Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SELF-SEALING THREADED FASTENER
This invention relates generally to self-
sealing fastener assemblies which includes a fastener member
and a washer member. More particularly, this invention has
to do with a fastener/washer assembly which exhibits
improved characteristics over those conventionally known.
BACKGROUND OF THIS INVENTION
In general, self-sealing washers used in conjunc-
tion with fastening members consist of a metal backing in
conjunction with a soft sealing component such as rubber,
neoprene or plastic. The sealing component may be either
bonded directly to the metal backing, or may itself be
a separate washer. When the fastener is driven, the soft
sealing component is compressed between the metal backing
and the surface through which the fastener is inserted,
creating a watertight seal. In some self-sealing assemblies,
the metal backing is designed so that metal-to-metal contact
is achieved between the backing and the wall. This pro-
vides a more reliable seal by preventing excessive compression
of the soft sealing component, and also increases the pull-
over and shear resistance of the system.
It is an aspect of this invention to provide afastener and washer assembly with improved characteristics,
capable of self-sealing while attaining metal-to-metal con-
tact. A further aspect of this invention is to provide a
fastener/washer assembly which is economical to manufacture.
Acc~rdingly, this invention provides a self-
sealing threaded assembly comprising a threaded fastener and
a resilient washer.
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The threaded fastener has a head with means by which
tor~ue can be applied to the head. The head has a sub-
stantially flat undersurface and there is provided a
shoulder portion beneath the undersurface of smaller diameter
than the undersurface. An inward step is provided at the
bottom of the shoulder portion, and the fastener includes
a threaded shank sized diametrally such that an opening
engageable by the threaded shank is too small to accept
the shoulder portion, and would abut the inward stop.
The resilient washer is of integral, resilient
material throughout and has a maximum thickness greater than
the axial dimension of the shoulder portion. The cross-
section of the washer, taken in a plane extending radially
from and including the axis of the washer, has a thicker
portion closer to the axis and has a tapering portion ex-
tending with diminishing thickness away from the thicker
portion and remote from the axis. The tapering portion is
defined by two outwardly convergent surfaces, one of which
slopes away from the undersurface of the head, the other of
which slopes ~oward the undersurface. In this manner, when
the threaded shank is screwed into an opening in a wall,
the shoulder portion will abut the wall so that the
resilient washer is squeezed between the wall and the
undersurface.
Two embodiments of this invention are illustrated
in the accompanying drawings, in which like numerals denote
like parts throughout the several view, and in which:
Figure 1 is an elevational view of a screw
together with a sectional view of the first embodiment of
the resilient washer when the latter is in unstressed con-
dition;
Figure 2 is a sectional view through a panel
through which the screw has been inserted, showing the
screw again in elevation, and showing the panel and the
first embodiment of the washer in section; and
Figure 3 is a sectional view of the second embodi-
ment of the washer.
In Figure 1 the~e is shown a self-sealing
screw assembly lO which includes a screw 12 and a resilient
2a
washer 1~. The screw includes a head 16 with means by
which torque can be applied to the head. More
specifically, the head shown in Figure 1 has a hexagonal
portion 17 of conventional construction surmounting an
integral annular portion 18 of circular outer circl~ference.
The integral annular portion 18 has a larger diameter
than the maximum diameter of the hexagonal portion 17
and has a flat under-
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surface 20. The upper surface of the integral annularportion 18 slopes inwardly and upwardly to join the
hexagonal portion 17, as shown at 21 in Figure 1.
The screw 12 has in addition an integral shoulder
portion 23 located coaxially beneath the integral annular
portion 18, the shoulder portion being of cylindrical
configuration but of smaller diameter than the integral
annular portion 18. Below the shoulder portion 23 is a
standard threaded shank 25.
In the figures, the threaded shank is of
self-tapping construction, although any other form of
thread could also be utilized, as will be obvious to those
skilled in the art.
More particularly, the threaded shank 25 is
sized diametrally in such a way that an opening which
is engageable by the threaded shank is too small to accept
the shoulder portion 23. In the specific embodiment shown,
the shoulder portion 23 has a diameter larger than the
outside thread diameter of the threaded shank 25.
The resilient washer 14 may have various
sections, and could include the standard resilient washer
section, in which an annulus is simply stamped from resilient
sheet material. ~owever, the section preferred for use with
the screw of this invention is that shown in Figures 1 and
2. The main characteristic of the section of the resilient
washer is that it must have a maximum thickness, i.e. a
dimension measured parallel to the ax~s of the washer, which
is greater than the axial dimension of the cylindrical
shoulder portion 23. This is clearly illustrated in Figure
1. By providing this configuration, one ensures that
the resilient washer, after it rides up around the shoulder
portion 23, will be resiliently squeezed between the
undersurface 20 and the surface 28 of a wall 30 through
which the screw is threaded (see Figure 2).
In Figure 1, this enlarged or thicker portion
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is seen to be located at the inward region of the washer,
and it is essential, of course, that this thicker portion be
located within the outer diameter of the integral annular
portion 18, so that the undersurface 20 can act in conjunc-
S tion with the surface 28 to squeeze the washer.
As further seen in Figure 1, the section of a
preferred form of washer, taken in a plane extending radially
from and including the axis of the washer (i.e. the axis of
the screw as seen in Figure 1), has a tapering portion 32
extending with diminishing thickness away from the thicker
portion 33, and extending remote from the axis of the washer.
Expressed in even more specific language, the
preferred washer shown in Figures 1 and 2 is of tetragonal
cross-section, having two walls 34 forming an o~tuse angle
with the apex toward the washer axis, and two walls 36
forming an acute angle with the apex away from the washer
axis.
It is preferred that the inner diameter of the
washer 14 be small enough to interfere with and hence locate
itself on the threads of the shank 25.
When the assembly is put into use, as shown in
Figure 2, the screw is threaded into the wall, with the
washer 14 in place on the threaded shank 25, until the
shoulder portion 23 abuts firmly-against the wall surface 28,
thus preventing further rotation of the screw. Because of
the comparative sizing of the washer 14 and the shoulder
portion 23, the washer 14 will be resiliently squeezed to
a controlled degree (determined by the axial extent of the
shoulder portion 23), and it is understood that the resilient
washer 14 will be sized in such a way that the degree of
compression applied to it in the Figure 2 configuration
will be sufficient to seal the opening 38 of the panel
30 against the ingress of water or other liguids.
It will now be appreciated that, by providing
the shoulder portion 23, angular misalignment can be
corrected as the shoulder portion 23 comes in contact with
the surface 28 of the panel 30. Moreover, even when the
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screw is initially threaded at an angle from the normal,
the taper ~walls 36) of the washer is such as to minimize
or eliminate spin-out of the washer during the turning.
Actual tests have shown that, whereas flat washers tend
to spin-out at about 15 angle from the normal, resulting
in poor appearance, the tapered washer shown in Figures 1
and 2 remains completely functional at a misalignment
angle of up to 20 from normal. Also, the shoulder portion
ensures that the screw remains tight in the event that the
resilient sealing member is subject to the long term creep
or relaxation which might otherwise lead to a degree of
loosening of the clamped members.
The provision of the shoulder portion 23 provides
excellent clamping and pull-over resistance, on the basisS of tests which have been conducted.
The following general size ratios characterize
typical fasteners to which this invention may be applied,
although these ratios are not considered limiting in terms
of the scope of the invention. In what follows, the term
"nominal diameter" refers to the threaded shank of the
fastener.
(a) The ratio of the undersurface 20 (bearing
surface to the nominal diamete~ may be in the
range of about 1.7 to about 2.7.
(b) The ratio of the height of the shoulder
portion 23 to the nominal diameter would be
in the range of about 0.15 to 0.45.
tc) The ratio of the diameter of the shoulder
portion 23 to the diameter of the hole into
which the fastener would be driven is in the
range from about 1.1 to about 1.5. Normally,
members drilled in situ will have the same size
hole in all components to be clamped.
(d) The ratio of the washer thickness to the
thickness of the shoulder portion 23 would be
about l.Q6 to about 2.5, with a more typical
range being from about 1.2 to about 1.5.
7()1
As a further illustrative example, which is not
offered in any limiting sense, the following may be
considered t~pical dimensions for a ~" type AB self-sealing
tapping screw to which this invention would apply:
5 Undersurface 20 diameter (flange diameter of head) .550"
Diameter of cylindrical shoulder .275"
Height (axial dimension) of shoulder portion .075"
Washer thickness (maximum axial dimension) .110"
Outside diameter of washer .500"
10 Inside diameter of washer .225"
In regard to the material from which the washer
may be constructed, there are the following fundamental
requirements which should be met in order to achieve proper
function.
(a) There must be sufficient resilience to seal
the space between the fastener bearing surface (undersurface
20) and a clamped member (wall surface 28).
(b) There must be an appropriate relationship
between strength and coefficient of friction between the
fastener bearing surface (undersurface 20) and the washer,
or between the clamped member (wall surface 28) and the
washer, such that as the fastener head is rotated, generat-
ing frictional forces and compressive strain on the washer,
the washer material will be able to resist failure under
these resultant stresses and strains. The lower the coefficient
of friction between the washer and the member against which
it is sliding, the lower will be the resultant stresses
(shear stresses) in the material of the washer. Hence, for
a given material strength, a reduced coefficient ~f friction
will reduce the likelihood of failure. Conversely, for
a given coefficient of friction, the material strength
properties can be made sufficiently high that failure will
not occur.
For example, a neoprene rubber washer used in the present
application would have a high friction, in that it would grip
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the members well and resist sliding, resulting in very high
stresses and strains in the rubber, usually high enough
to fracture the rubber. On the other hand, polyethylene is
not significantly better in mechanical properties, but has
low friction and permits the members to slide easily, generat-
ing low shear stresses and strains in the polyethylene, and
hence usually avoiding failure. By contrast, a steel washer
has a high friction and resists sliding, frequently galling
the two surfaces, but at the same time it has sufficient
material strength that it can resist the stresses and hence
not fail. The problem with a steel washer is the lack of
a sufficiently high resiliency. Hence,
a steel washer would not provide the requisite seal.
Attention is now directed to Figure 3, which shows
the second embodiment of the washer of this invention.
The washer shown in Figure 3 at the numeral 40 is essentially
flat annular washer having a flat annular upper surface 42,
a flat annular lower surface 44, a cylindrical outside surface
45 and a cylindrical inside surface 46. The diameter of
the inside surface 46 is slightly less than the outside thread
diameter of the threaded shank 25, which will cause the washer
42 to undergo a slight mechanical interference with the threads
of the threaded shank 25, thus holding the washer 40 in place
on the threaded shank 25 prior to the time of driving or
turning of the screw 12. It is pointed out that the inner
diameter of the flat washer 40 does not need to be angled in
order to allow it to mount the shoulder portion 23 during
driving. It is found that, due ~o the resiliency of the
flat washer 40, it readily expands to accommodate the shoulder
portion 23. While an annular washer such as shown at 40 is
not as preferred as the tetragonal-section washer 14 seen
in Figures-~ and 2, it is nonetheless a functionally workable
embodiment, and considered to have definite advantages over
conventional sealing assemblies.
