US 2280855 A
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Description (OCR text may contain errors)
J. J, ROSE y April 2s, 1942.
PLUG l CAELE TERMINAL SPARK SHELDED Filed Oct. l5,
' /A/l/c/vra@ L/OH/V d. o sf ZL fr fToQA/pr Patented Apr. 28, 1942 UNITED STATE S PATENT OFFICE (Granted under the act of March 3, 1883, as
amended April 30, 1928; 370 0. G. '157) IZClallns.
'Ihe invention described herein may be manufactured and used by or for Government for governmental purposes. without the payment to me of any royalty thereon'.
This invention relates to improvements in the cable terminals for shielded sparky plugs whereby the life of the high tension cables conducting the high voltage sparking current to"`the plugs is greatly extended.
Shielded spark plugs as now generally used on aircraft engines for -ignition purposes include in addition to the plug proper, a grounded tubular metal shield into which the upper parts of the spark plug core and central electrode project, the inner surface of the shield being lined with a suitable insulating material. The spark plug cable is formed oi' a central conductor surrounded by rubber insulation which in turn `is covered by a braided fabric sleeve coated with lacquer to render it waterproof and to protect the rubber insulation against oxidation. The terminal end of the cable is pushed into an annular sleeve or thimble 1v1-ade of Bakelite or ceramic insulating material which is provided on its end with a suitable contact to which the cable conductor is connected. The lcable terminal ywith the assembled insulating sleeve is then pushed into the-tubular shield of the plug, the contact on the sleeve engaging a contact secured to the upper end of the central spark plug electrode. The spark plug cable is then pulledl through a curved tubular metal elbow forming part of the shielded plug and which is secured by a. union nut to the threaded upper portion of the shielding tube, the
. Joint being sealed by a resilient rubber sealing washer which encompasses the cable; The above noted elbow at its outer end serves as a connection for the usual braided metal conduit or harness through which the ignition cable is passed.
A great deal of difficulty has been experienced with shielded ignition apparatus of the character described which results from several different causes, among which are the high temperatures existing adjacent the plugs due to heat transmission therethrough; high temperatures in the shield elbows, due to the latter being in the vicinity of the engine exhaust conduits; moisturercol- Y lecting along the cable, and especialLv at the plug terminal; necking of'the insulation on the cable by the sealing washer, causing damage to the lacquer protecting the fabric covering of the cable; and damage to the lacquer coating in pulling the cable through the shield elbow.
In accordance with the present invention many of the above troubles are minimized thus greatly extending the service life of the ignition cables, and the improvements in accordance with the invention may be attained by simple means without changing any of the standard ignition units now employed in the art.
lite" or ceramic sleeve, or thimble, generally em-` ployed on the terminal end of the cable is retained and the cable itself is preferably chosen of a smaller outside diameter than would ordinarily be employed with the respective terminal sleeve or thimble. A thin walled rubber tube is slipped over the cable forich a length that the tube will extend at least to or beyond the outer end of the shield elbow when assembled. Aiiat rubber washer is placed over the cable conductor at the terminal end thereof so as to abut the end of the rubber tube and braided fabric covering on the cable. The end of the cable is then forced into the conventional terminal thimble with the rubber washer tightly seated on the bottom thereof thus sealing the cut end of the braided covering of the ignition cable against moisture. The protective rubber sleeve serves to materially protect theignition cable proper from damage from high temperatures both.with in the plug shield and in the shield elbow. and further distributes the sealing washer pressure at the junction of the plug shield and the shield elbow, preventing necking and fracture of the lacquer coating on the cable. The rubber sleeve also protects the lacquer coating on the cable against damage while being pulled through the shield elbow and is further effective to protect the cable against the effects of high temperatures in the vicinity of the shield elbow due to heat from the engine exhaust conduits. The protection of the lacquer coating on the cable at those points where the same is liable to be mechanically damaged is oi great importance because damage or abrasion of the lacquer coating will allow moisture to enter the braid which permits a corona discharge, causing the production of ozone which rapidly oxidizes the rubber insulation of the cable causing the same to swell and crumble and necessitating frequent renewal of the igniil tion cables.
'Ihe principal object of the invention is therefore the provision of a spark plug terminal for ignition cables employed in conjunction with shielded plugs which provides a greater protection to the cable insulation in those portions adjacent and within the plug shielding elements subject to high temperatures. and insures adequate protection of the insulating cover of the cable from moisture and mechanical damage or abrasion.
A further object of the invention is the proyision'of a terminal for high tension ignition cable employed with shielded spark plugs; comprising a rigid thimble of insulating material having a contact thereon adapted to be electrically connected to the cable conductor, a washer of rubber or like material adapted to encompass the projecting portion of the cable conductor and to abut the end face of the cable insulation in sealing relation thereto, and a sleeve of rubber or like material surrounding the cable for a length at least sufficient to prevent contact of the cable with the shielding elements of the spark plug and the walls of the thimble and at its lower-end engaging said sealing washer in sealing relation.
Other objects of the invention will appear by reference to the detailed description hereinafter given and to the appended drawing in which:
Fig. 1 is a side elevation partly in section illustraurig the novel cable terminal assembled in a shielded spark plug, and;
Fig. 2 is a sectional view to an enlarged scale illustrating the details of the terminal of Fig. 1;`
Fig. 3 is a view similar to Fig. 1, illustrating a modified form of the invention.
Referring now to Figs. 1 and 2, the reference numeral I, generally indicates a conventional type of shielded spark plug employed in aircraft engines and having a metal body 2, insulating core y3 and a central high tension electrode I provided at its upper end with a contact button 5. The plug is provided with the usual grounded metal shield 6 screwed into the upper end of the spark plug body 2 and lined with an insulating material such as fBakelite or a ceramic composition, as indicated by the reference numeral 1. The upper end of the tubular shield 6 is threaded as at 8 to receive a coupling 9 which serves to connect a tubular shield elbow IIB to the shield 6, the outer end of the elbow being suitably threaded to receive the coupling member of a well'known type of braided metal conduit II which serves to electrically screen the cable between the distributor and the spark plug.
The high tension cable is generally indicated by the reference numeral I4 and comprises an electrical conductori5 covered by a thick layer of rubber insulation I6, which in turn is protected by an external layer of braided fabric Il coated with lacquer which protects the braid from moisture, and if undamaged, greatly minimizes the possibility of corona discharge which causes rapid deterioration of the rubber insulation.
The spark plug terminal on the cable comprises an annular insulating sleeve or ferrule I8 made of Bakelite or of ceramic material substantially closed at its lower end by a moulded in metal washer or disc I3 to which the cable conductor yi5 is soldered or otherwise electrically connected on assembly. The disc I is provided with a contact 2U in the form of a spring which is adapted to engage the contact on the central spark plug electrode 4. l The ierrule or sleeve I8 with the closure I9 forms av thimble which encloses the end of the cable and is a sliding nt in the tubular shield l. The cable Il instead of directly contacting the thimble vI8 is separated therefrom along the bottom face of the cable by a sealing washer 2l made of rubber or similar material which seats on the bottom I9 of the thimble land abuts the insulation I6 and cut end of the braided fabric covering I1 of the cable in water tight relation, preventing the entrance of moisture on the frayed end of the covering I1 and tightly encompassing the conductor I5 of the cable. The cable is further insulated from the inner walls of the thimble by a tubular rubber sleeve 22 which extends along the cable for a length suiilcient to pass through the shield elbow I0 into the metal conduit or shielding harness II. A rubber or other resilient packing washer 23 encompasses the sleeve 22 and serves as a packing seal for the coupling S.
In practice the cable I4 is chosen of a diameter slightly smaller than would otherwise be employed and the tubular sleeve 22 made of rubber or similar material is slipped over the end of the cable, the sleeve then being inflated with air under pressure is slipped on the cable for the desired length. The sleeve 22, cable covering I1 and insulation I6 of the cable Il is cutV transversely and the proper length of conductor is left projecting for connection to the spring contact 2li. The rubber washer I9 is then forced over the projecting portion of the conductor and seated against the transverse end faces of .the sleeve 22 and the protective covering and insulation ofthe cable, and may be cemented to the tubular sleeve 22 to form a bond therewith. After passing the cable through the elbow Il the insulating thimble I8 is forced over the end of the sleeve 22 until the washer 2l is tightly seated on the closure member I9, the conductor I5 then being connected to the contact 2l and the thimble I8 inserted in the plug shield l to completethe assembly.
The tubular sleeve 22 protects the lacquer coating on the cable from mechanical damage while being pulled through the shield elbow IU and further materially protects the cable covering from the effects of high temperatures in the vicinity of the elbow I0 due to heat from the engine exhaust manifolds. The portion of the sleeve 22 within the thimble I8 serves in conjunction with the washer 2l to protect the cable against any entrance of moisture and further materially reduces the effects of heat transmitted from the plug body and core from. damaging the lacquer coating on the cable exterior. The sleeve 22 further serves to distribute the pressure of the sealing washer 23 so that the cable proper is not necked beneath the washer and the lacquer coating at this point is not damaged as frequently occurs in prior constructions.
The modified form of the invention illustrated in Fig 3 is similar in all respects to the construction of Figs. I .fand 2 except that the ferrule or thimble I8 is'shortened and a second tubular sleeve 24 is placed over the sleeve 22 and terminates at its upper end in an integrally moulded radially extending sealing washer 25 which serves the same function as the sealing washer 23 of Figs. l and 2, but more effectively distributes the sealing pressure to eliminate necking of thecable and to prevent moisture from passing along the cable to the terminal connection thereof.
It is to be 'understood that while the invention has been illustrated as applied to spark plugs employing shielding elbows, the terminal construction is equally applicable to those plugs in which the ignition harness is coupled directly to the plug shield and in such cases the tubular sleeve is made of such a length that the cable is well protected adjacent the coupling member and 4constri'ictio'n :isf -standpointz Itis finther-to i vfor 2a considerable :distance-beyond' from lieat--.` Y transmittedfrom;` thefexhaustaswell as froni'rr mechanical abrasion due" to' bending *of the cable to-makefc'oupling connectionss': It is-further noted; that 'fthe Masher"- 21 may be integrally .foi-meri*withfthesleevefinv though the separate?.
preferableffrom' fassembly- 'L1 f "insulation, protective covering and said sleeve, a
- `tseunderstood mit the temi `rubber as? referringf1toftheItubular. protecting equal-toer greater than the length ofthe shielding. `elements of an associated-spark plug, said .sleeve being-slippedronover said cable to surround the. terminal fend thereof, a resilient sealing' washer -adapted tou.; tightlyv encom- .pass -the projecting; portlonhof theycable conductor and. to'abut the end faces of the cable -1 rigid ferrule `of insulating material having a subsleeve and sealing disc is intended to refer to' having properties somewhat similar to natural rubber and generally referred to in the art as "synthetic rubber.
Though preferred forms of the invention have been illustrated and described other forms and modications will become apparent to those skilled in the art as falling within the scope of the invention as dened by the appended claims.
l. A moisture proof terminal construction for ignition cables of the character used in conlunction with shielded'spark plugs having a grounded shielding sleeve projecting from the plug body; comprising an annular thimble of a rigid insulating material having a substantially closed lower end provided with a contact adapted to engage the central electrode of the spark plug, said contact being adapted to be electrically connected to the cable conductor, a resilient sealing washer seated on the bottom of said thimble and adapted to tightiy encompass the conductor of said cable and to abut the end face of the insulation and protective covering thereof in sealing relation, and a tubular sleeve of rubber or like material surrounding said cable, the end face of said sleeve also engaging said washer in sealing 'relation and the said tubular shield tightly engaging the interior walls of said thimble in sealing relation, said tubular sleeve extending along said cable from the said thimble a distance sufficient to prevent contact between said cable and any of the shielding elements of the associated spark plug.
2. A terminal construction for high tension cables of the character employed with shielded spark plugs having grounded tubular shielding elements extending from the body of the plug; comprising a tubular rubber thimble adapted to encompass the exterior of the cable for a length equal to or greater than the length of the shielding elements of the associated spark plug, the said thimble abutting the end face of the insulation and protective covering of the -cable in sealing relation and seallngly encompassing the projecting portion of the cable conductor, an insulating ferrule made of a rigid insulating material having a substantially closed end provided with a contact adapted to be electrically connected to the cable conductor, said ferrule being adapted to be forced onto said cable over the end portion of said tubular rubber thimble and the contact thereon being adapted to engage the central spark plug electrode when the ferrule and cable are inserted in assembled relation into the shielding elements of the plug.
3. A moisture proof terminal construction for high tension shielded spark plug cables of the character in which the electrical conductor ls surrounded by insulation made of rubber or the like and covered by a braided fabric material coated with a water proofing material; comprising a tubular sleeve madeof a resilient water proof insulating material having a length bothvnatur'al rubber and the various compounds stantially closed end provided withan electrical contact', said. ferrule being forced onto the end of said'fcable. overr. the tubularV sleeve until the sealing washer is compressed against the closed end of the ferrule, and an electrical connection between said contact and the conductor in said cable.
4. The structure as claimed in claim 3, including a second tubular sleeve surrounding said rst named sleeve the lower end of which engages the upper end of the said ferrule, and a radially extending annular sealing washer integrally formed with said second sleeve and serving as a packing seal for the junction of the spark plug shield with other cable shielding elements.
5. A terminal construction for high-tension cables `of the character employed with spark plugs having grounded tubular shielding elements extending from the body of the plug; comprising a tubular thimble of rubber or like material adapted to encompass the exterior of the cable for a length equal to or greater than the length of the shielding elements of the associated spark plug, said thimble abutting the end face of the insulation and protective covering of the cable in sealing relation and sealingly encompassing the projecting portion ofthe cable conductor, an insulating ferrule made of a rigid insulating material having a substantially closed end provided with a contact adapted to be electrically connected to the cable conductor, said ierrule being adapted to be forced on to said cable over the end portion of said thimble and a resilient tubular sleeve made of rubber or like material surrounding said thimble in sealing relation and abutting the upper end of said feriule, the upper end of said sleeve having integrally formed therewith, an annular radially extending sealing element adapted to serve as a seal for the junction of the spark plug shield .with other cable shielding elements.
6. In a terminal construction for high tension ignition cable for use in conjunction with shielded spark plugs, said cable being of the character in which the Velectrical conductor is surrounded by insulation of rubber or like material; comprising a rigid feri'ule of insulating material having a substantially closed end provided with an electrical contact, said ferrule being forced over the end of the cable, an electrical connection between the said contact and the cable conductor, a resilient cylindrical tubular sleeve made of rubber or like material encompassing the cable in sealingrelation, one end of'said sleeve being placed inabutting and sealing relation with the top lof said ferrule and the upper end of said sleeve being provided with an annular integrally formed radially extending disc or washer adapted to serve as a packing seal for the junction of the shielding `well of the spark plug with other cable shielding elements, the overall length of said tubular sleeve being substantially equal to or greater than the outside diameter of the sleeve so that radial pressure exerted on the sealing washer will be distributed into the sleeve without necking the protective insulation of the cable.
of said thimble in sealing contact, said sleeve having formed thereon adjacent its upper end, a radially extending annular disc or washer adapted to serve as a packing seal for the shielding well of the spark plug with other cable shielding elements, said sleeve having an overall length materially greater than the thickness o! said washer portion so as to distribute the radial stresses on the washer into the body of the sleeve to thereby reduce necking oi the insulation on the cable.
JOHN J. ROSE.