GB2236372A

GB2236372A - Tensioning method and device

Info

Publication number: GB2236372A
Application number: GB9020267A
Authority: GB
Inventors: Roger Joseph Ensor; Paul Raymond Egginton
Original assignee: SPS Technologies Ltd
Current assignee: SPS Technologies Ltd
Priority date: 1989-09-15
Filing date: 1990-09-17
Publication date: 1991-04-03
Anticipated expiration: 2010-09-17
Also published as: GB2236372B; GB9020267D0

Abstract

When the hydraulic unit 14 is supplied with pressure fluid to produce a first tensile stress in the fastener 10, an abutment washer 29 or head 29 of an elongate element E are adjusted into engagement with the fastener 10 so as to give an increased degree of frictional restraint to rotation of the washer 29 or to the head 27 of the element E, thus giving an indication that the first tensile stress has been attained. The nut 14 is then supplied with further fluid to produce a second tensile stress in the fastener 10, which is maintained by shims between the nut 14 and the workpiece 16, or by scewing part of the nut 14 into engagement with the workpiece 16. Loss of tension is also indicated. A torque-limiting clutch may be provided on the head 27 of the element E. The abutment may move transversely to the element E. <IMAGE>

Description

Title: "Tensioning method and device" Description of Invention This invention relates to a method of and a device for ascertaining that a member has been tensioned to a predetermined extent. The member may be a fastener having a screw thread such as a bolt, screw or stud.

EPO-049537 discloses a fastener having an elongate element extending lengthwise of the fastener and having abutment means wherein, when the fastener is unstressed, the abutment means is free to turn relative to the fastener and, when the fastener is subjected to a predetermined tensile stress, the fastener is elongated to engage the abutment means with a part of the fastener to thereby impede turning of the abutment means relative to the fastener.

Such a fastener suffers from the disadvantage that the extent of tightening which is indicated by the device varies in accordance with service conditions and hence the device cannot be relied upon to indicate a predetermined tensile stress obtaining in the fastener.

An object of the invention is to overcome or to reduce the above mentioned problem.

According to one aspect of the invention we provide a method of ascertaining that a member is tensioned to a predetermined degree comprising taking a second member having an elongate element extending lengthwise of the member and having abutment means wherein, when the member is unstressed, the abutment means is free to move relative thereto and, when the member is subjected to a tensile stress, the member is elongated to engage the abutment means with a part thereof to thereby impede movement of the abutment means relative to the member, characterized in that said member is tensioned to a first predetermined extent and then said abutment means is adjusted so that said abutment means is engaged with said part to impede movement of the abutment means relative to the member to a predetermined degree and thereafter the member is tensioned to a second, greater, predetermined extent.

The member may be provided with a male screw thread with which a nut is engaged and said tensile stress being applied by said nut.

The nut may comprise a hydraulic nut and sensing means may be provided to sense the pressure of the fluid in the nut, said sensing means enabling determination of said first and second predetermined extents.

Said adjustment of said elongate element and abutment means may comprise causing relative movement between said abutment means and said part of the member in the longitudinal direction of said elongate element.

According to another aspect of the present invention we provide a device for ascertaining that a member has been tensioned to a predetermined extent comprising a second member having an elongate element extending lengthwise of the member and having abutment means wherein, when the member is unstressed, the abutment means is free to move relative thereto and, when the member is subjected to tensile stress, the member is elongated to engage the abutment means with a part thereof to thereby impede movement of the abutment means relative to the member and wherein adjustment means are provided to permit adjustment of the position of the abutment means relative to said part of the member and means are provided to maintain the abutment means in a thus selected position.

Said adjustment means may comprise means to permit of the abutment means being adjusted longitudinally of the elongate element or, alternatively, means to permit of the elongate element being adjusted lengthwise of the member or, alternatively, means to permit of-the length of the elongate element being adjusted.

The adjustment means may comprise a screw threaded connection between the abutment means and the elongate element.

Alternatively, the adjustment means may comprise mutually engaging retaining means between the elongate element and the member at a position remote from said abutment means and said retaining means permitting of adjustment of the elongate element lengthwise of the member.

The retaining means may comprise inter-engageable screw threads.

Alternatively, the adjustment means may comprise a screw threaded connection between first and second parts of the elongate element intermediate the ends thereof.

The member may comprise a fastener having a male thread with which a nut is engaged to apply said tensile stress to the fastener.

The nut may be a hydraulic nut.

In this case a pressure indicating means may be included in a circuit to the nut to sense the tensile stress in the member.

Said abutment means may comprise an abutment member formed separately from the remainder of the elongate element and carried thereby so as to be movable relative thereto in a direction transverse to said longitudinal axis whilst being prevented from movement away from said part of the member.

The abutment member may be disposed between the member and a head on the elongate element.

Alternatively the abutment member may be fixed to the remainder of the elongate element.

Said screw threaded connections of the adjustment means may be provided with a prevailing torque configuration to resist relative rotation between the inter-engaged screw threaded members.

Preferably, when the abutment means is in the thus selected position, it is impeded by friction from further movement relative to the member.

A degree of frictional restraint may be detected by a part of the second member which comprises a torque limiting clutch.

The torque limiting clutch may comprise a pair of clutch members having inter-engaging torque transmitting teeth which are disengageable from one another when the torque exceeds a predetermined maximum value.

In a preferred arrangement the teeth on at least one of the clutch members may comprise a deformable material such as plastics material, rubber or the like.

An embodiment of the invention will now be described with reference to the accompanying drawings wherein: FIGURE 1 shows a nut and bolt combination embodying the invention; FIGURE 2 shows part of an alternative embodiment of the invention, and FIGURE 3 is a diagrammatic transverse section through a head of an elongate element which head includes a torque limiting clutch arrangement.

Referring to the Figures, a bolt is indicated generally at 10 and comprises a shank 11 having a male screw thread 12 at one end and a head 13 at the other end. Between the thread 12 and head 13 the bolt is unthreaded in conventional manner. Of course, if desired, the invention may be applied to a screw where the male thread extends over the whole length of the shank to the underside of the head 13 or to a stud, i.e.

a wholly or partly threaded shank without a head.

A hydraulic nut 14 is threadedly engaged with thread 12 and is of conventional type having an annular piston part 15 which is urged into engagement with a workpiece 16 to be clamped between the nut 14 and the head 13 by hydraulic fluid, fed to a cylinder 17 through a circuit 18 incorporating a pressure gauge 19, from a source of hydraulic pressure 20, all in conventional manner.

The bolt 10 is provided with a blind bore 21 which extends in a lengthwise direction X-X of the member, i.e. the direction in which the member is elongated when subjected to a tensile stress. The blind bore 21 is provided, at an inner end part 22, with a female screw thread 23.

An elongate element E comprising a generally cylindrical rod is disposed within the bore 21 as a clearance fit and has an inner end part 24 provided with a male thread 25 which is threadedly engaged with the thread 23.

The inter-engaged threads 23, 25 are provided with a prevailing torque configuration, for example a configuration of the type known as "ESLOK".

One end of the elongate element E extends from the free end 26 of the shank 11 and has a head 27 formed integrally therewith. Mounted on the elongate element E between the head 27 and a free end surface 28 of the shank 12 is a washer 29. If desired the head 27 may be formed separately from the remainder of the elongate element E and fixed thereto, for example by being swaged or welded thereto.

In use, a bolt embodying the present invention is supplied to a user, or is adjusted by the user, so that there is a gap between the end surface 28 of the shank 11 and the underside 30 of the head 27 sufficiently great so that the washer 29 can turn freely about the longitudinal axis X-X of the elongate element E.

The nut 14 is then tightened into engagement with the workpiece 16 and the nut is pressurised by the hydraulic fluid through the circuit 18 in conventional manner until the gauge 19 shows that the bolt has been subjected to a tensile stress of a first predetermined extent. This can be determined either by use of a table or other means which indicates that a given pressure indicated by the gauge 19 corresponds to a predetermined tensile stress or by calibrating the gauge 19 directly to provide an indication of stress. At this stage, although due to stressing the shank 11 will have elongated, the washer 29 is still free to rotate since the gap between the surfaces 28 and 30 is originally set so as to accommodate this extension.Then, the elongate member E is rotated to move it inwards in the lengthwise direction X-X of the shank 11 until the under-surface 30 urges the washer 29 against the end surface 28 with sufficient force for a frictional resistance to rotation of the washer 29 to be created of a magnitude which is easily detected by a user. For example, the degree of frictional restraint to rotation may be such that the washer 29 is impeded from movement under a torque applied by the fingers of a user. The elongate element is adjusted so that the restraint to rotation gradually increases from zero up to the above described position where rotation is just not possible under the torque applied by the fingers of the user.Of course, the precise torque will vary from user to user but the range of torques which can be applied in practice by typical users are sufficiently similar for this variation not to affect the desired accuracy of the device.

If desired, other means may be provided to detect a degree of frictional restraint to rotation, for example by providing the head 27 with a torque limiting clutch such as a pair of clutch members having inter-engaging torque transmitting teeth which can be disengaged to permit of relative movement between the clutch members when the torque exceeds a predetermined value. Such an arrangement is shown in Figure 3 in which a transverse section through a head 40 is diagrammatically represented, which head comprises an external part 41 which may be turned by a user of the device and an internal part 42 which is rigidly secured to the cylindrical rod of an elongate element E (not shown).

The internal and external parts 41, 42 inter-engage by means of teeth 43, 44 respectively such that a torque applied to the external part 41 is transmitted to the internal part 42 and thus to the cylindrical rod of the elongate element E.

However, the internal and external parts 41, 42, comprise plastics material, rubber or the like, that is to say, materials which are capable of deformation, such that if a frictional resistance to turning is encountered which resistance exceeds a certain limit, the teeth 43, 44 will not properly engage. For example the teeth 43, 44 may distort such that they slip relative to one another.

Of course if desired, the teeth of one only of the parts 41, 42 may comprise a deformable material, the teeth of the other of the parts 41, 42 comprising, say, metal.

Alternatively, a conventional torque wrench may be engaged with the head 27 to tighten the elongate element E.

Thereafter the nut is further pressurised to further stress the bolt to a second predetermined extent.

The load is then maintained in the bolt in conventional manner, for example by suitable mechanical means such as by inserting semi-circular shims between the underside of the nut 14, outwardly of the piston part 15 and the workpiece 16, or by providing the nut 14 with a collar threadedly engaged with the remainder of the nut and capable of being screwed into engagement with the workpiece 16.

If, in use, the connection afforded by the fastener loosens so that the tensile stress in the shank 11 falls to just below the first predetermined stress, then the washer 29 will become free to rotate thus indicating that the stress has fallen to below an acceptable level.

In the specific example illustrated the first predetermined extent may correspond to a pressure of 25,000 psi and the second predetermined extent may correspond to a pressure of 33,000 psi.

Of course, although in the above described example the extent to which rotation of the washer 29 is impeded is described in terms of being in a situation where the washer cannot be turned by a user, if desired other degrees of restraint may be preferred, for example a situation where, although the washer 29 can be rotated, it is clearly apparent to a user that a certain amount of restraint applies and the washer is not completely free of restraint. In practice, the difference in stress occurring between extremes where the washer is subject to the smallest possible restraint detectable and where rotation is impossible by a particular user is sufficiently small not to create any unacceptable inaccuracies.

Although in the above described example the washer 29 comprises an abutment member which in the unstressed condition is free to rotate, if desired, an abutment member may be provided which is fixed to rotate with the elongate element. In this case the determination of the first degree of restraint would be selected so as to be greater than the resistance to rotation afforded by the prevailing torque thread configuration.

In the above described example the adjustment of the position of the abutment means has been by virtue of adjusting the position of the whole elongate element lengthwise of the bolt 10. If desired, in an alternative embodiment the inner end of the elongate element E may be fixed at the inner end of the bore 21 and the head 27 may be made adjustable lengthwise of the remainder of the elongate element, for example by providing a screw threaded connection therebetween which could again be provided with a prevailing torque configuration. In this embodiment a separate washer may again be provided or not provided as desired. In a further alternative the length of the elongate element itself may be made adjustable, for example by making the elongate element rod in two parts which are threadedly inter-engaged and by rotating the parts relative to one another the length of the elongate element can be adjusted. In this case the head 27 may be fixed relative to the shank and at its inner end the shank may be fixed within the bore 21 by any suitable means, for example by adhesive or by a press fit. Again, a washer free to rotate about the head 27 may be provided if desired or rotation between the two parts of the rod may be relied upon.

Figure 2 shows part of an embodiment of the device which employs an abutment member which is fixed to rotate with the elongate element.

In Figure 2 part of a bolt 50 is shown which provides a lengthwise blind bore 51 including a first narrow part 52, a second wider part 53 and a third still wider part 54 which constitutes a recess in the end of the bolt 50. The parts 52, 53, 54 communicate with one another and provide first and second shoulders 55, 56 respectively therebetween.

The elongate element E includes a screw threaded end portion 57 which engages a female thread in a base of the narrow bore part 52, and a smooth shank portion 58 which connects the threaded end portion 57 with an abutment member in the form of a collar 59. The collar 59 is formed integrally with the remainder of the elongate element E and cannot move relative to the remainder of the elongate element E.

The collar 59 connects with a head 60 of the elongate element E which, if desired, on an external surface thereof may provide knurlings to permit of grasping by a user of the device.

In use, a hydraulic nut is threaded onto the bolt and pressurised to stress the bolt to a first predetermined extent at which the collar 59 of the elongate element does not abut the first shoulder 55 of the bore. The head 60 of the elongate element E is then turned by a user of the device so as to bring the collar 59 into abutment with the first shoulder 55 so that a frictional restraint is applied to further turning of the head 60.

Following this procedure the nut is further pressurised so as to impart to the bolt 50 a stress of a second predetermined extent.

The load on the bolt may then be maintained by suitable means such as described earlier in this specification.

A retaining plate 61 is provided in the but still wider bore part 54 and assists in retaining the elongate element in the bore 51.

The head 60 includes an annular recess 62 in which is an 0-ring 63 which acts as a seal between the retaining plate 61 and the head 60 to exclude dust from the bore 51.

A further dust excluding means 64 is provided in the form of a removable protective cover which releasably engages in a recess in the retaining plate 61. The cover is removable to permit of access to the head 60 for initial adjustment when the device is first used to establish a stress in a bolt and thereafter for periodic checking of the device.

A drop in the tensile stress in the shank of the bolt 50 to just below the first predetermined stress is evidenced by the head 60 of the elongate element becoming turnable once again.

If desired of course, the protective cover 64 and/or the retaining plate 61 and 0-ring 63 could be omitted from this embodiment of the device.

Although in the above described example the device has been described when used in connection with a hydraulic nut, the invention may be applied in other applications so long as means are provided whereby an initial tensioning of the bolt to a predetermined extent can be determined.

In an alternative embodiment, not illustrated, instead of the member with which the elongate element is associated comprising a bolt with a blind bore, it may comprise a bolt with a longitudinally extending through bore or a stud with a longitudinally extending through bore. In this case the elongate member extends throughout the through bore and projects outwardly of the bore at each end and is provided with an abutment means at each end for engagement with opposed surfaces of the bolt or stud.Means are provided to adjust the position of at least one of the abutment means relative to the associated surface and, for example, the elongate member may have a fixed head at one end and a nut threadedly engaged therewith at the other end so that the separation between the nut and head can be adjusted and thus permit of adjustment of the restraint to rotation at a first degree of stressing, as described hereinbefore. Again, a freely rotatable washer may be provided at one or each end or the abutment means may be arranged to rotate with the remainder of the elongate member. In a modification of the alternative embodiment the elongate member may be provided with a fixed head at each end and be of adjustable length itself, for example by virtue of being provided with two threadedly inter-engaged parts, as described hereinbefore.A freely rotable washer may be provided at least at one end, if desired.

In a further modification the abutment means may be adjustable lengthwise of the elongate member at one or each end, for example by virtue of comprising two nuts, one at each end threadedly engaged with opposite ends of the elongate member. A freely rotatable washer may be provided at least at one end if desired.

Although in the above described embodiments the member in which the tensile stress is to be ascertained has comprised a bolt, screw or stud, it may comprise any other member in which it is desired to ascertain that a predetermined stress has not been diminished and, for example, the member may comprise a sleeve provided with an internal thread which is engaged by another member having a male thread to apply the tensile stress to the sleeve member and in this case the elongate element or a plurality of elongate elements may be provided in the wall of the sleeve.

Although the abutment means described hereinbefore are adapted for rotary movement relative to the member, if desired they may be adapted for sliding movement in a direction having a component transverse to the member.

For example by providing a washer with an elongate slot which permits of transverse sliding movement, or by providing a member adapted to rotate about an axis transverse to the lengthwise direction of the member and having a circumferential surface which is adapted to engage said part of the member.

The features disclosed in the foregoing description, or the accompanying drawing, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, or a class or group of substances or compositions, as appropriate, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS:

1. A method of ascertaining that a member is tensioned to a predetermined degree comprising taking a second member having an elongate element extending lengthwise of the member and having abutment means wherein, when the member is unstressed, the abutment means is free to move relative thereto and, when the member is subjected to a tensile stress, the member is elongated to engage the abutment means with a part thereof to thereby impede movement of the abutment means relative to the member, characterized in that said member is tensioned to a first predetermined extent and then said abutment means is adjusted so that said abutment means is engaged with said part to impede movement of the abutment means relative to the member to a predetermined degree and thereafter the member is tensioned to a second, greater, predetermined extent.

2. A method according to Claim 1 in which the member is provided with a male screw thread with which a nut is engaged and said tensile stress is applied by said nut.

3. A method according to Claim 2 in which the nut comprises a hydraulic nut and sensing means are provided to sense the pressure of the fluid in the nut, said sensing means enabling determination of said first and second predetermined extents.

4. A method according to any one of Claims 1 to 3 in which said adjustment of said elongate member and abutment means comprises causing relative movement between said abutment means and said part of the member in the longitudinal direction of said elongate element.

5. A method substantially as described hereinbefore and with reference to the accompanying drawings.

6. A device for ascertaining that a member has been tensioned to a predetermined extent comprising a second member having an elongate element extending lengthwise of the member and having abutment means wherein, when the member is unstressed, the abutment means is free to move relative thereto and, when the member is subjected to tensile stress, the member is elongated to engage the abutment means with a part thereof to thereby impede movement of the abutment means relative to the member and wherein adjustment means are provided to permit adjustment of the position of the abutment means relative to said part of the member and means are provided to maintain the abutment means in a thus selected position.

7. A device according to Claim 6 in which said adjustment means comprises means to permit of the abutment means being adjusted longitudinally of the elongate element or, alternatively, means to permit of the elongate element being adjusted lengthwise of the member or, alternatively, means to permit of the length of the elongate element being adjusted.

8. A device according to Claim 6 or Claim 7 in which the adjustment means comprises a screw threaded connection between the abutment means and the elongate element.

9. A device according to Claim 6 or Claim 7 in which the adjustment means comprises mutually engaging retaining means between the elongate element and the member at a position remote from said abutment means and said retaining means permitting of adjustment of the elongate element lengthwise of the member.

10. A device according to Claim 9 in which the retaining means comprises inter-engageable screw threads.

11. A device according to Claim 6 or Claim 7 in which the adjustment means comprises a screw threaded connection between first and second parts of the elongate element intermediate the ends thereof.

12. A device according to any one of Claims 6 to 11 in which the member comprises a fastener having a male thread with which a nut is engaged to apply said tensile stress to the fastener.

13. A device according to Claim 12 in which the nut is a hydraulic nut.

14. A device according to Claim 13 in which a pressure indicating means is included in a circuit to the nut to sense the tensile stress in the member.

15. A device according to any one of Claims 6 to 14 in which said abutment means comprises an abutment member formed separately from the remainder of the elongate element and carried thereby so as to be movable relative thereto in a direction transverse to said longitudinal axis whilst being prevented from movement away from said part of the member.

16. A device according to any one of Claims 6 to 15 in which the abutment member is disposed between the member and a head on the elongate element.

17. A device according to any one of Claims 6 to 15 in which the abutment member is fixed to the remainder of the elongate element.

18. A device according to any one of Claims 6 to 17 in which said screw threaded connections of the adjustment means are provided with a prevailing torque configuration to resist relative rotation between the inter-engaged screw threaded members.

19. A device according to any one, of Claims 6 to 18 in which the abutments means, when in the thus selected position, is impeded by friction from further movement thereof relative to the member.

20. A device according to Claim 19 in which a degree of frictional restrainst is detected by a part of the second member which comprises a torque limiting clutch.

21. A device according to Claim 20 in which the torque limiting clutch comprises a pair of clutch members having inter-engaging torque transmitting teeth which are disengageable frdm one another when the torque exceeds a predetermined maximum value.

22. A device according to Claim 21 in which the teeth on at least one of the clutch members comprise a deformable material such as plastics material, rubber or the like.

23. A device substantially as described hereinbefore with reference to the accompanying drawings.

24. Any novel feature or combination of features disclosed herein.