GB2267944A - Hydraulic nut and bolt (or stud) assembly - Google Patents

Abstract

In order to reduce the overall diameter of a hydraulic nut 60, 60', the nut body 61, 61' has its inner wall 65 reduced in thickness so as to be elastically deflected by pressure in the annular chamber 71, 71' inwards into supported engagement with an unthreaded portion 58 of a bolt (or stud) 53. When a desired tension is achieved in the bolt (or stud) 53, a locking ring 67 is screwed down the nut body 61, 61' into abutment with the load ring 62, 62' (or with the flange 12). Where the locking ring 67 is formed as a cap 87, a fluid duct is formed through the load ring 62, 62'. <IMAGE>

Description

Hydraulic nut and bolt fastening arrangement This invention relates to a nut and bolt fastening arrangement and in particular to an arrangement of the type in which a precise compressive force is imposed on bodies held between the nut and bolt by pretensioning the bolt by means of a hydraulic load cell incorporated into the nut and maintaining a residual tensile stress by juxtaposition of physically retained parts of the nut.

Nuts of this general type are known as hydraulic nuts, being used on conventional threaded bolts in place of conventional nuts where conditions of size, space loading and accuracy preclude the use of such conventional nuts.

A typical nut and bolt fastening arrangement employing such a hydraulic nut known per se is shown in sectional elevation in Figure 1.

Two bodies, such as flanges 11 and 12, are held in compression by a bolt 13 passing through aligned aperture pair 14 and having a head 15 abutting one flange 11. The shank of 16 of the bolt has at its other end a threaded end portion 17.

A hydraulic nut 18 has a first annular component 19, the nut body, having an inner annular surface 20 threaded along its length and is disposed on the bolt such that the threaded surface 20 operatively surrounds and coacts with the threaded end 17 of the bolt. The end face 21 of the first component facing towards the flange 12 has an annular recess 22 therein.

A second component of the nut comprises an annular load ring member 23 that has a substantially flat lower face 24 for bearing against flange 12 and an upper face 25 carrying an upstanding annular rib 26 which extends into the recess 22. The upstanding rib 26 forms a sliding fit in the recess and has its upper end formed as, or supportive of, a fluid seal 27 that defines with the recess a fluid chamber 28. A fluid duct 29 extends through the first body and provides access to the chamber for hydraulic fluid at high pressure, and the recess thereby forms a cylinder and the rib 26 a piston member axially displaceable with respect to the cylinder.

The outer annular surface 30 of the first component is optionally threaded and supports coaxially therewith a co operatively threaded third annular component 31 of the nut, which can be rotated about the first component to be displaced axially to and from the second component 23 to form a locking ring.

In operation, with the piston formed by rib 26 fully retracted into the cylinder formed by recess 22, the hydraulic nut is screwed onto the end portion of the bolt until the load ring abuts the flange 12. Fluid at high pressure is introduced the annular chamber 28 and attempts to displace the rib axially from the recess, reaction to abutment pressure between the load ring and the flange tending to displace the nut body and the bolt end away from the flanges and introduce a level of pre-tension to the bolt in accordance with the level of fluid pressure.

When a suitable level of pre-tension has been achieved on the bolt the locking ring is run along the nut body into abutment with the load ring, or into direct abutment with the flange 12, after which the fluid pressure is vented and the residual tension is maintained in the bolt by way of the relatively long threaded inner and outer annular surfaces of the nut body 19 and the axial force exerted on the end of the locking ring by the flange 12, either directly or indirectly by way of the load ring.

It will be appreciated that the maintenance of residual bolt tension by the physical juxtaposition of the component parts of the hydraulic nut may be achieved by other than a separate threaded locking ring 31, such as by the introduction of shims into the gap 32 formed between the lower face 21 of the nut body and the upper face 26 of the load ring.

It is often necessary to install such nut and bolt arrangements in close proximity and where the minimum obtainable bolt spacing pitch, that is, distance between adjacent bolt centres, is inadequate but limited by the dimensions of the bolt and nut that are necessary to withstand the various forces involved.

Although the stresses which occur on the bolt are mitigated by the hydraulic nut configuration in respect of distribution of forces at the threads compared with a conventional nut, wherein such stresses tend to be concentrated in the vicinity of the thread end nearest the bolt head, and the bolt may be of smaller diameter than when using such conventional nut the diameter of the bolt is still limited by the tensile stress to be retained without permanent deformation.Insofar as the load carrying capacity of the nut is concerned, the overall diameter of the nut body need be little more than minimal and is determined in practice by the need to supply the necessary pre-tensioning forces from a practicable fluid pressure, that is, the radial dimension of the recess 22 needed to give the necessary axial force from the fluid pressure in chamber 28 and the need to retain such pressure in the chamber by the seal 27.When the arrangement is employed in a situation where it is exposed to high temperatures for a longer period, possibly years, it may be necessary to employ a metal-to-metal seal, for example as described in UK Patent specification No 2245681, of limited flexibility, so that the radial thickness of the nut body comprising the parts 33, 34 between the outer annular surface 31 and the recess and between the recess and the inner annular surface 20 respectively is limited by the need for such structural strength to withstand lateral, or radially-directed, chamber pressure without deforming sufficiently for the seal to become ineffective.

Clearly, a greater fluid pressure that would ostensibly require a small radial piston/cylinder dimension would require compensatory increase in cylinder wall strength which, in addition to handling and sealing problems associated with increased pressure, offers little scope for reducing the overall diameter of the first component/nut body.

The use of shims in the gap 32 instead of a locking ring 31 in practice requires an increase in the diameter of the nut body to increase the radial width of part 33 to withstand the additional loading without deforming and problems associated with access to the gap 32 to insert shims may result in no dimensional savings in the nut as a whole that permits a reduction in bolt spacing pitch.

It is an object of the present invention to provide a nut and bolt fastening arrangement that permits the reduction in bolt spacing pitch.

According to the present invention a nut and bolt fastening arrangement comprises a bolt, having on its shank a threaded end portion and an unthreaded cylindrical radialload-bearing portion, and an annular hydraulic nut having axially displaceable first and second components, the first component comprising a nut body having an inner annular surface threaded along part only of its length such that the threaded part of the inner annular surface operatively surrounds and coacts with the threaded portion of the bolt and the unthreaded part of the inner annular surface surrounds the load-bearing portion of the bolt, and the second component comprising a load ring member movable axially with respect to the first component by fluid pressure in an annular chamber, defined by and between the first and second components, and surrounding the unthreaded inner annular surface of the first component, the chamber wall, defined by the first component radially inwardly of the chamber and by the unthreaded annular surface being relatively thin and adapted to deflect radially inwardly under operative fluid pressure within the chamber to be supported against significant and unsealable deflection by the unthreaded inner annular surface on the load bearing portion of the bolt.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:- Figure 1 is a sectional elevation through a known nut and bolt fastening arrangement showing the principles of producing a compressive force between flanges by pretensioning a bolt with a hydraulic nut that is adapted to retain tension by a physically positionable locking ring, Figure 2 is a sectional elevation through a nut and bolt fastening arrangement in accordance with a first embodiment of the present invention and also shown installed in compressive relationship with a pair of flanges, and Figure 3 is a sectional elevation through a nut and bolt fastening arrangement in accordance with a second embodiment of the present invention shown installed in compressive relationship with a pair of flanges.

Referring to Figure 2, the nut and bolt arrangements 40 is basically similar in configuration and operation to the arrangement shown in Figure 1 and the following description will concentrate on the distinctions that result from the invention.

Flanges 11, 12 having aligned aperture pair 14 have a bolt 53 extending therethrough, the bolt having head 55 abutting flange 11 and on its shank 56 a threaded end portion 57.

Adjacent the threaded end portion the shank is formed as an unthreaded cylindrical radial-load-bearing portion 58, the diameter of the portion being equal to that of the outside diameter of the threaded portion 57.

An annular hydraulic nut 60 has axially displaceable first and second components 61 and 62. First component 61 comprises a nut body having an inner annular surface 63 threaded along part only 64 of its length such that the threaded part operatively surrounds and coacts with the threaded portion 57 of the bolt and unthreaded part 65 of the inner annular surface surrounds the load-bearing portion 58 of the bolt with minimal clearance that permits relative rotation and translation as the threaded part 64 is run along the threaded portion 57 of the bolt. The outer annular surface 66 of the nut body is threaded along at least part of its length and carries a locking ring 67 of conventional form. The lower face 68 of the nut body has an annular recess 69 formed therein.

The second component 62 comprises a load ring having an upstanding rib 70 that is disposed as a sliding fit in the recess to define by and between said first and second components an annular chamber 71 served by fluid duct 72 such that the load ring is moveable axially with respect to the first component by fluid pressure in the annular chamber. The upper part of the rib is formed as, or carries, a seal 73 to engage with the walls of the chamber.

The annular chamber 71 surrounds the unthreaded inner annular surface 65 of the nut body, the chamber wall, defined by that portion 74 of the nut body between the chamber and the unthreaded annular surface part 65, being formed relatively thin and adapted to deflect radially inwardly under operative fluid pressure in the chamber 71 to be supported against significant and unsealable deflection by support of the unthreaded inner annular surface part 65 on the load bearing portion 58 of the bolt.

The other chamber wall 75, is of conventional structural thickness and is resistant to significant deflection by chamber pressure.

Thus during assembly, the hydraulic nut 60, with the piston formed by the rib 70 retracted into the recess 69, is threaded onto the bolt and run along the threaded portion until the load ring 62 abuts the flange 12 and the unthreaded part of the inner annular surface 65 is disposed in close proximity to the load-bearing portion 58 of the bolt. When fluid in introduced into chamber 71 the wall 74 after very limited deflection is supported on the loadbearing portion 58 of the bolt and the seal 73 continue to be effective.

The nut body displaced axially with respect to the load ring and flanges by the fluid pressure moves the end portion of the bolt by virtue of the thread engagement so that there is no requirement for relative movement between the deflected unthreaded inner annular surface part 65 of the nut body and the surface of the bolt.

When the nut body and bolt end have been displaced to the extent necessary to install a defined tension in the bolt the locking ring 67 is run into abutment with the load ring (or flange 11) and chamber pressure vented.

Preferably, the thickness of the deflectable wall portion 74 and the clearance with the load bearing portion of the bolt is selected with the material of the nut body to provide for only elastic deflection such that the nut is (after relief of bolt tension) readily removable from the bolt. The seal 73 must be compatible with it to permit re pressurisation of the chamber. Thus the wall 74 which results in plastic deformation requires a plastically deformable seal which conforms to the chamber walls irrespective of any recovery.

The reduction in the radial thickness of the chamber wall 74 from the corresponding wall (34, Figure 1) of the prior art shows the extent to which an arrangement of such a nut and bolt 40 can permit the reduction in bolt spacing pitch.

Referring now to Figure 3, this shows in sectional elevation a second embodiment of nut and bolt arrangement 80. Parts thereof identical to those of Figure 2 are given like reference numerals and those corresponding but not identical being given the like reference numbers primed.

The bolt is identical to that 53 of the arrangement 50. The hydraulic nut 60' has first and second components 61' and 62', the first component comprising a nut body having its inner annular surface 63 formed into said threaded and unthreaded parts 64 and 65. The lower face of the nut body is provided with a recess 69' that extends also to the outer annular peripheral surface 66 which is formed unthreaded along part of its length. The second component 62' comprises a load ring having an axially extending inner annular surface divided into two parts 80, 81 separated by a radially extending shoulder 82 having substantially the same radial dimension as the recess 69.One inner annular load ring surface 80 slidably surrounds a descending nonstructural, radially deflectable portion 74' of the nut body between the recess and the inner annular unthreaded part 65 that defines the deflectable chamber wall. The other inner annular load ring surface 81 slidably surrounds the outer annular surface 66 of the nut body to define the outer wall 83 of the chamber with an outer annular surface part 84.

The inner annular surface 80 of the load ring carries a seal member 85 extending radially into sealing relationship with the wall portion 74' of the nut body and the outer annular surface 66 of the nut body carries a seal member 86 extending radially into sealing relationship with the wall portion 83 of the load ring.

The radial thickness of the outer chamber wall 83 is chosen to be the minimum consistent with providing the outer chamber wall of such structural strength as to resist unsealable radially outward deflection by operative fluid pressure in the chamber.

Part of the outer annular surface 66 of the nut body is threaded and carries a coaxial locking ring 67 of such length as to bear on the outer wall 83 of the load ring.

Preferably the locking ring is of substantially the same radial dimension as the outer wall 83 of the load ring so that the overall diameter of the hydraulic nut does not exceed that demanded by the hydraulic pressure.

Operation is substantially as described above in relation to Figure 2 in that hydraulic fluid supplied to chamber 71' under pressure causes relative axial displacement of the first and second components, the nut body and the load ring, such displacement extending the chamber axially and causing radial deflection of the chamber wall portion 74 to the limited extent possible before it is supported on the bearing surface 58 of the bolt and accommodated by seal member 85.

Notwithstanding this requirement for the radially outer chamber wall 83 to be of such structural strength as to resist deflection, whether formed by the nut body as in Figure 2 or the load ring as in Figure 3, the use of a thin deflectable wall for the radially inner chamber wall permits a corresponding reduction in overall nut radius and improvement in bolt spacing pitch.

Furthermore, it will be apparent from Figure 3 that the diameter of the nut is further reduced from that shown in Figure 2 by having the radially outer wall of the chamber defined by the load ring and axially in-line (67, 83 Figure 3) rather than side-by-side (67, 75 Figure 2).

Clearly with minor modifications to either embodiments the locking ring can be replaced by an arrangement of inserting shims between a radially outwardly extending part of the nut body and the load ring.

Where such a locking ring is employed with or because of limited access to the side of the nut and bolt arrangement, the upper end of the locking ring may be formed as a cap that extends in a taper over the end of the nut body and bolt as shown by ghosted lines 87 to provide both an accessible means of rotation (particularly if seized after a long period of retaining the residual bolt tension) and to protect the bolt and nut from ingress of dirt. Such a cap arrangement may impede access to the fluid duct 72 in the nut body and as an alternative, a fluid duct may be provided through the load ring 62 or 62' to the chamber.

Other variations to the arrangement shown in Figures 2 and 3 are possible without departing from the invention. The bolt 53, whose head bears upon flange 11, may be formed with its head other than at the end of the shank and/or with the head as other than integral with the shank. The shank may be threaded at least in the region where it passes through flange 11 and carry a conventional nut that abuts the flange to form a functional head. Because there is no requirement to access such functional head with such pre-tensioning apparatus as a hydraulic nut, and the size of such a functional head is not a limiting factor in bolt spacing pitch, the diameter of the threaded shank engaged by such conventional nut may be greater than at the other flange where tension is applied and readily accommodate uneven thread stresses associated with such conventional nuts.

The provision of such a functional bolt head by means of a nut which can be assembled on the shank in situ permits the hydraulic nuts 60 (Figure 2) or 80 (Figure 3) to be preassembled on the end of the bolt shank to lessen the possibility of contamination of the threads 57 and clearance between the nut body surface 65 and bolt loadbearing portion 58. This in turn enables the use of less clearance between the nut body and load-bearing portion and the use of finder threads at 57 and 64 or 57' and 64', the latter minimising the effective loss of bolt diameter by thread depth and thus facilitating a further small reduction in bolt spacing pitch.

Claims (6)

1. A nut and bolt fastening arrangement comprising a bolt, having on its shank a threaded end portion and an unthreaded cylindrical radial-load-bearing portion, and an annular hydraulic nut having axially displaceable first and second components, the first component comprising a nut body having an inner annular surface threaded along part only of its length such that the threaded part of the inner annular surface operatively surrounds and coacts with the threaded portion of the bolt and the unthreaded part of the inner annular surface surrounds the load bearing portion of the bolt, and the second component comprising a load ring member movable axially with respect to the first component by fluid pressure in an annular chamber, defined by and between the first and second components, and surrounding the unthreaded inner annular surface of the first component, the chamber walls defined by the first component radially inwardly of the chamber and by the unthreaded annular surface being relatively thin and adapted to deflect radially inwardly under operative fluid pressure within the chamber to be supported against significant and unsealable deflection by the unthreaded inner annular surface on the load bearing portion of the bolt.

2. An arrangement as claimed in claim 1 in which the annular chamber extends radially of the nut body from said deflectable chamber wall to an outer annular surface of the nut body and the load ring has axially extending inner annular surfaces stepped radially by said chamber radial dimension such that one inner annular load ring surface slidably surrounds the deflectable chamber wall of the nut body in sealing relationship therewith and the other inner annular load ring surface slidably surrounds the outer annular surface of the nut body in sealing relationship therewith to define the outer annular wall of the chamber.

3. An arrangement as claimed in claim 2 in which said one inner annular load ring surface carries a seal member extending radially into sealing relationship with the deflectable chamber wall surrounded thereby and said outer annular surface of the nut body carries a seal member extending radially into sealing relationship with the other inner annular load ring surface surrounding it.

4. An arrangement as claimed in claim 2 or claim 3 in which the outer annular surface of the load ring extends axially and surrounds said other inner annular load ring surface to define the outer annular wall of the chamber of minimal radial thickness consistent with providing structural strength to resist unsealable radially outward deflection by operative fluid pressure in the chamber.

5. An arrangement as claimed in claim 4 in which the outer annular surface of the nut body threadedly supports a coaxial locking ring of substantially the same radial dimensions as the outer annular wall of the chamber adapted to move axially with respect to the nut body into abutment with the end of the outer annular wall of the chamber.

6. A nut and bolt fastening arrangement substantially as herein described with reference to, and as shown in, Figure 2 or Figure 3 of the accompanying drawings