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While most, if not all oil analysis programs include at least some means of detecting active machine wear, all too often problems can be missed or misdiagnosed due to a basic lack of understanding of the strengths and weakness of the test method used. In this edition of Oil Analysis 101, we look at some of the more common test methods used for monitoring and analyzing wear.
Monitoring and controlling problems that lead to active machine wear are critical to an effective oil analysis strategy. For this reason, educated oil analysis users focus their attention on contamination monitoring and control, and on ensuring that the physical and chemical properties of the oil are in good condition. Nevertheless, no matter how effective a proactive lubrication management program might be, at sometime or another, a component will start to show signs of wear. This is where wear analysis comes into play.
Determining a Strategy
When it comes to wear analysis, there are a number of test methods available. From simple tests such as elemental spectroscopy, to sophisticated tests such as complete analytical ferrography, each test has its advantages and limitations when detecting and analyzing active machine wear. For this reason, it’s important that users of oil analysis become familiar with which test is appropriate for specific situations, enabling the selection of the most appropriate test for routine and exception sample analysis.
The most common tests for routine and exception testing are elemental analysis, ferrous density, particle counting, X-ray fluorescence, analytical ferrography and LaserNet FinesTM, the new technique that shows great promise for the future of wear particle analysis.
Elemental analysis, sometimes referred to as spectrochemical analysis or atomic emission spectroscopy, is one of the most basic oil analysis tests. The test measures the concentrations of 15 to 25 atomic elements, including wear metals such as iron, copper, lead and tin; contaminants such as silicon, sodium and potassium; and oil additive elements such as phosphorus, zinc and calcium. Elemental analysis is fundamental to routine oil analysis and was covered in the first Oil Analysis 101 article in the January-February edition of Practicing Oil Analysis magazine.
It is important to understand that both commonly used atomic emission spectrometers, Inductively Coupled Plasma (ICP) and Rotating Disc Electrode (RDE) instruments, suffer from an inability to detect wear debris in excess of 3 to 8 microns in size, depending on instrumentation, and are reliable only for particles smaller than 1. While this is not a major limitation in detecting slow, incipient wear, any system prone to severe sliding wear, adhesive wear or contact fatigue-type problems, such as gears and large rolling element bearings, is likely to generate particles in excess of 10 microns should a wear problem occur. This may result in poor detection sensitivity, or problems going unidentified if elemental analysis is relied upon as the sole source of wear particle detection. For this reason, it is advisable to include additional tests capable of detecting these larger particles when setting up routine test slates for heavily loaded components where fatigue and adhesive wear is possible.
The term ferrous density describes a number of instruments capable of detecting the presence of large (>5 micron) iron or steel particles in an oil sample. While the operation of each type of instrument is beyond the scope of this article, all instruments generally rely on magnetism to either trap ferrous particles or to detect particles directly using the magnetic Hall effect.
Figure 2 illustrates one such test method, referred to as Direct Reading (DR) Ferrography. In this apparatus, the test sample is pumped through a precipitator tube, held on an incline across a magnet. As the sample flows through the tube, large ferrous particles fall out of suspension at one end of the tube and are trapped by the magnet, while smaller ferrous particles remain in suspension until they too precipitate out farther down the tube. Thus, the quantity of debris in two size ranges, approximately equivalent to particles greater-than or less-than 5 microns, can be determined to provide an early warning of an active wear problem. In addition, by looking at the ratio of large (that is ferrous particles >5 micron) to small particles (<5 micron), the severity of an active wear problem can also be gauged and trended over time.
While ferrous density testing should be considered routine for large gearboxes and other components prone to wear mechanisms that produce large quantities of ferrous particles, the test is limited to magnetic particles and may not indicate problems with nonferrous components. Similarly, oxidized ferrous particles (for example, rust), can also be nonmagnetic while still indicative of wear in progress.
While many oil analysis users have come to rely on particle counting to determine fluid cleanliness levels, there is often a misconception that particle counters are always measuring the amount of dirt ingress. ISO particle counting reports the number of particles in three size ranges (>4 micron, >6 micron and >14 micron), without any discrimination between dirt, nonferrous wear, ferrous particles, etc. Nevertheless, ISO particle counting is an excellent tool for determining the onset of an active wear problem.
Particle counting has two major drawbacks when using it for wear debris detection. The first is obviously its inability to differentiate between large wear particles and nonwear particles. The second, unless the system is reasonably clean, with little background noise, it can often be difficult to detect active wear before it becomes serious due to random fluctuations occurring with the system not experiencing wear. Despite these limitations, particle counting has proven to be highly effective at determining the onset of component wear.
X-ray fluorescence (XRF) is a comparatively obscure test method, which is gaining in popularity with the development of instruments capable of online and onsite elemental analysis. The test method is similar to spectrochemical analysis except that instead of measuring elemental concentrations using atomic emission in the visible and UV range, XRF works in the X-ray region of the spectrum. Like X-rays used as a medical diagnostic tool, the higher energy of the X-ray radiation can penetrate into the particles, allowing the detection of larger particles than conventional spectrochemical analysis. To be effective, it is necessary to filter the particles for detection prior to presentation to the spectrometer. This is typically achieved using a filter patch arrangement.
Due to the development of low-cost X-ray sources and detectors, X-ray fluorescence will become an increasingly important tool in used oil analysis.
Cutting Wear on a Ferrogram
(after heat treatment)
Copper from Ring Gear
on a Worm Drive
Surface Fatigue Particle
Corrosive Wear Particles
Red Iron Rust
Figure 3. Common Wear Particles
Complete analytical ferrography is often referred to as the oil analysis equivalent of criminal forensic science. The test method relies on a visual, microscopic evaluation of particles, extracted and deposited on a microscope slide called a ferrogram. Based on an examination of the shape, color, edge detail, the effects of a magnetic field and other diagnostic tests such as heat treatment and the addition of chemical reagents, an assessment of the active wear mechanism can be made. This allows a skilled diagnostician to determine the root cause of a specific tribological problem.
While ferrographic analysis is an excellent tool when attempting to diagnose an active wear problem, it too has its limitations. The test is a qualitative test, which relies on the skill and knowledge of the ferrographic analyst. While this can have definite advantages, the interpretation is somewhat subjective and requires detailed knowledge, not just of analytical chemistry, but also machine and tribological failures. Also, because of the time and skills required to perform the test, it is usually considered too expensive for routine oil analysis. Nevertheless, used as an exception tool when a wear problem is suspected based on other test results, complete ferrographic analysis is one of the most enlightening of all wear analysis methods.
One of the most promising techniques for wear particle analysis to emerge in the past few years is that of the LaserNetTM Fines instrument. Details of this test method can be found on page 40 in this edition of POA. In short, the instrument uses a CCD (closed coupled device) array and image processing software to categorize particle shapes and sizes, defining particles into common wear categories such as cutting wear and severe sliding wear. While this method is similar to analytical ferrography in attempting to categorize particles based on their morphology, it does not suffer from the subjectivity limitation of the analyst’s skill. Because of this, it is likely that LaserNet Fines will become an invaluable field and lab-based instrument in helping to detect active wear.
While oil analysis users should focus on proactive initiatives such as contamination monitoring and control, and maintain tenure of good quality oil in good condition, even the best laid plans sometimes go awry. When this happens, machine wear results. However, by setting up appropriate routine and exception test slates to both detect and analyze the root cause of common machine wear problems, the diligent oil analyst can quickly and effectively recognize the onset of a problem and take corrective action before the problem reaches a critical stage.