US 3348186 A
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Description (OCR text may contain errors)
et, 17, H967 s. R. ROSEN 3,348,186
HIGH RES I STANCE CABLE Filed Nov. 16, 1964 2 Sheets-Sheet 1 INVENTOR 54AM/Z /e /P/V BY f frs-8m a,
W 17, 1967 s, R, ROSEN HIGH RESISTANCE CABLE Filed NOV. 15, 1964 ffii/0161495 United States Patent 3,348,186 HIGH RESISTANCE CABLE Samuel R. Rosen, Lorain, Ohio, assignor to Nordson Corporation, Amherst, Ohio, a corporation of Ohio Filed Nov. 16, 1964, Ser. No. 411,251 15 Claims. (Cl. 3268-214) ABSTRACT OF THE DISCLOSURE High tension electrical cables and conductors and particularly cables having relatively high electrical resistance and low electrical capacity per unit length. More particularly, electrical cables in combination with electrostatic discharge elements and with spray guns having electrostatic discharge elements for use in electrostatic spray coating systems.
Electrostatic spray coating systems of Various types and kinds have been known and used for some time now. Broadly, such systems comprise a spray gun, a high potential discharge electrode, a high voltage power pack and a high tension electrical cable of high conductivity for delivering energy from the power pack to the discharge electrode. In spraying electrostatically, the work to be coated is maintained at a low potential, such as ground, so that an electrostatic field is set up between the high potential discharge electrode and the work. The material intended to coat the work is delivered by the spray gun into the electrostatic field and atomized by one means or another so that the particles of atomized material are charged and, thereafter, guided, directed, pushed, and pulled by the electrostatic field toward the lower potential work for a more effective coating of it.
Various known devices and methods are employed for atomizing the coating material and delivering it in its desired condition into the electrostatic field. The discharge electrode may be the nozzle of a gun or a wire grid, for example, carried on a gun or it may be a wire grid or a single or series of antenna-like wires supported separately from a spray device. In accordance with wellknown electrostatic principles, the discharge element is of a sharp line or point configuration. Power packs supplying a direct current output voltage of the order of 50 kilovolts to 150` kilovolts are employed in such systems. This potential is commonly delivered to the discharge electrode or element by a high conductivity, high tension coaxial cable having a light gauge central or axial conductor of low resistivity surrounded by a radially thick, high dielectric sheath. A cylindrical conductive braid is woven around the dielectric and thereby positioned coaxially of thecentral conductor. An outer insulating and protective sheath usually covers the conductive braid. Normally, the central conductor is connected to one side of the power pack output and the discharge element and the braided outer conductor is connected to the other side of the power pack output and to the potential of the work being coated or to ground.
Electrostatic spray coating systems of the type described above and particularly the means they employ to establish electrostatic fields have certain undesirable operational features arising from hazards inherent in system components themselves, such as the high conductivity coaxial cable, and from combinations of system components. Should the discharge electrode at high potenial and a grounded object come close enough together, a substantial arc will be drawn. Such a discharge may often contain enough energy to ignite a combustible atmosphere or be highly distressing if not lethal to a human.
The size and strength of the discharge arc depends, among other things, upon the amount of stored electrostatic energy that can be delivered at the discharge electrode and the rate of delivery thereto. Any metallic masses in the electrical circuit, as parts of the gun, for example, and especially the ordinary high conductivity coaxial cable used to interconnect the power pack and the discharge electrode, normally have negligible resistance and more than enough electrostatic capacitance at the high potentials employed to produce a dangerous disruptive discharge and/or undesirable amounts of leakage current.
Various attempts have been made to prevent or minimize the possibility of such dangerous and undesired discharges. For example, a fence or other mechanical means may be provided for physically preventing persons or objects and the discharge electrode from approaching each other too closely. Also, electrical means have been employed to reduce the danger of electrical shock. Most commonly, a very high resistance is placed at the output of the high voltage power pack to protect against damage caused by tbe accidental discharge of capacitive energy stored in the power pack as well as to prevent damage to the power pack in the event of a short circuit at the output terminal or at some point beyond it. The effective electrostatic capacity available at the discharge electrode can be limited by a high ohmic value dropping resistor in series with and immediately ahead of and as close as possible to the discharge element. Such a resistor tends to isolate the capacity of the cable from the discharge element or at least to limit the rate of discharge of the stored energy to a safer value.
Although the inclusion of high ohmic value series connected dropping resistors in the electrostatic circuit at the power pack output and immediately ahead of and as close as possible to the discharge element substantially reduces the possibility of deleterious discharges, other disadvantages and undesirable features remain. ,When the discharge element is mounted on a spray gun in an electrostatic system, the necessity of locating a large dropping resistor as closely adjacent as possible to the discharge element proves to be a substantial distadvantage. First, the resistor, typically of l0() megohms or more, must be sufficiently well insulated to prevent arc-over. It is commonly placed in a dielectric bath of transformer oil. A suitable resistor, properly insulated, is heavy and bulky. Furthermore, it must be mounted in an awkward and inconvenient location at or near the outward end of the gun so that the size and electrostatic capacity of the electrostatic system beyond the end of the resistor may be kept to a minimum.
Another problem introduced by the high dropping resistor at the gun is that of providing a suitable anchor for and termination of a high conductivity, high electrostatic capacity cable and of making interconnections between the cable and the resistor. The anchoring of the cable to the gun and the connection of the cable to the resistor mounted on the forward end of it must be met chanically strong enough to withstand ordinary rough usage; because, should the cable become unattached from the gun and disconnected from the resistor, the exposed free end of the cable could freely discharge substantial amounts of energy. In terminating the cable, the grounded braid or shielding must be separated a sucient electrical distance from the exposed end of the central cable conductor to resist a discharge or leakage of this stored energy in the cable from the central conductor at full power pack output potential to the braid.
`The possible heat produced by such a high value resistor closely adjacent the discharge electrode is also a problem. If the discharge element is grounded, a heavy load is imposed on the dropping resistor, generating a substantialV amount of heat. A heat sink or some means must be employedV to dissipate the heat or it will degrade the resistor.
It will be seen that the high value dropping resistor conventionally employed at the forward end of the spray gun and the necessary arrangements for it are physically undesirable and electrically of only limited benefit. The very design of the gun is adversely affected and its ease of operation impaired. Various of its disadvantages electrically have been mentioned and others will be mentioned.
Of the individual components of conventional electrostatic systems, the ordinary high conductivity coaxial cable itself constitutes a separate and substantial hazard. One such cable, for example, has a light gauge stranded copper central conductor surrounded by a polyethylene dielectric insulating layer of approximately 4 mm. radial thickness. A copper ground sheath is braided over the dielectric and covered by an outer rubber covering or sheath. An accidental break or separation in such a cable may permit the capacitive energy stored in the cable to be discharged with unfavorable consequences. Since only a-few feet of ordinary coaxial cable of the type normally employed has substantial capacity to store energy at the voltages normally employed, an interruption of theeable at almost any point throughout its length can be dangerous.
Another problem connected with the use of ordinary high tension coaxial cables apparently concerns the high frequency voltage oscillations which commonly occur in such cables. Such oscillations manifest themselves through electromagnetic radiations of frequencies involved in television transmissions, for example. It is believed that a disruptive discharge at the discharge electrode or an arc-over at the dropping resistor adjacent the discharge electrode in an electrostatic system can excite very high frequency oscillations in the cable and produce standing voltage waves along the cable so that, at certain points along its length, the applied voltages are reinforced and multiplied into voltage peaks of much greater value than the normally applied voltage. These peaked voltages seem to oscillate at a very high frequency.
The high frequency peaked voltages such as described above are believed to be the cause of a phenomenon sometimes experienced with high-tension coaxial cable. At the time of an arc-over of the dropping resistor, for example, a large number of dancing arc fingers have been observed to appear on the outer sheath of the cable, especially along any portion which is lying on the ground. Also, a break or rupture in the cable is likely to occur concurrently somewhere in the same length of cable. It is believed that the peaked voltages excited by the arcover are of suicient strength to penetrate the dielectric surrounding the central conductor of the cable. The potential peaks arelnot carried off by the grounded braid surrounding the dielectric, however, but penetrate the braid and thev outer sheath to appear along its exterior surface and go directly to ground. It is believed that-this is dueto the higher impedance of the current path through the grounded braid compared with the impedance through the outer covering of the cable to ground at the frequency of the voltage peaks. Once the current path is formed from the central conductor through the dielectric and/or the outericovering, chemical and/or mechanical deterioration of the dielectric takes place and the cable fails.
The problem in terms of the interfering electromagnetic radiations in and from high tension cables is known, for example, in connection with automobile ignition sys tems and the interference they produce in televisionreceivers. Among the solutions that have been proposedfor this problem are ignition cables having a longitudinally distributed resistance which apparently tends to damp the troublesome high frequency oscillations. In the past,
ithas been proposedto provide distributed resistance cable for ignition systems ofinternal combustion engines by making the entire flexible cable an electrical resistance.
This can be done, for example, by having a molded or extruded central semi-conducting flexible element comprised of a mixture of carbon particles and a suitable plastic material and an ordinary insulating sheath enclosing this poor conductor. Such a cable is described in the T. F. Peterson Patent No. 2,790,053, issued Apr. 23, 1957.
Although such cables might be satisfactory in automobile ignition systems, they are electrically unstable under the circumstances encountered in the operation of electrostatic spray coating systems. Repeated flexing of such cables and the continuous application of high potentials in the order of 50 kilovolts to 150 kilovolts generally results in a gradual decreasing of the resistance per unit length of cable, or at least a change from the desired values. In addition, in such a molded form or construction, any air trapped in the semi-conductive element may well be subjected to differences of potential that ionize the trapped air and produce voltage concentrations which break down the resistance of the material by mechanical and/or chemical means. For whatever reasons, such cables are electrically unstable and their resistance value cannot be maintained under operating conditions. An additional difficulty with such cables is encountered in attempting to provide suitable terminations or means for connecting other circuit elements to the ends of such a cable.
Another way that has been proposed to build a cable having a high total resistance distributed along its length is disclosed in the E. I. Shobert, II, Paten-t No. 3,045,199, issued July 17, 1962. This cable is intended for use as internal combustion engine ignition cable and comprises a prefabricated string of carbon resistors longitudinally spaced a fixed distance apart by wire leads extending from their ends and soldered or electrically connected together. The string of soldered-together resistors is covered with a exible insulating sheath.
Such a cable suffers from several disadvantages. The wire leads extending from commercially available carbon resistors are able to withstand only a limited amount of flexing after which they break. Further, no way is known to me and no Way is disclosed in the patent mentionedA above to apply the flexible insulating sheath to such a string of soldered-together resistors without leaving voids between the insulating sheath and the resistors at the squared-olf resistor ends where the small diameter wire leads are joined to the larger diameter encapsulated carbon portion. The dielectric sheath will not follow abrupt changes in the diameter of the core or conductor being covered. Nor can the sheath be applied without obtained irregular outside diameters. Such voids trap air which is easily ionized by the difference in applied potentials. Resulting voltage concentrations then produce mechanical and/or chemical deterioration of the insulating sheath and the cable fails. Furthermore, along the small diameter wire leads of the resistors, the electrical stress towhich the insulating sheath is subjected is generally muchV above optimum at the voltages that might be applied in electrostatic spray coating systems.
The objects of this invention include the solutions of the problems mentioned above as well as the provision of the necessary mechanisms and means which are economical to manufacture, use, and maintain.
A more specific object of this invention is to provide a high tension electrical cable having resistance and capacitance characteristics at high applied voltages of the order` of 50 kilovolts to 150v kilovolts, for example, which cooperate to provide a cable that is less hazardous and moresafe than ordinary high conductivity coaxial cables and the like-employed in the same or similar uses. A further object is to provide such a high tension cable whoseconstruction permits various impedance patterns to be easily provided alo-ng the length of the cable according to a;predetermined plan of uniformity or variation. Yet'another object is to provide a cable having the necessary-mechanicall strength. and the necessary electrical strength and stability to withstand ordinary industrial production usage without unusual care.
A more specific object of this invention is to provide a high tension electrical cable and electrostatic discharge element combination having all of the safety and protection against dangerous and deleterious disruptive discharges and leakages to ground found in the ordinary electrostatic system having high conductivity coaxial cable and a dropping resistor closely adjacent a discharge element as described above and without having any of the undesirable features and disadvantages found in that cornbination. In particular, it is an object to provide such a high tension electrical cable and discharge element combination that achieves its safety with respect to deleterious discharges without the use of a large and cumbersome dropping resistor closely adjacent the discharge element.
This invention also has as an object the provision of such a high tension cable and discharge element combination that may be simply and conveniently mounted and carried on and combined with a spray gun so that electrostatic spray coating may be easily and safely performed.
In particular, it is an object of this invention to providel a high tension, high resistance cable in combination with a discharge element mounted on a spray gun for establishing an electrostatic held by direct connection of the cable to the discharge electrode without a high ohmic value dropping resistor interposed.
The foregoing objects and advantages, together with other notable features of this invention, will become apparent from the following description taken together with the accompanying drawings of a preferred form of cable embodying the invention and the combination of such a cable with an electrostatic spray gun. In the drawings:
FIGURE 1 is an axial cross section through a cable embodying the invention;
FIGURE 2 is a partially axially sectioned View in enlarged scale of a portion of the cable of FIGURE l, but With one element modified and shown in its preferred form;
FIGURE 3 is the same as FIGURE 2, but with a modied form of said one element; and
FIGURE 4 is an elevation View o-f an electrostatic spray gun in full axial cross ,section and in combination with the high resistance cable disclosed herein and shown in FIGURE l.
Briefly, the invention comprises a tiexible electrical cable having a high total resistance. The resistance is longitudinally distributed along the cable. The cable consists of an axially central high resistance conductor made up of alternately arranged and series connected high and low resistance segments. The high resistance segments are preferably rigid plastic encapsulated carbon resistors of cylindrical form and having short wire leads extending from the center of their circular ends. The low resistance segments are preferably resilient and somewhat bendable cylinders of the same outside diameter as the carbon resistors and having cavities in their ends or bores through them for telescopingly receiving the wire ends of adjacent resistors. The central conductor made up of such segments may lbe held together and restrained by a nonconducting braid of Fiberglas which can be easily and smoothly woven over the serially arranged segments of uniform outside diameter. The central conductor, with or without a confining non-conducting braid, is enclosed by a dielectric having a radial dimension appropriate to its electrical properties and the service intended for the particular cable. The dielectric may be covered or surrounded by a conductive braid of copper having a coaxial relationship with the axially central conductor. The entire cable may be provided with an outer insulating covering.
FIGURE l shows a short length of cable embodying this invention in axial cross section. The cable includes an axially central high resistance conductor indicated generally at 9 and made up of a number of cylindrical high 6 resistance segments 10 aligned on the axis of the cable and longitudinally spaced apart from each other and, at the same time, electrically connected together by a number of cylindrical low resistance segments 11. Segments 10 are conveniently commonly available carbon resistors such as used in electronic circuitry. Short wire leads 12 extend axially from the generally circular end faces of high resistance segments 10 and are telescopingly received within an internal bore or cavity 14 opening on each end of the low resistance segments 11.
Vinyl, heavily loaded with carbon black, has been found to be a suitable ymaterial for use in forming low resistance connecting links or segments 11. Such material is a very good conductor by virtue of the presence of the canbon black. It is also resilient enough so that, when properly dimensioned with respect to the size of the cooperating wire leads, it continuously grips the wire leads inserted into its ends and provides good electrical contact through the connection While tending to maintain the connected adjacent segments always in end-to-end abutting engagement. The resilience also permits some deformation of the low resistance segment when the cable is bent or curved.
It will be seen that the high and low resistance segments are alternately arranged along the axis of the cable in a serially connected and end-to-end relationship. Each wire lead 12 is positively gripped by the resilient side wall of a ibore or cavity 14. Thus, electrical contact between each high resistance segment and its adjacent low resistance segment is made through the outer surface of each wire lead and the interior surface of the receiving bore or cavity in the adjacent low resistance segment. There is ordinarily no contact between the outer cross-sectional end of eachwire lead 12 and any part of low resistance segment 11. It will be apparent that, upon :bending or curving of the cable, these ends are urged to one side and may come in contact with some portion of the adjacent low resistance segment 11 as the resilient segment yields and the stitf wire lead 12 digs into the wall or the bore of cavity 114.
The axially central high resistance conductor '9 described above is preferably confined and held together, in part at least, by a non-conducting braid woven around and alongit. Such a braid, indicated generally at 15, need not be of a particularly close weave and has been successfully woven of Fiber-glas. The virtue of such a braid is mechanical. It tends to fixedly hold together in end-t-o-end engagement the string of segments making up the central conductor even when the cable is subjected to bending and/ or axial loads so that air pockets cannot develop between the conductor at high potential and the dielectric sheath.
A heavy dielectric cylindrical sheath, indicated ,generally at 16, lies closely around and along central conductor 9. The uniform outside dia-meter of central conductor 9 permits such a dielectric sheath 16 to be applied by extrusion, for example, without producing air pockets or voids between the high potential portions of the central conductor and the dielectric. A uniform outside diameter is also achieved. It Will be observed that the void in the axially extending bore 14 of each low resistance segment 11 as shown in kFIGURE 1 is not subjected to different potentials and, therefore, not ionized. The radial thickness of the -dielectric sheath should be appropriate to the electrical properties of the materials employed and the operating conditions and electrical values likely to be encountered. It will be observed that the voltage stress on the dielectric sheathis considerably less for a given voltage difference across the conductor and sheath than would be developed in a cable in which the conductor links between resistors are only much smaller diameter wire leads.
A coaxial and/ or shielded cable may be constructed by weaving a conductive braid, indicated generally at 17, closely around and over the exterior of the dielectric sheath 16. When the outer conductive braid 17 is employed to provide a coaxial cable, the relatively large 7. diameter high potential, high resistance, central conductor 9 naturally resulting from the cable constructlon embodying this invention cooperates to provide a high quenching configuration. That is, the tendency of a specified applied voltage to produce an arc or discharge to a neighboring ground and to maintain such an. arc is less from a larger diameter or less pointed or less sharp element than from a smaller, `liner, and sharper element. The entire cable may be provided with an outer insulating covering 18.
Cables similar to the one described above have been built and successfully used. A particular cable, twenty-live feet long and intended for use in an electrostatic spray coating system at an applied voltage of about 100 kilovolts, consists of two hundred and forty 1A watt carbon resistors of 1.5 megohms each. The cable, therefore, has a total resistance of approximately 360 megohms, the resistance of the conductive connecting links being negligible. The resistances in this cable are all of the saine value and uniformly spaced on a 11/4 pitch pattern. The val-ue of theV resistors and/ or the spacing `between them, of course, can be Varied to provide a cable having a tapered resistance characteristic such as for loading the ends of a cable or approaching connections at which impedance mismatches occur. The resistors and conductivelinks or segments used in this cable are approximately three thirtyseconds inch in diameter and one-eighth` and one inch lon-g, respectively.
FIGURE 2 shows in enlarged scale the combination of a single high resistance segment or resistor similar to segments 10 shown in FIGURE 1 and a preferred form of conductive connecting link or low resistance segment 20. Low resistance segment is several times longer and of the same outside diameter as high resistance segment 10. Rather than ibeing tubular like low resistance segments 11 in FIGURE l, connecting link or segments 20 is substantially solid except for cavities 21 opening on each circular end of the segment and extending axially inwardly thereof as revealed iby the partial sectioning of FIGURE 2. Each cavity 21 is formed with an axially short cylindrical entrance portion 22 of `generally uniform diameter adjacent its open end. Inwardly of entrance portion 22, each cavity 21 is enlarged and has a greater inside diameter than that of entrance portion 22. As. shown in FIGURE 2, entrance portion 22 of each cavity 21 is enlarged into a generally conical or ared shape. The purpose of enlarging the inner end of cavities 21 is to enhance the flexibility and curvability of the cable and to lessen the strain imposed on the wire-leads 12 during llexing and bending of the cable by providing room for the wire leads to swing more freely about and from side to side as required when the cable is not straight. Very satisfactory continuous electrical contact is maintained-between the outside cylindrical surface of each wire lead 12 and the inner cylindrical surface of entrance portion 22 of cavity 21 receiving it. Further limited swinging movement between the wire lead andA the low resistance segment is possible without endwise or butting engagement and interference because wire leads 12 are shorterthan the depth of cavities 21.
FIGURE 3 shows a modied form`25 of preferred low resistance segment 20 shown in FIGURE 2.. Segment 25 is formed in three separate parts which, when assembled, form a whole not unlike segment 20y of FIG- URE 2. As shown, modified segment 25 consists of two tubular end members 26 and a central double-ended stem member 27. This arrangement provides what may be a less difficult manufacturing problem. Also, the configuration may contribute to a lesser resistance to bending by permitting a certain very slight relative shear stress relieving motion along the cylindrical surfaces of engagement between each stem and the end portion telescoped over it, thereby contributing to an increased llexibility of the cable. The end portions are dimensioned so that the stems of the central portion are grippingly engaged and electrical contact through this modified segment 25 is,
8 atl all times, as high as that enjoyed by segment 20 in FIGURE 2.
In electrostatic spray coating systems, high resistance cable comprehended by this invention and described above can be used in combination with discharge electrodes for establishing electrostatic ields at locations remote from D.C. power sources of high Ivoltage. The resistance distributed along the length of a piece of such cable, when electrically connected to a discharge electrode of low effective electrostatic capacity, provides an electrostatic circuit combination as safe from disruptive discharges to ground as the known series combination of a high conductivity cable, a high resistance dropping resistor and an electrostatic discharge element electrically closely adjacent the dropping resistor. In addition, however, the use of distributed resistance cable connected directly and closely with the discharge electrode has the additional substantial advantage of being lighter, smaller, and less complicated. The problem of terminating and connecting the cable to the resistor and then the resistor to the discharge electrode is simplified to one of connecting the cable to the discharge electrode. Additional safety features are enjoyed by use of the distributed high resistance cable. Any unintended separation of the cable and the discharge electrode will have much less tendency to produce a dangerous and distressing discharge than a similar separation of the low resistance cable from the dropping resistor of the previously known combination. Further, any accidental cutting of the cable intermediate its ends is less likely to produce a deleterious discharge than would a similar cutting of conventional low resistance cable. This is due, of course, to the high resistance-to-capacitance ratio per unit length of the new cable `as well as its favorable quenching configuration.
The distributed resistance cable and electrostatic discharge electrode combination may be combined with a spray gun as shown in sectioned side elevation in FIG- URE 4. While the gun illustrated and described below is intended particularly for use in systems in which hot paintis introduced into and contained in the chamber of the gun at high pressure and is intended to be shot from the gunat frequent intervals by manual or remote controlled trigger actuation, this invention is not limited to such a gun, this particular gun being shown for purposes of'illustration. The gun, as shown in FIGURE 4 in combination with one end of a high resistance electrostatic cable comprehended by this invention connected to a low capacity discharge electrode may be considered to be an electrostatically adapted version of the paint spr-ay gun disclosed by me together with E. T. Nord in Patent No. 3,116,020, issued Dec. 3l, 1963, the description therein of the structure and operation of the paint handling features of the gun being incorporated by reference in this description.
Referring now to FIGURE 4, it will be apparent that the gun G comprises a hand grip R with .an integrally formed stocks, having, conveniently, a suspension hook K at the top of the stock, a trigger T pivotally swinging about and depending from a pivot pin P near the top of the stock, a high pressure paint head H attached to and electrically insulated from the forward end of the stock and preferably formed of a non-conducting material, a barrel B removably attached to the forward end of high pressure head H and preferably formed of a non-conducting material, a spray nozzle N at the extreme forward end of the barrel with the discharge valve V adjacent'thereto within the barrel and controlling the flow of paint through the nozzle. Paint head H contains a high pressure fluid chamber C to which paint is conducted and/ or through which paint is circulated by hoses O by connectors E that have free fluid connection with the chamber C. Barrel B|is also in communication with lluid chamber C. The means for operating valve V by the squeezing and'swinging of trigger T so as to effect discharge of paintr from chamber C through theinside ofA barrel B and valve V 9 to spray into atmosphere through the nozzle N is completely described in Patent No. 3,116,020 mentioned above.
As shown in FIGURE 4, high pressure head H, barrel B and all of the valve and nozzle supporting structures,
including adaptor nut A and retaining nut U, are formed of a non-conducting material. The valve closure member and the valve seat member are preferably made of a very hard material such as carbide as is the nozzle orifice piece. The nozzle element N and the valve seat closure members are, however, mounted, retained, and supported by electrically non-conducting structures.
- A high resistance cable comprehended by this invention and indicated generally at 30 in FIGURE 4 is introduced into the gun through the lower end of handle R. The outer covering 31 is removed for la distance adjacent the end as shown and the cable is gripped by cooperation of terminal sleeve 32 having an annular radial flange 33 held against internal radial shoulder 34 by cable retainer nut 35. A protective spring surrounding the cable and extending away from handle R may be employed to limit bending of the cable adjacent the gun G. At the inward end of terminal sleeve 32, the conductive braid 36, located just beneath outer covering 31, is terminated and a porjust described are all formed of electrically conductive tion folded back and held tightly against the outer surface of terminal sleeve 32 by crimping sleeve 37. Handle R, stock S, trigger T, and the cable anchoring means material and, thus, the handle and that portion of the gun gripped and held by an operator are in circuit with the low resistance conductor or braid 36 of the coaxial cable 30 which, in turn, is normally connected to ground at the D.C. power source.
' From handle R where the cable 30 is anchored and connection made between the grounded braid 36 and the handle, the high resistance central conductor, indicated generally at 38 in FIGURE 4 and surrounded by the dielectric 39, is fed upwardly through stock S and then forwardly through longitudinal passages Q provided in paint head H and barrel B to a point near its forward end. The cable is terminated at a point in its length ending in one of its low resistance segments 381. A conductive button 39 is screwed into the opening in the exposed end of a low resistance segment 38 and is engaged by and in electrical contact with contacting and conducting spring 40, The forward end of this spring terminates in a conducting button which is in electrical contact with a second conducting spring 41. This latter interconnection is made to permit the disassembly of the parts ofthe forward end of the gun barrel B. The forwardmost end of the second conducting spring 41 engages and is in electrical contact with conductive seal washer 42 which, in turn, is in electrical contact with a iine wire-like antenna or discharge electrode 43 located radially beside but close to the nozzle oriiice and extending slightly forwardly thereof.
The mass of electrically charged conducting material between the end of high resistance cable 30 and the pointed end of discharge electrode 43 in the electrostatic spray gun in FIGURE 4 is relatively very small and provides a low effective electrostatic capacity. The combination of the high resistance cable and the low capacity discharge electrode assembly tends to inhibit and/or prevent deleterious discharges from the discharge electrode caused by its proximity with grounded objects to as great an extent as the known combination of a low resistance cable and low capacity discharge electrode with a high resistance dropping resistor interposed closely adjacent the discharge electrode. The construction shown in FIG- URE 4 employing the high resistance cable of this inven tion, however, enjoys additional substantial advantages. The high resistance distributed along the length of the cable obviates the necessity for a long and bulky high resistance dropping resistor located on and in the region of the nozzle end of the barrel. Also, the necessity for a heat sink for the dropping resistor is avoided. Of great importance is the increase in safety achieved in the event the high resistance cable is accidentally cut or pulled out of the gun.
The particular high resistance cable with its resistance distributed along its length and described above has its own inherent safety features which operate for the cable users benefit whenever the cable is employed in any application involving high potentials. For example, because of the high resistance-to-capacitance ratio per unit of length, an accidental cutting of the cable at any point is not nearly so dangerous as cutting a conventional high conductivity cable because, no matter where the improved cable is cut, the high resistance-tocapacitance ratio is enjoyed and its beneiits in suppressing and/or retarding the dangerous outflow of stored energy is obtained. Another advantage of the improved cable of this invention is the relatively low voltage stresses able to be obtained by the appropriate selection of the external diameters of the segments comprising the axially central conductor for a given external diameter insulating sheath. Known formulae existfor guidance in selecting such external diameters. For example, the particular external diameter of the central conductor of a coaxial cable having a grounded sheath or braid that will produce the minimum voltage stress possible for a dielectric sheath of a predetermined external diameter can be calculated by known methods. The various external diameter relationships between central conductor and insulating sheath that result in minimum voltage stresses possible have not been able to be used in conventional high tension coaxial cable hav ing a low resistance central conductor and used in ap paratus used in establishing electrostatic fields because, as the external diameter of the low resistance central conductor is increased toward the size required to reduce the voltage stress in the cable of a predetermined outside diameter to the minimum possible, the capacitance of any appreciable length of the cable increases at least to a value sufficient to draw a harmful shock or spark to ground from the exposed end of the central conductor of optimized external diameter. Also, the value` of capaci` tance built into a cable by dimensioning the central conductor for minimum possible stress cannot be accommodated in a shock-proof or explosion-proof apparatus for providing an electrostatic leld. In other words, such a low resistance cable adds more capacity to such apparatus than can be safely accommodated at the high voltage levels contemplated, i.e., 501 kilovolts to 1501 kilovolts, even by using a dropping resistor between the cable end and the discharge electrode. For example, the order to stress obtained with known conventional low resistance, high tension cables intended for use in apparatus for providing electrostatic fields with an accepted amount of safety from dangerous discharges is substantially more than within ten percent of the minimum possible stress obtainable for their external diameters.
My present best information is that the capacity of a distributed resistance cable embodying and comprehended by this invention does not increase more or less linearly or in the same disadvantageous fashion or to corresponding unacceptable values as it does in a conventional low resistance, high tension cable when the external diameter and/or length and mass are increased and that this favorable result is due, to a substantial extent at least, to the distributed arrangement of the high total resistance of the cable. Consequently, the external diameter of the axially central high resistance conductor in cable comprehended by this invention can be chosen to provide a voltage stress within at least ten percent of the minimum possible stress, if not the minimum possible, for a predetermined external diameter insulating sheath without building in capacities that cannot be safely accommodated in any length of cable or in combination with anA 11 viding an electrostatic field. Among other things, this advantage contributes to the successful building and uses a. more compact, more `flexible, and a safer high tension cable than was formerly possible.
While a preferred and modied form and embodiment of the invention is herein described and illustrated, including specific examples of sizes and proportions of parts that have been found to be useful in the best present embodiment that has been perfected, still, changes, modifications, and improvements will occur to those skilled in the art who practice this'invention and come to understand it, all without departing from the scope and substance thereof and hereof. Therefore, I do not care to be limited inthe scope and effect of my patent for my invention to the forms, examples, and embodiments thereof herein specifically illustrated and described, nor in any other way or fashion inconsistent with the progress in the art which my invention has promoted.
1. A flexible, high-resistance, low-capacitance, electrical cable comprising a plurality of rigid, high-resistance segments axially spaced from each other throughout the length of the cable and having small metallic conductors at their opposite axial ends in said cable, and alternately disposed tlexible, non-metallic, conductive segments axially aligned with and axially longer than said high-resistance segmentsand conductors, and electrically connected to said high-resistance segments through said conductors, all of said segments having about the same external diameter and being closely contained in air-free engagement in an insulating sheath.
2. The cable of?` claim 1 in which said high-resistance segments are relatively hard plastic encapsulated carbon resistors having wire leads extending axially from their ends and said conductive segments have axially extending interior cavities opening into their ends for telescoped mechanical connection and elastically stressed gripping mechanical engagement and electrical connection with said wire leads.
3. The cable of claim 2 in which said axially extending interior cavities each has only a short portion of axial cavity length closely adjacent its opening into a segment end in mechanical engagement and electrical contact with one of said wire leads, whereby saidstiff wire lead entering each of said cavities has continuous resiliently stressed mechanical engagement and electrical contact with said short portion of axial cavity length when said cable is straight and when said cable is curved and only possible intermittent touching mechanical engagement and electrical contact with said segment elsewhere than said short portion of axial cavity length inside said cavity when said cable is curved.
4. The cable of claim 3 in which said conductive segment comprises a pair of end elements, each having a longer continuous axial bore' of larger diameter at one end and a shorter and smaller diameter bore at the other end, a longitudinally central and extending connecting element having stem portions extending into resiliently stressed mechanical engagement and good electrical contactwith said larger diameter bores of said pair of end elements, said stem-like extensions of said central element being short of reaching said smaller diameter bores of said end elements.
5.A flexible, high-resistance, low-capacitance, electrical cable comprising a plurality of rigid, high-resistance segments axially spaced from each other throughout the length of the cable, and .alternately disposed flexible, nonmetallic, low-resistance, conductive segments axially aligned with said high-resistance segments and electrically connected to said high-resistance segments, all of said segments having about the same external diameter and being closely contained in an air-free insulating sheath.
- 6.A flexible, high-resistance, low-capacitance, electricalcable comprisingga plurality ofV rigid, high-resistance segments axially spaced-"from each other throughout the length of the cable and having connectors at opposite axial ends and flexible, elastic, non-metallic, conductive segments axially aligned with and alternately disposed between said high-resistance segments fixedly elastically and electrically connected to said connectors, said plurality of segments being closely contained in an insulating sheath.
7. A flexible, high-resistance, low-capacitance, electrical cable comprising a plurality of rigid, high-resistance segments and flexible, elastic, low-resistance segments alternately arranged and electrically connectedV together and mechanically connected to each other in fixed end-toend engagement externally and peripherally and in stressed elastic male and female engagement internally and centrally said segments being closely contained in an insulating sheath.
8. The cable of claim 7 in which said segments are all about 1A; inch in external diameter, the rigid segments being about 2 to 4 diameter-s in length and the flexible segments being about 2 to 4 times longer than said rigid segments.
9. Al flexible, high-resistance, low-capacitance, electrical cable comprising aninsulating sheath, a plurality of rigid, high-resistance segments and flexible, elastic, lowresi-stan-ce segments alternately 'arranged and electrically connected together and mechanically connected to each other in tight end-to-end engagement externally and peripherally and in elastic and yielding male and female engagement intern-ally and centrally, all said segments having substantially the same external diameter and having tight, air-free, continuous engagement with the interior of said insulating sheath.
10. The cable of claim 9 together with a longitudinally continuous non-conductive braid extending around and along said plurality of segments radially between said segments and said sheath tending to maintain and insure the tight end-to-end engagement of said segments.
11. A flexible, high-resistance, low-capacitance, electrical cable comprising a plurality of rigid, high-resistance segments and flexible, elastic, low-resistance, non-metallic segment-s alternately arranged and electrically connected together and mechanically connected to each other in stressed, elastic male and female engagement said segments being closely contained in an insulating sheath.
12. A flexible, low-stress, low-capacitance, high-resistance, high-voltage, cable adapted for electrostatic transmission, comprising lan outer conductive sheath, an inner insulating body and a central coaxial internal conductor, said conductor comprising alternately arranged and electrically connected axial segments of rigid resistors and flexible conductors of the substantially same external diameterA and of length from about 2 to 16 times said diameter; the ratio of thev external diameter of said insulating body to said conductorndiameter being approximately that for minimum stress therebetween.
l 13. The cable of claim 12 in which said ratio is comprised not more than about ten percent for the sake 0f reducing the capacitance of said flexible conductors.
, 14. A flexible, high-resistance, low-capacitance, electrical cable comprising a plurality of rigid, high-resistance segments axially spaced from each other throughout the length of the cable, and alternately disposed flexible, lowresistance, conductive segments axially I.aligned with said high-resistance segments and electrically connected to said high-resistance segments, all of said'segments having about the 'same external diameter and being closely contained in an air-free insulating sheath, said external diameter having a relationship-to the external diameter of said insulating sheath tending to provide a voltage stress between said segments and said sheath of approximately the minimum possible stress for a given external diameter insulating sheath;
15. Av flexible, high-resistance, low-capacitance, electrical cablecomprising a plurality of rigid, high-resistancesegments axially spaced from each other throughout the length of the cable, and alternately disposed flexible, 10W-resistance, conductive segments axially aligned with said high-resistance segments and electrically connected to said high-resistance segments, all of said segments having about the same external diameter and being closely contained in an air-free insulating sheath, said external diameter having a relationship to the external diameter of said insulating sheath tending to provide a voltage stress between said segments and said sheath within ten percent of the minimum possible stress for a given external diameter insulating sheath.
References Cited UNITED STATES PATENTS 1,909,248 5/1933 Benkelman 338-66 X 14 Geiger et al. 240-41 Setterblade S15-58 Webber 339-60 Wood 338-232 X Schrotter et al. 338-214 Shobert 338--66 Juvinall et al. 239-15 X Sibary 338-64 X Great Britain.
RICHARD M. WOOD, Primary Examiner. 15 V. Y. MAYEWSKY, Assistant Examiner.