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Effect of water on lubricants
Q. How does water affect a lubricant’s performance?
A: Lubricants not only reduce wear, but they prevent corrosion. Both of these functions are affected by water or moisture.
The way a lubricant performs in the presence of water depends on the amount of water, its corrosive nature (salt water, alkali), and the operating conditions — dynamic or static. For example, some water-resistant greases are suitable for static sealing but not for lubricating roller bearings under dynamic conditions.
Even systems that don’t operate in a wet environment are susceptible to moisture. After equipment is shut down at the end of the day, it cools to room temperature. During cooling, condensation from the air is drawn into the bearings and other lubricated components.
Water in a lubricant can deteriorate its lubricity and low-temperature performance, and cause corrosion of the lubricated parts.
Lubricity. Lubricants absorb water to different degrees depending on the lubricant type and the degree of mechanical action in the lubricated components. Most oils have low resistance to water absorption. Therefore, greases are usually applied where water resistance is needed.
Most lubricating oils (mineral, PAO, silicone, PFAE) require mechanical action of the lubricated part to cause water absorption and the formation of a wateroil emulsion. But, certain oils (hydrophilic polyglycols and some ester types) readily absorb water without mechanical action.
In small quantities, water has little effect on lubricant performance. However, if the amount of water reaches as little as 5%, it can reduce the lubricant’s adhesion and load-carrying capability, leading to increased wear.
Grease is normally chosen where adhesion properties are required. But, if a grease absorbs water, it becomes softer, loses adhesion, and washes out of the lubricated part.
To ensure that water doesn’t affect lubricant performance, choose a water-resistant lubricant. Water-resistant greases are a must for applications that are subject to water and where the currently used grease is being washed away. Such water-resistant greases include lithium complex, calcium complex, and barium complex, plus polyurea and clay types (See Table).
Corrosion. Though reduced load capacity and washing of lubricant are commonly associated with water absorption, corrosion of lubricated components (gears and bearings) is often a more serious problem. Lubricants that readily absorb water reduce this corrosion by preventing free water from contacting these components.
The degree to which grease absorbs water depends mainly on the thickener. For example, some lithium soap greases absorb up to 100% of their weight in water; barium and calcium-thickened greases absorb up to 40%; and polyurea greases absorb as little as 20%. Clay greases have good water resistance, but poor corrosion resistance.
Water works like a double-edged sword. Allowing it into a lubricant degrades the lubricant’s performance. But, if water is not captured by the lubricant, it can contact bearings and gears, causing corrosion. Unlike oil, grease can either absorb or resist water, depending on the lubricant composition. Because free water leads to corrosion and absorbed water degrades grease, the lubricant composition must strike a balance between the two. Today’s calcium complex, barium complex, and polyurea-thickened greases offer the best performance in this regard. To make sure you have one of these types, check the label on the container. Where condensation corrosion is the only concern, lithium complex is best suited.
Temperature. As lubricants absorb water they become more sensitive to low temperatures. If the temperature of grease drops below the freezing point, water trapped inside will freeze. This causes the grease to become very stiff, so that it doesn’t lubricant well, even if it has a low-temperature rating.
Q. How can I prevent water from damaging bearings and gears?
A: The best way to prevent damage is to adequately seal bearings and gearboxes. Even with the best contact seals, though, water may get into the system (especially during cooling-down periods). Therefore, you should frequently inspect the lubricant, replacing grease if it looks runny, or oil if it looks milky white.
Where water contamination is unavoidable, many companies use chromeplated parts for corrosion protection, coupled with water-resistant grease to optimize lubrication. Others use a dual approach: applying water-resistant lubricant to the seals to prevent water from entering the bearing or gearbox, and using water-absorbing lubricant in the bearings and gears to prevent corrosion. Next: Lubricating electrical devices.