Just 1% water in gearbox oil can reduce bearing life by up to 90%.

With water in gearbox oil, the effectiveness of a gear lubricant is compromised when the oil and water do not separate and the oil becomes diluted.

Its destructive effects on bearings can be worse than that of particle contamination, depending on conditions. As with dirt ingress, vigilant control must be exercised over water in gearbox oil to minimize its accumulation in the lubricants and its damage to bearing surfaces.

Experts agree that using a gear oil that quickly separates from water is the most effective way of
reducing the risk and consequences of water contamination. Gear oil with improved water separation properties not only allows faster and easier water draining, it also reduces the frequency of oil changes.

Water in gearbox oil can rust gear surfaces and, in the extreme, destroy the gear completely.

How water in gearbox oil reduces the effectiveness of the lubrication.

To a certain extent oil is hygroscopic, meaning it can absorb moisture directly from the air.

The amount of moisture absorbed is influenced by the relative humidity of the air and the saturation point of water solubility of the oil. Depending on temperature and pressure, this solubility limit will vary from about 100 ppm for low additive oil to several thousand ppm for high additive and many synthetic oils.

For any given water in gearbox oil saturation point an equilibrium will eventually be attained where the moisture moving from the air to the oil, and also from the oil to the air, is equal.

At first absorbed water is always dissolved in the oil, but may later, due to temperature/pressure changes, be condensed out to a free or emulsified state.

Condensation can also result in water in gearbox oil.

Humid air entering oil compartments will often cause moisture condensation on the walls and ceilings above the oil surface. Frequent temperature changes can greatly increase the rate of condensation. Eventually the condensation will coalesce and run down the walls forming a layer of free water in the gearbox oil.

With few exceptions, the chemical and physical stability of lubricants are threatened by even the slightest amount of suspended water.

Water in gearbox oil can promote several chemical reactions.

Water can promote a host of chemical reactions (hydrolysis) with compounds, oil additives, base stock and suspended contaminants. When combined with oxygen, heat, and metal catalysts, water is known to promote the oxidation and formation of free radicals and peroxide compounds.

Oxidation inhibitors are sacrificed by both neutralizing peroxides and breaking oxidation chain reactions to form stable compounds. Several oxidation inhibitors are known to form hydrogen sulfide and sulfonic acids when reacting with water.

The protection provided by zinc dialkyldithiophosphate (ZDDP), a common antiwear additive and antioxidant, is destroyed by as little as one drop of water in 5 lt gearbox oil, when oil temperature exceeds 82 degrees C.

Water is also known to attack rust inhibitors, viscosity improvers, and the oil's base stock resulting in undesirable by-products such as varnish, sludge, organic and inorganic acids, surface deposits and lubricant thickening (polymerization).

Large quantities of emulsified water in gearbox oil can lower viscosity, thereby reducing a lubricant's load carrying ability. When water is combined with metal catalysts such as iron or copper, accelerated stressing of the oil can occur. This results in base stock oxidation and the forming of free radicals (which continue the oxidation process), hydroperoxides, and acids.

How to manage water in gearbox oil.

One way to manage water contamination is to identify potential contamination points and institute measures to reduce the ingress and counteract the effects.
 The best strategy is to prevent water from entering the system in the first place.

A secondary strategy is to limit the water’s ability to damage components. A good way to do that is to opt for a lubricant designed to maintain its properties even when small amounts of water enter the system. Habot Synthetic Lubricants produce high quality synthetic gear lubricants that do just this.

Whilst combating the effects of water in gearbox oil synthetic industrial gear oils also contain rust and oxidation-inhibiting additives and antiwear EP additives resulting in longer lubricant life, better performance and ultimately value for the end-user.