Clamping device in main shaft driving device for machine tool

A clamping device in a main shaft driving device for a machine tool, the main shaft driving device rotationally driving a main shaft by a driving device, the main shaft being rotatably supported by a frame, the main shaft driving device including a member that is rotationally driven, the member that is rotationally driven being secured to an end portion of the main shaft, the clamping device comprising:

• a sliding disc that is secured to the main shaft;
• a clamp piston that is provided so as to be movable in an axial direction of the main shaft in the frame, the clamp piston including a braking portion that causes a press-contact force to act upon the sliding disc, the clamp piston causing the press-contact force to act upon the sliding disc by the braking portion, so that the sliding disc comes into contact with the frame, as a result of which, by frictional force generated between the braking portion and the sliding disc and frictional force generated between a clamp surface at the frame and the sliding disc, rotational resistance is applied to the main shaft through the sliding disc, the clamp piston having a first pressure-receiving surface and a second pressure-receiving surface, which are pressure-receiving surfaces that receive pressures of operating fluids and which extend orthogonally to a direction in which the braking portion causes the press-contact force to act upon the main shaft and which face a direction that is the same as a direction in which a surface of the sliding disc that contacts the braking portion faces;
• a first pressure chamber formed by the first pressure-receiving surface and the clamp piston and the frame;
• a second pressure chamber formed by the second pressure-receiving surface of the clamp piston and the frame;
• and
• an operating fluid supplying mechanism for supplying the operating fluid to at least one of the first pressure chamber and the second pressure chamber, when applying the rotational resistance to the main shaft, the operating fluid supplying mechanism including a switching portion that selectively supplies the operating fluids to the first and second pressure chambers,
• wherein the clamp piston is formed so that the braking portion causes press-contact forces that differ from each other to act upon the sliding disc when the pressures of the operating fluids that are equal to each other act upon the first pressure-receiving surface and second pressure-receiving surface, respectively.