WO1986000118A1

WO1986000118A1 - Improvements in fastenings and seals

Info

Publication number: WO1986000118A1
Application number: PCT/GB1985/000264
Authority: WO
Inventors: John Orr
Original assignee: John Orr
Priority date: 1984-06-16
Filing date: 1985-06-13
Publication date: 1986-01-03
Also published as: GB8415410D0; CA1264914A; JPS61502410A

Abstract

A bolted joint between two parts (10, 12) is strengthened by means of an annular reinforcement (20) having a sloping face (22) engaging a groove (18). Tightening of the bolt produces a compressive stress around the hole which counteracts ovalisation and fatique failure. The reinforcement can be a washer (20) as shown, or can be embodied in a bolt head, nut, rivet insert, or other item. Application of the invention to seals is also described.

Description

"Improvements in Fastenings and Seals"

This invention relates to improvements in fasten¬ ings, seals and the like used in conjunction with structures which may be attached to massive foundations or which may comprise two or more parts of a structure or machine which may be attached together with bolts,, screws, studs, rivets, or other such elongate fasteners, or which may contain cavities which require to be sealed, all of which will exhibit high stresses at the periphery of the holes through which pass the various types of fasteners or which constitute cavities.

Such structural parts may be attached together at one or more joint faces which be flat, spherical, cylindrical, conical, ellipsoidal, or any such surface shapes, and the bolts, studs, or rivets are disposed through holes which will normally to be perpendicular to the joint faces at their individual localities: only in exceptional cases will holes be at an oblique angle to the joint faces. Such holes in adjacent structural members will normally be aligned to permit the passage of the bolts, studs or rivets so that when these are placed in tension by means of any of several possible end fixings, such as bolt or rivet heads, nuts, cotters, cotter pins, collars, colletts, spring retainers, screw threads tapped in one or more of the adjoining structural parts, or other such device, then the several parts of the structure or the structure to the foundation will be closely bound together so that they constitute one integral structure in effect.

Also when such holes constitute cavities which require to be sealed, then the central axes of the holes in adjacent parts will normally be aligned, particularly when such cavities contain reciprocating parts such as pistons or spools or rotating parts such as pump or compressor shafts.

In the special case where a force requires to be transmitted in shear from one surface to the other of a pair constituting a joint face, an adapt- ation of the invention will transmit such shear forces without recourse to placing in shear the shank of the fastener nor resort to bolts of very high strength which create very frictional forces to permit the transmission of the shear load. (High- Strength Frictional-Grip Bolts.)

The invention will be described principally in relation to parts having aligned holes through which an elongate member is passed and is placed in tension by the clamping arrangement of an annular shoulder or shoulders at least on one end of the member with an outer surface or surfaces of one of the parts around the hole therein. The elongate member, may, for example, be the shank of a bolt and the shoulders may be presented by the bolt head and a nut or washers. The interengaging surfaces and shoulders are usually flat and present little or no resistance to deform¬ ation of the hole when the parts of the structure are subjected in use to applied forces which result in stresses which may be steady, impulsive, repeatedly^• impulsive, or cyclic in nature, and these are normally enhanced in magnitude in the immediate vicinity of the holes by reason of the geometry at these areas.

If a structural part such as a plate or casing with a hole through it is subjected to a tensile stress in one direction, it is well known that the hole elongates to an elliptical shape, with the major axis of the ellipse in the direction of the applied tension. When a bolt or such fastener is fitted through the hole and placed in tension (as described above) the friction under the bolt head or nut or any interposing washer or such device tends to grip the surface of the material around the hole, but it is normally of limited effectiveness in prevent- ing the ovalisation of the hole, particularly if the applied force is impulsive or cyclic in nature. In the vast majority of engineering or other assemb¬ lies therefore the fastener heads or nuts or washers play no significant part in resisting the deformation of the bolt holes, whether the applied load is static or dynamic.

In addition to the ovalisation, or straining, of a hole when the containing plate or other structural part is subjected to a tensile stress, it is well known also that a concentration of such tensile stress occurs at the surface of the hole tangential to the applied stress. In a simple hole and plate config¬ uration this concentration, known as the Stress Con¬ centration Factor or S.C.F. has a value of 3.0. When the designer is aware of this S.C.F. it is usual to increase the thickness of the material either locally by means of a raised boss in castings or by welding doubler plates around the hole, or sometimes by trebling the thickness of the plate or other such part by a factor of 3.0 to counter this stress concen¬ tration. When no provision or inadequate provision is made for the stress concentration then the structure is likely to fail from a crack or cracks originating from the surface of such holes. It is known also that such cracks are most likely to arise from irregularities on the surface of the hole coinciding with the area of maximum tensile stress.

An object of the present invention is to provide arrangements in which deformation of the holes is greatly reduced, the stress concentration at the surface of the hole is greatly reduced, and the area of maximum stress may be removed from the surface of the hole so that it will not coincide with surface irregularities existing there, so that the tendency to failure by cleavage from steady or impulsive forces or by fatigue from fluctuating forces is eliminated or greatly re¬ duced.

The invention accordingly provides an assembly comprising first and second parts having aligned holes and means for strengthening the parts in the area of the holes, said means comprising an annular groove formed in at least one surface of at -least one of the parts so as to circumscribe the hole at that surface, an annular reinforcing member shaped to engage with the groove, and means for applying force to the rein¬ forcing member in a direction parallel to the axis of the holes.

In preferred forms of the invention, said means comprises a counterbore prepared around the hole in at least one of the parts , which has the shape of a trunc¬ ated cone whose apex is toward the outer surface of the part, and which intercepts or approaches closely to the outer edge of the hole through the part, an annular reinforcing member such as a bolt head or rivet head or nut or washer which has a hollow cone shaped on the underside/that side which matches exactly, or very closely and in a controlled manner, the shape of the counterbore around the hole, and means for applying force to the reinforcing member in a direction parallel to the axis of the holes.

The invention further provides washers, inserts, bolts, rivets, screws, studs, nuts and adaptations of the matching surfaces of structural members fo^'r use in particular forms of this invention. Where the hole is in the form of a cavity which is required to be sealed, the invention further provides seals located in similarly shaped counterbores for use in further forms of the invention. Preferred forms of the invention are based on the principle of arranging a fastener, seal, or the like in such a manner that the surfaces of the aligned holes and the material of the structural part radially outward from the surface of the hole for some small distance are placed in a state of pre-compressive stress, within the design parameters. For the achieve¬ ment of the optimum state of pre-stress it will be preferred also that such bolt or rivet heads, nuts, washers, inserts or other such devices which will bear directly on the surface of the counterbore shall^" be manufactured of materials which have higher- yield stresses and equal or greater moduli of elasticity that the material of the structural parts. In the case of washer or seals which may be made of a polymer material or of composite construction of mainly polymer with bounding wall or walls of- a rigid material, the polymer part in use must be completely bounded on all sides whether by the walls of the structure or by metal parts suitably embedded in the material of the seal or washer, so that the bulk modulus of the polymer will be brought into operation by the complete enclosure and compression of that material, which will have the effect of imposing a compressive stress on the surface of the counterbore in the same manner as the solid bolt or rivet heads, or nuts, or washers or such device.

I am aware of certain prior art devices which might appear to be similar to devices embodying the present invention. For example, the well known Belle- ville washer has an angled lower face, as do washers described below. However the Belleville washer is invariably made of spring steel or some other similar spring material, has a very thin walled construction, and in use is normally compressed to a flat shape when used as a washer, and like all other forms of spring washer it relies upon the strain energy of the compressed spring to maintain a high axial force in the bolt through the hole which acts by friction between the surface of the structure around the hole and the underside of the bolt head or nut to prevent rotation and loosening of the fastener.

There are also so-called 'flanged* or 'Big-Head' bolts having relatively wide heads with the underside sloped at a small angle of 0°-3° which is designed to bear on a flat surface around the relevant bolt hole: these operate in a similar manner to a Belleville washer in combination with a plain bolt.

There are known also various types of 'ring- wedge', but these are employed mainly to create a frictional grip between a rotary shaft and an outer rotary member such as a flange or a. gearwheel: in these a force parallel to the axis of the shaft causes the opposing halves of a 'wedge-ring' device to expand radially inward and outward respectively to grip on the afDreshaft and outer rotary member, in order that a torque may be transmitted between the two at the relevant design speed.

There are known also various types of washer on the surface of which are embossed shallow serrations in various patterns which might appear to resemble certain implementations of this invention, but these act essentially by frictional forces enhanced by the deliberate roughening of one of the surfaces: other washers have sharp protruding edges which might cause local plastic deformation of a softer material of the structure, but these are intended to prevent rotation and thus loosening of the fastener, and plays no part in creating a favourable condition of compressive pre-stress in the material of the structure immediately around the hole.

Many configurations of rubber or polymer seal located in circumferential grooves around cavities in structural parts which require to be sealed against leakage of fluid into or out of the said cavity are known, but in no case is the combination of seal and groove profile so designed to create a favourable condition of compressive pre-stress at the bore of the cavity in the locality of the seal.

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

Fig. 1 is a cross-section of a fastening embody¬ ing the invention;

Fig. 2 is a detail of Fig. 1; Fig. 3 illustrates possible relationships between the washer and groove of Figs. 1 and 2;

Fig. 4 illustrates circumferential compressive pre-stress in Fig. 1 and related arrangements;

Fig. 5 is a schematic cross-section of an alter- native embodiment showing stress distributions;

Fig. 6 illustrates a modified figure of Fig. 4;

Fig. 7 is a cross-section of another embodiment; Fig. 8 is a cross-section of a practical embodi- ment of washer for use in the arrangements of Figs. 1-3;

Fig. 9 illustrates the invention applied to a spring washer;

Figs. 10, 11 and 12 illustrate bolt heads embody- ing the invention; Figs. 13-17 illustrate alternative types of washer for use in the invention;

Fig. 18 illustrates a socket head screw embodying the invention; Fig. 19 is a partially sectioned view of a bolt and washer incorporating a further embodiment of the invention;

Fig. 20 shows, in cross-section, the invention embodied in rivets; Figs. 21a and 22b are respectively a side view and a cross-section of the invention applied to rails and fishplates;

Fig. 22 illustrates structural components adapted for use with the invention; Fig. 23 illustrates the invention applied to flanges;

Fig. 24 shows inserts embodying the invention; Fig. 25 is a cross-section of another type of insert; and Fig. 26 is a cross-section of a fastening incor¬ porating another form of the invention.

Referring to Fig. 1, two plates^" 10 and 12 are fastened by a nut 14 and bolt 16 passing through aligned holes. In accordance with the invention, the plates are provided with annular grooves 18 which are engaged by shaped washers 20. The pressure face 22 of each washer is angled with respect to the bolt axis. Thus, as the fastening is tightened, a compressive force is created which has a major component forcing the plates together, and a second compressive component applying force radially inwardly towards the hole. This second component results in a circumferential compressive pre-stress around the hole.

As seen in more detail in Fig. 2, the groove 18 is preferably formed with an arcuate base to re¬ duce stress concentration at this point. The "pressure angle" ^ may suitably be in the range 3°- 50°. It is believed that the optimum angle is in the region of 10°-30° where the load is principally in bending, and approximately 4°-12° where the load is principally in tension or shear.

Fig. 3 illustrates^' the relationship between the inner edge 21 of the washer 20 and the inner edge 19 of the groove 18. Ideally these should be aligned, as in Fig. 3a. However, this is difficult to achieve in practice, and it is preferred that the washer should overlap, as in Fig. 3b. The opposite arrangement, as in Fig. 3c, produces an area of weakness at 17 in the part 10.

Fig. 4a illustrates at 24 the forces applied in Fig. 1 by the washers 20, and at 26 the distribution of the circumferential compressive stress produced thereby along the length of the aligned holes. Fig. 4b is similar but for thicker plates, and shows that the compressive circumferential stress is substantially reduced at the centre of the thickness. This can be overcome as shown in Fig. 4c by s.haping the grooves 18 and washers 20a or 20b such that there is a tapered (20a) or arcuate (20b) clearance opening towards the hole. This produces, on tightening down, non-uniform force distributions 24a, 24b which produce the more uniform pre-compressive stress 26a.

Fig. 5 illustrates an alternative approach in the case of thick plates. In this embodiment an intermediate washer 30 of double conical cross-section is engaged in internal grooves 32 and thus increases the precompressed radial stress in the inner part of the joint.

The arrangement shown in Fig. 5 can additionally be used to provide sealing in items such as high pres- ure hydraulic or fuel pumps. A development of this is illustrated in Fig. 6 in which the rigid washers 20 and 30 are replaced by similarly shaped flexible members -20a and 30a of rubber or polymeric material. These materials are equally effective in producing the desired radial inward force component provided that they are rigidly confined. Thus the outer washers 20a are confined by annular plates 34, and the inter¬ mediate washer 30a can either be confined by the geometry of the slots 32 as shown in the left-hand part of Fig. 6 or partly by the slots and partly by a metal rein¬ forcement hand 36 as shown in the right-hand part of Fig. 6. This arrangement can be used to seal a cylinder formed by aligned bores in more than two parts.

Fig. 7 illustrates a bolted joint between parts 10,12 loaded in shear, which is suitable for use in place of high strength friction grip bolts. In Fig. 7a, the bolt 16 and nut 14 are plain and without washers, and a double conical washer 30 is engaged in grooves 32 in the mating faces. The arrangement of Fig. 7b is similar, but the washer 30 is replaced by a multi- faced ring 31 seating in corresponding grooves in the parts 10 and 12.

Fig. 8 shows a washer 28 whose action is exactly analogous to that described above in Fig. 1-3. However, whereas the washer 20 would probably require to be turned to shape and would thus be expensive, the washer 28 could be cold-formed from sheet material.

Fig. 9a is an elevation and Fig. 9b a cross- section of a washer 29 which operates, in use, as des- cribed above. The washer 29, however, is split and has a free shape suitable to act additionally as a spring washer.

Instead of using separate washers the invention can be incorporated directly in bolt heads and nuts. Fig. 10 shows such a bolt head 37. Fig. 11 shows a similar bolt head with a captive washer 38. In Fig. 12 the captive washer 38 is formed such that in the unstressed condition it is at a small angle β to the mating face of the bolt head. When tightened down 5 the washer 38 deflects against the bolt head to provide a frictional force opposing unfastening.

The above embodiments rely on a single angled annular surface. It is equally possible to use a plur¬ ality of such surfaces, which might require more complex 0 fabrication and machining but has the advantage of requiring less penetration into the thickness of the articles to be fastened for a given compressive stress. Fig. 13 illustrates a washer 40 of this type whose action will be understood from the above description. 5 Figs. 14, 15 and 16 illustrate variations of the washer 40.

Fig. 17 illustrates in greater detail modifi¬ cations of these formed to provide additionally a self- locking and friction grip capability. In Fig. 17a 0 the flanks on both members are straight, while in Fig. 17b one flank is straight and the other arcuate or involute, in Fig. 17c both are arcuate or involute. In each case, the geometry is such . that on tightening down an interference fit is produced which resists 5 unfastening.

A similar approach is illustrated in Fig. 18 in which a socket head screw 42 is formed with a plur¬ ality of multiple annular- formations of conical cross- section. It is contemplated that this form could be

30 manufactured by cold heading, since the material dis¬ placed from the socket would provide material for form¬ ing the shape of the head.

Fig. 19 shows an arrangement which combines features of Figs. 8 and 13. In this embodiment a washer

35 44 has an angled engagement face 46 for engagement with a groove 48 in the member 10. The upper face of the washer 44 is provided with plural angled surfaces 50 for engagement with complementary surfaces formed on the underside of the head of a bolt 52. This embod- iment is believed to be of particualr utility in high tensile applications such as heavily stressed cylinder head bolts.

Fig. 20 shows the invention applied to various forms of rivet. In Fig. 20a, plates 10,12 are provided with grooves 18 in which a rivet 66 is fastened with flush heads. In some applications, it will be accept¬ able for only one head to be flush, as- in Fig. 20b. Fig. 20c shows a bolt-type rivet 68 with snap head, and Fig. 20d a tubular rivet 69. The action of all these rivets will be clear from the above discussion of threaded fasteners.

Turning to Fig. 21, the application of the inven¬ tion to rails for railways will now be described. These tend to suffer fatigue cracking at the bolt holes. In Fig. 21, rails 70 are connected by fish¬ plates 74 and bolts 76 and nuts 78.. An annular groove 72 is formed or machined around each bore in the rails 70 and is engaged either by a conical formation 80 on the fishplate as at 74a, or by a conical washer 82 engaged by a plain fishplate as at 74b via a res¬ ilient member 84.

In a similar manner, the invention may be used in the design of structural components as an integral feature. Fig. 22a shows a cast or forged boss 86 engaged by a bolt 88 similar to the bolts previously described. Fig. 22b shows a distance piece 90, and Fig. 22c a pillar 92.

Fig. 23 illustrates the application of the inven¬ tion to flanges. A main seal 30 is provided, as in Fig. 6. Additionally, the fasteners embody the inven- tion by washer 94 or shaped heads, as at 96. To minimise the overall diameter of the flange, the fasteners engage arucate grooves 98 which are smoothly blended into the flange profile, as best seen in the detail of Fig. 23a. Since the flanges are relatively thick, intermediate shaped rings 100 or 102 may be used in a similar manner to Fig. 5.

Fig. 24 shows metal inserts 104 which may be incor¬ porated in a member 106 to give a flush outer surface while providing the compressive prestress when a plain fastener is applied. The exterior of the insert may be screw-threaded (Fig. 24a), ribbed (24b), or knurled (Fig. 24c); the latter two forms are particularly useful with plastics members. Fig. 24d shows a part-sectional elevation and a plan of an insert 104 secured in a member 106 by a circumferential roller peen 105. Fig. 2^'4e similarly shows an insert 104 secured by ball-peening at 107.

Fig. 25 illlustrates another form of insert 108. This has a self-retaining capability, having projections 110 similar to the forms shown in Fig. 17. This allows the insert to be positioned in the part 106 and then machined xn situ, for example to accept a stud 112. An advantage which certain of the above embodiments exhibit is that stress applied transverse to the hole axis results in the application of an axial force com¬ ponent to the bolt or equivalent, due to the angled faces. Thus part of the applied force is absorbed as strain energy in the bolt. It is also within the purview of this invention to provide the shoulders with cross-sections in the form of rectangles or otherwise so as to present cylin¬ drical inwardly-facing faces engaging outwardly-facing cylindrical faces of the parts faced outwardly from the edges of the holes. These latter faces may be presented by annular recess or rims around the holes. With cylindrical faces there would be no pre-compressive circumferential stresses set up. However, the confining effect of the engagement between the cylindrical faces would still strengthen the holes against deformation by applied tensile stress across the holes. Fig. 26 shows such an embodiment, which a straight- sided ring 60 is seated in a complementary groove 62 in the structural part 10, with its top face inward compressive prestress on the hole. However, the ring 60, which is preferably of material with a substantially greater Young's modulus and yield stress than the part 10, substantially prevents any elliptical distortion of the hole right up to a load which would fracture the ring 60 (this load would normally be beyond the tensile strength of the bulk material of the part 10). The groove 62 is preferably formed with radiused under¬ cuts at its edges, as seen at 64 in the scrap detail in Fig. 26. This avoids stress concentrations at the groove.

An advantage of all the above embodiments is that their performance is not affected by the presence of lubricants. In contrast a plain bolted connection, which is essentially frictional, is substantially de¬ graded by lubricants.

Structures incorporating the invention have a number of advantages in comparison with conventional structures:-

1. Increased static strength in tension perpendicular to hole.

2. Increased bending strength and stiffness. 3. Increased torsional strength and stiffness.

4. Increased fatigue endurance.

5. Increased impact strength.

6. Increased shear strength when load is transferred between parts of the structure held together by fast- eners.

7. Elimination of fretting fatigue. 8. Structure of more reliable due to insensitivity of fastener assembly to friction effects and thus insen¬ sitive to spilled lubricant or other contaminant on joint faces.

9. Fatigue and strength limits of structure can be predicted with greater accuracy due to insensitivity of inverted cone fasteners and derivatives thereof to contamination.

Claims

1. An assembly comprising first and second parts having aligned holes and means for strengthening the parts in the area of holes, said means comprising an annular groove formed in at least one surface of at least one of the parts so ^• as to circumscribe the hole at that surface, an annular reinforcing member shaped to engage with the groove, and means for apply¬ ing force to the reinforcing member in a direction parallel to the axis of the holes.

2. The assembly of claim 1, in which the groove has a major face at an angle __^_ to the normal to said axis, and the reinforcing member is shaped to engage the groove only on said face, whereby said force causes a resultant compressive stress component in the res- pective part directed towards said hole axis.

3. The assembly of claim 1 or claim 2, in which the force applying means comprises a bolt or screw passing through the aligned holes and a nut.

4. The assembly of claim 3, in-which the reinforcing member is a washer engaged by the head of the bolt or screw, or by the nut.

5. The assembly of claim 3, in which the reinforcing member is the head of the bolt or scr.ew, or is the nut.

6. The assembly of claim 5, in which the bolt or screw, or the nut, is provided with a captive washer interposed between it and the respective first or second part.

7. The assembly of claim 6, in which the captive washer has a free shape such that, on tightening down, it is deformed to provide frictional resistance to unscrewing.

8. The assembly of claim 4, in which the interface between the washer and the head of the bolt or screw, or the nut, is formed with one or more angled annular surfaces .

9. The assembly of claim 8, in which the angled annular surface(s) and the corresponding groove(s) are shaped to provide an interference fit.

10. The assembly of claim 2, in which the force applying means is a rivet whose head provides the rein¬ forcing member.

11. A washer for use in the assembly of claim 4, the washer being an annular member having a first face for engagement with a bolt, screw or nut, a second face for engagement with a groove in a structural part, the second'face comprising one or more annular surfaces at an angle < . to the first face.

12. The washer of claim 11, in which the first and second faces are joined by a third, outer face, and the junction between the second and third faces is a radius .

13. The washer of claim 12, in which the outside diameter of the first face is less than that of the second face, whereby the third face is angled.

14. A bolt or screw for use in the assembly of claim 5, comprising a shank and a head, and in which the face of the head facing towards the shank is formed with one or more annular surfaces at an angle .< to the normal to the axis of the bolt or screw.

15. A nut for use in the assembly of claim 5, in which one transverse face of the nut is formed with one or more annular surfaces at an angle -- to the normal to the axis of the nut.

16. A rivet for use in the assembly of claim 10, comprising a shank and a head, and in which the face of the head facing towards the shank is formed with one or more annular surfaces at an angle ex^. to^" the normal to the axis of the rivet.

17. The subject matter of claim 2, claim 11, claim 14, claim 15 or claim 16, in which the angle --< is approx¬ imately 30°.

18. The subject matter of claim 2, claim 11, claim 14, claim 15 or claim 16, in which the angle =_ is in the range 8°-12° .