Precision on controlling contamination is important for lubricants to do their job. A contaminant is anything that’s in the lubricant but shouldn’t be. Water and particles are the most common, so we’ll look at them and their effects in more detail.
An obvious risk of water contamination is rust and corrosion on metal surfaces. Less obvious is hydrolysis.
Water contamination reacts with many lubricant additives, reducing their effectiveness and, in some cases, producing hydrogen sulfide and sulfuric acid. Water also reacts with metals to produce oxidizing agents that attack the base oil. Even less obvious, and potentially more deleterious, is how water affects the lubricant’s film strength. As previously discussed, viscosity is the lubricants most important property because it determines film thickness.
Oil possesses a physical property whereby viscosity increases as a function of pressure. The higher the load, the higher the viscosity and the film thickness. Water does not possess this same physical property, so when it’s present, the viscosity-pressure relationship in the oil is compromised, which decreases film strength and increases the likelihood of surface to surface contact.
Water is particularly damaging in rolling contacts, where the load forces are very high – in the hundreds of thousands of pounds per square inch. Rolling element bearings, for example, depend on the viscosity-pressure relationship in oil to protect components. Water contamination can increase wear rates by as much as 40 times. Target levels for moisture should range between 100 and 300 ppm or better for most applications. Except for rare cases, water contamination should never be allowed to exceed 500 ppm.
The lubricant provides a blood cell-sized separation between moving surfaces. If it’s not present in contacts that are in relative sliding motion, surface to surface contact and abrasion (two-body) occur. In rolling contacts, the lack of a lubrication film results in surface fatigue. When clearance-sized and larger particles are present in sliding contacts, abrasion (three-body) occurs even when there is film separation.
The particles in oil and grease act like the bits of grit on sandpaper to wear away surfaces. In rolling contacts, the process is somewhat more complex. Rolling contacts (e.g. rolling element bearings) transfer load via very small point or line contacts. The momentary load is extremely concentrated – in the hundreds of thousands of pounds per square inch – and the lubricant film - is very small – rarely exceeding half the diameter of a red blood cell.
A hard particle can bridge the gap provided by the lubricant film and transfer the load to the component surfaces, often concentrating it further. If a 250,000 psi normal load is transferred via a particle to an area one-tenth the normal area, the resulting load is 2.5 Million psi. This extreme load typically exceeds the fatigue limit of the metal and produces subsurface cracking.
Over time, the cracks propagate (grow) to the surface and the damage material is released. This is called pitting wear. The surrounding material is damaged and dented and overtime may lift away from the surface – a wear mechanism called spalling. Particles are involved in an estimated 80-90% of all wear, though other forcing functions like vibration, water contamination and insufficient lubrication contribute and influence the rate at which this occurs.
For most applications, cleanliness should be maintained to ISO 4406 15/12/9 to 19/16/13, depending upon the criticality of the application and the machine’s sensitivity to particle contamination. Contamination should never be allowed to exceed ISO 4406 21/18/15.
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