquid, and may remain suspended, causing air entrainment.
Entrained air can be caused by agitation or other actions. Though this occurs in healthy oil, it is more common in oil containing high amounts of moisture or contaminated oils. This can cause pump cavitation, erratic hydraulic operations, loss of precision, vibrations and safety problems.
Foaming is caused by air being whipped into the oil by gears or pumps, the cascading of oil into a reservoir, insufficient resident time, and grease contamination. It can displace oil, resulting in oil starvation, metal-to-metal contact and high operating temperatures.
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