CHAPTER 1–INTRODUCTION TO Wear debris analysis
1.1 Introduction:-
The technique of Wear Debris Analysis (Analytical Ferrography) is gaining popularity in the field of Condition Based Maintenance. WDA is a method of predicting the health of an equipment in a non-intrusive way, by the study of wear particles. The continuous trending of wear rate monitors the performance of Machine / Machine components and provides early warning and diagnosis. Oil condition monitoring can sense danger earlier than Vibration technique. This technique holds good for both oil and grease samples.
Analytical Ferrography with supporting physical and chemical tests can determine the following
/ The start of abnormal wear / / The components which are wearing
/ Root cause of wear/failure / / Usability of lubricant beyond its rated life
The Software developed to measure the MACHINE CONDITION INDEX ( MCI™) through Ferrography analysis for predicting the wear status of machine is a unique achievement of its own.
Some Typical Ferrogram
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FATIGUE WEAR ALONG WITH NORMAL RUBBING WEAR / SPHERICAL PARTICLES FROM A/F BRG
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SEVERE SLIDING WEAR / FATIGUE CHUNK FROM GEAR
Figure 1.1 Some typical ferrograms

1.2 Wear Particle Analysis or Ferrography:-

Ferrography is a technique that provides microscopic examination and analysis of wear particles separated from all type of fluids. Developed in the mid 1970’s as a predictive maintenance technique, it was initially used to magnetically precipitate ferrous wear particles from lubricating oils.

 This technique was used successfully to monitor the condition of military aircraft engines, gearboxes, and transmissions. That success has prompted the development of other applications, including modification of the method to precipitate non-magnetic particles from lubricants, quantifying wear particles on a glass substrate (Ferrogram) and the refinement of our grease solvent utilized in heavy industry today.

 Three of the major types of equipment used in wear particle analysis are the Direct-Reading (DR) Ferrograph, the Analytical Ferrograph System and the Ferrogram Scanner.

CHAPTER 2 - Wear Debris Analysis methods

2.1 Various methods:-

As a supplement to oil analysis, ALS Tribology Division offers wear debris analysis services. There are several analysis methods available.

2.1.1 Analytical Ferrography:-

Analytical Ferrography utilizes a skilled analyst examining a prepared ferrogram slide with a computer-aided microscope to identify the composition of the material present in a used lubricating oil sample.

Wear material and other debris suspended in a lubricant is deposited and separated onto a ferrogram slide maker. The sample is diluted to improve particle separation onto the ferrogram slide. Magnetic separation of wear material from the lubricating fluid attracts ferrous particles out of the oil onto the ferrogram slide maker. Though the method is biased to ferrous material, other nonferrous wear particle and contaminants are also captured and identified. The slide is examined under a microscope to distinguish composition, morphology, particle size, and relative concentration of the ferrous and non-ferrous wear particles. Treatment of the ferrogram with heating and chemicals will further distinguish identification of the metallurgical composition of the wear material.

The skilled analyst performs the analytical ferrography to provide a root cause for wear mechanisms based on the morphology and composition of the particles. The analyst will report material composition and wear morphology that will include, but is not limited to:

  • Ferrous wear particles
  • High alloy steel
  • Low alloy steel
  • Dark metallic oxides and cast iron
  • Red oxides (rust)
  • White nonferrous metal particles
  • Yellow metals wear particles
  • Contaminants, dirt (silica), fibers and other particulates
  • Fatigue wear
  • Sliding wear
  • Cutting wear - abrasive wear
  • Adhesive wear
  • Corrosive wear

2.1.2 Filter Patch Test (FPT, filtergram or patch test)

A common method for making a detailed determination of wear occurrence, especially for non-ferrous materials, is to employ a Filter Patch Test examination using a microscope for wear particle analysis. A measured portion of used oil is filtered through a filter patch. Trapped wear particles and debris are then visually examined microscopically for a qualitative report. Observation will generally be accompanied by a photo of the filtered wear material on a test report. The debris is assessed and the particles graded. The FPT can tell us a number of things:

  • Is there abnormal wear taking place?
  • Is the wear ferrous or non-ferrous?
  • Is there any evidence of abrasive contaminants e.g. dirt?

2.1.3 LaserNet Fines

Some of our ALS Tribology laboratories employ Lasernet Fines instrumentation, which was developed by Lockheed Martin with the Naval Research Laboratory for military application. Using direct digital imaging Lasernet Fines, test results classify particles larger than 20 micron into cutting wear, severe sliding wear, fatigue wear, and nonmetallic material. The analysis economically combines features of particle count determination with quantifying wear particle classification for industrial, gear and drivetrain components without subjective interpretation.

The test data complements other wear analysis techniques by using laser imaging and advanced image processing software to identify and measure:

  • Type of wear mechanism
  • Rate and severity of wear processes
  • Wear particle size distribution
  • Particulate contamination and oil cleanliness

2.1.4 Particle Quantifier Index (PQI)

The Particle Quantifier is a magnetometer that measures the mass of ferrous wear debris in a sample and displays this as a PQ Index. Test results are quantitated as a relative number of ferrous material within a sample; this can then be trended for useful wear monitoring. PQI is a simple, cost-effective test that can easily be incorporated into routine trending analysis.

Chapter 3 - Wear Particles

3.1 Types of wear partical:-

There is six basics wear particle types generated through the wear process. These include ferrous and nonferrous particles which comprise of:

3.1.1. Normal Rubbing Wear:

Normal-rubbing wear particles are generated as the result of normal sliding wear in a machine and result from exfoliation of parts of the shear mixed layer. Rubbing wear particles consist of flat platelets, generally 5 microns or smaller, although they may range up to 15 microns depending on equipment application. There should be little or no visible texturing of the surface and the thickness should be one micron or less.

3.1.2. Cutting Wear Particles:

Cutting wear particles are generated as a result of one surface penetrating another. There are two ways of generating this effect.

 A relatively hard component can become misaligned or fractured, resulting in hard sharp edge penetrating a softer surface. Particles generated this way is generally coarse and large, averaging 2 to 5 microns wide and 25 microns to 100 microns long.

 Hard abrasive particles in the lubrication system, either as contaminants such as sand or wear debris from another part of the system, may become embedded in a soft wear surface (two body abrasion) such as a lead/tin alloy bearing. The abrasive particles protrude from the soft surface and penetrate the opposing wear surface. The maximum size of cutting wear particles generated in this way is proportional to the size of the abrasive particles in the lubricant. Very fine wire-like particles can be generated with thickness as low as .25 microns.Occasionallysmall particles, about 5 microns long by 25 microns thick, may be generated due to the presence of hard inclusions in one of the wearing surfaces.

 Cutting wear particles are abnormal. Their presence and quantity should be carefully monitored. If the majority of cutting wear particles in a system are around a few micrometers long and a fraction of a micrometer wide, the presence of particulate contaminants should be suspected. If a system shows increased quantities of large (50 micrometers long) cutting wear particles, a component failure is potentially imminent.

3.1.3. Spherical Particles:

These particles are generated in the bearing cracks. If generated, their presence gives an improved warning of impending trouble as they are detectable before any actual spalling occurs. Rolling bearing fatigue is not the only source of spherical metallic particles. They are known to be generated by cavitation erosion and more importantly by welding or grinding processes. Spheres produced in fatigue cracks may be differentiated from those produced by other mechanisms through their size distribution. Rolling fatigue generates few spheres over 5 microns in diameter while the spheres generated by welding, grinding, and erosion are frequently over 10 microns in diameter.

3.1.4. Severe Sliding:

Severe sliding wear particles are identified by parallel striations on their surfaces. They are generally larger than 15 microns, with the length-to-with thickness ratio falling between 5 and 30 microns. Severe sliding wear particles sometimes show evidence of temper colors, which may change the appearance of the particle after heat treatment.

Figure.3.1: Severe Sliding Wear

3.1.5. Bearing Wear Particle:

These distinct particle types have been associated with rolling bearing fatigue:

Fatigue Spall Particles constitute actual removal from the metal surface when a pit or a crack is propagated. These particles reach a maximum size of 100 microns during the microspalling process. Fatigue Spalls are generally are flat with a major dimensions-to-thickness ratio of 10 to 1. They have a smooth surface and a random, irregularly shape circumference.

Laminar Particles are very thin free metal particles with frequent occurrence of holes. They range between 20 and 50 microns in major dimension with a thickness ratio of 30:1. These particles are formed by the passage of a wear particle through a rolling contact. Laminar particles may be generated throughout the life of a bearing, but at the onset of fatigue spalling, the quantity generated increases. An increasing quantity of laminar particles in addition to spherical wear is indicative of rolling-bearing fatigue microcracks.

3.1.6. Gear Wear Two types of wear have been associated with gear wear:

Pitch Line Fatigue Particles from a gear pitch line have much in common with rolling-element bearing fatigue particles. They generally have a smooth surface and are frequently irregularly shaped. Depending on the gear design, the particles usually have a major dimension-to-thickness ratio between 4:1 and 10:1. The chunkier particle result from tensile stresses on the gear surface causing the fatigue cracks to propagate deeper into the gear tooth prior to spalling.

Scuffing or Scoring Particles is caused by too high a load and/or speed. The particles tend to have a rough surface and jagged circumference. Even small particles may be discerned from rubbing wear by these characteristics. Some of the large particles have striations on their surface indicating a sliding contact. Because of the thermal nature of scuffing, quantities of oxide are usually present and some of the particles may show evidence of partial oxidation, that is, tan or blue temper colors.

Many other particle types are also present and generally describe particle morphology or origin such as chunk, black oxide, red oxide, corrosive, etc. In addition to ferrous and non-ferrous, contaminant particles can also be present and may include: Sand and Dirt, Fibers, Friction polymers, and Contaminant spheres.

CHAPTER - 4 A New Technique for Filter Debris Analysis

4.1 Introduction

op of Form

  • Due to the increasing fineness of filter elements in high-precision machinery lubricating oil systems, monitoring of filter debris analysis (FDA) is gaining increased significance for the early failure detection of moving parts. These considerations led to the development of a new method to recover filtered debris particles efficiently, productively and economically.

Figure.4.1: Typical PST for Solid Debris Separation

Methods for detecting damage to rotating components in high-precision machinery lubricating systems operate on the determination of types, size, shape and concentration of wear particles in the lubricating oil. Detecting still relies on an oil sample. Apart from the oil sampling technique, however, FDA is increasingly growing in acceptance. Filter inspection is a method of long standing, where the chance of detecting damage varies with the method used to recover the particles from a filter element specimen. FDA, in general, can therefore be thought of as consisting of three discrete steps: removal and cleaning of the oil filter, recovery of the removed debris, and examination of the debris. Typically, cleaning of the used oil filter is accomplished by immersing the filter in a suitable solvent and removing entrapped debris by ultrasonic agitation and/or air pulsation.

Figure.4.2: A Filter Element Specimen

Major drawbacks of conventional FDA are: particle stacking gives an erroneous result, and the method is a fairly cumbersome, time-consuming process. A new FDA approach is proposed in this article. A special particle separating tube (PST) is introduced. Figure 1 shows a typical PST; the component also can be used for separation of solid particles from used lubricants1,2.

Figure.4.3: Particle Separating Tube (PST) for FDA

Figure.4.4: Put the Sample into the PST

4.2 Filtersonicgram Maker Procedures

Here is a step-by-step walkthrough of the process.

  1. Collect a used oil filter (i.e. hydraulic, turbine, engine).
  2. Remove the filter housing with a suitable tool. (Do not use a hacksaw to open up the housing as the metal saw dust will have a significant effect in the solid debris analysis stage.)
  3. Cut part of the whole filter element as a “specimen” (Figure 2).
  4. Put the filter element specimen into the top chamber of the PST unit (Figure 3).
  5. Pour proprietary solvent into the PST until the filter element specimen is submerged under the solvent (Figure 4).
  6. Put the PST(s) into the fixture inside the ultrasonic washing machine (Figure 5).
    A set of PSTs can be used to extract solid particles in multiple samples simultaneously (Figure 6).
  7. The samples are now ready to be “washed” inside the ultrasonic washing machine (Figure 7).
  8. Operate the washing machine, which has an intensity of the “ultrasonic wave” approximately at 42 kilohertz for an “appropriate” duration, which depends on the type of filters – i.e. engine oil filter, hydraulic oil filter, turbine oil filter, etc. (Figure 8).
  9. Switch off the washing machine and take the PSTs out of the unit.
  10. Up to this stage, the solid particles have been extracted from the used filter element and also have been classified as per their sizes.
  11. Remove the drain plug to get rid of the unwanted solvent (Figure 9).
  12. Disconnect each section of the PST and remove the “patches” which are now ready to be analyzed under an optical microscope or similar device for debris classification and identification by: size; color; shape; edge detail; thickness ratio; surface texture; response to light (reflected or transmitted light); and response to heat ( the “wire mesh” can be used as a filter patch which can be heated up to certain temperatures, depending on the wire mesh materials). This process can be used to identify fiber, elastomer and alloy composition (i.e. copper, aluminum, tin, lead). Sample slides are shown in Figure 10.
  13. The patch also can be weighed, which can be used to quantify the extracted debris due to their size ranges.
  14. Debris morphology can be done in a more comfortable manner as the particle-stacking problem in the conventional “filtergram” technique (by the conventional vacuum filtration technique) is partly solved.
  15. The wire mesh patch may be reused, if needed.

Figure.4.5: Typical Ultrasonic Washing Machine

Figure.4.6: Insertion of the PSTs into the Fixture

Figure.4.7: Inside of Washing Machine After the PSTs are Put in Place

Figure.4.8: Utilization of Ultrasonic Washing Machine

4.3 A Unique Assessment and Examination Tool

“Filtersonicgram” is a novel method to recover solid particles trapped in filter elements with the simultaneous utilization of ultrasonic wave and a conventional filtration approach. The recovered particles on the multi-patch filters can be assessed with the aid of a microscope or other device. Careful examination of the debris morphology can give specific information about the condition of the moving parts of precision machine elements from which they were generated, and the wear mode and/or wear mechanism in operation in the system from which they were filtered. This technique is at present being tested in the field and it is the field operators who will judge the efficacy of solid debris separation and examination by this technique.

Figure.4.9:. Filtersonicgram Slides Have Been Prepared

Figure.4.10:. Typical Filtergram Slides

CHAPTER – 5 An SEM Approach to Wear Debris Analysis

5.1 Introduction – The Scanning Electron Microscope (SEM)

The SEM isfundamentally an imaging tool, which uses electrons instead of light in order to createhighly magnified images. The use of an electron microscope has several advantages overthe optical microscope. In the first place, the SEM can provide magnifications far beyondthe capability of a conventional microscope and the images have much better depth-of fieldat high magnification. In addition, the interaction of the electron beam with thespecimen causes the sample to emit highly localized signals, such as x-ray photons,which can be monitored with specialized detectors. The energy or wavelengths of thesex-rays indicate the elemental composition at the focal point of the beam.

The SEM can be especially useful for wear particle studies due to its specificity – that is,its ability to characterize a particle population while retaining the distinct characteristicsof each particle analyzed. In this way, the size, shape, morphology, and elementalconstituents of each particle can be reviewed and can be used for making decisions basedon the data generated. When evaluating the trade-offs of using SEM versus conventionalwear particle analysis, this specificity must be weighed against the speed and cost of thelatter techniques.

5.2 The SEM as Particle Analyzer

Historically, one would perform particle analysis byplacing a sample in the SEM chamber, and then sequentially observing fields-of-view at amagnification sufficient to see particles of interest. The operator would then zoom up oneach feature and place the beam on the sample to collect an x-ray “spectrum” to identifythe elements. He or she would then tally that information, perhaps take a photograph, andthen move on to the next particle. Clearly, this process is slow, tedious, and error-prone,especially as the operator becomes fatigued.