From the Lubrication Excellence/Reliability World 2005 Conference Proceedings. An excerpt from the paper "Using Oil Analysis as a Root Cause Analysis Tool".

The term "lubrication failure" is widely abused in industry. It is generally applied to any failure in which the lubricant is suspected. In some cases, it is assigned as a matter of convenience simply because no other cause was readily revealed. Ineffective lubrication often lies at the root of mechanical wear and failure, but one must develop a clearer understanding of lubrication failures and investigate them individually. There is no single definition for lubrication failure, rather multiple possible failures with multiple possible causes. Evaluate each significant failure independently of previous failures, avoiding the temptation to casually apply the scenario from a previous failure to the current one.

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QUESTION: If rust suddenly appears on oil wet machine surfaces, what tests should be performed on the oil?

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"As oil ages, it oxidizes. What is the best indicator that the oil has oxidized too much and should be changed based upon the results given in a typical oil analysis? We analyze the oil from our mixer gearboxes and will typically run the same oil in the gearbox for several years as long as the oil analysis comes back with good results."

The first issue here is to define a "typical" oil analysis. Most labs have several test packages to choose from that may or may not give an overall solid feel for lubricant oxidation. In order to identify lubricant oxidation, it is important to include oil properties monitoring as part of your regular test package. This will allow for the extension of drain intervals based on lubricant condition as well as help to identify adverse operating conditions that may exist.

Common tests that cover lubricant properties include viscosity, neutralization number (acid number for industrial-based lubricants, base number for mobile equipment), and FTIR. The values of the results in all of these tests will increase as the level of oxidation increases (with the exception of base number, which will decrease as the level of acids increase via the oxidation process). A couple of common field tests that can help to indicate possible oxidation are darkening color and foul odor. As with the laboratory tests, it is important that sensory testing is compared to that of a virgin oil base sample.

Oxidation will generally happen at a slow rate under optimum conditions; however, as the health of the lubricant decreases, the rate of degradation will increase. There are several other factors that if not controlled, can lead to increased rates of oxidation. These factors include heat, air, water, and metallic particles.

Using a combination of these tests will assist in pinpointing positive levels of oxidation. Combined with appropriate equipment monitoring tests and contaminant tests, root causes of oxidation can also be found and controlled.

Matt Spurlock, Noria Corporation

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