Failure Analysis of an Aero Engine Ball Bearing

Transcription

1 JFAPBC (2006) 6:25-31 ASM International DOI: / X / $19.00 Failure Analysis of an Aero Engine Ball Bearing N. Ejaz, I. Salam, and A. Tauqir (Submitted June 10, 2006; in revised form September 9, 2006) An aero engine failed due to the misalignment of the ball bearing fitted on the main shaft of the engine. The aero engine incorporates two independent compressors: a six-stage axial flow low-pressure compressor and a nine-stage axial flow high-pressure compressor. The bearing under consideration is a high-pressure-location bearing and is fitted at the rear of the nine-stage compressor. It was supposed to operate for at least 5000 h but failed catastrophically after 1300 h of operation and rendered the engine unserviceable. Unusually high stresses caused by misalignment and uneven axial loading resulted in the generation of fatigue crack(s) in the inner race. When the crack reached the critical size, the collar of the race fractured, causing subsequent damage. The cage also failed due to excessive stresses in the axial direction, and its material was smeared on the steel balls and the outer race. Keywords: aero engine component, ball bearing, bearing steel, fatigue, misalignment, tool steels Background The ball bearing assembly consisted of an inner race, an outer race, a cage, and balls. This bearing supported the high-speed assembly, compressor, and turbine that rotated at 13,970 rpm. The sketch in Fig. 1 shows the cross section of the bearing assembly. The bearing inner race was a split type, in the form of two complementary rings. One ring had a recess on the outer side, for locking purposes. To facilitate the discussion in this paper, the inner race with the locking recess will be termed IRA, while the other race will be termed IRB. In general, the life of the bearing was much higher than the specified 5000 working hours. However, in this specific case, after 683 operating hours, metal particles were observed on the magnetic chip detector fitted on an oil scavenge line. The composition of the particles was 18% W, 4% Cr, 1% V, with the balance iron. The oil temperature was 150 C. The engine remained in service after the particles were detected, at the advice of the manufacturer. After completion of 1300 h of service, catastrophic failure of the bearing rendered the engine unusable. Fig. 1 Investigation Results Visual Observation Figure 2 shows the components of the failed ball bearing. The observations are given in the following subsections. Inner Race. The IRB was comparatively undamaged and had no significant scratches/marks. The IRA was damaged, and its one collar was fractured (Fig. 3). Figure 4(a) shows the fracture surface of the collar. The crack was propagated in the race, as shown in Fig. 4(b). The broken pieces Schematic showing the ball bearing in cross section N. Ejaz, I. Salam, and A. Tauqir, KRL, GPO Box 502, Rawalpindi, Pakistan. Contact noveedejaz@yahoo.com. 25

2 Failure Analysis of an Aero Engine Ball Bearing (continued) of the collar were deformed, and a yellowish material was observed on the fracture surfaces. The inner surface of the race where the balls rest was rough and worn. Deformation was also observed in the groove of the IRA. Outer Race. The outer race also contained a smeared yellowish material. Axial smearing bands were observed on the ball course (Fig. 5). All the bands extended to one shoulder of the groove and were not observed on the other shoulder. Cage. The two sides of the damaged cage are shown in Fig. 6(a) and (b). The damage was comparatively higher on one side of the cage. The cage fractured into six pieces. Small pieces were deformed and fractured between the ball recess regions. Additionally, a number of cracks were found in the periphery between the ball pockets (Fig. 6c). On the damaged side, the collar of the cage appeared deformed between the ball(s) and the outer race. Debris was observed between the ball pockets and on the damaged side. Scuffing, rubbing, and deformation were also observed on the outer surface of the cage (Fig. 6d). Balls. All the balls were deformed in a similar manner (Fig. 7). The deformed material was accumulated on a side of the deformed surface. The deformation seemed to be due to ball movement on the outer race shoulder. Cage material was embedded on the surface of the balls. Cavities, probably due to spalling, were also present on the balls. Fig. 2 Failed bearing assembly Fig. 4 The IRA. (a) Fracture surface. (b) Crack near the fracture Fig. 3 Split rings of inner race showing fracture area Fig. 5 Outer race showing axial smearing bands (arrows) 26 Volume 6(6) December 2006 Journal of Failure Analysis and Prevention

3 Materials The material of the inner race, outer race, bearing balls, and cage was identified by energydispersive spectroscopy, and the carbon and sulfur content was determined by using a carbonsulfur analyzer. The results for the ferrous materials are given in Table 1 and for the nonferrous materials in Table 2. Optical Microscopy Two sections from the cage, inner race, outer race, and balls were prepared for metallography. The ferrous and nonferrous samples were etched in 5% nital Fig. 6 solution and a solution of 2 g K 2 Cr 2 O 7, 8 ml H 2 SO 4, 1 ml HCl, and 100 ml H 2 O, respectively. Microstructural observations are summarized in the following sections. Inner Race. At low magnifications, the cross section in the etched condition showed bowed lines, reflecting the curvature of the race (Fig. 8a). The microstructure consisted of tempered martensite containing undissolved carbides in the matrix. [1] Carbides were aligned in the direction of the work, which was probably carried out during the material processing. [2] Samples were prepared from the region f, as marked in Fig. 4(b), to examine the crack features in the as-polished condition. The crack propagated toward the outer shoulder of the groove (Fig. 9ac). Similar cracks were also observed at other locations on the IRA. The upper layer of the damaged race groove was deformed, compared with the undamaged comple- Cage. (a) Comparatively less damaged side. (b) Damaged side. (c) Deformation and cracks between the ball pockets. (d) Deformation and rubbing marks on outer periphery Fig. 7 Table 2 Balls showing smearing, deformation, and spalling Chemical Analysis Nonferrous Materials Element, wt.% Component Cu Ni Si Cage bal 2.4 ± ± 0.1 Standard C bal Table 1 Chemical Analysis Ferrous Materials Element, wt.% Component Fe C W Cr V Mn S Inner race bal ± ± ± ± Outer race bal ± ± ± Balls bal ± ± ± ± Standard AISI T1 bal

4 Failure Analysis of an Aero Engine Ball Bearing (continued) mentary part. Such deformation bands were observed at many other locations on the IRA, while such deformation was observed only in a small region on the IRB (Fig. 10). The deformed bands on the IRA and IRB, respectively, are shown in Fig. 10(a) and (b). Outer Race. The microstructural features of the outer race were identical to features on the inner race. A crack was observed in a section of the outer race (Fig. 11a, b). This crack propagated from inside the ball groove to the outer shoulder. Yellow- and silver-colored smeared material was found on one side of the groove, as shown in Fig. 11(c). Cage. The microstructural features of the cage consist of alpha grains containing very fine nickelsilicon particles in the matrix, [2] as shown in Fig. 12. The surface of the cage was silver coated. Bearing Balls. The microstructural features of the balls were identical to the inner race. A cross section of a ball was prepared for metallography. In the center, three large cracks, parallel to each Table 3 Component Hardness Fig. 8 (a, b) Inner race; aligned carbides in the matrix of tempered martensite Component Hardness, HV Inner race Undeformed regions 788 ± 3 Deformed regions 850 ± 10 Outer race Undeformed regions 807 ± 10 Deformed regions 800 ± 9 Ball Undeformed regions 782 ± 9 Deformed regions 824 ± 5 Cage 179 ± 2 Fig. 9 Cracks in the IRA 28 Volume 6(6) December 2006 Journal of Failure Analysis and Prevention

5 other, were observed (Fig.13). Some cracks were also observed near the deformed surface. Hardness A Vickers hardness tester was used to measure the hardness of the components. The results are given in Table 3. Inner Race On the IRA, the groove was found to be deformed and chipped from the ball rest location. Wear marks were observed on the deformed region across the track of the balls on the race. This is indicative of the offcenter vibration of the balls, which entrapped hard particles in the process. The particles could be generated from the balls/groove wear or from some other source. The material of the inner and outer races and balls was T1 (tungsten high-speed steel). The high alloying and high carbon content produced a large number of hard, wear-resistant carbides in the microstructure [3] to achieve high hardness and wear resistance. The material is tempered after quenching. The higher hardness values in the deformed regions of the inner and outer races are probably due to the excessive deformation. The fatigue crack causing the fracture of the collar started from the inner side of the damaged groove and propagated toward the outer side. Fatigue Fractography Samples from the fractured region of the inner race were examined by scanning electron microscopy. Numerous cracks were observed in the vicinity of crack initiation (Fig. 14a). Fatigue striations were observed on the fracture surface, as shown in Fig. 14(b). Discussion On the basis of all the visual and metallographic observations, a schematic was drawn of the bearing assembly just before final failure (Fig. 15). Fig. 10 Fig. 11 Photograph of etched inner race showing deformation band on damaged side Smearing and crack in the outer race 29

6 Failure Analysis of an Aero Engine Ball Bearing (continued) striations on the fracture surface indicate that these cracks were generated during high-speed rotation and axial loading. A small oxidized region (etched dark) on the fracture surface shows that the crack remained open during the process of fatigue crack propagation. Deformation bands observed on the two halves of the inner race groove indicated the off-center movement of the balls. This off-center movement generated stresses that resulted in the off-center deformation of the inner race surface. This deformation was clear on the IRA groove, while it was present only on the inner edge of the other half (IRB). This was confirmed by studying samples from various locations (from the IRA as well as the IRB), including the fracture region. Deformation bands were present in all the samples. The band width decreased from the outer edge to the inner on the IRA, while a very thin area was observed on the inner side of the IRB. Fig. 12 Fig. 13 Cage section showing general microstructure and silver coating Cracks in the cross section of a ball Outer Race On the outer race, the deformation and nicks were found on one shoulder. The crack found in the outer race was started from the groove and propagated to one shoulder. Cage material was smeared in the groove of the race. The deformation decreased from one shoulder of the groove to the other. Axial smearing marks present on the race could be due to the axial displacement between the inner and outer races under load. [4] Cage The damage on the cage was limited to one side, and cracks were present between the ball pockets. This should be due to axial loading. [4] The material of the cage was silicon bronze with a silver coating on it. Silver coating is used to give good resistance to fretting and improves Fig. 14 Fracture surface of the IRA. (a) Cracks. (b) Fatigue striations 30 Volume 6(6) December 2006 Journal of Failure Analysis and Prevention

7 the bedding-in and running properties of harder bearing materials. [5] Balls The balls had the shoulder impression of the outer race. It indicated that the balls were off-center and rode over the shoulder of the outer race before failure. The cage material was smeared on the balls. This may be due to jamming of the rolling element in the cage pockets. [4] The cracks were found in the center of the ball, which would be due to excessive loading before jamming/failure. All of the above discussion indicated misalignment of the bearing and the axial loading of the bearing. The conclusions made from these observations are consistent with the conclusions of previous studies. [4] Sequence of Failure Misalignment and uneven axial loading resulted in unusually high cyclic stresses. In the inner race, where stresses were high and material was soft, fatigue cracks initiated. When the most severe crack reached an appropriate size, the race fractured from the collar. The rotating balls shifted from their mean path, causing excessive wear and ultimately deforming the cage collar. The whole assembly became jammed and rendered the engine unserviceable. Conclusions This study demonstrates the importance of alignment in bearing service. Although this fact is well known, failures can be prevented when better attention is given to assuring proper alignment during service. References 1. W.F. Smith: The Structure and Properties of Engineering Alloys, 2nd ed., McGraw-Hill, 1986, pp Metallography and Microstructures, vol. 9, Metals Handbook, 9th ed., American Society for Metals, 1985, pp. 264, Properties and Selection: Stainless Steels, Tool Materials, and Special-Purpose Metals, vol. 3, Metals Handbook, 9th ed., American Society for Metals, 1980, pp Fig. 15 Schematic showing the cross section of the bearing just before failure 4. Failure Analysis and Prevention, vol. 11, Metals Handbook, 9th ed., American Society for Metals, 1986, pp , Eng. Fail. Anal., 1998, 5(4), pp