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Monday, 12 December 2011

How water in Gearbox Lubricant causes bearing Failure

Have you ever given thought to the severe operating environment of an industrial gearbox? Even if you have correctly lubricated the bearing, elevated temperatures, airflow and loads as often have dust and water that enter through breathers and failing seals often take their toll! Water in the gearbox lubricant is perhaps the most common cause of bearing failure, so today we’re going to examine how water in Gearbox lubricant causes bearing failure.

We all know that water is harmful to machinery in general, and bearings in particular. As little as 500ppm of water in gearbox oil can damage machine surfaces directly, through a sequence of events and often with a variety of helpers. In many cases, the most severe damage is as a result of secondary causes. For instance, water may initially lead to premature oxidation of the base oil. When the oxides combine with more water, a corrosive acidic fluid environment exists.

Likewise, oxidation can produce sludgy insolubles thereby increasing oil viscosity. Both processes can impede oil flow and lead to damage of the gearbox bearing. Not to be left out, the water and oxidative environment can aerate the oil, amplifying lubrication problems even further.

1) Corrosion. Rust requires water. Even soluble water can contribute to rust formation. Water gives acids their greatest corrosive potential. Etched and pitted surfaces from corrosion on bearing raceways and rolling elements disrupt the formation of critical elastohydrodynamic (EHD) oil films that give bearing lubricants film strength to control contact fatigue and wear. Static etching and fretting are also accelerated by free water.

2) Oil Flow Restrictions. Water is highly polar, and as such, has the interesting ability to mop up oil impurities that are also polar (oxides, dead additives, particles, carbon fines and resin, for instance) to form sludge balls and emulsions. These amorphous suspensions can enter critical oil ways, glands and orifices that feed bearings of lubricating oil. When the sludge impedes oil flow, the gearbox bearing suffers a starvation condition and failure is imminent. Additionally, filters are short-lived in oil systems loaded with suspended sludge. In subfreezing conditions, free water can form ice crystals which can interfere with oil flow as well.

3) Aeration and Foam. Another interesting aspect of how water in Gerbox lubricant causes bearing failure is that water lowers an oil's interfacial tension (IFT), which can cripple its air-handling ability, leading to aeration and foam. It takes only about 1,000 ppm water to turn your bearing sump into a bubble bath. Air can weaken oil films, increase heat, induce oxidation, cause cavitation and interfere with oil flow; all catastrophic to the bearing. Aeration and foam can also incapacitate the effectiveness of oil slingers/flingers, ring oilers and collar oilers.

4) Hydrogen embrittlement. This failure mode is perhaps more acute and prevalent than most tribologists and bearing manufacturers are aware. Once again water in the gearbox lubricant can be the source of the hydrogen, but also electrolysis and corrosion (aided by water). There is evidence that water is attracted to microscopic fatigue cracks in balls and rollers by capillary forces. Once in contact with the free metal within the fissure, the water breaks down and liberates atomic hydrogen. This causes further crack propagation and fracture of the gearbox bearing. High tensile-strength steels are at greatest risk. Sulfur from additives (extreme pressure (EP), antiwear (AW), etc.), mineral oils and environmental hydrogen sulfide may accelerate the progress of the facture. Risk is posed by both soluble and free water.

5) Oxidation. Many bearings have only a limited volume of lubricant and, therefore, just a scintilla of antioxidant. High temperatures flanked by metal particles and water can consume the antioxidants rapidly and rid the lubricant from the needed oxidative protective environment. The negative consequences of oil oxidation are numerous but include corrosion, sludge, varnish and impaired oil flow to the gearbox bearing.

6) Additive Depletion. We've mentioned that water aids the depletion of antioxidants in the gearbox lubricant, but it also cripples or diminishes the performance of a host of other oil additives. These include AW, EP, rust inhibitors, dispersants, detergents and demulsifying agents. Water can hydrolyze some additives, agglomerate others or simply wash them out of the working fluid into puddles on sump floors. Sulfur-phosphorous EP additives in the presence of water can transform into sulfuric and phosphoric acids, increasing an oil's acid number (AN).

7) Impaired Film Strength. Rolling element bearings depend on an oil's viscosity to create a critical clearance under load. If the loads are too great, speeds are too low or the viscosity is too thin, then the fatigue life of the bearing is shortened. When small globules of water in the gearbox lubricant are pulled into the load zone the clearance is often lost, resulting in bumping or rubbing of the opposing surfaces (rolling element and raceway).

Lubricants normally get stiff under load (referred to as their pressure-viscosity coefficient) which is needed to bear the working load (often greater than 500,000 psi).

However, water's viscosity is only one centistoke and this viscosity remains virtually unchanged, regardless of the load exerted. It is not good at bearing high-pressure loads. This results in collapsed film strength followed by fatigue cracks, pits and spalls. Water can also flash or explode into superheated steam in bearing load zones, which can sharply disrupt oil films and potentially fracture surfaces.

8) Microbial Contamination. Water is a known promoter of microorganisms such as fungi and bacteria. Over time, these can form thick biomass suspensions that can plug filters and interfere with oil flow. Microbial contamination is also corrosive.

9) Water Washing. When grease is contaminated with water, it can soften and flow out of the bearing. Water sprays can also wash the grease directly from the bearing, depending on the grease thickener and conditions.

The obvious solution to the problem is to exclude water from the gearbox lubrication system. This may sound simple but in practice can be very difficult to implement. Here are some common areas of water ingress:
  • Breather systems: Ensure these are clear and have no areas that are immersed in water. Make sure any one way valves are operating correctly.
  • Bearing seals: Inspect regularly for leaks and hardening of the seal. If there’s fluid leaking out, chances are there’s an avenue for water to enter.
  • Machine cleaning: Be very careful when washing the machine – any high pressure streams must not be directed at bearings and breathers, as this can readily contaminate the gearbox lubricant with water.
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Dick Olson said...

hose looks like quality made bearings. Great to look at. babbit bearings

mathew hayden said...

Paper mills uses industrial gearbox. .But their maintenance should be at proper time for smooth running of gearbox and bearing.

Thanks for Sharing..............

Industrial Gearbox said...

Paper mills uses industrial gearbox. .But their maintenance should be at proper time for smooth running of gearbox and bearing.