US3494120A - Rotating ring spinning or twisting frame - Google Patents

Description

Feb." 1'0,- 1970- K. c. mum a 3,494,120

ROTATING RING SPINNING OR TWISTING FRAME Filed Jan. 2. 1968 4 SheetS-Sheef 1 Feb. 10, 1970 K. c. CHILPAN ETAL 3,494,120

ROTATING RING SPINNING OR TWISTING FRAME Filed Jan. 2. 1968 4 Sheets-Sheet &

Feb. 10, 1970 K. c. CHILPAN ETAL 3,494,120

ROTATING RING SPINNING OR TWISTING FRAME Filed Jan. 2. 1968 4 Sheets-Sheet 3o 1 C 50 3|. 2O IFIG.4 38

I 39 29 17/ 27 37 h C P '6 II I Feb. 10, 1970 c. CHILPAN ETA'L 3,494,120

ROTATING RING SPINNING OR TWISTING FRAME I Filed Jan. 2, 1968 4 Sheets-Sheet 4 United States Patent US. Cl. 5775 16 Claims ABSTRACT OF THE DISCLOSURE A rotating spinning or twisting ring is automatically braked when its supporting ring rail is moved to a dofiing position below the normal traversing range, and a retainer element limits upward displacement of the ring caused either by traverse reversals or braking.

This invention relates to textile ring spinning and twisting frames and the like and more particularly to such frames of the rotating ring type.

It has long been realized that the use of a rotating spinning or twisting ring carrying a free floating traveler would make possible higher spindle speeds and hence higher production, present production rates being limited by the presently accepted maximum permissible traveler velocity with respect to its ring, the latter conventionally being held stationary on the ring rail of the frame. Until the present invention, however, attempts to provide a practical ring spinning or twisting frame of the rotating ring type were not successful, and for a number of reasons.

One of these reasons arises from the necessary traversing movement of the ring and traveler relatively to the spindle. At the quick reversals of the ring and traveler at the ends of their traverse, the pull of the yarn through the traveler then occurring tends vertically to displace the rotating ring element and so alfect its stable running, as by causing physical contact between the rotating ring and its support, resulting in yarn damage at each reversal, or even tending to lift the rotating ring from its support.

Another of these reasons arises at the time the frame is stopped as for dofiing purposes, at which time the ring rail is moved either by hand or automatically to a position lowered beneath its usual low traverse position and the spindles are stopped. With the conventional arrangement, the spindles stop quickly, but any rotating ring, being free from restraint and of substantial inertia, continue to rotate for a considerable time, thus unwinding yarn from the wound bobbin, causing yarn breakage.

Accordingly, it is a major object of the invention to provide a stabilized rotating spinning or twisting ring which does not cause yarn damage upon reversal of the yarn traverse.

It is another major object of the invention to provide a rotating spinning or twisting ring having braking means automatically operable upon the lowering of the ring rail to dofi'ing position.

These and further objects of the invention have been accomplished by providing a rotating spinning ring assembly for spinning and twisting frame and the like having, in combination with a spindle mounted on a spindle rail and a ring rail traversable relatively thereto, a rotating ring element having an external cylindrical bearing surface preferably with its lower end exposed below the ring rail, and an external flange extending radially outwardly with upper and lower bearing surfaces. A stationary air bearing housing element is mounted on the ring rail and has an internal cylindrical bearing surface surrounding the ring element providing air clearance therebetween and an upper bearing surface underlying the ring element flange lower bearing surface providing air clearance therebetween during operation. The air bearing housing preferably has air apertures through at least one of its said bearing surfaces for supplying pressurized air thereto, to suspend and center said ring element with respect to the housing free of contact therewith.

In one major aspect of the invention, a stationary ring element retainer member is preferably both removably and adjustably mounted on the housing element suitably spaced therefrom as by spacing means and has a lower bearing surface overlying the ring element flange providing air flow solely induced by centrifugal force radially outwardly throughout the air passage between the retainer member lower bearing surface and the flange upper bearing surface to suspend the ring element for rotation free of contact with the retainer member.

In another aspect of the invention, there is provided a brake pad element mounted for friction braking engagement preferably with the lower end of the rotating ring element when the ring rail is moved to a lowered position in order to stop it quickly and avoid broken yarn, preferably together with means limiting the upward movement of the rotating ring caused by its contact with the brake pad element.

Still further objects and features of the invention will become apparent from the following detailed description of a preferred embodiment thereof, taken together with the accompanying drawings, wherein:

FIG. 1 is a diagrammatic view of the operating element of a spinning or twisting frame embodying the invention;

FIG. 2 is a side elevational view, partly in section, of the spindle and ring portion of the frame of FIG. 1;

FIG. 3 is a cross-sectional view of a portion of the frame of FIG. 1, taken on the line 33 of FIG. 2;

FIG. 4 is an isometric detail view, partly in section, showing the assembly of the rotating spinning ring of FIGS. 1 and 2;

FIG. 5 is an exploded view of the elements of the assembly of FIG. 4; and

FIG. 6 is a side elevational view, partly in section, of a modification of the spindle and ring portion of the frame of FIG. 1.

As shown in the drawings, especially in regard to the overall schematic showing of FIG. 1, the invention is herein disclosed as embodied in a spinning frame which may be of the type generally shown in Patent No. 3,312,405, for example, consisting of an elongated frame structure having at each side thereof vertical posts 11 extending upwardly from duplicate fixed spindle rails 12 on which are mounted a series of spindles 13 on which bobbins 14 are carried. A main frame motor (not shown) is provided for driving spindles 13 in pairs by driving tapes 15.

The movable yarn tensioning guiding and control elements by means of which the yarns are guided for winding on the bobbins on the spindles of each spindle rail 12 comprise a spindle ring rail 16 mounted for vertical movement along posts 11 having mounted thereon along the length of the machine a series of spinning ring assemblies, generally designated 20 and hereinafter described in detail. The spinning ring rail 16 is supported at intervals along its length by means of tapes 17 which extend along the length of the machine, and are provided at intervals with extensions passing around idler pulleys 18. Each tape 17 is secured at the base end to a pulley 42 mounted on a Windlass, generally designated 40, which extends transversely across one end of the frame.

The illustrated machine is provided with a builder mechanism for producing the traversing movement of the yarn guiding and control elements which is similar in many respects to that shown in Patent No. 3,072,350. As generally shown in FIG. 1, the builder mechanism comprises a builder cam 70 mounted on a builder cam shaft 71 driven by the main frame motor and adapted to be engaged by a follower 72 on a builder cam lever 73 extending horizontally beneath the builder cam. The cam lever turns about a pivot 74 on a machine bracket 75, and is connetced with the lower end of an adjustable link chain 76. At its upper end the chain 76 is connected with a pick mechanism, generally designated 100, which is in turn connected by means of a link chain 78 with a sprocket 46 secured to the Windlass 40.

There is also connected to the Windlass 40 the compensating tension assembly including a torsion bar 80' which acts to rotate the Windlass in a direction counter to the spinning ring supporting tapes. Said compensating tension assembly comprises a chain 82 which is connected at its upper end to a sprocket 47 mounted on windlass 40. The chain 82 extends downwardly around a guide roller 83 and is connected at its lower end to a cam member 84 which is adjustably mounted on a pivot pin '85 between a pair of arms 86 formed on a bracket 87 which is rotatably supported on a bearing member 88 and is rigidly secured at one end to torsion bar 80 to rotate therewith about the torsion bar axis. The torsion bar 80 is placed under a twisting or torsion strain which serves to exert a downward bias on the chain 82 suflicient to overbalance the bias of the several tapes attached to windlass 40. An adjustable stop 89 is provided for cam member 84.

The builder mechanism above generally referred to is arranged to operate in the following manner:

The builder shaft 71 and cam 70 are positively driven by the main motor to produce a cyclical up-and-down movement of the builder cam follower lever 73 to produce a reciprocatory movement of the spinning ring rail 16 with a recurring winding pattern. At regular intervals during this operation, the pick mechanism 100 is operated to increase slightly the length of chain 78 and thereby to gradually lift the reciprocating pattern of winding effected by the operation of the builder cam follower 72.

For a more complete illustration and description of the builder mechanism, reference may be had to Patent No. 3,072,350, above referred to.

Means including hydraulic and mechanical operating means with their controls are provided for operating the spinning frame at the conclusion and initiation of the operation of winding a bobbin 14. The resulting functions at the conclusion of bobbin winding may include raising the spinning ring assemblies 20 for tip winding if desired, followed by lowering the spinning rings below their usual traversing bottom winding position to a lowered position to Wind a few turns of yarn around the bottom of the spindle 13. Bobbins 14 may be doffed either in such lowered position or in a somewhat higher position. Upon restarting the spinning frame with empty bobbins, the spinning rings are restored to their initial bobbin winding positions, the spindle bottom wind making possible initation of winding without piecing up.

As shown generally in FIG. 1, the means for accomplishing this sequence includes a main hydraulic piston and cylinder 90 having a rack 91 for moving Windlass gear 52 mounted on Windlass 40, a hydraulic clutch control of pick mechanism 100 and electric control means including a multiple cam driven switch mechanism driven from Windlass gear 52 as described in said Patent No. 3,312,405. Handle 95 is provided for manual operation of Windlass gear 52.

The rotating spinning ring assembly 20 of the invention, as best shown in its preferred embodiment in FIGS. 2, 4 and 5, in general includes a stationary air bearing housing element 22 mounted on ring rail 16 concentrically with the spindle opening therein and supporting therewithin a rotating ring element, generally designated 30. The rotating ring element 30 surrounds spindle 13 and has, on the upper portion thereof, the usual generally T- shaped track 31 supporting a 'C-shaped traveler 50. Rotating ring element 30 has a bottom tubular portion 32 terminating in an end brake surface 33 exposed beneath the bottom of ring rail 16. It. also has an external cylindrical bearing surface 34 which extends downwardly from track 31 and an external flange portion 35 located vbetween track 31 and cylindrical bearing surface 34 which extends radially outwardly with bothits upper. bearing surface 36 and its lower bearing surface 37 perpendicular to cylindrical bearing surface 34.

Air bearing housing element 22'is provided with an internal cylindrical bearing surface 24 for cooperating with the cylindrical bearing surface 34 of rotating ring element 30 and a horizontal top bearing surface 25 for cooperation with the lower bearing surface 37 of ring element flange 35. Housing element 22 is also provided with an internal air conduit 26 extending therearound and communicating with its cylindrical bearing surface 24 by air apertures 27. Compressed air is supplied to conduit 26 through a conventional air supply line 21, which line may be used to supply air to all of the rotating ring assemblies mounted on ring rail 16. A relatively small air clearance is provided between the vertical cylindrical bearing surfaces 24 and 34 so that when pressurized air is provided to housing element 22, rotating ring element 30 is suspended and centered with respect to housing element 22 free of contact therewith, the flow of compressed air creating clearance between horizontal bearing surfaces 25 and 37 as well as between vertical bearing surfaces 24 and 34.

As an important aspect of the invention, rotating ring element 30' is provided with retainer means which performs the important function of limiting upward displacement of ring element 30 relative to its housing element 22, both during reversals of the ring rail traverse motion while the frame is in normal operation and also during ring braking, all as hereinafter more fully set forth following the description of its mechanical arrangement.

More specifically as to the latter, an annular disc-shaped retainer member 38 is preferably removably mounted in adjustably vertically fixed position on locating pins 23 which extend upwardly from circumferentially spaced points on the surface of housing element 22 and is maintained at a desired predetermined distance above ring. flange 35 by spacing means which preferably take the form of a plurality of spacing washers 39 having central apertures to receive the locating pins 23. The spacerthickness provided by spacer washers 39 must be determined to provide a top and bottom air bearing clearance for the rotating ring element flange 35 adapted for the best running performance, being in any case greater than the thickness of said flange. To avoid impeding movement of traveler 50, the inner edge portion of retainer member 38 is preferably of reduced thickness, sloping downwardly and inwardly to an inner edge 44 of limited vertical dimension encircling the upper portion of rotating ring element 30 and closely spaced therefrom providing a vertical air passage therebetween. This passage and the open areas between Washers 39 provide free communication, at radially-separated locations, between the atmosphere and the space between the adjacent surfaces of retainer member 38 and ring flange 35.

Retainer member 38 may be heavier than ring element 30 and should retain it against displacement by an amount in excess of the clearance provided by the spacing washers 39. However, to take care of extreme conditions which might include very heavy yarn pull, as well as ring braking as hereinafter explained, retainer 38 may be secured to ring rail 16 by one or more spring clips 28 retained in ring rail slots 29.

In operation, with pressurized airsupplied to housing element air conduit 26 through air' supply line 21 to float and center ring element 30, during start-up,.the yarn willv pull traveler 50 around ring element 30 in the usual manner while ring element 30 is gradually accelerated to its running speed by the drag of traveler 50 along track 31 until the speed of ring element 30 comes up to approximately that of traveler 50. As the speed of ring element 30 increases, centrifugal force induces a concentrated air flow C radially outwardly, from the passage between inner edge 44 of retainer member 38 and ring element 30, through the upper radial passage between the upper bearing surface 36 of flange 35 and the lower bearing surface 43 of retainer member 38, and thence through the open areas between washers 39 to the atmosphere. Such air flow, which is continuous during normal operation of the frameand is solely induced by centrifugal force, has by virtue of its concentration a cushioning effect which minimizes contact between the adjacent surfaces of retainer 38 and ring 30 to such an extent that the retention function of the former is performed without detrimentally affecting the speed, stability or other desired operating characteristics of the latter.

It is understood that when stable running conditions are achieved, the spinning ring element 30 will operate without physical contact with either the stationary bearing surfaces of housing element 22 or of retainer member 38 during the complete spinning or twisting cycle, even during traversing reversals, with traveler 50 relatively stationary on the rotary ring element 30, with pressurized air being supplied solely through the air aperture 27 beneath flange 36 of rotating ring element 30, with its flow P radially outwardly along said flange being aided rather than opposed by centrifugal force.

One adavntage of the illustrated construction, which includes the readily removable spacer Washers 39 and top retainer member 38, consists in the fact that the washers 39 are readily substituted by ones of a different thickness vertically to vary the control clearances to a predetermined value between flange 35 and the opposed housing and retainer member surfaces between which the ring element 30 is suspended during operation, and thus to control the aerodynamic drag of the rotating ring element 30, which affects yarn tension. A further advantage consists in the fact that top retainer member 38 and the ring element 30 are readily removable from housing element 22 for cleaning, replacement or repair, as for example, if bearing contamination due to trapped lint particles should become a problem.

As noted above, upon completion of the spinning or twisting cycle, the frame spindles 13 are braked to a stop while, at the same time, the spinning ring assemblies 20, which are carried on the ring rail 16, are lowered to a low position, so that the bobbins 14 are underwound with a few turns of yarn, as the spindles are brought to rest. In the draw twisting of synthetic filaments, the feeding yarn is similarly brought to a low position to be Wound onto a waste collar. Normally, rotatable spinning rings, running on friction-free air bearings as above described, and having a considerable amount of inertia, continue to rotate with the result that yarn is unwound from the bobbins causing end breakage.

In this regard, another important aspect of the present invention consists in the provision of a simple and effective brake means automatically for arresting the rotation of the rotating ring elements 30 at the same time the spindles 13 come to a stop.

More specifically, the rotating ring braking means provided by the present invention comprises a brake pad element of friction material, which is mounted in a position to be engaged automatically with the individual rotating ring element 30 preferably against its bottom end brake surface 33 exposed beneath the ring rail just as the frame reaches the lowered position below its normal traverse range preparatory to doffing. Preferably, the invention also includes means limiting the upward displacement of ring element 30 caused by the contact of its brake surface 33 with said brake pad element.

In FIGS. 1 through 5 is shown a preferred construction wherein a friction brake pad element 54 is resiliently mounted on a U-shaped spring steel bracket 56 mounted by screws 58 on spindle rails 12 between each pair of spindles 13 in position to engage the lower ends 33 of rotating ring elements 30 thereof to arrest their rotation as they reach the extreme low position assumed for bottom winding. Upward displacement of ring element 30 is minimized by the resiliency of bracket 56 and is further limited by top retainer member 38, held in place by spring clips 28, its bottom surface 43 providing additional braking if ring element 30 should be raised thereagainst by contact of its surface 33 with brake pad 54. It will be apparent that during braking such physical contact between the adjacent surfaces of retainer 38 and ring element 30 is permissible, although highly undesirable during normal operation and when avoided by the cushioning effect of the centrifugal air flow C passing between those surfaces.

FIG. 6 illustrates a modification of the braking means structure of FIGS. 1 through 5 wherein a friction brake pad element 55 is mounted on the free end of a curved spring 57 mounted at its other end on ring rail 16' in position such that movement of the ring rail 16 to its lowered position beyond the normal traverse range causes spring 57 to contact spindle rail 12' and deflect its friction brake pad element 55 upwardly into contact with the lower end 33 of rotating ring element 30. Otherwise, the structure is the same as that illustrated in FIGS. 1-5 and described above, except that the whirl of spindle 13 is positioned below spindle rail 12 and is there driven by belt 15.

The spinning ring brake device of the invention is well adapted to operate in a reliable and satisfactory manner to arrest the rotation of the spinning rings 30 when the spindles 13 are moved to a lowered position before bobbins 14 are doffed, and may effectively be utilized in the absence of a retainer member, such as at 38, although the latter is generally advantageous. In the form shown, the spinning ring braking device has the important advantage in that it may be incorporated in a conventional spinning or twisting frame without any addition of new moving parts and without any substantial change in the operating structure of the frame.

It will be understood that the invention in its broader aspects is not limited to the specific embodiments shown, and is useful in frames of the moving spindle rail type as well. Still other changes within the spirit and scope of the invention will occur to those skilled in the art.

The invention having been described, what is claimed 1. A rotating spinning ring assembly for spinning and twisting frames and the like, having, in combination with a spindle mounted on a spindle rail and a ring rail traversable relative thereto:

a rotary ring element and a cooperating air bearing housing element carried by said ring rail in encircling relationship to said spindle for traversing movement relative thereto:

said elements having contiguous bearing surfaces thereon and said ring element being adapted to be suspended and centered relative to said housing element, for rotation during normal operation free of contact with said housing element, by an air flow between said contiguous bearing surfaces thereof;

ring element retainer means carried by said ring rail in closely-spaced overlying relationship to a portion of said ring element for therewith inducing a concentrated centrifugal flow of air therebetween and for limiting vertical displacement between said ring element and said housing element; and

ring element brake means carried by the frame and engageable when actuated with said ring element for retarding rotation thereof, said brake means during normal operation of the frame being spaced from and out of engagement with said ring element and being actuated by relative movement of said spindle and ring rails to a position lowered with respect to the normal traverse thereof, said breaking means including a friction braking surface engaging with said ring element during said actuation.

2. A rotating spinning ring assembly as in claim 1 wherein said retainer means defines with said portion of said ring element a generally radially-extending passage for said centrifugal flow of air, said passage communi- Ioating freely at a radially-inward location and at a radially-outward location with the atmosphere for respectively permitting the introduction of air therein and the exhausting of air therefrom.

3. A rotating spinning ring assembly for spinning and twisting frames and the like, having, in combination with a spindle mounted on a spindle rail and a ring rail traversable relative thereto:

a rotary ring element and a cooperating air bearing housing element carried by said ring rail in encircling relationship to said spindle for traversing movement relative thereto;

said elements having contiguous bearing surfaces thereon defining passageways for air flows adapted to lift and center said ring element for rotation during normal operation relative to and out of contact with said housing element;

and retainer means carried by said ring rail in closelyspaced overlying relationship to a portion of said ring element for limiting vertical displacement of said ring element relative to said housing element, said retainer means defining with said portion of said ring element a generally radially-extending passageway separate from said first-mentioned passageways and communicating freely at a radially-inward location and at a radially-outward location with the atmosphere for respectively permitting the introduction therein and the exhausting therefrom of a concentrated flow of air induced by centrifugal force during operation of the assembly.

4. A rotating spinning ring assembly as in claim 3 wherein said ring element has a flange extending generally radially outwardly therefrom and defining said portion thereof, and said retainer means includes an annular retainer member, and means removably mounting said retainer member a predetermined distance above said flange, the centrifugal flow of air being between adjacent surfaces of said annular member and said flange of said ring element.

5. A rotating spinning ring assembly as in claim 4 wherein said mounting means is adjustable for adjusting said predetermined distance, and is of an open construction for passing to the atmosphere the centrifugal flow of air exhausted from between said adjacent surfaces of said annular member and said flange of said ring element.

6. A rotating spinning ring assembly as in claim 4 in which said mounting means includes locating pins on said air bearing housing element engaged in locating holes formed in said retainer member.

7. A rotating spinning ring assembly as in claim 6 in which said mounting means includes circumferentially spaced and removable spacing means between said air bearing housing element and said retainer member.

8. A rotating spinning ring assembly for spinning and twisting frames and the like, having, in combination with a spindle mounted on a spindle rail and a ring rail traversable relatively thereto:

a rotary ring element encircling said spindle and having an external cylindrical bearing surface and external flange means extending radially outwardly with upper and lower bearing surfaces thereon;

a stationary air bearing housing element mounted on said ring rail and having an internal cylindrical bearing surface surrounding said ring element cylindrical bearing surface, an upper bearing surface underlying said ring element flange lower bearing surface, and

air passage means communicating through said housing with at least one said housing bearing surfaces; means providing pressure-induced air flow from said air passage means between said cylindrical bearing surfaces and between said housing upper bearing surface and said flange ,lower bearing surface to suspend and center said ring element for rotation with respect to said housing free of contact therewith;

a stationary ring element retainer member carried by said ring rail and having a lower surface closely spaced above said flange upper bearing surface and providing therewith a concentrated air flow, directed radially outwardly between said retainer member lower bearing surface and said flange upper bearing surface and induced solely by centrifugal force, normally maintaining said ring element out of contact with said retainer member;

said centrifugal air flow being in the same radial direction along said upper surface of said flange of said ring element as said pressure-induced air flow along said lower surface of said flange.

'9. A rotating spinning assembly as in claim 8, and further including mounting means connected to said housing element and removably and adjustably mounting said ring element retainer member above said flange of said ring element and in spaced relationship to said housing element.

10. In a spinning or twisting frame having a spindle rail and a ring rail carrying rotary spinning rings relatively movable within a normal traversing range and to a lowered position with respect to said normal traversing range, the combination of:

brake means carried by said frame and engageable when actuated with said rings for retarding rotation thereof, said brake means during normal operation of said frame being spaced from and out of engagement with said rings and being actuated by relative movement of said ring and spindle rails to said lowered position thereof.

11. In a spinning or twisting frame as in claim 10, wherein said brake means includes a friction-type braking surface and wherein said surface and said rings are in engagement during actuation of said brake means.

12. A rotating spinning ring assembly for spinning and twisting frames and the like, having, in combination with a spindle mounted on a spindle rail and a ring rail traversable relative thereto:

a rotary ring element and a cooperating air bearing housing element carried by said ring rail in encircling relationship to said spindle for traversing movement relative thereto;

said elements having contiguous bearing surfaces thereon and said ring element being adapted to be'suspended and centered relative to said housing element, for rotation during normal operation free of contact with said housing element, by an air flow between said contiguous bearing surfaces thereof;

brake means carried by one of said rails of the frame and engageable when actuated with said ring element for retarding rotation thereof, said brake means during normal operation of the frame being spaced from and out of engagement with said ring element and being actuated by relative movement of said spindle and ring rails to a position lowered with respect to the normal traverse thereof;

said brake means including a friction braking surface engaging with said ring element during said actuation.

13. A rotating spinning ring assembly as in claim 12 wherein said rotary ring element is generally cylindrical and has a lower end portion accessible from beneath said ring rail, and wherein the engagement during said actuation of said brake means is between said lower end portion of said ring element and said braking surface of said brake means.

14. A rotating spinning ring assembly as in claim 12,

9 wherein a force having an upwardly directed component is imparted to said ring element during actuation of said brake means, and further including retainer means carried by said ring rail for limiting upward displacement of said ring element relatively to said housing element.

15. A rotating spinning ring assembly as in claim 14, wherein said rotary ring element is generally cylindrical and has a lower end portion projecting downwardly beneath said ring rail, and wherein said brake means includes a brake pad having said friction braking surface thereon, and further including resilient means carried by said spindle rail and mounting said brake pad adjacent the lower end of said spindle for engagement between said friction surface thereof and said lower end portion of said ring element upon relative movement of said spindle and ring rails to a position lowered with respect to the normal traverse thereof.

16. A rotating spinning ring assembly as in claim 15, wherein said ring element has a flange extending generally radially outwardly therefrom intermediate its height, and wherein said retainer means includes a generally disc-shaped annular retainer member, means removably and adjustably mounting said retainer member in encircling relationship about said ring element and above said flange thereof with said retainer member closely spaced a predetermined distance therefrom, and said retainer member having an inner edge portion of reduced thickness.

References Cited UNITED STATES PATENTS 2,541,238 2/1951 Goree 57124 2,563,187 8/1951 Pennati 57124 2,907,165 10/1959 Adams et a1. 57124 2,932,152 4/1960 Jackson 57124 3,025,657 3/ 1962 Noordenbos 5775 3,324,643 6/1967 Kluttz 57124 FOREIGN PATENTS 1,000,365 8/1965 Great Britain.

JOHN PETRAKES, Primary Examiner US. Cl. X.R.

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