The fatigue performance of the rod end body of the rod end self-lubricating spherical plain bearing is tested, and the cause of the end face cracking of the bearing is analyzed.
Abstract: The fatigue property test is carried out for rod end body of self - lubricating spherical plain bearings for rod end, the microcracks are found on bearing end faces after 132.9 × 104 cycles of loading. The reason of cracking on bearing end faces is analyzed by means of chemical composition analysis, macroscopic test, microstructure analysis, energy spectrum analysis，metallographic examination and so on. The results show that the initiation of cracks on bearing end face is mainly due to adhesive wear between end faces and tooling plane, resulting in metal plastic flow on bearing end faces. The cracking is caused by plastic deformation on surface of metal when the bearing end faces are subjected by a larger shear force. Some of microcracks propagate inwards, and some lead to spalling of plastic deformation layer, eventually the pits are formed at bearing end faces．
Key words: self - lubricating spherical plain bearing; adhesive wear; fatigue fracture; failure analysis
Wear is the main failure form of materials and parts, which mainly includes adhesive wear, abrasive wear, fatigue wear, corrosion wear and fretting wear. Among them, fretting wear is a composite wear that can lead to oxidation wear, abrasive wear and adhesive wear, and adhesive wear is the most common form of wear. Self lubricating spherical plain bearing has the advantages of low friction coefficient and no need to add lubricant. It has been widely used. Its failure is closely related to adhesive wear. Therefore, it is necessary to deeply study the wear mechanism of self-lubricating spherical plain bearing.
1. Description of failed parts
In order to evaluate the fatigue performance of rod end body, the fatigue test of rod end self-lubricating spherical plain bearing was carried out. The self-lubricating spherical plain bearing is composed of inner ring, outer ring and self-lubricating gasket. The inner ring material is precipitation hardening stainless steel PH13 － 8Mo, and the hardness requirement is 43 ~ 47 HRC; The spherical surface is sprayed with ceramic, and the thickness of ceramic layer is 0.2 ~ 0.25 mm; The outer ring material is precipitation hardening stainless steel 05Cr17Ni4Cu4Nb, and the hardness is required to be 28 ~ 37 HRC. The structural parameters of the bearing are: outer diameter 64 mm, outer ring thickness 32 mm; The inner diameter is 22 mm and the inner ring thickness is 59 mm.
The test is carried out on MTS testing machine. It is an overall loading test. The loading method is shown in Figure 1. The clamps at both ends of the test piece shall be able to bear the tensile and compressive load. The test is divided into three stages: 1) radial load fr = ± 48 kN, frequency 4 Hz, and the number of test loading cycles is 5 × 105 times. After the test, it is confirmed that the test piece is not damaged; 2) The load increases by 20%, i.e. 57.6 kN, and the cyclic load is 5 ×105 times, the test piece is still not damaged; 3) Continue to load to 69.12 kN and load 32.9 kN × 104 times, microcracks extending from the end face to the inside were found on the end face of the bearing inner ring.
1- Test loading tooling 2- Rod end body 3- Outer ring 4- Inner ring
Fig． 1 Loading diagram of test
2. Physical and chemical analysis
2.1 Chemical composition analysis
The chemical composition of the cracked failed bearing inner ring is analyzed. The results are shown in Table 1. It can be seen from the table that the content of each element complies with GB / T 11170-2008 Determination of Multi Element Content in Stainless Steel Spark Discharge Atomic Emission Spectrometry; GB / T 20123-2006 Iron and Steel - Determination of Total Carbon and Sulfur Content - infrared Absorption Method after Combustion in High Frequency Induction Furnace; Technical requirements for the composition of ph13-8mo steel in GB / T 20124-2006 determination of nitrogen content in iron and steel - inert gas melting thermal conductivity method.
2.2 Macro morphology of fracture
The macro morphology of the crack on the end face of the bearing inner ring is shown in Figure 2. It can be seen from Fig. 2a and Fig. 2b that there is no obvious plastic deformation at the crack, and it extends inward along the bearing end. Manually open the crack along the propagation direction to form a fracture sample (Fig. 2c). Observing the fracture, it is found that the fracture surface is flat, fine and has fatigue characteristics. The fracture surface close to the end face of the inner ring is the crack source area, as indicated by the black arrow in Fig. 2c. The source area is point source, grayish black; Wear and rolling characteristics can be clearly seen on the side surface of the source area (bearing end face), and the wear direction is circumferential; Obvious fatigue growth stripes can be seen on the whole fracture (Fig. 2d).
Fig． 2 Macro morphology of fracture of bearing inner ring
2.3 Micro morphology
After the inner ring fracture is cleaned by ultrasonic cleaning machine, the scanning electron microscope magnification observation is carried out. The results are shown in Fig. 3-fig. 5. It can be seen from the figure that the fracture source area is located at the end face, and obvious extrusion marks can be seen in the observation source area, indicating that the crack initiation time is earlier. Under the action of alternating load, the section of the source area opens and closes repeatedly, resulting in serious extrusion and wear of the section (Figure 3). Fatigue growth striations and fatigue glow striations can be seen in the fracture propagation area (Fig. 4). Obvious wear, rolling and falling marks can be seen on the side surface of the crack source area (inner ring end face), showing the characteristics of adhesive wear (Fig. 5).
2.4 Energy spectrum analysis
The energy spectrum analysis of the fracture source area is carried out, and the results are shown in Fig. 6. It can be seen from the figure that the oxygen content in the fracture source area (gray black area) is high, but no large-size inclusions and other metallurgical defects are found. It can be seen that the crack initiated earlier and the source region has been oxidized.
Fig． 3 Morphology of fracture source area of inner ring
Fig． 4 Micro morphology of fracture propagation area of inner ring
Fig． 5 Morphology of side surface of source area( bearing end face)
2.5 Metallographic and hardness inspection
The metallographic picture of fracture is shown in Figure 7. Among them, figure 7a and figure 7b are metallographic pictures of one side of the fracture. It can be seen from the figure that there are secondary cracks near the fracture and extend to the inside of the bearing, the tail of the microcrack is distorted and deformed, and the metal streamline is clearly visible. There is no decarburization and burn near the crack, which can rule out the possibility of cracks in smelting, forging, cold and hot processing and other processes. As obvious wear and rolling marks are found on the bearing end face in the electron microscope observation, the longitudinal section of the bearing end face (Fig. 7c-fig. 7e) is observed and found that there is an obvious plastic deformation layer on the surface of the bearing end face (the double arrow in the figure indicates the thickness of the plastic deformation layer), and its thickness is uneven, and the thinnest area is about 20 μ m. The thickest area is about 50 μ m. In addition, microcracks are found in the plastic deformation layer. In some areas, the plastic deformation layer has partially fallen off to form pits, and the non falling off area forms protrusions on the end face, which belongs to typical adhesive wear characteristics.
Fig． 6 Energy spectrum analysis of fracture source area of inner ring
Use Rockwell hardness tester to test the hardness of the inner and outer rings respectively, and randomly test 3 points. The hardness of the inner ring is 45.5, 45.0 and 45.0 HRC; The outer ring hardness is 35.0, 34.0 and 34.0 HRC, which all meet the product hardness requirements.
3. Cause Analysis
Through macroscopic observation, it can be seen that the crack has no obvious plastic deformation, the fracture is flat and fine, and obvious fatigue glow lines can be seen in the propagation area, which has fatigue characteristics. According to the observation of scanning electron microscope, there are obvious adhesive wear characteristics on the bearing end face, and the wear direction is circumferential. According to the metallographic analysis, there is a secondary crack near the fracture and extends to the inside. There is also distortion at the tail of the crack, and the metal streamline is clearly visible; There is no abnormality around the main crack, so it is ruled out that there is a crack in the manufacturing process of the bearing. There is an obvious plastic deformation layer on the surface of the bearing end face, and the thickness is uneven. The plastic deformation layer in some areas has fallen off and formed pits, indicating that the stress on the bearing end face is uneven, and there are microcracks in the plastic deformation layer.
Fig． 7 Metallographic structure of fracture of inner ring and morphology of crack
The rod end self-lubricating joint bearing is subjected to tension and compression load during fatigue test. If the flatness of the bearing end face and the tooling is not well matched and there is a gap, there will be extrusion and relative displacement between the bearing end face and the tooling plane during the test, resulting in adhesive wear between the two planes. Each tension and compression of bearing will produce the slip of metal microstructure, and the plastic rheological layer will be formed after the accumulation of tissue slip. The thicker the plastic rheological layer is, the more serious the wear is. There are obvious wear and rolling marks on the end face of the failed bearing, which is a typical adhesive wear feature. The initiation of the crack on the end face of the bearing inner ring is mainly due to the adhesive wear between the bearing end face and the tooling plane, resulting in the plastic rheology of the metal on the bearing end face, resulting in the sliding and folding of the metal. When the bearing end face is subjected to large shear stress, the surface metal of the end face will undergo plastic deformation and crack. Some microcracks will expand to the inside, and some microcracks will lead to the peeling of the plastic deformation layer, and finally form pits on the bearing end face.
4. Conclusion
Due to the clearance between the flatness of the end face of the self-lubricating spherical plain bearing and the tooling, the bearing end face is worn, rolled and metal plastic rheology, resulting in the cracking of the bearing inner ring along the end face, that is, the adhesive wear between the bearing end face and the tooling plane is the main reason for the cracking of the end face of the bearing inner ring. It is suggested to improve the bearing test and assembly environment and try to avoid fretting wear between the bearing end face and the tooling plane during the test.
More about Marginal Self - Lubricating Bearings:
As a specialized manufacturer of self-lubricating bearings & bushings, Marginal Bearing is devoted to researching and producing new self-lubricating bearing materials.
Self-lubricating bearings, as the name suggests, provide their own lubrication during operation without requiring application of grease or oil lubricants. Due to this, self-lubricating bearings are also referred to as maintenance-free or greaseless bearings as they require no relubrication or grease.
An important distinction to make is that self-lubricating bearings are not bearings that come pre-applied with grease or oil lubricant – these bearings are instead referred to as prelubricated bearings. Prelubricated bearings will require relubrication at some point in their service life.
Self-lubricating bearings work by having lubricant impregnated within the sliding layer of the bearing. This lubricant can either be liquid (oil) or solid (graphite, MoS2, lead) based on the requirements of the application (such as operating temperature). As the bearing operates, the lubricant is released through pores in the sliding layer, lubricating the bearing surface. The lubricant is uniformly dispersed throughout the sliding layer and thus there is no reduction in low friction bearing performance, even if the sliding layer becomes worn. A “running-in” surface is also usually included at the top of the sliding layer to provide low friction bearing performance at start up before the impregnated lubricant reaches the bearing surface.
