1. Field of the Invention
The present invention relates to a linear motion guide bearing apparatus which is used in an ordinary industrial machine or in a delivery apparatus and, in particular, to a guide rail or a slider which is a composing part of the linear motion guide bearing apparatus.
2. Description of the Related Art
Generally, a linear motion guide bearing apparatus, as shown in FIG. 8, as composing parts thereof, comprises a guide rail 1 extending in a given direction and a slider 2 crossingly mounted on the guide rail 1 so as to be movable with respect to the guide rail 1 along the guide rail 1.
In the two side surfaces of the guide rail 1, there are formed rolling element rolling grooves 3 which respectively extend in the axial direction of the guide rail 1. Also, in the inner surfaces of the slider 2 as well, there are formed rolling element rolling grooves 4 which are respectively disposed opposed to the rolling element rolling grooves 3. And, within a load rolling passage which is formed between the mutually opposed rolling element rolling grooves 3, 4, there are inserted a plurality of rolling elements (such as rollers or balls).
In FIG. 8, reference character 6 designates a return passage and 7 stands for an end cap.
FIG. 9 is a section view of a conventional guide rail 1, when it is viewed from the axial direction (extending direction) of the rolling element rolling groove 3. And, FIG. 10 is a section view of a conventional slider 2, when it is viewed from the axial direction (extending direction) of the rolling element rolling groove 4.
Each of the rolling element rolling grooves 3, 4 has a substantially circular section shape; and, boundary portions A, B, which are formed between land surfaces existing continuously with the two sides of the rolling element rolling grooves 3, 4, are respectively formed in a shape having an angle formed by the mutual intersection of an arc and a straight line (which is hereinafter sometimes referred to as an edge shape). By the way, in the rolling element rolling grooves 3, 4, in some cases, there can be formed such a relief as shown in FIG. 8.
In the case of the guide rail 1 shown in FIG. 9, the upper-side rolling element rolling grooves 3 are formed in the corner portions between the side surfaces and upper surface of the guide rail 1. In the case of the present rolling element rolling grooves 3, the section shape thereof provides a substantially ¼ arc shape; and, the respective boundary portions A between the land surfaces (side surfaces and upper surface) existing continuously with the present rolling element rolling grooves 3 and the present rolling element rolling grooves 3 also provide an edge shape having an angle formed by the intersection between an arc and a straight line.
However, when the linear motion guide bearing is in operation, in case where there exists a clearance between a load rolling passage and the half-arc-shaped circulation passage formed in a rolling element circulation part such as an end cap, a rolling element, when passing through the clearance, can shift from its normal passage and interfere with the land portion.
At the then time, in case where the rolling element 5 is higher in hardness than the guide rail 1, for example, the guide rail 1 is made of metal and the rolling element 5 is made of ceramic, there arises a problem that the boundary portion A softer than the rolling element 5 is easy to wear.
That is, as in the above-mentioned conventional structure, when the boundary portions A, B are respectively formed in an edge shape, in case where there occurs such a load state in which the rolling element 5 is pressed against the boundary portions A, B, since the contact area of the rolling element 5 with the boundary portions A, B is small, the maximum contact pressure increases. As a result of this, the boundary portions A, B are easy to wear. Depending on the progress of such wear, there is a fear that such wear can have an ill effect on the linear motion accuracy of the slider 2.
This problem become obvious especially in such a using condition that the slider 2 must be moved at a high speed with respect to the guide rail 1, or the slider 2 must be positioned with high precision.
Especially, since the moving speed of the rolling element has been enhanced, when a corner portion exists in the land portion 2b (see FIG. 11), in the case of the guide rail 2 made of steel, there arises a problem that the rolling element 5 and land portion 2b are easy to flake.
Also, in case where the guide rail 2 is made of ceramic material, since the guide rail 2 is hard, the surface of the rolling element 5 can be damaged; and, because the ceramic material is fragile material, a crack can be caused in the land portion 2b. 
On the other hand, when the moving speed of the rolling element 5 is low, in the case of the guide rail 2 made of steel, the surface of the rolling element 5 and the groove surface of the rolling element rolling groove 4 of the guide rail 2 are worn due to the rolling motion of the rolling element 5 and the roughness of these surfaces is thereby reduced (the surfaces are smoothed), which makes it hard for the surfaces to wear any further.
However, in case where the guide rail 2 is made of ceramic material, since the guide rail 2 is much harder than the rolling element 5, there hardly arises the possibility that the groove surface of the rolling element rolling groove 4 of the guide rail 2 can be worn due to the rolling motion of the rolling element 5 and the roughness of the groove surface can be thereby reduced (the groove surface can be smoothed). Therefore, as the linear guide apparatus is operated, the rolling element 5 continues to wear. As a result of this, in case where the preload amount of a linear guide apparatus is small, especially, in the case of a small-size linear guide apparatus, the rigidity of the linear guide apparatus is lowered due to shortage of the preload, so that the durability of the linear guide apparatus is reduced.