SEALING NUT
The present invention relates to sealing nuts in general, and in particular to nuts
comprising a sealing ring.
Sealing nuts are used in the aeronautical industry to assemble structural
elements through which a bolt or equivalent component having an outer thread is
inserted, since said elements require an assembly that is impermeable to fluids and
vapors.
When structural elements that undergo mainly shearing stresses, traction
10 stresses or a combination of the two have to be assembled, bolts are used that have a
smooth cylindrical or conical shank portion having a length that allows said portion to
completely pass through a range of thicknesses of the elements to be assembled. The
length of the shank allows the assembly of thicknesses that vary between a minimum
thickness ("grip min") and a maximum thickness ("grip max"), the difference usually
15 being 1.6 mm (1/16 inch in the imperial system). The range of thicknesses is called
"grip capacity" in English.
The cylindrical or conical portion of the shank of the bolt used thus protrudes by
the length of said range when the bolt passes through elements having the minimum
thickness that the bolt can assemble. On the contrary, the cylindrical or conical portion
20 does not protrude when the bolt passes through elements having the maximum
thickness that the bolt can assemble.
Moreover, shear bolts generally comprise a shank having an outer diameter
greater than the maximum outer diameter of the thread.
To assemble the structural elements, it is therefore necessary to use, together
25 with a shear bolt or tension bolt, a nut 10 shown in figure 1, comprising a space 12
made in the bearing surface 14 intended to come in contact with a surface S of one of
the elements to be clamped. This space, made between the bearing surface 14 and
the first thread of the nut, is conventionally called a counterbore and can have various
shapes and dimensions. In the example given above, the counterbore can receive up
30 to 1.6 mm of the shank of the bolt that can protrude from the structure having a
minimum thickness.
In certain cases, these nuts must also be sealing nuts. Such a nut is shown in
figure 2. A deformable sealing ring 16, generally made of Teflon®, is positioned in a
bore 18 made in the counterbore 12. The ring 16 has an inner diameter greater than
35 the diameter of the inner thread and the diameter of the smooth shank in order to not
interfere with the bolt. The ring is mounted via interference in the bore 18 in order to
ensure that the ring is maintained in the nut during transportation.
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The sealing ring has a conical shape that generally protrudes from the bearing
surface of the nut Thus, when the nut is pressed against a structural element, the
sealing ring is both axially compressed and radially deformed towards the inside of the
counterbore and against the shank and/or the threads of the bolt, thus forming a
s sealed joint.
The applicant has noticed that in certain assembly configurations comprising a
bolt 20and a sealing nut 10 shown in figurej,the ring 16 does not deform entirely into
the counterbore 12 but partly deforms 22 into the space between the bearing surface
14 of the nut and the surfaceS of the element to be clamped. This can occur when the
10 bolt is installed in a structure having a minimum thickness and the entire shank length
that protrudes from the structure is located in the counterbore.
Such deformation must be completely avoided since it seriously compromises
the mechanical strength of the bolt/nut assembly. In fact, the presence of a lubricating
material between the bearing surface of the nut and the surface of the element to be
15 clamped affects the coefficient of friction between these surfaces. When the coefficient
of friction is reduced, the tension in the bolt increases beyond the acceptable threshold
for the bolt, which can cause the bolt to break.
The present invention relates to a sealing nut that prevents the sealing ring from
deforming into the space between the nut and the structure, regardless of the
20 assembly configuration, without reducing the strength, capacity or structural integrity of
the assembly or of the components of the assembly.
More specifically, the sealing nut comprises a nut and a deformable sealing ring,
the nut comprises an annular body extending in a direction of an axis of revolution, a
base comprising a counterbore, and a bore made in the base, and the bore comprises
25 an annular back wall having a length extending between a first inner radius and a
30
second radius. The sealing ring comprises an annular body and an end face suitable
for resting against the back wall, and the end face has a length extending between an
inner radius and an outer radius. The sealing nut is such that the ratio betwe.en the
length of the back wall of the bore and the length of the end face of the ring is between
20 and 45%.
Such a configuration allows the sealing ring to always be guided towards the
inside of the. counterbore, "regardless·· of the. configuration of·the assembly, without
deforming beyond the counterbore or out of the bore.
Moreover, the nut according to the invention can comprise one or more of the
35 following features:
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- The sealing nut comprises a bevel between the back wall of the bore and a
wall of the counterbore of the nut,
- the bevel makes an angle of between 45• and so• with the axis of revolution,
- the back wall and the end face have complementary shapes,
- the back wall and the end face each comprise a portion positioned
substantially perpendicular to the axis of revolution,
- a wall of the bore comprises a groove extending radially
- inside the base, and an outer wall of the ring comprises a protrusion having a
shape complementary to the shape of the groove,
- the ring has an outer radius greater than an inner radius of the bore,
- the ratio between the outer radius of the ring and the inner radius of the bore
is between 1.010 and 1.016,
- the volume of the ring compared to the available volume of the nut is between
70 and 85%, said available volume being the smallest hollow volume inside the nut
15 between a bearing surface and a first thread of the nut, at an end of the inner thread
located next to the base, once a bolt has been placed in the nut and when the bolt
takes up a maximum volume.
The invention will be better understood after reading the description that follows,
in combination with the drawings that illustrate examples of embodiments of the
20 invention.
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Figure 1 (already described) is a cross-section of a nut of the prior art having a
counterbore,
Figure 2 (already de~cribed) is a cross-section of a sealing nut of the prior art,
Figure 3 (already described) is a partial cross-section of an assembly comprising
a bolt and a sealing nut of the prior art,
• Figure 4 is a side view of a sealing nut according to an embodiment of the
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invention,
Figure 5 is an exploded cross-sectional view of the sealing nut of figure 4,
Figure 6 is a cross-section of a sealing nut according to a second embodiment,
Figure 7 is a partial cross-section of an assembly comprising a bolt and a sealing
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nut according to an embodiment of the invention,
Figure 8 is a cross-section of a sealing nut according to a third embodiment.
A sealing nut 100, shown in figures 4 and 5, comprises a nut 101 and a sealing
35 ring 130. The nut 101 extends in the direction of an axis of revolution A of said nut
4
and comprises a wrench grip 102 and a base 104. The wrench grip 102 is a hexagon
here. The base, having an outer diameter wider than the largest dimension of the
hexagon, has an upper surface 106, in the shape of a truncated cone, comprising six
faces, recessed with respect to this surface in the shape of a truncated cone, like the
5 faces described in the patent application FR 2937386 from the applicant. Each
recessed face is aligned with a flat surface of the hexagonal wrench grip.
. More sp~dfi~ally, figure'5'sfiows that the base 104 comprises a bearing surface
1 08 opposite the surface having the shape of a truncated cone, and a counterbore
11 0 having a cylindrical wall 112 and an end wall 114 connecting the cylindrical wall
10 112 and the first thread of the inner thread 116. In this example, the end wall114 is a
surface in the shape of a truncated cone.
The thread 116 extends on the inner surface of the nut, between the end of the
end wall 114 and the upper end 118 of the nut, opposite to the bearing surface 1 08.
The base 1 04 also comprises a cylindrical bore 120 made in the counterbore
15 110, opening onto the bearing surface 108 and having a first inner radius R1 greater
than the radius of the counterbore 11 0. The bore 120 comprises a cylindrical wall 122
and a flat back wall 124. In this example, the back wall is substantially perpendicular to
the axis A. By substantially, we mean that the angle can vary by a few degrees.
A bevel 126 connects the back wall 124 of the bore 120 to the cylindrical wall
20 112 of the counterbore 11 0.
25
30,
35
The back wall 124 is an annular surface A 1 extending between the first inner
radius R 1 of the cylindrical wall 122 and a second inner radius r1 defined by the
distance between the axis A and the distance at which the bevel 126 intersects the
endwall124.
The annular surface A 1 has the value
A1 = (R1 2
- r1 2
). 1T
In two dimensions, the back wall 124 has a length L 1 defined by the difference
between radii (R1-r1).
The bore 120 is intended to receive a sealing ring 130. A bevel 128 is made
around the open contour of the bore in order to guide the insertion of the ring into the
bore.·
The ring 130 is an annular ring having the axis of revolution A and comprising an
upper end face 132 intended to rest against the back wall 124 of the bore 120.
Preferably, the upper end face comprises at least one surface having a shape
complementary to the shape of the back wall 124 of the nut. In the example illustrated
5
in figure 3, the upper end surface 132 is entirely flat and substantially perpendicular to
the axis A.
The sealing ring 130 has a hole 134, the diameter of which, equal to twice the
inner radius r2 indicated in figure 3, is greater than the diameter of the shank of the
5 bolt in order to not interfere with the threads and the shank of the bolt onto which the
nut will be screwed. The ring may enter into contact with the shank of the bolt if a bolt
10
· having a largerdiameter, called oversize bolt, is used during a maintenance operation
instead of a bolt having a nominal diameter, but the function of the ring is not to lock
the bolt and/or limit the captiveness of the nut.
A lower end face 136 of the sealing ring, opposite to the upper end face 132,
has the shape of a truncated cone, and the wall of the hole 134 is longer in the axial
direction A than the outer wall 138 intended to enter into contact with the cylindrical
wall122 of the bore.
The outer radius R2 of the ring 130 is greater than the first inner radius R1 of the
15 bore, and thus the ring must be inserted into the bore by force. The interference
created allows the ring to be maintained in the bore, in particular during transportation
of the ring, storage thereof and the installation of the nut. Preferably, the interference
ratio between the outer radius R2 of the ring and the radius R 1 of the bore is between
1.010 and 1.016. The greater the nominal diameter - the diameter measured at the
20 root of the threads- of the nut, the smaller the interference ratio.
The bearing surface 132 of the ring is also an annular surface A2 extending
between the outer radius R2 and the inner radius r2 and having the value
A2 = (R22
- r22
). 11
In two dimensions, the bearing surface 132 of the ring has a length L2 defined
25 by the difference between radii (R2-r2).
30,
In order to prevent any unwanted deformation of the ring, the ratio between the
bearing lengths L 1 and L2 must be greater than or equal to 20% and less than or
equal to 45%, i.e.:
20% _::: L 1 I L2 _::: 45%
This ratio defines the contact length ratio between the upper end face 132 of the
ring 130 and the back wall 124 of the nut 101 in a plane substantially perpendicular to
the axis A.
If the contact length ratio between the ring and the nut is less than 20%, there is
35 a risk that during installation, the ring may deform into the counterbore 110 and into
the threads of the inner thread 116: the deformation of the ring during insertion into the
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nut is not controlled. If material from the ring is in the threads, the coefficient of friction
between the threads of the bolt and the threads of the nut will be reduced, and the
tension in the bolt could rise well above the acceptable limit, which could cause the
bolt to break.
If the contact length ratio between the ring and the nut is greater than 45%, there
is a risk that the ring 130 may deform into the space between the bearing surface 108
· of the nut and the surface of the structure when the nut 130 is installed on a bolt. The
risk is also that of causing uncontrolled tension in the bolt because of the reduction of
the coefficient of friction between the contact surfaces.
Table 1 below gives examples, by diameter, of the ratio between the bearing
lengths L 1 and L2 for a range of nuts with various diameters tested by the applicant:
Table 1
nominal nut L1 (mm) L2 (mm) Ratio
5 0.22 1.06 21%
6 0.24 1.09 22%
7 0.31 1.10 28%
8 0.35 0.96 36%
10 0.37 1.11 33%
12 0.6 1.48 41%
14 0.63 1.66 38%
16 0.41 1.96 21%
As previously described, a bevel126 connects the back wall124 of the bore 120
to the cylindrical wall 112 of the counterbore 110. Here, the bevel 126 allows the
material to be guided towards the counterbore 110. It increases the space available in
the counterbore without removing too much material from the base, in order to
guarantee the )ntegrity of the base when the base undergoes compression.
20 The bevel 126 can have various angles a, measured with respect to the axis of
rey()lution A. depending Of1 the nominal. diar(l~~erpf Jpe nut. Table 4.. indicates the
preferred angle of the bevel 126 for each nut diameter.
7
Table 2
nominal nut 0 angie a (0
)
5 60
6 45
7 45
8 45
10 45
12 45
14 45
16 60
In certatn configurations, the counterbore 110 can be totally cone-shaped: as
shown in figure 6, the end wall114 of the counterbore and the bevel126 are combined
5 into a single wall having a single angle. In the example of figure 6, the angle a with
respect to the axis of revolution A is 60°.
The applicant carried out multiple test assemblies and simulated, using finite
elements in two dimensions, these same assemblies, each comprising a bolt having
the maximum allowable shank diameter, a structure having a minimum thickness, a
10 nut having the minimum allowable volume of the counterbore, and a ring, all the
dimensions of which are at the maximum tolerance.
In the first assembly of the prior art, shown in figure 3, the nut is a nut of the prior
art, with a contact length ratio between 78 and 96%. In a second assembly, shown in
figure 7, the nut comprises a contact length ratio between 20 and 45%. The bolts 20
15 are identical in the two assemblies having the same structural thicknesses.
20
As previously indicated, in the assembly in figure 3, the material of the ring 16
does not deform entirely into the counterbore 12, but partly deforms 22 into the space
between the bearing surface 14 of the nut and the surface S of the element to be
clamped.
In the assembly of figure 7 comprising the nut of figure 5, no material of the ring
has deformedjnto the space between the nut and tbe structure, and .there is no space .
between the wall of the counterbore and the shank of the screw. The impermeability is
therefore better, and the risk of exceeding the UTS of the bolt is eliminated.
Preferably, a volume of the ring 130 formed by the volume of the material of the
25 ring 130, when compared to the available volume of the nut 101, is in a range between
70 and 85%. The available volume in question here is the smallest hollow volume
8
inside the nut 101 between the bearing surface 108 and the first thread of the inner
thread 116 once the bolt has been placed in the nut, when the bolt takes up a
maximum volume. By convention, the maximum volume of the bolt in the available
space is calculated as a height of the counterbore 11 0 between the bearing surface
5 108 and the first thread of the inner thread 116 on the end next to said counterbore,
multiplied by a surface area of a cross-section of the smooth shank of the bolt
perpendicular to the axis of revolution A. Below 70%, the volume of the ring is too
small with respect to the volume of the counterbore, and the nut can no longer be
impermeable. Above 8S%, there is a risk that the ring may deform out of the
10 counterbore towards the threads or under the base and compromise the integrity of
the nut during installation.
The nut is preferably made of a titanium alloy, and the ring is, for example, made
of PTFE (also sold under the brand name TeflonTM by the company Dupont de
Nemours), in order to keep the nut light. Of course, the nut and the ring can be made
15 from another material if the weight is not a major criterion.
The wrench grip comprises, in a known manner, a locking means, such as an
elliptical deformation or a three-point deformation of the inner thread, made on an
outer surface of the nut near the upper end 118. If the nut is made from a titanium
alloy, the locking will preferably be carried out according to the method described in
20 the patent FR2947S97 from the applicant.
The invention is not limited to the only example described above. Thus, the nut
can comprise a base in the shape of a truncated cone, without a hollow with respect to
the upper surface in the shape of a truncated cone.
The wall of the bore 120 can be conical, with an angle 13 between 2° and so
25 between the wall 122 and the axis A of the nut 101 and opening from the bearing
surface 108 towards the back wall 124, in order to improve the retention of the ring
130 in the bore, in particular at low temperature (figure 8). The ring 130 itself has the
same angle 13 between 2o and so between the outer wall 138 and the axis A. Beyond
an angle of so, the ring can no longer be inserted into the conical bore 120. Below 2°,
30 the difference in angle iS not sufficient to improve the retention of the ring with respect
to a cylindrical wall. The interference ratio between an outer radius R2 of the ring and
··a ·radius·R1 oHhe bore, said radii being positioned at the same· axial dislance·from the
back wall 124 and the upper end surface 132, respectively, is also between 1.010 and
1.016.
35 A surface other than the bevel 126 can be used in order to connect the end wall
124 of the bore and the cylindrical wall 112 of the counterbore, for example a rounded
or multi-rounded surface.
9
Likewise, the back wall of the counterbore can be conical or can comprise, from
the cylindrical wall of the bore, a conical portion and then a portion perpendicular to
the axis of revolution A. In these cases, the end face of the ring that is in contact with
the back wall will, respectively, be conical or comprise a conical portion
5 complementary to the portion of the back wall and a portion perpendicular to the axis
of revolution. The measurements of the lengths L 1 and L2 can be either carried out in
the angular direction or projected onto a direction substantially perpendicular to the
axis of revolution. Since the two surfaces have the same angle with respect to the axis
of revolution, the ratio of the two projected lengths will be equal to the ratio of the
10 lengths measured in the angular direction.
In another variant, the bore can comprise a groove extending radially inside the
wall122 of the bore- whether the wall is cylindrical or conical- and having a radius
greater than the radius R1 of the bore. The ring can comprise a protrusion extending
radially outside of the wall 138 and having a shape and dimensions complementary to
15 the shape and dimensions of the groove, that is to say, that allow said protrusion to be
inserted into said groove. The groove/protrusion assembly allows the retention of the
ring in the nut to be improved. In this case, the additional dimensions of the grooves
and protrusions are not taken into account for the measurements of the lengths L 1 and
L2. The grooves and protrusions can be circular or extend only partly around the bore
20 and the ring .

CLAIMS
1. A sealing nut (100) comprising a nut (101) and a deformable sealing ring
(130), wherein the nut comprises an annular body extending in a direction of an axis of
5 revolution (A), a base (104) comprising a counterbore (110), and a second
counterbore (120) made in said base, the second counterbore (120) comprises an
annular back wall (124) having a length extending between a first inner radius (R1)
and a second inner radius (r1 ), the sealing ring (130) comprises an annular body and
an upper end face (132) suitable for resting against the back wall (124) of the second
10 counterbore, and said upper end face has a length extending between an inner radius
(r2) and an outer radius (R2), characterized in that the ratio between the length (L 1) of
the back wall ( 124) of the second counterbore and the length (L2) of the upper end
face (132) of the ring is between 20 and 45%.
15 2. A sealing nut according to claim 1, wherein the nut (1 00) comprises a bevel
(126) between the back wall (124) of the second counterbore and a wall of the
counterbore (11 0) of said nut.
3. A sealing nut according to claim 2, wherein the bevel (126) makes an angle
20 (a) between 45° and 60° with respect to the axis of revolution (A).
25
30
4. A sealing nut according to one of the claims 1 to 3, such that the back wall
(124) of the second counterbore and the upper end face (132) of the ring have
complementary shapes.
5. A sealing nut according to claim 4, such that the back wall (124) and the
upper end face (132) each comprise a portion positioned substantially perpendicular to
the axis of revolution {A).
6. A sealing nut according to one of the claims 1 to 5, such that the ring (130)
has an outer radius (R2) greater than a first inner radius (R1) of the bore.
7. A sealing nut according to claim 6, such that a ratio between the outer radius
(R2) of the ring and the first inner radius (R 1) of the second counterbore is between
35 1.010and1.016.
'------------ ---------- ~~-
5
11
8. A sealing nut according to one of the claims 1 to 7, such that a wall (122) of
the second counterbore (120) comprises a groove extending radially inside the base
(104), and an outer wall (138) of the ring (130) comprises a protrusion having a shape
complementary to the shape of said groove.
9. A sealing nut according to one of the claims 1 to 8, such that a volume of the
ring (130) is between 70 and 85% of an available voli.une of the nut (101), said
available volume being the smallest hollow volume inside the nut between a bearing
surface (108) and a first thread of said nut, at an end of an inner thread (116) located
10 next to the base (1 04), once a bolt has been placed in the nut and when the bolt takes
up a maximum volume.