Does Size Matter in Oil Analysis?

During a recent meeting with a client, I was explaining wear limits for various gearboxes. I casually mentioned, “These gearboxes are very small, so they will naturally have more wear in them than than larger ones.” One of the attendees raised an eyebrow and replied, “I’m not convinced why size should make a difference.” His colleagues burst into laughter, with one of them adding, “He says the same to his wife!” Amidst the chuckles, I quickly regained my composure and replied, “Pardon the puns, but in oil analysis, size really does matter.” It was this that prompted today’s article.

The Role of Surface Area in Wear Analysis

When it comes to oil analysis, size influences wear levels more than many realise. In large systems, the volume of oil to metal is larger. This means the wear particles generated are dispersed over a much larger oil volume, resulting in lower concentrations in oil samples. Smaller systems, on the other hand, have less oil volume but similar or even greater surface area-to-oil ratios. Consequently, wear particles become more concentrated, and alarm limits need adjusting to reflect this disparity.

To illustrate, let’s consider two gearboxes I have done a rough diagram below of:

  • Gearbox A (illustrated on the left): A small system holding 5 litres of oil.
  • Gearbox B (illustrated on the right): A large industrial system containing 500 litres of oil.

Both gearboxes might generate 10 mg of iron wear particles during normal operation. In Gearbox A, that wear equates to 2 mg/L, while in Gearbox B, it’s only 0.02 mg/L. If alarm limits were set identically for both systems, Gearbox A would trigger alarms frequently, despite operating within acceptable wear ranges.

If you pack more metal into a volume of oil then you will have more concentrated wear metals Hence limits should take into consideration the size of the system too Note diagram not to scale and purely to illustrate volume of oil to metal ratios

Understanding Alarm Limits

Alarm limits should account for system size, application, and operational factors. Blanket thresholds often lead to unnecessary maintenance or overlooked issues. Customising limits based on oil volume and machine characteristics ensures reliable results. As a reminder size counts and it’s not just the amount of wear, but the SIZE that’s the key.  This is where modern techniques, such as LubeWear, provide deeper insights.

LubeWear and the Big Picture

Traditional oil analysis methods focus on total wear particle concentration but often overlook particle size distribution and in fact miss most abnormal wear particles. As a result the limitation of many oil analysis samples is it doesn’t detect all the wear present. LubeWear’s rapid acid digestion technique measures dissolved wear metals, highlighting trends often invisible to conventional techniques. Being able to compare small to large particles indicates what’s occurring in the machinery. For instance:

  • Large particles may indicate severe wear or a potential failure mode.
  • Smaller particles might signify normal rubbing wear.

By combining LubeWear with traditional elemental analysis, we can assess not only how much wear is present but also the size and severity of the wear particles so you have the complete insight into the wear process. This is not the case with most labs technologies which in fact miss most abnormal wear. Why? Well read on.

Large Particles in ICP Analysis

Inductively Coupled Plasma (ICP) spectroscopy, a cornerstone of oil analysis, has a limitation: it only detects particles smaller than approximately 5-8 microns. Large particles (>15 microns) are classified as abnormal, which can be crucial indicators of machine health, often go unnoticed unless supplementary methods like ferrography are used. This is another reason why understanding particle size alongside concentration is critical for accurate diagnostics.

1 to 438ppm difference.

An example happened only the other day when I was forwarded a report by another laboratory that had correctly identified magnetic material in the sample when they passed a magnet over it but in the lab data only showed 1ppm of iron. We subsequently tested the same sample that actually had a little less visible debris than the other lab’s sample and we detected 438ppm of iron. Why? Because we measured ALL the iron, not just the small stuff and that makes a big difference when detecting abnormal wear.

It’s Not Just About How Much – It’s About How Big

In oil analysis, size influences everything: from alarm limits to wear particle interpretation. Smaller systems with higher wear concentrations shouldn’t always be a cause for alarm, nor should larger systems with low concentrations always be deemed safe. By considering both the size of the system and the size of the particles, you can make informed decisions about machine health.

The next time someone asks, “Does size matter?” you can confidently answer, “In oil analysis, it absolutely does.” With the right tools such as LubeWear and insights, you’ll ensure every particle tells its story, no matter the size of the system it came from.

If you enjoyed the article and want to find out more about oil analysis then get in touch.