US2507723A - Safety electrical connection - Google Patents

Description

y 1950 A. H. LEJA 2,507,723

SAFETY ELECTRICAL CONNECTION Filed Nov. 25, 1946 Patented May 16, 1950 UNITED STATES PATENT OFFICE 9 Claims.

This invention relates to safe electrical connections for electric appliance cords and more particularly to plug connections of the renewable cord type.

The present invention combines several features of improvement over previously known plugs of related types.

One object of the present invention concerns an improvement through which separable parts of a plug body of novel form. are locked in safely assembled relation by the cord when in use.

Another objectconcerns provision, of an improved solderless connection between an. insulation sheathed, expandable conductor and another electrical conductor, in which the two conductors are pressed into firm contact by elastic strain in the insulating sheath.

A still further object is tocombine anv improved solderless electrical connection held under compression between parts of a housing, with safety locking structure assuring that the connection will not become loosened or exposed during use.

The invention having the above objects, and additional objects and advantages which may appear in this specification, can be carried into practical effect as described below with reference to the accompanying drawing, in which like reference characters have been employed to indicate similar parts throughout the several views.

Fig. l of the drawing is an isometric view of an assembled safety plug connection according to the invention;

Fig. 2 is an exploded view" of the plug connection shown in Fig. 1, showing the several parts in position for assembly;

' Fig; 3 is an isometric View of a partial assembly of parts of the plug shown in Fig. 1;

Fig. 4 is an exploded view similar to; Fig. 2' of a modified form of plug connection according to the invention;

Fig. 5 is a view", similar to; Fig. 3,, of a partial assembly of they parts of the modified plug, connection of Fig. 4';

Fig. 6 isa plan view of an assembled plug connection of. the modified form shown in Fig. 4;

' Fig. 7 is a cross sectional view taken on the line VII-VII of Fig. 6;

Fig. 8 is a longitudinal section of a second modified plug connection embodying the: invention taken. on line: VIII,--VIII of Fig. 10;

Fig. 9 is an end elevation of the plug, connection of Fig. 3 with a portion broken away; and

Fig. 101s a front elevation. of the modified form of plug connection shown in. Fig. 8.

, In-the. embodiments of the invention illustra ed in the, drawing, a unique type of electrical connection has been employed. For reasons of simplicity it is highly desirable, in a plug connection of the replaceable type, to be able to make connection between the insulated conductors of an electrical fixture cord and the contact fingers of the plug connection, without removal of any of the cord insulation, and without the use of tools. The present invention provides such a connection.

In the present invention, electrical contact pieces such as the plug contact fingers I, generally of the type used in plugs for home appliances, are provided with transversely ridged, or threaded, prongs 2, tapered to a point, similar to the ordinary wool screw; Prongs 2 may be inserted endwise among the strands of a flexible stranded conductor 3 having a more or less resilient covering of insulation 4, without removing or disturbing the insulating covering in any way. The act of insertion of the tapered prong 2 among the strands of the conductor 3 will result in an expansion of the conductor, and of the insulating sheath 4 surrounding the same. If the portion of the conductor surrounding the inserted prong 2 is confined laterally between surfaces which resist expansion of the insulating sheath 4, the insulating sheath will be compressed between the strands of conductor 3 and such surface; and a compressional stress in the material of the insulating sheath will result, which will exert a pressure tending to force the strands of the conductor firmly into electrical contact making relation with the prong 2. This pressure will tend to deform the strands of conductor 3 with respect to the ridges or threads of the prong 2, in such a mannerthat there will be a strong mechanical resistance to withdrawal of the prong 2 from conductor 3 by relative longitudinal, translational, movement. The compressional stress in the insulating sheath 4 will also result in a lateral pressure of the sheath against the restraining surfaces, creating a frictional resistance against movement of the insulating sheath relative to such restraining surfaces. It will be readily understood that a, good electrical connection between the strands of conductor 3 and prong 2v of contact I will be maintained by the elastic stresses in the insulating sheath 4, and that, at the same time, a strong frictional resistance against separation of the component parts of. the connection will also be maintained by the same elastic stress.

The electricalconnection described in the foregoing paragraph is employed to practical advantage. in. a preferred form of safety plug con- 3 struction as illustrated in Figs. 1-3. The assembly shown in Fig. 1 consists of a plug body 6, a pair of contact fingers I equipped with prongs 2, a cap member I and a flexible electric cord consisting of two flexible conductors 3, each sheathed with insulation 4.

The plug body member 6, as illustrated in Figs. l-3, consists of a thick disk, or short cylinder, of a suitably rigid insulating material, such as, for example, one of the common, so-called plastic or synthetic resin materials. Other materials,

such as glass, ceramics and other compositions may obviously be used. The disk or cylinder 6 is pierced by two suitably shaped apertures 8, through which the contact fingers I may be extended. The base ends of contact fingers I, from which the prongs 2 extend, are suitably formed so that the contact fingers may not pass entirely through the apertures 8. When the contact fingers are inserted in operative relation to body member 6, the prongs 2 will extend in laterally spaced parallel relation to an endsurface 9 of the member 6, which will be referred to hereinafter as the rear end surface of such body member. This rear end surface of body member 6 may be provided with a raised boss 9a between the apertures 8, such boss having lateral surfaces in spaced parallel relation to prongs 2 of contact fingers I when the same are in assembled relation thereto. A third aperture or passage I I extends from the rear through the front surface of body member 6, and opens also through the cylindrical, peripheral surface thereof, for reasons to be described hereinafter. Passage H, in the illustrated embodiments, takes the form of an axially extending groove in the peripheral surface of plug body 6. A groove II a across the front of disk 6 may also form a part of passage II for reasons described hereinafter. The'body member 6 is externally threaded for engagement with the cap member I. Cap member I has an internally threaded, hollow, cylindrical portion, or annular flange I2, adapted to threadedly engage body member 6, and to peripherally surround the body member when fully engaged therewith. The can thu has a cylindrical recess open only at one end, through the flat front face formed by the edge of flange I2. Cap member I and body member 6, when fully engaged, enclose a cavity between the rear end surface of body member 6 and the internal surface of cap I, for a reason described hereinafter, the inner surface of cap 1 having portions opposed to the rear end surface of body member 6. A lock slot, or notch I3 is formed in the annular flange I2 of cap 1, opening through the edge of the flange, as shown in Figs. 1 and 2, for a purpose described hereinbelow.

In assembling the safety plug connection of the present invention, a pair of contact fingers I are connected with the unstripped ends of a pair of flexible, insulated, cord conductors 3, by

insertion of the prongs 2 among the strands of the conductors. In the case of threaded prongs, this may be accomplished by manually inserting the point of the prong endwise among the conductor strands, and rotating the same as an ordinary wood screw, the rotation resultin in an axial advance of the prong, relative to the conductor strands, in the same manner as a wood screw will advance into a piece of wood. After connection of the contact fingers I with the cord conductor tips, the contact fingers I are inserted in apertures 8 and the flexible cord conductors are flexed so that the standing parts pass through passage or aperture I I and through the front end surface of the body member 6, as clearly i1- lustrated in the partial assembly of Fig. 3. With the conductors in this position, cap I is threadedl engaged with body member 6 and rotated, relatively thereto, until the flange I2 fully engages and surrounds the threaded peripheral surface of the body member 6. As the cap 1 approaches full engagement with body 6, the internal surface of cap I approaches the rear end surface of body 6 in such a manner as to engage the insulated ends of conductors 3 just prior to reaching a limiting position of full engagement. It will be clearly understood that the elastic sheath of insulation 4 surrounding the prongs 2 will be compressed between opposed surface portions of the body member 6 and cap I, so that when the cap "I is fully engaged with body 6, portions of the insulating sheath 4 will be under compression between the expanded conductor 3 and the engaged surface portions of plug body 6 and cap I. The elastic stresses in the insulating sheath l will-exert pressures tending to maintain electrical andmechanical connection between the prong 2 and the strands of conductor 3, and frictional engagement between the sheath 4 and engaged portions of the rear end surface of the plug body 6 and internal surface of cap I. When the cap I and plug body 6 are thus fully engaged, it will be seen that contact fingers I will be firmly held in place with relation to the plug body 6 and that the electrical connection between the contact fingers and the conductors 3 will be fully enclosed in the space between cap I and body member 6.

The lock slot I3, more than one of which may be provided, at angularly spaced intervals if desired, will be so placed that it will register with the lateral opening of passage II through the peripheral surface of body member 6 when the plug and body are substantially fully engaged by relative rotation. The cap member I may now be firmly locked in place by flexing the conductors 3 so that their standing parts extend radially outward from passage II, through lock slot I3. In order that some latitude may be allowed for as to the final tightness of the assembly without providing a plurality of lock slots, groove Ila across the face of body 6 may be used to accommodate the standing part of the cord when passage I I is turned past, or short of register with lock slot I3.

It will be seen, as illustrated in Fig. 3, that portions of the conductors 3 must protrude from the front end surface of plug body 6 in order to permit assembly and disassembly of cap I relative to the plug body. The protruding cord portions will interfere with the insertion of the contact fingers I into full engagement with a cooperating socket fixture, so long as they so protrude, since the front end face of the body member 6 should engage the external insulating face of the usual mating socket fixture, when the connection is made. Thus, the act of flexing the conductors 3 so as to pass radially through the lock slot I3 is an act necessary to condition the plug connection of the invention for operative engagement with a mating socket fixture. And it will be seen that the same act causes a locking of the cap 1 against rotation relative to the body member 6. The locking action results from the fact that any relative rotation of the cap and body, when so locked, will cause the conductor to be stressed in shear between surfaces of the pasage II (or groove II'a)'and lock slot I 3. This locking action is of' double importance, since it positively prevents any relative movement of cap I and body member 6 which would tend to release the pressure on the connection between conductor 3- and prong 2, and also will positively prevent removal of the cap 1 when the plug is engaged with a mating socket fixture. The ex posed surface of the mating socket fixture will effectively prevent any dislodgement of the conductors 3 from lock slot 13- solong as the plug connection is engaged with such a mating socket fixture. Accordingly, when the plug connection of the invention is engaged in the normal manner in the ordinary type of mating socket fixture, the cap 1' will be securely locked against removal such as would expose any live conductor.

7 The modified form of plug connection shown in Figs. 4-7 differs from that illustrated in Figs. 1-3 in respect to the type of threaded connection between the body member 6 and cap 1. In the modified form, the cap 7, instead of having its flange portion [2' internally threaded, is provided with a coaxial screw projection H. The body 6' instead of being externally threaded is provided with a tapped hole l6, coaxial with its peripheral surface. The cap 1 is assembled,

relative to body member 6, by threadedly engaging screw projection M in tapped hole i6, and relatively rotating the cap and body. As is clearly illustrated, the flange I2 is formed to closely surround the peripheral surface of body 6', so that it will be necessary to flex the conductors 3 so that they will extend from the rear end surface of body 6 through the passage II and the front end surfaceof the body 6 as shown in Fig. 5 in order to condition the parts for assembly of the cap 1' by rotation relative to body 6'. As in the preferred form, the standing parts of cords 3 will interfere with proper operative engagement of the plug connection of the invention with a mating socket fixture so long as they extend through the front surface of body member 5'. And, in order to condition the plug for use, it will be necessary, as in the preferred form, to flex thestanding parts of the conductors so as to make them extend radially outward from passage I I through lock slot I 3, in order that the plug connection may be conditioned for use with a mating socket fixture.

Fig. '7 clearly illustrates the modified form of plug connection in mating engagement with a socket fixture of the usual type, the exposed insulatingsurface of which is indicated in dotted lines at ll. Since the construction of such socket fixtures is well known, no illustration is believed necessary in this specification.

A second modified form of plug connection is illustrated in Figs. 8-10.. In this form, the electrical connection of the present invention is employed in a very simple form of cord tip plug. The plug body I 8 is a simple piece of more or less rigid. insulating material pierced by two parallel holes IS the internal surface of which may be serrated, threaded, or otherwise roughened, if desired. A pair of contact fingers 2| are provided with taper-threaded prongs 22. The simple plug connection is assembled with relation to a pair of flexible cord conductors 3 by inserting the unstripped conductor tips into holes l9, which are of a diameter substantially equal to the normal external diameter of the insulation sheath conductor. The plug assembly is completed' by inserting the prongs 22 among the strands of conductors 3 with a screwing action, which will result in an expansion of the conductors 3 and a compression of insulation sheath "4' between the expanded conductor 3 and the internal surface of holes IS. The contact. fingers 21 are preferably locked against relative rotation by use of a double slotted, piece of insulating material 23,. arranged to be slidaible into position against body member l8, and to fit snugly enough to be retained in position by frictional engagement with the contact fingers 2 I. As the contact fingers 2| are rectangular, or otherwise noncircular in cross section, thepiece 23', being slotted to fit snugly on contact fingers 2|, Will prevent the rotation thereof relative to each other and to body l8. As in the other forms of the present invention, the elastic stresses caused in insulation sheath ll, by the expansion of conductor 3 therein while the sheath is surrounded by a more or-less rigid restraining wall, will result not only in a maintenance of a good electrical and mechanical connection between the prong 22 and the strands of conductor 3, but will also result in the maintenance of a frictional resistance against displacement of the body member [8 from its position surrounding the connection. The body member l8, accordingly, will be frictionally locked in position, guarding and maintaining the electrical connection between prongs Z2 and conductors 3, :by the act of assembly. And, since non-circular contact fingers are used, the frictional locking action will prevent disassembly of the plug connection while the contact fingers 2! are engaged with a mating socket fixture of usual type. Any attempt to pull body member I8 axially, relative to the connection between prongs 22 and conductors 3, would, of course, result in pulling contact fingers 2| out of engagement with the live socket contacts in connection with which such a plug would be employed.

It will be understood that the elastic stress maintained electrical and mechanical connection is common to all of the fixtures described hereinabove, and that such a connection will also be advantageous for use in single conductor connections as well as in the ordinary two conductor plug connection, as illustrated and described I hereinabove. It will also be understood that the body surrounding the connection need not be absolutely rigid, but may be resilient.

While specific embodiments of the present invention are. particularly described and illustrated hereinabove, such specific embodiments are by way of illustration only. It will for example be readily apparent that the principles involved may advantageously be employed in socket fixtures as well as plug fixtures, andv not only with stranded conductors but with all. conductors which can be expanded by insertion of a Iprong. And the present invention includes such modifications and equivalents as may occur to persons skilled in the art, within. the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. An electrical contact plug assembly comprising; a body of insulating material having front and rear end' surfaces and having an aperture extending therethrough; a metal contact finger extending through said aperture from the rear through the front end surface of said body and having its rear end formed to prevent passa e through said aperture; a prong extending laterally from said rear end of said contact finger, tapering to a point and having: a transversely ribbed surface, the longitudinal axis of said prong lying substantially parallel to said rear end surface of said body acap of insulating material cooperating with said body to enclose the rear end surface of said body, said cap having an internal surface portion substantially parallel to said rear end surface of said body; and interengaging means on said body and cap member, operable to move said cap member relatively to said body, in the direction of the said rear end surface of said body.

2. A safety cord and contact plug assembly comprising; a body in the form of a cylinder of insulating material, having front and rear end surfaces; a metal contact finger projecting from said front end surface and having a rear end extending through said rear end surface; a flexible insulation sheathed conductor electrically connected at one end with said rear end of said contact finger adjacent said rear end surface; a cap member of insulating material, coaxially threadedly interengaged with said body and having a cylindrical recess open at one end only and closely fitting said body, said cap cooperating with said body to enclose said rear end surface and the connection between said one end of said conductor and said rear end of said contact finger; an axially extending groove in the cylindrical periphery of said body, capable of accommodating the portion of said sheathed conductor extending from said connection, in a manner to permit relative rotation of said cap and body to screw said body in and out of said recess during assembly and disassembly; and a notch in the open end of said cap, through which said conductor must be extended to prevent its extending axially beyond said front end surface of said body when said body is fully within said recess, whereby said conductor will undisengageably lock said cap against rotation relative to said body when said plug is normally engaged with a mating socket.

3. An electrical cord terminal fixture comprising; a cylindrical plug body of insulating material having a contact-finger-holding aperture extending axially therethrough and a cord accommodating groove penetrating both front and rear end surfaces and the cylindrical surface of said body; a cap of insulating material having a fiat front face and a recess opening in said front face, said recess being of substantially the same diameter as said plug body and somewhat deeper axially; coaxially threaded means on said cap and plug body interengageable by relative rotation of said cap and plug body to draw said body into said recess to a position in which its front end surface is substantially flush with said fiat front face of said cap; and a notch in said fiat front face of said cap, extending laterally from said recess and registerable with said groove of said body where it penetrates said front end surface and the adjacent cylindrical surface of said body when said front end surface is substantially fiush with said flat front face of said cap.

4. In combination with the structure of claim 3, a metal contact finger held in and extending through said contact finger-holding-aperture, one end of said contact finger being exposed at said front end surface of said plug body and the other end having a wood screw-threaded prong tapering to a point and extending laterally from said finger with its longitudinal axis substantially parallel to said rear end surface of said plug body and between it and the closed end of said recess of said cap in assembly.

5. In combination with the structure of claim 4, a flexible stranded cord conductor sheathed with elastic insulating material and having one .end expanded by insertion of said screw threaded prong into its end along its longitudinal axis and among its conducting strands within said sheathing, said expanded end being interposed between and compressed by said rear end surface of said plug body and said closed end of said cap recess when said front end surface of said plug body is drawn substantially fiush with said flat front face of said cap, said sheathed conductor extending from said expanded end through said groove and front end surface of said plug body to permit relative rotation in assembly of said cap and plug body and being extensible laterally from said groove through safd notch of said cap to clear said front end surface for normal plug use and thereby to undisengageably lock said cap against rotation relative to said plug body when the terminal fixture is in ordinary use with a mating socket.

6. An electrical contact plug assembly comprising in combination: a cord having multiple insulation sheathed flexible stranded conductors: a plurality of fiat-sided electrical contact fingers each having at one end a tapered screw-threaded prong integral therewith and tapering to a point, said contact fingers being individually connected with said conductors by screwing said prongs endwise among and lengthwise of the strands thereof within said sheaths of elastic insulation; and a pair of plug parts of rigid insulating material cooperating to completely house in the prong ends of said contact fingers and. the connected ends of said conductors, one of said plug parts having parallel flat-sided apertures through which said contact fingers protrude, said one part thereby preventing disengagement of said prongs from said conductors by relative rotation thereof.

7. An electrical contact plug assembly comprising: a cord having a pair of elastic insulation sheathed flexible stranded conductors; a pair of flat bladelike contact fingers each having at one end an integral tapered screw-threaded prong inserted by screwing endwise and longitudinally into the end of one of said conductors within its sheath of insulation; a hollow plug part of rigid insulating material receiving and having internal surface portions flanking the ends of said conductors into which said prongs are screwed; a second plug part of rigid insulating material having a pair of parallel spaced slots through which said contact fingers protrude in non-rotatable relation, said second plug part engaging and cooperating with said hollow part to house in the connections between said prongs and conductors and to provide an insulating holder and hand grip for the prong ends of said fingers, said internal surface portions of said hollow plug part serving to transmit pressure from said hollow plug part to said contact fingers and fix their posiitons relative to said hollow part through pressure on said sheaths of insulation. 8. The structure of claim '7 in which said hollow plug part has the form of a block with a pair of parallel spaced apertures extending therethrough, the internal surfaces of which constitute said internal surface portions, and said second plug part having the form of a plate with said'slots alined with said apertures of said block. -9. The structure of claim '7 in which said hollow plug part has the form of a cap with a cylindrical recess and'a. notch in its open end extending laterally from said recess through the wall thereof, said second plug part having the form of a cylindrical plug body of slightly less axial length than said recess, with a longitudinal groove in its peripheral surface, and fitting flush in the recess of said cap, said cap and body being threadedly engaged by relative rotation, whereby said cord must extend laterally through said notch into said groove and through said groove to its connection with said prongs when said plug is in use in normal engagement with a conventional outlet socket.

ALFRED H. LEJA.

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

Number Number 15 519,572

10 UNITED STATES PATENTS- Name Date Welch Jan. 21, 1913 Conklin Mar. 22, 1921 Goodwin Dec. 27, 1927 Gaynor et a1 June 19, 1928 Malling Dec. 25, 1928 McFadden Aug. 11, 1936 Walter May 23, 1939 Benander Aug. 13, 1940 McLarn Dec. 31, 19450 FOREIGN PATENTS Country Date France Jan. 27, 1921 England Dec. 3, 1925

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