This invention concerns a fastener that is installed through structures
on one side of the assembly only, commonly referred to as the "accessible"
side. This type of fastener is used, for example, in the assembly of aircraft
structures.
In particular, this invention concerns a fastener used to attach
structural components that have aligned perforations with a nominal diameter
(D) and that have a nominal thickness that varies between a minimum
thickness and a maximum thickness, with the fastener comprising a minimum
grip plane (Gmin) and a maximum grip plane (Gmax), and
10 - A screw comprising an enlarged head at one end and a threaded
portion at an opposite end;
- A sleeve comprising an enlarged head capable of accommodating
the head of the screw and designed to come into contact with the first side of
the structural components, a tubular body and a tapped portion that engages
15 the threaded portion of the screw, where the tapped portion is adjacent to an
internally smooth portion of the body of the sleeve. This smooth portion has a
nominal thickness (E), a non-deformable area adjacent to the head of the
sleeve and a deformable area adjacent to the non-deformable area;
- The resistance of the deformable area of the sleeve is reduced
20 compared with the resistance of the non-deformable area, in order to
facilitate the radial deformation of the deformable portion into a bulb intended
to come into contact with one side of the structural components opposite the
first side, commonly called the "blind" side.
Also known as a "blind rivet", this type of fastener is described, for
25 example, in the US 3236143 document, and the fastener also includes a
frangible gripping device in a shear groove.
Aircraft structures increasingly include composite materials that
present a risk of delamination when they are locally subjected to significant
compressive stress. In order to avoid delaminating the composite, the
30 external diameter of the bulb of a blind fastener should not be too small
compared with the nominal diameter (D) of the perforation into which the
fastener is inserted. Moreover, this type of fastener's bulb should have a
2
uniform span and must be formed repetitively, both at the minimum structural
thickness and the maximum structural thickness. When it is not possible to
inspect the blind side of the structure, it is mandatory to ensure that a bulb is
properly formed and sufficiently large.
5 The purpose of the invention is to eliminate the disadvantages of
earlier fasteners, and in particular to provide a fastener that enables the
formation of a bulb with an external diameter equal to one and a half times
the nominal diameter of the perforation of the structure and whose form is
uniform and repeatable, therefore stable, regardless of the structural
10 thickness to be tightened, with the given nominal thickness varying between
a minimum thickness and a maximum thickness.
For this, the fastener according to the invention is of the
aforementioned type, such that the deformable area extends over a length
that is greater than a minimum length (Lmin) and less than a maximum
15 length (Lmax), where the minimum and maximum lengths are defined by the
following relationships:
Lmin = D/2 + 2E + (Gmax- Gmin),
Lmax = (E I 0.092)
{1}
{2}
where the beginning of the deformable area is at most placed in the
20 fastener's minimum grip plane (Gmin).
This type of fastener offers the advantage of forming wide, stable
bulbs.
Ideally, the fastener according to the invention also has at least one of
the following characteristics:
25 The reduction in the resistance of the deformable area 1s achieved
through a local reduction in hardness;
The reduction in the resistance of the deformable area is achieved
through a reduction in the nominal thickness of the tubular body;
The sleeve is made of A286-type stainless steel or a Beta-C type titanium
30 alloy;
The deformable area has a hardness less than or equal to 300 HV;
The deformable area has a hardness less than or equal to 220 HV;
3
The fastener has a diameter of 6.32 mm, a tubular body nominal
thickness of 0.75 mm and a deformable area whose length is between
6.67 mm and 8.152 mm;
The fastener has a diameter of 4.80 mm, a tubular body nominal
5 thickness of 0.58 mm and a deformable area whose length is between
5.55 mm and 6.304 mm;
The sleeve includes titanium or a titanium alloy.
Other purposes, characteristics and advantages of the invention will
appear in the description of examples of embodiments of the invention, with
10 descriptions made in connection with drawings in which:
• Figure 1 is an isometric view of a fastener according to one
embodiment of the invention, in an uninstalled state;
• Figure 2 is a cross-sectional view of a fastener according to one
embodiment of the invention, in an uninstalled state;
15 • Figure 3 is a cross-sectional view of a fastener according to one
embodiment of the invention, in an installed state and forming a bulb;
• Figures 4A, 4B, and 4C are views of incorrectly formed bulbs;
• Figure 5 is a graphical representation of the hardness gradient
according to the length of the fastener's sleeve according to one
20 embodiment of the invention.
In order to make the drawings eas1er to understand, only those
elements required to understand the invention are shown. The same
references are used for the same elements in all drawings. The dimensions
given in the rest of the description are considered nominal. A tolerance, of
25 0.1 mm for example, can be applied to all or some of these dimensions, in
accordance with standard mechanical design practices.
With reference to figures 1 to 3, a fastener (1 0) according to one
embodiment of the invention comprises a screw (12) and a sleeve (30). The
screw (12) comprises a gripping device (14), a shear groove (16), a
30 countersunk head (18), a cylindrical shaft (20) and a threaded portion (22).
4
The shear groove (16) is sized in such a way as to have the smallest
screw (12) diameter capable of supporting a given installation tensile stress
and that breaks under a given torsional stress.
The screw (12) is inserted with slight clearance into the sleeve (30),
5 which comprises an enlarged flange (32) capable of accommodating the
countersunk head (18) of the screw and a tubular shaft (34). Before
installation of the fastener (1 0) in a structure, the external surface of the
tubular portion (34) is cylindrical.
At one end opposite the flange, the tubular shaft (34) has a tapped
10 portion (36), and between the flange and the tapped portion, a portion whose
internal surface (38) is cylindrical and smooth, in other words, not tapped. In
the example shown in figure 2, the nominal wall thickness (E) of the tubular
shaft is constant. The threading of the screw (12) and the tapping of the
sleeve (30) are complementary. For example, these threads comply with
15 standard AS8879, commonly used for aeronautical fasteners.
The length of the fastener (10) depends on the structural thickness to
be assembled, whose nominal thickness varies between a minimum
thickness and a maximum thickness. The nominal structural thickness
inteNal conventionally varies in steps of 1/16" (1.5875 mm). The fastener
20 (1 0) therefore has a minimum grip capacity and a maximum grip capacity that
enable the assembly of a nominal structural thickness that varies between a
minimum and a maximum. The plane corresponding to the minimum
thickness that the fastener can tighten is called "min grip" or "minimum
tightening plane" and is referred to as "Gmin" in the figures. The plane
25 corresponding to the maximum thickness that the fastener can tighten is
called "max grip" or "maximum tightening plane" and is referred to as "Gmax"
in the figures. On the fastener, the Gmax and Gmin distances are measured
from the end of the flange (32) when the head is countersunk (figure 2), and
from underneath the flange when the head is a protruding head.
30 The total length of the sleeve (30) is divided into three successive and
adjacent areas. An initial area A (figure 2) comprises the flange (32) and an
adjacent tubular shaft portion (34) with a smooth internal surface. Area A is
5
non-deformable. It extends over a length at most equal to the m1n1mum
structural thickness (Gmin) that the fastener can assemble.
A second area B, referred to as the deformable area, extends over the
remainder of the tubular shaft (34) that has a smooth internal surface. Area 8
5 of the sleeve is intended to be deformed to form a bulb, which will bear on
the blind side of the structures to be assembled. In order to facilitate the
formation of the bulb, area 8 must have reduced resistance compared with
the resistance of the non-deformable area. This reduction can be achieved
by reducing the hardness on deformable area B. In this case, the hardness of
1 0 deformable area B must be at least 20% lower than the hardness of nondeformable
area A, so that deformation ideally takes place in deformable
area 8, with this difference depending on the material actually used. The
reduction in hardness can be achieved through local annular annealing, for
example by means of an induction machine.
15 The sleeve may also be formed of various materials of different
hardnesses, welded together: deformable area B may be made of a material
that is more deformable than that of non-deformable area A.
The reduction in resistance can also be achieved through the local
reduction of the nominal thickness of the sleeve, for example by means of a
20 shoulder on a portion of the smooth internal surface of the sleeve, decreasing
the thickness of the sleeve along the length of this area.
When the fastener (1 0) is not installed, area 8 of the sleeve covers the
remaining portion of the smooth shaft (22) of the screw and a threading
portion (24).
25 The third sleeve area C extends over the entire tapped portion (36).
This area acts as a nut. When the fastener (10) is not installed, this area Cis
in contact with a terminal threading portion (22) of the screw (12).
Figure 3 shows the fastener (10) from figures 1 and 2 installed in two
structures (40, 42) to be assembled. The gripping device (14) has been
30 broken in the shear groove (16), so that only the head (18) of the screw and
the flange (32) of the sleeve remain, together forming the head of the
fastener, embedded in a countersink produced earlier in an accessible side
6
(44) of the structure (40). The first area A of the sleeve (30) is entirely
embedded in the structures (40, 42). The second area. B of the sleeve is
deformed and comprises a bulb (48), one side of which is in contact with the
blind side (46) of the structure (42), opposite the accessible side (44). The
5 tensile stress between the head of the fastener (20, 32) and the bulb (48)
makes it possible to hold the assembled structures (40, 42) in place. In the
installed position, the third sleeve area C covers a threading portion (24) of
the screw adjacent to the shaft (22). The fastener has the advantage of
including a mechanical or chemical braking component, in order to ensure
10 that the engaged threads do not come loose, for example by deforming the
tapped threads of the sleeve, or by adding a locking product to the threads of
the screw.
The fastener (1 0) according to the invention has an external diameter
that can be inserted with clearance into the perforation of a structure with a
15 nominal diameter D. Once the fastener (10) has been deformed, the bulb
(48) has an external diameter that is at least equal to one and a half times
the nominal diameter D of the perforation over the entire tightening range. A
correctly formed bulb is shown in figure 3. When the bulb is incorrectly
formed, the risk of matting and/or the risk of delamination increases. The bulb
20 can thus take the form of an "umbrella" (figures 4A, 4B) in which the bulb
forms an angle a with the blind side (46). The bearing surface is significantly
reduced, which leads to an increased risk of matting. Another incorrect form
is the formation of a double bulb (figure 4C), which produces a bearing
surface of an insufficient diameter on the blind side (46).
25 For example, the fastener (1 0) is installed using an installation tool that
initially tracts the gripping device (14) of the screw, while holding the sleeve
(30) in the structure by pressing the flange (32) against the structure (40).
The traction drives the threaded portion (22) of the screw and the
tapping (36) of the sleeve towards the blind side (46) of the structure. The
30 tubular shaft's deformation area B is deformed to create a bulb (48), one side
of which comes to bear against the blind side (46).
7
During a second stage, a rotation movement is imparted to the screw,
so that it is screwed into the sleeve (14) until the head (20) of the screw
bears on the flange (32) of the sleeve.
The last stage consists in finalizing the installation of the fastener (1 0),
5 by breaking the screw's gripping device (14). For this, the installation tool
continues to rotate in the same rotation direction, applying increasing stress
to the structure and the screw. The shear groove (16) is designed to break
beyond a certain torque generating a minimum level of stress in the structure.
The groove (16) therefore breaks once the torque threshold has been
1 0 reached, leaving the head (20) of the screw flush with the accessible surface
of the structure (40).
For example, the screw is made of Ti6AI4V titanium alloy, coated with
a layer of lubricant, and the sleeve is, for example, made of stainless steel
such as A286 passivated stainless steel. Deformation area B is produced
15 through local annealing. The resistance of this area is around 550 MPa, while
the resistance of the first and third areas A and C is around 1 ,200 MPa.
Other materials can be chosen for the screw and sleeves.
In order to form a correctly shaped bulb that reaches a diameter equal
to one and a half times the diameter of the perforation, the applicant has
20 established that the length (L) of deformable area 8 should be between a
minimum value and a maximum value defined by two relationships.
The applicant has thus established that in order to form a bulb equal to
one and a half times the diameter of the perforation for a given nominal
structural thickness, in the minimum to maximum thickness configurations,
25 the length (L) of deformable area 8 must be greater than a minimum length
(Lmin) according to the {1} relationship below:
Lmin = D/2 + 2E + (Gmax- Gmin), {1}
The applicant has also established that the shape of the bulb is correct
and reproducible if the length (L) of deformable area B is less than a
30 maximum length (Lmax) according to the {2} relationship below:
Lmax = (E I 0.092) {2}
8
In the {1} and {2} relationships, D is the nominal diameter of the
perforation in which the fastener (1 0) is intended to be inserted, E is the
nominal thickness of the wall of deformable area B of the tubular shaft (34) of
the sleeve (30) before deformation.
5 The value of 0.092 was established through testing. Below this value,
there is a proven risk of instability of the deformable area, which can lead to
the formation of "umbrella" bulbs or double bulbs, as shown in figures 4A and
48.
For example, a sleeve with a thickness of 0.625 mm and a length (L)
10 of deformable area B equal to 7.286 mm for a nominal diameter of 6.35 mm
deforms in the shape of an umbrella, as can be seen in figure 48. According
to relationships {1} and {2}, the length (L) of the deformable area must be
between Lmin = 6.408 mm and Lmax = 6.789 mm. A length of 7.286 mm
greater than the maximum length defined by the {2} relationship does not
15 therefore enable the formation of a proper bulb, whose entire surface bears
on the rear side of the structure.
In another example, a blind fastener marketed under the ERGOTECH
trademark, described in the US6868757 patent and comprising the
characteristics of the preamble of claim 1, with a thickness of 0.51 mm for a
20 nominal diameter of 6.35 mm and a length (L} of deformable area B equal to
4.11 mm, is deformed into a proper bulb but with a diameter equal to 1.30
times the nominal diameter of the perforation. The annealing length (L) is
less than the minimum length required, which, according to the {1}
relationship, is equal to 6.045 mm.
25 The deformable area B must begin at most in the fastener's minimum
grip plane (Gmin), in order to ensure that the bulb forms on a structure that
has a minimum thickness. Otherwise, the bulb will form at a distance from the
blind side of the structure and will not perform its structural component
tightening function.
30 Beyond the fastener's maximum grip plane (Gmax), the wall of the
sleeve must be sufficiently deformable over at least the distance (D/2 + 2E},
9
in order to form a bulb that is at least one and a half times the diameter of the
perforation.
When the length (L) of deformable area B does not comply with the {2}
relationship, the bulb no longer forms correctly. For example, it takes the
5 form of an umbrella, or forms a double bulb.
For example, a fastener with a diameter of 6.32 mm designed to be
inserted into a perforation with a nominal diameter D = 6.35 mm (8132") and
of "grip 8" type, that is to say designed to assemble structural components
with a nominal thickness of 12.70 mm (8116"), has a minimum grip capacity
10 (Gmin) of 10.914 mm and a maximum grip capacity (Gmax) of 12.898 mm. It
therefore has a tightening range 1.984 mm (1116" + 1164") greater than the
grip difference of 1116", in order to ensure the overlap between two
consecutive structural thickness ranges. The nominal thickness (E) of the
sleeve is selected to be equal to 0.75 mm, in particular to ensure a
15 compromise between the force required to deform the sleeve and a screw
diameter that is sufficient to maintain the tensile stress resulting from the
installation of the fastener in the structure. The length (L) of deformable area
B must therefore be
Greater than: Lmin = (6.35 I 2) + 2 x 0.75 + 1.984 = 6.67 mm
20 And less than: Lmax = (0.75 I 0.092) = 8.152 mm
In the case of a fastener ( 1 0) whose deformation area B has
undergone local annealing, the hardness must be sufficiently low from the
minimum grip plane (Gmin) over the length L, where L is chosen to be
between6.67 mm and 8.152 mm.
25 It should be noted that two fasteners ( 1 0) with the same external
diameter and the same sleeve thickness (E) but of different lengths, used to
assemble structures of different thicknesses, may both have the same length
(L) of deformable area B, since the dimensions used to establish the
minimum and maximum limits of this length depend only on the perforation
30 diameter (D) and on the sleeve thickness (E). The difference between these
two fasteners lies in the starting point of deformable area B, which will be at
10
most in the fastener's minimum tightening plane (Gmin), whose positioning
depends on the length of the fastener.
For example, for the previously described fastener (1 0) that has a
diameter of 8, table 1 indicates the positioning of the minimum tightening
5 plane (Gmin) and the Lmin and Lmax lengths of deformable area B, for
several examples of nominal thicknesses to be tightened:
Table 1
Nominal thickness to be grip plane Gmin Lengths Lmin - Lmax
tightened (mm) [1/16"] (mm) (mm)
6.35 [4] 4.564 6.67-8.152
7.93 [5] 6.152 6.67-8.152
9.52 [6] 7.739 6.67-8.152
12.70[8] 10.914 6.67-8.152
It is therefore possible for several fasteners of diameter 8 and of
1 0 different lengths to have an identical length (l) of deformable area B.
An example of a hardness gradient of a sleeve made of A286 is shown
in figure 5, with a length (L) of deformable area B selected at 6.68 mm from
the minimum tightening plane (Gmin) of the fastener (1 0).
The hardness graph in figure 5 comprises a "low" plateau in
15 deformable area B at around 155 HV and two "high" plateaus at around 405
HV in non-deformable areas A and C. The length of the low plateau can be
less than or equal to length L The level of hardness must be sufficiently low
over length L for area B to be effectively deformed. The applicant has
established that the deformable area B of an A286 stainless steel sleeve
20 should have a hardness 45% lower than the hardness of non-deformable
area A Deformable area 8 is produced, for example, through local
annealing, reducing the hardness of the A286 steel to around 220 HV. local
annealing results in transitional areas between the high plateau of nondeformable
area A and the low plateau of deformable area B, and between
25 the low plateau and the high plateau of area C. These slopes can be offset or
can have a different slope, provided that deformable area B has a length L
11
between the m1n1mum and max1mum values defined by the {1} and {2}
relationships.
The advantage of the A286 material is that local annealing enables a
significant reduction in the hardness of the material on the annealed area.
5 During the deformation of deformable area B, the material is subjected to
strain hardening and its hardness increases again. Consequently, the bulb
formed has sufficient resistance to oppose the shearing stress exerted on the
bearing side of the bulb against the blind side of the structure, when the
structural components are subjected to stress that tends to move them apart.
10 The starting points and slopes of the hardness values shown in figure
5 are also established by taking into account the tolerances due to the
annealing process, which may be in the order of mm.
Other materials can be used to form the sleeve. For example, these
may be one or more titanium grades, one or more titanium alloys or a
15 combination of titanium and titanium alloys welded to form the sleeve. The
advantage of titanium and titanium alloys lies essentially in their low density
and high resistance to galvanic corrosion, which makes it possible to install
them in various materials.
For example, the applicant carried out deformation tests on a sleeve
20 made of a Beta-C titanium, in other words, from the beta metastable family.
This alloy shows no phase transformation in the temperature range
immediately higher than solution treatment. It is for this reason that this alloy
was chosen. It was assumed that the possibility of using a high or very high
temperature for local annealing could sufficiently soften the material and
25 enable the deformation of the sleeve's deformable area. Thus, the applicant
found that an annealing temperature ranging from 850oc to 1, 150oC enabled
a deformable area with a lower hardness than that of the area that had not
been annealed to be obtained. On the non-annealed areas, the average
hardness value is 470 HV, while on the annealed deformable area, the
30 hardness value is around 300 HV, in other words, 130 to 150 HV more than
on an A286 sleeve. A Beta-C titanium alloy sleeve with a deformable area
whose hardness is less than or equal to 300 HV and whose length is
12
selected in the range defined by the {1} and {2} relationships forms a wide
bulb when the fastener is inserted into a structure that has a minimum
thickness that corresponds to the fastener's minimum tightening capacity.
In another example of embodiment, a fastener with a diameter of 4.80
5 mm designed to be inserted into a perforation with a nominal diameter D =
4.83 mm (6/32") to assemble a "grip 8" structure, that is to say, a structure
with a nominal thickness of 12.70 mm (8/16"), has a minimum grip capacity
(Gmin) of 10.914 mm and a maximum grip capacity (Gmax) of 12.898 mm. It
therefore has a tightening range of 1.984 mm (1/16" + 1/64"). The nominal
10 thickness (E) of the sleeve of a fastener of diameter 6 is 0.58 mm. The length
(L) of the deformable area B must therefore be
Greater than: lmin = (4.83/2) + 2 x 0.58 + 1.984 = 5.55 mm
And less than: Lmax = (0.58 I 0.092) = 6.304 mm
In the case of the fastener (10), the hardness of deformation area B
15 must be sufficiently low over a length L extending from the minimum
tightening plane (Gmin), where Lis chosen between 5.55 mm and 6.304 mm.
The fastener according to the invention is not structurally limited to the
examples described above. For example, the head of the fastener may be a
protruding head instead of a countersunk head. The gripping device may
20 have various forms, or may not exist. In this case, the fastener is installed, for
example, by screwing only the screw into the sleeve, while keeping the
sleeve stationary. The screw may not include a cylindrical shaft, and may
include only a threaded shaft from the lower part of the head.
The sleeve (14) may include two welded sleeve components made of
25 different materials, and compression grooves on the external surface of the
first component. Alternatively, the sleeve (14) rnay comprise a single sleeve
component and an annular groove on its external surface.
Similarly, the screw and sleeve materials may be different from those
described, and the thicknesses and hardness values of the various areas of
30 the sleeve should be adapted according to the materials used for the sleeve
and the screw.
13
As indicated above, the resistance of deformable area 8 can be
reduced by reducing the thickness of deformable area 8 over length L, where
length L must be between !he minimum and maximum values defined in the
{1} and {2} relationships, and where the thickness (E) of the wall is the
5 thickness of deformable area B.

CLAIMS
1. Fastener (1 0) to attach structural components (40, 42) that have
aligned perforations with a nominal diameter (D), and that have a nominal
5 thickness that varies between a minimum thickness and a maximum
thickness, with the fastener comprising a minimum grip plane (Gmin) and a
maximum grip plane (Gmax), and
10
- A screw ( 12) comprising an enlarged head ( 18) at one end and a
threaded portion (22) at the opposite end;
- A sleeve (30) comprising an enlarged head (32) capable of
accommodating the head (18) of the screw and intended to come into contact
with the first side of the structural components, a cylindrical tubular body (34)
capable of accommodating the cylindrical shaft (20) of the screw with
clearance and a tapped portion (36) that engages the threaded portion (22) of
15 the screw, where the tapped portion (36) is adjacent to an internally srnooth
portion (38) of the body (34) of the sleeve. This smooth portion has a nominal
thickness (E), a non-deformable area (A) adjacent to the head (32) of the
sleeve and a deformable area (B) adjacent to the non-deformable area (A);
- The deformable area (B) of the sleeve has reduced resistance
20 compared to that of the non-deformable area (A), in order to facilitate the
radial deformation of the deformable portion into a bulb (48) designed to come
into contact with one side (46) of the structural components opposite the first
side;
Characterized by the fact that the deformable area (B) extends across
25 a length .(L) .that is greater than a minimum length (Lmin) and less than a
maximum length (Lmax), where the minimum and maximum lengths (Lmin,
Lmax) are defined by the following relationships:
30
Lmin = D/2 + 2E + (Gmax- Gmin), {1}
Lmax = (E I 0.092) {2}
where the start of the deformable area (B) is at most placed in the
minimum grip plane (Gmin) of the fastener (10).
15
2. Fastener (10) according to claim 1, in which the reduction in the resistance of
the deformable area (B) is achieved through a local reduction in hardness.
3. Fastener (1 0) according to claim 1, in which the reduction in the resistance of
5 the deformable area (B) is achieved through a local reduction in the nominal
thickness (E) of the tubular body (34).
10
4. Fastener (10) according to claim 1 or 2, in which the deformable area (B) has
a hardness lower than or equal to 300 HV.
5. Fastener (10) according to claim 1 or 2, in which the deformable area has a
hardness lower than or equal to 220 HV.
6. Fastener (10) according to one of claims 1 to 4, in which the sleeve is made of
15 A286-type stainless steel or a titanium alloy.
20
7. Fastener (1 0) according to claim 5, with a diameter of 6.32 mm, a tubular body
(34) nominal thickness (E) of 0.75 mm and a deformable area (B) whose
length (L) is between 6.67 mm and 8.152 mm.
8. Fastener (1 0) according to claim 5, with a diameter of 4.80 mm, a tubular body
(34) nominal thickness (E) of 0.58 mm and a deformable area (B) whose
length (L) is between 5.55 mm and 6.304 mm.