The present invention relates to machines and methods adapted to exert a longitudinal traction on rod like members such as threaded studs used for securing together two mechanical parts .

The tensioning of rod like members or studs can be done by applying a tightening torque to a threaded nut resting axially on the surface of the structure to be tightened. This tightening method does not allow controlling precisely the axial force transmitted to the rod, because o f frictions between the nut in contact with the surface of the part to be tightened and between the nut and the rod. Moreover, this tightening method introduces torsion residual stresses into the rod, which results in using oversize rods. In addition, such a tightening generally leads to an important risk of damage of threads of the nut and o f the rod cooperating together during tightening. The surface against which the nut is resting during its rotation at the time o f tightening may also be damaged.

In order to avoid these disadvantages, a rod member or a stud may be axially tensioned before fitting-up a nut on the surface o f a structure to be tightened. The nut blo cks the rod when the axial force applied to the rod is released. The rod is thus pre-stressed only along its axis. An actuator can be used in order to transmit an axial traction force to the rod. Thus, after the nut is positioned on a threaded portion of the rod to be tensioned, the actuator is positioned so as to surround the head portion of the rod or stud and the nut. A traction force is applied in a longitudinal direction on a threaded extension of the head part of the rod member or stud and produces an elongation o f the stud. This elongation facilitates the rotation of the tightening nut adapted to cooperate with a threaded portion of the stud so as to tighten the nut. The two mechanical parts can thus be secured together by the rod member or the stud without inducing a twist or a residual torsion stress to the stud. Large mechanical assemblies must often be maintained by several threaded rods or studs. Multiple stud tensioning machines are then used to position, screw and unscrew as well as pre-tension a plurality o f threaded studs. Examples are the attachment of parts of wind turbines or of the protecting cover of nuclear power reactor shells. A number o f threaded studs are usually arranged in a circular row for attachment of such kind of large mechanical assemblies.

In order to facilitate the attachment operations, for example of the protecting cover to a nuclear reactor shell, a supporting ring assembly is provided, said assembly being moved from above on the cylindrical cover installed on the nuclear power shell. The complete set of attachment studs is supported by the supporting ring which is also provided with all necessary means for screwing and unscrewing the studs as well as tensioning them in a longitudinal direction before tightening the corresponding nuts . In order to fulfill those tasks, the supporting ring is usually equipped with at least one and preferably two robot units capable of being moved along the periphery of the supporting ring for screwing, unscrewing and tightening the nuts one by one within the nuclear power shell. The longitudinal traction force for pre-stressing each individual stud before tightening is produced by a plurality o f hydraulic stud tensioners, each of which is mounted on the head o f a respective stud.

All the studs around the cover of the nuclear power shell must be submitted to the same longitudinal traction before the corresponding tightening nuts can be rotated so as to secure the cover to the nuclear power shell. It is therefore necessary to carefully monitor the traction force applied to all individual studs so as to guarantee an identical tension, and o f course to maintain such force within specific limits depending on the tensile strength of the material used. Such monitoring is preferably made by measuring the elongation of each stud with elongation sensors, each sensor being mounted on a respective stud. The individual elongation sensors are usually mounted manually on each stud, which is particularly cumbersome. The sensors are supplied with electrical energy by electrical connections linked to a central power station. The measurement signals are transmitted from each individual elongation sensor by wire connections . Therefore, the mounting operation as well as the dismounting operation of the multip licity o f elongation sensors is particularly long and difficult.

One aim o f the present invention is to avoid those drawbacks and to simplify the operations of assembling and disassembling a plurality o f elongation sensors on a plurality of rod like members or studs to be tensioned in a multip le stud tensioning machine.

Another aim of the present invention is to permit a simpler monitoring of the elongation of a plurality of studs during a pre- tensioning operation.

More generally, the present invention aims at providing quicker and easier assembling and disassembling of a plurality o f elongation sensors . A further aim o f the invention is to provide a safer and cheaper monitoring of the elongation of a plurality o f studs, during attachment of two mechanical elements together, for example, a protecting cover to a nuclear reactor shell.

In an embodiment, a multiple stud tensioner machine adapted to longitudinally tension a plurality of studs, comprises a sensor supporting assembly adapted to receive a plurality o f elongation sensors arranged in a row and positioning means adapted to move one of said sensors from said row into an operating engagement with one of said studs.

All the sensors are therefore initially maintained, preferably in a stand-by state, on the supporting assembly. The assembling step o f each sensor onto the corresponding stud can be made automatically thus avoiding the long and cumbersome manual operations which were previously necessary.

Each elongation sensor is preferably provided with individual wireless communication means adapted to transmit to a computer system measurement and identification signals. The wireless communication means may also receive control signals from said computer system for contro lling the sensor. No electrical wire is therefore anymore necessary which makes the assembling operations of the plurality o f sensors considerably quicker and easier. The identification and measurement signals transmitted by each sensor allow the computer system to correctly monitor the tensioning operation of all the studs simultaneously. The signals are preferably sampled with an appropriate frequency, for example less than I s.

The machine also preferably includes indexing means capable of transmitting a position signal corresponding to the position o f said stud having said sensor in operating engagement. The computer system receives such a position signal as soon as an individual sensor is placed in operating engagement with a corresponding stud. The computer system is therefore able to detect which individual sensor corresponds to which stud, so as to safely and correctly monitor the tensioning o f each stud.

In an advantageous practical embo diment, the sensor supporting assembly is adapted to receive said row of sensors so that said sensors are urged by gravity toward an end position.

The sensor supporting assembly may comprise a stop means adapted to be moved between an open po sition and clo sed position in which said stop means block a sensor in said end position.

Such a structure using gravity to automatically supply the elongation sensors is particularly reliable and simple.

A movable grasping fork may be provided to receive a sensor in an intermediate position when said stop means is moved to said open position. A means for actuating a switch provided in the sensor may be mounted on the grasping fork. A seizing arm may be provided to seize said sensor in said intermediate position and to position said sensor in operating engagement with one of said studs.

The grasping fork may be withdrawn from said intermediate position as soon as the seizing arm has seized the sensor.

The seizing arm may be provided with mechanically actuatable j aws adapted to cooperate with a protruding portion of said sensor. Alternatively an electromagnetic lifting means may be provided at the end o f the seizing arm for lifting the sensor and moving it in operating engagement with said stud.

After the sensor has been placed in said operating engagement with the stud, the seizing arm is moved back to its initial position and the grasping fork may also be moved back to the intermediate position so as to cooperate with the next sensor.

In a particularly simple and advantageous embodiment, the sensor supporting assembly may comprise two elongated guides adapted to receive the lateral sides o f guiding plates secured to each said sensor, said elongated guides comprising a vertical portion and a curved portion ending substantially horizontally. The sensors are therefore maintained in the supporting assembly by their respective guiding plates . The gravity is sufficient to urge the queue of sensors toward the substantially horizontal end of the curved portion of the two guides where the last sensor is blo cked in position by the stop means. If necessary, additional weights having the same shape as the guiding plates of the sensors may be inserted within the two elongated guides at the top of the queue so as to better urge the sensors toward the end o f the curved portion.

The grasping fork is preferably adapted to receive the forward edge o f the guiding plate o f a sensor in said intermediate position.

The elongated guides preferably comprise electrically conducting slides cooperating with contacting portions of said plates for charging a battery mounted on each said sensor. Each individual sensor is equipped with its own battery as source of electrical energy.

The batteries of the sensors maintained in the supporting assembly by their respective guiding plates, can thus be charged during the inactive time when the sensors are in stand-by state.

In an embodiment particularly adapted to a multiple stud tensioner machine for attachment of the cover dome o f a nuclear power reactor shell, the sensor supporting assembly is attached to a vertical side of a robot device having at least one screwing head, said robot device being adapted to be moved over a series of studs . In a preferred embodiment, the elongation sensor adapted to measure the deformation of a stud during a tensioning operation, comprises a rechargeable battery and also individual wireless communication means adapted to transmit to a computer system measurement and identification signals and to receive control signals from said computer system. The positioning o f the sensors is thus greatly simplified since no connecting wires are necessary. This also makes possible to store the sensors in the supporting assembly before they are individually positioned on each respective stud.

According to a further aspect, the invention also relates to a method for automatically controlling the elongation o f a plurality o f studs during a tensioning operation of said studs. The method comprises the steps of providing a series of individual elongation sensors, positioning one of said sensors in operating engagement with each said stud prior to the tensioning operation, transmitting by wireless communication to a computer system a position signal corresponding to the position o f each said stud having said sensor in operating engagement and continuously transmitting by wireless communication to the computer system, measurement and identification signals issued by each said sensor during the tensioning operation.

The method may also comprise the further step of transmitting by wireless communication from the computer system to each sensor, control signals for charging a battery included in said sensors and/or for energizing said sensors or setting said sensors in a stand-by state.

The invention will be better understood on the basis o f the description o f an example which is to be considered in a non limiting way, and which is illustrated on the enclo sed drawings, in which:

Figure 1 is a schematic partial view o f a multip le stud tensioner machine taken from above, and showing a robot with a sensor supporting assembly,

Figure 2 is a side view taken in the direction of arrow F2 o f Figure 1 , showing the robot with the sensor supporting assembly, Figure 3 is a three-dimensional view of a part of the sensor supporting assembly illustrating more particularly two elongated guides supporting a row of sensors,

Figure 4 shows an example of an elongation sensor which can be used in the machine o f the present invention, and

Figure 5 schematically illustrates the main elements of an electronic control system for each sensor.

As illustrated on Figures 1 to 3 , a multip le stud tensioner machine generally comprises a main ring 1 , having a plurality o f ho les for accommodating and supporting a plurality o f studs 2 (see Figure 2) . The ring 1 has lateral rails 3 along which a robot 4 can be displaced. As illustrated on Figure 2, the robot 4 has supporting wheels 5 cooperating with the rails 3 . Each stud 2, which is to be put under traction by longitudinal tensioning, is equipped with a tensioner device 2b mounted on the head of the respective stud 2.

The robot 4 has, on one of its lateral faces, a sensor supporting assembly 6. This assembly comprises vertical elements 7 as well as horizontal elements 8 , which together build a frame within which two elongated guides 9 are mounted, as best shown on Figure 3. Said elongated guides 9 comprise a vertical portion 9a, and a curved portion 9b which terminates substantially horizontally at an end portion 9c (Figure 3) . The two guides 9 have guiding grooves 1 0 provided internally and facing each other. The two grooves 10 are adapted to guide a plurality o f elongation sensors 1 1 which are ranged in a row within the two elongated guides 9.

In the illustrated example as shown on Figure 4, each sensor 1 1 comprises a guiding plate 12. The dimensions of the guiding plates 12 are such that they can be mounted in the grooves 10 of the two guides 9 and maintained in those two guides 9 for sliding movement under the effect of gravity as illustrated on Figure 3. To this end, the two lateral end portions 12b o f the guiding plates 12 are received within the respective grooves 10. Said two end portions 12b are advantageously of substantially trapezoidal shape as shown on Figure 3 , with a reduced width toward their free end.

As illustrated on Figure 3 , the sensor supporting assembly 6 comprises two stop fingers 13 , which in the illustrated example, are horizontally extending fingers adapted to be moved from a closed position illustrated on Figure 3 , to an open position. In the clo sed position, the two fingers 13 block any further movement of the last sensor 1 1 a, which is in an end position in the row of sensors 1 1 . In this end position namely, the guiding plate 12a of said last sensor 1 1 a is located substantially in the end portion 9c of the two guides 9. The sensor 1 1 a is pushed by the row of all other sensors 1 1 acting by gravity and in sliding engagement with the two guides 9. The two fingers 13 in the clo sed position illustrated on Figure 3 blo ck the exit of the grooves 10 and therefore also block the plate 12a of the last sensor 1 1 a. In said closed position, the two fingers 13 extend only in front of the grooves 10. The movement of the fingers 13 from said closed position to an open position is produced by any suitable means . As illustrated on Figure 3 , two pistons 14 are mounted horizontally on both sides of the two guides 9 at their end portion 9c. The respective rods of the two pistons 14 bear the stop fingers 13.

In the embodiment illustrated on Figure 3 , a movable grasping fork 15 is provided on the sensor supporting assembly. The fork 15 has two substantially horizontal branches 16, which are split into an upper part 16a and a lower part 16b . The gap between the lower part 16b and the upper part 16a is slightly greater than the thickness of each guiding plate 12 of the sensors 1 1 . The overall width of the grasping fork 15 is smaller than the distance between the two guides 9. The grasping fork 15 is illustrated on Figure 3 in a waiting position. In this position, in the illustrated example, the forward edge portion of the guiding plate 12a of the sensor 1 1 a in the end position, is already inserted within the gap between the upper part 16a and the lower part 16b o f the branches 16. The grasping fork 15 can be moved from the waiting position illustrated on Figure 3 to a back off position by an actuating piston 17 having a movable rod 1 8 at the end o f which the grasping fork 15 is attached. The grasping fork 15 , the rod 1 8 and the piston 17 are mounted substantially horizontally.

When the fingers 13 are retracted to an open position, they leave passage o f the guiding plate 12a, which is pushed by the guiding plates 12 of the other sensors 1 1 . Consequently, the guiding plate 12 slides further within the gaps provided in the two branches 1 6 of the grasping fork 15. The sensor 1 1 a is then in an intermediate position corresponding to the waiting position o f the grasping fork 15.

In this position, the guiding plate 12a is disengaged from the grooves 10 of the guides 9 thanks to the reduced width of the trapezoidal shaped free ends of the guiding plate. The lateral edge o f the guiding plate 12a is still in contact with the lateral edge o f the subsequent sensor 1 1 . The stop fingers 13 may be again moved in the closed position so as to block the guiding plate of the subsequent sensor 1 1 in the end position.

The grasping fork 15 having the sensor 1 1 a between its branches 16, supports the sensor 1 1 a in the intermediate position which is aligned with the axis o f the stud 2a illustrated on Figure 2. The axis o f both the stud 2a and the sensor 1 1 a also corresponds to the axis o f a seizing arm 19 which is illustrated on Figure 3 as a piston 20 with a movable rod 21 having at its end a mandrel 22 with gripping j aws not visible on Figure 3. The seizing arm 19 is actuated while the guiding plate 12a of sensor 1 1 a is still maintained in the intermediate position by the fork 15 . The gripping j aws of the mandrel 22 can thus ho ld the sensor 1 1 a by grasping a protruding portion 24a o f sensor 1 1 a. At that time, the fork 15 is moved to its back off position, clearing the passage of the rod 21 and allowing the seizing arm 19 to position the sensor 1 1 a in operating engagement with the stud 2a.

As illustrated on Figure 4, each sensor 1 1 has a rechargeable electrical battery 23 mounted within a box 23 a on one side of the guiding plate 12. A protruding portion 24, located on the top of box 23 a, can be grasped by the j aws of the mandrel 21 upon actuation o f piston 20. The elongation sensor 1 1 has a measuring head 25 mounted at the end of supporting rods 26, for example three supporting rods at 120° from each other, attached to the guiding plate 12 on the face opposite to the box 23 a o f the rechargeable battery 23 . The elongation of the studs 2 can be measured for example by using an elongated bar lo cated within a ho llow internal cavity of each stud 2, said cavity extending from one end to the other end of the stud 2. A nut secured at the lower end of each stud 2 serves as a support for the measuring bar. The measurement head 25 simply bears on the top of said bar and is therefore able to measure a difference in length position between the measuring bar which is not submitted to any traction, and the stud, which is submitted to a pre-tensioning traction by the corresponding tensioner device 2b .

Each elongation sensor 1 1 is also provided with an individual wireless communication means mounted within box 23 a, and adapted to transmit measurement and identification signals, as will be explained further. The robot 4 comprises indexing means, which are not illustrated on Figure 2, and which are capable of transmitting a position signal o f the robot 4 during operation. The position signal corresponds to the position o f the robot when a given sensor, for example sensor 1 1 a, is in operating engagement with a given stud, for example stud 2a. The position signal which is transmitted by said indexing means, can be used by a computer system as will be explained further so as to precisely locate any given sensor in connection with its respective stud.

In order to charge the rechargeable batteries of each individual sensor 1 1 , the guiding plates 12 are provided, on one of their sides, with contacting plugs 27 (Figure 4) which are in electrical contact with the inside surface of the grooves 10. Said inside surface is made of an electrically conducting material so that the charging electrical energy can be fed through those grooves.

Figure 5 illustrates schematically the main parts of a control system of a sensor 1 1 .

The measuring head 25 o f the sensor issues an analog signal which is converted into a digital signal by a converter 28. The signal is then transmitted to a microprocessor 29 incorporated into the sensor 1 1 . After suitable conditioning, the signal is transmitted by a wireless communication device 30, for example a Bluetooth device, to a central computer system not illustrated on the figures. The electrical energy necessary for the function of the individual sensor is provided by the rechargeable battery 23. Signals from the sensor transmitted by the wireless communication device 30 , can be treated by the central computer system, which is able to receive the signals transmitted by all the elongation sensors 1 1 during the simultaneous tensioning operation of all the studs. The central computer system may also transmit control signals to the respective sensors 1 1 , said signals being received by the wireless communication device 30 and treated by the microprocessor 29 of each individual sensor 1 1 . The control signals can for examp le set one or several sensors in operation, or, on the contrary, in a stand-by state to avoid that the individual electrical batteries of the sensors be discharged unnecessarily during the time the sensors are supported by the supporting assembly 6.

In operation, a multiple stud tensioner machine such as illustrated on Figures 1 to 3 is able to efficiency control the elongation of all the studs during the tensioning operation. The elongation sensors which are preferably in a stand-by state, are supported by the sensor supporting assembly attached to the robot unit 4 as shown on Figure 1 . The robot unit 4 can be moved along the periphery o f ring 1 so as to position an individual elongation sensor 1 1 on the head o f each stud. The robot unit 4 is moved step by step in the direction of arrow F3. Each time the robot unit 4 reaches a position corresponding to a given stud, the movement of the robot unit 4 is stopped and an individual elongation sensor 1 1 is positioned on the corresponding stud as previously explained. The central computer system receives a signal indicating the exact position o f the stud and the exact correspondence with a given elongation sensor, which is identified by a specific code .

In a preferred embodiment, the grasping fork 15 is provided with means capable o f energizing the elongation sensor as soon as it is moved in engagement with said grasping fork. As an example, the grasping fork may have a permanent magnet which is capable o f actuating an ILF type switch provided on the sensor. The corresponding sensor is then in an active state allowing transmission by wireless communication o f the identification signal corresponding to the given sensor and the subsequent measurement of the elongation of the corresponding stud.

Due to the fact that each sensor has its own electrical energy source and is able to transmit and receive signals by wireless communication, the entire operation o f mounting o f the multip le sensors becomes particularly quick and easy. The sensor supporting assembly in which the sensors are kept in a vertical row and simply moved by gravity allows a particularly simple function. For the case the last sensors in the supporting assembly would not slide until the end position o f the two guides 9, particularly toward the end portion 9c, it is advisable to add supplemental weights 3 1 having the shape o f guiding plates 12 on the top of the row of sensors as illustrated on Figure 3.

The present invention provides therefore a more simple and efficient way of mounting elongation sensors on a plurality o f studs to be tensioned in a multiple stud tensioning machine. The invention also allows an easy monitoring o f the elongation of the plurality o f studs during the pre-tensioning operation.