1. Field of the Invention
The present invention relates to a structure of a nonmagnetic raceway assembly that has raceway surfaces forming a raceway through which rolling elements such as balls or rollers travel.
2. Description of the Prior Art
In recent years, in home electric appliances, semiconductor manufacturing equipment, image processing equipment and inspection devices, there are growing demands for nonmagnetic raceway assemblies such as nonmagnetic bearings, in addition to such properties as corrosion resistance and surface pressure withstandability. Materials that have conventionally been used to manufacture nonmagnetic raceway assemblies such as bearings include stainless steel, aluminum, and beryllium copper.
A vacuum bearing disclosed in the Japanese Patent Laid-Open No. 255224/1991 forms rolling elements with ceramics such as silicon carbide and silicon nitride and coats their surfaces with molybdenum disulfide by sputtering. This vacuum bearing has the inner race and the outer race formed of ceramics such as silicon carbide and silicon nitride or stainless steel and coats their surfaces with molybdenum disulfide by sputtering. A retainer is formed of polytetrafluoroethylene. The vacuum bearings are used where lubrication cannot be realized by oil or grease, as in satellite-borne equipment, semiconductor manufacturing equipment and medical X-ray tubes. The vacuum bearings, as mentioned above, does not use lubricating oil or agent, nor does it improve brittleness of ceramics.
When the raceway assembly of a bearing is formed of such nonmagnetic materials as stainless steel, aluminum, and beryllium copper, characteristics such as stiffness, wear resistance, seizure resistance and durability of the raceway assembly itself are not good enough, nor is it possible to reduce the manufacturing cost of the raceway assembly. To overcome these drawbacks, however, attention is being focused on ceramics as a material for the raceway assembly, which has good characteristics such as stiffness, wear resistance, seizure resistance, durability and corrosion resistance.
Although ceramics, when compared with metallic materials, has excellent properties such as heat resistance, corrosion resistance, wear resistance and stability as well as light weight, it has drawbacks of low tensile strength and bending strength and fragility. Hence, various problems must be overcome before ceramics can replace metals as the material of the raceway assembly.
Materials with small strength are not preferred as the material forming the raceway surface of the bearing because the raceway surface is subjected to heavy load. It is therefore problematical to manufacture a bearing with ceramics. When the raceway assembly is formed of ceramics and if the bearing should fail, the effect of the bearing failure goes beyond the bearing itself, leading also to a failure of the equipment in which the bearing is used.