From "Bench Testing of Industrial Fluid Lubrication and Wear Properties Used in Machinery Applications"

The antiscuffing or load-carrying performance of a lubricant is obtained by introducing additives that become active only under “stressed” conditions. In general, a hypoid gear oil will have both the extreme pressure type and antiwear type additives whereas crankcase lubricants will be based on the multifunctional ZDDP antiwear/antioxidant additive. The anti-wear additives react at lower temperatures than extreme pressure agents and ensure that the lubricant provides protection over a wide temperature range.

The test equipment and procedures developed for assessing these lubricants have always assumed that it is necessary to reproduce the highly stressed conditions of load and speed. A common feature of the procedures is that they push the lubricated contact to a failure point and so claim to make a measurement of the film rupture strength or load carrying capacity.

While this approach helps ensure that lubricants are fit for purpose from a quality assurance standpoint, they provide little insight into the specific strengths or weaknesses of competing formulations or offer much useful guidance on how they might be improved. Detailed observations of lubricant behavior are more easily obtained from simple laboratory rig tests (as opposed to gearbox, motored or fire engine tests) but the link with service life needs to be established. Indeed, even industry-standard rig tests have been observed to correlate with neither engine performance nor with each other.

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QUESTION: What type of wear in gears and bearings is controlled by using EP (antiscuff) additives?

Get the answer.

"What happens to the additives in oil over time, and is there any way to replenish them?"

Over time, additives are depleted performing the function for which they were intended, degraded by hydrolysis, mechanical shearing, condensation settling, water washing, particle scrubbing, etc.

The rate of depletion or degradation depends upon the application and the environment. In particular, heat, pressure, shear rate, fuel sulfur, soot, dirt, water, aeration and the presence of catalytic metals (copper, iron, etc.) affect the rate of depletion.

Regarding replenishment - whenever you top-up a system, you are replenishing additives. Likewise, one can perform a partial drain and replacement (often referred to as bleed and feed).

A bleed and feed can work if the base oil is not degraded. If the base oil has been degraded, adding new oil is analogous to sending a healthy person into a room full of sick people with the hope that his or her good health will be contagious - it doesn't work that way. The additives in the new oil might be compromised within the first hours of use, leaving you right back where you started.

Casual addition of additives into a formulated oil can be dangerous and should be avoided. When in doubt, consult your lubricant supplier.

Drew Troyer, Noria Corporation

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