GB2054113A - Spark ignition electrode - Google Patents

Abstract

A spark ignition electrode device comprising a wire electrode 10, a hollow elongate ceramic insulator 11, and a shaped metal mounting member 13 in which the electrode is bent to form a holding portion 23 which is displaced bodily by a fraction of the electrode diameter so as to be non-co-axial with the rest of the electrode, said displacement being such that the holding portion is a non- rotatable press-in interference fit in a non-circular bore 30 of the insulator to secure together the electrode and the insulator without requiring the use of cement or separate mechanical fittings. A spark tip 20A is bent after the electrode has been inserted into the insulator. <IMAGE>

Description

SPECIFICATION Improvements in or relating to electrode devices for spark ignition apparatus for gas appliances This invention concerns electrode devices for spark ignition apparatus for gas appliances.

Electrode devices are known in which a wire electrode is cemented into an elongate hollow ceramic insulator, and a shaped metal mounting member is fitted onto the insulator for mounting the device on the appliance. The use of cement has well known disadvantages, and many expedients have been tried to obviate such disadvantages, e.g. providing mechanical fixings to connect one end part of the insulator to the electrode, but such expedients are relatively expensive.

What is needed is a simple and inexpensive way of securing the electrode and insulator together.

According to the present invention there is provided an electrode device comprising a wire electrode, a hollow elongate ceramic insulator, and a shaped metal mounting member, wherein: (a) the wire electrode has an elongate body of substantially constant diameter disposed between a spark tip and a cold terminal end portion; (b) a hot end portion of the body is a co-axial sliding fit in the insulator; (c) the body is bent to form a holding portion such that part of which holding portion is displaced bodily by a fraction of the body diameter so as to be non-co-axial with the hot end portion; (d) the insulator has a bore of non-circular transverse cross-section; and (e) said displacement is such that the holding portion is a non-rotatable press-in interference fit in said bore.

The bore is preferably partially defined by a plurality of angular shoulders which provide edges extending longitudinally within the insulator, which edges engage in said holding portion. Said shoulders are preferably arranged in pairs, and the shape of the holding portion is preferably such that parts of the holding portion are wedged between said edges.

The amount of interference is preferably between 2 and 4 thousandths of an inch ("thou."), and within this range an interference of 3 thou. is preferred when said diemeter is between 60 and 80 thou.

The mounting or an extension of the mounting preferably surrounds and grips an internally loaded portion of the insulator around said holding portion, or a separate collar may be provided around said internally loaded portion, whereby to support the latter against the loads generated by the interference fit. The mounting or a part of the mounting is preferably disposed between the hot end portion and said loaded portion to conduct heat from the insulator and thereby thermally protect the loaded portion.

The invention also provides an improved method of making an electrode device comprising a wire electode and a hollow elongate ceramic insulator having attached thereto a shaped metal mounting member, wherein the wire electrode is formed by serving a straight piece of wire of predetermined length from a supply of wire bending the piece to form a holding portion having a part which is displaced bodily by a fraction of the diameter of the piece so as to be non-co-axial with the unbent remainder of the piece, and, before or after the formation of the holding portion, forming a terminal end portion at one end of the piece, the bending and forming being such that the holding portion is nearer to said one end than to the other end of the piece, is disposed at a predetermined angular relationship to the terminal end portion and is an interference fit in the insulator; wherein the other end of the piece is inserted into the insulator; wherein an axial thrust is applied to the terminal end portion to drive the holding portion into the insulator so that the holding portion is disposed in a predetermined position relative to the mounting member; and wherein a transverse thrust is applied to the other end to form a spark tip projecting from the insulator.

The invention will be described further, by way of example, with reference to the accompanying drawings, wherein: Figure 1 shows a first embodiment of electrode device of the invention, in longitudinal crosssection; Figure 2 shows a second embodiment of electrode device of the invention, in longitudinal cross-section; Figure 3 is an enlarged side view of part of an electrode employed in the electrode device; Figure 4 shows a transverse cross-section through the electrode on the lines IV--IV of Figure 3; Figure 5 shows a transverse cross-section through an insulator employed in the electrode devices; Figure 6 shows a cross-section on the lines VI--VI of Figure 2; Figure 7 shows a plan view of press apparatus used in a method of the invention for making the electrode device; and Figure 8 is a side view of said press apparatus.

Referring to Figures 1 and 2, each electrode device generally comprises a wire electrode 10, a ceramic insulator 11 and a metal mounting member 12.

Each wire electrode 10 is formed from 72 thou.

diameter wire, as hereinafter described, so as to comprise a shaped spark tip 20A or 20B at one end of a body and a terminal portion 21 at the other end of the body. The body includes a straight hot end portion 22 (i.e. a portion which in use receives heat from the spark tip) disposed between a holding portion 23 and the spark tip. As shown in Figures 3 and 4, the holding portion 23 is bent so that a part 24 of the holding portion 23 is displaced bodily by about eleven thou. so as to be non-co-axial with the portion 22, whereby to provide convex bends 25 and 26 on opposite sides of the holding portion 23 which are separated by a distance A perpendicular to the axis 27.

Each insulator 11 is of known form and has a bore 30 which in cross-section has the shape of an 80 thou. diameter circle combined with a 110 by 50 thou. rectangle so as to have two pairs of internal shoulders 31,31 and 32, 32 defining defining edges running longitudinally of the bore, as shown in Figures 1,2 and 5. (The rectangle was dimensioned to allow the terminal portion to be inserted into the bore as was necessary in known methods of constructing the cemented devices hereinbefore mentioned. The insulator has a hot end 33 (i.e. an end which is heated in use) and a cold end 34.

The mounting member 12 is of known form and comprises a cylindrical collar 13 having a shaped planarflange 14.

During manufacture, the electrode 10 is formed by cutting a straight piece of wire of predetermined length from a supply of wire.

The piece is then worked to form the holding portion 23 and the terminal portion 21, the latter being made to British Standard No: BS 5057 and being disposed at one end of the piece, so that a predetermined angular relationship exists between said portions 21 and 23.

The mounting member 12 is pressed onto the insulator so that a predetermined angular relationship exists between the bore 30 and the flange 14 in the assembly so formed.

The other end of the piece is inserted into the cold end 34 of the insulator and placed in the apparatus shown in Figures 7 and 8 so that the insulator is located in a support 70 with its hot end 33 abutting a former 71 and with the flange 14 located in a locating recess shaped to hold the insulator assembly in a particular orientation; and so that the terminal portion 21 projects towards a slotted locator 72 mounted on a first hydraulic or pneumatic ram 73 which, upon being actuated, thrusts the piece into the bore 30 until a first predetermined position is reached. The slotted locator 72 is shaped to engage the portion 21 so as to ensure that the portion 21 is held at a predetermined angular orientation relative to the locating recess.

As can be seen from Figures 3, 4 and 6, the distance A is such that the holding portion 23 is a three thou. interference fit in the bore 30, which interference produces a linear resistance which is overcome by the thrust of the ram 73, and an internal load on the insulator 11, as the piece is thrust into the bore 30. The internal load on the insulator causes the edges of the shoulders 31, 32 to engage in the bends 25, 26; cause the bend 25 to become wedged between the shoulders 31 and the bends 25 to engage between the shoulders 32; and cause the holding portion to be compressed frictionally against the inherent stiffness and resilience of the wire so as slightly to reduce the distance A.

When said first predetermined position is reached the other end of the piece protrudes from said hot end 33 and the ram 73 actuates a limit switch 74 which in turn actuates a transverse ram 75. The ram 75 has a forming head 76 which is moved to bend the other end against the former 71 to form the spark tip 20A, and thereafter to actuate a limit switch 77 which causes the rams to retract, so that the electrode device can be removed from the apparatus.

The apparatus is constructed so as to permit variation of the strokes of the rams, the positions of the limit switches, the location of the locating recess and the locator, and the shape of the former, to enable variations to be made in the forms of devices produced. A guide member 78, shown in broken lines, may be provided to guide the forming head 76 for forming the spark tip 20B.

The invention is not confined to the precise details of the foregoing examples, and many variations are possible within the scope of the invention. For example the shape of the holding portion may be varied to increase the number of convex bends, and the position of the holding portion may be varied provided that it is nearer to the cold end than the hot end of the insulator. The position of the mounting member may be varied, but it is preferred to locate the holding portion either partially within the mounting member or between the latter and said cold end. The distance A may be varied to suit the bore and to produce an interference fit which secures the electrode in the insulator sufficiently to withstand the forces necessary to engage and disengage a connector on and from the terminal portion.

The shape of the bore may be varied, but a bore partially defined by substantially robust shoulders, e.g. shoulders having a large included acute angle or an obtuse angle, providing sharp or blunt edges, is preferred. The amount of interference may be varied to suit the nature of the bore and to avoid cracking the insulator as the holding portion is inserted or becomes warmed in use. In the foregoing example, a maximum for the interference is about four thou. if cracking is to be avoided.

Claims (11)

1. An electrode device comprising a wire electrode, a hollow elongate ceramic insulator, and a shaped metal mounting member, wherein (a) the wire electrode has an elongate body of substantially constant diameter disposed between a spark tip and a cold terminal end portion; (b) a hot end portion of the body is a co-axial sliding fit in the insulator; (c) the body is bent to form a holding portion such that part of which holding portion is displaced bodily by a fraction of the body diameter so as to be non-co-axial with the hot end portion; (d) the insulator has a bore of non-circular transverse cross-section; and (e) said displacement is such that the holding portion is a non-rotatable press-in interference fit in said bore.

2. A device as claimed in claim 1, wherein the bore is partially defined by a plurality of angular shoulders which provide edges extending longitudinally within the insulator, which edges engage in said holding portion.

3. A device as claimed in claim 2, wherein said shoulders are arranged in pairs.

4. A device as claimed in claim 2 or 3, wherein parts of the holding portion are wedged between said edges.

5. A device as claimed in any preceding claim, wherein the amount of interference is between 2 and 4 thou.

6. A device as claimed in any preceding claim, wherein the mounting, an extension of the mounting or a collar surrounds and grips an internally loaded portion of the insulator around said holding portion.

7. A device as claimed in claim 6, wherein the mounting or part of the mounting is disposed between the hot end portion and the internally loaded portion.

8. An electrode device substantially as hereinbefore described with reference to Figures 1 to 6 of the accompanying drawings.

9. A method of making an electrode device comprising a wire electrode and a hollow elongate ceramic insulator having attached thereto a shaped metal mounting member, wherein the wire electrode is formed by serving a straight piece of wire of predetermined length from a supply of wire bending the piece to form a holding portion having a part which is displaced bodily by a fraction of the diameter of the piece so as to be non-co-axial with the unbent remainder of the piece, and, before or after the formation of the holding portion, forming a terminal end portion at one end of the piece, the bending and forming being such that the holding portion is nearer to said one end than to the other end of the piece, is disposed at a predetermined angular relationship to the terminal end portion and is an interference fit in the insulator; wherein the other end of the piece is inserted into the insulator; wherein an axial thrust is applied to the terminal end portion to drive the holding portion into the insulator so that the holding portion is disposed in a predetermined position relative to the mounting member; and wherein a transverse thrust is applied to the other end to form a spark tip projecting from the insulator.

10. A method of making an electrode device substantially as hereinbefore described with reference to Figures 7 and 8 of the accompanying drawing.

11. An electrode device made by the method claimed in claim 9 or 10.