Abstract: A rotating holder for tools or work pieces includes a chuck in the form of a hydraulic clamp bushing. The clamp bushing comprises a thin inner sleeve or wall, a solid outer sleeve and a plastic or hydraulic pressure medium which is entrapped between the sleeves. The thin inner sleeve is arranged to both center and firmly clamp a rotating tool or work piece when the hydraulic pressure medium is being pressurized. The clamp bushing is integrated with a mounting cone or with a machine spindle by a welding or soldering joint. The clamp bushing is formed with a bore at the periphery thereof for providing an opening to some type of means for pressurizing the hydraulic pressure medium. A protection ring is formed around the periphery of the clamp bushing and is rotatable to at least two positions, in one position of which the protection ring covers the bore for the pressurization means and in another position of which a passageway is opened to the pressurization means.