Wear, 126 (1988) 31 - 55 31
AN ASSESSMENT OF QUANTITATIVE AND QUALITATIVE FERROGRAPHY
G. R. WAKEFIELD and H. LEVINSOHN
Defence Scientific Establishment, Ministry of Defence, Auckland Naval Base, Auckland (New Zealand)
(Received July 14, 1987;revised November 2, 1987; accepted February 2, 1988)
summary
The direct reading and analytical ferrographic processes have been examined in detail to evaluate their use as quantitative and qualitative failure prediction tools. Particular attention was paid to the quantitative aspects of ferrography and the broad assumptions on which they are based.
Experimental work has shown that, for the type of samples analysed, the repeatability of ferrographic data is inadequate for the ferrograph to be used in isolation as an engine health monitoring instrument. A number of minor alterations were made to the instrument to enhance its repeatability, and the quantitative results can now be successfully used in conjunction with both qualitative ferrographic information and data from other engine health monitoring techniques such as spectrometric oil analysis.
1. Introduction
A project was undertaken with the following objectives: (a) to deter- mine whether a spectrometric oil analysis program (SOAP) based on atomic absorption spectrometry (AAS) chemical analysis data trends could be correlated with the results obtained from ferrography, (b) to assess the repeatability of the ferrographic procedures and (c) to determine whether the ferrograph could be successfully used to characterize abnormal wear situations.
A duplex ferrograph analyser was used for this study [ 11.
2. Initial tests
Early tests showed that routine analytical ferrographic analysis of the oil from a particular model of turbopropeller engine used in some RNZAF aircraft-produced data from which detection of wear rate trends was difficult (Fig. l(a)). The severity of wear index (Appendix A) of consecutive samples
0043-1648/88/$3.50 @ Elsevier Sequoia/Printed in The Netherlands
Figure 1A
/ Figure 16
5500 Ina 8500
Engine hours Fig. 1. (a) Comparison of analytical ferrograph results with (b) corresponding SOAP-AAS data.
often followed a pattern of transient fluctuations with up to an order of magnitude of variation. A number of reasons for this phenomenon were suggested.
(a) The sampling procedure from the aircraft was incorrect. (b) There was a lack of sample homogeneity. (c) There was a lack of repea~bi~ty in the ferrographic process. (d) The values were actually a true indication of fluctuating wear debris
levels within a lubricating system. These hypotheses prompted the investigation of a number of factors,
in order to determine the reliabifity of ferrographic data.
33
3. Initial comparison atomic absorption spectrometry-ferrography data
Plots of analytical and direct reading (DR) ferrographic data were compared with the SOAP-AAS data for the same samples (Figs. 1 and 2) in order to establish possible causes of erratic results.
From Fig. 1, it is apparent that it is not possible to correlate fluctua- tions in the severity of wear index with the iron concentration as determined by AAS. The AAS results showed a gradual increase in iron concentration between successive oil samples, as expected. However, the severity of wear index plot tended to show large random fluctuations with no definite trends. Unlike the AAS data, the ferrographic data did not change consistently when an engine had undergone an oil change. Plots of readings from both the large-particle (Dl) sensor and the small-particle (Ds) sensor of the DR ferrograph also did not correlate well with the corresponding SOAP results (Fig. 2).
4. The direct reading ferrograph: preliminary investigations and alterations
Early trials with the DR ferrograph showed that variations in the ambient light intensity directly affected the optical density readings. This was found to be caused by the leakage of stray light into the exposed ends of the precipitator tube. The interference was overcome by enclosing the photodetectors, precipitator tube and clamping assembly in a light-tight black polyethylene cover.
In order to facilitate continuous monitoring of sensor recordings as a sample flowed through the precipitator tube, a chart recorder terminal was connected to the DR ferrograph, enabling the output of the large or small particle sensor amplifier [2] to be recorded.
4.1. Physical measurements A series of physical measurements were carried out on the DR unit to
obtain a better understanding of the significance of the particle deposition patterns within the precipitator tubes and the implications of the Dl and Ds readings. The DR unit was dismantled and measurements were made to establish the positions of the particle sensors relative to the precipitator tube (Fig. 3). The DR readings could then be correlated with the observed debris deposition within selected tubes.
4.2. Magnetic field strength measurements A series of measurements was carried out to determine the magnetic
field strength along the length of the DR magnet. Measurements were carried out with a longitudinally sectioned precipitator tube in position in order to simulate the actual magnetic field within a tube during running conditions. Figure 4 shows the change in gaussmeter readings as the probe is moved along the magnet, up field from the start position. Any readings closer than
34
P - ________----
______________-__--------“-~., “\
/- “\ /” “A
,’ ‘.\ / I - “\.
Fig. 2. (a) Comparison of SOAP-AAS data with (b) and (c) corresponding DR ferrograph results.
4.5 mm from the extremities of the magnet were disregarded because of inaccuracies caused by the fact that the probe overlapped the ends of the magnet. By extrapolation the field strength in the vicinity of the large- particle sensor is estimated to be 2050 - 2100 G. The field strength in the vicinity of the small-particle sensor rwed from 2175 to 2225 G. As the magnet of the DR ferrograph is positioned parallel to the precipitator tube, it would be expected that the magnetic field strength along the precipitator tube would be constant. This is not the case, as Fig. 4 indicates. The field
35
Cross section
I sensor sensor Datum line
(leaoqintgube,dge
clamping assembly)
Stab of magnet
Fig. 3. Position of photodetectors relative to magnet (DR). (Dimensions in millimetres.)
0 10 70 00
Fig. 4. Variation in magnetic field strength along the analytical and DR magnets.
strength decreases at both ends of the magnet with a more pronounced effect towards the tube entry. These decreases in magnetic field strength are due to end effects at the extremities of the magnet which cause a lower- ing of the magnetic flux density above the magnet. The more pronounced
36
decrease at the entry region has been attributed to the presence of the sensors which attenuate the magnetic field. Unlike the analytical ferrograph which provides an increasing magnetic field along the entire oil flow path by virtue of the inclined ferrographic slides, the DR unit provides increasing magnetic field only in the region of the sensors.
5. Repeatability trials
In order to examine the degree of repea~bility of the DR fe~o~aphic process, a series of trials was carried out on various oil samples contaminated with different types of debris.
Two different samples were tested for repeatability of readings. Sample A consisted of oil extracted from a helicopter main rotor gearbox. Sample B consisted of unused NATO O-156 (MIL-L-23699C) oil artificially contami- nated with iron microspheres to a concentration of 0.001%. Chemical analysis had previously shown the spheres to be 99 wt.% Fe and they had a diameter distribution ranging from 0.6 to 6.2 pm [3].
The results obtained from the repeated testing of the above samples are shown in Table 1.
TABLE 1
Direct reading ferrograph repeatability
Sample A Sample B
Dl RSD (%) 5.0 8 Ds RSD (%) 5.6 75 Is RSD (W) 11.3 -
Relative standard deviation (RSD) = standard deviation/mean) x 100%. Severity of wear index Is = D12 - Ds2, where Dl is the standardized large-particle sensor reading and Ds the standardized small-particle sensor reading.
Two significant factors arose from these tests. (a) The RSD of the severity of wear index value is higher than that for
individual Dl and Ds readings. It is important to note this large variation, particularly if the wear index is to be used in a practical engine health monitoring system.
(b) The repeatability for the artificially contaminated sample was significantly lower than for the used sample which had Dl values of a similar magnitude. There is no apparent explanation for this anomaly, The fresh oil used to prepare the synthetic samples was free of any detectable particulate contamination.
The effect of ultrasonic agitation of samples passed through the DR ferrograph was investigated. The specimens analysed were used NATO O-156 (MIL-L-23699C) from aircraft turbopropeller engines which usually have
31
very low levels of wear debris contamination and produce DRs of less than 10. The trial involved analysing the samples by two different methods: the standard method of analysis; the standard method of analysis preceded by an ultrasonic agitation treatment. This involved soaking the sample at 65 + 5 “C as in the standard routine. The sample container was removed from the oven, hand shaken for 10 s and immediately placed in an ultrasonic bath containing water which was preheated to a nominal 65 “C. After ultra- sonic agitation for 45 - 60 s the specimen was removed from the bath and hand shaken for a further 10 s. Oil was then drawn off for analysis. Each specimen was individually treated immediately preceding the analysis.
It was clearly shown (Figs. 5 and 6) that ultrasonic agitation, in this case, improved the repeatability of both the Dl and the Ds readings. The presence of isolated unusually high readings could be attributed to a small quantity of relatively large non-magnetic particles which deposited on one of the two sensor locations. (Experience has shown that this effect is fre- quently encountered in analytical ferrography.) If the single high reading in both Fig. 5 and Fig. 6 are omitted, the following statistical values are obtained: from the standard procedure,
Dl average = 9.6
Dl standard deviation = 2.6
Ds average = 8.5
Ds standard deviation = 3.2
0 I 2 3
dun ““m&M
a 7 I
Fig. 5. DRs from a series of tests performed on a single used oil sample from an Allison T-56 turbopropeller engine.
Ds reading 0 DI reading m
! 2 3 1 5
Run &umb& B 9 ,o
Fig. 6. As in Fig. 5 with ultrasonic agitation of sample prior to testing.
and, with ultrasonic agitation,
Dl average = 4.4
Dl standard deviation = 0.6
Ds average = 2.5
Ds standard deviation = 0.7
The ultrasonic agitation treatment has significantly improved the repeatabil- ity of the readings. A second notable fact arising from these results is that the values of both the Dl and the Ds readings have decreased significantly with the use of the ultrasonic treatment. This suggests that ultrasonic agita- tion has caused agglomerates of small particles to disperse. Agglomerates of small particles would have the physical behaviour of a large particle and be deposited in the vicinity of the large-particle sensor. However, the decrease in the Ds reading is not fully understood but could be a particle orientation effect. Ultrasonically treated samples left for a number of hours and then retested gave readings similar to the untreated samples. This indicated that the dispersion effect was temporary and the small particles had con- glomerated again.
Tests on a sample from a similar engine produced no change either in the repeatability or in the magnitude of the readings despite the ultrasonic treatment. Therefore the effects of ultrasonic agitation appear to be sample dependent. The size, shape and composition of the particles would all affect the response to this treatment. The benefit of ultrasonic agitation on DR samples is uncertain and, because this process is very time consuming, each application needs to be preceded by a comprehensive study.
39
6. Particle deposition patterns
6.1. Method of tube preparation Particle deposition patterns within selected DR precipitator tubes were
examined by axially sectioning a number of specially prepared tubes. Three methods of tube preparation were utilized.
Method 1. Tubes were prepared for examination by slicing along section AA (Fig. 7) prior to use using a diamond cut-off wheel. The segment was then replaced using a silicone rubber cement. Readings of the Dl and Ds sensors were not taken because of the optical interference caused by the silicone cement. After testing, the tube and its contents were washed with fixer solvent and the tube was then carefully drained. When dry, the tube with the deposited debris in place was carefully removed from the DR unit and the sections were separated by splitting the silicone rubber joint. The segment was then ready to be examined under an optical microscope. An
A
Deposition i
zone -
A
1 Section removed
enable observation
Method 1
Debris position
TD
Section BB
Method 2
Fig. 7. Precipitator tube preparation. Section orientations for internal examination.
40
external clean with an organic solvent was required to prepare the specimen for scanning electron microscopy (SEM).
Method 2. The selected sample was processed in a normal precipitator tube and the sensor readings were recorded. The fixer solvent remaining in the tube was drained carefully. When dry, the precipitator tube was carefully filled by injecting an epoxy encapsulating resin. Once the resin had set, the tube could be removed from the DR unit. The section over the sensors was cut from the precipitator tube using a diamond cut-off wheel. This section was mounted in a transparent medium (acrylic resin). The orienta- tion was such that polishing of the mounting revealed a vertical cross section through the debris deposit (Fig. 7, section BB). The polished sample was then examined under an optical microscope.
Method 3. A tube which had been impregnated with epoxy resin was placed in concentrated hydrofluoric acid for approximately 3 h. This dis- solved the glass tube but left the resin intact, allowing the exact position of the debris deposits to be measured.
6.2. Investigation of particle deposition patterns The first deposition patterns to be investigated were produced by
passing samples of iron microspheres (10 ppm in NATO O-156 lubricating oil) through presplit tubes. These were examined by SEM to observe the microsphere size distributions and positions (Fig. 8). When the three micro- graphs (Figs. 8(a), 8(b) and 8(c)) were compared, it was evident that a decrease in the average sphere size occurred along the flow path. The large- particle sensor aperture occupies a position between 4.75 and 7.50 mm from the datum line, as shown in Fig. 3. All the spheres in Fig. 8 are deposited within this region. No spheres were deposited past this zone despite the presence of significant quantities of spheres less than 2 pm in diameter. The position of the small-particle sensor, 10.25 - 13.00 mm downstream from the datum line (Fig. 3), is a considerable distance beyond any micro- sphere deposits.
Deposition patterns for a number of samples were studied, and the results are given in Table 2. Sample preparation method 3 gave a more accurate representation of deposit position than the method 1 sectioned tube samples. Measurements from the sectioned resin-filled tubes cannot be accepted as an absolute measure of the deposit position because of the difficulty in terminating the polishing process at the instant that the longest length of deposit has been exposed.
A sample of 10 ppm Fe microspheres prepared by method 2 was examined at 1000X magnification under an optical microscope (Fig. 9). The deposit was photographed and measurements of the microsphere section diameters were made from a series of enlargements of Fig. 9. The average section diameter was calculated for each of six equal divisions over the 0.5 mm deposit length. From the average section diameters the average sphere diameter for the particular division can be calculated from the follow- ing equation [ 41:
E
42
TABLE 2
Positions of microsphere deposits within precipitator tubes
Sample number
Sample volume
(ml)
Start of deposit
(-1
End of deposit
0-1
Length of deposit
(mm)
Preparation method; measurement method
1 1.0 5.47 6.95 1.48 2 1.0 5.07 6.53 1.46 3 0.5 5.24 6.33 1.09 4 0.2 5.36 6.38 1.02 5 0.2 5.2 5.7 0.5 6 0.2 5.6 6.0 0.4 7 0.2 5.2 5.7 0.5 8 0.2 5.6 6.0 0.4 9 0.2 5.2 5.7 0.5
10 1.0 5.4 6.1 0.7
aOM, viewed in an optical microscope with the tube unsectioned. bOM, viewed in an optical microscope with the tube sectioned.
Method 1; SEM Method 1; SEM Method 1; SEM Method 1; SEM Method 3; OMa Method 3; OM Method 3; OM Method 3; OM Method 3; OM Method 2; OMb
where R is the average sphere radius and 2 the reciprocal of average section diameter. The results of the calculations are shown in Table 3.
There is a significant decrease in sphere size with increasing distance from the tube entry. The presence of all the spheres (including many of 2 pm or less) at the large-particle sensor is contrary to accepted theories [ 5,6]. The observations and measurements made on tubes prepared by this method were confirmed by observations made on tubes prepared by method
TABLE 3
Calculation of average sphere diameters from average section diameter
Distance f*om datum Average section diameter Z
(mm) (cun)
Average sphere diameter D
(Ctm)
5.10 - 5.18 5.71 8.96 5.18 - 5.27 4.49 7.06 5.27 - 5.35 4.68 7.36 5.35 - 5.43 3.54 5.56 5.43 - 5.52 3.63 5.70 5.52 - 5.60 3.68 5.78
Position of large-particle wuior, 4.75 - 7;50 mm. Position of small-particle aenaor, 10.75 - 13.00 mm.
Sta
rt
of
End
of
depo
sit
depo
sit
t Dis
tanc
e fr
om
datu
m
5.1
mm
0.5
mm
t
Dis
tanc
e fr
om
datu
m
5.6
mm
Pos
ition
of
la
rge
part
icle
se
nsor
: 4.
75
- 7.
50 m
m
Fig
. 9.
Iro
n m
icro
sph
eres
in
sec
tion
ed
DR
tu
be
(mag
nif
icat
ion
ap
prox
imat
ely
350X
).
I I
Tub
e en
try
end
Fig
. 10
. S
ecti
oned
D
R t
ube
con
tain
ing
plat
elet
-typ
e w
ear
part
icle
s (m
agn
ific
atio
n
appr
oxim
atel
y 10
5X).
44
1 (see Fig. 8). Both methods showed a slight but measurable gradation of sphere sixes and both showed spheres deposited only at the large-particle sensor.
Following the laboratory trials with synthetic samples, a used industrial gearbox oil sample was examined. This sample was very heavily contami- nated and required a l/100 dilution and only a 1 ml test volume to produce acceptable DR readings. The bulk of the magnetic debris was in the form of flat platelets of up to 30 pm in major dimension. Typical Dl and Ds readings for this sample were 70 and 10 respectively. DR tubes containing this debris were prepared for microscopic examination by method 2. Figure 10 shows the type of deposit observed. This particular test sample had Dl and Ds readings of 62.4 and 6.7 respectively.
From the examination of these tubes a number of important observa- tions were noted.
(1) A very small proportion of the platelet-type particles were lying parallel to the glass tube.
(2) A significant amount of concentrated stacking of platelet particles had occurred within the tubes despite the fact that the Dl reading was well below 100. This was the limit specified by the manufacturer past which particle stacking results in non-linearity of readings [ 7 3.
(3) The total length of the deposit was 1.3 mm starting at 5.9 mm and ending at 7.2 mm from the datum (Fig. 3). There was no visible metallic debris at the small-particle sensor position. It was possible, however, that the lack of any visible magnetic debris at the Ds position was due to the tube preparation method and the fact that only a single longitudinal plane through the deposit was examined.
As the experimental work progressed, it became apparent that the interpretation of DRs required considerable caution. Scott et al. [5] stated that the large-particle sensor measures the fractional area covered by par- ticles greater than 5 pm and the small-particle sensor measures the fractional area covered by particles in the less than 2 pm size range. This experimental work has shown that this statement is not true for all cases. It has also been shown that for platelet wear debris the DR ferrography readings were produced by a significant number of particles oriented perpendicular to the DR tube (Fig. lo), thus presenting a minimum silhouette to the sensor. The evidence showed that a proportion of the particles tended to become aligned to the magnetic field direction. Possible causes for this phenomenon are as follows*
(1) The magnetic field imposes a transverse and vertical orientation on the platelets.
(2) Flat particles are presented to the DR magnet such that their flat surfaces are oriented normal to the oil flow direction by virtue of the flow dynamics within the capillary tube. The particles then deposit on the precipi- tator tube without changing their orientation.
(3) The initial particles which deposit place a physical restraint on later particles, causing them to deposit VerticalIy.
45
All these factors would have the effect of reducing the optical density readings for a given particle size distribution. It appears that in certain samples the small-particle sensor reading is often due to the presence of non-magnetic or non-metallic contamination rather than to the magnetic debris smaller than 2 pm in size. It can be concluded that the use of optical density readings taken at two positions within the DR tube therefore gives only a very broad indication of the particle size distribution. Visual examina- tion of the debris is needed to qualify any deductions made from optical density readings.
7. Photodetector signal response
An x-y-t chart recorder was used to permit the continuous monitoring of particle sensor readings while a sample was passing through the precipi- tator tube. Only one sensor reading could be recorded at a time by this method.
The initial sample examined by this technique consisted of a mixture of various turbopropeller engine oil specimens which were relatively free from non-metallic contamination. Figures 11 and 12 are plots showing the build-up of the large- and small-particle sensor readings respectively. The solvent-washed small- and large-particle sensor readings are also marked on the plots as S and L. The large transient fluctuations superimposed on the steady state increase in the readings were caused by the passage of air
Fig. 11. Varying rate of increase in large-particle sensor reading as the sample (ex Allison T-56) flows through the precipitator tube.
46
1. t I
I L, .
SL. SL 0.
Fig. 12. Constant rate of increase in small-particle sensor reading as the T-56) flows through the precipitator tube.
sample @x Allison
bubbles passing through the sensors. The main point to emerge from Fig. 11 is that the build-up of the large-particle sensor reading was non-linear. Initially, there was a rapid increase in the reading followed by a characteristic plateau towards the end of the cycle. This phenomenon was consistently repeatable when testing this particular type of sample. The final value of DI for this sample had an average of 4.9, well below the non-linear upper limit of 100 specified by the manufacturer. The small-particle sensor readings for this sample showed a more linear increase with time as the sample passed through the DR tube (Fig. 12).
These samples contained a very small concentration of magnetic debris and the reason for a non-linear rate of increase in readings cannot be attri- buted to pile-up due to excess contamination. The non-linear rate of increase in readings may be at~ibu~ to particle deposition mechanics. The initial particles to form a deposit in the vicinity of the large-particle sensor may have created an obstruction, causing a localized pile-up of subsequent particles. However, a more likely mechanism may be that the primary deposit of particles caused an ~~ns~ication of the magnetic field. The primary deposit would then attract subsequent particles to the same region, similar to the mechanism behind the deposition of magnetic particles in strings.
The second type of oil sample to be examined by the chart recorder method contained a high concentration of platelet debris. As with the previous sample, the increase in the large-particle sensor readings with time was non-linear and the build-up of the small particle readings linear. The Dl readings obtained from this sample had an average of 74.5. The non-linearity
41
was found to be due to the pile-up of particles. It was apparent that the initial deposition of wear debris directly onto the surface of the precipitator tube determined the optical density reading and subsequent superposition of particles produced only small changes in the reading. This occurred despite the fact that the average Dl reading of 74.5 was below 100, the quoted value of the non-linearity threshold.
The significance of the non-linearities is that making the comparison of optical density readings from differing sample volumes would be difficult. It does not necessarily follow that a sample volume of 6 ml will produce twice the optical density reading of a 3 ml sample.
8. The analytical ferrograph repeatability
Results from a series of duplicated tests on a variety of samples were compared to give an indication of the repeatability of the process.
Table 4 shows that the variation in optical density values between readings of duplicate tests can be as high as 36%, with an average variation of 20% for both sets of readings. Other researchers [8] have also found variations of over 40% in the entry point readings, and up to 20% variations in the 50 mm readings. These researchers [8] have made some improvements to the repeatability of quantitative analytical ferrography by using a more precise sample volume measurement device. The use of a micropipette for dispensing samples was standard practice for all work covered in the current study detailed here.
Microscopic examination of a large number of ferrograms indicated that the variations can be caused by one or any combination of the following considerations: (a) variations in deposit shape and size, particularly in the length and width of strings of magnetic particles; (b) irregularities in the
TABLE 4
Analytical ferrograph repeatability
Sample Entry reading
First Second
Difference
(%I
50 mm reading
First Second
Difference
(%I
1 1.00 0.68 32 0.17 0.22 22 2 1.40 0.90 36 0.23 0.15 35 3 11.00 9.80 11 1.80 1.60 11 4 22.10 17.70 6 3.70 3.00 23 5 1.40 1.20 14 0.23 0.20 13 6 1.10 1.30 17 0.18 0.22 18
Average difference between Dl values, 19%. Average difference between Ds values, 20%. It should be noted that the values in the above table are standardized to a 1 ml sample volume.
deposition patterns apparently caused by localized irregularities in the magnetic field (Fig. 13); (c) the unavoidable presence of non-magnetic or non-metallic debris within the reading areas. It is not practical to eliminate any of these sources of variation. It can be concluded that the interpretation of the optical density values must be based on the microscopic field selected for observation. To obtain maximum consistency of optical density readings, the entry position must be made repeatable and the effect of flow rate on optical density readings must be assessed.
Fig. 13. Irregular deposit patterns caused by the deposition of magnetic debris in prefer- ential locations.
49
9. Introduction of improvements: entry point repeatability
Using the standard turret tube positioning method, large variations in entry point positions from one sample to the next and also when retesting the same sample were noted. If the entry point is not consistent, the relative position of the second reading at 50 mm also varies (Fig. 14). As the severity of wear index depends upon optical density readings recorded at the entry point and 50 mm mark, any changes in the relative position between these will affect the value of the results. Also, if the delivery tube could be re- positioned at exactly the same distance above the substrate for each test, then the flow conditions in the entry region should be consistent and give more reproducible readings.
In order to fix the position of the delivery tube above the substrate, two locating jigs were made (Fig. 15), one to set the tube protrusion distance and the second to set the delivery tube height. Prior to the use of the locat- ing jigs the average entry point position was 53.9 mm with a standard deviation of 0.77 mm. Use of the jigs fixed the average entry point position at 54.1 mm and halved the standard deviation to 0.39 mm.
Standard optical density readngs taken at entry ragm (Al) and 50 mm position (As)
L Entry region where Wid fist touches
\ down on ferrogram
~5OlllTOOSltii
- Non-wetting barrier
Exit end
Fig. 14. Schematic diagram of a ferrogram.
10. Flow rate variation experiments
By varying the peristaltic pump speed the effect of variations in the oil flow rate on the optical density readings were evaluated. Variations in flow rate ranging from 0.3 to 0.7 ml min-’ were obtained and the effect of this
50
Tube positioning gauge to position tube protuding a repeatable distance out of the delivery arm
Sectlon AA Elevation
Delivery aml podtknhg gauge accurately positiis delivery arm to ensure that the twrel tube su@ss ol to the s&&ate at the same entry poht for each analysis
Seats on aluminiun magnet -5 housing above substrate
_+ “&y&y&r:
while in Material: alumtim Diinsions in mm
& operation
Elevation
Fig. 15. Delivery arm and turret tube positioning jig.
on optical density values are shown in Fig. 16. All optical density values, except the entry point readings, decreased with an increasing flow rate. The different rates of decrease for each reading position are given in Table 5. It should be noted that variations in optical density with flow rate will be sample dependent and these results apply only to this particular sample. The position that particles will take on the substrate will be dependent upon their density, magnetic susceptibility and hydrodynamic shape. It follows that, if the dynamic conditions of flow are altered, then the distri- bution of particles along the substrate will change accordingly. It is therefore important that the sample flow rate be included as part of the analysis procedure so as to maintain consistency within and between laboratories.
This is particularly important if results are to be compared between U.S. laboratories using a 60 Hz power supply, and laboratories in other countries whose supply is 50 Hz, because the ferrograph peristaltic pump is driven by an induction motor using local supply power.
51
I - Position entry point - 0 ~.~~~~~~~‘~~~,~~~~~‘~~~,~~~~~‘~/~,~~~~~ 0.3 0.4 0.5 06 DI
6
Position - 45 mm 0 ~.ll~~~~‘~.~~~~~l~/~~~~~~~~~‘~~~.l~l~~ 0.3 0.4 0.5 0.1 07
Flow rate (ml/mid Fig. 16. Variation in analytical ferrograph readings with flow rate: 0, percentage area covered ; -, least-squares line.
11. Magnetic field strength along a sloping substrate
A series of measurements with a substrate in position were carried out on the analytical ferrograph magnet unit to determine the field strength along the length of the glass slide. Readings were taken with the probe just in surface contact along the centre line of the substrate.
The magnetic field strength along the substrate, with one end resting on the shelf above the magnet, varied linearly from 2000 G at the entry
52
TABLE 5
Variations in the rate of change of optical density readings with flow rate
Reading position Slope ma
Entry 0.04 50 mm -4.67 45 mm -4.15 40 mm -4.19 30 mm -3.76 20 mm -3.03
ay = mx + c where y is the optical density reading, x the sample flow rate, m the slope of flow rate vs. optical density reading plot and c the y axis intercept (i.e. x = 0, a hypo- thetical point in this case).
to approximately 2500 G at the exit (Fig. 4). A comparison between the strength of and variation in the magnetic fields of the DR ferrograph with that of the analytical ferrograph is shown in Fig. 4. The slope of the field strength measured from the entry to the 50 mm region is less than the increase between the Dl and Ds sensor on the DR ferrograph. These two areas tend to be interpreted as having similar significance in terms of optical density readings, but the lower magnetic field strength and gradient would have caused the wear particles to form a deposit of greater spread on the analytical ferrograph.
12. Particle size distribution along the substrate
The lack of spread of iron microspheres less than 2 pm diameter along the DR ferrograph precipitator tubes prompted the examination of particle deposition patterns on the analytical ferrograph. An oil sample containing 10 ppm of the microspheres was tested in 6 ml portions.
Entry point optical density readings for this type of sample were in the vicinity of 45, with readings at the 50 mm position of less than 1 and often 0. Microscopic examination of the ferrogram at a magnification of 400X showed no microspheres present at the 50 mm position although there were frequently sparse deposits of microspheres at 51 mm.
The magnetic force acting on a particle is proportional to its volume [9] and this is the major property which determines the position of a par- ticle on the substrate. A 2 pm particle with a typical diameter-to-thickness ratio (i.e. aspect ratio) of 5 to 1 [lo] has a volume of 1.26 pm3 compared with that of a sphere 2 pm in diameter whose volume is 4.19 pm3. This could explain why there were no 2 pm microspheres present at the 50 mm position in the synthetic samples prepared for these trials, whereas platelets of similar diameter have been observed at this position when analysing certain samples. This effect must be accounted for when analysing samples
53
which contain particles with a small major-dimension-to-thickness ratio such as those produced by a wearing gear system [9]. Optical density readings from differing types of sample cannot be compared without considering the type of particles and their possible variations in the magnetic mass.
13. Conclusions
Experiments have shown that ferrography does not possess the same standards of repeatability as AAS and emission spectroscopy, for the chem- ical analysis of wear metal in oil. The use of SOAP is a tried and proven method for detecting abnormal trends in the wear metal accumulation rate within certain aircraft engine lubricating systems [ 111. Throughout the ferrography evaluation program a number of abnormal wear trends high- lighted by SOAP remained undetected by ferrography. Ferrography has shown some success, however, in detecting and aiding the interpretation of abnormal wear trends in certain aircraft engines, particularly those systems which produce larger quantities of wear debris or those which generate cutting wear.
Experimental work has initially produced ferrograhic data with a repeatability which was considered inadequate for a quantitative evaluation, particularly for samples with low debris levels. It appears that the instrument was not able to reproduce debris deposits with the same light-interrupting properties from separate tests on a single sample. A number of alterations were made to the instrument which have enhanced the repeatability of results to a certain extent enabling quantitative determinations. Johnson and coworkers [8,12] have made significant advances in the field of quanti- tative analytical ferrography. These researchers [8,12] have improved the precision of analytical ferrography by the use of a zero-volume intercept which is a correction factor to enable results from oil samples of differing volumes to be more accurately compared. It is expected that the findings of Johnson and coworkers applied in conjunction with the studies and improve- ments discussed in this paper will enable quantitative analytical ferrography to be used more effectively and with a greater understanding. Although the quantitative application of ferrography has distinct limitations, significant benefits are achieved by the examination of deposits on substrates under optical microscopy and SEM as well as energy-dispersive X-ray analysis.
Relatively low levels of wear metal concentration in certain turbo- propeller engines necessitated the use of a large sample size, resulting in a run time of approximately $ h for either DR or analytical ferrography. The latter procedure required an additional 5 - 10 min for microscopic analysis and optical density readings. Hence the monitoring of a large number of aircraft engines would be very time consuming.
It has been stated elsewhere that (a) particles larger than 5 pm deposit at the large-particle (Dl) sensor position and particles less than 2 ,um in size deposit at the small-particle (Ds) sensor position [5, 61 and (b) non-linearity
54
of optical density readings will occur only at sensor readings of greater than 100 [7]. The experimental work described here has produced results which have shown that neither of the statements are necessarily true.
This study has shown that the optical density readings were not neces- sarily related to size distributions and interpretations should be made with caution since the significance of the readings depends on the type of debris present in the sample. Particles with a low aspect ratio (e.g. spherical par- ticles) will deposit closer to the entry or Dl sensor region than flat particles of similar diameter (i.e. high aspect ratio). The DR values and their inter- pretation in terms of the severity of wear index is questionable. This con- clusion is based not only on the lack of repeatability when retesting the same sample but also on the scatter of results between samples where other engine health monitoring methods showed distinct trends.
Combined quantitative and qualitative information produced by analytical ferrography conforms more closely with SOAP, but in some instances there is unacceptable scatter of the quantitative results. The main advantage of the analytical ferrograph is that the process deposits magnetic debris on a glass slide. The wear particles can then be examined in detail by optical and electron microscopy and the information obtained in this way is extremely useful in assessing the causes and origins of the debris.
In general, the analytical ferrograph has proved to be a valuable adjunct for an engine health monitoring programme but the performance of the DR ferrograph was disappointing.
References
1 D. Scott and V. C. Westcott, Predictive maintenance by ferrography, Wear, 44 (1977) 173 - 182.
2 Tellus Incorporated duplex ferrograph analyser, Tech. Inform. hstruct., 19 (Tellus Incorporated).
3 H. Levinsohn, Limitations of atomic absorption spectrophotometry applied to Spectrometric oil analysis, ASLE Trans., 2 7 (1) (1984) 24 - 32.
4 R. DeHoff and F. N. Rhines, Quantitatiue Microscopy, McGraw-Hill, New York, 1968.
5 D. Scott, P. J. McCallagh and G. W. Campbell, Condition monitoring of gas turbines - an exploratory investigation of ferrographic trend analysis, Wear, 46 (1978) 373 - 389.
6 DR ferrograph system, Foxboro-TransonicsPubl., 19 (Foxboro; Transonics). 7 Sample processing with the model 7067 direct reading ferrograph, Foxboro Instruct.,
September 1980, Ml, 611 - 113 (Foxboro). 8 D. N. Anderson, C. J. Hubert and J. H. Johnson, Advances in quantitative analytical
ferrography and the evaluation of a high gradient magnetic separator for the study of diesel engine wear, Wear, 90 (1983) 297 - 333.
9 W. W. Seifert and V. C. Westcott, A method for the study of wear particles in lubri- cating oil, Wear, 21 (1972) 27 - 47.
10 D. D. Anderson, Wear Particle Atlas (revised), in NAEC Rep., June 1982, 92 - 163. 11 R. C. Clark, A spectrometric oil analysis programme for the Fleet Air Arm, J. R. Nav.
Sci. Serv., 25 (6) (1970) 330 - 340. 12 J. H. Johnson and C. J. Hubert, An overview of recent advances in quantitative
ferrography as applied to diesel engines, Wear, 90 (1983) 199 - 219.
55
Appendix A: The severity of wear index concept and standardization of readings
Rl is the large-particle sensor reading or entry reading, and Rs is the small-particle sensor reading or 50 mm reading.
For the standardized optical density, direct reading (DR) ferrography,
Dl R1 =- s
where S is the sample volume. Similarly,
Ds=!? S
For analytical ferrography,
and
As Rs =- S
The severity of wear index Is is
Is = D12 - DC?
for DR ferrography or
Is = Al2 - As2
for analytical ferrography. It should be noted that, since the readings Rl and Rs are always stan-
dardised to 1 ml, it is common, for simplicity, to refer to the large- and small- particle sensor readings as Dl and Ds respectively.
Recommended
