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1 September 2020
Differences Between Normal Wear, Abnormal Wear, and Sudden Failure

Used oil analysis (UOA) is the most-effective tool for identifying wear within oil lubricated machinery. When fluids are sampled over consistent intervals, UOA provides a glimpse into the inner workings of the machine, allowing one to trend the development of wear-metal particles and the condition of the oil.


All equipment experiences some amount of wear while in use and all equipment has a life span. Even the most well-designed and manufactured machines that are maintained under the best practices will, at some point, cease to be operational… unfortunately, normal wear is going to eventually win the battle.


However, wear can also be abnormal or accelerated due to many causes… from incorrect assembly or installation, to misuse or abuse, to poor maintenance, as well as other causes. Whether the rate of wear is normal or abnormal, the ability to trend it using UOA allows one to take action to prevent failure, extend the life of the equipment, or to plan unavoidable repairs. Avoiding the unanticipated, unplanned failures and associated downtime that comes with them is a primary reason for implementing a UOA program. 

However, unanticipated, unplanned failures do sometimes occur… even with the best UOA program. This can lead to frustration and questions as to why UOA didn’t catch the impending failure before it occurred. There are several reasons why this can happen, but fundamental among them is the difference between a failure resulting from a long-term (or even a short-term) wear condition and a sudden failure, such as the fracture of a gear or the breaking of a shaft. 

In order to understand these differences, a basic understanding of the technology used to identify wear particles in UOA is needed. A type of spectroscopy is used to excite certain elements if they are present in the oil. There are a few different types of spectroscopy used in the UOA industry, but all of them are focused on finding very small particle sizes… typically less than 10 microns in size (1 micron is equal to 1 millionth of a meter). Particles larger than this are invisible to this technology. 

Certain failures can occur without emitting enough particles of the size that would be identified by spectroscopy… usually because of the size of the particles being too large. For instance, when a gear fails it often results in a sudden release of several very large pieces that can often get wedged between other gear teeth, which can cause further damage. 

Prior to the original failure, there may have been no release of sub-10-micron wear particles that could be identified using UOA. Customers sometimes complain that the oil sample they pulled and sent in for analysis had plenty of visible metal in it, so they ask why the sample results look normal. This question indicates a lack of understanding of how the UOA technology works, because any particles that you can see in the oil are too large to be visible to the spectrograph. As a side note, there are additional tests that can be performed to identify larger particles in a sample; thus, if you find an oil sample that has large, clearly visible particles in it, you should ask the lab to complete additional testing for that sample.

Differences Between Normal Wear, Abnormal Wear, and Sudden Failure

Another issue is that these types of failures can occur in between oil sample intervals. For example, if you pull a sample from a component at the normal interval and the sample report shows nothing abnormal, the machine goes back to work. However, before the next oil sample interval arrives, something may happen to cause a weakened part to fail catastrophically. In this example, maybe a crack that had already started in the gear finally reached the point where it failed completely. 

There are many possible failure scenarios, so it’s impossible to list all of them. The takeaway is that sudden failures can and do occur to components that do not involve a prolonged process of wear, so these types of occurrences can be missed completely by UOA analysis. 

So, you may be thinking that if this is true, then why bother with a UOA program at all? The right response is that these types of failures are the exception, not the rule. The most common failure mode is one that occurs over a longer period and involves generating wear particles that can be identified by UOA. 

UOA is still the most effective tool to identify potential or impending failure, even if it is not perfect and even if there are failure modes that exist that it cannot identify. Besides, there are plenty of other reasons to employ a UOA program, such as the monitoring of contaminants like dirt, fuel dilution, water, coolant, as well as monitoring the viscosity, oxidation or nitration levels, and acid or base levels of a lubricating fluid. 

Used oil analysis is a necessary component in any maintenance program that is focused on achieving best practices, reducing costs, and extending the life of equipment. 

Castrol® has a team of experienced, field-based lubrication engineers who can help you to better monitor wear in your equipment using the Labcheck® used oil analysis program. The team can also help you to identify the right product for each application, identify opportunities to improve your maintenance practices, and reduce your operational costs. Please contact your Castrol sales representative for assistance. 

Important Considerations When Cross-Referencing Lubricants