GB1572152A

GB1572152A - Bolts

Info

Publication number: GB1572152A
Application number: GB3562675A
Authority: GB
Original assignee: Pilgrim Engineering Developments Ltd
Current assignee: Pilgrim Engineering Developments Ltd
Priority date: 1976-11-29
Filing date: 1976-11-29
Publication date: 1980-07-23
Also published as: FR2381198A1; DE2706301A1

Description

(54) BOLTS (71) We, PILGRIM ENGINEERING DE VELOPMENTS LIMITED, a British Company, of Navigation House, One Aldgate, London, E.C.3, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to improvements in bolts.

According to the present invention there is provided a compound bolt assembly comprising a hollow shank and a core member within the shank, the shank and core member being secured to each other at or adjacent a thirst end of the bolt assembly, releasable force transfer means at or adjacent the second end of the assembly for holding the core member in tension and the hollow shank in compression, the core member being capable of withstanding in compression a force equal to the normal tensile force generated in the shank in use and having at or adjacent the second end coupling means through which tensile and compressive forces may be applied to the core member.

Also according to the present invention there is provided a method of establishing a tensioned compound bolt connection between two spaced regions comprising the steps of pre-tensioning a hollow bolt shank by pre-compressing a core member in the shank, positioning the shank and core member assembly in the required position for the bolt connection without end play; releasing the pre-compression in the core member, whereby the pre-tension of the hollow shank is applied between the two regions, applying tension to the core member by exerting reaction against the shank, creating a play-free force transmitting path between the two ends of the tensioned core member and corresponding ends of the hollow shank, and releasing the tension applied extermally to the core member so that the tension in the core member is applied in the direction to compress the shank.

By making use of the invention, it is possible to establish a high tension bolt connection by using bolt tightening devices which do not project beyond the circumscribed cylinder around the head or load transmitting flange of the bolt connection.

This arrangement is particularly desirable with large highly-stressed closely-pitched bolts or studs used for example in securing the casings of large steam turbines which operate at high temperatures. The present method of tightening such bolts is by thermal means in which the bolt or stud is rapidly elongated by inserting an electrical heating element into a centrally drilled hole in the core of the bolt or stud.

When the bolt has increased its length by the required amount, the nut is secured down and measurements made through the centrally drilled hole indicate the true extension achieved in the bolt or stud and hence the true tension achieved in the bolt or stud.

However, this method is time-consuming and there is always a danger that the rapid heating from the core of the bolt or stud will cause cracking due to plastic deformation of the bolt material resulting from the very high thermal stress gradients which such heating causes. To achieve uniform tightening around a complete turbine casing, it is often necessary to repeat this time-consuming operation two or three times on each bolt or stud.

The present invention avoids the need for any thermal stressing of the bolts or studs but instead makes use of hydraulic stressing techniques (which have been proved by wide experience in other fields) while at the same time, avoiding the need for any equipment of greater diameter than the head of the bolt or stud which is to be tightened.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which the single figure shows one compound bolt together with tensioning apparatus therefor in axial section and the two adjacent bolts in elevation.

In the following description and in the claims, reference is made to "a compound bolt"; it will of course be understood that this term is used generically for the sake of simplicity and will incIude compound studs where appropriate.

The compound bolt shown in the drawing comprises a hollow tubular shank 1 which is both internally and externally screwthreaded at its first or lower end at 2 and 3 respectively and has an integral circular flange 4 which forms the base of an externally screw-threaded head portion 5 for the bolt.

A core member in the form of a solid rod 6 (which may however have a central narrow axial bore) has its lower end screwthreaded and in tight engagement with the threads 2 at the lower end of the hollow shank 1. The rod 6 is somewhat longer than the shank 1 so that it projects above the top of the head 5 and the upper end portion of the rod above the flange 4 is screwthreaded at 8 and has engaged thereon a cylindrical nut 9 the upper part of which projects above the head 5 and is formed with blind castellations 10 while the lower part of the nut 9 is received in a counterbore 11 in the shank head 5.

The tensioning apparatus or jacking head for tensioning the compound bolt comprises a generally cylindrical housing 21 the lower end of which is internally screwthreaded to engage with the external threads on the shank head 5. The external diameter of the housing 21 is equal to the diameter of the shank head flange 4.

An internal flange 22 extends inwards from the housing 21 to make sliding contact with a cylindrical surface 23 on a combined piston and nut 24 which has internal screwthreads engaged on the upper part of the threads 8 on the rod 6 and has its external cylindrical surface 25 in sliding contact with the inner wall surface of the housing 21 above the flange 22. There is thus defined an annular chamber 26 containing a tyre or load-cell 27 made of nitrile rubber which has a connection 28 for receiving a high pressure hydraulic supply.

The top surface of the radially inner part of the piston nut 24 is substantially flush with the top surface of the rod 6 and the two form a seating for a diaphragm 29 which may be formed by a stack of thin discs of nitrile rubber. An end cap 30 has external screw-threads engaged in internal screw-threads in the upper part of the housing 21 and has a central connector 32 for delivering high pressure hydraulic fluid to the upper side of the diaphragm 29.

At the level of the blind castellations or notches 10, the housing 21 is formed with horizontal arcuate slots 33 through which a suitable tommy-bar or the like can be inserted to engage the notches 10 for mani pulation of the nut 9.

Installation and fitting of each compound bolt are as follows: Before installation in the work W, the compound bolt is pre-assembled and pretensioned in the workshop. First, the rod 6 is screwed firmly home by means of its screw-threads into the screw-threads 2 in the lower part of the shank 1. The nut 9 is then engaged on the screw-threads 8 on the upper part of the rod 6.

The components of the tensioning head within the housing 21 are then assembled into the head 5 of the shank 1 and onto the upper end of the threaded portion 8 of the rod 6 as shown in the drawing. In particular, the upper surface of the piston nut 24 is flush with the upper end of the rod 6 and then the nut 9 is screwed up to abut the underside of the piston nut 24. With the end cap 30 tightened down to trap the outer margin of the diaphragm 29, hydraulic fluid under a pressure for example 35,000 p.s.i. can be introduced through the connection 32 to compress the rod 6 and simultaneously elongate the hollow shank 1. By closing an isolating valve (not shown) upstream of the connector 32, this pre-stressing of the rod and shank may be maintained.

Alternatively, the nut 9 may be screwed further up into contact with the underside of the flange 22 which for this purpose would have to be of thicker construction than shown in the drawing.

A micrometer reading of the state of elongation of the shank 1 may be taken through a suitable aperture (not shown) or the change in length in the rod may be recorded by means of a thin rod inserted along the length of the hollow axial bore if the latter is provided.

The pre-tensioned bolt can now he screwed into the work W which has a suitable internally threaded bore to receive the screw-threads 3. The shank is tightened by hand and by applying a hammer and chisel to appropriate notches 36 in the periphery of the flange 4 to take up any end play. The micrometer reading is then repeated to ensure that there has been no loss of pretension. If there has been a loss, for example, due to leakage, the diaphragm 29 may have to be replaced, but the compound bolt may be again pre-tensioned on site by using a small hand pump to achieve the same micrometer depth gauge reading as was previously obtained with the predetermined working pressure of about 35,000 p.s.i.

If, as in the case of the drawing, a ring of bolts is being installed, it is desirable that the final tensioning operation on all the bolts should take place simultaneously. For this purpose, all connections 32 of individual jacking or tensioning heads for each of the bolts are connected together as are the connections 28. Then, the isolating valve controlling all the connectors 32 is opened simultaneously releasing the pressure as a result of which the shanks 1 contract in length thereby applying a uniform load to the work W. The micrometer depth gauge readings for each bolt should then be taken again and recorded after checking that they are within prescribed limits.

The isolator valve connections are then removed from the connectors 32 and applied to the connectors 28. The high working pressure of about 35,000 p.s.i. is then applied to pressurise each of the annular pressure cells or tyres 27 thereby stretching each of the rods 6. Micrometer depth gauge readings are again taken and recorded as a check on the tension in each of the rods 6. With the hydraulic pressure maintained at the working pressure, each nut is screwed down on the threads 8 into contact with the bottom of the counterbore 11 by hand, using a suitable tommy-bar engaged through the slots 33 into the notches 10, thereby establishing a play-free force transmitting path between corresponding ends of the core member and shank.

The hydraulic pressure has now applied the tensile load of the rod to the head 5 and flange 4 of the bolt shank. By releasing the pressure in the load cells 27, the load which was transmitted by the load cells is now transmitted through the nuts 9 again to the heads 5 and flanges 4 of the bolt shanks. As a final check, the micrometer readings (for example through the access slots 33) are taken to ensure that the nut 9 has been correctly nipped down onto the bottom of the counterbore 11.

The tensioning heads may now be removed by disconnecting the hydraulic pipe connections from the connections 28, removing the top covers 30 by unscrewing them, unscrewing the piston nuts 24 and then finally unscrewing the outer casing 21 from the bolt head 5.

Removal of a bolt is effected by reassembling the tensioning head onto the head 5 of the bolt in question applying the high loading fluid pressure to the load cell or tyre 27, unscrewing the nut 9 by hand by means of a tommy-bar passed through the slots 33 until the nut 9 touches the underside of the piston nut 24, applying the high fluid pressure through the connector 32 and thereafter closing the isolator valve, using the hammer and chisel to slacken off the bolt or bolts by means of the notches 36, completely unscrewing the bolt or bolts in question and removing the bolt and tensioning head assembly for further work as required.

As can be clearly seen in the drawing, the housings 21 of the tensioning heads do not come into contact with the work W and are situated within the imaginary oircumscribing cylinder of the flanges 4. As a result a nominal clearance only is required between adjacent flanges 4 and tensioning head housings 21.

WHAT WE CLAIM IS: 1. A compound bolt assembly comprising a hollow shank and a core member within the shank, the shank and core memmember being secured to each other at or adjacent a first end of the bolt assembly, releasable force transfer means at or adjacent the second end of the assembly for holding the core member in tension and the hollow shank in compression, the core member being capable of withstanding in compression a force equal to the normal tensile force generated in the shank in use, and having at or adjacent the second end, coupling means through which tensile and compressive forces may be applied to the core member.

2. A compound bolt assembly according to claim 1, wherein the releasable transfer means comprises a nut in screw-threaded engagement with the core member, the nut being rotatable into abutting contact with a shoulder on the shank.

3. A compound bolt assembly according to claim 2, wherein the nut is in screwthreaded engagement with a threaded portion of the core member and the threaded portion projects beyond the nut, when the latter is in contact with the shoulder on the shank.

by a sufficient amount to form the coupling means through which tensile force may be applied to the core member.

4. A compound bolt assembly according to claim 2 or 3, wherein the nut projects beyond the end of the hollow shank for engagement by an adjusting tool, 5. A compound bolt assembly according to any of the preceding claims wherein the shank has an externally screw-threaded surface at or adjacent the second end for engagement by a tensioning head.

6. A compound bolt assembly according to claim 5, wherein the second end of the shank has a flange for engaging a workpiece and the screw-threaded surface of the shank lies beyond the flange and is of smaller diameter than the flange.

7. A compound bolt assembly according to any of the preceding claims, wherein the shank is pre-tensioned and the core precompressed by a force transmitted through the transfer means.

8. A compound bolt assembly substan- tially as hereinbefore described with reference to the accompanying drawings.

9. Apparatus comprising a bolt assembly according to any of the preceding claims and a jack conneotable to coupling means on the shank and adapted to apply a force to the end face of the core member.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. controlling all the connectors 32 is opened simultaneously releasing the pressure as a result of which the shanks 1 contract in length thereby applying a uniform load to the work W. The micrometer depth gauge readings for each bolt should then be taken again and recorded after checking that they are within prescribed limits. The isolator valve connections are then removed from the connectors 32 and applied to the connectors 28. The high working pressure of about 35,000 p.s.i. is then applied to pressurise each of the annular pressure cells or tyres 27 thereby stretching each of the rods 6. Micrometer depth gauge readings are again taken and recorded as a check on the tension in each of the rods 6. With the hydraulic pressure maintained at the working pressure, each nut is screwed down on the threads 8 into contact with the bottom of the counterbore 11 by hand, using a suitable tommy-bar engaged through the slots 33 into the notches 10, thereby establishing a play-free force transmitting path between corresponding ends of the core member and shank. The hydraulic pressure has now applied the tensile load of the rod to the head 5 and flange 4 of the bolt shank. By releasing the pressure in the load cells 27, the load which was transmitted by the load cells is now transmitted through the nuts 9 again to the heads 5 and flanges 4 of the bolt shanks. As a final check, the micrometer readings (for example through the access slots 33) are taken to ensure that the nut 9 has been correctly nipped down onto the bottom of the counterbore 11. The tensioning heads may now be removed by disconnecting the hydraulic pipe connections from the connections 28, removing the top covers 30 by unscrewing them, unscrewing the piston nuts 24 and then finally unscrewing the outer casing 21 from the bolt head 5. Removal of a bolt is effected by reassembling the tensioning head onto the head 5 of the bolt in question applying the high loading fluid pressure to the load cell or tyre 27, unscrewing the nut 9 by hand by means of a tommy-bar passed through the slots 33 until the nut 9 touches the underside of the piston nut 24, applying the high fluid pressure through the connector 32 and thereafter closing the isolator valve, using the hammer and chisel to slacken off the bolt or bolts by means of the notches 36, completely unscrewing the bolt or bolts in question and removing the bolt and tensioning head assembly for further work as required. As can be clearly seen in the drawing, the housings 21 of the tensioning heads do not come into contact with the work W and are situated within the imaginary oircumscribing cylinder of the flanges 4. As a result a nominal clearance only is required between adjacent flanges 4 and tensioning head housings 21. WHAT WE CLAIM IS:

1. A compound bolt assembly comprising a hollow shank and a core member within the shank, the shank and core memmember being secured to each other at or adjacent a first end of the bolt assembly, releasable force transfer means at or adjacent the second end of the assembly for holding the core member in tension and the hollow shank in compression, the core member being capable of withstanding in compression a force equal to the normal tensile force generated in the shank in use, and having at or adjacent the second end, coupling means through which tensile and compressive forces may be applied to the core member.

4. A compound bolt assembly according to claim 2 or 3, wherein the nut projects beyond the end of the hollow shank for engagement by an adjusting tool,

5. A compound bolt assembly according to any of the preceding claims wherein the shank has an externally screw-threaded surface at or adjacent the second end for engagement by a tensioning head.

10. Apparatus according to claim 9,

wherein the jack is a hydraulic jack having a diaphragm adapted to apply fluid-pressure generated forces to the end surface of the core member.

11. Apparatus according to claim 9 or 10 including a compound bolt assembly according to claim 6, wherein the jack has an internally screw-threaded sleeve engageable on the coupling means of the shank and the external diameter of the jack sleeve is equal to that of the flange on the shank.

12. Apparatus according to any of claims 9 to 11, wherein the body of the jack is apertured to provide access for adjusting the position of the force transfer means.

13. Apparatus according to any of claims 9 to 12, wherein the body of the jack has an internal abutment surface for engagement by the force transfer means to retain the pre-compression of the core member when the jacking forces are released.

14. Apparatus comprising a compound bolt assembly according to any of claims 1 to 9 and core jacking means engageable with the coupling means on the core and adapted to bear against the hollow shank.

15. Apparatus according to claim 14, wherein the core jacking means comprises a cylindrical housing having an internal flange, a piston having an external flange slidably received in the housing and defining in the housing a closed annular chamber, the piston being engageable with the coupling means on the core, and a toroidal flexible wall tube in the annular chamber and connected to a source of fluid pressure.

16. Apparatus according to claims 10 and 15, wherein the outer surface of the piston is adapted to form a radial extension of the end surface of the core, and the diaphragm has its outer margin clamped between the said outer surface of the piston and an end cap screwed into the said housing and formed with an inlet for fluid under pressure.

17. A method of establishing a tensioned compound bolt connection between two spaced regions comprising the steps of pretensioning a hollow bolt shank by pre-compressing a core member in the shank, positioning the shank and core member assembly in the required position for the bolt connection without end play; releasing the pre-compression in the core member, whereby the pre-tension of the hollow shank is applied between the two regions, applying tension to the core member by exerting reaction against the shank, creating a play-free force transmitting path between the two ends of the tensioned core member and corresponding ends of the hollow shank, and releasing the tension applied externally to the core member so that the tension in the core member is applied in the direction to compress the shank.

18. A method according to claim 18, wherein a compound bolt according to any of claims 1 to 9 is tensioned while forming part apparatus according to any of claims 10 to 16.

19. A method of establishing a compound bolt connection, substantially as hereinbefore described with reference to the accompanying drawings.

20. A compound bolt connection produced by a method according to any of claims 17 to 19.