US3051296A

US3051296A - Oven stripper

Info

Publication number: US3051296A
Application number: US855854A
Authority: US
Inventors: Ralph F Mertz
Original assignee: Miehle Goss Dexter Inc
Current assignee: Miehle Goss Dexter Inc
Priority date: 1959-11-27
Filing date: 1959-11-27
Publication date: 1962-08-28
Anticipated expiration: 1979-08-28

Description

Aug. 28, 1962 R. F. MERTZ 3,051,296

OVEN STRIPPER Filed Nov. 27, 1959 4 Sheets-Sheet 1 INVENTOR.

RALPH F. MERTZ ATTO R S Aug. 28, 1962 R. F. MERTZ 3,051,296

OVEN STRIPPER Filed Nov. 27, 1959 4 Sheets-Sheet 2 INVENTOR.

RALPH F. MERTZ o,- NEYS R. F. MERTZ OVEN STRIPPER Aug. 28, 1962 4 SheetsSheet 3 Filed Nov. 27, 1959 .INVENTOR.

RALPH F. MERTZ ,fldodaf Qyw ATTOR YS Aug. 28, 1962 R. F. MERTZ 3,051,296

OVEN STRIPPER Filed Nov. 27, 1959 4 Sheets-Sheet 4 INVENTOR.

RALPH F. MERTZ 36w M/Qww ATTO NEYS ilnited tates Fatent @iiim 335L295 Patented Aug. 28, 1952 3,051,296 OVEN STREPER Ralph F. Mertz, Valley Cottage, N.Y., nssignor to Miehle- Goss-Dexter, Incorporated, (Zhicago, Ill., a corporation of Delaware Filed Nov. 27, 1959, Ser. No. 855,854 11 Claims. (Cl. 198134) This invention relates to sheet handling apparatus and more particularly to apparatus for receiving sheets of material from the conveyor means of a sheet treating apparatus, such as a drying oven, and moving the same out of operational contact with said conveyor means.

In the sheet metal art it has been found desirable to treat metal sheets in various ways such as coating said sheets with a protective and/ or decorative material. The sheets of metal carrying the protective coating are then transported through a drying oven on a conveyor means. This invention is directed to apparatus for removing the sheets from the oven conveying means without stopping said conveying means.

One object of the present invention is to provide an oven stripping apparatus which sequentially receives a series of sheets of material and quickly moves each sheet out of operational range of the means for conveying the sheets through the oven.

Another object of the present invention is to provide an oven stripping apparatus wherein the sheets are firmly seated and retained on a conveyor means with a minimum of sheet surface contact with the conveyor means.

Another object is to provide means associated with a conveyor for attracting sheets toward the conveyor and to firmly retain said sheets on the conveyor to prevent relative motion between the surface of the sheets and the conveyor whereby marring of the sheet surfaces is prevented.

Another object is to provide a drive means for a conveyor having means for adjusting the speed of the con veyor during the operation of the conveyor.

Another object is to provide an oven stripping apparatus having means for preventing the stopping of the oven conveyor means when a jam occurs between said oven conveyor means and a sheet seated on said stripper conveyor.

Another object is to provide novel means for supporting an oven stripping apparatus.

A still further object is to provide an oven stripping apparatus having novel support means therefor and novel means responsive to a jam between a sheet on the stripper conveyor and the oven conveyor to cause said support means to be actuated whereby said stripper conveyor and sheet are moved away from said oven conveyor.

The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.

In the drawings, wherein like reference characters refer to like parts throughout the several views,

FIG. 1 is a top plan View of the oven stripper;

FIG. 2 is a side elevational view taken along line 22 of FIG. 1;

FIG. 3 is a side cross-sectional view of the suction means taken along line 33 of FIG. 1;

FIG. 4 is an end crosssectional view of the suction means taken along line 4-4- of FIG. 3;

PEG. 5 is a cross-sectional view of the supplemental magnetic hold down means; and

FIG. 6 is an end elevational view of the oven stripper taken along line 6-6 of FIG. 1.

Referring to the drawings, one form of oven stripping apparatus constructed and operated in accordance with the present invention is herein illustrated as being operatively associated with a drying oven conveyor system. It is to be understood that this apparatus could equally be associated with other types of sheet treating apparatuses wherein it is desired to quickly and efliciently remove sheets from a continuously moving conveyor system. In an apparatus for removing sheets of metal having coated or printed surfaces from the continuously operating oven conveyor, it is necessary to provide means to receive each sheet and quickly move the same away from the oven conveyor without permitting the surfaces of the sheets to be marred and also to release any jams which might occur between the oven conveyor and a sheet on the oven stripper. To prevent any slippage between a sheet and the stripper conveyor there is illustrated a suction means to hold the sheets on the conveyor. The suction comprises a low pressure differential on each side of the sheet with the suction applied over the large area of the sheet to provide a downward force of sufficient intensity to retain the sheets on the conveyor without slippage. At the same time this downward force is not great enough to prevent the sheet from being transferred from the stripper conveyor to another conveyor.

Referring to FIGS. 1 and 2, an oven stripper 15 is mounted to one end of an oven 14 and retained in alignment with an oven conveying means. Sheets of metal are conveyed through a conventional drying oven (not shown) in a vertical position and are supported by spaced apart wicket members it which are in turn secured at each side to a pair of endless conveyor chains 11. The oven conveyor chains t1 are continuously operated at a uniform speed. At the discharge end of the oven the conveyor chains are driven over three pairs of sprocket members arranged in a generally triangular position to each other as shown in H6. 2 so that each wicket and sheet 12 supported by it are first carried at a gradual downward slope toward a horizontal plane and then each wicket abruptly reverses its direction as it moves away from the horizontal plane and continues to the lower path of travel of the oven conveyor chains 11 to be returned to the position for receiving additional sheets for conveying through the oven. The oven stripper comprises a pair of spaced apart side arms 13 having the rear portion of each pivotally mounted to the discharge end of the oven frame. As shown in FIG. 2, the side arms 13 are maintained in a horizontal plane slightly below the center line of the pair of sprockets around which each of the wickets carried by the oven conveyor chains reverses its stripper mechanism and then the wicket passes the stripper at an increased radial speed leaving the sheet resting on the stripper. At the same time this increases the distance between wickets at this point which allows time for the sheet to be removed before the next wicket passes the stripper.

A shaft 16 extends transversely of the forward end of the side arms 13 and is rotatably mounted in suitable hearings in said side arms.

Conveyor belts

17, 18 and 19 are driven from pulleys secured to shaft 16. A pair of downwardly extending support plates 20 are secured to the forward ends of side arms 13. A motor adjustment shaft 21 extends through the lower portion of the support plates 20 and is rotatably supported therein by suitable bearings. An electric motor 22 is secured to the one end of the shaft 21 by a pair of brackets 23, as shown in FIG. 1. A plate 28 having an aperture for receiving an end of shaft 16 extends between that shaft and the motor adjustment shaft 21. An arm bracket 29 is secured at one end to the motor shaft 21 and is bifurcated at the upper end with a block member having a threaded aperture therethrough for receiving a threaded portion of rod 30. A rod end bearing 31 is securely attached to the plate 28, and permits rod 30 to align itself with the threaded aperture in the block retained by arm 29. A wheel is secured to the upper end of rod 30 to facilitate turning the same. As the rod 30 is manually rotated the arm 29 is pivotally moved causing the shaft 21 to rotate. As shaft 21 rotates it carries with it the motor brackets 23 and the motor 22. A drive pulley 3 2 is secured to the motor shaft and a drive pulley 33 is secured to the conveyor drive shaft 16 with a V-belt 34 connecting said pulleys. The drive pulley 32 is of the type wherein one half is secured to the motor shaft and the other half is spring-loaded along the plane of the motor shaft and toward the fixed half of the pulley. The motor 22 and its mounting are rotated with shaft 21 which, in turn, is spaced from but in vertical alignment with shaft 16. The rotation of shaft 21 will vary the distance between the center of pulley 32 and pulley 33 so that this distance is either increased or decreased depending upon the direcion of the rotation of the shaft 21. Since the V-belt 34 is of a fixed length the lengthening or shortening of the distance between the center of these two pulleys will cause the V-belt to ride up and down in spring-loaded pulley 32 thus changing the pitch ratios. In this manner the speed of the

conveyors

17, 18, '19, 39 and 40 can be selectively controlled while the sheet is in operation. The rear portions of

conveyor belts

17 and 19 are supported on pulleys which are secured to

shafts

35 and 36 which, in turn, are supported in shaft blocks secured to the inside of side arms 13.

Shafts

37 and 38 are posi tioned parallel to

shafts

35 and 36 and likewise are retained in shaft blocks which are secured to the inner surfaces of the side arms 13. A pair of conveyor belts 39 are mounted on suitable pulleys which are secured to

shafts

35 and 37, respectively, and a pair of conveyor belts 40 are mounted to suitable pulleys which are secured to the ends of

shafts

36 and 38. Each of shafts 3538 are additionally rotatably supported by a pair of

brackets

45 and 46 each of which is positioned parallel to one of the side arms 13 and secured at one end to the torque tube 44. As shown in FIG. 1,

conveyor belts

39 and 40 are of a lesser length than

conveyor belts

17 and 19. The pair of conveyor belts 18 are mounted intermediate the pairs of

conveyor belts

39 and 40. The rearward pulleys for the conveyor belts 18 are rotatably mounted to a shaft which extends between a pair of upwardly extending shaft support plates 41. The bottom portion of each plate 41 is secured to a horizontally extending bracket 42, Bracket 42 is maintained in a horizontal position by having its forward end secured to the torque tube 44 which extends transversely of the side arms 13 and is mounted in the support plates 20. Tube 44 is positioned in vertical alignment with and between the conveyor shaft 16 and the motor adjustment shaft 21.

A suction device 43 is positioned between each of the pairs of conveyor belts 18, '39 and 40. Each of the suction devices 43 are generally identical in construction and operation and therefore a description of one is sufficient for the understanding of all. As shown in FIGS. 3 and 4, each suction device comprises a chamber 49 positioned below the conveyor belts, an outlet 50 at the bottom of the chamber, and a hose 56 which is connected to the suction side of a blower (not shown). A pair of spaced apart wall members 51 and connecting end walls 53 form a narrow rectangular shaped passage 52 communicating with the chamber 49 at its lower end and with the atmosphere at its upper end. Secured to the upper end of each wall 51 and extending horizontally outwardly away from the passage 52 are a pair of flanges 54 which are positioned in contact with the undersurface of the upper reaches of the conveyor belts. It will be seen from FIG. 1 that the shape of the wickets will control the positioning of the

conveyors

17, 18, 19, 39 and 40 so that each of the wickets, carried in their predetermined path by the conveyor chains 11, will pass these conveyors Without contact and at the same time will leave each sheet 12 retained on the top surface of these conveyor belts. When a suction is created in each of the suction devices 43, air will be drawn into the passages 52, thus creating an increased flow of air above the conveyor belts toward these suction devices between the belts. This, in turn, creates a downward movement of air toward the conveyor belts on the underside of each sheet as it advances toward said conveyor belts and results in an additional downward pull on each advancing sheet toward the conveyor belts. The upper surface of each of the conveyor belts is composed of a series of parallel tranver-sely extending grooved members. When each sheet has been seated on the conveyor belts the flow of air is restricted to these grooved areas as well as the narrow area defined by the undersurface of the sheet, the upper surface of the flanges 54 and the inside edge portions of the upper reaches of the conveyor belts. The grooved areas in the conveyor belts distribute the hold down force over a larger sheet surface area and the restricted flow of air increases the suction on the lower surface of each sheet which, in turn, more firmly secures the sheet to the conveyor belts. In addition to the more even distribution of the suction force on a large surface area, the grooved belts provide for a minimum of contact of the sheet surface with the conveyor belts which, in turn, minimize the potential damage to the sheet surface. The conveyor belts are continuously driven and quickly move each of the sheets away from the oven and onto the continuously driven endless take-off conveyor belts 55 positioned adjacent the forward end of oven stripper. In this manner each sheet is quickly removed from the path of the wickets '10 and the stripping conveyors are cleared ready to receive the succeeding sheet 1 2. It will aso be observed that the sheets are not drawn tightly down into contact with the open end of the passages 52. By having the upper end of passage 52 ending at a horizontal plane which also passes through the undersurfaces of the conveyor belts, there is provided a passageway equal to the length of the slot 52 and having a width equal to the spacing between the conveyor belts and a depth equal to the thickness of the conveyor belts through which air is constantly drawn. This permits a partial suction on the undersurface of each sheet carried on the conveyor to create a holding force but at the same time the force is not great enough to impede the efficient transfer of each sheet onto the take-off conveyor belts 55 nor to damage the sheets. Should the undersurface of the sheets 12 be permitted to seal off the upper end of passages 5-2, the suction would have to be cut off to release the sheet to permit its transfer onto the take-off conveyor belts 55. A frame comprising vertically-extending supports 60 and horizontally-extending side frames 61 is positioned forward of the oven stripper 15. The supports 60 are secured at their lower ends to a base or floor (not shown) and have a conveyor belt shaft 62 extending between their upper ends and rotatably mounted therein by suitable. bearings. Conveyor belts 55 are operatively connected at one end to pulleys which are secured to shaft 62 and at their other end to pulleys and a drive shaft (not shown) for continuous operation.

The forward end of the oven stripper 15 is pivotally retained in a horizontal plane by a stripper holding means generally designated as 65 and more specifically described as follows. Secured to frame 60 is an air cylinder 66 of standard construct-ion having a reciprocating piston and a piston rod 67 which extend exteriorly of one end of the casing of the cylinder. Aconduit 68 is secured at one end to a source of compressed air (not shown) and at the other end to a valve 69 which is of the commercial type made by the Bellows Co., Model No. 4SS-V3/8 and which is electrically actuated to selectively direct the compressed air to either end of the air cylinder, which, in turn, will move the piston to one end or the other of the cylinder. A shaft 72 is rotatably mounted between the upper ends of the support 60 by suitable bearings and has a sprocket 73 secured to one end thereof. A chain is secured at one end to the sprocket 73 and at its other end to the outer end of piston rod 67. Secured to the forward surface of the torque tube 44 are a pair of

control devices

74 and 75. Since each control device is of identical construction and operation, a description of one will be sufficient for an understanding of both. Secured to the torque tube 44 is a base plate 76. A lever arm 77 is pivotally monted at 78 to the base plate 76 and compresses a short arm 79 and a long arm 80. A pin 81 is secured at its upper end to the bracket 76 and extends downwardly through an aperture in the end of arm 80. A spring 82 is telescopically mounted over the end of pin 81 and is retained under slight compression between the undersurface of arm 80 and a nut which is threadedly received on the end of pin 8. A microswitch 83 having an actuating pin 84 is mounted to plate 76 so that the pin 84, in its normal nonactivated position, contacts the upper surface of arm 89.

The microswitch is of the type which has a springloaded pin 84 and the switch can be wired either in the normally opened or normally closed position when the pin is recessed into the casing, as shown in FIG. 6. The microswitch of the control 74 is wired so that the switch is normally opened when the spring-loaded pin 34 is recessed into the casing. The microswitch of control device 75 is wired so that the switch is normally closed when the spring-loaded pin 84 is recessed into the casing. Spring 81 holds arm 89 in an upward position and arm 81, in turn, retains the spring-loaded pin 34 in its recessed position. A chain 88 is secured at one end to the outer end of arm 79 and has its other end secured to a sprocket 89 in such a manner that a portion of the chain engages the teeth in this sprocket. As shown in FIGS. 1 and 6, there is a pair of sprockets 89 and chains 88, each of which is operatively connected to the

control devices

74 and 75, respectively. The spring member 82 associated with control device 75 is larger than the spring associated with control device 74 in order to compensate for the added weight of the motor and the drive pulleys attached to that side of the oven stripper. The microswitch associated with the control device 74 is electrically connected to the Bellows valve 69. The microswitch associated with the control device 75 is electrically connected to the control circuit for the drive of the oven conveyor chain 11 (not shown).

In normal operation the oven stripper will remain in a horizontal position as shown in FIG. 2 and the sheets will be conveyed onto the take-ofi conveyor belts 55 by the conveyor belts which form a part of the oven stripper. The forward end of the oven stripper is supported by the pairs of

chains

88 and 74 and the air cylinder 66. Any downward pivotal movement of the oven stripper is resisted by the compressed air on the topside of the piston in air cylinder 66. Should a sheet be jammed between the conveyor belts of the oven stripper and the next advancing wicket It the entire oven stripper is slightly pivotally moved downwardly. When this occurs, the arm will be pivotally moved downwardly and the arm 79 will be moved upwardly which, in turn, will permit the spring-loaded pin 84 to be moved downwardly thus closing the microswitch 83 in each of the

control devices

74 and 75. The closing of the microswitch in the control device 74 will cause the Bellows valve 69 to direct the compressed air to the underside of the piston in the air cylinder 66 thus moving the piston rod 67 upwardly (as viewed in FIG. 2) which, in turn, permits the

sprockets

73 and 89 to be rotated in a counterclockwise direction. The forward end of the oven stripper is thus permitted to drop rapidly to relieve the tension between the wicket and the sheet. At the same time the microswitch of the control device 75 has been closed, which, in turn, opens the circuit for driving the wickets 10. As soon as the jammed sheet has been moved onto the take-off conveyor belts 55, the springs 81 will return the lever arms to their normal position thus opening the microswitch 83 and the cycle is reversed, wherein the Bellows valve 69 is deactivated, compressed air is directed to the top side of the cylinder of the pistion in the air cylinder 66, which, in turn, pulls the chains 74 downwardly and the chain 38 upwardly and returns the oven stripper to its normal horizontal position. It will be observed from FIG. 2 that the conveyor belts of the oven stripper 15 are normally maintained at a slightly higher elevation than the take-off conveyor belts 55. This arrangement is designed to permit a sheet on the conveyor belts of the oven stripper to be fed directly onto the conveyor belts 55 when the oven stripper and its conveyor belts are pivotally depressed downwardly, as described above, at the time that a jam occurs.

A distributing manifold 92 is mounted to the supports 60. A source of suction (not shown) is connected through conduit 93 to the manifold 92. A hose 56 connects the manifold to each of the suction devices 43 and the intensity of the suction is selectively controlled by varying the size of the hose 56.

On each side of the stripper there is a vertically-extending and freely rotatable guide roll 96 which is mounted to a horizontally extending rod 97 which, in turn, is mounted to the inner surface of the side rail 13. The rolls 96 are adjustably secured to shaft 97 so that they may be moved toward or away from the side rails to accommodate sheets of various widths.

A pair of support rollers 97 are mounted to shaft 16 and are so spaced thereon to support each sheet between the conveyors. When the sheets are formed of magnetizable material the rollers 97 may be in the form of magnets. As shown in FIG. 5 the core 93 surrounds shaft 16 which, in turn, magnetizes the end plates 99 and 1%. In the embodiment shown, plate 99 is the north pole and plate 10!} is the south pole with the magnetic [field extending generally horizontally as shown by the arrows. The magnets not only serve to support each sheet intermediate the conveyors, but the magnetic field serves to attract and retain a metallic sheet onto the conveyors to further prevent slippage.

While the foregoing description and the accompanying drawings do set forth with more or less particularity one embodiment of the invention, is to be expressly understood that said invention is not limited to said embodiment. Various changes may be made in location, size and intensity of the suction means as well as in the design and arrangement of the other parts illustrated, and in the mode of operation and manner of use, without de parting from the spirit and scope of the invention, as will now be clear to those skilled in the art.

What is claimed is:

1. In an apparatus for receiving and conveying sheets of material the combination of a plurality of spaced apart conveyor means, means to sequentially feed sheets onto said conveyor means, means for creating a suction between said conveyor means, a frame pivoted at one end for supporting one end of said conveyor means, means operatively secured to the other end of said frame to retain the same in a substantially horizontal position, means to release said frame retaining means, and means responsive to a downward pressure on each conveyor in excess of the weight of said sheet to actuate said frame releasing means.

2. In an apparatus for receiving and conveying sheets of material, the combination of a plurality of spaced apart conveyor means, means to sequentially feed sheets onto said conveyor means, means for creating a restricted flow of air on the undersurface of each sheet and distributed evenly about the peripheral edge of each sheet for retaining each sheet on said conveyor means, a frame pivoted at one end for supporting one end of said conveyor, means to releasably retain said frame in a substantially horizontal position, means to release each frame retaining means, and means responsive to a downward pressure on said conveyor in excess of the weight of said sheet to actuate said frame releasing means.

3. Apparatus as defined by claim 2. wherein said frame retaining means comprises an air cylinder having a reciprocable piston and a connecting piston rod, a source of compressed air selectively connected to either the first orsecond side of said piston, chainmeans connected to said piston and to the forward end of said frame, control means between said cylinder and said source of compressed air to normally direct said compressed air to the first side of said piston whereby the forward end of said frame is retained in a substantially horizontal position.

4. Apparatus as defined in claim 3 wherein said means responsive to an excessive downward pressure on said conveyors actuates said control means to direct compressed air to the second side ofsaid piston whereby the forward end of said frame is permitted to rapidly drop.

5. In an apparatus for receiving and conveying sheets of material the combination of a plurality of spaced apart conveyors, means to sequentially feed sheets onto said conveyors, means for creating -a suction between said conveyors, a frame for supporting said conveyors and pivotally mounted on one end, means to drive said conveyors, means to adjust the speed of said drive means while said conveyors are in operation, means at the other end of said frame to retain the same in a substantially horizontal position, means to disconnect said frame retaining means, and means responsive to a downward pressure on said conveyor in excess of the weight of each sheet to actuate said frame disconnecting means.

6. Apparatus as defined in claim 5 comprising a mo tor support shaft, a motor secured to said support shaft, a spring loaded split drive means connected to said motor, and means to rotate said support shaft to selectively vary the distance between the centers of said conveyor drive means and said motor drive means.

7. A sheet handling apparatus comprising a plurality of sets of spaced apart pairs of conveyors, means for creating a suction between said conveyors, means for sequentially depositing sheets of material on said conveyors, said last named means mounted to pass between said pairs of conveyors and to leave each sheet resting on said conveyors, a frame for supporting said conveyors and pivotally mounted at one end, means at the other end of said frame to retain the same in a generally horizontal position, means to disconnect said frame retaining means, and means responsive to a jam between said sheet depositing means and a sheet on said conveyors to actuate said frame disconnecting means.

8. In a sheet conveying apparatus the combination of a first and a second conveyor means the respective paths of which intersect at planes substantially normal to each other, the first conveyor comprising at least a pair of endless belt members, means to cause a low velocity and high volume of air to continuously pass downwardly only through the space between said belts whereby each sheet is retained on the top surface of the upper reaches of said belts and wherein each sheet moves with said belts without slippage therebetween, said last mentioned means having an opening which is positioned below the top surface of the upper reaches of said belts whereby said sheets are prevented from adhering to the said openings, the second conveyor comprising a plurality of sheet support means each of which is adapted to retain a sheet thereon only by gravity and each of said support means being so positioned that they pass through the plane of the belts of said first conveyor without contact.

9. In a sheet conveying apparatus as defined in claim 8 wherein the top surface of said belts on said first conveyor comprises a plurality of parallel transverse grooves communicating the surrounding atmosphere with the opening in said means to cause a low velocity and high volume of air to pass between said belts.

10. In a sheet conveying apparatus the combination of a first and a second conveyor means the respective paths of which intersect at planes substantially normal to each other, the first conveyor comprising at least one pair of endless belt members, means to cause said belt members to move at a predetermined surface speed, means for continuously creating an atmospheric pressure only between said belt members which is lower than the atmospheric pressure surrounding said belt members, whereby each sheet is retained on the top surface of the upper reaches of said belts and wherein each sheet moves with said belts without slippage therebetween, said last mentioned means having an opening which is positioned below the top surface of the upper reaches of said belts whereby said sheets are prevented from adhering to the said openings, the second conveyor comprising a plurality of means each adapted to support a sheet thereon and each of said support means being, so positioned that they pass through the plane of the belts of the first conveyor without contact and to deposit each sheet on the belts of said first conveyor.

11. In a sheet conveying apparatus the combination of a first and second conveyor means the respective paths of which intersect at planes substantially normal to each other, said first conveyor comprising at least a pair of endless belt members, means to cause a low velocity and high volume of air to continuously pass downwardly only through the space between said belts, said last mentioned means having an opening which is positioned in a plane below the top surface of the upper reaches of said belts, a frame pivoted at one end for supporting said conveyor belts and said air means, means operatively secured to theother end of said frame to retain the same in a substantially horizontal position, means to release said frame retaining means, and means responsive to a downward pressure on said conveyor in excess of the weight of a sheet to actuate said frame releasing means.

References Cited in the file of this patent UNITED STATES PATENTS 2,339,268 Huffman Jan. 18, 1944 2,648,420 Best et al. Aug. 11, 1953 2,680,615 Edgar June 8, 1954 2,706,552 Key Apr. 19, 1955 2,789,704 Lewin Apr. 23, 1957 gs-adv.