US 3808573 A
An electric strip heater of the so-called "mica type" comprises a a sheet metal channel with a mica sheet disposed in the channel flatwise against the base thereof. The terminals of the heater have thin flat heads and threaded shanks. The shanks pass through aligned openings at each end of (1) a flat mica resistor plate having a resistor wound therearound, (2) a mica cover plate overhanging the resistor plate and (3) a flat metal cover plate overlying the mica cover plate. A dish-shaped metal washer is disposed over the end of each terminal shank and is deformed by a pressing action in a direction toward the terminal head to provide a subassembly which is disposed within the metal channel with the flat terminal heads positioned against the mica sheet therein. The sides of the channel are pressed over the metal cover plate to complete the assembly. A nut is threaded on each terminal shank outwardly of the dished washer for securing an electrical conductor to the shank.
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Description (OCR text may contain errors)
United States Patent [191 Cappell [451 Apr. 30, 1974 [541 ELECTRIC HEATER ASSEMBLIES  Inventor: Charles W. Cappell, Allison Park,
 Assignee: Emerson Electric Co., St. Louis, Mo.
22 Filed: Jan. 16, 1973 i  Appl. No.: 324,064
219/541, 219/544, 219/551, 338/276, 338/301  Int. Cl. H05b 3/30, HOlc H02  Field of Search 219/528, 538, 541, 542,
 References Cited UNITED STATES PATENTS 2,797,296 6/1957 Fowler et a1 219/528 Boecher et al...
Trent Primary ExaminerA. Bartis Attorney, Agent, or Firm-Michael Williams ABSTRACT An electric strip heater of the so-called mica type comprises a a sheet metal channel with a mica sheet disposed in the channel flatwise against the base thereof. The terminals of the heater have thin flat heads and threaded shanks. The shanks pass through aligned openings at each end of l) a flat mica resistor plate having a resistor wound therearound, (2) a mica cover plate overhanging the resistor plate and (3) a flat metal cover plate overlying the mica cover plate. A dish-shaped metal washer is disposed over the end of each terminal shank and is deformed by a pressing action in a direction toward the terminal head to provide a subassembly which is disposed within the metal channel with the flat terminal heads positioned against the mica sheet therein. The sides of the channel are pressed over the metal cover plate to complete the assembly. A nut is threaded on each terminal shank outwardly of the dished washer for securing an electrical conductor to the shank.
3 Claim, 4 Drawing Figures 1 ELECTRICAL lNst/LATION PATENTED APR 30 m4 SREU 1 0F 2 ELECTRlCAL SULANON ELECTRIC ASSEMBLIES BACKGROUND AND SUMMARY Strip heaters, as previously manufactured, are shown in cross-section in FIG. 4 of the drawings. The terminals are of special manufacture since they are required to provide an intermediate shoulder and a tubular end portion which has a plurality of longitudinal slits so that the end portion may be upset. In the heater shown in FIG. 4,.only the mica plate which supports the resistor may be assembled with the terminals outside of the channel. Further, in final assembly with all other parts in the channel, the upset end portion of the terminals frequently had burrs which have a tendency to dig into the mica sheet which insulates the terminals from the channel base and sometimes punctured or fractured such sheet.
The present invention makes it possible to effect a complete subassembly of parts outside of the channel so that the final assembly with the channel is relatively simple. Further, the invention enables the use of lowcost terminals which require only a thin, flat head and a threaded shank. The flat heads provide a smooth, unbroken surface which maybe pressed tightly against the mica sheet within the channel without puncturing or fracturing the sheet.
Briefly, the invention comprises a sheet metal channel, with a first mica sheet withinthe channel and disposed flatwise against the channel base. A subassembly is independently formed' and includes superposed second and third mica sheets and a metal cover plate, all three parts having aligned holes to respectively pass the shanks of tenninal members, each of which has a fiat head bearing against the outer surface of the second mica sheet. Securing means, in the form of a dished washer, is disposed over that part of each terminal shank which extends outwardly of the cover plate, and pressure is applied to both washers to tend to flatten them and cause them to grip the terminal shanks. This pressure also presses the mica sheets and cover plate in close face-to-face engagement.
The second mica sheet has a resistor wound thereon, and'the third mica sheet overhangs the second sheet so that when the sub-assembly is placed in the channel, the resistor is held spaced from the side walls of the channel. The flat heads of the terminal members bear against the first mica sheet. To complete the assembly, the free parts of the side walls of the channel are bent over the cover plate.
DESCRIPTION OF THE DRAWINGS In the drawings accompanying this specification and forming-a part of this application, there is shown, for purpose of illustration, an embodiment which the invention may assume, and in these drawings:
FIG. 1 is a separated, perspective view of parts included in the preferred embodiment, 7
FIG. 2 is an enlarged, transverse sectional view through the terminal portion of a completed heater,
FIG. 3 is an enlarged, sectional view of a dish-shaped metal washer used in the assembly, and
FIG. 4 is an enlarged, transverse sectional view through the terminal portion of a heater as previously manufactured.
DESCRIPTION OF THE PREFERRED EMBODIMENT With particular reference to FIG. 1, the' embodiment herein disclosed comprises a sheet metal channel 10 having a flat base 11 and upstanding sides 12-12. A flat sheet 14 of mica, or similar electrical insulating material, is adapted to be disposed within the channel 10 and preferably is of a length and width equal to that of the base 11.
Adapted to overlie the mica sheet 14 is a further flat sheet 15 of mica, or other suitable insulation material, which is of a length and width less than that of the base 11 so that in final assembly it will be held out of electrical contact with the latter. The sheet 15 has holes 16-16 adjacent to the ends thereof, and a current resistor 17, either wire or ribbon, is wound about the sheet 15 in a flat helix. Each end of the resistor is wound several times around the sheet 15, between an end hole 16 and a side portion of the sheet, to provide the plurality of closely spaced windings 18.
A terminal 19 is provided for each end of the resistor, each terminal having a thin, flat head 20 and a threaded shank 21. The head 20 is adapted to be clamped between the mica sheet 14 and the underside of the resistor sheet 15 and thus held in good electrical contact with the plural windings 18 adjacent to a respective hole 16. The threaded shank 21 of each terminal fits closely through a respective hole 16 and electrically engages the plural windings passing through the hole to provide further electrical contact between the resistor 17 and the terminals.
Another flat sheet 23 of mica, or other suitable insulating material, is adapted to overlie the resistor sheet 15. The sheet 23 is about the same width and length as the mica sheet 14 and has end holes 24-24 which are about the same size as the holes 16 so as to closely pass the shanks 21 of respective terminals. Thus, the resistor sheet 15 is anchored between the mica sheets 14 and 23 and cannot shift transversely relative thereto.
A flat metal cover 25 is adapted to overlie the mica sheet 23 and this cover is about the length of the base 11 but just enough less in width so that the channel sides 1212 may be closely bent thereover, as seen in FIG. 2. The metal cover 25 has end holes 2613 26-- which are large enough to closely receive mica washers 27, each washer having a hole to closely pass the shank 21 of a respective terminal 19. Thus, the terminals are electrically insulated from the metal cover.
Overlying the metal cover 25 and assembled on each terminal are a mica washer 28, a metal washer 29, a dished metal washer 30 (seen in enlarged section in FIG. 3) and a metal washer 31, all of the foregoing washers having holes to closely pass the shank 21 of a respective terminal 19. A nut 32 is adapted to be threaded on each terminal shank 21.
The preferred method of producing the assembly shown in enlarged section in FIG. 2 is to dispose the terminal shanks 21 through respective, holes 16 in the resistor sheet 15 and through respective holes 24 in the mica sheet 23. The mica washers 27 may be fitted on the extending portions of the terminal shanks 21 and the metal cover 25 disposed over the mica sheet 23 with the mica washers 27 fitted with respective cover holes 26. It is also possible to first fit the mica washers within the cover holes 26 and then dispose the cover 25 over the mica sheet 23.
Mica washers 28, metal washers 29, and dished washers 30 are then disposed over the extending ends of the terminal shanks. By means of suitable pressing apparatus (not shown) pressure is applied downwardly on each dished washer 30 to force the metal cover 25, the mica sheet 23 and the resistor sheet into tight side-toside interengagement and to draw the head 20 of each terminal tightly against the underside of the resistor sheet 15 and the plural windings 18 at each hole 16. The pressure tends to flatten each dished washer 30, as seen in FIG. 2, so that metal margining the hole in each dished washer bites into the threads of a respective terminal shank to maintain this sub-assembly.
The mica plate 14 may then be placed within the channel and the sub-assembly above described placed over the mica plate 14 and lined up lengthwise therewith. Thereafter, the channel sides 12--12 may be bent over the cover plate 25 and the assembly is completed. The washers 31 may then be disposed over the extending ends of the terminal shanks 21 and the nuts 32 threaded onto the latter.
The resultant assembly provides an electric heater of a lower profile than that of the prior art heater shown in FIG. 4. In such prior art heater, a metal channel 10a is required, but of a greater depth than the channel 10. Also required is a lower mica plate 14a, a resistor plate 15a, an upper mica plate 23a and a metal cover plate 25a. The sides 12a were also bent over the cover plate, as in the improved construction.
Heretofore, the terminals 19a were quite different in that an intermediate portion had a shoulder and the lower end was formed tubular and longitudinally split at a plurality of places. The lower end of each terminal was passed through a respective end hole in the resistor plate 15a so that the shoulder 35 abutted against the upper surface of the resistor plate and the plural windings 18a. A Belleville washer 36 was inserted over the lower end of each terminal, and such lower end was then deformed by a pressing operation to upset the longitudinally split portions in rose-pedal manner, as seen at 37. i
It will be noted that the sub-assembly thus formed only of the terminals wound a and the resistor plate 15a. Therefore, the mica plate 23a, cover plate'25a, mica washers 27a and 28a and metal washers 29a constituted loose parts which had to be added to the sub-assembly of the mica plate and terminals.
In contrast, my invention provides a sub-assembly including the terminals 19, the resistor plate 15, the mica plate 23, the cover plate 25, the mica washers 27 and 28, and the metal washers 29, all held tightly together by the deformed dished washers 30, and with the terminals held against rotation. It'will be appreciated that this sub-assembly may be more efficiently-manufactured. Additionally, the need for the Belleville washer 36 (to hold the terminals against rotation) is eliminated. v
A further advantage of my invention resides in the fact-that the deformed dished washers tightly hold all parts of the sub-assembly in side-to-side relation, with only the smooth flat head 20 of theterminals extending from the underside of the resistor plate 15a. In the prior art construction shown in FIG. 4, when the sides l2a12a were pressed over the cover plate 25a, the legs of the rose-pedal 37 had a tendency to dig into the mica plate 14a and fracture or puncture the latter. This will be more fully appreciated when it is understood that the plates 14, 15 and 23 are insubstantial interengagement on final assembly in the channel, rather than the spaced relation shown in FIGS. 2 and 4. Thus, any puncture or fracture of the mica plate could lead to dangerous shorting of the heating element, a condition which is avoided by the smooth, flat heads 20 of the terminals 19.
1. A strip heater assembly, comprising:
a sheet metal channel providing a base and upstanding sides, a first sheet of insulating material within said channel and having one side surface bearing flatwise against the inner surface of said base, said sheet being of a width substantially equal to the inner distance between channel sides and of a length to overlie a major portion of the length of said inside surface,
a second sheet of insulating material of a width less than the width of saidfirst sheet, said second sheet having a hole at each end,
a resistor wound around said second sheet and having each ot its ends would between the margin of a respective hole and the adjoining side of said second sheet,
a pair of terminal members, "each having a flat head and a shank, each terminal member having its shank passing through a hole in said second sheet and in contact with a resistor end portion in said hole, and having its head extending from a side surface of said second sheet and in contact with a resistor end portion adjoining said hole,
a third sheet of insulating material of a size generally equal to that of said first sheet and adapted to overliethe opposite sidesurface of said second sheet and the resistor wound thereover, said third sheet having a hole at each end corresponding to the holes in said second sheet and through which the shank of a terminal passes to hold said second and third sheets in predetermined alignment wherein opposite longitudinal portions of said third sheet overhang the corresponding margins of said second sheet, 1
a sheet metal cover plate of a size generally equal to that of said third sheet, saidcover plate having a hole at each end, slightly larger than the holes in said third sheet and through which the shank of a terminal passes in an electrically insulated manner,
electrically insulated securing means connected to each terminal shank and pressingagainst the adjacent surface of the cover plate to provide a subassembly comprising said second and third sheets and said cover plate held tightly together in faceto-face relation between said securing means and the heads of said terminal members, said sub-assembly being disposedwithin said channel with said flat terminal heads bearing against theopposite side surface of said first insulating sheet, the sides of said channel being bent over said cover plate to firmly hold said sub-assembly within said channel. I l 2. The assembly according to claim 1, wherein said securing means comprises a dished, metal washer having a hole to closely pass a respective terminal shank and an insulating member interposed between the cover plate and the washer.
3. The construction according to claim 2 wherein further including a nut threaded on each terminal each of said terminal shanks is threaded, and wherein shank outwardly of said cover plate for securing an the marginal surface defining the washer hole tightly electrical conductor to a shank. interengages with the threads on said shank; and