PRECISION CONTAMINATION CONTROL

Water contamination reacts with many lubricant additives, reducing their effectiveness and, in some cases, producing hydrogen sulphide and sulfuric acid. Water also reacts with metals to produce oxidising agents that attack the base oil. How water affects the lubricant's film strength is even less obvious and potentially more harmful. As previously discussed, viscosity is the lubricant's most important property because it determines film thickness.

The 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.

Particle Contamination

The lubricant provides a blood cell-sized separation between moving surfaces. If it's not present in contacts in relative sliding motion, surface-to-surface contact and abrasion (two-body) occur—the lack of a lubrication film results in surface fatigue in rolling contacts. When clearance-sized and larger particles are present in sliding contacts, abrasion (three-body) occurs even when the film is separated.

The particles in oil and grease act like 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 small point or line contacts. The momentary load is highly concentrated – in the hundreds of thousands of pounds per square inch – and the lubricant film - is tiny – rarely exceeding half the diameter of a red blood cell.

A complex 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 average 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, releasing the damaged material. This is called pitting wear. The surrounding material is damaged and dented and may lift away from the surface over time – 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 to 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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