Quantitative Baseline with the DR Ferrograph
Philosophy
All machines, in the course of normal operation, produce microscopic wear particles which are formed by removal of deformation of surface material from bearings, gears, cylinders, pistons, and other interacting surfaces. The rate at which particles are generated is called the “wear rate”. Wear rate tends to be constant when a machine operates normally at steady state. The particles are washed away by the lubricant and are carried by the oil for a period of time until they are lost by settling, filtering, and other loss mechanisms. Starting from a perfectly clean lubricant system the wear particle concentration would increase linearly, given a constant wear rate and no particle losses. However, the loss rate is proportional to the population of particles. In practice, the concentration increases until the loss rate comes into balance with the wear rate.  It can be demonstrated mathematically and in practice that this wear particle concentration (WPC) equilibrium is reached very soon after arriving at steady state after start up, speed increase, etc. The time constant for most machines is less than one hour. Since machines do run normally with a characteristic or baseline WPC, this is a useful and fundamental parameter for determination of machine health.

Establishing the US Army WPC Baseline

Ferrographic analysis of (7, OH-58A) hydraulic samples, (15, UH-1H) hydraulic samples, and (30, CH-47D) hydraulic samples revealed a variety of normal wear particles with small amounts of abnormal wear debris present. Certain hydraulic samples did not have enough sample volume to provide the appropriate standard test. Judgment or evaluation is based on the samples submitted to this lab. Therefore, without other oil analysis information, a total of 52 samples were tested with 6 samples indicating a Marginal and 46 indicating a Normal. One Critical was present coming from the OH-58A helicopter. However, a pattern was established to set-up a guideline criteria for the CH-47D hydraulic system using the DR-Ferrograph instrument. Details of the guideline will be provided later in this report.

DR-5 Ferrograph Instrument

A table from each helicopter type shows the logistic and quantitative information providing all the necessary data for sample comparison from each Army base. The Wear Particle Concentration, (WPC) is calculated using the Large and Small readings from the DR-Ferrograph instrument when enough samples were available.  The DR Ferrograph senses particles at two locations. First, at the entrance deposit, usually referred to as “L” (Large) and second, about five millimeters downstream, usually referred to as “S” (Small). The DR Ferrograph senses the presence of particles by measuring the amount of light attenuated at the two locations. The Wear Particle Concentration (WPC) is the sum of L+S, divided by sample size (Volume). In most cases the sample volume is one ml, so WPC is simply L+S.

The formula for WPC is as follows:

The WPC = Direct Large + Direct Small/ Sample Volume

Details for each of the samples are presented on the individual Equipment Condition Report with Photomicrographs and comments. Each sample analysis was done by our senior analysis with over 15 years of industry experience. Some samples had abnormal wear with low WPC readings, whereas, some samples had high WPC readings with normal wear. This is why each hydraulic sample was treated with special care. With the lack of sample volume and the samples coming from different helicopter types we felt the control factors for this evaluation was compromised. Therefore, each sample was reviewed on their own individual merit with a full analysis evaluation. Hence, a partial conflict with the WPC readings and the overall rating was displayed in the table. Nevertheless, our guidelines for the following equipment are as follows based on experience and the small amount of samples submitted:

                   Wear Particle Concentration Army Guideline

These guidelines are used as a reference and may need adjusting after more hydraulic samples are submitted.  In addition to the WPC reference, other oil analysis data will play an important role on the diagnosis of the machine components. Therefore, any information will be an added benefit to the Army Oil Analysis Program, (AOAP), to increase flight safety and reduce any mechanical failure.

Listed below is Table I showing the quantitative data.

Summary of Results

Based on the quantitative data shown, 3 marginals from the UH-1H, 2 marginals from the CH-47D and 1 Critical from the OH-58A helicopter were noted. All of the marginals reported revealed bearing wear particles present with small amounts of cutting wear indicating an abnormal situation. A critical was indicated due to a high WPC value. However, not enough sample oil was available to make an analytical ferrogram to identify any wear particles. Normally, when we need to establish a baseline we need to run a DR Ferrograph, which provides us a WPC, and a Analytical Ferrograph which provides a substrate to identify the wear particles present. In this way, we are measuring the baseline properly not using abnormal wear to set the baseline criteria.

The next batch of samples sent to a lab should have enough volume to perform the entire basic test at our facility. The volume needed is about 30 ml.

Heat Treatment of Ferrograms                          

The photomicrographs shown above illustrate a prime example of the abnormal bearing wear particles both before and after heat treatment. The photo on the left shows bearing wear taken at 500X magnification, with white reflected and green transmitted light source. The photo on the right is taken after heat-treating the ferrogram to 330ºC for 90 seconds on a laboratory hot plate to oxidize the particle surface. If the oxidized surfaces changes to a blue temper color this is an indication of low alloy steel, whereas, if it had change to a yellow temper color this would indicate medium alloy steel. 

These are just some of the ferrography techniques being used to develop a data baseline for the CH-47D. Once more samples are submitted, we can further improved the baseline and provide better accurate laboratory results.

CH-47D Helicopter

Conclusion

In summary, the next batch of samples submitted must have enough sample volume, accurate machine information and most of all, a clean reference sample to compare against. Having these pieces of information can make it much easier for the program to be established and hopefully eliminate any machine component problems using Ferrography as your diagnostic tool. In addition, sampling techniques is paramount. You must consider “where to draw the sample”, ‘use a sample tap”, and “retrieve the sample at similar RPM or at the same time interval”. If not, you will be gathering misleading data which will give you erroneous conclusions regarding the wear mode of the machine. This is an on-going and developing program constantly looking for improvement and with increase sample oil population, the more accurate your diagnostic capabilities will be.

UH-1H Helicopter

If you have any questions regarding this report, please do not hesitate to contact Ray Dalley at 216-642-3223 x 430 or e-mail me at rdalley@predictinc.com .

References:

Anderson, D.P., “Ferrographic Analysis for Hydraulic Fluids” S.A.E./SP-79/477 (October 1979).

Lockwood, F.E. Dr., & Dalley, R.J., Reprint from Metals Handbook Volume 18: Friction, Lubrication, and Wear Technology. (October 1992).