Patent Number: 
Section: claims

1. A device for axially holding an element of elongate shape of lengthways axis and for setting it in rotation around its' axis, said device of lengthways axis comprising:means configured to exert radial tightening forces on the periphery of said elongate element aligned towards the lengthways axis, by bringing said means configured to exert radial tightening forces close to said lengthways axis,means to cause said means configured to exert said radial tightening forces closer to one another, said means to cause comprising a pusher to apply an axial force on to said means configured to exert radial tightening forces, a counter thrust bearing to force said means configured to exert radial tightening forces to come closer to the lengthways under the effect of the axial force, a linear actuator of axis parallel to the lengthways axis of the device, a yoke which is translationally secured to said pusher and configured to rotate around a first axis orthogonal to lengthways axis and not secant with it, said first orthogonal axis being fixed, said yoke being hinged to the linear actuator around a second orthogonal axis parallel to the first orthogonal axis located opposite the first orthogonal axis relative to the lengthways axis, such that the linear actuator causes the yoke to rotate around the first orthogonal axis and a control unit to control the linear actuator according to the position of the yoke, said pusher being configured to rotate freely in said yoke,means to cause rotation of the means configured to exert radial tightening forces on the periphery of said elongate element and said pusher. 2. A device for axial holding and for setting in rotation according to claim 1, comprising:a hollow shaft of lengthways axis in which said pusher is installed such that it can slide freely, said pusher being rotationally secured to said shaft (15),bearings in which the hollow the shaft is mounted,means to cause rotation of the means configured to exert radial tightening forces on the periphery of said elongate element and said pusher, directly causing rotation of said hollow shaft. 3. A device for axial holding and setting in rotation according to claim 2, in which said means causing rotation comprise an electric motor, a first gear engaged with the electric motor, a chain surrounding said first gear and a second gear rotationally secured to said shaft and coaxial with said hollow shaft. 4. A device for axial holding and setting in rotation according to claim 1, in which the control unit to control the linear actuator comprises at least a first sensor to detect the position of the yoke in the tightened position. 5. A device for axial holding and setting in rotation according to claim 4, in which the control unit to control the linear actuator comprises a metal part rigidly connected to the yoke, and in which said first sensor is an inductive sensor, and where the detection of the presence of said metal part by said first sensor corresponding to a tightened position. 6. A device for axial holding and setting in rotation according to claim 4, comprising a second sensor to detect the untightened position. 7. A device for axial holding and setting in rotation according to claim 1, in which the linear actuator is a pneumatic jack, said device also comprising an axial force transmission shaft connecting the jack and the yoke, a hinge between said axial force transmission shaft and said yoke, said hinge comprising a universal joint hinged to the yoke around the second orthogonal axis, said universal joint being traversed by said axial force transmission shaft, and said axial force transmission shaft being connected to said universal joint. 8. A device for axial holding and setting in rotation according to claim 7, comprising elastic means installed compressed between said universal joint and a free end of said axial force transmission shaft. 9. A device for axial holding and setting in rotation according to claim 8, comprising an adjuster for adjusting the load of said elastic means. 10. A device for axial holding and setting in rotation according to claim 1, comprising mechanical stop device to limit the angular displacement of the yoke. 11. A device for axial holding and setting in rotation according to claim 10, in which the mechanical stop device comprises one first mechanical stop installed on the free end of the axial force transmission shaft and an immobile counter thrust bearing, said axial force transmission shaft traverses the immobile counter thrust bearing, and said first mechanical stop is positioned on the other side of the counter thrust bearing relative to the hinge between the yoke and the axial force transmission shaft. 12. A device for axial holding and setting in rotation according to claim 11, comprising a second mechanical stop on the axial force transmission shaft between the hinge and the counter thrust bearing. 13. A device for axial holding and setting in rotation according to claim 12, comprising an adjuster for adjusting the position along the axis of the first stop and/or the second stop. 14. A device for axial holding and setting in rotation according to claim 1, in which the yoke comprises a bore made between the first and second orthogonal axes, a hub installed such that it is hinged in said bore around an axis parallel to the first and second orthogonal axes, and a rolling bearing installed in said hub, the pusher being installed in said rolling bearing. 15. A welding installation for welding the top plugs on the nuclear fuel rods, comprising:a device for axial holding and setting in rotation nuclear fuel rods, said device of lengthways axis comprising:means configured to exert radial tightening forces on the periphery of said elongate element aligned towards the lengthways axis, by bringing said means configured to exert radial tightening forces close to said lengthways axis,means to cause said means configured to exert said radial tightening forces closer to one another, said means to cause comprising a pusher to apply an axial force on to said means configured to exert radial tightening forces, a counter thrust bearing to force said means configured to exert radial tightening forces to come closer to the lengthways under the effect of the axial force, a linear actuator of axis parallel to the lengthways axis of the device, a yoke which is translationally secured to said pusher and configured to rotate around a first axis orthogonal to lengthways axis and not secant with it, said first orthogonal axis being fixed, said yoke being hinged to the linear actuator around a second orthogonal axis parallel to the first orthogonal axis located opposite the first orthogonal axis relative to the lengthways axis, such that the linear actuator causes the yoke to rotate around the first orthogonal axis and a control unit to control the linear actuator according to the position of the yoke, said pusher being configured to rotate freely in said yoke,means to cause rotation of the means configured to exert radial tightening forces on the periphery of said elongate element and said pusher,a welding chamber, the rods forming elongate elements, said device comprising a through housing of lengthways axis in which the rod is intended to be held axially by tightening, and to pivot around its axis, said welding chamber being positioned at one end of the through housing opposite the end the rod being introduced, the end of the rod fitted with the plug emerging from said through housing in a welding area of said welding chamber, and said welding chamber-comprisings means of welding the top plug on the rod. 16. A welding installation according to claim 15, in which the welding chamber comprises a stop for the top plug of the rod aligned with the lengthways axis of the device. 17. A welding installation according to claim 15, in which the welding chamber comprises suction means to create a vacuum, an injector for injecting an inert gas in the welding chamber, and an analyser for analysing the atmosphere in said welding chamber, and a fluid channel connecting the suction means and the injector to the welding area. 18. A welding installation according to claim 16, in which the stop is pierced with a lengthways channel, where the fluid channel is connected to the welding area through the lengthways channel of the stop. 19. A welding installation according to claim 18, in which the fluid channel is formed by a tube rotationally secured to the stop, where said tube is installed in bearings, and where the stop is rotated by the rod. 20. A welding installation according to claim 15, comprising means to view the position of the welding means relative to a mating surface between the top plug and the cladding of the rod. 21. A welding installation according to claim 15, wherein said device is configured to check a speed of rotation of the rod and/or a quality of the atmosphere and/or a level of tightening and/or a position of the welding means relative to the mating surface. 22. A welding method implementing a welding installation for welding the top plugs on the nuclear fuel rods, comprising:a device for axial holding and setting in rotation nuclear fuel rods, said device of lengthways axis comprising:means configured to exert radial tightening forces on the periphery of said elongate element aligned towards the lengthways axis, by bringing said means configured to exert radial tightening forces close to said lengthways axis,means to cause said means configured to exert said radial tightening forces closer to one another, said means to cause comprising a pusher to apply an axial force on to said means configured to exert radial tightening forces, a counter thrust bearing to force said means configured to exert radial tightening forces to come closer to the lengthways under the effect of the axial force, a linear actuator of axis parallel to the lengthways axis of the device, a yoke which is translationally secured to said pusher and configured to rotate around a first axis orthogonal to lengthways axis and not secant with it, said first orthogonal axis being fixed, said yoke being hinged to the linear actuator around a second orthogonal axis parallel to the first orthogonal axis located opposite the first orthogonal axis relative to the lengthways axis, such that the linear actuator causes the yoke to rotate around the first orthogonal axis and a control unit to control the linear actuator according to the position of the yoke, said pusher being configured to rotate freely in said yoke,means to cause rotation of the means configured to exert radial tightening forces on the periphery of said elongate element and said pusher, according to claim 1, anda welding chamber, the rods forming elongate elements, said device comprising a through housing of lengthways axis in which the rod is intended to be held axially by tightening, and to pivot around its axis, said welding chamber being positioned at one end of the through housing opposite the end the rod being introduced, the end of the rod fitted with the plug emerging from said through housing in a welding area of said welding chamber, and said welding chamber comprising means of welding the top plug on the rod,said welding method comprising the following steps:insertion of the rod fitted with the top plug into the passage of the device, until the top plug comes to rest against the stop of the weld chamber,operation of the rod tightening means,setting the rod in rotation,generation of a vacuum in the welding area,injection of inert gas,welding. 23. A welding method according to claim 22, in which, prior to welding, the speed of rotation of the rod and/or the quality of the atmosphere and/or the tightening level and/or the position of the welding means relative to the mating surface are checked.