WATER AND BALL BEARINGS W014
Moisture can lead to the loss of oil additives
- Water aids in the depletion of antioxidants, but it also cripples or diminishes the performance of many other 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 oilвЂ™s acid number (AN).
It will cause aeration and the formation of foam.
- Water lowers oilвЂ™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.
Corrosion will increase.
- 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 oil films that give bearing lubricants film strength to control contact fatigue and wear.
- Static etching and fretting are also accelerated by free water.
It will accelerate hydrogen-induced fractures.
- Often called hydrogen embrittlement or blistering.
- The sources of the hydrogen can be water, 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. High tensile-strength steels are at greatest risk.
- Sulfur from additives, mineral oils and environmental hydrogen sulfide may accelerate the progress of the fracture. Risk is posed by both soluble and free water.
Oxidation will increase.
- Many bearings have only a limited volume of lubricant and, therefore, very little 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.
It will restrict oil flow through critical clearances
- Water is highly polar, and as such, can attract oil impurities that are also polar (oxides, dead additives, particles, carbon fines and resin) 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 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.
Water will reduce film strength.
- Rolling element bearings depend on oilвЂ™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 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.
Water will accelerate 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.
Wash down hoses, packing leakage and high humidity are the main sources of water entering the bearings.
- 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.
WhatвЂ™s the answer to this water problem?
- Replace grease or lip seals with labyrinth seals or positive face seals.
- Try not to aim wash-down hoses at the bearing case and bearing housing vent.