Wear, 90 (1983) 11 - 19

CONTINUOUS WEAR MEASUREMENT

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BY ON-LINE FERROGRAPHY*

WOLFGANG HOLZHAUER and S. F. MURRAY

Department of Mechanical Engineering, Aeronautical Engineering and Mechanics, Rensselaer Polytechnic Institute, Troy, NY 12181 (U.S.A.)

(Received September 24,1982)

Summary

An on-line Ferrograph was used to monitor the wear rates of oil- lubricated ball-bearings. Periodically, the test bearings were also removed from the test stand and cleaned and weighed on an analytical balance.

A comparison of the mass loss data obtained by each of these two methods showed that the Ferrograph readings did provide on-line quantita- tive wear data for each individual test. However, the rates at which the ball-bearings wore were not consistent from test to test. For the four ball- bearings that were evaluated, plots of the Ferrograph concentration readings uersus bearing mass loss gave slopes that varied between 1.1 and 2.5. Never- theless, the results showed that semiquantitative data on the wear rates of machine components could be obtained with this on-line instrumentation.

1. Introduction

During the last 20 years, much technical effort has been devoted to monitoring the condition of machinery by evaluating the quantity and characteristics of the wear particles in the lubricant. Two promising tech- niques have evolved; the spectrographic oil analysis programs (including the Army Oil Analysis Program), which identify and analyze the quantity of each metal present in the oil, and ferrography, which evaluates the concentration, shape and particle size distribution of metallic wear debris. Each method has certain unique advantages; however, both share a common disadvantage, i.e. the need for transmitting a sample of oil to the laboratory for evaluation. The time lag between sampling and analysis, plus the need for constant up- dating and evaluation of records, detracts from the value and cost effective- ness of these methods.

To fill the need for a real-time monitoring system, an on-line Ferro- graph has been developed [ 11. This instrument can be installed in a cir-

*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 0 Elsevier Sequoia/Printed in The Netherlands

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culating oil-lubricated system to provide continuous readings of wear particle concentration and the per cent concentration of large particles in the oil. However, much remains to be learned about the significance and inter- pretation of the readings that are obtained with this instrument.

As one step in filling this need for background information, an experi- mental study was undertaken to monitor the wear rates of oil-lubricated ball-bearings with the on-line Ferrograph and to compare the data with the actual mass loss measurements obtained by periodically removing, cleaning and weighing the bearing. The results obtained are summarized in this paper.

2. Test equipment

The on-line Ferrograph consists of two main elements, a sensing cell, which is piped into a bypass line in the oil circulation system, and a remote wear analyzer which displays the measurements transmitted from the sensor. During operation, a small percentage of the oil being circulated in the system is diverted through the sensing cell. This instrument is designed to capture wear debris from the oil with a high gradient magnetic field. It then uses a surface-effect capacitive sensor to detect the presence of large and small wear particles. A quantitative measure of the total debris concentration and the percentage of the reading due to large particles (greater than 5 pm in size) are derived by relating the sum of large and small particle readings to the volume of the sample. The concentration readings are produced in units of parts per million and are measured in two ranges by the Ferrograph analyzer. There is one range of 0 - 1000 ppm and, for improved accuracy where small quantities of wear are involve& a 0 - 100 ppm range.

Once a measurement has been completed, the system recycles automatically by flushing the sensing cell and the procedure is repeated. The measurement time varies from 30 s to 30 min, depending on the concentra- tion of wear debris in the oil. Thus cycling time is also an indication of the amount of wear debris in the oil, since high debris concentrations result in more rapid cycling.

In designing the system and selecting a practical machine component for these wear rate evaluations, the following criteria were considered to be most important.

(1) The lubrication system had to be a circulating oil system with the specified pressure drop and oil flow rates that were required for sensor operation.

(2) The test component had to be the only source of wear particles in the system.

(3) The component had to weigh less than 160 gf so that it could be weighed on an analytical balance.

(4) The machine component had to wear fast enough to provide useful data.

On the basis of these criteria a circulating oil system, driven by a peristaltic pump, was designed and a conventional deep-groove size 202

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ball-bearing (bore, 15 mm) with a stamped and riveted mild steel retainer was selected as the test component. The primary disadvantage of the ball- bearing was the fact that it normally operates on an elastohydrodynamic film and, once the bearing has been run in, the wear rate should be negligi- ble. To circumvent this problem, the oil was deliberately contaminated with periodic additions of fine abrasive particles (1 E.trn ar-Al,O, polishing powder). Previous studies [2] had shown that abrasive particles of this size would cause mild abrasive wear in ball-bearings.

A schematic diagram of the bearing test rig and lubricant supply system is shown in Fig. 1. The inner race of the test bearing was a close slide fit on the lower end of a rotating vertical stub shaft which was supported on top by a ball-bearing spindle. An aluminum cup served as the housing to support the outer race of the bearing.

The test bearing was thrust loaded by means of a lever arm arrange- ment. In these tests a thrust load of 269 N (60 lbf) was used.

The vertical spindle shaft was driven by a timing belt which coupled the shaft to a variable-speed electric motor. All tests were run at a constant speed of 960 rev min-’ .

An SAE 10 W non-detergent petroleum oil, with a viscosity of 50 cSt at the oil operating temperature (35 “C), was used as the lubricant. This oil was circulated by a peristaltic pump to avoid extraneous contamination by wear products from conventional oil pumping devices. The pump supplied oil at a pressure of 429 kPa (60 lbf ine2) which is twice the pressure drop (30 lbf inV2) required for operation of the sensor. The total volume of oil in the system was 350 ml and the flow rate was 600 ml min-’ .

ON-LINE IN TEST CUP

FERROCRAPH

RNALVZER

Fig. 1. Schematic diagram of lubricant system and ball-bearing test rig.

3. Test procedure

A new ball-bearing and a fresh sample of oil were used for each test. Before installing the test bearing, the lubricant system was charged with

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350 ml of fresh oil. This oil was then circulated through the system and through a filter to remove any loose debris that might have been trapped in the lines during the previous run. While this oil clean-up step was in process, the test bearing was ultrasonically cleaned with solvent grade Freon 113. The bearing was stored for at least 30 min in a desiccator to allow any residual solvent to evaporate. Then the bearing was weighed on an analytical balance to an accuracy of +O.l mgf.

As soon as the on-line Ferrograph sensor indicated that the contamina- tion level of the oil was approaching 1 ppm, and the readings were reasonably consistent, the clean-up filter was removed, the test bearing was installed in the system and the test was started. For the first few hours, the bearing was run in with the freshly cleaned oil. At the end of this run-in period, the test bearing was removed, cleaned and reweighed. A series of one- day tests was then made. Each morning, the bearing was cleaned, weighed and mounted in the test rig. About 30 - 50 mg of 1 pm CY-A1,03 particles were added to contaminate the lubricant. As soon as the oil was circulating at the desired pressure and flow, the sensor was turned on and the test was started. The bearing was then run for several hours until the sensor readings appeared to be leveling off (indicating that the abrasive particles had been ground to a size too small to affect the bearing wear). The test was then shut down. The following day the same procedure was repeated with fresh abrasive being added to the oil before the test was restarted.

The test was concluded after a significant amount of wear had taken place. This required about 25 h of operation. In all cases the oil appeared dark at the end of the test.

4. Test results

Four new ball-bearings were evaluated in these tests. All four bearings were manufactured by the same company, but two bearings (B and C) appeared to have come from a different lot than the other two (A and D) because of slight differences in design.

Figure 2 shows the bearing mass losses as a function of running time. These mass changes were measured by using the analytical balance. The corresponding Ferrograph readings of wear particle concentration were then reduced and average values were calculated for 30 min intervals. Graphs of the wear particle concentration uersus time were plotted. One of these graphs is shown for bearing B in Fig. 3. The measured mass loss of the ball- bearing is also shown on the same plot. (It should be noted that the scale for mass loss is on the right-hand side of the graph.) When plotted in this manner, the concentration readings and mass loss readings correlated very well. The same plot for bearings A, C and D also showed good correlation between concentration readings and measured mass loss.

In Fig. 3 it can be seen that the Ferrograph instrument was switched from its low range to its high range when the low range concentration

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0 1000

TIM (mid

Fig. 2. Measured ball-bearing wear.

a+,,,,~,,,,,,.,,,,,..,,,,,, III,! to 0 IO00 2000 3000

TIMEtmin)

Fig. 3. Particle concentration readings (0, low scale; 0, high scale) and total bearing mass loss (0) us. time for ball-bearing B.

reading was 39.8. It was found that the instrument’s two scales overlap well at a concentration reading of about 40. There was one exception to this: for bearing C, the range was switched when the low scale concentration read 34.7. The next high scale concentration reading, after a cycle time of about 8 min was 55. This jump could not be accounted for by any sudden wear of the bearing. Both points, i.e. 34.7 and 55, are included in all plots for bearing C.

A summary of the data for all four bearings is given in Fig. 4. Here the concentration reading just before shut-down is plotted against the total mass loss measured, including that at shut-down. Because bearing D wore much

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Fig. 4. Plot of particle concentration readings us. ball-bearing wear measurements (points (121, 298) and (165, 369) for curve D are off the figure).

more than the other three bearings, two points have not been included within the scale of the plot for bearing D.

All four bearings showed a linear relationship between concentration reading and wear. However, the slope is different in each case. Calculated slopes of the concentration versus mass loss plots are given in Table 1. The concentration-mass loss slopes for bearings A, B and C are fairly close to one another, but the slope for bearing D is somewhat higher. By inverting these numbers and calculating mass loss-concentration slopes, this difference is masked considerably.

TABLE 1

Calculated slopes from concentration uersus mass loss curves

Bearing Concentration/ (Mass loss)/ (mass loss) concentration

A 1.63 0.61 B 1.41 0.71 C 1.12 0.89 D 2.45 0.41 Mean 1.65 0.66

5. Discussion of results

Although bearings A, B and C gave similar results, reference to Fig. 4 shows that the differences among the data for the three bearings are not due

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to random scatter of the instrument readings. Each bearing has its own distinct linear relationship between concentration reading and weight loss.

In an attempt to determine why the results for bearings A, B and C matched more closely than the results for bearing D, a comparison was made of the “per cent large” readings recorded for each of the bearings. Figure 5 shows this comparison. Bearings A, B and C produced comparable readings while those for bearing D were higher. If we recall that bearing D also gave higher wear concentration readings (steeper slope) than the other three bearings, this suggests that the instrument has emphasized the higher per cent large values for bearing D when computing concentration values.

Fig. 5. Comparison of per cent large readings from four ball-bearing tests.

Preliminary tests were also run with a slider bearing consisting of a radi- ally grooved thrust washer and counterface. Both the thrust washer and the counterface were annealed SAE 4340 (approximately 0.40% C, 0.70% Mn, 0.80% Cr, 1.80% Ni and 0.25% MO) steel. The bearing was run under a thrust load of 1800 N (400 lbf) at a rotational speed of 450 rev mm-l, yielding a mean sliding velocity of 0.51 m s -’ (100 ft min-“). The nominal contact area was 2.14 cm2 (0.332 in2). The results for this test are shown in Fig. 6. Once again, good correlation between particle concentration and bearing mass loss is indicated.

There are two reasons why the thrust washer test was not pursued further. One is that the bearing actually wore too quickly. During each shut- down for weighing, the particles in the oil settled out somewhat. It took some time to redistribute the particles in the oil after restarting and thus it also took some time for the concentration readings to increase to their pre- shut-down levels. However, the bearing wore so quickly that it had to be shut down again soon after the concent~tion readings recovered. Strong conclusions could not be drawn from the data produced. Attempts to reduce

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04 = / i 0 50 IO0 150 200 250 300 360

TIMI

Fig. 6. Particle concentration readings (- ) and total bearing mass loss (0) us. time for a radially grooved thrust bearing.

the wear rate by lessening the severity of the operating conditions resulted in the bearings developing a fluid film and thus yielding no measurable wear.

The second difficulty with the thrust washer geometry was that the friction (and presumably instantaneous wear rate) would sometimes increase suddenly. The wear particle analyzer was apparently not able to handle these sudden jolts of debris and behaved as if it were plugged or jammed. The readings would become fixed at some unrealistic value. It was necessary to pressure flush the sensor with Freon solvent to produce meaningful readings again. It should be noted that such sudden increases in wear debris concen- tration in the oil are probably a result of the relatively low oil volume (about 300 ml in this case) used in this test. In critical applications, it would be prudent to investigate this further and to avoid the extremely high debris concentrations encountered in the thrust washer tests.

6. Conclusions

The on-line Ferrograph does provide valid wear rate measurements. For each individual bearing tested, a good correlation was found between mass loss and the Ferrograph concentration readings. However, the correlation was unique to each bearing.

Acknowledgments

Financial support for this work was provided under U.S. Army Research Office Contract DAAG 29-79-C-0204. The authors also acknowl-

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edge with thanks the encouragement and technical contributions of Mr. Vernon Westcott and also the kindness of Mr. Roger Rotondi, formerly of Foxboro Analytical and now president of the Telus Corporation, who provided the on-line Ferrograph instrument for this work.

References

1 958 PF series on-line Ferrograph, Brochure TZ-612-111, 1980 (Foxboro Analytical, Burlington, MA 01803).

2 J. A. Perrotto, R. R. Riano and S. F. Murray, Effect of abrasive contamination on ball bearing performance, Lubr. Eng., 35 (12) (1979) 698 - 705.