Electrical cartridge-type heater
United States Patent [1 1 Lindroth et al.  
 [ 1 Apr. 29, 1975 ELECTRICAL CARTRlDGE-TYPE HEATER [75] Inventors: Herbert L. Lindroth, Ballwin; Carl E. Barker, Overland, both of Mo.  
 [73] Assignee: Industrial Engineering and Equipment Co., St. Louis, Mo.  
 [22] Filed: May 28, 1974 [21] Appl. No.: 473,288  
 [52] vU.S. Cl. 338/302; 29/511; 29/619; 219/541; 338/240; 338/270; 338/331 [51] Int. Cl 1101c 3/00 [58] Field of Search 338/238, 240, 302, 331, 338/276, 270; 219/541; 29/511, 619  
 [56] References Cited UNITED STATES PATENTS 3,310,769 3/1967 Simmons 338/270 3812580 5/1974 Drugmand 338/240 X Primar E.\&#39;uminerE. A. Goldberg [5 7] ABSTRACT An electrical cartridge-type heater has a metal sheath containing a heating coil embedded in an insulating core. Connections between the coil and terminal pins are made by means of flat conductor strips which are wound around the pins and compressed by surrounding insulation. An improved method for securing the connector strips to the embedded portions of the terminal pins comprises twisting the strips about their longitudinal axes, inserting the twisted strips in preformed passages in the core member, pressing the terminal pins into the passages against the spiral surfaces of the twisted strips, to cause the strips to become entwined around the pins, installing the core assembly in a metal sheath, and then swaging the assembled unit.  
 10 Claims, 5 Drawing Figures 1 ELECTRICAL CARTRIDGE-TYPE HEATER BACKGROUND OF THE INVENTION The present invention relates to electrical cartridgetype heaters, and methods of manufacturing thesame. A failure of many cartridge heaters is the result of faulty connections between the heater coil and the terminal pins. In heaters where there is no positive coupling of these parts, as where the coil ends and pins are held together merely by the surrounding insulating material, a high resistance junction and in some instances internal arcing may occur.  
  The conditions is particularly manifest in heaters wherein the coil ends extend along the terminal pins in the surrounding insulating core and which, at best make only linear contact with the pins. Contact and conductance may be further diminished by the entry of powdered insulating core material between the confronting conductors. Such poor connections often result in excessively high internal temperatures that cause early failure of the heater.  
  The principal object of the present invention is to provide improved means and a method for producing a positive surface-tosurface, low resistance electrical connection between the heat coil and terminal pins, inside the ceramic core of the heater unit, and to accomplish the same by a simple procedure and means that do not materially increase manufacturing costs.  
 SUMMARY OF THE INVENTION In an electrical cartridge-type heater of the present invention connections between ends of the heater coil and terminal pins are made with the use of short strips of conductor ribbon. The connector strips are initially twisted, then inserted into restricted passages in a ceramic core body upon which the heater coil is wound. Ends of the strips are secured to the coil ends outside the core, and terminal pins are then inserted into the passages against the twisted surfaces of the strips to cause the strips to become tightly entwined around the pins. Thereafter the core assembly is installed in a tubular metal sheath and the unit is laterally compressed by swaging.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 of the drawings is a plan view of a preformed connector strip;  
  FIG. 2 is a plan view, partly in section, illustrating the core assembly with connector strips in place, prior to installation of terminal pins;  
  FIG. 3 is a longitudinal sectional view of the heater assembly with all parts in place but prior to a swaging and trimming operation;  
  FIG. 4 is a longitudinal sectional view through the unit following swaging and trimming, and  
  FIG. 5 is a transverse sectional view taken at line 5-5 of FIG. 3.  
 DESCRIPTION OF THE PREFERRED EMBODIMENT As appears in FIG. 4 the cartridge heater of the present invention comprises a tubular metal casing 6 containing a cylindrical core member 7 formed of a compacted body of refractory insulating material, such as magnesium oxide. A helical coil 8 of resistance wire, wound on core 7, is embedded in a layer 9 of refractory material that surrounds the core assembly and insulates the coil from the casing. A pair of terminal pins 10 extend longitudinally through core 7 and are connected each to one end of the coil 8 by elements 11 formed of flat wire ribbon. These connector elements are entwined around the pins and are held in intimate surface contact therewith within the compacted core 7. Pins 10 project outwardly from one end of the unit, extending through a closure plug 12 at that end, and at the opposite end of the unit the pins abut an insulating disc that separates the pins from a metal closure disc 14 which is welded to the casing wall.  
  Curciform elements 15, FIG. 5, are formed of refractory material and are primarily employed to support the core assembly centrally within the casing during the assembly process, as will be explained.  
  As shown in FIG. 2 the preformed core member 7 is provided with a pair of parallel passages 15 for receiving the terminal pins, and also with lateral recesses 17 at opposite ends of the core, each recess communicating with one of the passages. Passages 16 are slightly larger in diameter than that of the terminal pins, but desirably such that the passage diameter does not exceed the pin diameter by more than twice the thickness of the ribbon from which the connector elements 11 are made. The coil 8 is wound on the core so that a number of the end turns 8a thereof extend across the passages 15 at the recesses 17.  
  The connector elements 11 are simply formed from short strips of conductor ribbon by twisting the strips about their longitudinal axes one or more full turns, and bending an end portion of each twisted strip to form a crook 110, as shown in FIG. 1.  
  After the coil 8 has been wound on the core member 7 as above described the twisted elements 11 are inserted into the core passages 15, one from one end of the core member and the other from the opposite end, and the transversely bent end portions 11a of the elements are hooked over end turns of the coil and clamped thereto. With the twisted strips held in place as described the terminal pins are slid into the passages against the spiral surfaces of the elements 18, the effect of which is to cause the elements to entwine around and come into surface contact, as distinguished from linear contact, with the pins.  
  With the pins thusly in place in the core-coil assembly, projecting a substantial distance from one end of the core and protruding slightly from the opposite end, the cruciform spacer elements 15 are threaded on the pins, one at each end, and the assembly is inserted into the casing containing the insulating disc 13 resting upon the inner surface of the metal closure disc 14. At this stage of the assembly process the elements 15 serve to support core assembly coaxially within the casing, with the coil 8 narrowly spaced therefrom.  
  The interior space surrounding the core assembly is then packed with powdered magnesium oxide or the equivalent. Such material is introduced into the open end of the casing, and, while the assembly is being vibrated, caused to pass through the marginal openings in spacer discs 15 to fill completely all voids in the casing. Thereafter, the lava closure plug 12, threaded on the terminal pins, is pressed into place against spacer disc 15, and a temporary plug 20 (FIG.3), of plastic, is installed in similar manner within the open end of the casing and against the plug 12.  
  The assembled unit is subjected to lateral compaction, as by swaging, which has the effect of crushing the core member 7, pressing the material thereof tightly against the pins and connector strips 11 and binding all parts together into a solid unit. Finally. the casing is severed in the plane of the outer face of the plug 12 and the excess casing portion with temporary plug removed. In the same operation the extreme end of the tubular casing may be turned inwardly over the outer edge of plug 12 to secure and seal that member in place.  
  The herein-described method and construction has important advantages over known constructions and methods, particularly with reference to the described provisions for obtaining a superior internal connection. mechanically and electrically. between the resistor coil and the terminal pins.  
 We claim:  
 1. An electrical heating unit comprising:  
 a. a core of insulating material having a pair of passages extending therethrough,  
 b. a resistor coil wound on said core,  
 0. a pair of terminal pins in said passages. and  
 d. means interconnecting the ends of said resistor coil each to one of said terminal pins, comprising strips of flat, longitudinally twisted metallic ribbon extending longitudinally in said core passages and making surface to surface contact with said terminal pins.  
 2. The electrical heating unit described in claim 1 wherein:  
 e. said metallic ribbon connector strips extend spirally around said pins and are held in intimate surface contact with said pins by said insulating material of the core.  
 3. The electrical heating unit of claim 2 wherein:  
 f. said strips have U-shaped end portions clamped to opposite end portions of the resistor coil.  
 4. An electrical cartridge heater including:  
 a. an insulating core having passages extending longitudinally therethrough,  
 b. a resistor coil wound on said core,  
 0. terminal pins in said core passages, and  
 d. strip of flat wire connected to opposite ends of said coil, said strips extending individually into said passages and winding spirally around portions of the pins lodged in said passages.  
  e. said strips have bent end portions thereof clamped to opposite end portions of the resistor coil. 5 6. An electrical cartridge heater as set forth in claim 4, wherein:  
 f. said core is provided with lateral recesses at opposite ends thereof, each recess communicating with one of said passages, and  
 g. wherein said resistor coil has end turns traversing said recesses, and  
 h. said strips have reveresely bend end portions located within said recesses in clamping engagement with the portions of the resistor coil that traverse said recesses.  
 7. The method of manufacturing electrical heaters which includes the steps of:  
 a. winding a resistor coil on a preformed core member of insulating material having a pair of narrow passages extending therethrough,  
 b. inserting a strip of twisted metallic ribbon in each of said passages,  
 c. connecting end portions of said twisted strips to the end portions of the resistor coil, and  
 d. inserting terminal pins into said passages against the twisted surfaces of said strips to cause said strips to entwine the pins.  
 8. The method described in claim 7 wherein:  
 e. said terminal pins are comprised of slender metallic rods of diameter to fit loosely within the core passages.  
 9. The method described in claim 8 wherein:  
 f. said terminal pins are inserted into the core passages from the same ends thereof in which said twisted metal strips are inserted.  
 10. The method defined in claim 7 wherein:  
 e. said core member is formed to provide lateral recesses each communicating with one of the core passages, and i f. a connection between the twisted strips and the resistor coil is made by clamping end portions of the strips to end turns of the resistor coil at said lateral COI&#39;C recess. 4 5