Demystifying Oil Analysis

Jeremie Edwards, Condition Monitoring International
Tags: oil analysis

Oil analysis is a touchy subject for some people. I have been to places where reading an oil analysis report was viewed about the same as reading tea leaves; other places treat oil analysis like a religion. The true nature of oil analysis is somewhere in between. It is one of the many tools that reliability folks should be familiar with since it is meant to be a window into how our equipment is wearing, what type of stuff is getting into our equipment and if any of the contamination detected is hurting the machine or the oil.

A lot of the places I go to are doing some type of oil analysis, or at least attempting it. I often find that samples are being pulled from equipment and getting shipped out for analysis, but nothing is being done with the results. One recent site visit revealed that only one piece of equipment was having oil samples pulled, but nobody knew who was pulling the sample, where the sample was pulled from, who was getting the results or anything else. It is hard to say you are performing oil analysis if you don’t know who is doing it or how it is being done. Maybe it’s time to demystify oil analysis a bit.

First things first: before we can even think about what we want to learn from our oil, we need to find out what our oil is supposed to look like. So, when oil first shows up on-site, make sure you pull a sample and run a couple of tests. Since we don’t necessarily know what kind of information we will be looking for in the future, it is a good idea to run a few different tests. Fourier Transform Infrared Spectroscopy (FTIR) and Elemental Spectroscopy are great places to start. The results from these two tests will set you up for success later on when you’re trying to figure out what is going on in your equipment.

Since we now have that initial sample, also known as our “baseline” sample, we can look at what we want to learn from our oil. There are three major categories that people typically think about when it comes to oil analysis:

  1. The condition of the oil.
  2. The contaminants in the oil.
  3. The wear taking place in the machine.

Whether or not you need to know all three things from your oil is a bit of a complicated question. Let’s say that you have a very small gearbox that only holds around a quart of oil. Oil analysis may or may not be warranted on this gearbox; it all depends on how critical this little gearbox is to operations. If this gearbox could shut down an entire facility by failing, then it is probably a good idea to do some analysis on it. For argument’s sake, let’s say that this gearbox has the power to shut the whole place down if it goes out.

So, what type of oil analysis should we do on it?

Well, the oil volume isn’t large, so the cost of an oil change shouldn’t be too bad. This means that information about the oil’s condition may not be high on our priority list.

How about contaminants?

Certainly, solid particles would be of interest. These are the ones that cause a lot of mechanical wear with three-body abrasion. Something as routine as a particle count would be a good idea, even with this small volume of lubricant.

Machine condition?

Absolutely! Machine condition would be the number one reason to perform oil analysis on this gearbox. Even if you don’t care about the condition of the oil or have little thoughts as to the contaminants getting in there; if this gearbox can shut the entire place down, you definitely want to know what kind of wear is taking place inside of it (not to mention how much wear).

Now that we know the type of information we want to gain, what tests should we run? Since we want to know more about the wear of the machine components, we need to look for what those components are made from. Firstly, ferrous density is going to tell us the concentration of ferrous metals. This is an excellent resource since components are typically made with such metals.

A step further is analytical ferrography. Analytical ferrography can be done in a lab with a technician looking at the actual wear metal’s particles. These technicians are highly trained and can offer insight as to what type of wear is taking place - an extremely handy tool for figuring out how healthy the machine is.

Last but not least is elemental spectroscopy. Unlike FTIR (which looks for compounds), elemental spectroscopy looks for elements (or the building blocks). This can be a fantastic tool, especially if you are familiar with the types of metals that exist within your equipment, particularly in the given components (like the bearing material, if there are babbitt materials). If I know that a particular gearbox is a worm drive and I see signs of copper, lead or bronze on my elemental analysis report, I know where those elements are coming from. This goes a long way in telling me how much wear has taken place.

Now let's move on to a large hydraulic reservoir. This system isn’t mission-critical; no catastrophic failures will be caused if this system goes down, but the system holds a lot of high-dollar fluid in it. My focus switches a bit: I am no longer quite as interested in the wear metals as I am in the health of the oil. I mean, if it costs thousands of dollars just to change the oil in this system, I should probably try to get as much life out of that oil as possible. So, what information do we want from this system?

Since the fluid is the part we are worried about, we need to look at the physical and chemical properties of the oil. We need to ask ourselves:

What is the most important property of a hydraulic fluid?

The viscosity of the fluid. Since the fluid is doing the work, we need to make sure that it is still the right viscosity.

What are some of the things that influence viscosity?

Acid/base number influences viscosity, so it’s a good idea to keep tabs on these. Oxidation also influences viscosity; this is where that FTIR comes in handy.

What other desirable things are in the oil?

Our additives. These typically show themselves in either the FTIR or Elemental Spectroscopy. Good thing we thought ahead and got that baseline of the oil as it came in, right?

How about all of the outside factors? We know about the debris that gets generated within the machine from the work it does. We know about the health of the lubricant, the chemical and physical properties. The one thing we haven’t touched on a whole lot is the stuff getting into the oil/equipment that we aren’t putting in there on purpose: the contaminants.

The contaminants are going to affect both the oil and the equipment, and none of it in a good way. So we need to look at what types of contaminants are being introduced to our equipment/lubricants.

Does this piece of equipment breath through the headspace (does the oil level fluctuate depending on cylinder movement or operating conditions)? If it does, at the very least we need to look at particles and probably moisture as well. The cleaner and drier the oil, the longer it will last, and we have a lot lower likelihood of having early failures. Particle counts and moisture levels are key, and both are easily detected by and commonly included with FTIR and elemental spectroscopy. (Seriously, those two tests are staples in oil analysis for a reason.)

Maybe you want to get hands-on with testing for contaminants, and you are trying to go the extra distance. Patch testing is a fairly simple concept and lets you look at the contaminants yourself so you can see exactly what is going on.

Now, if you are looking at motor oils, you might have a bit of a different concern, especially with older equipment that might have a bit of wear but a lot of life left. Usually, one of the biggest worries is blow-by or getting a bit of fuel into your oil. This would typically show itself as a change of viscosity or a change in aroma. Something that could be a bit more worrisome is the flash point. Depending on the application, this could be a safety concern as well.

Keep in mind that this is not an exhaustive list of tests available but more of an introduction and an explanation as to why one test might be more relevant to your situation. The real takeaway should be the answer to this question: “What do I want to learn and what is important for this piece of equipment?”