US2731254A - Annealing furnace structure - Google Patents

Description

1956 G. J. CAMPBELL ET AL 2,731,254

ANNEALING FURNACE STRUCTURE 3 Sheets-Sheet 1 Filed Oct. 11, 1951 IN VENTORS bell TTORiEY Gear 6 I Caryn fiew s G. (060%.

BY 67am. AW A F 1. m fl Jam 17, 1956 G. J. CAMPBELL ET AL 2,731,254

ANNEALING FURNACE STRUCTURE 3 Sheets-Sheet 2 Filed Oct. 1]., 1951 1N VENTORS be TTORNZY I 3 J: 0 Lelb' 0.623330 Jan. 17, 1956 G. J. CAMPBELL ETAL ANNEALING FURNACE STRUCTURE 3 Sheets-Sheet 3 Filed Oct. 11 1951 5 R O T N E V m United States Patent ANNEALING FURNACE STRUCTURE George J. Campbell, Sparrows Point, and Lewis G. Coscia, Baltimore, Md., assignors to Bethlehem Steel Company, a corporation of Pennsylvania Application October 11, 1951, Serial No. 250,852

Claims. (Cl. 263-40) Our invention relates to furnaces and more particularly to furnaces for heat-treating or annealing coils of rolled steel strip or the like.

One of the objects of our invention is to produce annealed strip at a high furnace rate, as the rate of production depends on the rate of heat transfer from the furnace through the inner cover or hood to the steel charge. The greater part of the heat is transferred to the charge by convection currents under the said inner cover. By maintaining a high forced circulation of these currents, the coils will heat faster. In this manner we can circulate larger volumes of higher temperature prepared gas than is now possible with the present constant speed fans and motors.

Another advantage of our invention is that by virtue of the increased production, the fuel rate will be proportionally lower.

A further object of our invention is to produce a completely deoxidized annealed strip with uniform quality by the improved circulation which will be maintained and by the closer temperature differentials between the top and bottom of the charge and'also between the inside and outside of the coils. The outside laps of the coils will not overheat because in general the inner cover temperature will be lower by virtue of the larger volume of recirculated prepared gas.

Another object of our invention is to produce annealed strip at a low maintenance cost.

Another object of our invention relates to the central pedestal and the construction and arrangement of the diffuser plates and their associated elements. As shown in the drawings the fan is located centrally of the pedestal. Surrounding the fan is a diffuser plate. This diffuser plate consists of a number of segments, each segment consisting of a base member and a plurality of upstanding vanes formed integral therewith which define-outwardly extending passageways. The gas streamfrom a fan is discharged at a fixed angle with a radial line which (the angle) is the resultant of the angle of discharge of the blades and a tangent to the periphery of the blades. This angle will be different for each different-design of fan blade. Therefore, the angle of the diffuser vanes must be the same as the resultant angle of the gas stream discharged by the fan and a'slight variation of the angle in either direction causes a large decrease in the volume of gas discharged by the fan and the static pressure developed by the fan.

If the angle of the diffuser is not correct eddy currents are set up by the flow of the gas through the diffuser causing the above mentioned losses. Furthermore, if the fan were allowed to discharge into a chamber, without a diffuser, the expansion of the gas would-be abrupt and the entire velocity pressure in the outlet stream would be lost. Most of the velocity pressure can be saved, however, if the outlet of the fan wheel is fitted with a proper evas diffuser. For the purposes hereof, thepassageways will be characterized by'extendingjoutwardly in a direction substantially tangent to theperiphery of I structure which is provided with a refractory base pedestal 'ice the fan. At the outer ends the said vanes are bent so as to extend in a direction substantially radially with respect to the center of the plate and in addition the base member between the vanes is curved upwardly at its periphery to deflect upwardly the gas flowing through the passageways for the purpose of A. Avoiding turbulence and high velocity pressure between the inner cover and retaining ring to prevent blowing and breaking of the sand seal.

B. To prevent the tangential flow of gas between the inner cover and the coils of steel, because tangential flow of the gas at high velocities would pick up sand from the sand seal and deposit the sand on the coils.

C. Another important point which must be considered in the design of the diffuser plate is that the upstanding vanes must have sufficient strength and bearing surface to support the weight of the charge under continued high temperature operating conditions. The weight of the charge must also be distributed over the total area of the refractory pedestal. The ratio of the passageway area to the supporting area of the diffuser plate is important and both areas must perform their functions properly, i. e. to support the charge of steel and to provide passageways for the circulation of gas.

The segments forming the diffuser plate are each provided with lugs and matching recesses to aid in locating them properly and each segment is also provided with a lifting bar cast therein to facilitate handling and installation of the diffuser plates. Superimposed on the diffuser plate is a hearth plate on which the coils rest during the heat treatment.

An important point which must be considered in the design of the diffuser plate is a method of converting relatively high and confined velocities into low and expanded velocities, at the periphery of the diffuser casting. This is accomplished by progressive and accurate increase of the free area between the vanes and the diffuser plate. The iner end of the diffuser plate acts as a cut-off to direct the air from the periphery of the fan into the evas outlet between the vanes. The multiple cut-offs also equally divide the output of gas from the fan so that it is equally distributed around the periphery of the base and up the annular space between the charge and the inner wall of the hood.

A further object of our invention relates to the manner of cooling the driving motor for the fan.

With these and other objects in view which will become apparent in the following description and disclosure, this invention comprises the novel construction and combination hereinafter described and more particularly pointed out and defined in the appended claims.

Having thus given a general description of the objects of our invention we will now in orderto make the same more clear, refer to the annexed three sheets of drawings forming a part of this specification and in which like characters of reference. denote like parts.

Figure'l is a vertical transverse section of our improved furnace embodying our invention.

Fig. 2 is an enlarged detail of a lower portion of the furnace.

Fig. 3 is an elevation of the outer end of one of the segments of the diffuser plate.

Fig. 4 is a detail section through the outer edge of one of the diffuser segments taken on the line 44 of Fig. 7.

Fig. Sis a detail section taken on the line 55 of Fig. 7.

Fig. 6 is a top plan view of the diffuser plate.

Fig. 7 is a top plan view of one of the diffuser plate segments, and

Fig. 8 is an enlarged detail in plan of one of the inner end portions of a diffuser plate segment. 1

Referring in the drawings in detail, 1 is the furnac 2, hearth retaining ring 3 and insulating blocks 4 resting upon a metal plate 5 supported by structural members 6. The hearth retaining ring 3, has its lower end welded to the metal base plate 5, thereby forming a gastight seal between the base plate 5 and the hearth retaining ring 3 to exclude all air infiltration from under the hood in order to produce a completely deoxidized annealed strip. The refractory base pedestal 2 is provided centrally with a vertically extending sleeve 7 through which the upper end of the motor shaft 8 for the furnace fan 9 extends.

The furnace 1 is provided with a casing it) having its lower end 11 extending into a sand seal 12, refractory walls 13 and a refractory roof 14 to prevent heat losses. The casing iii, refractory walls 13 and roof 14- are removable and means for facilitating its removal may be connected to the bar 15.

in order to provide heat to the furnace a number of standard methods may be used, namely electric heating elements mounted along the side walls and end walls of the furnace; gas or oil tired burners mounted in the end and side walls and firing against a low refractory inner wall or hollow radiant tubes is extend longitudinally of the furnace in two batteries as shown, one battery along each side wall of the furnace, end walls, or between the pedestals. in operation the tubes "M are filled with burning gas which heats them to a high temperature; they then transmit this heat to other parts of the furnace by radiation. The base pedestal 2 has a circular refractory base built-up within the retaining ring 3 upon which is mounted a flat cover plate 1'7 and a diffuser plate is which is covered by hearth plate 1? having a central aperture 29.

The refractory pedestal and component parts is so constructed that the total weight of the charge is transferred and distributed equally to the hearth plate 19, diffuser plate 13, cover plate l7, refractories 2, metal plate 5, and structural member 6. No portion of the weight of the charge overhangs the fan 9, or aperture 24 As shown in the drawings the fan 9 is located cen trally above the refractory base pedestal 2. The diffuser. plate 18, consists of a number of segments 231. each consisting of a base member 22 and a plurality of upstanding vanes 23 which define passageways 24 extending out wardly in a direction substantially tangent to the periphery of the fan 9 as at 25.

At the outer ends the said vanes are bent so as to extend in a direction substantially radially with respect to the center of the plate as at 25. The base member 22 between the vanes at their outer end portions are curved as at 27 to deflect upwardly the gas flow as it is discharged from the passageways for the purposes of avoiding turbulence and high velocity pressure between the inner cover and retaining ring to prevent blowing and breaking of the sand seal, to prevent the tangential flow of gas between the inner cover and the coils of steel, because tangential flow of the gas at high velocities would pick up sand from the sand seal and deposit the sand on the coils.

The segments forming the diffuser plate 18 are each provided with a spacing rib 2S and lugs 3-0 and matchrecesses iii. to aid in locating them properly and each segment is also provided with a lifting bar 32 cast therein acilitate handling and installation of the segments 1g the diflfuser plate. One of the segments is furrovide with an opening 33 for receiving a thermoe. The hearth plate l) is adapted to receive and support the coils of strip or like material during the heat treating operation is superimposed on the dilfuser plate and in order to hold the hearth plate in position the outer top edge of one of the vanes 2-3 for each segment is notched as at 35 to receive a square bar 36 attached to the lower face at the outer edge of the hearth plate 19 at spaced intervals.

To protect the coils of strip or the like material 34 being heat treated against oxidation the hood 28 which is made of thin sheet metal and annular in cross section completely covers the charge, the lower end of which is open and extends into a sand seal 37 and is seated on the insulating blocks 4 thereby producing a substantially gastight chamber for the material treated.

The lower open end of the hood 28 is reinforced by means of an angle 38 which is welded to the outer edge, while an angle 39 is welded to the inner surface of the hood adjacent to the discharge ends of the passageways 24 to assist in upwardly deflecting the gas at this point and to center the inner cover in order to have an equal annular space around the charge.

The motor 40 for rotating the fan 9 is attached to a top housing 41 which is mounted in the lower surface of the refractory base pedestal 2, and is further attached to the base plate 5 by means of cap screws 42.

The housing 43 for the motor 40 has only two external openings to the motor. The opening 44 is for electrical connections to the motor and is sealed gas tight after the wires are installed. The other opening 45 is for a pipe connection 46 for prepared gas and allows the gas to enter the furnace adjacent to the upper end of the motor shaft 8.

In order to eliminate openings in the motor housing for shafts and to eliminate shaft seals, an external air blower 47 is used which is driven by a motor 48 secured to the lower end of the motor housing 43 and mounted in an air duct W to circulate cooling air on the outside of the motor housing 43 which is ribbed as at St to increase the cooling surface. The motor windings, shafts and bearings are cooled by the prepared gas admitted through pipe connection 46 which is circulated through the motor and against the inside of the motor housing for cooling before it enters the furnace. The inside of the motor housing is also ribbed as at 51 to increase the cooling surface and an internal blower 52 is mounted on the motor shaft 8 above the motor housing 43 for this purpose.

The furnace fan 9 has a hub 53 which is secured to the upper end of the motor shaft 8 and is provided with a downwardly extending flange 54, the lower end of which surrounds in spaced relation the upper end of the sleeve 7.

As indicated by the arrows, the prepared gas enters the motor housing 43 through pipe connection 46, passes through the motor, and is then discharged through the opening 55 in the housing top which supports the motor. The gas then passes through the spaced channel 56 between the sleeve 7 and the upper portion of the fan motor shaft 8, then over the top of the sleeve and under the lower edge of the hub flange 54 to the furnace fan 9 which will circulate the gas through the passageways 24- under the hearth plate 19. Then it is deflected upwardly around the coil of strip 34 and the inside of the hood 28, then down through the center of the coil and through the central aperture 20 in the hearth plate 19, and will continue to circulate in this manner until the coil is brought up to the proper temperature.

The gas upon entering the space defined by the hood 28, will be heated, and by means of the fan 9 will be rapidly recirculated around the enclosure. Because of rapid recirculation it is possible to maintain a very high fuel input to the furnace without overheating the furnace, the inner cover or the charge, as the heat is quickly transferred to the coils by convection currents accelerated by the rapid recirculation of the gas.

It will at the same time flow through the passageways 24 in the diffuser plate 18 under the hearth plate 19. The hearth plate 19 will transmit the heat contained by the gas to the bottom portion of the coil of strip 34 which will then quickly attain a uniform temperature with the balance of the charge. This will not only result in a more uniform heating elfect on the metal, but because of the faster heating of the bottom portion of the coil will produce a faster treating cycle and result in increase in production. Another result will be a decrease in fuel consumption, which means a decrease in costs.

The word gas when used in conjunction with the introduction of gas under thehoodo'r inthe gas recirculating system includes the use of any prepared atmosphere which is used for producing a deoxidized strip and/or for surface preparation of the strip.

Although we have shown and described our improvements in'con'siderable detail, wedo not wish to be limited to the exact and specific details shown and described, but may use such substitutions, modifications or equivalents thereof, as are embraced'within the scope of ourinventio'n, or as pointed out in the-claims.

We claim:

1. A heat-treating furnacecomprising top, side and bottom walls, forming a closed heating chamber, heating means-disposed adjacent thereto, a fan mounted centrally above thebottom wall, a diffuser plate formed of a plurality of segments surrounding the fan each of said segments having a base member and a plurality of tapered upstanding vanes formed integral therewith which define passageways'of 1 greater width at'the outer periphery of the diffuser plate forgas'circulation, the inner ends of the upstanding vanes extending outwardly in a direction substantially thesame as that'of the 'gas'st'ream discharged by the fan'and the outer ends ofth'e vanes 'bentso as to extend in a direction substantially radial with respect to the center of the'fan, the base member between the outer ends of theva'nes having anupwardly curved surface to deflect upwardly the gas after passing throughthe passageways inthe diffuser=plate,-a hearth plate mounted on the diffuser plate and secured to the tops of the vanes for supporting the material to be heated, and means for rotating the fan whereby gas is forced outwardly through the passageways in the diffuser plate.

2. A heat-treating furnace comprising top, side and bottom walls, forming a closed heating chamber, a refractory pedestal centrally disposed in the bottom wall and extending above the "same, a hearth retaining ring surrounding the refractory =pedestal, a cover plate mounted on top of the refractory pedestal, a fan'mounted centrally on a shaftabo've the refractory pedestal-and cover plate, a diffuser plate formed of -a plurality of cast segments surroundingthe fan, each of said cast segments having a base member and a-plurality of upstanding vanes formed integral therewith whichdefine passageways for "gas circulation, the inner'end "portions of the said passageways having a gr'eaterwidth and extending outwardly in a direction'substanti'ally the same as that of the gas stream discharged by' the fan with theouterend portions of said vanes bent so as to extend in a direction radially with respect to the center-of the diffuser plate, the base member between the outer ends-of 'the'vanes having an upwardly curved surface to deflect the gas upwardly after passing through the passageways in "the diffuser plate, a hearth plate mounted on the diffuser plate for supporting the material to be heated, and means for rotating the fan whereby gas'is forced outwardly through the passageways in the diffuserplate.

3. A heat-treating furnace comprisingtop, side and bottom walls, forming a closed heating chamber, heating means disposed adjacent-thereto, said bottom wall having an upwardly-extending refractory pedestal, a cover plate mounted on top'of the refractory pedestal, a fan mounted centrally above the cover plate on the refractory pedestal, a diffuser plate mounted on the cover plate and surroundingthefan'which consists of a plurality of segments, said and secured to the tops of "the vanes for supporting the material to be heated, and the base member of the diffuser plate between the radially extending ends of the vanes curved upwardly at the periphery to deflect upwardly the flow of gas around the material treated after passing through the passageways.

4. A support for material to be heated in a heat-treating furnace having a bottom wall, comprising a diffuser plate formed of a plurality of cast segments having a central aperture mounted on the bottom wall, a fan mounted for rotation in the central aperture of the diffuser plate, each of said segments forming the diffuser plate having a base member with a plurality of vertically tapered upstanding vanes formed integral therewith, the inner ends of said vanes extending outwardly in a direction substantially the same as that of the gas stream discharged by the fan, and the outer ends of said vanes extending radially with respect to the center of said fan, passageways between the vanes for circulation of heated gas, a hearth plate mounted above the fan on the diffuser plate for supporting the material to be heated, the outer end portions of the base member of the diffuser plate between the vanes formed with upwardly curved surfaces adapted to deflect upwardly the gas discharged from the passageways, and means for rotating the fan for the circulation of heated gas in the passageways.

5. A heat-treating furnace comprising top, side and bottom walls, forming a-closed heating chamber, heating means disposed adjacent thereto, said bottom wall having an upwardly extending refractory pedestal, a hearth retainer ring surrounding the refractory pedestal, a sand seal surrounding the hearth retainer ring, a fan for producing gas circulation mounted centrally above the refractory pedestal for rotation about a substantially vertical axis, a diffuser plate formed of a plurality of segments surrounding the fan mounted on top of the refractory pedestal, said diffuser plate segments each having a base member with a plurality of tapered upstanding vanes, the inner ends of said vanes extending outwardly in a direction substantially the same as that of the gas stream discharged by the fan and the outer ends of said vanes extending radially with respect to the center of the said fan, passageways between the vanes gradually in creasing in width from the fan to the outer peripheral edge of the diffuser plate for gas circulation, a hearth plate mounted above the fan and seated on the tops of the vanes of the diffuser plate and secured thereto for supporting the material to be heated, a hood open at its lower end and seated in the sand seal arrangement to enclose the material and the refractory pedestal, the end portions of the base member of the diffuser plate between vanes formed with upwardly curved surfaces adapted to deflect upwardly the gas discharged from the passageways, and an angle secured to the inner surface of the hood adjacent to the discharge ends of the passageways in the diffuser plate adapted to assist in the upward deflection of the gas.

6. A heat-treating furnace for annealing coils of strip, comprising top, side and bottom walls forming a closed heating chamber, heating means disposed adjacent thereto, a refractory pedestal centrally disposed in the bottom wall and extending above the same, a hearth retainer ring surrounding therefractory pedestal, a sand seal disposed adjacent to the hearth retainer ring, a fan disposed centrally above the refractory pedestal, a diffuser plate surrounding the fan mounted on top of the refractory pedestal, said diffuser plate having a base member with a plurality of upstanding vanes formed integral therewith adapted to define passageways for circulating a gaseous atmosphere therein, said passageways gradually increasing in width from theinner ends adjacent to the'fan to the peripherial outer edge of the diffuser platc,'the inner ends of the passageways extending outwardly in a direction substantiallythe'same as that of the gas stream discharged by the fan, and the outer ends of the passageways extending substantially radially to the center of the fan, the base member between the outer ends of the vanes having an upwardly curved surface adapted to deflect upwardly the gaseous atmosphere after it passes through the passageways in the diffuser plate, a hearth plate having a central aperture mounted on top of the diffuser plate above the fan for supporting the coil having a vertically extending central opening, a hood open at its lower end and seated in the sand seal arranged to enclose the coil and the refractory pedestal in spaced relation, a motor for rotating the fan, means for circulating gas through the coils of the motor to cool the motor, means for discharging the gas for cooling the motor to the fan, and means for rotating the fan whereby the heated gas is discharged from the passageways in the diffuser plate into the space between the inner side wall of the hood and the coil whereby the heated gas is forced upwardly and then returned downwardly to the fan through the central opening in the coil and the aperture in the hearth plate.

7. A heat-treating furnace for annealing coils of strip, comprising top, side and bottom walls forming a closed heating chamber, heating means disposed adjacent thereto, a refractory pedestal centrally disposed in the bottom wall, a hearth retainer ring surrounding the refractory pedestal, a sand seal disposed adjacent to the hearth retainer ring, a cover plate mounted on top of the refractory pedestal, a fan disposed centrally above the cover plate, a diffuser plate formed of a plurality of segments surrounding the fan mounted on the cover plate, said diffuser plate segments each having a base member with a plurality of upstanding vanes formed integral therewith adapted to define passageways for gas circulation therein, the inner ends of the passageways extending outwardly in a direction substantially the same as that of the gas stream discharged by the fan, and the outer ends of the passageways extending substantially radially to the center of the fan, said passageways being of gradually increasing width from their inner ends outwardly the base member bet cm the outer ends of each of the vanes having an upwardly curved surface adapted to deflect upwardly the heated gas after it passes through the passageways in the diffuser plate, a hearth plate having a central aperture mounted on top of the diffuser plate above the fan for supporting the coil having a vertically extending opening adapted to register with the aperture in the hearth plate, notches formed in the outer upper edges of a plurality of the vanes, a plurality of square bars secured to the lower outer edges of the hearth plate adapted to be seated in notches in the vanes for holding the hearth plate in position in relation to the diffuser plate, a sheet metal hood annular in cross section and open at its lower end and seated in the sand seal arranged to form a gas-tight enclosure in spaced relation for the coil and the refractory pedestal, a motor for rotating the fan, a housing for the motor, a pipe connection to the motor housing for circulating gas through the coils of me motor to cool the motor, a connection between the motor housing and the fan for discharging the gas from the motor housing to the fan, and means for rotating the fan whereby the heated gas is discharged from the passageways in the diffuser plate into the space between the inner side wall of the hood and the coil whereby the heated gas is forced upwardly around the coil and. then returned downwardly to the fan through the central opening in the coil and the aperture in the hearth plate.

8. A heat-treating furnace for annealing coils of strip, comprising top, side and bottom walls forming a closed heating chamber, heating means disposed adjacent thereto, a refractory pedestal centrally disposed in the bottom wall, a hearth retainer ring surrounding the refractory pedestal, a sand seal disposed adjacent to the hearth retainer ring, a cover plate mounted on top of the refractory pedestal, a fan disposed centrally above the cover plate, a diffuser plate surrounding the fan mounted on the cover plate, said diffuser plate having a base member with a plurality of upstanding vanes formed integral therewith adapted to define passageways for gas circulation therein, the inner ends of the passageways being of less width and extending outwardly in a direction substantially the same as that of the gas stream discharged by the fan, and the outer ends of the passageways of greater width and extending substantially radially to the center of the fan, the base member between the outer ends of the vanes having an upwardly curved surface adapted to deflect upwardly the heated gas after it passes through the passageways in the diffuser plate, a hearth plate having a central aperture mounted on top of the difiuser plate above the fan and the coil to be heated, said coil having a vertically extending opening adapted to register with the aperture in the hearth plate, a sheet metal hood annular in cross section and open at its lower end seated in the sand seal and arranged to form a gas tight enclosure in spaced relation for the coil and the refractory pedestal, a motor for rotating the fan, an integral housing for the motor, a pipe connection for the motor housing adapted to circulate gas through the coils of the motor to cool the motor, a sleeve extending vertically through the center of the refractory pedestal adapted to form a gas passage for discharging the gas from the motor housing to the fan, a motor shaft extending through the sleeve in spaced relation therewith, cooling flanges on the fan motor housing, a motor connected to the lower end of the fan motor having an air blower for circulating air around the cooling flanges for cooling the motor housing, and means for rotating the fan whereby the heated gas is discharged from the passageways in the diffuser plate into the space between the inner side wall of the hood and the coil whereby the heated gas is forced upwardly around the coil and then returned downwardly to the fan through the central opening in the coil and the aperture in the hearth plate.

9. A heat-treating furnace for annealing strip, comprising top, side and bottom walls forming a closed heat ing chamber, heating means disposed therein, a base plate for supporting the bottom wall, a housing top for a motor secured centrally to the base plate, a refractory pedestal mounted centrally on the bottom wall and extending above the same, a hearth retaining ring surrounding the refractory pedestal having its lower end welded to the base plate, a cover plate mounted on top of the refractory pedestal, a fan disposed centrally above the cover plate, a blower mounted adjacent thereto, a diffuser plate surrounding the fan, a motor for rotating the fan, an integral housing for the motor secured to the housing top, a pipe connection for the motor housing adapted to circulate prepared atmosphere through the motor coils to cool the motor, a sleeve extending vertically through the center of the refractory pedestal adapted to form a passage for discharging the prepared atmosphere from the motor housing to the fan, and a motor shaft extending through the sleeve in spaced relation therewith.

10. A heat-treating furnace for annealing strip, comprising top, side and bottom walls forming a closed heating chamber, heating means disposed therein, a base plate for supporting the bottom wall, a housing top for a motor secured centrally to the base plate, a refractory pedestal mounted centrally on the bottom wall, a cover plate mounted on top of the refractory pedestal, a fan disposed centrally above the cover plate, a diffuser plate formed of a plurality of segments surrounding the fan, a motor for rotating the fan, an integral housing for the motor secured to the housing top, a blower and a pipe connection for the motor housing adapted to circulate prepared atmosphere through the motor coils to cool the motor, a sleeve for the introduction of prepared atmosphere from the motor to the fan, said sleeve enclosing the motor shaft and allowing the prepared atmosphere to flow in an annular space between the sleeve and the References Cited in the file of this patent UNITED STATES PATENTS Haskell Oct. 9, 1934 Harsch Aug. 1, 1939 Hough May 28, 1940 10 Wechsberg et a1 Dec. 31, 1940 Davis Dec. 2, 1941 Dailey, Jr., et a1 Apr. 6, 1948 Mumford Oct. 18, 1949 Bailey, Jr Nov. 22, 1949 Hoop June 26, 1951 Cone Dec. 25, 1951 Cone June 10, 1952

Images