US 3156840 A
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Description (OCR text may contain errors)
Nov. 10, 1964 N. ca. CLOTHIER ETAL 3,156,840
MAGNETIC com: FOR DYNAMOELECTRIC MACHINES Filed July 10, 1959 3 Sheets-Sheet l In venfors: Norman 6. C/ofh/r AQ-. Joseph h. Mc/(endree;
Vincent J. P/cozzi by l W The/r Afforney,
10, 1964 N. G. CLOTHIER ETAL 3,156,840
MAGNETIC CORE FOR DYNAMOELECTRIC MACHINES Filed July 10, 1959 Fig. 4. 48 Fig. 5.
.5 Sheets-Sheet 2 I 160 I Q 3010 40'0 //7 VeHIOfS unit of Pres's'dreflbsp Norman 6 C/of/fi'er I Joseph H. Ma Kendree;
Vince/7f J P/cozz/ Nov. 10, 1964 N. G. CLOTHIER ETAL 3,156,840
MAGNETIC CORE FOR DYNAMOELECTRIC MACHINES Filed July 10, 1959 3 Sheets-Sheet 3 F ig.
Twist Angle Before Press After Press Fig. /2
39 f 39 y 1 7% 28 A A 7\ m 28 38 l I A y- M Fig. /3.
lnvenfors; Norman 6. C/ofh/er; Joseph H. McKendrge; l/incem J. Picozzl; by M 1 MM The/r Afforney.
United States Patent Oflice 3,156,840 MAGNETIC CORE FOR DYNAMGELE CTRL MACHINES Norman G. Clothicr, West Albany, .loseph H. McKendree,
Schenectady, and Vincent J. Picozzi, Scotia, N.Y., as-
signors to General Electric Company, a corporation of New York Filed l'uly Ill, 1959, Ser. No. 826,358 5 Claims. (Cl. 310-459) The invention described herein relates to dynamoelectric machines and more particularly to an economically designed prewound or cartridge core adapted for use with induction, synchronous and other types of machines and electrical apparatus requiring a magnetic core for operation.
Prewound or cartridge cores generally are old in the art and many different designs and modifications have been developed in an effort to provide improved electrical performance and economy in manufacture. Generally such cores comprise a plurality of thin silicon steel laminations having conductor receiving slots and interlaminar insulation, such as varnish or an oxide deposit, coated on the adjacent lamination surfaces. The cores are placed under a suitable axial pressure and the means used for retaining such pressure in the laminated stacx consist of axially extending bolts, straps, rivets, welds, or the like which either pierce the laminations or are positioned axially in the peripheral surfaces thereof, and when fastened in place, serve to hold the laminations together tightly.
In those instances where the laminations are pierced, certain punching, machining and assembling operations are required which adds measurably to the cost of construction and precludes producing an economical core, particularly in intermediate and large size machines. Magnetic cores in this range of intermediate and large machines usually are segmented and special tabs or cutaway portions also must be provided on the punchings for coaction with slots or bars in the machine frame. Moreover, the existence of radial air ducts in the core presents ventilation and pressure problems because of the need for a large number of axially extending bars used in holding the assembly together.
Welding axially along either or both the inner and outer core surfaces in either of the segmented or nonsegmented types requires large annealing furnaces and large presses which also must be factored into the cost of the machines. Since all of the laminations are shortcircuited, particularly when welding or wedge rivets are used, the electrical losses are even larger than in cores constructed by other manufacturing processes. Furthermore, the weld material must be removed from the inner or outer peripheral surfaces of the core body thus calling for additional labor costs besides the expense connected with purchase and maintenance of special tools needed for weld removal.
The above discussion indicates in part, the extent and intensity of problems confronting inventors engaged in this type of development effort, but the demands for higher quality machines at lower costs create additional problems when other constructions or manufacturing processes are resorted to. For example, it has been observed that pressures in the neighborhood of 100-150 p.s.i. are representative of the pressures that are maintained in the core bodies after assembly. Pressures too low permit radial displacement of the laminations, noise, wearing of conductor insulation, and other factors detrimental to machine performance. Low pressures also cannot resist radial pressures imposed on the core by the frame. Pressures too great cause the varnish type inter- 3,156,840 Patented Nov. 10, 1964 laminar insulations to flow especially when the steel laminaticns expand as a result of heat generated in the core during operation. Since the laminations are locked in position, the added forces produced by lamination expansion becomes so great as to create fluidity in the insulating medium. Subsequent shrinkage of the lamination stack then permits the laminations to occupy that area formerly taken up by the insulation, with the result, that a decrease in the stack pressure takes place to such an extent that it may be reduced to as much as l0%-30% of the pressure incorporated in the core prior to the time of being placed in operation. Excessively high pressures also cause greater interlaminar losses and require stronger structural members for retaining the pressure in the core.
In spite of the disadvantages inherent in the prior art machines, a reasonable balance has been achieved between manufacturing costs and the electrical performance required for a particular type of core. Nevertheless, the need exists for a magnetic core of improved construction which not only will have desirable electrical characteris- 'cs, but also, contain a substantial reduction in the manufacturing costs and material needed in its construction.
The primary object of our invention therefore is to eliminate the disadvantages inherent in prior art construction by providing a prewound core which can be stacked and wound separately from the frame for reducing the manufacturing cycle time and without increasing the labor or material costs involved in the manufacturing process.
Another object of our invention is to provide a prewound core construction capable of retaining a predetermined amount of the pressure initially applied to the core during the manufacturing operation.
Still another object of our invention is to provide an economical and improved method for assembling a magnetic core prior to insertion in a macihne fra e.
In carrying out our invention We provide a segmented prewound core wherein a plurality of laminations are stacked into an assembly and space blocks and flange rings positioned on opposite ends thereof. The assembled core is then compressed to an extent sufficient to cause deflection of the flange rings, and straps are then positioned in axially extending slots formed in the core peripheral surface and welded to the flange rings while the core is under compression. Removal of the initial pressure provided by a press, then permits the straps and flange rings to deflect outwardly and thereby carry the load represented by the pressure remaining in the core assembly. The retained pressure is only a portion of that originally applied to the core but the deflection and original pressure chosen are such that an optimum pressure will remain in the stack; that is, a pressure high enough to prevent lamination displacement but still sufficiently low to permit use of reasonably sized structural members and for minimizing core losses. Constructing a core according to this method permits installing the winding therein before the core is set into the frame thus making the Winding end turns accessible for special treatment while also allowing parallel construction of the magnetic core, frame, coils and winding accessories.
The subject matter which We regard as our invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. Our invention, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:
FIGURE 1 is an end view in elevation of a prewound egrnented core illustrating the disposition of straps in the outer peripheral surface thereof;
FIGURE 2 is an enlarged view of a portion of the core illustrated in FIGURE 1;
FIGURE 3 is a sectional View in elevation taken on lines 33 of FIGURE 2;
FIGURE 4 is an enlarged view of a portion of a cage type of core illustrating a modification of the invention;
FIGURE 5 is a sectional view in elevation taken on lines 5-5 of FIGURE 4;
FIGURE 6 is an enlarged view of another embodiment of a prewound core illustrating the disposition of a Wedge key in a slot used for holding the laminations against axial displacement;
FIGURE 7 is a sectional view in elevation taken on lines 7-7 of FIGURE 6;
FIGURE 8 illustrates the design of slot used in the construction of FIGURES 6 and 7;
FIGURE 9 is a chart illustrating the relationship of pressure loss in a core with axial expansion or deflection upon removal of the external forces causing compression;
FIGURE 10 is a sectional view in elevation illustrating the disposition of a flange ring prior to deflection by a press;
FIGURE ll is a view similar to FIGURE 10 illustrating the arrangement of parts after the flange ring is in a compressed condition;
FIGURE 12 illustrates a spring arrangement for retaining pressure in the core; and
FIGURE 13 is a perspective View of a combined flange ring and spring used for retaining pressure in the core.
Referring now to the drawings wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIGURES 1-3, a prewound core comprising a plurality of segmented laminations 2d assembled into a stack and provided with axially extending slots 22 positioned around its outer peripheral surface. The laminated core is equipped with teeth 24 forming conductor receiving slots for accommodating either form or random wound windings a in the Space blocks 26 shaped to the same general configuration as the laminations are positioned on opposite ends of the core and flange rings 28 located on the outer sides thereof. Space blocks 27 also space the lan inations to provide air ducts 29. A core assembled in the manner described above is placed under compression and axially extending steel straps 30 are positioned loosely in each of the slots 22 and welded to the flange rings 23 when the core and rings are under compression. The external forces imparting pressure in the core is removed and the core is then placed in a frame comprising part of a motor or generator. Although the core illustrated comprises a series of segments 31 joined to form a complete core, it will be evident that the inventive teachings herein are applicable equally to cores of other constructions, such as Where each lamination comprises a circular punching.
Attractive benefits are derived from using a segmented construction since good quality control of stacking and slot alignment is made possible, primarily because the core is formed separately from the frame and in areas where the manufacturing process can be performed conveniently. Better resonance characteristics and reduction of flare at the bore also result. In other known constructions, the frame must be manufactured first and the laminations then stacked therein thus requiring the components to be manufactured in series which in many cases, ties up the manufacturing facilities. By making the prewound core as a separate entity which can be inserted in any one of a number of different size frames, the frames therefor can be made at either different or the same time as the core, thereby permitting simultaneous use of plant equipment and reducing the manufacturing cycle time.
The axial slots 22 formed in the core peripheral surface serve to align the straps and are of a depth just sufficient to take the strap 22 while their width is slightly greater than the expected width of the straps, taking into account strap manufacturing tolerances and those variations in slot dimensions in each punching. The straps also serve to prevent circumferential movement of the laminations. The disposition of each slot may be symmetrical about the center line of each segmented portion or may be po sitioned unsymmetrical thereto depending on flux considerations in each segment. It will be obvious that the slots 22 can be eliminated and the straps placed in contact with the core peripheral surface. Also, where sponginess at the bore and where heat is not a critical factor in machine operation, the space blocks 26 may be eliminated and the flange rings placed in direct contact with the core.
Control of pressure in the stack is especially important since pressure determines the frictional force between adjaccnt laminations and whether lamination displacement will occur after the straps are Welded in position. Dimensional stability and core strength is dependent on maintaining a predetermined pressure in the stack and if this pressure is lost or decays substantially for any reason, sponginess in the stack may occur with consequent es tablishment of vibrations, noise, lamination movement and other adverse effects detrimental to machine operation. Accordingly, the correct amount of pressure must be incorporated initially in the stack With an accompanying deflection in each of the flanges and straps when the compressive forces are removed, in order to obtain a compactly assembled core.
Early investigations showed that optimum pressure cannot be applied and then maintained conveniently in a core without providing some construction, in addition to axial straps and fiat rings on the core ends, for retaining the pressure after exterior forces creating stack pressure were removed. Referring to FIGURE 9 illustrating a typical deflection curve, it will be seen that if a core assembly of the prior art including fiat flange rings which are not capable of being deflected or distorted because both sides are parallel, is compressed to approximately 320 psi. as shown by the solid line 33, the core will be deflected about 16 mils/in. After the straps are welded to the flange rings, and the compressive forces removed, the core or lamination stack will grow or expand 6 mils/ in. approximately, but the pressure therein will drop radically to about 35 p.s.i., equal to about 10% of that originally imposed on the core as shown by the dotted curve 35. The sudden pressure drop results from extension of the straps and a slight dishing in of the flange rings as they assume the load directed outwardly upon removal of the external forces causing compression. The flat flange rings have parallel faces, within the limits of manufacturing tolerances, and they therefore cannot be prestressed by an applied compressive force acting on the rings and stacked laminations, before the straps are welded to the ring edges. However, the rings are subjected to stressing forces when the exterior forces are removed and the lamination stack expands axially against them until an equilibrum condition is reached. Calculation of strap and flange deflections confirms these observations and the conclusion has been reached that if the initial pressure in the press is kept below a load value which would permanently deform the outside space blocks used in the tests, the flange rings and strap deflections will be too great and looseness will occur in the laminations.
To overcome this relaxation problem, the flange rings, straps, and spacing blocks obviously could be made of heavier material to minimize deflection under load. The disadvantage of using heavier structural members is that the straps and flange rings may not have suflicicnt deflection to follow the core in the event of shrinkage. However, since our primary objection is to obtain optimum core pressure and to reduce the manufacturing cycle time without increasing labor and material costs, we have devised a method for incorporating deflection in the flange ring before the strap welding process is complete. In short, we prestress the flange.
As illustrated in FIGURES and 11, this is accomplished by using a closed flange ring or deflector 28 of generally trapezoidal cross section throughout and a pressing plate 34 having an angled surface suiflcient to produce 3 to 4 twist angle in the flange when pressed solid against the end laminations or space blocks on the core ends. The limit on the flange twist angle in the flare produced in the laminations at the bore where too much twist would create undue sponginess. Obviously, excessive twist in the flange itself should be avoided. FIGURE 11 illustrates the relationship of parts when the pressing process is complete and it Will be noted that the parts are in full surface engagement with each other when the twist angle is the same as that shown in FIGURES 10 and 11. When the compressive forces are applied by the press to the flange rings 28, they are deliberately distorted from their initial configuration, thus prestressing the rings prior to welding the axially extending straps to their edges.
With the core and flange ring deflected and welded by a structure of the type described above, when the compressive forces are removed, relaxation of the core is limited to the extension of the straps which is a small percentage of total deflection in the system. The prestressed flange rings hold or maintain pressure in the laminated stack. Reference to FIGURE 9 will show that for an approximate 3 mil deflection, most of which is in the straps, the pressure in the core drops to a highly desirable value of about l5lll60 p.s.i. This latter pressure is of a value sufficient to provide an amount of frictional resistance between the laminations for precluding movement with respect to each other. It also does not extend beyond the working values of the structural members. Moreover, this pressure represents an optimum amount necessary for providing a core of high strength and stability. The use of straps and prestressed flange rings is especially suitable with cores having one or more rows of radial air ducts along its length. A stronger and more economically constructed core than those presently available results, in addition to obtaining shrinkage benefits and discussed more fully hereinafter.
In order to retain substantially full pressure in the core after the compressive forces have been removed, it would be necessary also to prestress the straps. Although consideration has been given to constructions necessary for carrying out this function, since the straps can be prestressed in a number of different ways, the additional manufacturing costs required to perform the extra strap prestressing steps, are outweighed by the results achieved by practicing the process described above. in light of the successful performance of cores made by the process disclosed herein, economic considerations therefore dictate the latter method of manufacturing the core.
When the compressing plate prestresses the flange, the forces are applied on the outermost areas of the laminations with the result that a small amount of outward flare takes place at the core bore. The degree of flare however is so small, e.g., -60 mils on a 7 inch core, as not to present any problems of sponginess in the stack. If any unusual situations should arise where substantial consistency in pressure should be maintained throughout all portions of the core, pressure can be applied simultaneously on the flange rings 28 and space blocks 26 which have inwardly directed members 36 lying over the core teeth. Developmental efforts have shown that original pressures of 500700 p.s.i. can be applied without permanently deforming the outside space blocks. Welding of straps to the flanges would of course then be made in the manner described above.
Although specific angles and constructions have been used to illustrate this portion of the invention, it will be evident to those skilled in structural designs that other angles on the surface of the coacting parts can be varied between wide extremes depending on the degree of twist and therefore the deflection to be incorporated in the flange ring. The basic concept is to prestress the flange for the purpose of retaining pressure in the core and it will be obvious that many different designs and configurations of parts can be used for performing this relatively simple but highly important and novel function. The flange is prestressed or deflected in an amount suflicient to produce a force corresponding to that desired in the core after the straps are welded to the flange and the equipment compressing the core during manufacture has been removed. Corresponding means that a force is applied to deflect the deflectors an amount such that when the force is removed, the pressure of the desired value will be retained in the core by the straps and the deflectors. This will vary for diflerent size, diameter, and length of core.
The cores produced have been subjected to both dropping and resonating tests. The dropping tests consisted of allowing 21 /2 inch diameter cores to fall freely from a one foot height onto a thick steel plate resulting in a deceleration in the order of 5G to G. 28 inch diameter cores were lowered as fast as a crane could lower them onto a steel block positioned on a wooden block floor. Deceleration was estimated to be 15 to 30 G. All cores were dropped with the bore horizontal, vertical and inclined so that the center of mass was over the point of impact.
The drops with the bore vertical produced substantially no dimensional chance, but equalization of tension in the straps took place as indicated by strain gages attached to the straps during test. Drops with the bore horizontal created a slight out of roundness of approximately 5 mils. The drops with the bore inclined caused a relatively small distortion axially of about 1'3 mils for the large diameter cores of 7 inch stack length. These dimensional changes are so relatively slight as not to cause adverse performance when the machine is placed in operation.
The conclusions drawn from the drop tests indicate that final core pressures of to lbs./ sq. inch produce a core which can easily withstand normal factory handling and meet design criteria of high standards. Unless proper pressures are maintained, the prewound core will not have surlicient strength and dimensional stability.
The sample cores were excited in ring resonance up to the twelfth mode and the effective Youngs' modulus calculated to be 26 to 27 x 19* lbs/sq. inch. This confirms that the unit pressure is high enough to make the stack composed of segmental lamination approach the rigidity of a solid ring.
Another important aspect of the effects of pressure in the core is that encountered when the core shrinks after being heated to a temperature sufficient to cause the lamination enamel to oxidize or flow. Under such conditions of fluidity in the enamel, the pressure is relieved in the steel: thus causing loosening of the core laminations. It has been found that cores heat cycled for predetermined periods of time at temperatures where the insulation flows, cause shrinkage of 3-4 mils per inch and although this may be acceptable for small cores of about 8 inch lengths, it presents troublesome problems in cores of about 28 inch or greater lengths. Consider for example that a 20 inch core may have a shrinkage of 80 mils when subjected to high temperatures and pressures. The initial strap and flange detection of the small core ranges between 60 and 80 from zero to full pressure. In order to assure one-half average pressure after shrinkage, the straps and flange system should have 150-200 mils deflection which may call for very high initial pressures. It is evident that as lon er cores are used the problem of shrinkage becomes more severe.
There are several ways to overcome these disadvantages, such as using a non-shrinking or inorganic insulation on the adjacent surfaces of the laminations. Investigations show that the use of a commercially available insulation known as Sterling Core Plate did not allow shrinkage of a core when it was heat cycled in the same manner as cores which used a resinous enamel as an insulation '2" medium. Tests on such a core using a non-shrinking insulation show that shrinkage does not take place and that the core is tighter or more compact than when initially placed under pressure.
In order to minimize the decrease in pressure resulting from shrinkage in long cores, a spring 33, FIGURE 12, may be positioned between the space blocks and flange rings for maintaining the core under pressure. During core assembly, the spring is deflected an amount to account for maximum shrinkage which may occur when the core is subjected to heat. This is in addition to flange ring deflection. Different configurations of springs may be used for this purpose, such as those of helical, conical disk, leaf, wave-shape or other configurations capable of being deflected when subjected to axial forces. The conical disk and wave springs are most suited for this kind of application because of their ability to carry high loads at relatively small deflections. Where such a spring is used, it may be necessary to provide a second flange 3% which would serve as a seat for the sprin it will be evident that the flange ring conveniently may be used as a spring merely by designing it in such fashion as to have the flange ring also perform a spring function and thereby permit incorporating a greater deflection in the ring and strap assembly. As illustratecl in FIGURE 13, the combined flange and spring is designed as a conical disk spring to provide deflection without too much stress or it may be made as a wave spring, for example, having the required deflection and satisfactory flare.
The teachings of this invention directed toward utilizing a flange ring or a combined flange ring and spring may also be carried over and used with other prewound core constructions. Referring to FIGURES 4 and 5, a cage type of core may be used which consists of a plurality of stacked laminations 40 having space blocks 42 and flange rings 44 disposed on opposite ends thereof. A cage consisting of a number of axially disposed but spaced bars 46 are placed around the outer peripheral surface of the core with rings 48 welded or otherwise atlixed to the ends thereof. The internal diameter of the rings and therefore the bars is substantially the same as the outer diameter of the core. The core is then compressed in the same manner as illustrated in FIGURES 10 and 11 and stop blocks ft then placed in circumferential grooves provided in the bars 46 for holding the core under compression. In the alternative, the stop blocks can of course be welded or otherwise afiixed to the bars 46 to retain pressure in the core. In some cases, it is advantageous to apply the core compressive force directly to the stop blocks with welding of the blocks to the bars 46 taking place while the core is under compression. The stop blocks and their mating grooves may assume any con figuration, such as round, square, T-shape or of any other configuration capable of carrying out the function described above.
In the modification shown in FlGURES 6, 7, and 8 a plurality of laminations 4d, space blocks 42 and flange rings 44- are assembled in the same manner as previously described. A single or number of grooves 52 of a depth preferably greater than the thickness of a strap 54 is formed along the length of each segment of laminations. When the assembled parts are placed under compression, the strap is wedged into firm engagement with the exposed end surfaces of the laminations appearing in the slots for holding the laminations against axial and circumferential displacement. In doing so, the longitudinal edges of the straps wedge between the laminations and serve to hold them rigidly in position. While the core is still under compression, the ends of the straps are welded to the flange rings which have been deflected in the same manner as that described in the embodiment illustrated in FIGURES 1-3. In the event the interlaminar insulation on the surfaces should happen to oxidize or achieve a fluid state which permits shrinkage of the core, the
. i=3 deflection initially incorporated in the flange rings is nevertheless sufiicient to accommodate any shrinkage. In this case, the flange ring serves as a supplemental or auxiliary means for assuring compactness and strength in the core.
In view of the above, it will be evident that many modifications and variations are possible in light of the above teachings. The primary concept disclosed herein is that of providing a prewound core havng elements in a prestressed condition positioned on the ends thereof. The function served by these elements, in addition to providing strength and compactness to the core, is to supply a force component capable of maintaining the core under pressure even though some of the pressure originally incorporated therein may be lost as a result of shrinkage with the core body. It will be apparent that the teachings may be used in any kind of magnetic core construction wherein it is desirable to maintain predetermined pressures therein, such as transformers, and other mag netic core bodies. it therefore is to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
What we claim as new and desire to obtain by Letters Patent of the United States is:
1. A prewound core comprising a plurality of stacked laminations having conductor receiving slots therein, a deflector positioned on opposite ends of said laminations and straps extending axially thereacross and interconnecting said deflectors, said deflectors initially having a configuration such that the faccs thereof do not lie in parallel planes so that when subjected to a force exerted axially thereon and against the laminations, the deflectors are caused to deform out of their original plane thus creating a pressure in the lamination stock corresponding with the degree of distortion in said deflection, each of said defiectors comprising a combined flange ring and spring having a resiliency suflicient to impose said pressure in said laminations.
2. A prewound core comprising a plurality of stacked segmented laminations having conductor receiving slots therein, at least one axiallyextending slot formed in the peripheral surface of each of said segments, space blocks and flange rings positioned on opposite sides of said core, a strap positioned in each of said slots and afiixed to the flange rings on opposite sides of said laminations, said flange rings being of trapezoidal cross section such that when distorted by force out of a plane perpendicular to the core axis, a pressure is imparted to the stacked laminations in an amount corresponding to the degree of force.
3. A prewound core comprising a plurality of stacked segmented laminations having conductor receiving slots therein, space blocks and deflectors positioned on opposite ends of said laminations, a pair of cage rings positioned on opposite side of said stack of laminations and disposed outwardly from said deflectors, and bars interconnecting said cage rings, each of said bars having a circumferentially disposed slot formed therein, a key in each of said slots, and a surface of each key in contact with said deilector, so that when said stack of laminations, space blocks and deflectors are compressed to a degree corresponding with the amount of pressure desired in said core, said keys maintaining the pressure in said stack of laminations when the external forces applied thereto are removed.
4. A process for assembling a prewound core comprising the steps of assembling a stack of laminations, placing a deflector on opposite sides of said stack of laminations, applying an axially directed force against said deflectors and said stacked laminations in an amount sufficient to cause axial distortion of said deflectors and thereby create a pressure in said core corresponding to a pressure desired therein when said force is removed, positioning a plurality of axially extending straps at spaced intervals about said core, welding said straps to said distorted deflectors 9 and removing said force used in creating said pressure in the stack of laminations.
5. The process of assembling a prewound core comprising the steps of forming a segmented stack of laminations each having a series of spaced axially extending slots therein, aligning the slots in said laminations, placing a space block and flange ring on opposite sides of said stack of laminations, compressing the parts as assembled to a pressure corresponding with an ultimate pressure desired in said stack of laminations, distorting said flange rings in a direction toward said laminations and out of a plane originally extending parallel thereto and thereby W creating a force therein, loosely positioning a strap on each of said axially extending slots and welding said strap to said flange rings, and removing the force creating pressure in said stack thereby permitting the straps and flange rings to assume the axially directed load.
References Cited in the file of this patent UNITED STATES PATENTS 1,685,054 Hibbard Sept. 18, 1928 1,322,096 Hollander Sept. 8, 1931 1,936,744 Adams Nov. 28, 1933 1,957,380 Barlow May 1, 1934