Demands of ultra-precision machining systems have recently increased for machining three dimensional nano-geometries. Such systems require not only horizontal positioning systems but also vertical positioning systems with a nanometer positioning accuracy. However, the gravity load of moving mass negatively affects the positioning accuracy especially in a vertical positioning system. Therefore, it is necessary to compensate the influence of the gravity load for vertical ultra-precision positioning. Although the gravity compensation method using a non-contact vacuum cylinder has been proposed, it is difficult to improve the sealing performance of the non-contact seal. Thus, this study proposes the magnetic fluid seal for linear motion system with a gravity compensator. In general, magnetic fluid seals have been applied to rotary motion systems, however there are few studies on linear motion systems so far. When a magnetic fluid seal is applied to linear motion systems, the magnetic fluid leaks out of the seal clearance due to the movement, and it cannot keep the performance as a seal. In this study, it is possible to prevent leaking the magnetic fluid from the seal clearance and also to maintain the performance of the seal by holding a magnetic fluid by the two pole pieces shaped tapered tip and putting it near the piston made of non-magnetic material and coated with oil repellent layer. The proposed magnetic fluid seal performance has been evaluated. As a result, the result confirmed that the proposed magmagnetic fluid seal is useful as a linear motion seal, and the oil repellent layer on the piston surface can largely improve the sealing performance.
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