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Increasing Accuracy in Lubrication Testing

One of the most common procedures for maintaining plant equipment and increasing its reliability and usefulness is lubrication analysis. Reading the results of regular lubrication analysis can help plant engineers and maintenance staff schedule when oil changes are required on each machine based on findings that indicate the presence of abrasives, oxidation, and/or lubricant breakdown.

However, if the results are inaccurate, they may lead maintenance technicians to take inappropriate actions. To increase accuracy in sampling, preparation and consistency in testing are key.

Preparation of equipment

Because the results of lubrication analysis will be only as good as the oil sample itself, the sample must contain a representative selection of wear particles found inside the machine. The concern here is that in any lubrication system, wear particles and contaminants usually are not distributed evenly.

Uneven distribution is especially true of particles larger than a few micrometers. While these particles tend to be more easily removed from the lubricant through filtration, they can settle after being separated out. They often settle in pipe nipples and valves where, over time, they may become oxidized or otherwise chemically changed.

If sample lines and valves are not flushed properly, large numbers of these old particles will find their way into oil samples, yielding invalid results.

Once proper flushing procedures are followed, the concentration and size of wear particles can reveal considerable information about the condition of lubricated wearing surfaces inside a machine. As particle concentration and size increase, the wear process progresses from normal operating condition to incipient failure and finally to catastrophic failure.

Common Wear Particles and Possible Causes

Testing tips

The most important key to lubrication analysis is consistency. Results from randomly sampled oil can provide misleading results, often pointing to operating conditions that do not really exist. In the long run, this can lead to costly equipment failure for reasons that could have been detected through proper testing well before the problem reached critical mass.

A widely held misconception among plant personnel is that consistency simply means the lubricant should always be taken from the same place in the lubricating system. While this is true, it is only a small part of the process.

The following tips also lead to ideal testing conditions:

Tip one: The sample should be taken immediately downstream from lubricated surfaces. For example, take a sample from a drain line off an individual bearing prior to going through the filter. At worst, take the sample between the pump and the filter.

Tip two: Take the sample while the equipment is operating under normal conditions and temperatures. If the machine has been turned off, the sample should be taken within 10 minutes of turning it off.

Tip three: As mentioned earlier, take the sample at the same location each time, using the same procedures.

Tip four: For machines that have frequent oil changes, samples should be taken at the same time intervals after each oil change.

Tip five: In conjunction with the previous tip, do not sample immediately after each oil change or right after the addition of new fluid. New oil has not yet had the chance to make multiple circuits through the machine, preventing it from being subjected to the machine's operating conditions.

Tip six: To ensure that the sample taken is representative of the oil in the machine, make sure any oil that has been stagnant in the drain line has been cleaned out prior to taking the new sample.

Tip seven: Samples should always be taken in clean, nonmetallic containers and sealed before being sent for analysis. Oil should be analyzed within 48 hours of being sampled.
Admittedly, it can be difficult to achieve all of these conditions. Nevertheless, more consistent sample-taking equals more consistent results, and accurate results make it much easier to determine and predict equipment status.

Lubrication testing is an essential component of any preventive maintenance system. Only by regularly flushing sample lines and drawing those samples as consistently as possible can the results be of real value.

Anne Spano is director of customer training at Applied Industrial Technologies,

One Applied Plaza, Cleveland, OH 44115; (216) 426-4406;

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