The condition monitoring of heavily loaded spur gears

Wear, S8 (1983) 115 - 124

115

THE CONDITION MONITORING OF HEAVILY LOADED SPUR GEARS*

M. L. ATKIN

Aeronautical Research Laboratoridzs, De fence Science and Technology Organisation, Fisherman's Bend, Victoria 3207 (Australia) E. D. DOYLE

Materials Research Laboratories, Defence Science and Technology Organisation, Ascot Vale, Victoria 3032 (Australia) (Received April 13, 1983)

Summary A short-duration high-tooth-load test on case-hardened spta, gears was run on an experimental gear rig with a view to comparing the performance of various condition monitoring techniques. The condition monitoring techniques used all relie:l on the detection of wear debris in the oil and included atomic absorption spectrometry, X-ray fluorescence, particle counting and ferrography. They all proved to be successful in signalling ~Lccelerated wear after 100 h of operation. Examination of the gears at the termination of the test indicated that the gears had worn by a combination of surface plastic deformation and pitting.

1. Introduction The increasing complexity and cost of engine maintenance, together with a demand for improved safety of operation, has resulted in $.Teater emphasis being placed on research into the modes of wear occun~ing in oillubricated machinery and on monitoring methods to improve wear detection. To help in achieving these aims a number of gear rigs, operating under realistic conditions, have been studied [ 1 ]. To date, spectrometric oil analysis results have been colTelated with metal lost from gears in the annealed condition [1 ], and optical microscopy and scanning electron microscopy have been used to examine the worn gear teeth surfaces and wear particles [ 2 ]. As an extension of the previous work, a short-duration high-tooth-load test on case-hardened gears is described in this report. A comparison is made *Paper presented at the 9th Meeting of the Organization for Economic Cooperation and Development International Research Group on Wear of Engineering Materials, Neuch~tel, Switzerland, September 21 - 24, 1982. 0043-1648/83/$3.00

© Elsevier Sequoia/Printed in The Nethellands

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between atomic absorption (AA) spectrometry, X-ray fluorescence (XRF)~, particle counting and ferrography as means of monitoring wear debris in the oil. Also, metallurgicai examination of the gear teeth and wear particles using scanning electron microscopy and ol~tical metallography is described.

2. Experimental mei~od

2.1. Rig description The gear rig used in the tests, which is shown in Fig. 1, consists of two gear boxes, each containing three gears, located in a back-to-back configum.tion. The box in the foreground (with cover removed) contains the test gears; these are cantilevered on roller bearings to provide easy access. The gears are isolated from the bearing housing by seals to prevent oil contamination. The gear box shown at the rear of the figure contains a central idler gear which can be loaded by a lever arm, the pivot point of which is slightly displaced from the gear centre line to provide a mechanical advantage of 100 to 1. Lubrication of the test gears is by splash feed from the sump and the oil temperature is monitored by thermocouples and controlled by an immersed water cooler. Oil samples of 50 ml were bled from a multipoint probe located approximately 25 mm below the meshing point of the gears (designated "top "~' sample in Tables 2 and 3) and from a probe near the bottom of the sump (designated " b o t t o m " sample in Tables 2 and 3); equiv-

Fig. 1. View of the test rig ~howing the three test gears (arrowed) in the foreground.

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alent volumes were replaced with clean oil. A description of the test gears, the tooth loads and the lubricant used is given in Appendix A.

2.2. Test method Prior to assembly the gear box was thoroughly cleaned. The gears were washed and weighed, and the surface finish and involute form were measured. Selected teeth were identified and p h o t o ~ a p h e d using a Cambridge Stereoscan M K l l scanning electron microscope. After assembly, 4600 ml of oil from sealed cans were added to the sump until the oil level covered the full tooth depth of the gears. For a period of 1 h the gears were run at light load with the lever arm weight offset by a spring balance; oil samples were then taken from the tapping points described above. The load was then increased and additional oil samples were obtained after running periods of 50 and 100 h; the test was then concluded because of excessive wear. The following techniques were used to assess the condition of the gears: (a) gear weight; (b) spectrometric oil analysis, including (i) AA spectrometry and (ii) XRF; (c) particle counter; (d) ferrography; (e) metallurgical examination.

3. Results

3.1. Gear mass The mass lost by each gear after 100 h operation is given in Table 1. Prior to weighing before and after the test run, the gears were thoroughly cleaned by washing in solvent and vapour degreasing in trichloroethylene. TABLE 1 Gear mass loss Time

G e a r m a s s loss (g)

(h)

A

B (idler) a

C

100

0.185

0.218

0.105

a G e a r B is s u b j e c t e d t o wear on b o t h surfaces.

3.2. Spectrometric oil analysis 3.2.1. A t o m i c absorption spectrometry Two methods of AA analysis were used in these tests. The first, the direct dilution method [ 1], uses methyl isobutyl ketone to reduce the sample viscosity before it is fed to the spectrophotometer (Perkin-Elmer 403). The second method [ 1] involves filtering the oil sample through a 0.45 #m filter and heating the residue with dilute hydrochloric acid to dissolve the iron present and so to produce a homogeneous aqueous solution. In both

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methods the results are expressed as the concentration of iron in the oil as parts per million by weight and shown in Table 2. TABLE 2 Spectrometric oil analysis

Time

Sample

(h)

A A (direct method)

A A (filter method)

XRF

(ppm)

(ppm)

(ppm)

1

Top Bottom

0.4 0.0

0.8 0.8

2 0

50

Top Bottom

0.9 1.1

2.2 2.1

2 2

100

Top Bottom

8.6 9.1

24.3 26.0

28 28

3.2.2. X-ray fluorescence A substance irradiated with an X-ray beam will emit X-rays at wavelengths characteristic of the particular substance. If iron is the element of interest, stock solutions are prepared[ to provide a series of iron standards for instrument calibration. The partk:ular instrument used in this investigation was a Philips X-ray spectrometer (PW 1400). The results are given as the concentration of iron in the c~il and are expressed as parts per million by weight in Table 2.

3.3. Particle counter A Hiac particle counter was used to count the n u m b e r of particles in a series of size ranges. In this instrument the oil sample flows through a small rectangulm" passage past a window so that each particle partially interrupts a light beam which passes through the window. The light intensity is sensed by a photodetector, and the voltage o u t p u t from the detector is proportional to TABLE 3 Particle size distribution Hiac particle counter

Size (l~m)

Wear particles per I 0 0 ml o f sample 1 h flush

5-10 10-15 15-25 25-50 51-100 >100

50 h

1 O0 h

Top

Bottom

Top

Bottom

Top

Bottom

327136 78318 45850 11190 1692 104

356308 81934 45738 10906 1632 144

552480 86352 37424 5376 1960 200

655768 95480 38904 7344 1744 192

783432 506240 120320 13376 1984 288

9069568 661056 180992 41984 2944 384

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the blockage and thus to the particle size. The numbers of particles in six different size ranges are given in Table 3.

3.4. Ferrography In these tests the Duplex Ferrograph [3] was used to assess the iron conl~nt of the wear mets! in the oil samples. The two instruments are both based on the magnetic separation of wear metal from the lubricant.

3.4.1. Direct reading Ferrograph hi direct reading (DR) ferrography an off sample is treated to reduce viscosity and to improve particle separation and is siphoned through a glass cal:~illary tube which is located above the poles of a permanent magnet. W e ~ particles greater than 5 /~m in size (with a density D L ) in the oil are deposited near the entry point and the smaller particles (with a density Ds) are deposited 5 mm downstream from this point. At each location the wear deposit is assumed to be proportional to the optical density. Digital counters display the values for DL and D s , and the wear index is calculated as the product (DL + Ds)(DL - - D s ) . The results obtained from the current run are shown in Table 4. DR ferrography is used for the routine examination of oil s~unples to obtain rapid evaluation of a wear situation. If the wear index measured is abnormal (a significant increase on former values) the particular sample should be checked by analytical ferrography. In Table 4, there is a significm~t increase in going from the 50 h sample to the 100 h sample. TABLE 4 Wear evaluation by f e r r o g r a p h y : direc~ reader results (average o f four results normalized t o 1 ml o f sample)

Sample number

Time (h)

DL

Ds

DL + Ds

D L -- D s

DL 2 - - D s ~

1 2 3 4a

0 1 50 100

1.8 10.6 21.2 240

0.6 3.2 9.3 136

2.4 13.8 30.5 376

1.2 7.4 11.9 104

2.9 102 363 39104

°

a S a m p l e 4 was diluted 10:1 and was t h e n normalized to 1 ml o f sample.

3.4.2. Analytical Ferrograph The oil sample is fed across a glass substrate which is located above an inclined magnetic field. At the sample entry point the larger particles (greater than about 5 /~m) are precipitated, the remaining particles being deposited along the glass substrate roughly according to size. After washing and fixing, the substrate, now termed a Ferrogram, is examined by optical and scanning electron microscopy. The wear particles are characterized by their shape, size, colour etc. The normal wear process generates particles with a maxi-

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mum size of about 15 pm, the majority being 2/~m or less [4]. With abnormal wea~¢ a shift in the distribution occurs and the maximum size will increase beyond 15 /~m, the actual distribution depending on the severity of wear. A scg~ning electron micrograph of a Ferrogram of the wear deposit is shown in Fig. 2(a) and the corresponding backscattered electron image of the same field of view is shown in Fig. 2(b). o

(a)

(b)

Fig. 2. (a) Scanning electron micrograph of wear particles on a Ferrogram, showing that the majority are platelets with only one " c h u n k " (arrowed) in this field of view; ( b ) m i crograph showing the backscattered electron image from the field of view shown in (a).

4. Metallurgical examinations

4.1. Scanning electron microscopy The gears were examined before and after the test run in a Cambridge Stereoscan M K l l scanning electron microscope. Three teeth were selected for examination and it was found that they 'were sufficient~ly similar that it could be assumed that the surface condition recorded was an adequate representation of all the teeth or, the gear. Figure 3 is a set of scanning electron micrographs showing the surface condition of the gear teeth before and after the test run. Figure 3(c) shows the original ground finish of the gear teeth• Figures 3(a) and 3 ( b ) s h o w the surface damage on the addendum and dedendum of the gear teeth af'~er the test run. This surface damage is in the form of pitting and gross surface plastic deformation. Figure 4 shows a region of pitting at higher magnification, in which it is evident that some pits appear to have fatigue striations (,arrow A) while others appear to have oxide scale at their base (arrow B). The latter was particularly evident in regions well away from the pitch line. The surface regions between the pits are smooth with no evidence of the original surface finish grinding marks. This represents a change which can only have occurred as a result of ~ o s s plastic deformation of the surface layers.

121

Fig. 3. S c a n n i n g e l e c t r o n m i c r o g r a p h s o f t h e surface of a gear t o o t h before a n d after t h e te,c:t r u n s h o w i n g (a) t h e surface d a m a g e o n the a d d e n d u m o f a gear t o o t h after t h e test ru',a, (b) t h e surface d a m a g e o n t h e d e d e n d u m o f a gear t o o t h after the test r u n and (c) t h e original g r o u n d finish o f a gear t o o t h p r i o r to t h e test run.

Fig. 4. S c a n n i n g e l e c t r o n m i c r o g r a p h o f a gear t o o t h s h o w i n g a p i t t e d area at high magnification. A r r o w A indicates a pit which a p p e a r s to have fatigue striations. A r r o w B indic a t e s a pit with p r o b a b l e o x i d e scale at its base.

4.2. Optical metallography At the end o f the test run, the gears were sectioned for metaUographic examination. Besides the features evident in Figs. 3 and 4, this examination

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showed two further features of note. Firstly, there was extensive subsurface

cracking together with associated "white etching" layers, as shown in Fig. 5. Secondly, evidence of subsurface heating in the form of overtempering of the subsurface micro structure was observed in regions furthest a~vay from the pitch line and close to the t o o t h root, as shown in Fig. 6.

4)

~°~

15#m ,

Fig. 5. Optical micrograph showing the subsurface cracking and associated white etching layen~. Fig. 6. Optical mierograph showing the overtempered subsurface microstructure in the dedendum of the gear tooth 'well away from the pitch line.

5. Discussion As an exercise in monitoring heavily loaded gears the present experiments were too successful in that the gears wore out too quickly. However, the experimental techniques employed all provided a clear signal that the gears were suffering heavy wear. Both o f the AA techniques signalled a significant increase in the wear metal content of oil at 100 h; the longer more laborious filter m e t h o d has the advantage of a muc]~ enhanced signal, as shown in Table 2. Perhaps of even more interest is the equally good signal given by the X R F technique. This technique is clearly not too sensitive at around 1 - 2 ppm but is quite sensitive to larger amounts of wear debris. Tt-e particle counter indicated a significant increase in the number of wear particles in the different particle size ranges at 100 h compared with 50 h. In particular, there seems to be a trend to a greater percentage increase in the number of wear particles in the smaller particle size range (Table 3, bot~t~m sample). This m a y w e l l account for the success of the direc~ AA method in signalling increased wear, since it is considered to be insensitive to the presence of wear particles in the oil above 10 pm [5]. The DR Ferro,~aph also gave a clear signal of high wear in the rig as evidenced by the sudden increase in the 100 h reading (Table 4). Scanning electron microscopy and optical metallography both revealed e,xt~nsive plastic deformation of the surface layers of the gear teeth plus ext~nsive pitting over their full face width. The extent of the surface plastic

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deformation was most evident away from the pitch line in both the addendum and the d e d e n d u m of the gear teeth, i.e. corresponding to the regions of m a x i m u m sliding contact. Indeed, in the region of the dedendum evidence of significant surface heating was noted in the form of overtempering of the subsurface microstructure and in the apparent formation of oxide scale at the base of the pits. A further significant feature revealed in optical metallography was the extent of subsurface microcracking and the pronounced white etching layers associated with the cracks. The significance of these white etching layers is being further assessed by serial sectioning techniques [6], and the results of this examination will form the subject of a further paper. The metallurgical examination of the gears shows that the origin of wear debris arises from two possible sources: (i) plastic deformation giving rise to delamination and (ii) through pitting. The analytical Ferrograph, used to look in more detail at the morphology of the wear debris, revealed that the great majority of wear particles were of the fiat platelet type (see Fig. 2). It was surprising not to find more small chunks corresponding to the asymmetric pits arising from the extensive pitting. One can only assume then that the wear debris from surface pitting is flattened on release from the gear t o o t h surface or alternatively is re-entrained in the meshing contact. This raises doubts about the premise of particle tribology [7], for gears at least, that the wear particle morphology is an indicator of its mechanism of origin. The nature of mechanical systems is such that wear particles may suffer considerable ,~econdary deform~.tion before finding their way onto a Ferrogram slide. In conclusion, it is .clear from the present results that the condition of heavily loaded spur gears cam. be success:[ully monitored using a range of techniques such as AA spectrometry, XRF, particle counting and ferrography.

Acknowledgments ~I~e authors thank Messrs. J. Davis and G. Healy for their technical assistance in the project and Mrs. V. Silva for discussions and provision of the scanning electron microscope facilities. Acknowledgment is also made to, Superintendent, Chemistry Laboratory, HMA Naval Dockyard, Williamstown, for provision of the Hiac particle counting facility.

References I M. L. Atkin, R. A. Cummins, E. D. Doyle and G. R. Sharp, Trans. Inst. Eng. Aust. Mech. Eng., ~ (1979) 40. 2 R. A. Cummins and E. D. Doyle, A S L E Trans., 25 (1982) 502. 3 E. R. Bowen, D. Scott, W. W. Siefert and V. C. Westcott, Tribol. Int., 9 (1976) 109. 4 E. R. Bowen and V. C. Westcott, Wear Particle Atlas, Foxboro Transonics Inc., Burlington, MA, 197(i.

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5 B. B. Bond, Tech. Rep. TR9-73, May 1974 (U.S. Navy Oil Analysis Program, Washington, DC). 6 L. E. Samuels, Metallographic Polizhing by Mechanical Methods, American Society for Metals, Metals Park, OH, 3rd edn., 1982. 7 D. Scott, NEL Rep. 627, 1976 (National Engineering Laboratory, East Kilbride, Glasgow).

Appendix A

A.1. Description o f gears Type Pressure angle Number of teeth Diametral pitch Tooth width Surface finish

Involute spur 10 ° 3 8 , 3 8 , 37 10 m m 6.25 mm Less than 0 . 8 / ~ m centre-line average ( c .l.a.) (32 #in c .l.a.) E n 3 6 (3 wt.% Ni, 1 wt.% Cr) Material 6 7 0 - 8 2 0 HV Hardness of the case 3 3 0 - 3 9 0 HV Hardness o f the core Gears manufactured to BSS 436 Class A1 standard! for ground gears [A1 ].

A.2. Test conditions 3000 rev min -1 Shaft speed Power 37 kW Pitch line velocity 15ms -l Tooth contact stress (American Gear 1 6 6 0 MPa Manufacturers' Association) Tooth contact design stress for 100" h 1378 MPa or 1.1 X 107 cycles (It should be noted that the actual contact stress exceeds the design value iur 100 h operation; thus severe surface wear should occur within the test p;~od.) A. 3. Oil description Synthetic oil Viscosity Bulk oil temperature at operating condition

Mobil Jet II 27 cSt at 38 "C 5 cSt at 100 '~C 65 °C

Reference for Appendix A A1 Br. Stand. Specif. for Machine-cut Gears, October 1940.