Note: Descriptions are shown in the official language in which they were submitted.
~Im rovements in Fastenin s and Seals n
P 9
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
~2~9~
-- 2
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 shearforces 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-
Streng~h Frictional-Grip solts~)
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
12649i~
- 3 -
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 ifthe 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 3Ø
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
~Z6491~
- 4 -
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 fro~ 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, nu~s and adaptations ofthe
matching surfaces of structural members for use in
particular forms of this invention. Where the hole
12649~4
is in the form of a cavit~ which is required to be
sealed, the invention further provides seals located
in similarly shaped counterbores for use in further
forms of the invention.
S 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
10 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
15 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
20 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
25 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
30 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-
35 ville washer has an angled lower face, as do washers
126~914
-- 6described 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 OD-3D 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 aforeshaft 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
~2649~4
-- 7
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 circumfererltial grooves around cavities in
struc-tural 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 cornpressive
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 embodying
the invention;
Fig. 2 is a detail of Fig. l;
Figs. 3a-c illustrate possible relationships between
the washer and groove of Figs. 1 and 2;
Figs. 4a-c illustrate circumferential compressive
pre-stress in Fig. 1 and related arrangements;
Fig. 5 is a schematic cross-section of an alternative
embodimen-t showing stress distributions;
Fig, 6 illustrates a modified figure of Fig. 4;
Fig. 7a is a cross-section of another embodiment;
Fig. 7b is a cross-section of a modification of
Fig. 7a;
Fig. 8 is a cross-section of a practical embodiment
of washer for use in the arrangements of Figs. 1-3;
Fig. 9a shows a side elevation (upper) and plan view
(lower) of the invention applied to a spring washer;
Fig. 9b is a cross-section of the spring washer of
Fig. 9a;
Figs. 10, 11 and 12 illustrate bolt heads embody-
ing the invention;
1264914
-- 8
Figs. 13-16 illustrate alternative types of washer
for use in the invention;
Figs. 17a-c illustrate further types of washer;
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;
Figs. 20a-d show, in cross-section, the invention
embodied in rivets;
Figs. 21a and 21b are respectively a side view and
a cross-section of the invention applied to rails and fish-
plates;
Eigs. 22a-c are cross-sections illustratiny structural
components adapted for use with the invention;
Fig. 23 is a cross-section showing the invention
applied to flanges;
Fig. 23a shows a detail of Fig. 23 to an enlarged
scale;
Fig. 24a is a cross-section of an insert embodying
the invention;
Figs. 24b and 24c are similar view of modified inserts;
Fig. 24d is (upper) a half elevation and half section
and (lower) a plan view of another insert;
Fig. 24e comprises views similar to Fig. 24d of yet
another insert;
Fig. 25 is a cross-section of another type of insert;
and
Fig. 26 is a cross-sec-tion of a fastening incorporating
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 cleated which has a major component
forcing the plates together, and a second compressive component
. .
,, ,
12649~4
g
applying force radially inwardly towards the hole. This
second cornponent results in a circulnferential compressive
pre-stress around the hole.
As seen in more detail in E'ig. 2, the groove 18 is
preferably formed with an arcuate base to reduce 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.
Figs. 3a-c illustrate 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 cir-
c~nferential compressive stress produced thereby along the
length of the aligned holes. E'ig. 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 shaping 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 i-tems such as high pressure
hydraulic or fuel pumps. A development of this
~2~;~914
-- 10 --
is illustrated in Fig. 6 in which the rigid washers
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
13,12 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 andhas 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.
35 Fig. 10 shows such a bolt head 37. Fig. 11 shows a
lZ6~4
-- 11 --
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 ~ 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
10 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.
lS 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
20 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
25 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
126~
- 12 -
with a groove 48 in the member lO. The upper face
of the washer 44 is provided with plural angled surfaces
S0 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 hightensile applications such as heavily stressed cylinder
head bolts.
Fig. 20 shows the invention applied to vsrious
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
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-
12~;4914
- 13 -
tion by washer 94 or shaped heads, as at 96. To minimise
the overall diameter of the flange, the fasteners engage
arucate grooves 9~ which are smoothly blended into the
flange profile, as best seen in the detail of Fig. 23a.
S 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
10 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
15 elevation and a plan of an insert 104 secured in a member
106 by a circumferential roller peen 105. Fig. 24e
similarly shows an insert 104 secured by ball-peening
at 107.
Fig. 25 illlustrates another form of insert 108.
20 This has a self-retaining capability, having projections
110 similar to the forms shown in ~ig. 17. This allows
the insert to be positioned in the part 106 and then
machined in situ, for example to accept a stud 112.
An advantage which certain of the above embodiments
25 exhibit is that stress applied transverse to the hole
axis results in the application of an axial force com-
ponent to the bolt or e~uivalent, 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
35 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
126~
- 14 -
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 lO, 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
lO, substantially prevents any elliptical distortion
of the hole right up to a load which would ~acture
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 perpendicularto 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.
~2~
- 15 -
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.
g. Fatigue and strength limits of structur~ can be
predicted with greater accuracy due to insensitivity
of inverted cone fasteners and derivatives thereof to
contamination.
