Laboratory Analysis of Spalled FAG 6326 Deep Groove Ball Bearing

Transcription

1 Laboratory Analysis of Spalled FAG 6326 Deep Groove Ball Bearing Abstract : A 6326 bearing was submitted for analysis because of inner ring spalling. Laboratory analysis shows the bearing failed from overloading and overheating. The combination of these two conditions led to breakdown of the lubricant film, and resulted in surface initiated spalling of the inner ring. The contact angle of the balls against the inner ring was 14, indicating axial force may have significantly affected the loading in the bearing that led to failure. Author : Stu Smalinskas, P.E. Report N AT13A020 Project N Date 19 February (17)

2 Background A bearing was received with a spalled inner ring. Customer requested analysis and characterization of the damage to determine root cause of bearing failure. Figure 1. Bearing, as received. Sampling No. received: one bearing Date received: 11 February 2013 Part number: 6326 Factory: FAG Application: motor Service life: 2 years Number stamped on outer ring: FAG M.C USA E J Procedure Method Equipment Macroexamination NATC Work Instruction MATL 2110 NATC Equip. No SEM Examination NATC Work Instruction MATL 2120 NATC Equip. No Rockwell Hardness Testing NATC Work Instruction MATL 2300 NATC Equip. No Metallographic Analysis NATC Work Instruction MATL 2790 NATC Equip. No NATC Equip. No NATC Equip. No Fourier Transform infrared spectroscopy (FTIR) NATC Work Instruction CHEM 1048 NATC Equip. No Report N AT13A020 Project N Date 19 February (17)

3 Results Figure 2. Inner ring, showing the distinctive wear path on the raceway (upper photo). One part of the raceway is spalled (lower photo). Except for the spall, the wear path is uniform around the entire circumference. The center of the wear path is measured at a 14 angle from the true radial position. Report N AT13A020 Project N Date 19 February (17)

4 brightfield darkfield Figure 3. Close-up of the inner ring raceway using different lighting conditions (upper photos). The lower photo shows detail of the raceway wear, which includes spall debris denting, a mirror finish in the contact zone, a brown band in the center, and a line of crazing. Report N AT13A020 Project N Date 19 February (17)

5 A section of the raceway - about an inch or two from the spalled area was examined at higher magnifications using scanning electron microscopy (SEM). 700X 700X Figure 4. SEM images of the inner ring raceway. The upper image is the as-ground surface outside the wear track. It is presented as a control. The lower image is from the wear zone that has a mirror finish. A few kinematic (curved) wear marks can be seen on the surface. These marks are normal for a used bearing. Report N AT13A020 Project N Date 19 February (17)

6 60X 250X 750X Figure 5. A magnification series of SEM images shows the crazing marks are microcracks. These microcracks result from cold work at the surface caused by excessive scrubbing between the balls and raceway during operation. The cold work on the raceway reaches a point where the material breaks away and results in wastage of material from the surface. The darker areas are organic in nature (energy dispersive spectroscopy reveals chiefly carbon and oxygen). This is likely oxidized grease that is deposited on the surface, and gives the dark brown visual appearance on the surface. Report N AT13A020 Project N Date 19 February (17)

7 500X 650X Figure 6. Further examples of surface cold work, cracking, and material wastage on the inner ring wear path, as presented in Figure 5. Report N AT13A020 Project N Date 19 February (17)

8 700X 2000X Figure 7. Same as Figure 5, showing cold work material flow at the surface. Report N AT13A020 Project N Date 19 February (17)