J Fail. Anal. and Preven. (2012)12: 427-437 DOI10.1007/sl1668-0129583-z TECHNICAL ARTICLE-PEER-REVIEWED Failure analysis of fatigue fracture on the Outer ring of a Cylindrical Roller bearing in an Air Blower Motor Zhi-Qiang Yu. Zhen-Guo Yang Submitted: I November 2011/in revised form: 19 March 2012/Published online: 30 May 2012 C ASM International 2012 Abstract In order to identify the causes of a fracture roller, etc. are the most widely used type of bearings incident that occurred on the ring of a cylindrical because of their excellent tradeoff between size, cost, and oller bearing within an air blower motor, diverse charac- performance [1]. Therein, the cylindrical roller bearing, terization analyses were conducted his study. prototype of which in the form of logs with a large stone Metallurgical structures and chemical compositions of the block on them could even date back to ancient Egyptian era, bearings matrix materials were inspected by metallo- possesses its popularity in applications that are always graphic microscope and photoelectric direct reading involved with heavy radial loading and spectrometer. Scanning electron microscope and energy tion, such as power generation, oil field, and mining dispersive spectroscope were applied to detect the micro- industries. Concretely, a cylindrical roller bearing generally copic morphologies and micro-area compositions on the consists of four parts(Fig. 1),i.e. the inner ring, the outer fracture surfaces and contact surfaces. The compositions ring, the cage, and several cylinder-shaped rollers which are and thermal properties of the lubrication medium, the actually the reasons for the name cylindrical to originate grease, were also examined. Analysis results revealed that Indeed, cylindrical roller bearings have some specific interaction between dry friction and impact both led by the advantages, e.g., the avoidance of fretting which is usu- degraded grease due to decomposition and oxidation at ally encountered in ball bearings [2]. However, failure elevated temperatures, which resulted in serious wear of incidents still occurred on them different the outer cage, was the main cause that orig- unexpected causes. Gerdun et al. [3] analyzed a failure inated and propagated the fatigue cracks on the corners of case of one railway freight wagon wheel that was brought the outer ring, and eventually resulted in the fracture. about by the fatigue fracture on the inner rings of the Finally, countermeasures and suggestions have been bearings. Li et al. [4] even reported that a failed aero- engine was the result of fracture of the cage rivets of one cylindrical roller bearing. Moreover, it is a fact that the Keywords Bearing failure. Failure analysis. Fatigue greases applied between rollers and rings for providing brication are also key factors in ensuring the safe operation of bearings [5, 6]. Prashad [7] testified that the failure of some cylindrical roller bearings in alternators Introduction was just relevant to the chemical decomposition of the grease. Thus, thorough investigation on failure cases of Rolling-element bearings with diverse variants including cylindrical roller bearings, particularly, those aroused by ball, needle, cylindrical roller, spherical roller, tapered failed greases, should be paid special attention to for prevention of recurring incidents In this article, a fracture incident on the outer ring of a Z-QYu()·ZG.Yang Department of Materials Science, Fudan University, Shanghai cylindrical roller bearing that was sealed at the driving end 200433, People's Republic of China of an air blower motor that had a rotation speed of 990 rpm e-mail:yuzhiqiang@fudan.edu.cn in the electric power unit was reported and analyzed Spring
TECHNICAL ARTICLE—PEER-REVIEWED Failure Analysis of Fatigue Fracture on the Outer Ring of a Cylindrical Roller Bearing in an Air Blower Motor Zhi-Qiang Yu • Zhen-Guo Yang Submitted: 1 November 2011 / in revised form: 19 March 2012 / Published online: 30 May 2012 ASM International 2012 Abstract In order to identify the causes of a fracture incident that occurred on the outer ring of a cylindrical roller bearing within an air blower motor, diverse characterization analyses were conducted in this study. Metallurgical structures and chemical compositions of the bearing’s matrix materials were inspected by metallographic microscope and photoelectric direct reading spectrometer. Scanning electron microscope and energy dispersive spectroscope were applied to detect the microscopic morphologies and micro-area compositions on the fracture surfaces and contact surfaces. The compositions and thermal properties of the lubrication medium, the grease, were also examined. Analysis results revealed that interaction between dry friction and impact both led by the degraded grease due to decomposition and oxidation at elevated temperatures, which resulted in serious wear of the outer ring and the cage, was the main cause that originated and propagated the fatigue cracks on the corners of the outer ring, and eventually resulted in the fracture. Finally, countermeasures and suggestions have been proposed. Keywords Bearing failure Failure analysis Fatigue Fracture Introduction Rolling-element bearings with diverse variants including ball, needle, cylindrical roller, spherical roller, tapered roller, etc. are the most widely used type of bearings because of their excellent tradeoff between size, cost, and performance [1]. Therein, the cylindrical roller bearing, prototype of which in the form of logs with a large stone block on them could even date back to ancient Egyptian era, possesses its popularity in applications that are always involved with heavy radial loading and high-speed operation, such as power generation, oil field, and mining industries. Concretely, a cylindrical roller bearing generally consists of four parts (Fig. 1), i.e., the inner ring, the outer ring, the cage, and several cylinder-shaped rollers which are actually the reasons for the name cylindrical to originate. Indeed, cylindrical roller bearings have some specific advantages, e.g., the avoidance of fretting which is usually encountered in ball bearings [2]. However, failure incidents still occurred on them owing to different unexpected causes. Gerdun et al. [3] analyzed a failure case of one railway freight wagon wheel that was brought about by the fatigue fracture on the inner rings of the bearings. Li et al. [4] even reported that a failed aeroengine was the result of fracture of the cage rivets of one cylindrical roller bearing. Moreover, it is a fact that the greases applied between rollers and rings for providing lubrication are also key factors in ensuring the safe operation of bearings [5, 6]. Prashad [7] testified that the failure of some cylindrical roller bearings in alternators was just relevant to the chemical decomposition of the grease. Thus, thorough investigation on failure cases of cylindrical roller bearings, particularly, those aroused by failed greases, should be paid special attention to for prevention of recurring incidents. In this article, a fracture incident on the outer ring of a cylindrical roller bearing that was sealed at the driving end of an air blower motor that had a rotation speed of 990 rpm in the electric power unit was reported and analyzed. Z.-Q. Yu (&) Z.-G. Yang Department of Materials Science, Fudan University, Shanghai 200433, People’s Republic of China e-mail: yuzhiqiang@fudan.edu.cn 123 J Fail. Anal. and Preven. (2012) 12:427–437 DOI 10.1007/s11668-012-9583-z
J Fail. Anal and Preven.(2012)12: 427-437 The face material of the bearing with 18 rolling-elements side of the raceway of the detached failed bearing, turned was GCrl5 bearing steel. The out to become agglomerated, semisolid, and heavy machined brass cage (MA/C3). The lubrication Meanwhile, the outer ring of the bearing presented the was molybdenum disulfide(MoS2)lithium grease. During fracture failure. Hence, in order to confirm the actual its operation, the bearing suddenly failed when its opera- causes of this failure, a variety of characterization methods tional temperature exceeded the warning limit of 70C. were successively conducted according to our previous Afterward, the lubricating grease, which was found on the experiences [8]. Photoelectric direct reading spectrometer, metallographic microscope, and Rockwell hardness(HRC) tester were employed to inspect, respectively, the chemical compositions, the metallographic structures, and the hard- ness of the matrix materials of the failed roller bearing Meanwhile, scanning electron microscope (SEM) and energy dispersive spectroscope(EDS)were used to analyze the microscopic morphologies along with the micro-area compositions of the fracture surface and the contact surface of the bearings outer ring. Furthermore, Fourier transform outer r infrared spectroscopy(FTIR), Raman spectroscopy(RS thermogravimetric analysis (TGA), and x-ray diffraction (XRD)were utilized to characterize the degradation extent of the greases applied within the roller bearing. Based on conclu Isions were put forward hypothesizing that this fracture failure was brought about Fig. 1 3D schematic diagram of a cylindrical roller bearing by both the unqualified matrix materials and the degraded fracture position (c) (d) comer Fig. 2 External appearances of the fracture on the failed roller bearing:(a) total morphology, (b) magnification of front face, (c)magnification of side face, and(d) fractograph
The face material of the bearing with 18 rolling-elements was GCr15 bearing steel. The cage was a combined machined brass cage (MA/C3). The lubrication medium was molybdenum disulfide (MoS2) lithium grease. During its operation, the bearing suddenly failed when its operational temperature exceeded the warning limit of 70C. Afterward, the lubricating grease, which was found on the side of the raceway of the detached failed bearing, turned out to become agglomerated, semisolid, and heavy. Meanwhile, the outer ring of the bearing presented the fracture failure. Hence, in order to confirm the actual causes of this failure, a variety of characterization methods were successively conducted according to our previous experiences [8]. Photoelectric direct reading spectrometer, metallographic microscope, and Rockwell hardness (HRC) tester were employed to inspect, respectively, the chemical compositions, the metallographic structures, and the hardness of the matrix materials of the failed roller bearing. Meanwhile, scanning electron microscope (SEM) and energy dispersive spectroscope (EDS) were used to analyze the microscopic morphologies along with the micro-area compositions of the fracture surface and the contact surface of the bearing’s outer ring. Furthermore, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (RS), thermogravimetric analysis (TGA), and x-ray diffraction (XRD) were utilized to characterize the degradation extent of the greases applied within the roller bearing. Based on these analytic results, conclusions were put forward hypothesizing that this fracture failure was brought about Fig. 1 3D schematic diagram of a cylindrical roller bearing by both the unqualified matrix materials and the degraded Fig. 2 External appearances of the fracture on the failed roller bearing: (a) total morphology, (b) magnification of front face, (c) magnification of side face, and (d) fractograph 428 J Fail. Anal. and Preven. (2012) 12:427–437 123
J Fail. Anal. and Preven. (2012)12: 427-437 Table 1 Chemical compositions of the failed roller bearing (wt %o) Elemen Si Mo Outer ring 0.85l 0.314 0.007 0.009 0.269 0.137 1.751 0.186 0032 0.187 GB/T GCr15095-1050.15-0.35<0.025<0.0250.25-0.450301.401.65<0.10<0.25 AISIE521000.98-1.100.15-0.30<0.0250.025025-0.45 1.30-1.60 (b)R (c) Fig 3 Metallographic structures of the failed roller bearing:(a) outer ring, x500.(b)roller, x500, and(e)cage, x10 greases owing to excessive operational temperature of the exhibits relatively smooth, but contains some traces of bearing. Achievements of this article have critical impor- friction, Fig. 2d. tance in prevention of such failures on the cylindrical roller ngs running under similar operatic Matrix Material Examination Chemical Compositions Experimental and Results Chemical compositions of the matrix materials of the outer Visual observation ring and the roller of this failed roller bearing are listed in Table l, and are also compared with the requirements of Figure 2a presents the location of this fracture, i.e., on the GCr15 bearing steel in GB/T 18254-2002 standard of China bottom of the outer ring of the roller bearing. Moreover, it (approximately equals to AISI E52100). It is obvious that can be learnt from Fig. 2b, c that this fracture is straight, except silicon, other elements in the roller are all qu even crossing the thickness of the bearing. Its fractograph according to the specifications. However, as for the outer Spring
greases owing to excessive operational temperature of the bearing. Achievements of this article have critical importance in prevention of such failures on the cylindrical roller bearings running under similar operation conditions. Experimental and Results Visual Observation Figure 2a presents the location of this fracture, i.e., on the bottom of the outer ring of the roller bearing. Moreover, it can be learnt from Fig. 2b, c that this fracture is straight, even crossing the thickness of the bearing. Its fractograph exhibits relatively smooth, but contains some traces of friction, Fig. 2d. Matrix Material Examination Chemical Compositions Chemical compositions of the matrix materials of the outer ring and the roller of this failed roller bearing are listed in Table 1, and are also compared with the requirements of GCr15 bearing steel in GB/T 18254-2002 standard of China (approximately equals to AISI E52100). It is obvious that except silicon, other elements in the roller are all qualified according to the specifications. However, as for the outer Table 1 Chemical compositions of the failed roller bearing (wt.%) Element C Si S P Mn Ni Cr Mo Cu Outer ring 0.851 0.314 0.007 0.009 0.269 0.137 1.751 0.193 0.186 Roller 0.964 0.583 0.009 0.011 1.064 0.125 1.464 0.032 0.187 GB/T GCr15 0.95–1.05 0.15–0.35 B0.025 B0.025 0.25–0.45 B0.30 1.40–1.65 B0.10 B0.25 AISI E52100 0.98–1.10 0.15–0.30 B0.025 B0.025 0.25–0.45 1.30–1.60 Fig. 3 Metallographic structures of the failed roller bearing: (a) outer ring, 9500, (b) roller, 9500, and (c) cage, 9100 J Fail. Anal. and Preven. (2012) 12:427–437 429 123
J Fail. Anal and Preven.(2012)12: 427-437 ring, its carbon content does not meet the requirement, while of the matrix materials of the outer ring and the roller are its chromium content exceeds the requirement In terms of displayed in Fig. 3. As the heat treatment conditions of the the former one, it is common sense that steel hardness is GCr15 steel used in the bearing are quenching and temper proportional to the carbon content in it; as to the latter one, ing, it can be observed that both the materials are composed higher chromium content will bring about more retained of acicular martensites and evenly distributed carbides. austenites, which are apt to deform under stresses in the However, more grain boundaries of the retained austenites microstructure after heat treatment. Consequently, owing to were present in the outer ring, Fig. 3a. This phenomenon is these two factors, it can be concluded that hardness of the consistent with the results of the chemical compositional outer ring of the failed bearing is intrinsically unqualified. analysis. Besides, the cage consisted of both acicular a phase As for the cage, its chemical compositions are Zn 40% (white)and B phase(black), Fig 3c, showing the typical (wt %), Cu 59%0, and Pb 1%, in accordance with the AstM microstructure of casting brass alloy C37800 lead brass requiremen Hardness Survey Metallographic Structures Table 2 lists the hardness results of the outer ring and the Etched in agent of picric acid (2, 4, 6-trinitrophenol)1.0 g, roller. It clearly reveals that the outer ring possesses lower HCI5.0 mL, and ethanol 100 mL, metallographic structures hardness than GCr15 specification, which further testifies the results of its composition. Table 2 Hardness of the failed roller bearing(hrc) Test position Outer ring Roller Specification SEM and EDS Analyses 65.3 Fracture Surface 60.2 65.5 First, fracture surface of the outer ring was observed Average using SEM. As shown in Fig 4a, dissociation steps along so w spot Moon Set woo fiae Fig 4 SEM morphologies of the fracture surface:(a)total morphology, (b) trace of friction, (c) trace of squeeze, and(d) actinomorphou
ring, its carbon content does not meet the requirement, while its chromium content exceeds the requirement. In terms of the former one, it is common sense that steel hardness is proportional to the carbon content in it; as to the latter one, higher chromium content will bring about more retained austenites, which are apt to deform under stresses in the microstructure after heat treatment. Consequently, owing to these two factors, it can be concluded that hardness of the outer ring of the failed bearing is intrinsically unqualified. As for the cage, its chemical compositions are Zn 40% (wt.%), Cu 59%, and Pb 1%, in accordance with the ASTM C37800 lead brass requirement. Metallographic Structures Etched in agent of picric acid (2,4,6-trinitrophenol) 1.0 g, HCl 5.0 mL, and ethanol 100 mL, metallographic structures of the matrix materials of the outer ring and the roller are displayed in Fig. 3. As the heat treatment conditions of the GCr15 steel used in the bearing are quenching and tempering, it can be observed that both the materials are composed of acicular martensites and evenly distributed carbides. However, more grain boundaries of the retained austenites were present in the outer ring, Fig. 3a. This phenomenon is consistent with the results of the chemical compositional analysis. Besides, the cage consisted of both acicular a phase (white) and b phase (black), Fig. 3c, showing the typical microstructure of casting brass alloy. Hardness Survey Table 2 lists the hardness results of the outer ring and the roller. It clearly reveals that the outer ring possesses lower hardness than GCr15 specification, which further testifies the results of its composition. SEM and EDS Analyses Fracture Surface First, fracture surface of the outer ring was observed using SEM. As shown in Fig. 4a, dissociation steps along Table 2 Hardness of the failed roller bearing (HRC) Test position Outer ring Roller Specification 1 60.2 65.3 65–66 2 60.3 65.8 3 60.2 65.5 Average 60.2 65.5 Fig. 4 SEM morphologies of the fracture surface: (a) total morphology, (b) trace of friction, (c) trace of squeeze, and (d) actinomorphous fracture origin 430 J Fail. Anal. and Preven. (2012) 12:427–437 123
J Fail. Anal. and Preven. (2012)12: 427-437 with dimples were present on the fracture surface, which surface, which is also depicted in Fig. 2d. This shows represents a typical morphology of quasi-dissociation actually the distinct evidence of the fracture origin, and fracture. Furthermore, traces of mutual friction and will be concretely explained as follows ueeze can be also found on the fracture surface. as shown in Fig 4b, c, respectively. These two phenomena Contact Surface on the Outer ring may have resulted from the contact between the two counterfaces of the outer ring after fracture. It should be The contact surface between the outer ring and the roller particularly noted from Fig 4d that actinomorphous also was then analyzed under SEM and EDS. Figure 5a fringes have originated from the corner of the fracture displays neat machining fringes on the fresh surface of the (a) (d) 2.003.04.00s,006,0D7.00n,09, 1..。2.。03.004.00s0G.DO7.B.0分.o Fig 5 SEM morphologies and EDS of the contact surface on the outer ring:(a) normal morphology, (b) lar trenches, (c)concave, (d adhered substances (e) eDs of site nd (f eds of site B Spring
with dimples were present on the fracture surface, which represents a typical morphology of quasi-dissociation fracture. Furthermore, traces of mutual friction and squeeze can be also found on the fracture surface, as shown in Fig. 4b, c, respectively. These two phenomena may have resulted from the contact between the two counterfaces of the outer ring after fracture. It should be particularly noted from Fig. 4d that actinomorphous fringes have originated from the corner of the fracture surface, which is also depicted in Fig. 2d. This shows actually the distinct evidence of the fracture origin, and will be concretely explained as follows. Contact Surface on the Outer ring The contact surface between the outer ring and the roller also was then analyzed under SEM and EDS. Figure 5a displays neat machining fringes on the fresh surface of the Fig. 5 SEM morphologies and EDS of the contact surface on the outer ring: (a) normal morphology, (b) irregular trenches, (c) concave, (d) adhered substances (e) EDS of site A, and (f) EDS of site B J Fail. Anal. and Preven. (2012) 12:427–437 431 123
J Fail. Anal and Preven.(2012)12: 427-437 Fig 6 SEM morphologies of the greases:(a) fresh and(b) used outer ring. However, as shown in Fig 5b, irregular tren- present in it, which were then compared with those in the ches were produced on the contact surface. Even in some fresh one as well. As shown in Fig. 7a, wave numbers of locations, a few relatively large concaves can be also 2900, 1460, 1380, 970, and 720 cm are the absorption observed, Fig 5c. However, its chemical composition is peaks of aliphatic hydrocarbons groups, while 3500 and identical with the matrix material (Fig. 5e); in other words, 1570 cm -, respectively, represent the -OH and-COOH this concave was only brought about by exfoliation, rather groups. In other words, the main composition of the grease than by any type of corrosion. Meanwhile, some black- was that of hydroxy stearic acid. However, in terms of the colored substances adhering to the contact surface too are used grease(Fig. 7b), the stretching vibration absorptio red Fig. 5d. According to the eds results(fig peak of C=0 is observed at 1709 cm, which may have besides the elements of the matrix material, molybdenum, been due to the thermal oxidation of -OH at elevated Mo, and sulfur, S, are also present In other words, such temperatures. In other words, constituents of the used substances were the residua of the greases which may have grease had decomposed and probably could not provide the failed as well, and this phenomenon deserves further sufficient lubrication effect. Investigation RS Inspection of the greases Similarly, both the used and the fresh greases were As was discussed above the fracture of the outer ring was studied by RS. As displayed in Fig. &a, wave num relevant to the grease of the roller bearing; consequently, around 1570 and 1740 cm are ascribed to the vibratio the following research will be focused on the greases, both peaks of-COOH, and those from about 2830-2930 cify- the used and the fresh stand for the -CH3 and-CH2 groups, which are consistent with the FTIR results of the fresh grease compositions sem and eDs Nevertheless, in the case of the used grease, several distinct changes could be observed, Fig. 8b. In Fig. &b, the wave Compared with the morphology of the fresh grease in numbers around 416 and 568 cm are, respectively, Fig. 6a, the used grease was covered with several white ascribed to the peaks of copper and molybdenum oxides, substances, Fig 6b. According to the EDS results(not i.e. they correspond to the wear debris of the cage and the hown)of the six test sites shown in Fig. 6b, it was found oxidation products of the Mos2 additives in the grease hat the main elements of these substances were Cu and Zn. Meanwhile, 1335, 1681, 1767, and 1817 cm represent which are the dominant constituents in the cage It can be the peaks of C=o group, same as the Ftir result obtained suggested that the cage bars had become worn severely in the case of the used grease because of improper impact on the cage FTIR The used grease obtained from the failed roller bearing was the two kinds of greases. Compared with the starting tem- inspected by FtiR to detect the organic functional groups perature as 199.5"C for the fresh grease(Fig. 9a), the said
outer ring. However, as shown in Fig. 5b, irregular trenches were produced on the contact surface. Even in some locations, a few relatively large concaves can be also observed, Fig. 5c. However, its chemical composition is identical with the matrix material (Fig. 5e); in other words, this concave was only brought about by exfoliation, rather than by any type of corrosion. Meanwhile, some blackcolored substances adhering to the contact surface too are observed, Fig. 5d. According to the EDS results (Fig. 5f), besides the elements of the matrix material, molybdenum, Mo, and sulfur, S, are also present. In other words, such substances were the residua of the greases which may have failed as well, and this phenomenon deserves further investigation. Inspection of the Greases As was discussed above, the fracture of the outer ring was relevant to the grease of the roller bearing; consequently, the following research will be focused on the greases, both the used and the fresh. SEM and EDS Compared with the morphology of the fresh grease in Fig. 6a, the used grease was covered with several white substances, Fig. 6b. According to the EDS results (not shown) of the six test sites shown in Fig. 6b, it was found that the main elements of these substances were Cu and Zn, which are the dominant constituents in the cage. It can be suggested that the cage bars had become worn severely because of improper impact on the cage. FTIR The used grease obtained from the failed roller bearing was inspected by FTIR to detect the organic functional groups present in it, which were then compared with those in the fresh one as well. As shown in Fig. 7a, wave numbers of 2900, 1460, 1380, 970, and 720 cm1 are the absorption peaks of aliphatic hydrocarbons groups, while 3500 and 1570 cm1 , respectively, represent the –OH and –COOH groups. In other words, the main composition of the grease was that of hydroxy stearic acid. However, in terms of the used grease (Fig. 7b), the stretching vibration absorption peak of C=O is observed at 1709 cm1 , which may have been due to the thermal oxidation of –OH at elevated temperatures. In other words, constituents of the used grease had decomposed and probably could not provide the sufficient lubrication effect. RS Similarly, both the used and the fresh greases were also studied by RS. As displayed in Fig. 8a, wave numbers around 1570 and 1740 cm1 are ascribed to the vibration peaks of –COOH, and those from about 2830–2930 cm1 stand for the –CH3 and –CH2 groups, which are consistent with the FTIR results of the fresh grease compositions. Nevertheless, in the case of the used grease, several distinct changes could be observed, Fig. 8b. In Fig. 8b, the wave numbers around 416 and 568 cm1 are, respectively, ascribed to the peaks of copper and molybdenum oxides, i.e., they correspond to the wear debris of the cage and the oxidation products of the MoS2 additives in the grease. Meanwhile, 1335, 1681, 1767, and 1817 cm1 represent the peaks of C=O group, same as the FTIR result obtained in the case of the used grease. TGA Figure 9 presents the thermal decomposition properties of the two kinds of greases. Compared with the starting temperature as 199.5C for the fresh grease (Fig. 9a), the said Fig. 6 SEM morphologies of the greases: (a) fresh and (b) used 432 J Fail. Anal. and Preven. (2012) 12:427–437 123
J Fail. Anal. and Preven. (2012)12: 427-437 100.0 3.0081432 137714 3.92 2923.6 4000,0 3200 cm-1 I00.0 491154107 966.50 299.52 170988 1460.17 20 285463 4000.0 7 FTIR results of the greases: (a) fresh and(b)used temperature of the used grease is only 116.6.C(Fig. 9b-a operation and generated low-molecular-weight products reduction of about 83C. In contrast, the final temperature on the other hand, the grease was also oxidized simulta for the used grease(887.0C)is 102.C higher than that for neously at elevated temperatures and produced saturated the fresh one(7854C), but its residual weight percentage organic compounds that were hard to decompose. Anyway. (60.6%)is nearly 25 times the fresh grease's(2.5%). These such changes had greatly decreased the lubrication capacity two contrasting results can be explained as follows: On of the grease, and consequently, clearly exerted serious dry the one hand, the grease thermally decomposed during friction on the roller bearin Spring
temperature of the used grease is only 116.6C (Fig. 9b)—a reduction of about 83C. In contrast, the final temperature for the used grease (887.0C) is 102C higher than that for the fresh one (785.4C), but its residual weight percentage (60.6%) is nearly 25 times the fresh grease’s (2.5%). These two contrasting results can be explained as follows: On the one hand, the grease thermally decomposed during operation and generated low-molecular-weight products; on the other hand, the grease was also oxidized simultaneously at elevated temperatures and produced saturated organic compounds that were hard to decompose. Anyway, such changes had greatly decreased the lubrication capacity of the grease, and consequently, clearly exerted serious dry friction on the roller bearing. Fig. 7 FTIR results of the greases: (a) fresh and (b) used J Fail. Anal. and Preven. (2012) 12:427–437 433 123
J Fail. Anal and Preven.(2012)12: 427-437 Fig& RS results of the greases: (a)fresh and(b)used Wavenumber (cm-1) 会 wavenumber (cm-1) During the operation of the air blower motor, the tem- perature of the roller bearing within the motor would be In order to characterize the solid substances in the greases, inevitably increased to some extent. Under this condition, KRD analyses were employed. As shown in Fig. 10a, the the grease used in the bearing suffered two kinds of deg fresh grease exhibits clear peaks of the Mos2 additives in radation, i.e., the decomposition and the oxidation. The the grease. However, complicated compounds such as former one would produce several low-molecular-weight Co3 ZnC appear in the used grease(Fig. 10b), which even organic compounds, and the latter one would generate contained the Zn element originally in the cage. Moreover, products with higher saturation, which were not easy to the Mosz peaks have disappeared, which further confirms decompose, i.e., the hardening effect. Only because of such the failure of the grease and the serious friction from degradations, the lubrication capacity of the grease had roller and the cage, and therefore the cages have bece deteriorated. Consequently, two harmful results were worn severely. brought about. On the one hand, with the decrease lubrication capacity, the lubrication mode between the roller and the outer ring changed from oil friction to the dry Discussion friction, which would certainly aggravate the extent of wear on the outer ring[9-12]. In addition, as the hardness Based on the analytic results of the fractured outer ring of of the outer ring was relatively lower than that of the roller, the cylindrical roller bearing and the used grease, origin of owing to its unqualified chemical compositions, the contact his fracture failure could be clearly put forward surface of the outer ring was destroyed and, with irregular
XRD In order to characterize the solid substances in the greases, XRD analyses were employed. As shown in Fig. 10a, the fresh grease exhibits clear peaks of the MoS2 additives in the grease. However, complicated compounds such as Co3ZnC appear in the used grease (Fig. 10b), which even contained the Zn element originally in the cage. Moreover, the MoS2 peaks have disappeared, which further confirms the failure of the grease and the serious friction from the roller and the cage, and therefore the cages have become worn severely. Discussion Based on the analytic results of the fractured outer ring of the cylindrical roller bearing and the used grease, origin of this fracture failure could be clearly put forward. During the operation of the air blower motor, the temperature of the roller bearing within the motor would be inevitably increased to some extent. Under this condition, the grease used in the bearing suffered two kinds of degradation, i.e., the decomposition and the oxidation. The former one would produce several low-molecular-weight organic compounds, and the latter one would generate products with higher saturation, which were not easy to decompose, i.e., the hardening effect. Only because of such degradations, the lubrication capacity of the grease had deteriorated. Consequently, two harmful results were brought about. On the one hand, with the decrease of lubrication capacity, the lubrication mode between the roller and the outer ring changed from oil friction to the dry friction, which would certainly aggravate the extent of wear on the outer ring [9–12]. In addition, as the hardness of the outer ring was relatively lower than that of the roller, owing to its unqualified chemical compositions, the contact surface of the outer ring was destroyed and, with irregular Fig. 8 RS results of the greases: (a) fresh and (b) used 434 J Fail. Anal. and Preven. (2012) 12:427–437 123
J Fail. Anal. and Preven. (2012)12: 427-437 Fig.9 TGA results of the PerkinElmer Thermal Analysi greases: (a)fresh and(b)used Xl=199450℃ Delta Y=97.083%% Temperature(C) Perkin Elmer Thermal Analvsis Delta=39.4667% Y2=60.6199% X2=886.987℃ trenches and concaves emerging on it, and undoubtedly symmetrical corners of the bottom part of the outer ring. yielding some debris. On the other hand, owing to the Therefore, during operation, fatigue cracks emerged and presence of hardened grease particles and their accumula- propagated on the two sites, and finally connected mutu- tion on the bottom of the bearing, the rollers had to undergo ally, to form the macroscopic fracture crossing the slight bounces during operation, which would then create thickness of the bearing. Actually, once the roller bearing some kind of impact on both the cage and the bottom of the failed, the rotor could not work normally, and this led to the outer ring, yielding some cage debris due to serious wear of increase of the service temperature within the motor, the cages. As a result, the two species of debris-which finally brought about the unpredicted catastrophes. What a were, respectively, derived from the outer ring and the tremendous Butterfly Effect cage--were engendered on the contact surface between the roller and the outer ring. Actually, such debris would help Conclusions and recommendations in increasing the extent of wear in return; in other words, it was a self-circulation process. Then, under the interaction between dry friction and Conclusions impact from the roller on the outer ring, stresses were 1. Except the rollers, matrix material of the outer ring of generated, which began accumulating on the two the cylindrical roller bearing was unqualified accordin Spring
trenches and concaves emerging on it, and undoubtedly yielding some debris. On the other hand, owing to the presence of hardened grease particles and their accumulation on the bottom of the bearing, the rollers had to undergo slight bounces during operation, which would then create some kind of impact on both the cage and the bottom of the outer ring, yielding some cage debris due to serious wear of the cages. As a result, the two species of debris—which were, respectively, derived from the outer ring and the cage—were engendered on the contact surface between the roller and the outer ring. Actually, such debris would help in increasing the extent of wear in return; in other words, it was a self-circulation process. Then, under the interaction between dry friction and impact from the roller on the outer ring, stresses were generated, which began accumulating on the two symmetrical corners of the bottom part of the outer ring. Therefore, during operation, fatigue cracks emerged and propagated on the two sites, and finally connected mutually, to form the macroscopic fracture crossing the thickness of the bearing. Actually, once the roller bearing failed, the rotor could not work normally, and this led to the increase of the service temperature within the motor, and finally brought about the unpredicted catastrophes. What a tremendous Butterfly Effect! Conclusions and Recommendations Conclusions 1. Except the rollers, matrix material of the outer ring of the cylindrical roller bearing was unqualified according Fig. 9 TGA results of the greases: (a) fresh and (b) used J Fail. Anal. and Preven. (2012) 12:427–437 435 123
J Fail. Anal and Preven.(2012)12: 427-437 Fig. 10 XRD results of the greases: (a)fresh and(b)used 2Theta [deg. 1 000 40.000 to the gB/T GCrl5 specifications, which led to the current lubricating greases and the choice of the greases reduction of its hardness, and consequently, a highe with superior thermal stability to serve as the lubrica probability of deformation when under friction 2. The grease was decomposed and oxidized underelevated 3. In addition, shortening the cycle of lubricant replen- temperatures during operation, which decreased the ishment in the track of the bearing is probably the lubrication capacity of the grease. Meanwhile, part of simplest approach to ensure the running of a bearing the grease was hardened and then formed some solid under a well-lubricated condition all the time particles between the rollers and the outer ring. 3. Owing to the degradation of the grease, serious dry Acknowledgments This study was supported by Shanghai Leading friction and slight impact were engendered from the Academic Discipline Project(Project Number:B113).Further, the rollers, respectively, on the contact surface of the outer authors owe their gratitude also to Shanghai Research Institute ring and the cage, which resulted in serious wear of the Materials for providing various experimental facilities outer ring and the cage 4. Under the interaction between dry friction and impact, stresses were generated on the corners of the outer ring References from which sites fatigue cracks were originated and 1. Hamrock. B.L. Anderson,wJ:Rolling-Element Bearing.NASA propagated, and finally formed the macroscopic frac Reference Publication, Cleveland (1983) ture crossing the thickness of the outer ring. 2. Ost, W, De Baets, P: Failure analysis of the deep groove bearings of an electric motor. Eng. Fail. Anal. 12, 772-7 (2005) Recommendations 3. Gerdun. v. Sedmak. T. Sinkovec. V. et al nd axles in railway freight wagons. Eng. Fail. 1. Cylindrical roller bearings with qualified chemical oppositions of the matrix materials, especially for the 4. Li, YJ, Tao, CH, Zhang, W.F., Jiang, T: Fracture an outer ring should be used to substitute the current ones. cage rivets of a cylindrical roller bearing. Eng. Fail. Al 796801(2008 To extend the lifetime of the bearing, the best 5. Cann, P. M.: Grease degradation in a bearing simulation device countermeasure probably lies in the substitution of the Tribol.Int.39,1698-1706(2006)
to the GB/T GCr15 specifications, which led to the reduction of its hardness, and consequently, a higher probability of deformation when under friction. 2. The grease was decomposed and oxidized under elevated temperatures during operation, which decreased the lubrication capacity of the grease. Meanwhile, part of the grease was hardened and then formed some solid particles between the rollers and the outer ring. 3. Owing to the degradation of the grease, serious dry friction and slight impact were engendered from the rollers, respectively, on the contact surface of the outer ring and the cage, which resulted in serious wear of the outer ring and the cage. 4. Under the interaction between dry friction and impact, stresses were generated on the corners of the outer ring, from which sites fatigue cracks were originated and propagated, and finally formed the macroscopic fracture crossing the thickness of the outer ring. Recommendations 1. Cylindrical roller bearings with qualified chemical compositions of the matrix materials, especially for the outer ring, should be used to substitute the current ones. 2. To extend the lifetime of the bearing, the best countermeasure probably lies in the substitution of the current lubricating greases and the choice of the greases with superior thermal stability to serve as the lubrication medium. 3. In addition, shortening the cycle of lubricant replenishment in the track of the bearing is probably the simplest approach to ensure the running of a bearing under a well-lubricated condition all the time. Acknowledgments This study was supported by Shanghai Leading Academic Discipline Project (Project Number: B113). Further, the authors owe their gratitude also to Shanghai Research Institute of Materials for providing various experimental facilities. References 1. Hamrock, B.J., Anderson, W.J.: Rolling-Element Bearing. NASA Reference Publication, Cleveland (1983) 2. Ost, W., De Baets, P.: Failure analysis of the deep groove ball bearings of an electric motor. Eng. Fail. Anal. 12, 772–783 (2005) 3. Gerdun, V., Sedmak, T., Sˇinkovec, V., et al.: Failures of bearings and axles in railway freight wagons. Eng. Fail. Anal. 14, 884–894 (2007) 4. Li, Y.J., Tao, C.H., Zhang, W.F., Jiang, T.: Fracture analysis on cage rivets of a cylindrical roller bearing. Eng. Fail. Anal. 15, 796–801 (2008) 5. Cann, P.M.: Grease degradation in a bearing simulation device. Tribol. Int. 39, 1698–1706 (2006) Fig. 10 XRD results of the greases: (a) fresh and (b) used 436 J Fail. Anal. and Preven. (2012) 12:427–437 123