Understanding and trending machine wear is critical for predictive maintenance. Oil analysis is the best routine non-destructive technology (NDT) for detecting surface wear on a wide array of oil lubricated machines like engines, turbines, gearboxes, motors, and compressors. To make an accurate assessment of potential failures, mechanics like to know where the wear debris is coming from, what is causing the wear and how severe it is. The parameters measured using oil analysis that provide those clues are: wear material, particle shape, particle size, and quantity of debris. Knowing what material the debris is made from allows you to trace its origin back to specific components and contaminants. Shape helps identify the root cause, and size and quantity are related to severity. The techniques used to analyze machine wear are a sub category of oil analysis - particle analysis.

There are many different particle analysis techniques. Typically each technique is optimized to measure one of the four parameters previously mentioned. The most common techniques today are Ferrography, patch filter analysis, elemental analysis, particle count, SEM/EDXRF, Ferrous Index/PQ, and LaserNet Fines laser imaging particle analysis. The most deterministic particle analysis tool is the Scanning Electron Microscope with Energy Dispersive X-ray analysis. (SEM/EDX). This technique can identify each particle's material, shape, and size. However, due to the very high cost and low throughout, it is mainly used as a failure analysis tool, instead of a routine oil analysis tool.

Elemental analysis performed with Optical Emission Spectrometers (OES) such as ICP or RDE, provides concentrations of wear metals by element, giving clues of the potentially problematic components. It is widely used in oil labs worldwide. The technique is limited to dissolved (corrosive wear) and fine particles. It does not provide any particle shape and size information. Large particle elemental analysis is possible but only with time consuming and costly sample preparation techniques.

Particle counters provide information on particle size and quantity and are widely used for contamination control. Particle count does not identify the material or the shape of particles in oil.

Ferrography is favored by oil analysis experts, as it provides excellent information about material for common metals such as steel, copper and aluminum. Ferrography also identifies dirt and sand particles and gives detailed information on particle shape, size and quantity. The drawback is that it takes a long time to analyze one sample and it takes years of experiences to properly interpret the results. Ferrous Index or PQ provides ferrous wear concentration only. The table at the end of the article provides more detailed comparisons among these techniques (sorry mobile user, it does not look good on a phone).

It is common practice for a lab to combine two or three of the techniques to provide a complete picture of machine wear from a used oil sample. In addition to the obvious added cost of hardware, labor, consumables, and time, an oil sample has to be divided into several smaller sample bottles for each different analytical method. This adds additional time for sample preparation, but more importantly, creates the opportunity for additional errors. Keeping particles distributed uniformly across all samples is a lengthy process and is a main source of measurement error for particle analysis. Large particles settle to the bottom of the bottle quickly, so the sample has to be agitated thoroughly right before the measurement. The settling time also varies drastically by oil viscosity as well. Each time the sample is divided and put into different bottles, the chance of errors is increased.

An ideal particle analysis tool should be cost effective, fast, and comprehensive so that an accurate diagnosis of machine condition could be made from one sample analysis. Spectro Scientific’s LaserNet Fines (LNF) particle analyzer is such a tool. The LNF technique was developed jointly by the Naval Research Lab (NRL) and Lockheed Martin and commercialized by Spectro Scientific more than 10 years ago ^[1,2,3]. The original design goal was to combine the advantages of a particle counter and a Ferrography system to trend marine engine wear continuously in an in-line system. In 2013 Spectro Scientific purchased the intellectual property and manufacturing rights from Lockheed Martin and added ferrous particle analysis capability to the system. This yields the most comprehensive particle analyzer on the market with several unique characteristics:

1.    Automatic shape classification - one key feature of the original LNF was the use of a high speed CCD camera to capture particle images as the oil was flowing through a large area flow cell. These images are analyzed using Neural Network Artificial Intelligence and machine learning to automatically classify particles larger than 20 microns into categories such as: fatigue wear, sliding wear, cutting wear and non-metallic particles (sand and dirt) (see picture below). Water droplets and air bubbles (> 20 microns) can be identified and eliminated from the measurement. This helps to separate wear from contaminants, one key weakness of other particle counters on the market.

2.    Engine oil analysis. Engine oils are dark and contain soot from the combustion process. Engine oil samples are not normally recommended for particle count in labs because normal optical particle counters on the market have trouble seeing through the dark oil. The LNF was designed with engine oils in mind. An adjustable power laser is used with real time image processing to adjust the laser power based on the image brightness so that the LNF can see through dark engine oils with up to 2% soot and capture wear particle images - an impossible task for other optical-based particle counters.​

3.    Very high dynamic range. With a large viewing area of the flow cell and a high speed CCD camera, LNF can analyze very clean hydraulic samples and very dirty engine samples without dilution. The dynamic range goes from about 1 particle/ml (ISO 7/8) to over 2,000,000 particles/ml (>ISO 28), easily one to two orders of magnitude larger than other particle counters on the market.

4.    Ferrous Particle analyzer. The latest LNF Q230 contains two magnetometers. One is used to analyze total ferrous particle concentration with single PPM limits of detection (LOD), an order of magnitude better than others. The other magnetometer is used to count large (> 25 microns) ferrous particles and place them into different size bins. The two magnetometers are installed in-line with the other tests and only add 30 seconds to the total test time [4]. Figure a shows the correlation between PQL index and LNF ferrous concentration, and figure b shows the increase of iron wear in engine samples as a function of kilometers traveled, measured by both LNF and a RDE spectrometer[5]. Notice that iron measured by the LNF is typically higher than the same sample measured on the spectrometer. This is because large particles are included in the LNF measurement and not in the spectrometer measurement.

5.    Flow through system. The LNF was designed for both online and benchtop applications so it was designed as a flow through system, and can accommodate a relatively inexpensive auto sampling system for unattended operations.

Of all the commercial systems available today, the LNF provides the most complete picture of machine wear and solid contamination at the lowest cost, highest throughout and highest data integrity. Since it only uses one oil sample, errors due to sample preparation are minimized. It is a revolutionary oil analysis tool for both on-site and laboratory oil analysis needs. The table below lists detailed comparisons between common particle analysis techniques. For more information about the LNF, visit the Spectro Scientific webpage http://www.spectrosci.com/products/product/lnf-q200/.

References:

1: “LaserNet Fines – A new tool for the oil analysis toolbox”, David Filicky, Thomas Sebok, Malte Lukas, and Dan Anderson, machinery lubrication. 

2. Particle counters for oil analysis: design and specifications. Spectro Scientific white paper.

3. LNF Particle Count – How Low Can You Go? Spectro Scientific white paper.

4. LaserNet Fines® Q230 Technology. Spectro Scientific White paper.

5. Linking ferrous measurement to particle counting for machine condition monitoring, Thomas Barraclough, STLE 2015 presentation.