J Fail. Anal. and Preven. (2011)11: 158-166 DOI10.1007/11668-0109422-z TECHNICAL ARTICLE-PEER-REVIEWED Fatigue Failure Analysis of a Grease-Lubricated Roller bearing from an electric motor Zhi-Qiang Yu. Zhen-Guo Yang Submitted: 16 August 2010/in revised form: 30 November 2010/ Published online: 22 December 2010 C ASM International 2010 Abstract The grease-lubricated roller bearing of an elec- a roller bearing [1-4], failures of lubricating grease may be tric motor that drove a supply blower suddenly failed during the predominant cause of failure. The failed greases gen- operation. In order to identify the causes of the failure, a erally suffered from physical, chemical, and thermal variety of characterizations were carried out. The failed degradations during bearing operation [5-8], and the deg- surfaces of the bearing were observed visually and micro- radation brought about the loss of lubricating capacities, copically, and the characteristics of the lubricating grease especially under high temperatures and high velocity con were also investigated. Results showed that the surface of the ditions. Physical changes are commonly involved with the inner ring of the bearing contained contact fatigue damage, increase of oil separation, the loss of thickener structure, ind was covered with a multitude of debris and contact and the reduction of base oil content [9]. Chemical changes fatigue pits. What's more, the lubricating grease was sub- are mainly due to oxidation of base oil and thickener and jected to severe thermally induced degradation due to high loss of antioxidant additives, which will consequently service temperature, which consequently resulted in the increase the amount of acidic species and high-viscosity decrease of the lubricating capacity of the grease. Thus, the products. Komatsuzaki et al. [10] showed that the loss of lubricant film in the roller/raceway contacts was not formed base oil was generally caused by the evaporation of volatile effectively and the lubrication of the roller bearing was poor. oxidation products and was the predominant factor con- As a result, serious local wear as well as contact fatigue trolling the lubrication life of grease in cylindrical roller damage were brought about on the roller and raceway and the bearings wear finally led to the failure of the bearing most common failure event that results fror lubricating greases in rolling-element bearings is surface Keywords Bearing failure. Contact fatigue damage contact fatigue [11]. According to this failure mode, cracks Greases lubrication Lubricant degradation usually initiate at or near the contact surfaces, and subse quently form microscopic pits, which will then act as the stress concentration sites for further damages 3, 11] during Introduction continuing bearing operation. What's more, such stress concentration sites on the contact surfaces interact with Roller bearings are significant components of supply other pre-existing defects ing handling damage blower motors and play a critical role in normal operation surface inclusions, and dents to formed solid particles of the blower, although many factors can lead to failures of entrapped in the lubrication fluid [3]. These particles will accelerate the initiation of cracks and promote additional This article will present a failure analysis of one grease lubricated bearing that was sealed at the driving end of a Z.-Q.Yu()·Z.G.Yang Department of Materials Science, Fudan University, supply blower motor that had a rotation speed of 990 r/min Shanghai China in the electric power unit. The bearing was a cylindrical e-mail:yuzhiqiang@fudan.edu.cn roller bearing with 18 rolling elements. The face material
TECHNICAL ARTICLE—PEER-REVIEWED Fatigue Failure Analysis of a Grease-Lubricated Roller Bearing from an Electric Motor Zhi-Qiang Yu · Zhen-Guo Yang Submitted: 16 August 2010 / in revised form: 30 November 2010 / Published online: 22 December 2010 © ASM International 2010 Abstract The grease-lubricated roller bearing of an electric motor that drove a supply blower suddenly failed during operation. In order to identify the causes of the failure, a variety of characterizations were carried out. The failed surfaces of the bearing were observed visually and microscopically, and the characteristics of the lubricating grease were also investigated. Results showed that the surface of the inner ring of the bearing contained contact fatigue damage, and was covered with a multitude of debris and contact fatigue pits. What’s more, the lubricating grease was subjected to severe thermally induced degradation due to high service temperature, which consequently resulted in the decrease of the lubricating capacity of the grease. Thus, the lubricant film in the roller/raceway contacts was not formed effectively and the lubrication of the roller bearing was poor. As a result, serious local wear as well as contact fatigue damage were brought about on the roller and raceway and the wear finally led to the failure of the bearing. Keywords Bearing failure · Contact fatigue damage · Greases lubrication · Lubricant degradation Introduction Roller bearings are significant components of supply blower motors and play a critical role in normal operation of the blower, although many factors can lead to failures of a roller bearing [1–4], failures of lubricating grease may be the predominant cause of failure. The failed greases generally suffered from physical, chemical, and thermal degradations during bearing operation [5–8], and the degradation brought about the loss of lubricating capacities, especially under high temperatures and high velocity conditions. Physical changes are commonly involved with the increase of oil separation, the loss of thickener structure, and the reduction of base oil content [9]. Chemical changes are mainly due to oxidation of base oil and thickener and loss of antioxidant additives, which will consequently increase the amount of acidic species and high-viscosity products. Komatsuzaki et al. [10] showed that the loss of base oil was generally caused by the evaporation of volatile oxidation products and was the predominant factor controlling the lubrication life of grease in cylindrical roller bearings. The most common failure event that results from failed lubricating greases in rolling-element bearings is surface contact fatigue [11]. According to this failure mode, cracks usually initiate at or near the contact surfaces, and subsequently form microscopic pits, which will then act as the stress concentration sites for further damages [3, 11] during continuing bearing operation. What’s more, such stress concentration sites on the contact surfaces interact with other pre-existing defects including handling damage, surface inclusions, and dents to formed solid particles entrapped in the lubrication fluid [3]. These particles will accelerate the initiation of cracks and promote additional debris production [12]. This article will present a failure analysis of one greaselubricated bearing that was sealed at the driving end of a supply blower motor that had a rotation speed of 990 r/min in the electric power unit. The bearing was a cylindrical roller bearing with 18 rolling elements. The face material Z.-Q. Yu (&) · Z.-G. Yang Department of Materials Science, Fudan University, Shanghai, China e-mail: yuzhiqiang@fudan.edu.cn 123 J Fail. Anal. and Preven. (2011) 11:158–166 DOI 10.1007/s11668-010-9422-z
J Fail. Anal. and Preven. (2011)11: 158-166 of the bearing was GCr15 bearing steel, while the cage material was an alloy of copper and zinc. The lubricating grease was lithium based containing MoS2 particles. Dur its operation, the bearing suddenly failed when its operation temperature exceeded the warning limit of 70C. After that, the lubricating grease which was found on the side of the raceway of the detached failed bearing was agglomerated, semisolid, and heavily. Meanwhile, the raceway surface of the inner ring of the bearing showed the signs of contact fatigue and wear. Thus, in order to identify the causes of the failure, the lubricating grease used in the failed bearing was collected and then inspected by Fourier transform infrared spectroscopy(FT-IR), X-ray diffraction (XRD), and thermogravimetric analysis(TGA), while the micromorphologies and chemical compositions of the wear faces were examined by scanning electron microscopy Fig. I Dismounted samples of failure bea (SEM) and energy dispersive spectroscopy(EDS). Based on the analysis and relevant discussion, failure prevention methodologies for similar grease-lubricated roller bearing FT-IR Analysis were developed Figure 2 shows the FT-IR spectra of used and fresh grease samples. The fresh grease spectrum(Fig. 2a) shows char acteristic absorbance peaks of carboxylate stretch at Investigation Methods 1597 cm and hydroxyl at 3441 cm due to the presence of hydroxystearate thickener. The bands at 1460 and FT-IR with KBr discs, XRD with Co Ka radiation, and 1377 cm were assigned to Ch vibrations from the base TGA under N2 purge were applied to investigate the oil [9]. On contrast, the intense carbonyl(c=O) band at structural and thermal characteristics of the greases. 1709 cm occurred in the infrared spectrum of the used Besides that, microstructures of the greases and the wear grease(Fig. 2b), which was from the oxidation of base oil marks on the bearing inner-ring surface were observed by and thickener in the greases. In addition, that the thickener SEM. Chemical compositions of the bearing material were peaks were reduced to a broad ill-defined band indicates and the material hardness(HRC) was also measured determined by phe that the sample of used grease was mainly composed of base oil and carbonyl-containing degradation products Hence, it can be concluded that the gre raceway contact suffered heavily thermo-oxidation degra- Observation Results and Analysis dation under the high temperature The thermo-oxidation degradation of the grease fol- logy of the detached lowed the free radicals reaction mechanism [13], seen in bearing. It is obvious that the raceway surfaces of the bearing inner ring were worn heavily and there were large grease in the initial phase of the oxidation reaction with the number of pits and debris on the surface. The inner-ring temperature increase during the bearing rolling. In general, urfaces were a red-brown color and appeared polished the reaction speed was very slow. which may the result of direct contact between the rollers rH-Ar'+H and the raceway during operation. Also, the lubricating grease was found to be solidified and pushed out of the where RH denotes the base oil and thickener in grease, R and H were free radicals of the alkyl and hydrogen Characterizations of the Lubricating Grease The reaction between the alkyl free radicals and oxon) d quickly to generate peroxide groups( after alkyl radicals formation during the chain propagation Samples of used greases from the failed bearing were Successively, the reaction occurred between the peroxide identified by FT-IR, XRD, SEM with EDS, and TGA, and groups and Rh(the base oil and thickener) by direct was then compared with the fresh greases abstracting hydrogen atoms from RH and then generated Spring
of the bearing was GCr15 bearing steel, while the cage material was an alloy of copper and zinc. The lubricating grease was lithium based containing MoS2 particles. During its operation, the bearing suddenly failed when its operation temperature exceeded the warning limit of 70 °C. After that, the lubricating grease which was found on the side of the raceway of the detached failed bearing was agglomerated, semisolid, and heavily. Meanwhile, the raceway surface of the inner ring of the bearing showed the signs of contact fatigue and wear. Thus, in order to identify the causes of the failure, the lubricating grease used in the failed bearing was collected and then inspected by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and thermogravimetric analysis (TGA), while the micromorphologies and chemical compositions of the wear faces were examined by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Based on the analysis and relevant discussion, failure prevention methodologies for similar grease-lubricated roller bearing were developed. Investigation Methods FT-IR with KBr discs, XRD with Co Kα radiation, and TGA under N2 purge were applied to investigate the structural and thermal characteristics of the greases. Besides that, microstructures of the greases and the wear marks on the bearing inner-ring surface were observed by SEM. Chemical compositions of the bearing material were determined by photoelectric direct reading spectrometry and the material hardness (HRC) was also measured. Observation Results and Analysis Figure 1 displays the external morphology of the detached bearing. It is obvious that the raceway surfaces of the bearing inner ring were worn heavily and there were large number of pits and debris on the surface. The inner-ring surfaces were a red-brown color and appeared polished which may the result of direct contact between the rollers and the raceway during operation. Also, the lubricating grease was found to be solidified and pushed out of the bearing track. Characterizations of the Lubricating Grease Samples of used greases from the failed bearing were identified by FT-IR, XRD, SEM with EDS, and TGA, and was then compared with the fresh greases. FT-IR Analysis Figure 2 shows the FT-IR spectra of used and fresh grease samples. The fresh grease spectrum (Fig. 2a) shows characteristic absorbance peaks of carboxylate stretch at 1597 cm−1 and hydroxyl at 3441 cm−1 due to the presence of hydroxystearate thickener. The bands at 1460 and 1377 cm−1 were assigned to CH vibrations from the base oil [9]. On contrast, the intense carbonyl (C=O) band at 1709 cm−1 occurred in the infrared spectrum of the used grease (Fig. 2b), which was from the oxidation of base oil and thickener in the greases. In addition, that the thickener peaks were reduced to a broad ill-defined band indicates that the sample of used grease was mainly composed of base oil and carbonyl-containing degradation products. Hence, it can be concluded that the grease in the roller/ raceway contact suffered heavily thermo-oxidation degradation under the high temperature. The thermo-oxidation degradation of the grease followed the free radicals reaction mechanism [13], seen in Eq. 1. The alkyl free radicals (R• ) were formed in the grease in the initial phase of the oxidation reaction with the temperature increase during the bearing rolling. In general, the reaction speed was very slow. RH ! D R þ H ðEq 1Þ where RH denotes the base oil and thickener in grease, R• and H• were free radicals of the alkyl and hydrogen, respectively. The reaction between the alkyl free radicals and oxygen gas occurred quickly to generate peroxide groups (ROO• ) after alkyl radicals formation during the chain propagation. Successively, the reaction occurred between the peroxide groups and RH (the base oil and thickener) by direct abstracting hydrogen atoms from RH and then generated Fig. 1 Dismounted samples of failure bearing J Fail. Anal. and Preven. (2011) 11:158–166 159 123
J Fail. Anal and Preven.(2011)11: 158-16 Fig 2 FT-IR spectra of (a)fresh and(b) used grease 46050 m223763135 597 103802 1n1837 1377.2 146096 2924.12 (a)00300302000020008001001400120010090060040 1000 177449154107 129952 7130 13T09 1460.17 285463 292449 400003600320029002400200018001600140012001000 the hydrogen peroxide and secondary alkyl free radicals, as R"+h-Rh (Eq4) shown in reactions(2)and(3). In this way, the reactions R·+R·→R-R would not cease until the termination of the chain occurred via reaction with the hydrogen free radical, and/or mutual When the temperature increased the hydrogen peroxide coupling, as shown in reactions(4)and (5), respectively. (ROOH) decomposed into alkyl-oxygen radicals(RO) R+O→ROO (Eg 2)and hydroxyl radicals(HO), as shown in reaction (6) ould fur ROO·+RH→ROOH+R (Eq 3) produce the final alkyl radicals (R,), as shown in
the hydrogen peroxide and secondary alkyl free radicals, as shown in reactions (2) and (3). In this way, the reactions would not cease until the termination of the chain occurred via reaction with the hydrogen free radical, and/or mutual coupling, as shown in reactions (4) and (5), respectively. R þ O2 ! ROO ðEq 2Þ ROO þ RH ! ROOH þ R ðEq 3Þ R þ H ! RH ðEq 4Þ R þ R ! R R ðEq 5Þ When the temperature increased the hydrogen peroxide (ROOH) decomposed into alkyl-oxygen radicals (RO• ) and hydroxyl radicals (HO• ), as shown in reaction (6), below. These groups could further react with RH to produce the final alkyl radicals (R• ), as shown in Fig. 2 FT-IR spectra of (a) fresh and (b) used grease sample 160 J Fail. Anal. and Preven. (2011) 11:158–166 123
J Fail. Anal. and Preven. (2011)11: 158-166 reactions(7) and(8), which would return to the above XRD Analysis and SEM Observation chain propagation again. As a result, thermal-oxidative he lubricating rease was repeatedly Figure 3a and b shows the XRD patterns of the fresh and decreased used grease sample, respectively. From the X-ray analysis, ROOH→→RO·+HO° (Eq 6)the patterns. The strongest diffraction peak from the fresh HO+RH→H2O+RO (Eq 7) sample, as shown in Fig. 3a, appeared at about 20=14 RO·+RH→ROH+R cq 8) corresponding planar spacing d=0.63 nm, another strong diffraction peak presents at about 20= 26, corresponding Thus, during the chain termination the content of the planar spacing d=0.34 nm. According to PDF cards, they arbonyl-containing oxidation products including acidic could be attributed to(100)and (101)crystalline planes of species and high-viscosity products increased in the the molybdenum disulfide(Mos2). In addition, some lubricating grease and heavily deteriorated the lubricating diffraction peaks of MoSz also appeared in thi properties of the grease profile. This means that the fresh grease was a According to Xue et al. [14], the intensiy egree of the observation and EDS analysis of the fresh grease confirmed of C=0 peaks lubricating grease containing MoS2. Actually, SEM was directly proportional to the degradation grease.As illustrated in Fig. 2b, the C=0 peak was sharp this fact as well. Figure 4a and b shows SEM and EDS and the peak intensity was high(compared with the fresh results of the fresh grease sample, respectively. As shown grease, see Fig. 2a). Consequently, it could be concluded in Fig. 4a, there were some even particulates(arrows)with that large amounts of compounds containing C=0 group a size of around 5 um dispersing in a fine microstructure of were produced in the used greases, i.e., the greases in the the greases. The EDS results indicated that those particu roller/raceway contacts were oxidized and degraded lates consisted of molybdenum (Mo) and sulfur (S) heavily. elements, namely MoS2. In comparison, the intensity of ig. 3 X-ray analysis of (a) fresh and (b)used grease 6000 0.000 40.000 4000 30 Spring
reactions (7) and (8), which would return to the above chain propagation again. As a result, thermal-oxidative stability of the lubricating grease was repeatedly decreased. ROOH ! D RO þ HO ðEq 6Þ HO þ RH ! H2O þ RO ðEq 7Þ RO þ RH ! ROH þ R ðEq 8Þ Thus, during the chain termination the content of the carbonyl-containing oxidation products including acidic species and high-viscosity products increased in the lubricating grease and heavily deteriorated the lubricating properties of the grease. According to Xue et al. [14], the intensity of C=O peaks was directly proportional to the degradation degree of the grease. As illustrated in Fig. 2b, the C=O peak was sharp and the peak intensity was high (compared with the fresh grease, see Fig. 2a). Consequently, it could be concluded that large amounts of compounds containing C=O group were produced in the used greases, i.e., the greases in the roller/raceway contacts were oxidized and degraded heavily. XRD Analysis and SEM Observation Figure 3a and b shows the XRD patterns of the fresh and used grease sample, respectively. From the X-ray analysis, it is clear that there were significant differences between the patterns. The strongest diffraction peak from the fresh sample, as shown in Fig. 3a, appeared at about 2θ = 14°, corresponding planar spacing d = 0.63 nm, another strong diffraction peak presents at about 2θ = 26°, corresponding planar spacing d = 0.34 nm. According to PDF cards, they could be attributed to (100) and (101) crystalline planes of the molybdenum disulfide (MoS2). In addition, some weak diffraction peaks of MoS2 also appeared in this XRD profile. This means that the fresh grease was a lithium lubricating grease containing MoS2. Actually, SEM observation and EDS analysis of the fresh grease confirmed this fact as well. Figure 4a and b shows SEM and EDS results of the fresh grease sample, respectively. As shown in Fig. 4a, there were some even particulates (arrows) with a size of around 5 μm dispersing in a fine microstructure of the greases. The EDS results indicated that those particulates consisted of molybdenum (Mo) and sulfur (S) elements, namely MoS2. In comparison, the intensity of Fig. 3 X-ray analysis of (a) fresh and (b) used grease sample J Fail. Anal. and Preven. (2011) 11:158–166 161 123
J Fail. Anal and Preven.(2011)11: 158-16 ZEKE 1,9818从m 26/NoU/85 ) C 1200 0 1800 SMo 600 SMo (b) Fig 5 SEM and EDS of the used grease sample (a)SEM micrograph ig. 4 SEM and EDS of the fresh grease sample. (a)SEM micro- and (b) EDS analysis )EDS analysis containing copper, would further accelerate the oxidation MoS2 diffraction peaks was decreased significantly in XRD of the base oil and thickener, and consequently degraded atterns of the used grease sample, as shown in Fig. 3b grease[9] [9]. Actually, the metallic elements Cu, Zn, Fe it, the strongest diffraction peak(20= 42, d=0.21 nm) etc. in the used grease were mainly derived from the could be attributed to(110) crystalline plane of the Cu-Zn bearing (ring or roller)and the cage materials (Cu, Zn intermetallic (identified by PDF cards ). At the same time, alloy ). That is to say, the roller bearings had suffered a the weak diffraction peaks of Fe Cr2O4 (about 20= 36, serious extent of wear. With regard to MoS2 diffraction d=0.25 nm, 20= 56, d=0.16 nm, etc. also existed in peaks being weakened in XRD curve of the used grease Fig. 3b. It revealed that the used grease sample was a sample, it suggested that the contents of Mos2 particulates omplex mixture of compounds like MoS2, Cu-Zn inter- in the used grease had decreased since the content of met metallic, FeCr2O4, and so on. Figure 5a shows sEM debris increased in the grease. Thus, the relative value of icrograph of the used greases. Compared to the micro- the Mos, content was reduced. In addition, another factor graph of the fresh grease(Fig. 4a), it is obvious that may be that part of the MoS2 particulates had been disso- were a large amount of irregular particulates in the ciated from the grease and agglomerated on the raceways greases. The EDS analysis of different particulates showed due to loss of the grease network structure induced by that these particulates mainly contained elements like Cu, thermal degradation/oxidation Zn, Fe, Mo, S, etc(see Fig 5b). It can be inferred that the The above analysis demonstrated that the phase com- uneven particulates were resulted from agglomeration of ponents of the used grease samples were quite different different compounds made from these elements, which with that of fresh ones. The structure of the grease in was consistent with above XRD results of the used grease. bearing had significantly changed during the bearing The presence of metallic particulates, particularly those operation
MoS2 diffraction peaks was decreased significantly in XRD patterns of the used grease sample, as shown in Fig. 3b. In it, the strongest diffraction peak (2θ = 42°, d = 0.21 nm) could be attributed to (110) crystalline plane of the Cu–Zn intermetallic (identified by PDF cards). At the same time, the weak diffraction peaks of FeCr2O4 (about 2θ = 36°, d = 0.25 nm, 2θ = 56°, d = 0.16 nm, etc.) also existed in Fig. 3b. It revealed that the used grease sample was a complex mixture of compounds like MoS2, Cu–Zn intermetallic, FeCr2O4, and so on. Figure 5a shows SEM micrograph of the used greases. Compared to the micrograph of the fresh grease (Fig. 4a), it is obvious that there were a large amount of irregular particulates in the used greases. The EDS analysis of different particulates showed that these particulates mainly contained elements like Cu, Zn, Fe, Mo, S, etc. (see Fig. 5b). It can be inferred that the uneven particulates were resulted from agglomeration of different compounds made from these elements, which was consistent with above XRD results of the used grease. The presence of metallic particulates, particularly those containing copper, would further accelerate the oxidation of the base oil and thickener, and consequently degraded the grease [9]. Actually, the metallic elements Cu, Zn, Fe, etc. in the used grease were mainly derived from the bearing (ring or roller) and the cage materials (Cu, Zn alloy). That is to say, the roller bearings had suffered a serious extent of wear. With regard to MoS2 diffraction peaks being weakened in XRD curve of the used grease sample, it suggested that the contents of MoS2 particulates in the used grease had decreased since the content of metal debris increased in the grease. Thus, the relative value of the MoS2 content was reduced. In addition, another factor may be that part of the MoS2 particulates had been dissociated from the grease and agglomerated on the raceways due to loss of the grease network structure induced by thermal degradation/oxidation. The above analysis demonstrated that the phase components of the used grease samples were quite different with that of fresh ones. The structure of the grease in bearing had significantly changed during the bearing operation. Fig. 4 SEM and EDS of the fresh grease sample. (a) SEM micrograph and (b) EDS analysis Fig. 5 SEM and EDS of the used grease sample. (a) SEM micrograph and (b) EDS analysis 162 J Fail. Anal. and Preven. (2011) 11:158–166 123