US2577501A - Electric igniter for use in electric initiating devices - Google Patents

Description

Decf4, I l

D. G. ASHCROFT 2,577,501 ELECTRIC IGNITER FOR USE IN ELECTRIC INITIATING DEVICES Filed April e, 1948 4 sheeis-sheet l Filed April 6, 1948' Dec. 4, 1951 ASHCROFT I 2,577,501

ELECTRIC IGNITER FOR USE IN ELECTRIC INITIATING DEVICES 4 Sheets-Sheet 2 v Inventor:

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Attorneys.

Dec. 4, 1951 D. G. ASHCROFT 2,577,501

ELECTRIC IGNITER FOR USE IN ELECTRIC INITIATING DEVICES Filed April 6, 1948 4 Sheets-Sheet s A ttorn 251 1386- 1951 D. G. ASHCROFT ELECTRIC IGNITER FOR USE IN ELECTRIC INITIATING DEVICES 4 Sheets-Sheet 4 Filed April 6, 1948- INVENTOR" DONALD GEOR'GE ASHCROFT ATTORNEY heads.

Patented Dec. 4, 1951 NIT, srrss ELECTRIC IGNITER FOR USE IN ELECTRIC INITIATING DEVICES Donald George Ashcroft, Saltcoats, Scotland, as-

signor to Imperial Chemical Industries Limited, a corporation of Great Britain Application April 6, 1948, Serial No. 19,356 In Great Britain April 14, 1947 14 Claims.

The present invention relates to improved electric igniters for use in electric initiating devices for explosive compositions, incendiary compositions and the like, and of the kind wherein the pole pieces are constituted by the bared ends of the insulated leading wires extending beyond a spacing element. The invention also relates to an improved method for the produc-- tion of the aforesaid electric igniters.

The present invention does not relate to electric igniters wherein metal foil elements are united to an insulating sheet element and soldered to the bared ends of the insulated leading wires.

The term electric igniters as used herein includes structures in the manufacture whereof at least one easily ignitable deflagrating composition present in the electric initiating device as well as the resistance element connecting the pole pieces forms an integral part and structures whereof the resistance element connecting the pole pieces is intended to be embedded in an easily ignitable deflagrating composition that is present in the initiating device but does not form part of the igniter structure. The former kind of electric igniter structures are usually referred to as electric fuseheads. Electric fuseheads wherein the resistance element is a resistance wire embedded in a match head composition of non-conducting properties including a deflagrating ingredient ignita-ble by the heated resistance wire are known as low tension fuseheads, while those in which there is no resistance wire but the match head composition joining the pole pieces includes in addition to the deflagrating ingredient an ingredient rendering it semi-conducting e. g. graphite, are known as high tension fuse- The present invention is more especially but not exclusively applicable to electric igniter structures having a resistance wire bridging the pole pieces.

In electric igniters of the kind wherein the pole pieces are constituted by the bared ends of the leading wires the spacing and insulating element beyond which the pole pieces extend usually consists of a plug of hard, soft or resilient material cast, moulded or vulcanised around a portion of the two leading wires, at least partly bared where they pass through the plug, only a short length of the bared portion of each wire being allowed to 1 protrude through the plug so that these will be -maintained apart by their own rigidity. When a resilient plug is used it is ultimately crimped into the casing of an initiating device to form-a waterproof joint. '-This operation tends to introduce a certain element of uncertainty about the position and spacing of the protruding bared portions. Electric igniters insulated by means of cast or moulded hard or soft compositions may also become unserviceable owing to disintegration or flow of the material of the plug. Moreover in the manufacture of the electric igniters some time must elapse before the structures can be removed from the moulds in which the plugs are cast, moulded or vulcanised.

It is an object of the invention to provide electric igniters of the aforesaid kind wherein relative lateral displacement of the pole pieces will not occur either on storage or when the electric igniter is inserted into an initiating device.

It is also an object of the invention to provide a more expeditious method of manufacturing electric igniters, and in particular to provide a method of fixing portions of the leading wires in the proximity of the pole pieces spaced from one another in parallel relationship on a strip member by a mechanical operation which can bev repeated at intervals along the length of the strip so that subsequent operations can be conducted before the strip is severed to form. individual igniters.

According to the present invention an electric igniter of the kind wherein the pole pieces are constituted by the bared ends of insulated leading wires extending beyond a spacing element consisting of a strip of compressible material is characterised in that a portion of each of the said wires is in the form of an open loop in contact with at least the two faces and the four corners of the said strip, with the end portions of the loops on the same face of the strip and each end portion bent over on itself against said face.

These extending bare ends of theleading wires and if desired also the loop forming portions are advantageously flattened.

Preferably the length of the leading wires constituting the loops as well as their continuations constituting the pole pieces are bare. In this case the compressible sheet material must be an insulating material.

According to the present invention the method for the production of electric igniters comprises gripping a length of a strip of compressible sheet material at intervals along it with an individual pair of conducting wire lengths each pair consisting of two parallel and spaced insulated leading wires at least partly bared at their corresponding ends by shaping each wire of this pair against the faces and four corners of the said strip into an open loop with the end' portions of the loop on the same face of the strip and bending over on itself each end portion against said face.

The bare ends of the end portions of the loop of the wire projecting beyond one edge of the strip may advantageously be trimmed to length and the resulting pole pieces may be connected by a resistance wire and/or any match head composition applied in known manner before the strip of insulating material is cut.

The open loops may be doubled around the edges of the strip of sheet material, and if desired both edges of the strip may be provided with pairs of spaced notches corresponding to the desired positions to be occupied by the loops in order to assist in locating said loops. Alternatively the strip of insulating material may be provided with one or more lines of perforations through which doubled over portions of the preferably flattened wire representing the commencement and/or termination of each open loop are passed from one face of the strip and folded over the other face, preferably so as to bring the points of refiection towards one another and thus tightly grip the part of the thickness of the strip between the single portion of the open loop and the double thickness portions thereof. The strip may consist of for instance rigid or semi-rigid insulating material into the material of which the wire can be slightly impressed so as to locate the wire loops securely in their parallel spaced position. The wires are preferably flattened before the loops are formed.

It is an advantage of the invention that there is obtained by it a continuous web structure which can ultimately be cut to form separate igniters, and to which the application of the elements positioned at the pole pieces may be applied before this is done. Another advantage is that the production of said web can be accomplished as fast as the mechanical operations can be carried out with the wire and without waiting for changes in temperature to change the consistency of any material used in the process.

A further advantage is that the length of the strip to be present in each igniter after the cutting operation and any desired further trimming operation may be predetermined so that it will limit the motion of the igniter within the initiator into which it is ultimately inserted and thus prevent the bare wires from touching the walls of the initiator.

The portion of the preferably flattened leading wires that is formed into the open loop and caused to embrace the strip of insulating material may be a portion stripped of its insulation, but alternatively it may still retain its insulating coating. In the former case the sheet material must be an insulating material. The wires may be flattened by passing the ends between pressure rollers, and usually if the wires are insulated when they are passed through the nip the insulation will be ruptured as the wires are flattened and can be removed easily when the wires are withdrawn from the nip, as for instance, when the insulating material consists of a cotton covering.

The wires may be spaced and rendered parallel by pulling them through guide channels. If

desired the insulated portions near each bare end of the leading wires may be partly twisted together and threaded through a plug of preferably resilient material on to which a' detonator tube or the like will ultimately be crimped to provide a waterproof joint.

The strip of compressible sheet material, preferably of insulating material, may be fed in the direction of its length to a fixed position at which there is located a combination of suitable folding and pressing tools adapted to apply a pair of the preferably flattened portions of the leading wires already brought into spaced parallel relationship, these wires being presented transversely to the strip. As will be understood, there may be a series of positions along the path of the strip at which a series of pairs of spaced and parallel portions of the leading wires are simultaneously applied to the strip by a row of combinations of folding and pressing tools. The resulting strip having a series of. parallel flattened wire loops attached to it may now have the bared ends of the projecting wires trimmed.

If the pole pieces are to be bridged by a resistance wire, the latter may be attached to the bared ends of the wires, for instance, by spot soldering or by bending the bare flattened ends of the leading wires over the resistance wire so as to grip it tightly. A continuous length of bridge wire may be used and the unwanted portions in the spaces between the pairs of leading wires may be cut off. The attachment of the bridge wire may be carried out as a repetition operation by feeding the strip to the appropriate tools. The match head composition may advantageously be applied to the pole pieces and the bridge wire if used before the strip of insulating material is out.

One form of the invention is further illustrated in the diagrammatic drawings Figs. 1 through 5, whereof Figure 1 represents a side elevation and Figure 2 a plan view of a pair of leading wires partly twisted together and prepared for attachment to an insulating strip. Figure 3 is an isometric view showing a number of pairs of partly twisted leading wires attached to a strip of press board or the like insulating material and Figures 4 and 5 show partly sectioned side elevations and plan views of a low tension electric fusehead made in accordance with the invention. Figures. 6 and 7 depict isometrically a, modification of the invention of Figures 1 and 2.

In the diagrammatic drawings, 2, as is best seen in Figure l, are cylindrical portions of the leading wires covered by textile or. plastic insulating material I. 3 are flattened, spaced parallel and bared end portions of the wire and 4 is a sheet of pressboard. As is best seen in Figure 3, a space is left on one side of the sheet of insulating material between the reflexed folds 5 and 6 at the commencement and termination of the open loop tightly embracing the strip of insulating material 4. In Figures 4 and 5 numerals i to B have the same significance as in Figure 3. I is a tubular plug of resilient waterproof material such as neoprene threaded on to the twisted portion of the insulated portion of the leading wires.- 8 is a high resistance bridge wire attached by folding over it the extremities of the trimmed ends of the pole pieces as shown at I0. 9 is a bead of match composition capable of being ignited by the heat developed in the bridge wire 8 when an electric current is passed through the latter. The igniter illustrated in Figures 4 and 5 is a low tension electric fusehead, to which a charged detonator tube may be attached by .figures represent a modification of this invention'wherein the strip of insulating material 4 is provided with a line of perforations l through which doubled-over portions of the preferably flattened wire 3 representing the commencement and/or termination of each open loop are passed from one face of the strip 4 and folded over the other face preferably so as to bring the points of reflection towards one another and thus tightly grip the part of the thickness of the strip 4 between the single portion of the open loop and the double thick portions thereof. Figure 7 represents the completely formed electric igniter whereas Figures 6 represents the electric igniter in one stage of its formation.

I claim:

1. An electric igniter wherein the pole pieces are constituted by the bared ends of insulated leading wires extending beyond a spacing element consisting of a strip of electrically non-conducting compressible material characterised in that a portion of each of the said wires is in the form of an open loop in contact with at least the two faces and the four corners of the said strip, with the end portions of the loops on the same face of the strip and each end portion bent over on itself against said face.

2. An electric igniter as claimed in claim 1 wherein the said bared ends are flattened.

3. An electric igniter as claimed in claim 1 wherein the open loop portions are flattened.

4. An electric igniter as claimed in claim 1 wherein the said open loop portions are bare.

5. An electric igniter as claimed in claim 1 wherein the continuations of the loops constituting the pole pieces are bare.

6. A method for the production of electric igniters which comprises gripping a length of a strip of electrically non-conducting compressible sheet material at intervals along its length with individual pairs of conducting wire lengths, each pair consisting of two parallel and spaced insulated leading wires at least partly bared at their corresponding ends, by shaping each wire of this pair against the faces and four corners of the said strip into an open loop with the end portions of the loop on the same face of the strip and bending over on itself each end portion against said face.

'7. A method as claimed in claim 6 wherein the bared ends of the end portions of the loop are trimmed to length, then connected by a resistance wire, and thereafter the strip of compressible sheet material is cut in lengths, each length having gripped thereon one pair of conducting wire lengths.

8. A method as claimed in claim 6 wherein the said bending over on itself of each end portion is extended to the opposite face of the strip.

9. A method as claimed in claim 6 wherein op- 5 posite points on the strip of compressible sheet material are provided with pairs of spaced notches corresponding to the desired positions to be occupied by the loops.

10. A method as claimed in claim 6 wherein the strip of compressible sheet material is provided with at least one line of perforations through which the doubled end portions of the wire representing the commencement of each open loop are passed from one face of the strip and folded over the other face.

11. A method as claimed in claim 10 wherein the said folding is such as to bring the points of reflection towards one another.

12. A method as claim d in claim 6 wherein the bared ends of the wire are flattened before the loops are formed.

13. A method as claimed in claim 6 wherein the spaced and parallel relationship of the pairs of insulated wires has been effected by passing the wires through guide channels.

14. A method as claimed in claim 6 wherein the insulated portions near each bare end of the leading wires are partly twisted together and threaded through a plug adapted to permit a detonator tube or the like to be ultimately crimped thereon to provide a waterproof jacket.

DONALD GEORGE ASHCROFT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,046,816 Lightfoot Dec. 10, 1912 1,473,866 Phelps Nov. 13, 1923 1,608,005 Schoeppler Nov. 23, 1926 1,706,659 Fahnestock Mar. 26, 1929 1,970,051 McWeeny Aug. 14, 1934 2,046,102 Abbott June 30, 1936 2,257,621 Schellenger Sept. 30, 1941 FOREIGN PATENTS Number Country Date 543,500 Great Britain Feb. 27, 1942 546,533 France Aug. 25, 1922 568,831 Germany Jan. 24, 1933 571,289 Germany Feb. 25, 1933

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