Mandrel holding and releasing means
The invention pertains to a detachable connection between a mandrel and a mandrel rod of a piercing mill for seamless tube, where an intermediate piece fits into the mandrel rod at the mandrel side, and where the rear surface of the mandrel rests against a ring-shaped surface of the intermediate piece. The mandrel, having a centrally located holding bored or machined opening is mounted on a corresponding centrally located projection of the intermediate piece, and at least one radially elastic hook is inserted into a radial opening of the intermediate piece. The elastic hook, under the force of a spring has a nose which engages beyond the external circumference of the projection of the intermediate piece into a ring-shaped radial recess of the holding opening. 
     The invention is based on a mandrel-mandrel rod connection known in the art as illustrated, for example, in DT-OS No. 25 14 720 corresponding to U.S. Pat. No. 4,022,043. In this case, the elastic hook is designed as a double hook. Its base is mounted to the intermediate piece by a bolt, and it has elastic protruding parts extending from the center of the intermediate piece which serve to release the hook from the outside. This means for releasing the hook allows the use of hooks with relatively hard spring action or high initial spring tension, which prevents an unintentional stripping of the mandrel during the return movement of the mandrel rod, but in addition, it also requires an external releasing device. Moreover, the mandrel rod must assume a specific radial position relative to the releasing device. 
     It is an object of the present invention to provide a mandrel-mandrel rod connection where, even with a high initial spring tension of the hook, the connection can be reliably released upon the backward movement of the mandrel rod simply by bringing the protruding part of the rear surface of the mandrel into contact with a retractable flap or a similar device having a surface which engages the surface of the mandrel to resist the backward movement of the mandrel rod. This arrangement prevents an overloading of the nose of the hook usually caused by a sudden hard impact. 
     More particularly, the present invention provides an elastic hook which can be moved forward axially against the effect of a pressure spring. As is generally the case with similar mandrel-mandrel rod connections, the rear holding surface of the holding part of the hook is slightly tapered, so that the hook can be pushed toward the inside of the intermediate piece to release the mandrel. When the mandrel is released, the hook can make a limited forward movement against the effect of the pressure spring, which avoids an overload of the hook. The holding part of the hook is always provided with a nose, whose contact surface pushes the holding part toward the inside when the mandrel is mounted. A preferred embodiment of the present invention provides a recess in axial extension of the tip of the nose which is tapered in the same direction as, and at an acute angle relative to, the axis of the mandrel rod, and that the distance of the tip of the nose from the frontal limitation is smaller than the axial displacement of the hook. 
     It is thus possible that the hook, moving forward against the effect of the pressure spring, runs against the tapered section of the recess and is automatically moved toward the inside. This makes it also possible to release the holding part of a hook whose rear surfaces are not tapered. 
     Another object of the present invention is to provide an arrangement having a radial recess in the intermediate piece which is contiguous to an axial bored opening which extends to the rear surface of the intermediate piece; a hook which is held in position in the axial opening with its thicker rear clamping part fitted into the axial opening; the thicker clamping part being tapered at the rear surface of the intermediate piece into a bolt of a smaller diameter; a setting nut fastened on the free threaded end of this bolt; a pressure spring clamped between the nut and a thrust washer resting on the front surface of the intermediate piece and contacting the thicker part of the clamping part. These elements are easy to manufacture and, moreover, this arrangement of elements offers the possibility to change the pressure spring and replace it with a spring of higher or lower initial tension. Moreover, all parts are protected when the mandrel is in place, and the ingress of scale which could affect the moveability of the parts is prevented. 
     Finally, it should be pointed out that the invention makes a three-point-mounting of the mandrel possible by providing three hooks evenly distributed over the circumference of the intermediate piece. 
    
    
     These objects, as well as other novel features and advantages of the present invention will be better understood and appreciated when the description of the invention is read along with the accompanying drawings of which: 
     FIG. 1 is a longitudinal sectional view of the present invention, and 
     FIG. 2 is a transverse sectional view taken along lines A--A of FIG. 1. 
    
    
     FIG. 1 shows a detachable connection between a mandrel 1 and a hollow mandrel rod 2 of a seamless tube piercing mill, where an intermediate piece 3 having threads is screwed into the mandrel-end of the mandrel rod 2. The slightly protruding rear surface 7 of mandrel 1 is in contact with a ring-shaped surface 8 of intermediate piece 3. The mandrel, having a centrally located bored opening 5, is mounted on a correspondingly shaped centrally located projection 10 of intermediate piece 3. Furthermore, the holding opening 5 has a radial recess 8a, into which the holding part 11 of a hook 4 in radial recess 14 engages. 
     The recess 14 has a slanted surface front limitation 15 at an acute angle relative to the axis of the mandrel rod. Surface 15, upon operation of the present invention, is contacted by a surface 12 of a nose 13 provided at the holding part 11 of the hook 4. This contacting action causes nose 13 to move into the recess 14 at an acute angle relative to the axis of the mandrel rod. A conical shaped opening 9 of mandrel 1 permits projection 10 to enter opening 5 of the mandrel. The longitudinally-stretched hook 4 is located in an axial bored opening 17, into which radial recess 14 changes over. Hook 4 is held in axial opening 17 with its thicker rear clamping part 18 fitted into axial opening 17. Adjacent clamping part 18 at the rear surface 23 of intermediate piece 3 is a smaller diameter threaded portion in the form of a bolt 20, onto which a setting nut 24 is fastened to the free end of bolt 20. 
     A pressure spring 19 is clamped between nut 24 and a thrust washer 21, which abuts front surface 23 of intermediate piece 3, and clamping part 18, and which is secured to intermediate piece 3 by a screw 22. 
     FIG. 2 illustrates three hooks 4, 4&#39;, 4&#34;, which are evenly distributed over the circumference of the intermediate piece 3. 
     After the piercing operation and upon the return motion of the mandrel-mandrel rod assembly, the slightly protruding part of the rear surface 7 of the mandrel impacts against an element not shown in the FIGURES, which the mandrel rod 2 with the intermediate piece 3 moves further backward. This continuing backward movement releases the conical seat 9 and brings a slanted ring surface 16 of the recess 8a into contact with the holding part 11 of the hook 4. After a short axial movement, surface 12 of nose 13 contacts the slanted surface limitation 15 of the recess 14. It slides along toward the inside thereby causing hook 4 to pivot inward so that the mandrel 1 can be removed and replaced. 
     In accordance with the provisions of the patent statutes, we have explained the principle and operation of our invention and have illustrated and described what we consider to represent the best embodiment thereof.