Technical Field of the Invention

The present invention relates to a separation fastener assembly comprising a nut having a plurality of nut segments, a movable element which is movable from a first position, in which it restrains the nut segments from radial expansion, to a second position, in which it permits the nut segments to radially expand, a bolt having one end coupled to the nut, and a retaining element which is arranged to hold the nut in place.

Background Art

Separation fastener assemblies have been used for various

applications and are generally responsive to a signal for releasing secured components. One type of separation fastener includes a pyrotechnic charge that is actuated to release the components. A typical assembly may include a nut having a plurality of discrete segments. The discrete segments are arranged within the fastener assembly to normally retain a bolt. When the separation fastener assembly is pyrotechnically actuated, the discrete nut segments move outward in response to an axial load on the bolt. As a result, the bolt is released.

US 2010/0021265 discloses a separation fastener assembly. The assembly comprises a movable element. In a first position the movable element restrain nut segments from radial expansion. In a second position the nut segments are permitted to radially expand for releasing the fastener assembly.

With respect to previously known solutions, it is desirable to provide a more robust separation fastener arrangement.

Summary of the Invention

It is an object of the present invention to provide a separation fastener assembly that can be made robust in a cost efficient manner. This and other objects that will be apparent from the following summary and description are achieved by a separation fastener arrangement according to the appended claims.

According to an aspect of the present invention, there is provided a separation fastener assembly comprising a nut having a plurality of nut segments, a movable element which is movable from a first position, in which it restrains the nut segments from radial expansion, to a second position, in which it permits the nut segments to radially expand, a bolt having one end coupled to the nut, and a retaining element which is arranged to hold the nut in place, wherein the retaining element comprises an energy absorbing portion which is sleeve shaped and adapted to be permanently deformed by the movable element when the movable element is moved from the first position to the second position.

Such a separation fastener assembly may e.g. be used to connect a structural member of a vehicle to a chassis thereof in a releasably manner. As long as the separation fastener assembly is not activated the movable element is held in a first position in which it secures the connecting function of the fastener assembly. The separation fastener assembly is activated, e.g. by an initiator device, when there is a need to destroy the coupling function of a bolt joint system formed by the bolt and nut of the separation fastener assembly. As long as the separation fastener assembly is not activated the movable element is positioned in a position in which it restrains the nut segments. Upon activation of the separation fastener assembly the movable element is moved from this position to a position in which it no longer restrains the nut segments from radial expansion. The movable element may be moved by, e.g., a force generated by a pyrotechnical charge. During the movement towards the second position energy is absorbed by the energy absorbing portion. Hence, the retaining element combines the function of holding the nut in place and the function of load limiting the movable element when the arrangement is activated. By this combination a separation fastener assembly that can be made robust in a cost-efficient manner is achieved.

When the movable element is moved from the first position to a second position, parts of the separation fastener assembly, such as e.g. a housing accommodating the nut, may be subjected to high dynamical loads by the movable element. Such dynamical loads may be very high since a very rapid movement of the movable element is often desired and they may thus destroy e.g. a housing accommodating the nut and even separate parts from the separation fastener assembly. Often a pyrotechnical charge is used to provide a robust and rapid release of the bolt of a separation fastener assembly. Then parts of the separation fastener arrangement are subjected to high loads generated by the movement of the movable element following the detonation of the pyrotechnical charge.

The energy absorbing portion absorbs energy when deformed in a longitudinal direction thereof by the movable element. Hence, the energy absorbing portion provides for load limiting of the movable element, which has the advantage that loads to parts, such as a housing accommodating the nut, during separation of the fastener assembly may be significantly reduced.

The movable element is thus movable to the second position against a force from the energy absorbing portion. Since the energy absorbing portion is adapted to be permanently deformed, i.e. to absorb energy, the retaining element serves as a load limiter limiting the load on the housing

accommodating the movable element.

According to one embodiment adjacent nut segments are

interconnected at fracture lines, thereby forming a frangible nut.

Preferably, at least a sub portion of the energy absorbing portion is narrowing as seen in a direction from a first end to a second end thereof in order to absorb energy in an efficient manner.

Preferably, at least a sub portion of the energy absorbing portion has a progressively increasing cross section as seen in a direction from a second end to a first end thereof.

in order to further enhance the energy absorbing characteristics. A progressive load limiting of the movable element is thereby achieved. A progressive load limiting is advantageous since the risk of destroying the housing is thereby further reuced.

According to one embodiment at least a wall section of the energy absorbing portion is curvilinear with a radius of curvature R in the range between 3 mm and 30 mm as seen in the longitudinal direction of the retaining element.

According to one embodiment a sub portion of the energy absorbing portion is linearly narrowing as seen in a direction from a first end to a second end thereof. A sub portion of the energy absorbing portion may taper at an angle with respect to the longitudinal axis of the retaining element

corresponding to an angle of 1 -30 degrees. Preferably around 20 degrees.

According to one embodiment a first sub portion of the energy absorbing portion has a progressively increasing cross section as seen in a direction from a second end to a first end thereof and a second sub portion of the energy absorbing portion is linearly narrowing as seen in a direction from the first end to the second end thereof.

According to one embodiment the retaining element is arranged such that the second end of the energy absorbing portion is closest to the nut.

According to one embodiment the retaining element further comprises at least one tab projecting in a direction from a first end to a second end thereof, the projected tab being arranged to prevent the nut from rotational movement. This embodiment has the advantage that rotation of the nut is prevented during screwing and unscrewing of the bolt, thereby facilitate mounting of the separation fastener assembly.

According to one embodiment the separation fastener assembly further comprises an actuator for initiating movement of the movable element from the first position to the second position. Preferably, the actuator comprises a pyrotechnical charge and an igniter.

The igniter may be adapted to receive an ignition signal from a sensor system, such as an in-crash based sensor system or a pre-crash based sensor system.

The separation fastener assembly may be arranged in a motor vehicle for releasably connecting a structural member of the vehicle to the vehicle chassis.

These and other aspects of the invention will be apparent from and elucidated with reference to the claims and the embodiments described hereinafter. Brief Description of the Drawings

The invention will hereafter be described in more detail and with reference to the appended schematic drawings.

Fig. 1 is a side view of a part of motor vehicle structure.

Fig. 2 is a schematic cross sectional view illustrating a separation fastener assembly according to one embodiment.

Fig. 3a is a schematic cross sectional view of the separation fastener assembly shown in Fig. 2.

Fig. 3b is a schematic perspective side view of parts of the separation fastener assembly shown in Fig. 3a.

Fig. 4a-d illustrates the function of the separation fastener assembly shown in Fig. 2 and 3.

Fig. 5 is a schematic cross sectional view of a part of a separation fastener assembly according to a second embodiment.

Detailed Description of Preferred Embodiments of the Invention

Fig. 1 shows parts of a vehicle structure 1 which comprises a front subframe 2 and a chassis 3. A separation fastener assembly according to an embodiment of the present invention may be arranged for releasably connecting the rear end of the front subframe 2 to the chassis 3. The front subframe 2, which comprises longitudinally and transversally extending structural members, is an important member of the vehicle front end structure 1 a. The front end vehicle structure 1 a further comprises collapsible

longitudinal structural members 6 that are adapted to form a deformation zone 7 in which collision energy is absorbed during an impact event, such as in a collision with another vehicle or an object. Due to the deformation zone 7 the acceleration experienced by an occupant inside the passenger compartment 1 b during an impact event is reduced. Consequently, the risk of injuries to the occupant(s) is reduced. In optimizing the stiffness of the front end structure 1 a different load cases are considered.

Fig. 2 shows shematically a portion of a vehicle front subframe 2 of which a rear end 2a is connected to a vehicle chassis 3 by means of a separation fastener assembly 9 according to an embodiment of the present invention. The separation fastener assembly 9 comprises a housing 8, a threaded bolt 10 and a nut 1 1 , which is illustrated in Fig. 3, having internal threads. The nut 1 1 is disposed in a cavity of the housing 8. The bolt 10 extends through a sleeve 2b formed at the rear end 2a of the front subframe 2 and through an aperture in the chassis 3, and is threadably engaged with the nut 1 1 .

The separation fastener assembly 9 further comprises a pivotable means, in the form of a lever arm 12, which at a front end 13 thereof is mounted to the rear end 2a of the front subframe 2 and at a rear end 14 thereof is mounted to the chassis 3, for assisting release of the bolt 10 from the chassis 3 when the connecting function of the fastener assembly 9 is destroyed. The housing 8 is attached to the vehicle chassis 3 by means of bolt connections where bolts (not shown) are extending through holes in a bottom flange of the housing 9.

Now referring to Figs. 3a and 3b the separation fastener assembly 9 will be further described. Fig. 3 is cross sectional side view schematically illustrating the separation fastener assembly 9. As illustrated in Fig. 3a the separation fastener assembly 9 comprises a housing 8, a nut 1 1 , a threaded bolt 10, a movable element 17, a retaining element 19 and a lid 21 . The bolt

10 and the nut 1 1 together form a bolt joint system 15.

The nut 1 1 may include a plurality of nut segments connected to one another through fracture lines such that the nut 1 1 is a frangible nut. For other applications within the scope of the present teachings, however, the plurality of segments may alternatively be a plurality of discrete segments. In this embodiment the nut 1 1 includes four nut segments 1 1 a, 1 1 b, 1 1 c, 1 1 d, as best illustrated in Fig. 3b. Adjacent nut segments of the nut 1 1 are

interconnected at fracture lines and the nut segments 1 1 a, 1 1 b, 1 1 c, 1 1 d, together form a frangible nut 1 1 . The frangible nut 1 1 may be manufactured from a powdered metal process. The powdered metal process may yield a nut

1 1 having high compression strength an low elongation values, resulting in a component having brittle characteristics. Alternatively, the nut 1 1 may be manufactured with any other suitable material capable of functioning as described herein.

The movable element 17, which is formed by a collar, is arranged around the nut 1 1 , as best illustrated in Fig. 3a. The movable element 17 includes a portion 23 which normally abuts a corresponding portion of the nut 1 1 such that the movable element 17 restrains the segments 1 1 a, 1 1 b, 1 1 c, 1 1 d of the nut 1 1 from radial expansion. The abutting portion 23 includes a protrusion, in the form of a circumferential protrusion, extending radially inward from the side wall of the movable element 17. The nut 1 1 includes a portion 25 which normally abuts a corresponding portion of the movable element 17. The abutting portion 25 includes a protrusion, in the form of a circumferential protrusion, extending radially outward from the side wall of the nut 1 1 . A circumferential channel 22 is formed between a flange portion 27 of the movable element 17 and the housing 8, as illustrated in Fig. 3a. The movable element 17 is movable from a first position, in which it restrains the nut segments 1 1 a, 1 1 b, 1 1 c, 1 1 d from radial expansion, to a second position, in which it permits the nut segments 1 1 a, 1 1 b, 1 1 c, 1 1 d to radially expand. Fig. 3a illustrates the separation fastener assembly 9 when the nut segments 1 1 are prevented from radial expansion by the movable element 17, i.e. when the movable element 17 is positioned in the first position. The movable element 17 is arranged to hold the nut 1 1 in place to secure the function of the bolt joint system 15 under normal conditions.

The retaining element 19 comprises a flat base portion 18 and a sleeve shaped energy absorbing portion 20. Each of the flat base portion 18 and the energy absorbing portion 20 is formed as an integral part of the retaining element 19. The energy absorbing portion 20 is narrowing as seen in a direction from a first end 19a to a second end 19b thereof. The energy absorbing portion 20 thus has a narrowing cross section as seen in a direction from the first end 19a to the second end 19b thereof, i.e. in the longitudinal direction A of the retaining element 19. Hence, the cross section of the energy absorbing portion 20 narrows from a large cross section at the first 19a end to a small cross section at the second end 19b, as best illustrated in Fig. 3a. The retaining element 19 is arranged such that the second end 19b, i.e. the end having the smaller cross section, is closest to the nut 1 1 and thus faces the nut 1 1 .

The sleeve shaped energy absorbing portion 20 may have a

progressively increasing cross section as seen in a direction from the second end 19b to the first end 19a of the retaining element 19. Preferably, at least a portion 20a of the wall of the sleeve shaped energy absorbing portion 20, as seen in a cross section, is curvilinear with a radius of curvature R in the range of 3- 30mm. In one embodiment a part of the wall of the energy absorbing portion 20, as seen in a cross section, is curvilinear with a radius of curvature corresponding to 8mm.

Alternatively, or additionally, a portion of the wall of the sleeve shaped energy absorbing portion 20b may have a cross section which tapers at an angle, as seen with respect to an axis parallel to the longitudinal axis LA of the retaining element 19, such as e.g. an angle, corresponding to an angle in the range between 1 -30 degrees.

In the embodiment shown in Fig.3a the energy absorbing portion 20 comprises a first sub portion 20a having a cross section which is curvilinear with a radius of curvature corresponding to R8 and a second sub portion 20b having a cross section which tapers at an angle, as seen with respect to an axis parallel to the longitudinal axis LA of the retaining element 19,

corresponding to an angle of 20 degrees, as illustrated in Fig. 3a.

The energy absorbing portion 20 is arranged to be permanently deformed in its longitudinal direction by the movable element when the movable element is moved from the first position to the second position, thereby limiting the load on the housing 8. To this end the energy absorbing portion 20 is formed from a plastically deformable material, such as e.g. steel. The movable element 17 is thus movable from the first position to a second position against a force from the energy absorbing portion 20. A portion of the energy absorbing portion 20 has a progressively increasing cross section as seen in the longitudinal direction thereof, i.e. in a direction from the second end 19b to the first end 19a.

The energy absorbing portion 20 may have a uniform thickness.

Preferably the energy absorbing portion 20 has a uniform thickness of 0.5 - 2 mm. In the embodiment described hereinbefore the energy absorbing portion 20 has a uniform thickness of 1 mm.

The retaining element 19 comprises four downwardly extending tabs 29, of which two are visible in Fig. 3a. The tabs 29 are received in recesses 31 of the nut 1 1 and prevents the nut 1 1 from being rotated with respect to the housing, e.g. when the bolt 10 is tightened.

The retaining element 19 need to be attached to the housing 8 in a rather robust manner in order to withstand the forces to which it is subjected during deformation of the energy absorbing portion 20 upon movement of the movable element to the second position. In this embodiment the retaining element 19 is attached to the housing 8 at a number of riveting joints (not shown) along the circumference of the flat base portion of the retaining element 19 by means of radial riveting.

Upon assembly of the separation fastener assembly, a base of the housing 8 reacts the torque of the nut 1 1 and bolt 10. For certain applications, a low level of torque may be introduced during assembly to fracture the nut 1 1 into the discrete segments 1 1 a, 1 1 b, 1 1 c, 1 1 d. In this manner, the nut 1 1 may be pre-broken prior to activation of the assembly 9. Alternatively, the nut 1 1 may be fractured into the discrete segments 1 1 a, 1 1 b, 1 1 c, 1 1 d during activation.

It is understood that in a crash situation it may be required to rapidly release a bolt 10 from a nut 1 1 . The movement of the movable element 17 to the second position thus need to be carried out rapidly. To this end, an initiator device comprising a pyrotechnical charge may be used to achieve a rapid movement of the movable element in case a release of the connection established by the bolt joint system 15 is desired.

In this embodiment the separation fastener assembly 9 comprises an initiator device, in the form of a pyrotechnical squib 33, of a type known in the art. The pyrotechnical squib 33, which is carried by the housing 8, comprises a pyrotechnical charge and an electrical igniter. The pyrotechnical squib 33 is received in a receiving portion 35 of the housing 8, as illustrated in Fig. 2. In order to fasten the squib 33 it may have external threads that are engaged with internal threads of the receiving portion 35. The squib 33 is connected to a control unit 37 by means of a cable 39, as schematically illustrated in Fig. 2. The receiving portion 25, and thus the squib 33, is in communication with the circumferential channel 22 through an apperture (not shown) of the housing 8. Upon ignition of the squib 33 the circumferential channel 22 is pressurized.

The flat base portion 18 of the retaining element 19 is provided with a plurality of holes 41 , as best illustrated in Fig.3b, preventing the movable element 17 from being displaced from its normal position due to e.g. pressure leakage. As described hereinbefore the separation fastener assembly 9 comprises a protecting lid 21 , in the form of a plastic lid. The lid, which may be formed from a thermoplastic elastomer, protects the joint bolt sytem 15 from e.g. water.

Now referring to Figs. 4a-4d, the function of the separation fastener assembly 9 will be described. Upon ignition of the squib 33 (not shown in Figs. 4a-4d) it produces a volume of gas at elevated pressure which is received in the circumferential channel 22. Due to elevated pressure inside the circumferential channel 22 a force is applied on the flange 27 of the movable element 17, thereby causing the movable element 17 to perform a movement away from the segmented nut 1 1 , as indicated by arrows A in Fig. 4a. The movable element 17 is moved into engagement with the energy absorbing portion 20 which is then deformed by the movable element 17, as illustrated in Fig. 4b. The sleeve shaped energy absorbing portion 20 is permanently deformed by the movable element 17. Hence, the energy absorbing portion 20 absorbs impact energy of the movable element 17 during its movement toward the second position.

The movable element 17 is moved to a second position, illustrated in

Fig. 4b, in which it no longer restrains the nut segments 1 1 a, 1 1 b, 1 1 c, 1 1 d, thereby enabling movement of the nut segments 1 1 a, 1 1 b, 1 1 c, 1 1 d in radial direction. At this stage the bolt 10 is subjected to forces in its longitudinal direction A due to crash forces. As a result the fracture lines of the frangible nut 1 1 are broken. The nut segments 1 1 a, 1 1 b, 1 1 c, 1 1 d are then separated from each other and from the bolt 10 by forces from the bolt 10, as illustrated by arrows B in Fig. 4c. The fracture lines are broken at this stage or they may alternatively be already pre-broken. Consequently, the connecting function of the bolt joint system 15 is destroyed which enables displacement of the bolt 10, as illustrated by the arrow C in Fig. 4d, and thereby displacement of the rear end 2a of the front subframe 2 relative to the vehicle chassis 3.

Hereinafter a separation fastener assembly according to a second embodiment of the present invention will be described with reference to Fig. 5. Many features disclosed in the first embodiment are also present in the second embodiment with similar reference numerals identifying similar or same features. Having mentioned this, the description will focus on explaining the differing features of the second embodiment.

The second embodiment differs in that the energy absorbing portion

120 comprises three linearly narrowing sub portions 120a, 120b, 120c as seen along the longitudinal axis of the retaining element 1 19. A first sub portion 120a extends from the flat base portion 1 18 to a second sub portion 120b of the retaining element 1 19. The second sub portion 120b extends from the first sub portion 120a to a third sub portion 120c of the energy absorbing portion 120. The third sub portion 120c extends from the second sub portion 120b to the second end 1 19b of the retaining element 1 19. The cross section of the first sub portion 120a tapers at a first angle a1 as seen with respect to an axis parallel to the longitudinal axis LA of the retaining element 1 19. The cross section of the second sub portion tapers at a second angle a2, as seen with respect to the longitudinal axis of the retaining element 1 19, which is less than the first angle. The cross section of the third sub portion 120c is tapering at a third angle a3, as seen with respect to the longitudinal axis of the retaining element, which is less than the second angle a2.

Preferably, the cross section of the first sub portion 120a tapers at a first angle α1 , as seen with respect to the longitudinal axis of the retaining element 1 19, corresponding to an angle of 1 - 30 degrees.

Preferably, the cross section of the second sub portion 120c tapers at a second angle a2, as seen with respect to the longitudinal axis of the retaining element 1 19, corresponding to an angle of 30 - 60 degrees.

Preferably, the cross section of the third sub portion 120c tapers at a third angle a3, as seen with respect to the longitudinal axis of the retaining element 1 19, corresponding to an angle in the range between 60 - 90 degrees.

In one embodiment a1 corresponds to an angle of 20 degrees, a2 to an angle of 40 degrees and a3 to an angle of 60 degrees.

It is realized by a person skilled in the art that features from various embodiments disclosed herein may be combined with one another in order to provide further alternative embodiments.

Hereinbefore, it has been described that the movable element includes a protrusion which normally abuts the nut such that the movable element restrains the nut segments from radial expansion. It is appreciated that the movable element may include further portions which normally abut corresponding portions of the nut 1 1 . Alternatively, the movable element may not include such a protrusion. Then, the nut need to have at least one protrusion which normally abuts the movable element.