A New Technique for Filter Debris Analysis

Surapol Raadnui

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 1. Typical PST for Solid Debris Separation

A Brief Introduction

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 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 3. Particle Separating Tube (PST) for FDA

Figure 4. Put the Sample into the PST

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 5. Typical Ultrasonic Washing Machine

Figure 6. Insertion of the PSTs into the Fixture

Figure 7. Inside of Washing Machine After the PSTs are Put in Place

Figure 8. Utilization of Ultrasonic Washing Machine

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 9. Filtersonicgram Slides Have Been Prepared

Figure 10. Typical Filtergram Slides


The work reported here has been funded by the Thailand Research Fund (www.trf.or.th).


  1. System and Method for Filter Debris Analysis, International Patent Application (PCT) Number PCT/SG2009/000465, date of filing December 3, 2009.
  2. An Apparatus and Method for Particle Analysis, International Patent Application (PCT) Number PCT/SG2009/000264, date of filing July 27, 2009.
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