Abstract: This invention relates to improvement of a chuck unit for machine tools consisting of main body, plunger, and master jaw. In a chuck where the master jaw and plunger engagement part constitutes an inverted T shape in cross section, slide surfaces of engagement between the wedge part of the master jaw and the plunger are formed into circular arc shape.

Abstract: This invention relates to a cooling mechanism of a rotary hydraulic cylinder for chucking operation of machine tools. In a rotary hydraulic cylinder which consist of a rotor member having a reciprocable piston inside and in which working oil is fed to the piston in said rotor member, many fins are mounted, parallel to the movements of said piston rod, on the outer periphery of said working oil supplying member, and a cylinder-like cover whose side facing to said rotor member is folded inside to make a flange, is mounted at the edge, of said fins, facing said rotor, so that it may partially enclose the outer peripheral surface of said rotor member. The side of the rotor member enclosed by said cylinder-like cover, of said oil supplying member is provided with a rotary fan which rotates together with said roller member, thereby causing an air inlet opening of a fixed clearance between said cover and the rotary fan to be formed.