1. Field of the Invention
This invention relates to a method of manufacturing hollow ceramic rolling elements in which rolling elements used for a bearing are formed out of a ceramic material, such as Si.sub.3 N.sub.4.
2. Description of the Prior Art
The ceramic materials have excellent properties including thermal resistance, corrosion resistance, abrasion resistance, non-magnetizability, self-lubricating ability and weight reducing ability as compared with metal materials but substituting ceramic materials for metal materials has problems due to the fragility thereof. Therefore, although the ceramic material itself and sintering and processing techniques therefore have been advanced, steel bearings have not yet been superseded by ceramic bearings.
The ceramic bearings which have heretofore been available include a bearing in which the constituent parts thereof, such as an inner race, an outer race and rolling elements are formed out of a ceramic material, and a bearing in which one or two kinds of constituent parts thereof are formed out of a ceramic material, and these bearings have various effects. Since rolling elements roll in a bearing, simply reducing the weight of a bearing by forming rolling elements out of a ceramic material enables the centrifugal force of the rolling elements to be reduced. The ceramic rolling elements are superior in the seizure resistance, thermal expansion resistance and corrosion resistance owing to their material.
The methods of manufacturing balls for bearings include, for example, a method disclosed in Japanese Patent Laid-Open No. 249643/1989. This ceramic ball manufacturing method consists of the steps of impregnating green balls, which are obtained by molding ceramic powder of silicon nitride, with an organic compound which is in a solid state at normal temperature, grinding the green balls, degreasing the resultant balls, and then sintering the resultant products. The weight of these ceramic balls is smaller than that of metal balls but not greatly smaller since they have a solid structure.
In order to solve these problems, a method of manufacturing hollow balls for bearings has been developed (refer to Japanese Utility Model Laid-Open No. 107822/1990). This method of manufacturing hollow balls for bearings consists of the steps of forming spherical core members out of an organic material, such as polystyrene foam, forming spherical bodies by bonding pulverized bodies of a ceramic material, such as zirconia to the outer circumferential surfaces of the core members while rolling the core members thereon so that the core members are coated with the pulverized bodies, and then incinerating the spherical core members by burning the spherical bodies at 1450.degree. C., whereby hollow rolling elements are produced. In the steps of this method of manufacturing hollow balls for bearings, it is difficult to form a layer of coating of the pulverized bodies to a uniform thickness over the whole region of the outer circumferential surface of a spherical body. Unless the hollow balls for bearings have a uniform wall thickness, problems concerning the strength thereof arise, and, moreover, the weight distribution of each ball becomes unbalanced. Accordingly, when the bearings with such rolling elements inserted therein are rotated, especially, at a high speed, the rolling elements cannot roll smoothly.