This invention relates to securing body parts together by holding the parts face-to-face in compression provided by a plurality of junction bolts each extending through the parts and in which a residual tensile stress is maintained by a bolt head and a nut abutting the flanges.

The invention is concerned particularly but not exclusively with such body parts having or comprising flanges to be held face-to-face in compression but which flanges are not necessarily in mutual abutment prior to the securing by the bolts and is concerned with bodies, such as turbine casings, where a large number of bolts are required at close pitch and where the forces required to effect an adequate and uniform flange compression require the use of bolts up to 20cm. diameter placed under correspondingly large tensile stress.

It is well known in heavy engineering that obtaining a predetermined tensile stress by normal rotation of a nut threadedly engaging the bolt is difficult because of friction and accessibility problems and also that differences in residual tensile stresses obtained in the various bolts may result in failure of individual over- stressed bolts or in unacceptable distortion of the flanges, with possible leakage paths, where the bolts are under- stressed.

In such circumstances it is known to achieve a desired

residual tensile stress in a flange-joining bolt by creating a large tensile stress between opposite ends of the bolt along its length with consequential bolt elongation, running a nut along the threaded shank of the bolt until it abuts one flange with the bolt head abutting the other and then removing the source of stress such that as the bolt attempts to contract the bolt head and nut maintain a residual stress that is transferred into the flanges as a compressive force.

It will be understood that the relationship between bolt elongation and tensile stress permits as initial tensile stress to be chosen to enable a nut position to be taken up that in turn results in a predeter inable residual stress.

Hitherto it has been known in securing bodies such as turbine casings, where there is little room to apply complex bolt stressing devices, to employ bolts having an axial cavity extending through the bolt from end to end and for the bolt to be heated by way of such cavity by way of a bolt heater therein so as to expand it longitudinally by a predetermined amount before the nut is positioned, after which it is cooled to develop, by the accompanying contraction, the predetermined residual stress. Such an operation has been found to give consistent and controllable results but is very time consuming.

Such a bolt having such an axial cavity and suitable for joining two such body parts may be used in other ways and may be conveniently referred to, as in this specification, as a junction bolt.

Such a junction bolt having an axial cavity may be caused to undertake axial extension by "mechanical" forces applied instead of, or in addition to, thermal expansion, most practicably delivered by a form of hydraulic ram or load cell, conveniently considered as hydraulic stressing means. Such a bolt with such hydraulic stressing means for applying

such extension forces directly between opposite ends of the bolt may be considered as a self-stressing junction bolt.

An example of such a self-stressing junction bolt is disclosed in US-A-4884934 in which a large tensile stress is created between opposite ends of the bolt by means of a hydraulic stressing device comprising an actuating rod extending along a blind longitudinal central cavity of the bolt, one rod end bearing internally against the head region and the other rod end acting as a piston to a cylinder, formed in the other end of the cavity, to which hydraulic pressure is applied. The hydraulic-pressure-created longitudinally acting forces act and react through opposite ends of the bolt to impart a tensile stress whilst a nut is run down the shank with little torque into a desired position contacting a body, after which the hydraulic pressure is released to enable the bolt head and nut to develop residual stress. That patent specification also discloses the use of heat in combination with such hydraulic stressing means.

Another example of a self-stressing junction bolt is described in Patent Specification GB 1382191 which is directed towards a so-called radial fit bolt. As described therein a bolt, having an integral head at one end of an elongate shank that is threaded to accept a nut at at least the other end of the shank, has a axially extending blind cavity open at the head end and extending to the vicinity of the other end of the shank and an actuating rod is disposed in the cavity and a hydraulic ram mountable on the head end, whereby hydraulic pressure causes a tensile stress between opposite ends which effects both an elongation of the bolt shank and a radial contraction of the shank to permit its insertion into an undersize hole in which it becomes a tight fit when the hydraulic pressure is released.

In this specification the term "junction bolt" refers only to such a self-stressing junction bolt as outlined hereinbefore and distinguished from a bolt that is stressed by reacting tensile forces into a body from which the bolt extends and the term "hydraulic stressing means" is used to refer to such hydraulic apparatus and actuating rod that extends along the central cavity of the junction bolt to effect such stressing between opposite ends of the bolt.

Clearly, in general, in order to achieve any such tensile stress directly between opposite ends of the shank of a self-stressing junction bolt it is immaterial from which end of the shank the hydraulically derived stressing actuation is performed.

Furthermore, the term "bolt" is used in this specification, whether in relation to a junction bolt or otherwise, as meaning an elongate shank having a "head" comprising a radially extending flange or extension which is able or caused to take up a position of abutment against a body without rotation relative to the shank, and in this respect a functional bolt head may be integral with the shank or comprise a threaded nut at one position along the shank instead of, or in addition to, an integral head, and may be disposed other than at a physical end of the elongate shank.

A situation may occur in securing two body parts at flanges thereof where there is resistance to the flanges coming together into abutment, by reason of impedance to motion of one or both flanged body parts, by natural distortion or unevenness of the flanges or if there is a gasket or sealing material between the flanges. However, as it is a feature of such known bolt stressing installation procedures that by controlled direct stressing of the bolt the nut is run into position with minimal applied torque, a situation may arise where a nut run along such a pre-stressed junction bolt at low torque that is, in practice until it is resisted by

flange, is erroneously assumed to be correctly positioned. If the body flanges are not in fact properly together, when the bolt stressing device is relieved, the anticipated residual stress in the bolt may be dissipated in further moving the flanges rather than compressing them.

This situation might be resolved by enabling a considerable level of torque to be applied to the nut to effect any flange displacement necessary or by employing an alternative hydraulic bolt tensioning arrangement in which the tensile stress in induced by reacting the forces by way of the flanges.

The use of considerable nut running torques is seen to re- introduce the uncertainties of conventional nut tightening, whereas such bolt tensioning devices that react against the body have a problem of impeding access to the nut and/or requiring more space than is available between tightly packed bolts and/or risk local damage to the body where the tensioning force is reacted.

Furthermore, in all of the procedures and possible procedures outlined above, there is little scope for speeding up and/or automating the securing of two flanged bodies such as turbine casings where a large number of closely packed junction bolts are involved.

It is an object of the present invention to provide a method of securing two apertured body parts face-to-face in compression, and apparatus for so securing, that mitigates disadvantages hitherto known and permits of greater automation.

According to a first aspect of the present invention a method of securing two apertured body parts face-to-face in compression by means of a plurality of junction bolts (as herein defined) , comprises (1) disposing the body parts

facing each other with a plurality of apertures thereof mutually aligned as spaced aperture pairs, (2) disposing at ' least two bolts, at least one of which is a junction bolt, to extend through neighbouring aperture pairs such that a head end of each bolt is adjacent one body part and a threaded shank extends from the other body part to a shank end remote from the body part, (3) assembling on a junction bolt a nut and hydraulic stressing means (as herein defined) , (4) assembling on said remote end of at least one neighbouring disposed bolt a hydraulic nut, one part thereof in engagement with said remote end and the other part able to bear against the body part from which the bolt extends, (5) applying hydraulic pressure to both hydraulic nut and hydraulic stressing means of said bolts such that extension of the hydraulic nut forces the body parts together into abutment and extension of the hydraulic stressing means develops a predetermined installation tensile stress in the junction bolt, (6) rotating the nut until it assumes a desired position with respect to the stressed shank, and (7) releasing hydraulic pressure from both the hydraulic nut and the hydraulic stressing means.

According to a second aspect of the present invention a securing arrangement for two body parts in which a plurality of through apertures are mutually aligned as aperture pairs comprises (1) a hydraulic pressure source,

(2) hydraulic nut means operable to apply to a bolt shank extending from one aperture of a pair a tensile stress reacted against the body part containing the aperture so as to draw, with a head of the bolt adjacent the other body part, the two body parts into abutment, (3) hydraulic stressing means (as herein defined) responsive to hydraulic pressure therein to apply to a shank of a junction bolt (as herein defined) extending from a neighbouring aperture pair a tensile stress reacted against the opposite end of the bolt, (4) nut running means associated with the hydraulic stressing means operable to cause the junction bolt to be

held with respect to the abutted body parts by way of a bolt head in abutment with one body part and cause a nut on the shank to be positioned with respect to the other body part, and (5) control means operable to cause, in sequence, (a) actuation of both said hydraulic nut means and hydraulic stressing means in respect of said neighbouring bolts at such hydraulic pressure as to achieve a predetermined level of tensile stress in the junction bolt, (b) actuation of the nut running means and (c) de-actuation of said hydraulic nut means and hydraulic stressing means to achieve, by means of said junction bolt head and nut, at least an approximation to a predetermined level of residual stress in the junction bolt.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:-

Figure 1 shows a fragmentary, partly cut-away perspective view of a flanged body, two parts of which are secured to each other by means of compressive mating of corresponding flanges having aligned aperture pairs by junction bolts, illustrating the distribution of flange apertures and proximity of junction bolts,

Figure 2 shows schematically a securing arrangement in accordance with the present invention illustrating in particular the securing operation at two adjacent flange apertures, and

Figure 3 shows as a flow diagram a sequence of operations involved in initial installation of the junction bolt of Figures 1 and 2 and a subsequent monitoring and corrective sequence.

Referring to Figure 1 a body 10 such as a turbine casing is formed when two matching body halves 10- and 10 ₂ are brought

together with peripheral flanges, 11, and 11 ₂ respectively, face-to-face and held in compression. Such compression is achieved and maintained about the periphery of the body, that is, along the flange, by means of apertures, one in each flange, through which extend junction bolts 13 each maintained under tensile stress by a bolt head 14 and nut 15 bearing against the opposing flanges.

It will be understood that in such an application, as in many others, the flanges may benefit from being brought into compression at symmetrically disposed locations about the casing so as not to introduce distortion thereof. This is particularly true if for any reason the flanges do not naturally assume abutment, that is, there is resistance to even preliminary compression that could exacerbate distortion of a casing secured at one or more isolated point only.

Notwithstanding such consideration of uniform distribution of flange compression, it will be appreciated that the number and proximity of adjacent apertures places practical limits on the type of junction bolts.

It will also be understood that the above outlined problem associated with obtaining a constant residual bolt stress within predetermined limits where the flanges do not naturally abut may be considered separately for each junction bolt from any scheme of securing at multiple points.

Referring now to Figure 2 a small portion of each of the body flanges 11] and 11 ₂ is shown with the flanges disposed in face-to-face relationship but separated by a small gap 16, shown principally for clarity of description. Such gap may be due to inherent reluctance or inability of the flanges to meet at all points or may be due to the presence of a sealing or gasket material which requires some applied

force to deform or compress but not of the same order of magnitude as the forces to be finally exerted by the junction bolt. The flanges each contain through-apertures 17,18 which are aligned axially by virtue of the body shapes and each such pair is referred to hereinafter as being "an aperture pair" 19,, 19 ₂ .

A junction bolt 13 (as hereinbefore defined) extends through aperture pair 19 _! , and a further bolt 21, conveniently, also an identical junction bolt, extends through a neighbouring aperture pair, shown as immediately adjacent aperture pair 19 ₂ .

The bolt 13 comprises an externally threaded shank 22 at one end 22- of which is carried a nut 23 which effectively forms with the shank end a head 14 the bolt. The shank also contains a cavity 25 extending along the longitudinal axis between the other end 22 ₂ and the end 22 _j . For convenience the shank may be manufactured with the cavity 25 as a through-aperture extending between opposite ends but closed by a plug 26.

The outer surface of the shank 22, at least towards the end 22 ₂ , is threaded in conventional manner and so able to engage with a nut 15 that can be run along the shank. The bolt 13 is thus employed to apply a compressive force between the flanges 11, and 11 ₂ by means of the nut and head bearing thereon in maintaining a tensile stress in the shank.

The junction bolt 13 and the bolt 21 may be considered as part of the body on which a securing arrangement in accordance with the present invention operates.

The securing arrangement, indicated generally at 29, comprises five major components that will be described hereinafter, namely a hydraulic pressure source 30,

hydraulic nut means 40, hydraulic stressing means 50, nut running means 60 and control means 70.

The hydraulic pressure source 30 comprises a hydraulic pump 31 driven by compressed air from a source (not shown) by way of an air control valve 32. The air control valve determines when the pump is or is not to function and this timing is under the control of the control means 70. The hydraulic circuit includes a return control valve 33 which determines the pressure in fluid delivery line 34 which pressure is monitored by a pressure transducer 35. The signal output by the transducer is fed to the control means 70 which varies the state of the return control valve 33 as well as the air control valve to produce or maintain a particular flow rate or pressure. It will be appreciated that the pressure source in combination with the control means is able to be set to a high flow rate to deliver fluid at low pressure and in the same delivery to continue delivery at low rate but exert a higher controlled pressure.

Hydraulic nut means 40 comprises a conventional hydraulic nut 41 having first annular part or nut body 42, internally threaded to engage the threaded shank of the bolt 21 and including annular recess 43 forming a cylinder to receive hydraulic fluid from the source 30, and a second annular part or load ring 44 forming a piston in the recess 43 and reciprocable relative to the shank in response to fluid pressure in the recess to bear against the flange 11-,, the separating force between the two nut parts resulting from such pressure displacing the bolt head towards the nut and with it adjacent the other flange 11,, drawing the two flanges into close abutment. The relationship between the supplied fluid pressure and the cross-sectional area of the piston formed by the second part 44 is such that the clamping force on the flanges is sufficient to draw the flanges into abutment but insufficient to cause any damage

or distortion and the tensile stress induced in the bolt is sufficient to cause any permanent deformation of the bolt.

The hydraulic stressing means 50 comprises in association with the junction bolt 13 a hydraulic stressing device having an actuating rod 51, extending along the cavity 25 between the plug 26 at the head end 22, and beyond the other end 22 ₂ , and an operating head 52. The operating head comprises a piston 53 reciprocable within a cylinder body 54 whose walls extend beyond the piston and are threaded at 55 to removably engage the threaded end 22 ₂ of the shank and be carried thereby. The piston and cylinder body define a fluid chamber 56 and when the operating head is assembled on the shank as shown the actuating rod 51 extends into the cylinder body towards the piston so that hydraulic pressure applied by admitting hydraulic fluid from source 30 to the chamber 56, by way of supply line 57 and coupling 58, causes the piston and cylinder body to be forced apart in opposite directions and the forces being coupled into said opposite ends 21, and 22 ₂ respectively of the junction bolt to put the latter in tensile stress.

The fluid pressure supply line 57 is connected to the hydraulic pressure source 30 at 34 so that the fluid pressure applied to the operating head is identical to that applied to the adjacent hydraulic nut 41. However the area of the piston 52 on which the fluid pressure acts is greater than that of the adjacent hydraulic nut so that the tensile stress introduced into the junction bolt 13 is in excess of that in the bolt 21.

The nut running means 60 comprises for the junction bolt 13 a nut rotating motor 61, conveniently carried by the operating head 52, a toothed drive gear 62 carried by, or at least rotatable with, the motor shaft and a correspondingly toothed driven gear ring 63 either formed integrally with, or removably attachable to, nut 15. The nut running means

further comprises a biasing ram 64 which is conveniently removably attachable to the head of the adjacent bolt 21 and is operable to bias the head 23 of the junction bolt into abutment with the flange 11,. The nut rotating motor is controllable with respect to effecting a position change by control means 70. Conveniently the motor 61 is a stepper motor, enabling the nut to be changed in position by simply using a number of stepping pulses that correspond to the desired nut rotation,, but it may, alternatively, be provided by a conventional continuously rotatable motor equipped with a position sensor and forming part of a servo control loop. Notwithstanding the motor type and its manner of effecting position change, the motor 61 is arranged to exert only a relatively low torque that is sufficient to rotate the nut when otherwise unrestrained but not to generate any significant degree of axial force in the junction bolt when the nut abuts the flange 11 ₂ . This may result from an inherent power limitation on the part of the motor supply or the motor itself which causes the latter to stall or may result from sensing the load on the motor and limiting the power in accordance with a predetermined value.

The control means 70 comprises a central control unit 71 that exercises control over the hydraulic pressure means 30 and the nut running means 60 in response both to a predetermined sequence of operations effecting installation of the junction bolt that results in residual tensile stress exerted on the flanges, and to a further predetermined sequence governed by signals derived from a transducer 72 which measures the actual level of residual stress in the junction bolt, in case this differs from that ostensibly attained by said predetermined sequence of operations.

The control unit 71 is conveniently provided by a microprocessor suitably programmed in any conventional manner in accordance with the operations it is desired to

perform in accordance with the second aspect of the present invention.

Operation is considered firstly for securing flanges 11,, ll ₂ in the vicinity of the adjacent aperture pairs 19, and 19 ₂ by means of junction bolt 13 and using a bolt 21 which, as indicated above, is conveniently also a junction bolt.

Assuming that the flanges 11, and ll ₂ are aligned in respect of the through-apertures but spaced apart as indicated by gap 16, the bolts 13 and 21 are inserted through the adjacent aperture pairs 19, and 19 ₂ respectively. The hydraulic nut 41 is assembled onto the shank of bolt 21 such that the bolt head abuts the flange 11, and the hydraulic nut 41 abuts the flange 11 ₂ .

The nut 15 is threaded onto the shank 22 of bolt 13 after which the hydraulic stressing means 50 is assembled, the actuator rod 51 being disposed in the bolt cavity 25 and the operating head 52 being threaded onto the end of the shank. The nut running means 60 is then assembled such that the motor 61 is in drive engagement with the nut via gears 62 and 63 and with the biasing ram 64 attached to the head of junction bolt 21 in order to bias the head of bolt 13 against flange 11,.

The motor 61 is connected to the control unit 71 to receive a suitable drive signal therefrom on line 73 and the operating head 52 and hydraulic nut 41 are connected hydraulically to the common output line 34 of the hydraulic pump 31.

The control unit 71 is provided with data that represents the desired residual tensile stress for the junction bolt, having regard to its interengagement with the nut 15. Having such regard the control unit stores, or is able to compute,

the initial or installation tensile stress required to be imparted to the junction bolt to permit the nut to take up a position at which the predetermined residual stress is achieved. This installation tensile stress is, of course, provided by the force exerted between the operating head 52 and the actuating rod 51 and is directly related to the pressure of fluid in the chamber 56. Thus the control unit 71 may, in practice, store this pressure value that gives such installation tensile stress for the particular bolt instead of, or in addition to, the initial and/or residual tensile stress value.

Figure 3 shows in flow diagram form a summary of the steps of the securing method as performed by, or under the control of unit 71.

The control unit 71 actuates the hydraulic pressure means 30 to deliver the fluid at the appropriate pressure on line 34 to the operating head 52 and hydraulic nut 41.

The fluid pressure acts within the hydraulic nut 41 to separate the two parts 42 and 44, abutment of the bolt head with flange 11, and nut part 44 with flange 11 ₂ forcing the flanges into abutment. Simultaneously, the fluid pressure acts within the operating head 52 to introduce the predetermined installation tensile stress related thereto between opposite ends of the junction bolt 13 and within the ram 64 of the nut running means to bias the head 23 of the junction bolt the flange 11,. After a few seconds to permit the hydraulic nut 41 to draw the flanges together and for the junction bolt 13 to extend under tensile stress, and with the pressure maintained, the control unit further energises the nut running means 60, causing the motor 61 to run the nut 15 along the shank into abutment with flange 11 ₂ . The load on the motor is sensed to effect removal of motor supply, or after a suitable interval the supply is simply removed from the stalled motor, following which the

hydraulic pressure is removed from the nut 41 and operating head 52, the tensile stress on the junction bolt being held by nut 15 at a residual level possibly slightly reduced from the installation value.

The flanges, at least at this point, are thus held in compression with what should be the predetermined force. The nut running means, operating head and hydraulic nut may then be removed, and if desired the operating head and nut running means may be assembled on the bolt 21, after a suitable nut (not shown) corresponding to nut 15, and that bolt secured in a similar manner, but without the hydraulic nut or with the hydraulic nut moved to a bolt extending through an adjacent aperture pair (not shown) .

If it is not assumed that the predetermined residual stress is achieved in bolt 13 with a sufficient degree of accuracy, then, preferably before removal of the hydraulic stressing means, nut running means and hydraulic nut, a further controlled sequence of operations may be performed by the control means in response to the determination of actual residual tension in the junction bolt 13 from signals produced by the transducer 72.

The control unit is required to store the predetermined or desired value of residual tensile stress, rather than merely the analogous fluid pressure that relates to the associated initial tensile stress as indicated above, and determines directly or indirectly from the signals of transducer 72 the actual value of residual tension, comparing this with the desired value to ascertain the value of any error. If the error is in excess of a small threshold that represents an acceptable tolerance, the control means computes, or looks up from a table, a nut rotation amount that will change the residual tensile stress in the bolt to eliminate the measured error. If the residual stress has to be increased the bolt will clearly be required to be subjected to an

additional tensile stress elongating it sufficiently for the nut to be moved further towards the head, whereas if the ^' residual stress has to be reduced the bolt will clearly require stressing only sufficient for the residual load to be taken from the nut for it to be moved away from the head. Accordingly, the sign and magnitude of the required nut rotation is used to compute, or look up, either directly or by way of this corrective tensile stress/bolt extension required, the minimum hydraulic pressure required by the operating head.

When all such values have been ascertained, the control unit operates the hydraulic pressure means such that the pump 31 delivers fluid to both the hydraulic nut 41 and the operating head 52 and achieves the determined pressure to achieve the determined corrective tensile stress. When the junction bolt is considered to have been extended by this corrective tensile stress, the control means operates the nut running means such that the ram 64 biases head 23 against the flange 11, and operates the motor 61 such that it rotates the nut 15 by the determined amount to reposition it. The control unit 71 then causes pressure to be released from the hydraulic nut and the hydraulic stressing device.

At this stage, if the single correction is considered adequate, or after again measuring the residual stress and repeating the above procedure until no errors exist that require elimination, the hydraulic stressing device, nut running means and hydraulic nut may be removed from the nuts 13 and 21 and the whole operation repeated on other bolts.

The above described initial installation sequence and further sequence that responds to actual rather than predetermined conditions, illustrate a method of securing flanged bodies in which any forces necessary to bring the flanges together are separated from the forces imposed upon a junction bolt, whose tensile stress is to be kept within

defined limits, and/or upon a nut, which is to be positioned with a repeatable accuracy on the bolt, by not having to overcome forces other than between the nut and the bolt shank, and with apparatus which, once assembled, is able to perform both initial installation to predetermined values and, if necessary, follow up automatically with a series of corrective iterations to achieve the predetermined values.

As indicated above in relation to Figure 1, it is to be expected that a flanged body 10 such as a turbine casing has a plurality of aperture pairs and that it is required in practice to secure the flanges at several discrete sites on the body simultaneously and preferably, to minimise the total assembly time, to secure a plurality of junction bolts at each of the sites simultaneously.

The securing arrangement of the present invention is well suited to such operation. Referring again to Figure 2 , the hydraulic pressure means 30 may include a fluid distribution manifold 36 in pump output line 34 able to provide equal fluid pressure to a plurality (as shown six) of hydraulic nut and operating head pairs. That is, the hydraulic nut means 40 comprises six hydraulic nuts corresponding to 41 and the hydraulic stressing means 50 comprise six hydraulic stressing devices each having an operating head 52 and actuator rod 51. The nut running means 60 thus comprises six motors 61 and bias rams 64. Similarly, the control means 70 may include transducer multiplexing means 74, able to receive signals from a plurality of transducers (72) carried by each of the junction bolts being simultaneously installed, so that the transducer signals can be received or sampled from each such bolt in turn and acted upon by the control unit 71, and a nut running de-multiplexing means 75 able to distribute the nut rotation signals, produced in repositioning the nuts of any bolts, to bolts associated with the transducer whose residual stress signal determined the degree of rotation. ^~~ '

In operation, and with a group of six installed junction bolts, the control unit may poll each stress transducer in turn to determine the nut position correction, if any, required for each of the six bolts and to compute the pressure necessary to achieve the largest of corrective tensile stress values required. The control means sets the hydraulic pressure means to deliver such pressure to all operating heads and adjacent hydraulic nuts by way of the manifold 36 and then in sequence initiates the determined repositioning of the nuts associated with the six bolts.

The precise number of junction bolts in any one group, or their disposition about the body flange, is a matter of choice, although it is to be expected that if very large compressive force are to be retained by the bolts then during initial stages of installation a plurality of junction bolts will be installed together at one site in a series of alternate aperture pairs and with the bolts carrying the hydraulic nuts in the intervening aperture pairs. Where it is practicable the aperture pairs associated with the whole of a body may be considered as a single group such that all of the aperture pairs contain bolts carrying either the junction bolt stressing means or hydraulic nuts and for all bolts to be stressed together.

It will be understood that different installation schemes may be employed in dependence upon the number and proximity of apertures and the magnitude and distribution of flange compression forces and the behaviour of the flanges. In particular, it may be that at one site for installing a series of junction bolts they are more widely spaced than alternate apertures or unevenly spaced. For example, although each junction bolt is considered to be installed in an adjacent aperture to a bolt with an hydraulic nut 41 thereon it will be understood that a pair of junction bolts may be installed with one such bolt between them and shared

between each as regards constituting two pairs of adjacent apertures, or vice versa.

In some circumstances the flange structure and forces necessary for effecting abutment of the flanges may make it possible, or even desirable, for the installation method to be performed with the aperture pair through which the simultaneously stressed junction bolt (13) and hydraulic- nut-carrying bolt (21) extend being neighbouring aperture pairs that are not adjacent that is, next to each other, but sufficiently close, having regard to the above-mentioned flange structure and behaviour, for the flange clamping of hydraulic nut to be as effective as with adjacent aperture pairs.

Where the flanged body parts are particularly far from abutment (by gap 16) and/or susceptible to deformation if clamped together in initial stages at a small number of well spaced sites, it may be desirable, and rendered possible by the apparatus of the invention, to consider a more uniform spread of flange closing forces in addition to that effected within the neighbourhood of each junction bolt that is installed; that is, to have in addition a plurality of extra bolts, such as 21, each carrying a hydraulic nut in order to clamp the flanges over a greater region of the flanges than adjacent to a junction bolt.

It will be appreciated that the control means 70 may include apparatus associated with the installation that is most conveniently controlled from a central processor. For instance the measured values of residual tensile stress and determined corrective values and intermediates in achieving correction, such as pressures, may be recorded for the respective bolts and presented visually at a display unit 76 and/or printer 77 and data input at keyboard 78. Furthermore the control means may adapt the stored predetermined values used for initial installation of groups of bolts, in

response to determined errors, to converge upon installation values used for subsequent groups that meet tolerance with fewer, or no, further corrective procedures.

The transducer 72 may comprise an ultrasonic device that produces a signal indicative of tensile stress in the junction bolt related to the length of the bolt between its ends, that is, any extension induced therein. Such transducer requires to be attached with proper acoustic coupling and in such a position that acoustic reflections are properly received. Such transducers may be used with, or form part of, a commercially available apparatus 80 for measuring tensile stress that incorporates an oscilloscope to assist in placing of the transducer. It may be convenient for such apparatus to be coupled to the control unit 71 in respect of the locating of each transducer in accordance with any multiplexing scheme and possibly to illustrate each measured tensile stress graphically in real time during installation and/or possibly to perform some of the calculation of each stress value from the transducer signals.

It will be appreciated that many of the individual items may take a form other than precisely as described, both as mentioned at appropriate points in the above description and within the purview of conventional techniques. It will be understood that the form of the pressure supply means, hydraulic nut means and, to some extent, the hydraulic stressing means may vary in form provided the above- described functional effects are achieved.

The nut running means 60 may include ram means 64 for disposing the head of the junction bolt adjacent the flange that is supported on other than the head of an adjacent bolt, irrespective of the nature of that bolt, such as a different part of the (flanged) body or even on the floor if appropriate.

Furthermore, the nut running means 60, in association with the control means may vary the above described manner in' which the nut position is defined in the initial installation of the junction bolt.

Although it is convenient in the initial installation to define the nut position as in abutment with the flange when a predetermined installation level of tensile stress exists in the bolt shank, and that this position is readily attainable, irrespective of the initial nut position, by simply running it into abutment, it will be appreciated that if the nut of each junction bolt is initially placed in a known position, say in abutment with the operating head or some marker on the shank, then the nut may be displaced a specific distance in accordance with any predetermined or measured tensile stress in the shank in the manner described above for corrective positioning of the nut.

The form taken by the hydraulic stressing means may vary with design of junction bolt. For example, the operating head may be adapted to be secured to, or outwardly of, a head or effective head of the junction bolt when the central cavity opens to the head end. In such circumstances it may be desired or appropriate to assemble a hydraulic stressing device on such a bolt prior to the bolt being inserted into the aperture pair and possibly to induce such installation tensile stress therein before inserting it through the aperture pair, enabling the bolt to be used with undersized apertures and form a radial interference fit when under residual tensile stress.

It will be understood that the method of, and apparatus for, securing body parts together as described hereinbefore is not restricted to use with closed bodies such as turbine casings but may also be employed in respect of other bodies such as pipes with flanged ends, wherein the facility for drawing together the flanges separately from accurately and

consistently setting residual stress in the junction bolts is exploited. Furthermore, it is not restricted to securing flanges of bodies but may be applied to any such mating body parts having aligned aperture pairs and secured in compression by such junction bolts.