WO2008102087A2 - Pyrotechnic actuator and method for making same - Google Patents

Abstract

An actuator including a housing (12) containing a plunger (14), a pyrotechnic gas generator (17) and a frangible or releasable connecting element (20) defining a combustion chamber (19) with a predetermined starting volume in the housing prior to triggering.

Description

 Pyrotechnic actineur and its manufacturing process

The invention relates to a pyrotechnic actuator piston, mainly intended to be used in a security system, including automotive. The invention relates more particularly to an improvement to better control the initial volume of the combustion chamber. Another aspect of the invention is the use of this improvement in a new component assembly method of the actuator, compatible with a robotic production line at high rate.

The main applications envisaged mainly concern pyrotechnic actuators used for safety devices in the automotive field, such as seat-belt pretensioners, steering column dampers, etc. Such actuators can also be used in the aeronautical, space and maritime domains.

A conventional pyrotechnic actuator comprises, in a housing, a piston comprising an actuating rod projecting outside the housing and a pyrotechnic gas generator, triggered by an electric discharge. These actuators find their main application in the field of automotive safety. They are manufactured in very large series and must be of simple design and quick assembly while being made of simple mechanical parts that do not require complex machining.

At the moment of tripping, the generator emits a gas under high pressure into a combustion chamber, one of the walls of which is constituted by the piston.

The performance of such a pyrotechnic gas generator depend in particular on the initial volume of the combustion chamber. More particularly, it seeks to use very bright powders whose combustion is almost immediate to provide a driving pulse very quickly after tripping. The initial volume is given by the diameter of the cavity and the relative position of the generator with respect to the piston, before triggering.

Moreover, to ensure an effective and reproducible ignition of the gas generator, the piston must withstand a certain time the effect of thrust due to pressure. Excessively immediate displacement of the piston may cause a vacuum that could extinguish the generator. In addition, the actuator must withstand shock before mounting. A standardized test is to make it fall 1 m high on a concrete slab. In this case, an impact on the rod must not change the initial volume, that is to say the position of the piston relative to the gas generator.

We must therefore imagine solutions for maintaining the piston allowing:

to control the relative positions of the components,

- having a predetermined resistance, at the beginning of the race, and

- having a high resistance to shocks and external vibrations. US Patent 4,412,420 discloses a pyrotechnic actuator wherein the piston is initially blocked by means of a washer placed on the piston rod and being housed in a groove of the housing. Therefore, this locking system slides by deformation on the piston during operation of the actuator. This can lead to unnecessary restraint efforts and possible misalignment that could compromise the proper functioning of the system. In addition, this system does not allow to reserve a dead volume between the gas generator and the piston.

The whole is complex and requires the use of precision mechanical parts, relatively expensive.

In addition, the assembly of the components of such an actuator involves controlling steps with low mechanical tolerance, which makes difficult a robotic assembly at a high rate of production. The invention solves all these problems. More particularly, the invention relates to a pyrotechnic actuator comprising a housing, a piston displaceable in a cavity of this housing and a pyrotechnic gas generator mounted in said cavity, characterized in that it comprises a breakable or disconnectable connecting element connecting said piston to said generator so as to define in said housing, before triggering, a combustion chamber of predetermined initial volume. Preferably, said connecting element is a socket. Such a socket may be made of a semi-rigid material chosen both to be assembled to said piston and / or to said gas generator by elastic deformation and to absorb the energy of a shock on said piston.

For example, this sleeve may be plastic. In this case, the sleeve undergoes a melting and / or stretching resulting in its rupture, during operation of the gas generator.

In addition, zones of weakness can be arranged on this socket to locate and calibrate its breakage or disconnection.

The invention also relates to a pyrotechnic actuator consisting in installing in a housing a piston and a pyrotechnic gas generator, characterized in that it consists in pre-assembling a subassembly comprising at least said piston and a breakable or disconnectable connecting element before inserting this subassembly into said housing.

The invention will be better understood and other advantageous features thereof will appear better in the light of the following description of several embodiments of a pyrotechnic actuator according to its principle, given solely as examples and with reference to the accompanying drawings, in which:

FIG. 1 illustrates the various elements of a pyrotechnic actuator according to the invention, separated but aligned along a common X-X assembly axis; FIG. 2 is a perspective view of the connecting element of FIG. 1;

- Figure 3 is a sectional view of the gas generator assembled to the connecting element;

FIG. 4 is a view partially in section of a subassembly comprising the piston, the connecting element and the gas generator;

- Figure 5 is a sectional view of the actuator before tripping;

- Figure 6 is a sectional view of the same actuator during the trip;

FIG. 7 illustrates a variant of the actuator; - Figure 8 illustrates another variant of the actuator; - Figures 9 and 10 illustrate another variant and a method of assembling the constituent elements of this variant; and

- Figures 11 to 14 illustrating a method of assembling the elements of the actuator.

In Figures 1 to 5, there is shown a possible embodiment of a pyrotechnic actuator according to the invention. This actuator 11 comprises a housing 12 enclosing a piston 14, a pyrotechnic gas generator 17, here installed in a base 18 of molded plastic material and a connecting element 20, breakable or disconnectable connecting said piston 14 to said generator 17 so as to define in said housing, before tripping, a combustion chamber 19 of predetermined initial volume. Before triggering, the actuating rod 15 protrudes out of the housing 12 at an axial end thereof.

In the example shown, the connecting element 20 has the general shape of a socket (see FIG. 2). The housing 12 has the shape of a rectilinear tube, provided with two shoulders: a front shoulder 21 and a rear shoulder 22. The piston comprises two sliding seals 23, 24 housed in corresponding grooves and its rear end is provided with an annular groove 26 for the attachment of the connecting element 20.

As shown in Figure 2, the end of the bushing 20 which is connected to the piston 14 is provided with internal radial teeth 28 engaged in the annular groove 26 at the adjacent end of the piston.

In this example, the bushing is advantageously made of semi-rigid material chosen both to allow the assembly to the piston and / or to the gas generator by elastic deformation and to withstand the energy of an impact on the piston 14. In other words, the chosen material must be rigid enough to absorb without deforming the energy of a shock that would be communicated to the piston via the rod 15, this to maintain the initial volume, before release, of the chamber 19 and sufficiently flexible to allow mounting by axial interlocking.

For example, the sleeve 20 may be plastic.

The piston 14 also comprises an annular collar 30 of diameter corresponding to the internal diameter of the middle portion 31 of the housing 12. This annular flange 30 is capable of abutting against the shoulder before 21 when triggering the actuator, which determines the stroke of the rod 15.

The gas generator 17 is of a known type. It comprises two pins 35 protruding from the rear, for the electrical connection to a voltage generator may trigger the firing of the powder it contains. It is installed in the base 18 which has an open cavity 37 at the rear, in which the pins protrude.

In the example, the base 18 has an annular edge 38 for assembling the generator 17 to the base 18 (for example by hot crimping) as shown in FIG.

In the example, the base 18 has a peripheral groove 39 in which an annular rib 41 is engaged at the adjacent end of the bushing 20. Once the rib 41 is engaged in the groove 39 of the base, the assembly between the socket and the base (trapping the generator 17) can be confirmed by peripheral crimping. In this way, the socket-generator assembly comprises a collar 43 which bears against the rear shoulder 22 of the housing. This gives a subassembly 44 comprising the piston 14, the sleeve 20, the generator 17 and its base 18. This subassembly is shown in Figure 4, ready to be introduced into the housing 12. The assembly between the subassembly 44a and the housing 12 is confirmed by an additional crimping 45 between the rear end of the housing 12 and a rear shoulder of the base 18. Thus completed, the pyrotechnic actuator is compliant, before triggering, in FIG. understands that, before triggering, the volume of the chamber 19 is well determined and stable. In addition, if an impact is exerted axially on the rod 15, the sleeve 20 is able to guarantee the distance between the piston and the gas generator 17 and consequently the volume of the chamber 19.

If an electric pulse delivered by a source of electrical voltage not shown, is applied to the generator 17, it emits a gas which causes a sudden increase in the pressure in the chamber 19. In the first place, because of the presence of the sleeve 20, the piston 14 remains immobilized, which allows a rapid rise in pressure in the chamber 19. Then, the sleeve breaks (possibly by partial melting) or detaches from the piston and / or the base 18, which releases abruptly the piston 14. This situation Fig. 6. The movement continues until the flange 30 abuts against the front shoulder 21 of the housing.

In the variant of Figure 7, the piston 14 is similar to that of the actuator described above. In contrast, the sleeve 20a is part of a plastic body 47 overmolded around the gas generator. The bushing, overmolded, may be provided with teeth 28 as in the example of FIG. 2, for a force fit at the rear end of the piston 14. As in the previous case, the overmolded bushing may be preassembled to the piston ( with the generator to which it is linked) in the extension of said piston, at the rear of it. In this example, the overmolded plastic body 47 is provided with a two-wire electrical cord 49 for firing. The electric wires of this cord are connected to the gas generator, inside the overmolding. The plastic body has a transverse bore 50 for the passage of a pin 51. It also has a flange 48 for its axial immobilization in the housing 12. The pyrotechnic actuator can thus be hingedly mounted at its rear end.

As seen in Figure 8, the force fit between the bushing 20a and the rear end of the piston can be replaced by a bayonet type mounting. To do this, the end of the piston has notches 52 opening into the groove 26. The assembly is then performed by rotation of the plastic body overmolded relative to the piston.

Of course, the assembly according to FIG. 8 can also be retained for the embodiment described with reference to FIGS. 1 to 5.

FIG. 9 describes another embodiment where the bushing 20b, for example made of plastic material, comprises a solid portion 53 provided with an axial blind hole 54 in which an end 55 (of corresponding diameter) engages with the piston 14a. In this case, the assembly between the piston and the sleeve is by simple axial interlocking, possibly glued.

In this variant, the sleeve provided with radial teeth at its end has longitudinal slots 57 facilitating its implementation by elastic deformation, here to bypass an annular shoulder 59 provided around the gas generator. A seal 60 is provided around the plastic body 47a, in a groove defined at the rear of this shoulder. The teeth of the sleeve are inserted into the groove 61 containing the seal 60.

It is understood that in the examples of Figures 7 to 9, the assembly order of the various components is indifferent since it results in the formation of a subset comprising the piston, the bushing and the gas generator. The last operation is to slide this subassembly in the housing 12 by its rear end (Figure 10) until the flange 48 defined in the molded plastic body comes to bear against the rear shoulder 22 of the housing. For these embodiments, crimping between the plastic body and the rear end of the housing is possible. It is not essential since the pin 51 fixes and stabilizes the axial position of the plastic body 47 overmolded with respect to the housing 12. Therefore, when firing, it is always the connecting element in sleeve shape that breaks or disconnects to release the piston.

Alternatively, one can combine in other ways the connecting means between the sleeve and the piston on the one hand and the sleeve and the generator on the other hand, according to the examples which have been described above. In particular, both ends of the sleeve may be provided with teeth engaged by force fitting into corresponding annular grooves of the piston and the generator.

The invention also relates to a method of assembling the elements of a pyrotechnic actuator comprising a housing 12, a piston 14 and a pyrotechnic gas generator 17, according to the examples described above. The assembly method is mainly characterized in that it consists in pre-assembling a subassembly comprising at least said piston and a breakable or disconnectable connecting element and then inserting said subassembly into the housing 12. As mentioned previously, assembly order of the elements of the subset is indifferent.

Preferably, the connecting element is in the form of a socket. It can be snapped to a rear end of the piston. It can also be force-fitted to one end of the generator. It can be crimped at one end of the piston and / or at one end of the generator. Finally, as we have seen, the connecting element and the gas generator can be combined by overmolding.

As an advantageous example, FIGS. 11 to 14 describe the steps of a method of assembling the elements of the pyrotechnic actuator. We begin by making a subset 44 comprising

(at least) the piston 14 and the connecting element 20 described above (Figure 11).

In FIG. 12, this subassembly is inserted into the housing 12. In FIG. 13, a pyrotechnic gas generator 17 is inserted into the housing, at the rear of the subassembly. FIG. the rear end of the housing, on the gas generator side, to hold all components in the housing.

Of course, alternatively, the gas generator may be part of the preassembled subassembly that is inserted into the housing before crimping. The subassembly 44a shown in FIG.

4.

Other variants are possible. For example, in the same way as for the embodiment of FIG. 9, the groove 26 (FIG. 1) can be omitted, the sleeve then being clipped into the groove of the seal 24.

Claims

1. Pyrotechnic actuator comprising a housing (12), a piston (14) movable in a cavity of this housing and having a rod projecting outside the housing, and a pyrotechnic gas generator (17) mounted in said cavity , characterized in that it comprises a connecting element (20) breakable or disconnectable connecting said piston to said generator so as to define in said housing, before tripping, a combustion chamber (19) of predetermined initial volume. 2. Actuator according to claim 1, characterized in that said connecting element (20) is a socket. 3. Actuator according to claim 1 or 2, characterized in that said connecting element (20) is of semi-rigid material selected both to allow its assembly to said piston and / or said gas generator by elastic deformation and to absorb the energy of a shock on said piston. 4. Actuator according to claim 3, characterized in that said sleeve (20) is plastic. 5. Actuator according to one of claims 2 to 4, characterized in that the end of said sleeve connected to said piston is provided with radial teeth (28) internally engaged in an annular groove (26) at the adjacent end of the piston . 6. An actuator according to claim 5, characterized in that the edge of said annular groove, at the end of the piston, has notches (52) for mounting bayonet type. 7. Actuator according to one of claims 2 to 6, characterized in that the end of said sleeve connected to said generator is fixed thereto, for example by crimping. 8. Actuator according to one of claims 2 to 7, characterized in that the bushing-generator assembly comprises a flange (43, 48) bearing against an inner shoulder of said housing. 9. Actuator according to one of claims 2 to 7, characterized in that said sleeve (20a) is part of a plastic body (47) overmolded around said gas generator. 10. Actuator according to claim 9, characterized in that said plastic body (47) is provided with a two-wire electrical cord (49) for firing the gas generator (17). 11. An actuator according to claim 9, characterized in that said plastic body (47) has a transverse bore (50) for the passage of a pin (51). 12. Actuator according to one of claims 2 to 9, characterized in that said sleeve (20b) is assembled to said piston in the extension thereof, by interlocking. 13. Actuator according to one of claims 2 to 10, characterized in that said sleeve is provided with longitudinal slots (57) facilitating its implementation by elastic deformation. 14. Actuator according to one of claims 2 to 13, characterized in that the two ends of said sleeve are provided with teeth engaged by force fitting into corresponding grooves of the piston and the generator. 15. Actuator according to one of claims 2 to 13, characterized in that said sleeve engages in a groove (61) receiving a seal (60) of said generator. 16. A method of assembling the elements of a pyrotechnic actuator consisting in installing in a housing (12) a piston (14) and a pyrotechnic gas generator (17), characterized in that it consists in pre-assembling a subset comprising at least said piston (14) and a breakable or disconnectable connecting element (20) before inserting said subassembly into said housing. 17. The method of claim 16, characterized in that said pyrotechnic gas generator is inserted into said housing, in the extension of said connecting element and in that it closes or tightens an end of the housing, the side of said generator. 18. The method of claim 17, characterized in that pre-assembling a subset aforesaid further comprising said pyrotechnic gas generator and that it is installed in said housing. 19. Method according to one of claims 16 to 18, characterized in that said bushing-shaped connecting element is force-fitted to one end of said piston. 20. Method according to one of claims 16 to 18, characterized in that the bushing-shaped connecting element is force-fitted at one end of said generator. 21. Method according to one of claims 16 to 18, characterized in that one molds the socket-shaped connecting element around said gas generator. 22. The method of claim 21, characterized in that one secures the plastic body (47) consisting of the sleeve and the generator combined by overmoulding by means of a trunnion.