How to manage bearing water contamination from grease

Water contamination is a major issue affecting bearing life. It is a source of bearing failure that is often not well understood and is overlooked, particularly as bearings might not be in direct proximity to wet areas or obvious sources of water contamination. 

Most engineers seem to imagine that unless the bearing is in wet operating conditions or in equipment that has bearings in close proximity to liquids for example pumps, that water contamination will not be an issue. Not so.

Housing condensation 

Condensation inside the housing is a cause of water contamination of grease. This will occur as hot conditions during the day become cool or colder conditions at night, and the water in the air trapped in the housing becomes a condensate. Humidity will also cause degradation of the grease through water contamination.

Processes such as wash downs with high pressure hoses are likely to force water back into the housing though the seals, unless the seals are in good condition and continuously and fully grease-purged.

The limit for the absolute water content in mineral oil - the lubrication component of most greases - above which problems will occur, is 200 ppm[1]. Water contamination higher than this will cause a number of lubrication problems and failures of the grease’s lubrication and protection properties. 

Clearly the higher the water contamination, the worse the result but it is most important to understand that lubrication failures can start to occur with an extremely small amount of water contamination in the grease. Breakdown of the oil film strength, which reduces the oils ability to separate rolling elements under load can be expected. The result of this loss of oil film strength is damaged surfaces, the roughness preventing hydro-dynamic oil film separation of the rolling surfaces. This causes accelerated wear and surface damage leading to bearing failure. 

The performance of the oils and additives in the grease is critical to lubrication outcomes. Water not only is a cause of corrosion but the water also allows acids to be formed as a result of the lubricant break down, due to sulphurs released from the oil and the EP and anti-wear additives in the grease. Not only does corrosion follow from the formation of acids but the grease is additive-depleted and unable to perform the required lubrication as it originally did.

The presence of water causes hydrogen-induced embrittlement and cracking. Acceleration of the hydrogen-induced fracture process, started with the water contamination and advanced by corrosion and electrolysis, causes the rolling elements and raceways to become etched and pitted. This surface damage then causes high temperatures at the point of load as a result of the loss of oil film separation between the damaged surfaces, resulting in further oxidation of the oils in the grease and in turn adding to corrosion and bearing damage.

Methods of preventing water contamination:

1. Prevention of water ingress by maintaining the seals in good condition and if labyrinth seals are used ensuring that they are continuously fully grease-purged.

2. Maintaining continuous re-lubrication of the bearings to ensure that there is a progressive and continuous replacement of lubricant and that any contaminated grease is purged from the bearing.

GreaseMax lubricators provide proven results with continuous lubrication and grease replenishment for bearings and the maintenance of flow-through purging of seals. It delivers an effective solution to the problems of contamination and bearing damage caused by water ingress and from condensation. This low cost improvement can provide considerable maintenance and productivity benefits.