Wear, 90 (1983) 273 - 279 213

AN ATTEMPT TO QUANTIFY THE LIMITS OF FAILURE DETECTION BY FERROGRAPHY*

E. D. YARDLEY and G. MORETON

Mining Research and Development Establishment, National Coal Board, Ashby Road, Stanhope Bretby, Burton-on-Trent, Staffs. DE15 OQD (Gt. Britain)

(Received November 17,1982)

Summary

An attempt is made to determine whether ferrography is likely to be able to detect the levels of damage that would lead to catastrophic failure in mining gear transmissions.

A level of maximum sustainable damage before catastrophic failure would occur is set. The level is necessarily very approximate, but it provides an estimate of the amount of wear debris that would be released into the oil. This amount is considered against the background level of wear debris likely in underground equipment to give the increases produced by failures.

The’ increases in the direct reading Ferrograph values produced by given increases in ferrous wear debris are derived from experimental data and are related back to the levels of maximum sustainable damage in mining gear boxes.

It is concluded that for some mining gear transmissions ferrography is unlikely to be able to detect maximum sustainable levels of damage.

1. Introduction

The machinery used by the National Coal Board at the coal face oper- ates under some of the most arduous conditions found in any industry. Lubricant contamination is a constant problem with the ingress of coal dust, stone dust and water into gear boxes and hydraulic systems. The very nature of work underground means that, despite the best efforts of all concerned, equipment rarely achieves its design life. Failures arising from overload, contamination, corrosion and lubricant problems are common and break- downs are frequent. The actual amount of time that a coal-cutting machine spends cutting coal is about 30% of available time. Face machine break- downs account for a substantial proportion of the lost time. If this figure for

*Paper presented at the First International Conference on Advances in Ferrography, University College, Swansea, Gt. Britain, September 22 - 24, 1982.

0043-1648/83/$3.00 @ Elsevier Sequoia/Printed in The Netherlands

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machine utilization could be increased from 30% to 35% a major advance in production and productivity would have been achieved.

It is against this background that the Mining Research and Development Establishment has been looking at the use of ferrography as one of a number of condition monitoring techniques that might provide sufficient warning of machinery breakdown for rectification to be made so that loss of production is minimized.

This paper will be concerned with gear boxes of the type used at the coal face. These gear boxes typically contain about 50 1 of oil and are splash lubricated. Filtration is absent and ingress of solid contamination, in amounts of up to 3 g ll’ (or more in extreme cases), common. Spectro- metric oil analysis of samples taken from underground has shown total iron contents of over 1000 ppm in machinery operating satisfactorily.

With such high background levels of wear debris in the lubricating oil three questions need to be answered to indicate whether ferrography is likely to be a useful tool to the mining industry.

(1) How much damage is sustainable in mining gear boxes without cata- strophic failure occurring?

(2) By how much would this damage increase the amount of wear debris in the oil?

(3) Would this increase be detectable by ferrography? Before considering the answers to these questions it must be said that

for ferrography to be a viable technique it would have to operate success- fully substantially within the existing engineering framework and with the existing problems of oil leaks, contamination and so on that beset the in- dustry’s underground equipment. Additionally it would have to be usable by skilled operators rather than expert tribologists.

2. Discussion

How much damage is sustainable without catastrophic failure occur- ring? This of course depends on the type and cause of the damage and where it occurs. It has been estimated that 90% of gear box failures underground arise from bearing problems, and consideration can therefore be confined to these components. Damage tends to be extreme in underground machinery as it is often run until catastrophic failure occurs. Rolling fatigue damage extending all around bearing tracks and causing disintegration of races has been observed, but equally so has extreme cage and race wear due to abra- sion and lubrication failure leading eventually to the ejection of rolling elements from bearings.

To fix some parameters regarding this damage, let us consider a bearing of 100 mm bore 25 mm wide having a cage with a mass of, say, 200 g. Rolling fatigue damage all around the inner track of this bearing is assumed to be tolerable to a depth of 1 mm before the race disintegrates and would produce 88 g of metal. Cage wear amounting to perhaps half of the 200 g

might be tolerable before the rolling elements are lost from the bearing. These figures are very approximate but will be of the right order and define the amount of sustainable damage.

This leads then to the second question, by how much would this amount of damage increase the amount of wear debris in the lubricating oil in the gear box? If the background level of wear debris in the oil were 1000 ppm then calculations show that the amount of sustainable damage defined above would approximately double this figure to 2000 ppm debris. At a background figure of 250 ppm the damage would increase the level by a factor of 5 and so on. Now, would these increases be detectable by the direct reading Ferrograph?

To answer this question the change in the direct reader (DR) reading for known increases in the amount of wear debris in the oil must be estab- lished. This change can then be considered against the scatter in the readings constituting the previous trend to assess whether a significant deviation would be represented.

By taking an oil with a given amount of wear debris in it and passing samples from it through the DR at different dilutions a plot can be obtained of the large and small particle densities DL and Ds against dilution. If the assumption is made that at a dilution of, say, 1O:l the sample contains twice as many particles as at 2O:l and so on we then have a relationship between debris content and DR reading. Plots may then be derived of the DR reading against the reading that would be obtained were there n times the amount of debris present. This then allows the increase in reading produced by a given increase in wear debris level simulating failure to be obtained.

0 20 40 60 80 DILUTION h : 1)

Fig. 1. DR values plotted against dilution for two oils.

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20 60 100 140 180

DR VALUES WITH ‘T’ PARTICLES IN OIL

Fig. 2. Plot of the DR value with x particles in oil against the value with nx particles in oil.

Figure 1 is a plot of DR reading uersus dilution for two oil samples drawn from a gear box. Figure 2 is derived from Fig. 1 in the manner de- scribed above, with lines drawn for increases by factors of 2, 5 and 10 in the amount of. debris present in the oil. Table 1 shows readings from each of these lines that are obtained at three values of r>, or Ds (since both fall on the same line), i.e. 10, 50 and 100. This table illustrates the well-known fact that the Ferrograph is most sensitive at the lower end of its scale. Hence a failure occurring that doubled the amount of debris in the oil would increase a reading of 10 by 80% but would increase a reading of 100 by only 30%.

Experience has shown that with an iron content of about 1000 ppm in the oil a dilution of 500: 1 gives a Dr, value of about 50. Similarly at an iron content of about 250 ppm a dilution of 100: 1 produces a DL value of about 50. It is convenient, therefore, to use a value of I$, of 50 to examine the likelihood of detection of failures producing increases of two times and five times in the quantity of wear debris in the oil. Table 1 shows that increases of these magnitudes produce increases in 4, of 30 and 66 respectively. These increases must now be considered against the likely scatter in readings to examine whether they might fall outside confidence limits on a pre-existing trend. Work by Pocock and Courtney [l] has shown that for a mean DL value of 58, obtained from 20 determinations on ahquot parts from a single oil sample, a standard deviation of 8.9 was obtained. If a 90% confidence limit is put at very approximately two standard deviations this would mean that for a reading to be significantly different from 50 it would need to be in the upper 70s. If the additional variability in readings from sample to sample is superimposed it becomes clear that a reading of 80 might not

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TABLE 1

How values of 4, or IIS would change with increases in wear debris level

Initial value of 4. or 0s

10 50

100

Resultant value of DL or Ds for increases in wear debris by the following factors

2 5 10

18 40 50 80 116 132

130 160 170

DIRECT 100

READER 1 a VALUES 60

J

DL 60 i

0 0 0

AND

1 2 3 4 5 6 7 6

SAMPLE NUMBER OR WEEK NUMBER

Fig. 3. Plot of DR values against sample number (from ref. 1): 0, &; X, 0s.

represent an obvious and significant deviation. With 1000 ppm as a back- ground level of iron in the oil therefore it is unlikely that the maximum sustainable damage representing a doubling of the iron content would be detectable by a change in DL even if it occurred instantaneously between samples. To illustrate this, Fig. 3 shows the first seven & values from Pocock and Courtney’s work plotted against sample number as if the latter were the timebase. The point at sample number 8 is 30 4, points above the mean value for the previous seven readings and represents an increase by a factor of about 2 in the contamination level at a background iron content of around 1000 ppm. The point appears merely to sit at the upper end of a rising trend.

It will be seen, however, that Ds is also plotted in Fig. 3 and the devia- tion from the existing trend representing a doubling of the quantity of debris in the oil (from Fig. 2) is much more clear cut and certainly outside a con- fidence limit set at two standard deviations since the standard deviation on D, from ref. 1 was 1.04. Despite this it should be borne in mind that the eighth points for DL and Ds plotted in Fig. 3 represent the maximum damage before catastrophic failure, and the need would be to detect the failure well before this stage. Increases of one-third or even one-half of those plotted in Fig. 3 would not be seen as highly significant. The use of higher dilutions to bring the DR readings down would help to improve the sensitivity of the technique, but the problems involved in getting accurate and reproducible

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dilutions increase with the dilution, to the detriment of the accuracy of the DR values. It must be concluded, therefore, that the chances of detecting a failure at a background iron content of 1000 ppm are poor.

At 250 ppm iron in the oil the chances of detecting the maximum sustainable damage are much better and under the majority of industrial conditions would be adequate.

The unknown quantity in underground operations is the increased level of scatter on results that might be expected from the ingress of contamina- tion, the effect of oil leaks and top-ups and the sheer physical difficulty of taking oil samples at the coal face. Other factors that have not been so far discussed might cloud the issue further, but these tend to be even less quan- tifiable than those considered above. A principal factor is, for example, the proportion of failure debris produced that is of a form suitable for detection by ferrography and that will remain in the oil for a sufficiently long time. It would be reasonable to expect, however, that at a background level of 250 ppm iron the increase in reading occasioned by the maximum sustainable damage would still be significant. There remains then an area of uncertainty between 250 and 1000 ppm total iron in the oil.

Trials are currently being carried out to examine whether ferrography can be a useful tool for predicting failure in gear transmissions used under- ground. The information emerging to date suggests that levels of contamina- tion and background wear debris vary widely from one type of transmission to another. Rates of production of wear debris also appear to vary widely during satisfactory operation, and the scatter on the results for certain trans- missions is very large. An encouraging factor, however, is that one failure has been detected and sufficient warning was available for remedial action to have been taken.

3. Conclusions

The following tentative conclusions can be drawn from the work described in this paper.

(1) Ferrography is likely to be able to detect failures in systems where the background level of wear debris is below 250 ppm total iron but unlike- ly to be able to detect failures when this level is in the region of 1000 ppm as for certain mining gear transmissions.

(2) It has been confirmed that the chances of detecting failures rise as the DR values fall. Conversely, above a DL value of 100 ferrography is in- creasingly insensitive to changes occurring in the amount of debris in the oil.

Acknowledgments

The authors would like to express their thanks to Mr P. G. Tregelles, Director of Mining Research and Development, for the provision of

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facilities to carry out the work described and permission to publish the paper. The views expressed are those of the authors and not necessarily of the National Coal Board.

Reference

1 G. Pocock and J. S. Courtney, Some quantitative aspects of ferrography, Wear, 67 (1981) 287 - 301.