Planar guideways are crucial elements in precision planar positioning systems with two or even three DOF which are prevalent in semiconductor industry [1]. As guidance systems those high precision planar positioning systems mainly apply aerostatic guideways. It is often desired to operate positioning systems in the environment of a technical vacuum which demands elaborate solutions of air extraction systems for the aerostatic guideways [2]. These devices are cumbersome and have an influence on the motion path which is not negligible. Planar ball guideways offer an attractive alternative solution eliminating the mentioned disadvantages. Since air is not longer necessary, the guideway is applicable under atmospheric pressure and under technical vacuum conditions as well. The central problem concerning the design of ball guideways is the determination of the percentage contact area which describes the ratio (further on called "contact ratio") of the number of balls under stress to the total number of balls. The contact ratio defines properties like stiffness and load capacity of the guideway. A high stiffness is necessary for high precision positioning systems [3]. Currently the knowledge about the contact ratio in planar guides is based on assumptions which in turn are based on theoretical and experimental expertise about ball bearings. Utilizable scientific results are not available. Therefore the guideway properties are determined by experiment. There is a necessity to calculate the influences of the contact ratio on the decisive properties during the conceptual design stage of guiding systems in order to reduce development time. The measurement procedure and the experimental setup will be presented as well as the influence of the number of balls of guideway on the contact ratio. The aim of these investigations is to define dependent and independent influence quantities on the contact ratio.
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