Patent Number: 042808758
Section: summary

The invention relates to a safety device for preventing unscrewing of cylinder-head or pressure vessel cover screws having an inner polygon, which extends through a first structural part or member behind the latter and clamps it against a second structural part or member in which they are anchored, the screw heads being disposed in respective receiving holes of the first structural part, preferably for securing the internal parts of reactor pressure vessels. Saftey devices against unscrewing cylinder-head screws with a polygonal recess, which include in particular those with a hexagonal recess, also called "Imbus screws", allen screws or socket head cap screws, must meet a number of requirements. Thus, the safety device must secure the screw aganst dropping out if the shank of the screw breaks. The unscrewing protection should be easy to establish after the screw is tightened but it must also be releasable if the structural parts which are clamped together by the rotation-secured screws are to be detached from each other again. The above-mentioned requirements are especially important in nuclear reactor engineering, and in this field for securing the internal parts of reactor pressure vessels. For changing fuel assemblies in pressurized- or boiling-water reactors, it is necessary to open the pressure vessel after the reactor is shut down. On this occasion, a number of inspections of parts of the reactor pressure vessel are performed, including the screw connections at internal parts of the pressure vessel. The above-mentioned requirements have not been satisfactorily met by the known devices. It is therefore an object of the invention of the instant application to provide a device for preventing unscrewing which avoids the foregoing disadvantages of the heretofore known devices of this general type and, furthermore, that it not only meets the aforementioned requirements but, in addition, can be released without destruction and preferably in such a manner that it can be used at least a second time and, preferably, several times more. With the foregoing and other objects in view, there is provided, in accordance with the invention, a safety device for preventing unscrewing of cylinder head screws fastening two structural members together, one of the structural members having receiving bores formed therein, the screws having heads disposed in the receiving bores and being formed with inner polygonal cutouts, the screws extending through the one structural member and being anchored in the other structural member, comprising circular grooves cut into the one structural member at the receiving bores, plastically deformable ring collars disposed in the circular grooves and spaced from the screw heads so as to form annular gaps therebetween, at least two arcuate or lemon-shaped recesses formed at equal distances from one another about the periphery of the screw heads, the ring collars having a first set of portions thereof pushable into the arcuate recesses in tightened condition of the screws. The advantages that can be achieved with the invention are seen particularly in the fact that the device for preventing unscrewing can be released quickly and by means of a remotely controlled tool, which is especially important in applications to internal parts of reactor pressure vessels where such devices must be released quickly and without much radiation exposure for the operating personnel, after the reactor pressure vessel is opened. In this connection, in accordance with another feature of the invention, the first set of portions of the ring collars are reformable after being pushed into the arcuate recesses, the ring collars having a thickness, as compared to the shape of the arcuate recesses, whereby the loosening torque of the screws is at least equal to the tightening torque thereof. In accordance with a further feature of the invention, there are provided additional sets of portions of the ring collars pushable into the arcuate recesses after reformation of the first set of portions, the screw heads having a plurality of pairs of arcuate recesses formed at different locations about the periphery thereof. In accordance with an added feature of the invention, there are provided additional sets of portions of the ring collars pushable into the arcuate recesses after reformation of the first set of portions, each screw having a pair of arcuate recesses formed at different locations about the periphery of the screw head relative to a given reference point. In accordance with an additional feature of the invention, the maximum width of the arcuate recesses, in radial direction of the screws, is substantially 2 to 4 times the thickness of the ring collars. In accordance with yet another feature of the invention, the depth of the arcuate recesses is less than the thickness of the screw head in axial direction of the screw, and including stop flanges integral with the screw head below the arcuate recesses, in axial direction of the screw. In accordance with yet an additional feature of the invention, the safety device is used for securing a core structure hold-down system of a nuclear reactor pressure vessel having a cover and an inner surface, wherein the one structural part is in the form of a ring flange holding an upper part of the core structure against a support flange disposed on the cover surface of the pressure vessel, and including pretensioned cup springs disposed between the hold-down plates and the ring flanges, a flange on the pressure vessel cover disposed above the hold-down plates, and t-shaped clamping studs partially extending through a hole formed in the hold-down plates for pushing the cup springs downward through the weight of the pressure vessel cover flange. In this application, the invention also makes it possible to exchange the cup spring stacks after extended insertion quickly and without exposing the installation personnel to much radiation. The subject of the invention is also a tool for the plastic deformation of the ring collar for the above-described device for preventing unscrewing. Therefore, in accordance with yet a further feature of the invention, there is provided a substantially cylindrical deforming punch having a lower surface of the deforming punch, the centering pin being centrally receivable in the inner polygonal cutouts of the screw heads, and two deforming plugs protruding from diametrically opposite locations of the lower surface of the deforming punch, the deforming plugs having deformation profiles for deforming the ring collar so as to correspond to desired impressions. In accordance with still another feature of the invention, the deforming plugs are provided with downwardly and outwardly extending rounded edges. In accordance with a concomitant feature of the invention, the centering pin is slidably received in a blind guide hole formed in the lower end of the deforming punch, and including a compression spring disposed between the closed end of the blind guide hole and an end of the centering pin, the centering pin having an elongated slot formed therein and a crosswise pin extending in radial direction of the centering pin through the elongated slot so as to prevent the centering pin from falling out of the deforming punch. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in safety device for preventing unscrewing of cylinder-head screws having an inner polygon, and plastic deformation tool for use with the device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.