Factory Customized medical equipment usa from Xiamen China
A large number of application practices and life experiments have shown that bearing failures are mostly caused by contact surface fatigue. ISO25243-2004 lists fatigue as the first of the six common failure modes of bearings, and the fracture listed in the sixth place is also called fatigue fracture due to fatigue in the formation process. Typical fatigue failures are divided into subsurface origin and surface origin. Factory Customized medical equipment usa from Xiamen China
one. subsurface origin fatigue
The maximum contact stress of rolling contact occurs somewhere below the surface at a certain depth, where a fatigue source (microcrack) is formed under the repeated action of alternating stress. The crack source gradually expands to the surface under cyclic stress, forming an open flaky crack, which is then torn into flaky particles and peeled off from the surface, resulting in pits and pits. If there is a certain weak point or defect in the bearing steel (common ones such as non-metallic inclusions, air gaps, and grain interfaces of coarse carbides), it will accelerate the formation of fatigue sources and the expansion of fatigue cracks, and greatly reduce the fatigue life.
two. surface origin fatigue
There are damages on the contact surface. These damages may be original, that is, scratches and bumps formed during the manufacturing process, or they may be generated during use, such as hard particles in lubricants, and tiny scratches caused by relative motion of bearing parts ; There may be poor lubrication at the damage site, such as lack of lubricant, lubricant failure; poor lubrication state aggravates the relative sliding between the rolling element and the raceway, resulting in micro-cracks at the root of the asperity at the surface damage; crack propagation leads to Microprotrusions fall off, or form exfoliated areas. This spalling is shallow in depth and can sometimes be confused with dark gray plaques. Factory Customized medical equipment usa from Xiamen China
three. fatigue fracture
The origin of fatigue fracture is fatigue yield formed by assembly stress and cyclic alternating stress caused by excessive tight fit. Once the balance between assembly stress, alternating stress and yield limit is lost, fracture will occur along the axis of the ferrule, forming a through shaped cracks.
In practice, most of the bearings that fail in normal use are damaged as described above, that is, contact surface fatigue, and the three types of fatigue failure are subsurface origin fatigue. The most common bearing life calculation method recommended by ASO281 and ISO281/amd.2 is Based on subsurface origin fatigue.
Commonly used anti-fatigue methods are:
A. Heat treatment technology
Heat treatment is a commonly used process method to improve the mechanical properties of materials. In order to adapt to the different use requirements of different material parts, it is necessary to select different heat treatment processes, such as pre-heat treatment structure, quenching heating temperature, heating speed, cooling method (medium and speed), tempering Both temperature and time have a significant impact on mechanical properties. Many heat treatment parameters should be optimized and combined to obtain the best performance suitable for use conditions, thereby extending the fatigue life of parts. Build a virtual production platform for heat treatment, and promote the transformation of heat treatment technology to high-tech knowledge-intensive. The optimization of heat treatment process parameters and the development of digital heat treatment technology are important prerequisites for the realization of anti-fatigue manufacturing. Factory Customized medical equipment usa from Xiamen China
B. Surface chemical heat treatment
The modification effect of surface chemical heat treatment is mainly on the surface. The chemical elements infiltrated can be selected according to different application requirements, such as quenching and tempering after carburizing to improve the surface hardness, but the distortion of the workpiece is not easy to control: the formation of metal nitrides after nitriding can Obtain higher surface hardness and wear resistance, corrosion resistance and fatigue resistance, and the workpiece distortion is small, but the efficiency is not high; the co-infiltration process makes the hardness, wear resistance, corrosion resistance, fatigue resistance better, and quenching distortion Less, but the hardened layer is thin, not suitable for heavy-duty workpieces. The development direction of surface chemical heat treatment is to expand the application of low temperature chemical treatment, improve the quality of the infiltration layer, accelerate the treatment process, and develop environmentally friendly processes, composite infiltration processes and analog and digital processing technologies. Factory Customized medical equipment usa from Xiamen China
C. Application of Surface Strengthening Technology
The traditional surface hardening technology is derived from the principle of cold work hardening, such as shot blasting, sand blasting, shot peening, etc., new surface hardening technologies such as laser surface hardening, laser shot peening surface hardening, ultrasonic roll light hardening, chemical surface hardening, composite surface hardening New technologies for surface hardening of various processes have been successfully applied in many fields, such as laser-peening process (laser shock treatment), which uses high-energy pulsed lasers to form shock waves on the surface of parts, compressing and plastically deforming surface materials to form surface Residual compressive stress, thereby enhancing fatigue resistance (such as stress crack resistance, corrosion fatigue resistance, etc.).
D. Surface Modification Technology
Commonly used surface modification techniques mainly include ion implantation and surface coating.
Ion implantation is a non-high temperature process without the limitations of metallurgy and equilibrium phase diagrams. Different implantation elements and doses can be selected according to different needs to obtain the desired surface properties. Such as: implanting chromium ions to enhance the corrosion resistance and fatigue resistance of the base material; implanting boron ions to enhance the wear resistance of the base material. Factory Customized medical equipment usa from Xiamen China
Surface coating techniques include physical vapor deposition (PVD), chemical vapor deposition (CVD) radio frequency sputtering (RF) ion spraying (PSC), electroless plating, etc.
In addition, the ion infiltration process uses high-voltage direct current to make the infiltrated elements in an ionic state under a certain degree of vacuum, so that the generated ion current bombards the surface of the workpiece, and forms compounds on the surface to reduce friction and improve wear resistance.
E. Microfabrication and finishing technology
As an advanced manufacturing technology, high-precision micromachining and blending, and finishing technology also play an important role in improving the fatigue resistance of basic parts. Ultra-precision grinding and eddy current finishing are aimed at reducing the surface roughness of the workpiece. After processing, the surface physical and chemical properties, mechanical properties, and the contour shape of the contact point are all beneficially changed, which can correct the contact stress distribution and facilitate dynamic lubrication. The formation of oil film increases fatigue life.
F. Coordinate hardness matching
The hardness matching relationship of different parts can also coordinate the stress and strain transfer state at the rolling contact, which has a significant effect on prolonging the fatigue life of the parts. Factory Customized medical equipment usa from Xiamen China
