US3005737A - Method and apparatus for making laminated transformer cores - Google Patents

Description

Oct. 24, 1961 s. BLAIR ET AL 3,005,737

METHOD AND APPARATUS FOR MAKING LAMINATED TRANSFORMER CORES Filed June 28, 1956 [772/677/52'5 L/o qd 5'. B/air, I Jordan (*1 Non/(e77, [1 19%? 72? Q j Meir/{27627521.

This invention relates to a method and apparatus for making transformer cores and more particularly to improvements in the method and apparatus for forming and processing the larninations from which flat stacked transformer cores are made.

Practica ly all transformer core larninations are now made from cold rolled silicon alloy steel because of its superior magnetic properties particularly as to hysteresis loss and permeability. However, as is well known, such properties are adversely affected by strains in the larninations such as are caused by slitting, cutting or punching the larninations from the coiled steel strip as it is received from the steel mill. Accordingly, it has been standard practice to give such larninations a strain relieving heat treatment or anneal by placing them in high stacks or batches for a predetermined length of time in an annealing furnace maintained at a sufficiently high temperature to soften the steel and thus relieve its internal strains. However, such batch annealed larninations are ordinarily far from flat, and in the case of larninations for large power transformers which are several feet in length, they often have a pronounced wavey configuration known as edge ripple. It is believed that this is caused by differential heating and cooling with resultant flow of the material during the batch annealing cycle. Thus when a stack of larninations is placed in the annealing furnace its outer surface temperature rises more quickly than the temperature within the batch and within the larninations constituting the batch, and conversely when the batch or stack of larninations is cooled, the outer surfaces and edges cool more rapidly than the internal portions with the result that when the stack finally attains room temperature the edges of the larninations are slightly longer than they should be and consequently they have to assume a sinuous or wavey configuration in order to take up the excess length. The result is that when these laminations are clamped together in a core they are strained thus materially impairing the magnetic qualities of the steel and hence of the core itself. Also, the edge ripple is itself indicative of thermally produced residual strains. This is not to say substantially flat and strainfree batch annealed larninations can not be produced if practically unlimited time is available for very gradual heating and cooling, particularly the latter, so as to reduce the above mentioned differential effects to almost the vanishing point. However, the very long times involved make this solution of the problem impractical in most cases.

By this is meant that the larninations are so placed on a moving conveyor which carries them through the annealer that for substantially all of their journey through the heating and cooling sections of the annealer they have practically no greater internal temperature gradients or differential heating and cooling efiects parallel to their surfaces than they would have if they passed through the annealer as individual units which are not in contact with each other. Theoretically, for rapidly producing ltuninations with minimum internal strains, minimum magnetic losses and minimum curvature best results will be pro duced by placing the larninations on the conveyor out of any overlapping contact with each other because then their maximum top and bottom surfaces are directly exte States Patent posed to the ambient temperature conditions prevailing in the annealer. However, this limits the production rate of any given size annealer so that in order to increase the production rate two other ways of placing the laminations on the conveyor have been tried. One of these is to stack the larninations two high and the other is to partially overlap them like shingles on a roof. Results obtained in both these cases were indistinguishable from those obtained when the larninations were not in contact with each other. This is not as surprising as it might at first appear to be when it is realized that, neglecting the thickness of the larninations (which in all cases is very small compared with the other dimensions of the larninations), every point on each lamination is directly exposed on one side to the ambient temperature conditions prevailing in the annealer.

In order still further to increase the production rate the larninations have been stacked three high with only a slight resulting impairment in quality. This can be explained by the fact that the masking effect of the overlying and underlying larninations on the center lamination is compensated for by the fact that the latter receives heat through both its top and bottom surfaces from the other larninations and loses heat in the same way to those other larninations during cooling.

Stacking the larninations more than three high has also been tried in order to get still higher production, but the resulting quality of the larninations falls off rapidly in proportion to the stack height presumably because of the differential heating and cooling effects of temperature gradients mentioned above.

Furthermore, the preferred form of continuous annealing furnace is a so-called roller hearth furnace in which the conveyor over which the larninations pass consists of a series of horizontal, parallel, synchronously driven, spaced rollers. It is believed that in such a furnace while the steel is plastic that it sags slightly between the rollers where it is unsupported and it is lifted slightly as it passes over each roller. Consequently, the lamination is flexed slightly in successively opposite directions and in a progressive manner throughout the length of the lamination as it passes through the furnace and this serves to work, or in efiect massage, out more effectively any slight residual strains in the larninations.

The larninations are also cooled while passing over a similar roller type conveyor in a cooling section of the annealer so that the degree of flexing of the lamination as it passes over the rollers progressively decreases as the lamination cools and becomes stronger and more solid. The result is that when each lamination has cooled sufficiently to obtain substantially full strength or solidity it is practically dead flat and strain free.

By the term individually annealed, We therefore wish to be understood as including all placements of the larninations on the conveyor which produce results equivalent to those obtained when the larninations are placed on the conveyor out of contact with each other, and specifically we wish to be understood as including two and three high stacking and partial overlapping.

An object of the invention is to provide a new and improved method and apparatus for making laminated transformer cores and particularly the individual lamination pieces of such core.

Another object of the invention is to provide a new and improved method and apparatus for processing transformer core larninations.

A further object of the invention is to provide a new and improved method and apparatus for strain relief annealing silicon steel alloy lamination pieces for transformer cores.-

The invention will be better understood from the folindicated generally as 3 in FIG. 1.

3 lowing description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing, 7

FIG. 1 is a perspective view of a preferred mechanism for slitting Jumbo rolls of fully processed alloysteel strip as it is received from the steel mill into the proper Width with the introduction of minimum strains,

FIG. 2 is a perspective view of apparatus for edge grinding the slit strip and cutting it to length to form individual lamination pieces, 7 FIG. 3 is-a perspective view of a continuous roller hearth combined annealing furnace and two-step cooling chamber combined with apparatus for placing an insulating enamel coating on the 'laminations and baking this coating to form a hard insulating surface,

FIG. 4 is a cross-section of the annealing furnace taken on line 44 of FIG. 3, and

FIG. 5 is a partial longitudinal sectional view of the roller hearth portion showing in exaggerated form the progressive reverse bending of a lamination.

1 Referring now to the drawing and more particularly t FTG. 1, there isshowntherein a so-called Jumbo roll or coil 1 of electrical steel, such for example as the material known'to the trade as M-6X and M-7X. This is a fully processed steel as received from the steel mill, typically with nominal thickness of .012 inch or .014 inch plus or minus an industry tolerance. of .002 inch. It differs from partially finished or processed electrical steel such as so-called SX-lO in a number of respects. SX-lO steel is high reduction cold rolled silicon alloy steel having a silicon content of about 3.25% which has received a so-called normalizing treatment which consists of a heat treatment for relieving strains due to the high reduction cold rolling and also reduces the carbon content of the steel. The additional steps which convert SX-lO into M6X etc. consist of coating with magnesium oxide or equivalent, winding in a coil and annealing at about 1175 C. in a hydrogen atmosphere. It is then on wound, coated with magnesium phosphate, heat-tension flattened, and rewound into a coil.

Roll [is mounted on an arbor 2 of a slitting machine The arbor 2 is driven by an electric motor 4 so as to unwind the coil 1 and cause the wide strip of steel to form a loop 5 between spaced guides 6 and 7 over which it passes. The strip then passes between upper and lower slitting or shearing cutters 8 and 9 driven by an independent electric motor 19. These cutters 8 and 9 trim the steel strip to the exact width desired by removing edge strips 11 and 12 and at the same time they slit the strip into two or more separate strips of the desired width. The trimmed and slit strips then form a second loop 13 while passing over spaced guides 14 and 15 and are then wound up into separate rolls or coils 16 and 17 on an arbor 18 driven by a motor 19. The length of the loop 5 is automatically maintained between upper and lower limits by means of properly spaced photo-cells 20 in cooperation with light beams from spaced light sources 21 on the other side of the loop from the photo-cells. The photocells are arranged to control the speed of the motor 4 through a suitable control box 22 so that when the light beam from the upper light 21.is intercepted by the loop 5 but the light beam from the lower light 21 is not intercepted by the loop 5, the controller 22 will not act to change the speed of the motor. If the loop becomes long enough to intercept the light beam from the lower light 21, the speed of the motor 4 will be reduced so as to shorten the loop, and if the loop becomes short enough to rise above the light beam from the upper light 21 the upper photo-cell 20v will be activated to cause the controller 2210 increase the speed of the motor so as to lengthen the loop 5. Similarly upper and lower light sources 23 cooperate respectively with upper and lower photo-cells 24 control of the speed 19- through a control box 25 so as to maintain the length of the loop 13 within predetermined upper and lower limits as determined by the spacing of thelight sources 23.

This arrangement has numerous advantages. Thus there is practically no forward tension or back tension on the steel strip as it passes between the slitting cutters. This results in introducing minimum strains in the steel when it is slit and minimizes any longitudinal tearing action or flow of the steel as it is being transversely sheared by the slitting cutters. Consequently the slit strip and laminations produced therefrom have substantially no edge curl which characterizes strip drawn by tension through a slitter. Another advantage is that the speed of the slitter can very easily be adjusted by merely adjusting the speed of the motor 10 driving the cutters because due to the automatic controls of the length of the loops 5 and 13, the speed of the motors 4 and 19 will automatically beadjusted to the new speed of the motor 10 in order to maintain the loops 5 and 13 at their predetermined lengths. Still another advantage of the arrangement is that the loop '13 greatly improves the manageability of the individual slip strips in comparison with an arrangement where there is no loop 13 between the slitting cutters and the wind-up reel. In the latter arrangement, the individual strips have substantial lateral stiffness, and as the individual strips start to diverge or converge, it is verydifiicult for guides to control them with the result that the individual rolls have very ragged edges and there is excessive burr formed on the edge of the strip. However, the loop 13 removes practically all of this lateral stiffness effect from the slit strips so that they are very tractable or manageable and can easily be guided to form individual rolls having all of their edges in a straight line or in registration.

Referring now to FIG. 2, the precision slit narrower width roll 16 is mounted onan arbor 26 driven by an electric motor 27. This unwinds the roll to form a loop 28 the return side of which passes over a guide 29 and the strip then passes between two spaced sets of gripping rolls 30 and 31, roll 31 being driven by a motor 32. The rolls 30 act as a drag so as to maintain the strip in slight tension between these sets of rolls. In this space between the two sets of rolls any burrs on the edges of the strip caused by the slitting operation are removed by spaced upper and lower sets of rapidly rotating deburring wheels 33 which may be driven in any suitable manner.

It is important to remove burns from the edges of the strip because when a large number of laminations cut from a strip are stacked to form a magnetic core the cumulative length or build of the burrs may be substan tial so that when the core laminations are clamped together uneven or unbalanced strains will be introduced into the steel.

The strip next forms another loop 34 between spaced guides 35 and 36 over which it passes and it is moved forward by another set of upper and lower driving rolls 37 operated by a motor 38. The strip passes over spaced shearing anvils I39 and 40 above which are separate reciprocable cutting shears or punches 41 and 42 which may be operated by pressure cylinders 43 and 44 respectively. The strip also passes over a continuously moving endless belt 45. When the forward edge of the strip strikes a suitably positioned switch 46 a fluid pressure operated brake 47 is activated in any suitable manner so as to stop the strip and allow the shears or punches 41 and 42 to descend and sever the strip so as to out two laminations having the proper or desired end configuration. As soon as the shears 41 and 42 are released, the two cut punchings or laminations are carried forward by the belt 45 and they successively enter a stacking jig 48 which consists of' a pair of pivoted angle bars 49 operated by suitable servo-mechanisms 50 and gripping rollers 51 operated by motor 52 so as to carry the strips forward into the trough formed'by the channel shaped members.49. When the leading edge ofa lamination gets to the proper position it operates another switch 53 which by any suitable means can be made to operate the mechanisms 50 to pivot the angle bars 49 longitudinally so as to release the lamination and let it drop on a suitable support or plate 54 on which a stack of cut larninations is built up.

As soon as the second cut lamination clears the switch 46, the brake 47 is released and the shearing and stacking cycle is repeated. The loop 34, of course, grows longer while the strip is stopped by the brake 47 during the shearing and stacking operation and it is shortened during intermediate portions of the cycle. In other words, the average speed of the strip through the shearing and stacking mechanism is the same as the speed of the strip through the edge grinder.

Referring now to FIG. 3, the stack of laminations 55 has been transferred by any suitable conveyor equipment to the entering end of a continuous annealer-cooler 56, through which the larninations are passed individually in a manner to be described. The continuous annealer-cooler is preferably of the so-called roller hearth design instead of a moving web design. It consists essentially of a series of spaced parallel generally horizontal steel rollers 58 all driven in the same direction synchronously by any suitable means such as by a common chain and sprocket drive shown more clearly at 57 and in FIGS. 4 and 5. In one unit which has produced good results the over-all length of the roller hearth or bed is approximately 250 feet, the surface speed of the rollers being approximately .40 feet per minute although this is not critical and sub stantially greater speeds may be employed. The speed and capacity of the furnace are a direct function of its length. Approximately the first two-thirds of the entire length is the heating portion of the unit which is shown in cross section in FIG. '4. The final third of the entire unit is the cooling end and this is divided into a first or slow cooler and a second or fast cooler both of which are approximately of equal length corresponding to ap proximately one-sixth of the total length of the combined annealer-cooler.

The individual laminations from the stack 55 may be picked up and fed between conveyor rollers 59 in any suitable manner. There is shown a laterally reciprocable member 60 having a vertically reciprocal suction cup 61 the arrangement being such that the suction cup can descend, grip the upper-most lamination of the stack 55 and then raise this lamination from the stack and move it forward between the rollers 59 which then grip the lamination and introduce it into the furnace, through a slot 62 and on to the hearth of rollers 58. As shown more clearly in FIG. 4 the furnace is equipped with a plurality of spaced electric resistance heaters 63 both above and below the rollers 58. The heaters 63 which are below the roller 58 are preferably mounted on the sides of the furnace instead of on the bottom so that if a lamination piece should dive between rollers it could not short circuit the heaters. At the entering end of the furnace there are preferably additional lower heaters for the purpose of heating the lower surface of the entering edge of each lamination more rapidly than its upper surface so that as the leading end of the strip or lamination piece tends to curl it will curl upward rather than downward so as to prevent the piece from diving between adjacent rollers. As the lamination piece moves forward through the continuous annealer it tends to writhe somewhat in relieving thermal stresses caused by transistory inequalities in heating. The temperature of the annealing part of the furnace is maintained at such a value as to render the steel soft or plastic which is somewhere in the range of 700 875 C. it being typically 800 C. For annealing larninations several feet in length and of a thickness of 12 or 14 thousandths of an inch good results have been obtained with rollers 58 having a two-inch diameter maintained on fourinch centers.

It is believed that the improved flatness characteristics of larninations processed in accordance with the present invention andthe substantially lower hysteresis losses in such larninations is attributable to a progressive slight flexing or bending or wave motion of the plastic lamination as it sags between adjacent rollers and alternately passes over the successive rollers the curvature being concave upward in the space between the rollers and being concave downward as the lamination passes over the rollers. This working or kneading action of the plastic material appears to have an improved effect on working out latent strains in the larninations.

After the larninations have moved approximately twothirds of the way through the unit 56 they enter a slow cooling zone in which there are preferably no resistance heaters and air enters hollow sides of the unit through individual damper or butterfly valve controlled ports 64, the air being at room temperature and being drawn into the furnace walls by suitable blower equipment and exhausted through stacks 65. The valves in the ports 64 may be so adjusted that the larninations are cooled to approximately 500 C. at a rate within the range 300 C. to 375 C. per minute. Initially as the laminationsenter the slow cooler, without thermal shock, they are still plastic and as they progressively cool and thereby gain strength or solidity the flexing progressively decreases as they pass over the spaced rollers. When the larninations enter the fast cooling zone their temperature is reduced to from IOU-200 C. at a rate within the range of 400 C. to 875 C. per minute. In the fast cooler, air enters at one side of the unit through inlet ducts 66 and is removed through exhaust ducts 67. The result of this operation is that the larninations emerge from the unit 56 dead flat and with not the slightest sign of the edge ripple which characterizes typical batch annealed larninations. The larninations are then carried on an endless web conveyor 68 to an enamel coater, 69 which consists essentially of a plurality of rollers 70 between which a suitable coating enamel 71 is fed so that as the larninations pass between the rollers they are coated with the enamel, the operation being somewhat similar to printing a layer of the enamel on the larninations. Next the larninations enter a baking oven 72 maintained at a temperature of approximately 500 C. so as to cure and bake the enamel on the laminations. The back end of '72 is a cooling unit to cool the lamination before it is stacked. As the laminations emerge from the baking oven 72 they are conveyed in any suitable manner such as by an endless moving web 73 above the lower portion of which are mounted a plurality of electro-magnets 74 which hold the laminations against the surface of the moving web so that they are conveyed to a point at which the leading edge of the lamination will contact a switch 75 for momentarily de-energizing the magnets 74 so as to drop the laminations one on top of the other on a suitable base or support 76 at which point they are ready to be stacked into a core for transformers and similar apparatus.

By means of the above described process and apparatus, it has been possible to produce individual laminations which have a watts loss equal to or better than the mill Epstein classification test. Furthermore, extensive tests of. completed transformers having cores made of such larninations have demonstrated that this low loss in the individual larninations is carried over into the completed core and is directly related to final core loss in watts per pound when corrections are made for directional characteristics in the completed core which consists of a plurality of stacked larninations clamped together. Furthermore, extensive tests on a substantial number of completed transformers show a reduction in noise which is believed to be due to the reduced strains and substantially dead flatness of the individual laminations.

While there has been shown and described a particular embodiment of the invention, it will be obvious to those skilled in the art that changes and modifications may be made wjhout departing from the invention, and there- Patent of the 'United States is: e

a l. The method of flattening elongated electrical stee 'laminations which have been slit and sheared at room temperature from a twelve-thousandths of an inch or fourteen-thousandths of an inch thick strip of previously heat-tension flattened and phosphate coated electrical steel which comprises passing the laminations flatwise and lengthwise as separate units over a series of parallel two-inch diameter synchronously rotating circular cylindrical rollers mounted on four-inch spaced centers in transverse alignment in a horizontail'plane in a chamber heated to a temperature between 700 C. and 875 C. at

which said steel is plastic whereby continuous bending in successively reverse directions of each lamination as it is lifted over the rollers and sagsbetween the rollers in a plastic state causes sufficient flow of the steel to work out curvature producing internal mechanical strains, heating the bottom of each lamination to a higher temperature than its top as it enters said chamber so as to prevent itsleading edge from curving downward between adjacent rollers during the initial period of its passage through said chamber while-its thermal stresses are being relieved by writhing and before it becomes plastic, and progressively cooling said' laminations without thermal shock at an increasing rate to a temperature at which they are no longer-plastic by similarly passing them over a second set of generally similar rollers in a chamber into which controlled amounts of cooling air are introduced above and below said second set of rollers, the relative amounts of upper and lower air being so proportioned as to keep said laminations in contact with said second set of rollers and prevent them from being blown upward away from said secondset of rollers. g

2. The method of removing. internal mechanical strains from and flattening elongated electrical steel laminations which have been slit and sheared at room temperature from a twelve-thousandths of an inch or fourteen-thousandths of an inch thick strip of previously heat-tension flattened and phosphate coated electrical steel which comprises passing the laminations flatwise it is lifted over the rollers and sags betwen the rollers in a plastic state causes suflicient flow of the steel to work out curvature producing internal mechanical strains, heating the bottom ofieach lamination to a higher temperature than its top as it enters said chamber so as'to prevent its leading edge from curving'downward between ad- 'jacent rollers during the initial period of its pasag'e through said chamber while its thermal stresses are being relieved by writhing and before it becomes plastic, and progressively cooling said laminations Without thermal shock at an increasing rate to a temperature at which they are no longer plastic by similarly passing them over a second set of generally'similar rollers in a chamber into which controlled amounts of cooling air are introduced above and below said second set of rollers, the relative amounts of upper and lower air being so proportioned as to keep said laminations in contact with said second set of rollers 'and prevent them from being blown upward away from said second set of rollers.

3. The method of removing edge curl from and flattening elongated silicon steel laminations which have been slit and sheared at room temperature from a twelvethousandths of an inch or fourteen-thousandths of an inch thick strip of previously normalized, phosphate coated and heat-tension flattened steel which comprises passing the laminations flatwise and lengthwise as separate units over a series of parallel two-inch diameter synchronously rotating circular cylindrical rollers mounted on four-inch spaced centers in transverse alignment in a. horizontal plane in a chamber heated to a temperature between 700 ,C. and 875 C. at which said steel is plastic whereby continuous bending in successively reverse directions of each lamination as it is lifted over the rollers and sags between the rollers in a plastic state causes suflicient flow of the steel to work out curvature producing internal mechanical strains, heating the bottom of each lamination to a higher temperature than its top as it enters said chamber so as to prevent its leading edge from curvingidownward between adjacent rollers during the initial period of its passage through said chamber while its thermal stresses are being relieved by writhing and before it becomes plastic, and progressively cooling said laminations without thermal shock at an increasing rate in two steps to a temperature at which they are no longer plastic by similarly passingthem over a second set of generally similar rollers in a chamber intotwhich controlled amounts of cooling air are introduced above and below said second set of rollers, therelative amounts of upper and lower air being so proportioned as to keep said laminations in contact with said second set of rollers and prevent them from being blown upward away from said second set of rollers.

: References Cited in the file of this patent UNITED STATES PATENTS 1,752,490 Karcher Apr. 1, 1930 1,808,152 Baily June 2, 1931 2,085,092 Furth June 29, 1937 2,120,319 Wilson June 14, 1938 2,237,061 Scharschu"; Apr. 1, 1941 2,278,136 Otis et al. Mar. 31, 1942 2,287,466 Carpenter June 23, 1942 2,351,922 7 Biirgwin June 20, 1944 2,412,041 Gifford et al. Dec. 3, 1946 2,448,514 Butler Sept. 7, 1948 2,565,303 Garbarino Aug. 21, 1951 2,584,564 Ellis Feb. 5, 1952 2,588,439 Ward Mar. 11, 1952 OTHER REFERENCES The Making, Shaping and Treating of Stee United States Steel,,6th ed., p. 1134.

Metals Handbook, American Society for Metals, Cleveland,1l948, page 355.

Claims (1)

1. THE METHOD OF FLATTENING ELONGATED ELECTRICAL STEEL LAMINATIONS WHICH HAVE BEEN SLIT AND SHEARED AT ROOM TEMPERATURE FROM A TWELVE-THOUSANDS OF AN INCH OR FOURTEEN-THOUSANDTHS OF AN INCH THICK STRIP OF PREVIOUSLY HEAT-TENSION FLATTENED AND PHOSPHATE COATED ELECTRICAL STEEL WHICH COMPRISES PASSING THE LAMINATIONS FLATWISE AND LENGTHWISE AS SEPARATE UNITS OVER A SERIES OF PARALLEL TWO-INCH DIAMETER SYNCHRONOUSLY ROTATING CIRCULAR CYLINDRICAL ROLLERS MOUNTED ON FOUR-INCH SPACED CENTERS IN TRANSVERSE ALIGNMENT IN A HORIZONTAL PLANE IN A CHAMBER HEATED TO A TEMPERATURE BETWEEN 700*C. AND 875*C. AT WHICH SAID STEEL IS PLASTIC WHEREBY CONTINUOUS BENDING IN SUCCESSIVELY REVERSE DIRECTIONS OF EACH LAMINATION AS IT IS LIFTED OVER THE ROLLERS AND SAGS BETWEEN THE ROLLERS IN A PLASTIC STATE CAUSES SUFFICIENT FLOW OF THE STEEL TO WORK OUT CURVATURE PRODUCING INTERNAL MECHANICAL STRAINS, HEATING THE BOTTOM OF EACH LAMINATION TO A HIGHER TEMPERATURE THAN ITS TOP AS IT ENTERS SAID CHAMBER SO AS TO PREVENT ITS LEADING EDGE FROM CURVING DOWNWARD BETWEEN ADJACENT ROLLERS DURING THE INITIAL PERIOD OF ITS PASSAGE THROUGH SAID CHAMBER WHILE ITS THERMAL STRESSES ARE BEING RELIEVED BY WRITHING AND BEFORE IT BECOMES PLASTIC, AND PROGRESSIVELY COOLING SAID LAMINATIONS WITHOUT THERMAL SHOCK AT AN INCREASING RATE TO A TEMPERATURE AT WHICH THEY ARE NO LONGER PLASTIC BY SIMILARLY PASSING THEM OVER A SECOND SET OF GENERALLY SIMILAR ROLLERS IN A CHAMBER INTO WHICH CONTROLLED AMOUNTS OF COOLING AIR ARE INTRODUCED ABOVE AND BELOW SAID SECOND SET OF ROLLERS, THE RELATIVE AMOUNTS OF UPPER AND LOWER AIR BEING SO PROPORTIONED AS TO KEEP SAID LAMINATIONS IN CONTACT WITH SAID SECOND SET OF ROLLERS AND PREVENT THEM FROM BEING BLOWN UPWARD AWAY FROM SAID SECOND SET OF ROLLERS.

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