US2128222A - Resistance grid - Google Patents

Description

M. A. DU Bols 2,128,222

RESISTANCE GRID Aug. 30, 1938.

F'iled Dec. 2, 1935 2 Sheets-Sheet l l l f any wup:-

Patented Aug. so, 193s 2,128,222

UNITED STATES PATENT OFFICE RESISTANCE Gam Marvin A. Du Bois, Chicago, Ill., 'assignon by mesne assignments, to The Post Glover Electrkgiompany, Cincinnati, Ohio, a corporation of` o Application December 2, 1935, Serial No. 52,531

6 Claims. (Cl. B01-89) The p'resent invention relatesmtp electrical relight weight, and designed to have a high degree sstance grids and electrical resistances contanof practically. uniform self-ventilation. ing the same and has for an object the provision A further object is to provide resistance devices of a novel and relatively inexpensive grid eleof the character described wherein the grid element and, as a further object, the provision of r ments of a given gauge of sheet stock and at a an effectively ventilated, long lived and compact predetermined spacing have a higher continuous electrical resistance f or use in various installacarrying capacity than heretofore known grids tions. of similar thickness and width and like spacing A further object of the invention is to provide that have equal individual resistance. 10 a novel method of forming highly eiilcient resist- Another object is a simple and inexpensively l0 ance grids of sheet metal with a minimum of loss practiced method of cutting grids of the invenof sheet stock in scrap or waste. tion from an elongated steel sheet.

Another object is to provide a stamped sheet These and other objects are attained by the metal resistance grid characterized in that each method and means herein described and disgrid, between the terminals thereof, is constituted closed in the accompanying drawings, in which: 15 by a continuous series of angulariy related sec- Fig. 1 is a plan view showing in full lines the tions so disposed with relation to each other as resistance grid in relation to the sheet in which to provide for the closer disposition of the sec it is cut, and in dotted lines, the position of the tions thereof nearest the edges of the grid, and die to cut a succeeding grid.

for the wider spacing of certain non-adjacent Fig. 2 is a cross sectional view of a resistance 20 sections providing for the more ready dissipation device embodying the grid oi the invention. oi heat from the more closely spaced sections to Fig. 3 is a view taken on line 3-3 of Fig. 2. the edges of the grid, the said relation of these Fig. ,.4 isa cross sectional view of a resistance continuous sections further permitting of the device showing a modiiled form'of mounting the 25 stamping of successive grids from a ribbon or grids and of providing additional support there- 25 50 Another object is to provide electrical resistelongated sheet of electrical steel in a fashion for as may be required in rare instances -of exproductive of a minimum of waste of the sheet cessively close spacing and very thin grids.

stock from which the grids are stamped. Fig. 5 is a fragmental schematic view showing Another object of the invention is to provide a modified form of the mounting of the grids of a resistance grid of stamped sheet electrical steel the invention in multiple banks. 30', that is designed to provide a high continuous car- Fig. 6 is an enlarged view taken on line 6--8 rying capacity factor in proportion to the resistof Fig. 5. ance thereof and which in use in a resistance In the art 0f Constructing reSiStOrS. Practice device will readily combine the desirable factors has been largely with cast metal grid resistors of very nearly uniform temperature distribution or the so called ribbon resistors, and although the :3,5`V

throughout the major portion of each grid with use of sheet metal is not unknown, previous ata surprising1y low temperature at the terminals tempts to use it have resulted in products infethus. avoiding the destructive action of heat on rior t0 the present device and incurring large the insulating supports therefor and upon the Waste Of material The Present inventien DIO- insulation and other parts of the actuators and Vides a new Sheet metal grid which has advan' 40 lead Wires associated with the termina1s tages over the cast grid resistors in that it gives Another object of the invention is to provide a more even distribution wheat on the grid and sheet metal resistance grids of relatively thin 1s C00] on the ends of the grid during use' allow' gauge and electrical resistance devices compris- .ing the support to remain C001 and thus retard' y ing the carbonization of the mica tubing. Fur- 45 11g he sam Wlilten ghe gridsf may ibe ver thermore, the 4connecting terminals will therefore hls; sm wovglategrobugf ngurf remain cool and the possibility of destroying insuth id g g lation on wires connected to the terminals is e gr S- reduced. The devices made in accordance with the present invention also have specific advanance deViCeSCOmpriSing grids the afOIeStated tages over cast grid resistors in that they are characteristics ilhi Ieqilire a minimum -Of SUD- non-breakable and will withstand high overloads porting meteriel'end Which ere easily end rapidly without warpirig or drooping o f the grids whereas assembled and which, in general, provide an many cast-grid resistor grids droop when hot so ideally eiiicient resistance device of compact form, that the loops of the grids will touch each other. 55

In addition to this the resistance banks of the present invention will require a lesser amount of space than required for a cast grid resistor of equal resistance and carrying capacity because of the thin cross section of the grid which allows a higher ratio ofY airspace to space .taken up by grids. In addition to this the entire structure is lighter in weight for equal carrying capacity and resistance. Furthermore, the present device offers a lower thermal coefficient of resistance. The device furthermore has advantages over ribbon type resistors in that no porcelain need be used so that the resistor may be heated and drenched with water without danger of damage. Furthermore, the number of supporting bolts is less and of those used a portion can be arranged for floating mounting so as to allow for expansion. The type of terminal necessary may` be such as can be firmly attached but easily moved when desired.

The individual grid of the invention indicated generally by the reference numeral i0 is an integral stamping from suitable electrical sheet steel having a pair of end lugs ii with perforations I2 therein connected by a dat strip forming a circuitous path and comprising a succession of oppositely extending open ended loops connected in a series between said lugs. The shape of the loops is such that a pair of closely spaced sections II which are of uniform cross section and size and having a narrow space I4 between, are connected together at a common end Il and each of these sections i3 in turn connects at its remaining end with an angular-ly divergent portion i6. The portions i6 in turn connect the sections I3 of the next adjacent oppcsitely extending loops or with the perforate end lugs Il. Thus it will be noted that the narrow opening Il between the sections I3 of each loop communicates with a wider open? ing I1 which is bounded by sections i3 on adjacent oppositely extending loops and the intervening portions Il. It will further be noted from a further inspection of Fig. 1 that each of the larger openings Il is occasioned by the cutting out of the loops of a preceding or succeeding grid. Hence, the amount of scrap or waste stock occasioned by the production of these grids according to the present invention is very small. and in practice represents a seventy-ve per cent savingof scrap metal over heretofore known methodsof forming stamped sheet metal resistance grids. This factor of saving is deemed important because the nature of electrical sheet steel is such as to render the value of its scrap very low as compared with its initial cost because this type of metal does not have a wide field of utility for other purposes. Furthermore, a greater number of grids can be stamped from a given piece of metal than by heretofore known methods.

The method of forming these grids consists in feeding a steel sheet or strip i8 endwise beneath a punch and die, the sheet Il in Fig. 1 being indicated in full lines and exclusive of the completed grid shown in position prior to the next operation of the die, the position of which is shown in dotted lines. Thus a complete grid is struck from the sheet at each operation of the die and the outline of half of the succeeding grid is cut from the sheet in the same operation. This affords a very rapid and inexpensive fabrication of the metal into grids with a minimum of scrap which will be readily apparent from an inspection of Fig. 1. The method of forming the grids may be stated to be constituted by the steps of successively stamping out the point shaped openings I1 and the irregular openings I9. With each stroke leaving the forward profile of the grid with tongues shaped like opening I4 on the sheet and. on the next succeeding operation of the die, cutting a similar outline which severs the said tongues.

The shape of the grid thus formed is, as before indicated, not only productive of a maximum number of grids with a minimum amount of waste from a given quantity of sheet material, but it also affords the utilitarian features of a substantially even distribution of the heat over the major area of the grid and a comparatively low temperature at the terminal lugs thereof. It will be further noted that there is for all practical purposes uniformity of cross section of resistance metal in the circuitously shaped strip between the end lugs. The relatively close proximity of the sections il of a given loop is compensated by the fact that around all sides and ends of the said sections Il there is an enlarged Ventilating area communieating with the outside atmosphere and tending to effect relatively uniform distribution and dissipation of heat. It has been found that in use this uniform heating and efficient ventilation precludes warping of the grids when carrying rated current and that when the current is increased to bring the grids to a white heat, there is but a slight amount of warping which is not sufficient to cause even the more closely spaced grids to touch at any point. Furthermore, when the grids are allowed to cool. they return to practically original position. The warping just referred to obtained only under extreme conditions and under conditions wherein both of the lugs Il were rigidly fixed. As will be hereinafter described in greater detail, it is possible to eliminate observable warping by mounting the lugs of a bank of grids on a fixed through bolt at one end and on a floating through bolt at the other end so that when the grids are excessively overheated, e. g., to a white heat, the grids are allowed to expand so that the distance between the fixed and floating bolts would increase slightly but sutilciently to accommodate the elongation due to expansion.

The grids of the invention adapt themselves to the mounting in banks as illustrated in Fig. 2 or in a connected series of banks as suggested in Fig. 5. They are further adapted to mounting in double banks as suggested in Fig. 4 and in the latter embodiment when the grid thickness is #18 gauge or thinner and the grid spacings are very small, it has been found desirable to use the center bolts and spacers illustrated in Fig. 4 in order to preclude warping and possible touching of the rigidly secured grids under continued overload heating. The device of Fig. 2 may have a center bolt and spacers applied thereto if the grids are thin and are closely spaced.

It is well understood in the art that resistors of various types, for example, such as illustrated herein, are adapted for use upon electric locomotives such as mine locomotives, in mills. and in various environments. and that in the design and construction of the various devices and apparatus employing these various resistors. it frequently happens that the design or lay out has been made with compactness as an important object and that no particular provision has been made for the exact location of the necessary resistors. For all such conditions. it is particularly desirable and useful to have a fully efficient resistor as small and compact as possible so as to permit the installation of the resistors if need be in restricted remaining. space or spaces. In

the following detailed description, the resistors will be considered only from the viewpoint of the necessary end supports for the through bolts which carry the grids and these latter elements.

Referring now to Figs. 2 and 3 a resistor of the present invention employing the grids heretofore described, comprises essentially suitable end supports or frames 20 and 2l. One of these end frames such as 20 has a pair of elongated supporting bolts 22 inserted through it and upon these bolts are mounted insulating tubes 2l of mica or the like. A suitable number of insulating washers or spacing collars 24 are then placed over the tube and in abutment with the inner face of wall 20 to establish the spacing between said wall and the first grid lli, a terminal lug 25 which may constitute a mere perforated conductor plate contacting the ilrst grid and suitably arranged for connection with a ter? minal 26 of a lead wire 21 being mounted in posi- -tion at this point. Thereafter, successive washers orl groups of washers or spacing collars 24 of metal and of insulation are added to the stack vlugs are placed at desired intervals to secure the desired different resistances for the Several taps and the entire stack is finally bound as a rigid unit in this case by drawing the remaining end frame 2| tightly upon the stack by means of nuts 28 on the threaded ends of the bolts. This particular form of mounting may be used when the grids are stamped from relatively heavylgauge sheet metal, e. g., thicker than #18 gauge, and with relatively wide spacing. A structure so arranged may, when overloaded to a degree that brings the grids to a white heat, show some slight warping of the grids while at that heat but this has been found insuilicient to cause the grids to touch and short circuit and it has been found furthermore that upon cooling, the grids will return to substantially their original shape. It can be readily appreciated that the cost of resistors of this type is relatively low and that comparatively little skill is required in the assembly thereof.

It is preferred in general and not only for resistors employing very thin gauge grids, e. g., from #1.8 gauge to #26 gauge, to rigidly fix one of the bolts 22 in the end frames as specically illustrated in Fig. 3 and to provide a floating mounting of the remaining bolt in the end frames as detailed in Fig. 6. This constructionis provided by forming an elongated slot 29 in each end frame in lieu of a perforation which snugly receives the bolt and providing means whereby the stack is rigidly clamped together with the bolt 22 and a means such as a collar 3,0 and shoulder-forrning washers. which permits the rigidly clamped bolt and grid assembly to move longitudinally relative to the frame when the grids elongate slightly due to high heating. In ordinary construction the heating of a grid to a white heat will increase the distance between the rigidly anchored bolt and the floating bolt support suiiiciently to take care of expansion. Any other suitable mode of providing for the free floating of one of the support bolts may be employed in lieu of the means just illustrated and described.

It will have been noted from the foregoing that the shape of the stamped grids of electrical sheet metal has been designed to afford a relatively uniform heating of the grids throughout the area thereof so that by the use of floating bolt supports it is possible to simply and inexpensively construct resistors with only two supports for the bank in cases where the grid spaciny is threeeighths of an inch or more and the gauge of the grids is of #18 or thicker. When it is desired to use closer spacings'and thinner grids than the foregoing, it is possible to use a center bolt with spacer washers entered through one of the spaces I1 between the loops oi' the grid. This would be provided essentially as a matter of safety against extreme and continued overheating due to difllculties in the electrical system or the load. The use of additional supports intermediate the ends of the grids and the necessary spacing washers effects some restriction of the escape of heat although not to the degree encountered in resistors now on the market and in which a multiplicity of supports are utilized.

As shown in Fig. 4 it is possible to assemble resistors employing double interleaved banks of grids wherein the composite bank will have the same resistance per unit of length as a single bank of similar grids at half the spacing of either the top or bottom section. Under some conditions such a double bank may be found ad` vantageous even though, by reason of the fact that the air temperature will reach its permitted maximum before the grids have reached their maximum permitted temperature, the carrying capacity value is somewhat reduced. This is apparently due to the restriction of air ilow caused by the use of additional center supports 220. A s shown somewhat diagrammaticallyin Fig. 5 a plurality of banks maybe arranged by suspending the first bank of grids froma rigidly mounted bolt 22 iixed between the elongated end frames 33, which frames are provided with elongated lslots 34 at necessary intervals to receive the remaining bolts 22. In such 'installation the intermediate bolts 2 2 would have adjacent sides of adjacent grid banks mounted thereon so that the entire assembly is suspended from the uppermost rigidly mounted support. One or more of these elongated banks may be of course adaptable to assemblies which will t compactly into long narrow confined spaces but nevertheless with very effective operation.

What is claimed is: v

l. As a new article of manufacture a resistance grid comprising an integral sheet stamping having a pair of mounting lugs connected by an intermediate uniform strip forming a circuitous resistance path, said strip comprising a succession of parallel legged loops alternately extending in opposite directions from the longitudinal axis of the grid, the loopsv on each side of said axis being spaced apart by an opening greater than the combined width of the parallel legs of the intermediate loop on the opposite side of said axis, each leg of each loop being offset laterally from every other leg of the remaining loops, and each of said legs being longitudinally aligned with one of said openings.

2. A a new article of manufacture a resistance grid comprising` an integral sheet stamping having a pair of mounting lugs connected by an intermediate uniform strip forming a circuitous resistance path, said strip comprising a succession of parallel sided loops alternately extending in opposite directions from the longitudinal axis of the grid. the loops on each side oi said axis being spaced apart by an opening shaped like the reversed counterpart of one of said loops, the interior openings of the several loops on one side of the longitudinal center of the grid forming a continuation or the loop-spacing openings on the other side, the mounting lugs being disposed beyond the extreme loops of said grid.

3. A resistance grid o! stamped sheet electrical resistance material and comprising a pair of end mounting lugs connected by an intermediate succession of oppositelyextending loops, integral portions disposed angularly to the adjacent legs of successive loops and connecting said adjacent legs whereby said legs are offset from one another in staggered parallel relation and form, between alternate loops, enlarged ventilating openings.

4. A resistor comprising spaced end frames having aligned perforations therein at one side and aligned elongated slots in the other side, a pair of suitable support bolts extending respectively. through the said pair of perforations and the said pair of slots,-a suitably spaced stack of stamped sheet metal grids having perforate lugs at opposite ends receiving the support bolts, said grids each having oppositely extending open loops in staggered relation on opposite sides of the longitudinal axis of the grid and enlarged ventilating spaces on each side of each loop, means rigidly securing together the perforate lugs on one end of said stack, end frames and the supporting bolts in said perforations of the end frames and means rigidly securing the lugs on the remaining end of the stack on the remaining bolt. said bolt being adapted for movement in the slots in said end frames and permitting expansion of the grids.

5. As a new article of manufacture a resistance grid of stamped sheet material comprising a pair of end lugs and a connected series of spaced loops disposed on opposite sides of the longitudinal axis oi the grid in staggered relation whereby enlarged openings are provided between adjacent loops on a common side of said axis, said enlarged openings communicating with smaller interior openings of intermediate loops on the opposite side of said axis and providing, upon electrical energization of the grid, increased heat dissipation resulting in uniform heating of all of said loops.

6. A resistor comprising spaced end frames each having aligned pairs of elongated slots, support bolts each having opposite ends extending into slots in opposed end frames. a suitably spaced stack of stamped sheet metal grids having perforate lugs at opposite ends receiving the support bolts, said grids having oppositely extending open loops in staggered relation on opposite sides of the longitudinal axis of the grid and enlarged Ventilating spaces on each side of each loop, means rigidly securing the grids to the bolts and means retaining the bolts against accldental separation from the end frames, said bolts being adapted for movement in the slots in said end frames and permitting expansion of the grids.

MARVIN A. DU BOIS.

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