US3386149A - Fluid bearing table roll - Google Patents

Description

O June 4, 1968 1.D. RoBERTsoN 3,386,149

FLUID BEARING TABLE ROLL I NVENTOR JOHN D. ROBERTSON ATTORNEYS June 4, 1968 J. n. RoBERTsoN FLUID BEARING TABLE ROLL original med oct. 15, 1965 2 Sheets-Sheet 2 WVENTOIL JOHN D. ROBERTSON @MV/Mawdt United States Patent 3,386,149 FLUHD EEARlNG TABLE RSU.,

.lohn D. Robertson, Taunton, Mass., assigner, by mesnc assignments, to Mount Hope Machine Company, incorporated, Taunton, Mass., a corporation of Massachusetts Application Oct. 15, 1965, Ser. No. 505,101, which is a continuation-impart of abandoned application Ser. No. 451,455, Apr. 28, 1967. Divided and this application Sept. 19, 1967, Ser. No. 678,134

Claims. (Cl. 29-116) ABSTRACT QF THE DSCLOSURE A roll having an annular sleeve rotatably supported by a fluid bearing on a stationary cylindrical support, which has two rows of orifices for feeding fluid to the clearance space between the sleeve and the support. The rows of orilices are so oriented that the resultant of the sleeves working load is applied between them. This equalizes the flow on either side of the load to maintain clearance between the sleeve and support. The supports outer surface may be cylindrical or be formed with a number of portions of differing curvatures.

The present application is a divisional application of my earlier application Ser. No. 505,101 filed Oct. l5, 1965 which was a continuation-in-part of application Ser. No. 451,455 filed Apr. 28, 1967 and now abandoned.

This invention relates generally to rolls for supporting travelling webs, and more particularly concerns an improved table roll having fluid bearing means for supporting a rotatable sleeve -carrying a web, sheet, or wire screen. The invention nds particular utility in Fourdrinier paper machines for supporting a wire bearing a pulp furnish, but is also generally useful in the processing of other porous or non-porous sheets or webs.

In the manufacture of sheet paper, the present practice involves depositing a thin suspension of fibre in water on a ne mesh screen, commonly referred to as a wire, which is moving in a horizontal path at high speed and supported by a number of table rolls arranged transversely below the web. The water in the suspension is allowed to drain through the wire to leave a layer of pulp on the wire surface. The pulpy layer is subsequently pressed and dried into sheet paper and wound onto a roll.

During the initial stages in the formation of the paper sheet, it is important that the wire be maintained in as at a condition as possible. Should the wire sag along its center, the watery suspension would tend to drain towards the center-line with the result that the finished paper sheet would be thicker at its center than at its edges. In order for the wire to present a flat surface, it follows that the supporting table rolls must be axially straight throughout their entire length.

Some of these rolls, which run up to 2O feet or more in length, have -commonly been made slightly barrelshaped to compensate for the sag so that, when mounted horizontally, the upper screen-supporting portion will be longitudinally straight. This type of table roll is quite expensive to produce and has not given entirely satisfactory performance. Other table rolls have been made quite strong and with a relatively large diameter (16 inches for example) in order to support, without sagging, the weight of the wire and the partly formed paper. While this measure is effective in providing a generally straight roll, the resulting increase in roll diameter adversely affects the draining of water through the wire. It has been found that when table rolls of relatively large diameter are used to support the wire, a fillet of water tends to build up be- Patented June 4, 1958 "ice neath the wire along the upper quadrant on the discharge side of the roll. This fillet of water creates a rather high vacuum (up to 26 inches of mercury) which pulls the wire down and slightly around the roll. Where the wire finally pulls away from the roll, breaking the vacuum, the wire snaps up forming a line of spray across the sheet. Its forward velocity carries it several feet Idownstream disturbing the sheet formation where it lands. However, a table roll of relatively small outside diameter eliminates this problem while permitting a greater number of rolls to be used in the same length of Fourdrinier thus more effectively removing the water. But, for reasons already stated, any such small-diameter roll must be axially straight when in operation.

According to U.S. Patents No. 3,099,072 of July 30, 1963, and No. 3,094,771 of Sept. 16, 1963, both having a common assignee with the present application, a table roll of small diameter incorporating a non-rotating hollow axle supporting a shell roll may be straightened to remove transverse deflection or curvature under load, by tension rod means passing longitudinally through the axle. My US. Patent No. 3,094,769 of June 25, 1963, straightens the roll non-adjustably by deforming the axle into an upward bow. According to all of these patents, however, the outside surface of the roll is formed of a sleeve that is supported on annular spools, spaced along the length of the axle which in turn are rotatably mounted on the axle by means of conventional bearings. For an axle of given diameter (determined by overall stiffness requirements of the roll) the minimum attainable outside diameter of the roll is limited by the space occupied by the bearings.

lt is one object of the present invention to provide a roll with improved fluid bearing means, which make it feasible to reduce the space between the sleeve and axle to a minimum, affording minimum outside diameter of roll for a given axle stiffness. It is a further object to provide an improved iiuid bearing support for rolls having cylindrical as well as non-cylindrical surfaces.

lt has previously been proposed to lit an annular surface sleeve snugly but slidably about a cylindrical support or axle, to form a fluid bearing at the interface of the adioining surfaces of the sleeve and support by providing uid orifices in the outer surface of the support in communication with the interface, and to supply these orifices with pressure uid for establishing a fluid bearing. However, this soluton has not hitherto proven entirely satisfactory, especially where a rigid sleeve is used, or where there is any substantial clearance between the sleeve and the support. An early proposal arranged the orifices at random over the support surface. This resulted not only in excessively rapid fluid flow through the ends of the sleeve, but in eccentric positioning of the sleeve under the applied load, which causes rubbing between the sleeve and support with this arrangement.

A later modification, employing a flexible sleeve, provided orifices in the support only at its gravitational top, and at the point of load application if this was not at the gravitational top; the remainder of the support was imperforate. The construction is not satisfactory when the sleeve is rigid and/ or there is substantial clearance between the sleeve and support. The fluid ilow divides to move in both circumferential directions about the support, but in practice, does not divide equally. This tends to rock the sleeve to one side or the other resulting in lower velocity ows and consequently higher pressure on the side where the crack is widest. The effect is cumulative, and results in substantially all the fluid flowing on one side, with the corresponding surfaces of the support member and the sleeve on the opposite side coming in contact with each other. The resulting friction may be sufficient to prevent rotation from starting.

I have found, however, that by providing two longitudinal rows of orifices, one spaced in each circumferential direction from the line of application of the resultant of the applied load, this difficulty is avoided. The two rows of orifices provide two lines of support, so that the rocking action cannot occur, and the roll starts easily. The angle between the two rows of holes should be sufficient to provide stable support, but not so great as to lose the lifting effect. In this latter connection, it should be noted that before rotation starts, and with no fluid flowing through the supporting orifices, the sleeve is in contact with the support in the vertical plane of the center line; consequently the gap between the sleeve and the two rows of orifices increases as their angle from the vertical centerline increases, so that an excessive angle would destroy their effectiveness in lifting the sleeve from the support. I have found that an angle of about 60 degrees between the rows of orifices provides satisfactory results; but the angle may vary over a considerable range.

In a cylindrical roll, no flexing of the sleeve is required, and it is feasible to form the sleeve of a relatively hard and rigid material such as fiberglass, hard rubber or the like. In fact it is preferable to do so, since such materials generally are more resistant to wear than softer flexible sleeve materials. When a hard sleeve material is used, it is found that the foregoing arrangement of orifices for admitting the fluid under the sleeve is very beneficial in its reduction of the torque required to start the sleeve rotating` The remainder of the support is imperforate in the cylindrical form of roll having a rigid sleeve, Sufficient' clearance is provided to allow space for fluid flow between the sleeve and support, and to accommodate any slight ovality of these elements which may be encountered.

Recent and continuing research indicates that a circular cylindrical surface may not in all circumstances be the best form of surface for supporting the wire of a Fourdrinier machine. A varying-curvature surface or foil may produce better water drainage and less tendency to cause spattering as the wire breaks away from the surface. Further, the optimum shape appears to differ with the composition of the furnish, and the speed of the wire; and also to change over the series of rolls running from the headbox, where the furnish is'wetter, to the drier portion of the wire. In general, it is presently thought that the entering portion of the foil, upstream from its point of tangency with the Wire, may best meet the wire rather abruptly, so as to squeeze water from the underside of the wire rather than force it through; while the exit portion downstream of the point of tangency may best depart from the wire relatively gradually, so that it does not create too high a vacuum. The degree of pressure of the foil or roll against the wire is still another variable factor which is not yet fully understood. Generally, knowledge of the subject is in an early stage of development. Increased flexibility is needed to enable papermakers to adapt this equipment to different compositions, speeds, locations along the wire, and other factors, as this art becomes further advanced.

It is accordingly a still further objective of my invention to afford adjustable or interchangeable foil surface shapes and curvatures; and also to accomplish this with rotatable sleeves, so as to reduce the rate of wear of these foil surfaces. So far as I am aware, non-circular foils have hitherto had to be stationary, and therefore subject to rapid wear by the wire dragged over them; conversely, the wire, which itself is expensive, is worn by stationary foils. It is still another object of my invention to provide a roll whose curvature at an area of contact with a supported web, sheet, or wire, may be substantially different from a circular arc corresponding to the overall diameter of the support, so that the contact-area curvature may be less interdependent with the transverse rigidity of the support.

In carrying out these objects, I provide in a single roll for the selection of a variety of roll surface configurations, which may have circular arcuate forms or other curvatures of any desired form and dimensions suitable for a variety of uses. This is achieved by forming the supporting axle in two parts, one being an inner circular cylindrical annulus which remains stationary, and the other an outer annulus which is rotationally adjustable with respect to the inner annulus to present any of a series of differentlyshaped outer curved surfaces to the supported sheet or wire. The outer curved surfaces may for example have circular arcuate sections of different radii, or may have foil surfaces of any shape, or any combination of both. These active surfaces are connected by suitable tangent smooth areas which will allow the sleeve to slide freely.

The inner annulus has a single set of rows of pressure fluid orifices, which are aligned with one of a series of sets of rows of orifices in the outer annulus in any adjusted position; each outer set corresponds to one of the outer curved surfaces, so that the fluid pressure is released in opposition to the sapported load in all cases.

An abrasion-resistant sleeve is snugly received over the outer annulus, and rotates freely on the fluid bearing with the travel of the wire or sheet; the sleeve must be flexible to rotate about the noncircular section of the outer annulus. Each set of orifices includes the previously-described arrangement of two rows of orifices, one spaced in each circumferential direction from the point of application of the load resultant; in addition, a third row may be arranged in alignment with the load resultant, to prevent the load from deflecting the flexible sleeve into direct contact with the support at this point.

Identical rolls of this construction may be used in series in a run, each adjusted to employ a selected active surface appropriate to the conditions prevailing at its particular station. And the series of rolls may be readily re-adjusted when the working conditions are altered, eg., if the composition of furnish is to be changed in a series of runs of a Fourdrinier machine. Another important benefit in appropriate circumstances is that the active surface can have a radius much less than an equally-thick circular roll would provide, so that equivalent transverse roll stiffness is provided without the penalties accruing to a large radius of curvature.

While the specification concludes with claims clearly pointing out the subject matter which I regard as my invention, it is believed that the invention may be better understood from the following detailed description of preferred embodiments thereof, referring to the accompanying drawings, in which:

FIG. l is a cross-sectional view in elevation showing a first construction of the improved roll which provides for selective adjustment of the configuration of an active surface supporting the wire or sheet carried by the roll;

FIG. 2 is an end View in section of the roll of FIG. 1;

FIG. 3 is a sectional end view on an enlarged scale of another embodiment of the improved roll; and

FIGS. 4 and 5 are sectional end views of two further embodiments.

Referring to FIG. 1, one end only of a first form of the roll is shown, the other end being symmetrical. The roll has a tubular axle or support, formed in two parts, including `an inner annulus 12 of longitudinally-uniform circular cross-section, and an outer annulus 14 of varying surface form. The outer annulus is received for rotational adjustment on the inner annulus, by means of a Worm gear 16 pinned at 18, which is provided only at the illustrated end of the roll. A worm 20 having suitable operating and mounting means (not shown) positions the annulus 14 as desired, and visual indication of the rotational position may be provided by index markings (not shown) on the exposed face of the gear, and on a locating ring 22 to hold the parts in longitudinal assembly.

Extending lengthwise through the support is a tension rod 34 having threaded end portions 36'. Stout abutments or end plates 37 are received in opposite ends of the support, and are provided with aligned openings to receive the end portions of the rod 34. These openings are formed eccentrically in the abutments so that the rod will be arranged parallel to, but spaced vertically beneath, a neutral axis of transverse deflection of the roll. A pair of nuts 40 engage the ends of the rod and are tensioned against the abutments. By tightening the nuts 401, the rod 34 may be subjected to increasing tension, and the support placed under an eccentric longitudinal compressive force. This force is used to remove longitudinal curvature from the roll when the load is applied, as more fully described in the aforementioned U.S. Patents No. 3,099,072 and 3,094,771. The material of Iwhich member 14 is made is preferably one which is of relatively light weight and low rigidity, such as metal and molded or extruded plastic. By so doing, there is negligible variation of vertical stiffness and of location of the center of gravity of the completely assembly as member 14 is indexed to different posi tions. Thus, a single eccentric location of the through rod 34 will permit the roll to be straightened regardless of the position to which member 14- is indexed.

The end portions of the support extend through and are supported by spherical bearing elements 28 which are mounted for relative canting movement of the support within bearing members 30 on fixed supports 32. When the center of the roll is deflected upwardly by the tension rod 34 in an unloaded condition, the end portions cant downwardly slightly with respect to the longitudinal axis of the roll. However, the deiiection is very slight, and is removed by a load during actual. use of the roll; plain cylindrical supports may therefore be substituted if dc sired.

A sleeve 44 of flexible and barasion-resistant material is received snugly and slidably about the outer annular member 14, for supporting a wire, sheet, or other element 46, travelling longitudinally in a direction indicated by the arrow in FlG. 2. The sleeve is located longitudinally of the roll by a band 48 adjacent either edge, aiiixed by screws S0; the band should not grip the sleeve edges tightly, and a slight gap 52 should be provided to avoid binding, and to allow the escape of fluid from the interface 53 between the confronting surfaces of the sleeve and support.

The outer annular member 14 is formed with a number of surface portions 54A, 54B, 54C and 54D of differing curvatures; in the embodiment of FIGS. 1 and 2 these are all of circular arcuate section, having differing radii indicated by arrows. Any of these surface areas may be positioned opposite the supported element 46 to select a desired form for the sleeve 44 as it passes through engagement with the element 46, to afford optimum drainage action or other effect involved in any particular process. Thes active surface portions are interconnected by smoothly faired areas 56A, 56B, 56C and 56D, each mutually tangent to the pair of active areas which it connects. Thus, the fiexible sleeve 44 can rotate freely about the support.

I form three circumferentially-spaced rows of orifices 58 in the annular member 12, generally confronting the area of contact with the element 46; and a number of sets, each of three rows, of orifices 60A, 60B, 60C and 60D in corresponding active surface portions of the annular member 14. These orifices are arranged so that any one of the last-mentioned sets is aligned with the rows of orifices 58 when its corresponding surface portion is selected to be positioned opposite the element 46, and the remaining sets are sealed oli.

At the illustrated end of the roll, an opening 62 in the abutment 37 admits pressure iiuid from supply means including a conduit 64, to the interior of the annular member 12. The pressure fluid is passed through the orifices 58 and the aligned ones of the orifices 60A-60D', to the interface 53; it then passes to the ends of the sleeve and out the gaps 52. The pressure drop occurring in the flow along and about the interface provides a iiuid bearing action which supports the sleeve out of frictional engagement with the support, and allows it to rotate freely with the passage of the traveling element 46.

Two rows of the orifices in the active set, i.e., 60A in the position shown, are spaced in opposite circumferential directions from the points of application of the load resultant, where the wire 46 contacts the sleeve 44.

These two rows define between them a wedge-shaped sector of the support, indicated by angle et, such that the load resultant falls at an intermediate point. These rows provide two lines of support, each applying a horizontal as well as a vertical component of force against the sleeve, so that a rocking action is prevented and the roll starts easily. In this embodiment, a third row of orifices 60A, ete., is located between these two rows in opposition to the load, since the necessary flexibility of the sleeve 44 might otherwise permit enough deflection to cause rubbing at this point.

Another embodiment is shown in FIG. 3, in which a unitary -axle or support 12 has a circular cylindrical form. The sleeve 44 is of a hard material adapted for wearresistance, such as fiberglass, hard rubber, or the like, and is substantially rigid against deformation by the load. The sleeve must also have a loose fit over the support 12' to provide some clearance for ovality of the parts; the interface 53 assumes an eccentric shape as shown. Only two rows of orifices 58 -are provided for embodiments having a rigid sleeve, arranged one on each side of the vertical ,center line, at which the resultant load is applied in the illustrated arrangement. The rows define between them an arcuate sector a of the support 12.

Since fluid escaping from the orifices 58 provides two lines of support to the sleeve 44', rocking acti-on of the sleeve is Iprevented and the roll starts easily when pulled by the element 46. The angle a should be sufficient to provide stable support, but not so great as to lose the lifting effect against the applied load. In this latter connection, it will be noted that at the start, before the iiuid is applied and before rotation starts with the sleeve in contact with the support in the vertical plane of the center line, the space between the two rows of orifices and the sleeve increases as their angle from the vertical center line is increased, so that they would lose their effectiveness in lifting the sleeve if the angle is too great. I have found that an angle -of about 60 degrees between the rows of orifices provides satisfactory results. The pressure iiuid is supplied to the orifices 5S as before, and passes directly to the interface 53 to provide a uid bearing for the sleeve 44. This embodiment is less complex and cheaper to construct than the roll of FIGS. 1 Vand 2; and is preferred in those applications which do not call for la noncircular surface section, or for a .circular surface section of larger or smaller radius than is provided by a circular support section of appropriate thickness to meet the stiffness requirement.

Another construction shown in cross-section in FIG. 4 features -a very wide selection of surface forms, having an outer annular support member 68 whose half-section is a modified spiral of Archimedes, that is, shows a variation in radius proportional to the angular locus except for the terminal portions of the spiral which are faired together to provide a .continuous curve. A single set of orifices 58 in the inner annular member 12 communicates with any of sixteen orifices 7 ti spaced about the outer annular member 68, defining sixteen non-discrete surface portions of varying local radii of curvature. The right lobe is -a mirror image of the left. The inner support member 12 is of circular cross section. Surface portions in the left lobe, when aligned with the orifices 58, present an active foil which is approached by the element 46 relatively gradually, and departed from slightly more abruptly; while the converse is true of those in the right lobe. Those portions which appear lat the top and bottom positions in the drawing present equal rates of approach and departure to the element 46. It should also be noted that the various stations will press the traveling element more or less upwardly,

i7 producing a variable degree of angular wrap about the sleeve 44, provided that the axis of the support remains stationary; but the axis may be adjusted up or down to meet particular needs, by moving the bearing members 3G by means not shown.

Another embodiment shown in FIG. 5 is illustrated with four discrete surface portions 72A, 72B, 72C yand 72D in an outer annular member 74; the first two of these are foils of special form, and the latter two are circular arcuate areas of different radii and height relative to the axis of the support. The surface portions are connected by faired tangent areas MA-'76D as in the embodiment of FIGS. 1 and 2; each is also provided with a correspondingly-lettered set of three orifices 78A, 78B, 78C or 78D. The foils are similar in form to air-foils, having a rather abrupt approach to the element 46 and a more gradual departure. A scraping action of the leading edge of a foil 72A or '72B may tend to remove water rapidly from the lower surface of the element 46 in paper-making processes, while the trailing edge establishes a favorable distribution of suction.

However, the optimum forms of foils or other surfaces is not well established at present, and does not form a part of the present invention, which is rather concerned with the provision of a roll having an improved relationship of transverse stiffness to diameter and weight, incorporating improved bearing means which contribute to these qualities, and affording selective adjustability of su-pporting surface form and dimensions. This invention also makes available a rotating non-circular curved supporting surface or foil, whose frictional resistance to the traveling element and resultant wear ofthe sa-me is sharply reduced, relative to previously-available stationary foils.

It is to be understood that the foregoing description of preferred embodiments of the invention is given for purposes of illustration, and that various `changes and modifications may be made without departing from the true spirit and scope of the invention, which I intend to define in the appended claims.

The term cylinder as used herein refers broadly to a surface generated by any straight line moving parallel to a fixed straight line, whereas the term circular cylinder refers to the surface generated by one side of a rectangle rotated round the parallel side as axis.

What I claim and desire to secure by Letters Patent of the United States is:

i. A roll for supporting the applied load of a traveling element, comprising: a longitudinally-elongated stationary support having a smoothly-curved cylindrical outer surface; an annular sleeve received rotatably about said support; conduit means extending into said support for supplying pressure fluid; said support being formed with two longitudinal rows of orifices communicating said conduit means with the interface between said sleeve and said support and defining between them a llongitudinally-extending wedg-e-shaped minor arcuate sector of said support, said support sealing off direct communication between said conduit means and said interface over the remaining major arcuate sector of said support means mounting said support with a fixed orientation such that the resultant of the load on said sleeve acts within said wedge-shaped minor sector and at a distance from either of said rows which is a substantial portion of the arc length of said wedge-shaped minor sector; whereby fiuid is directly delivered from said conduit means to said interface at points spaced in either circumferential direction from points of load resultant application, to form a dynamically-balanced fiuid bearing support for holding the rotating sleeve in spaced-apart relation to said support against the load of said traveling element.

2. A roll as recited in claim l, in which said support is formed with one additional row of orifices communicating with said interface within said wedge-shaped sector and located in substantial alignment with points of application of the load of said traveling element, said sleeve being fiexible under the applied load.

3. A roll as recited in claim l, in which said outer surface has a cylindrical form differing from a circular cylinder.

li. A roll as recited in claim 1, in which said outer surface is formed with at least one surface portion having a cylindrical form generated parallel to the longitudinal axis of said support by a curve differing from a circular arc which is generated about the neutral axis of defiection of said support and tangent to said curve, said surface portion being opposable to the area of engagement of said traveling element by said sleeve; and said sleeve being flexible to conform to said one surface portion while rotating about said support.

5. A roll as recit-ed in claim 4, in which said one surface portion is formed by a circular arc of radius less than half the transverse thickness of said support.

6. A roll as recited in claim 4, in which said one surface portion is formed by a non-circular curve of foil configuration.

7. A roll as recited in claim 1, in which said outer surface is formed with a plurality of surface portions of cylindrical forms generated parallel to the longitudinal axis of said support by differing pre-selected curves; said support being rotationally adjustable to position any select-ed one of said surface portions in opposition to an area of engagement of said traveling element by said sleeve; and said sleeve being flexible to conform to said differing surface portions while rotating about said support.

S. A roll for supporting the applied load of a traveling element, comprising: a longitudinally-elongated support; said support being formed with an outer cylindrical surface having a plurality of circumferentially-spaced surface portions of differing pre-selected curvatures, interconnected by faired surface areas; means for positioning any of said surface portions in opposition to said traveling element; an annular flexible sleeve received rotatably about said support and being conformable to said outer surface thereof, to prevent a foil configuration to said traveling element determined by the form of an opposed one of said differently-curved surface portions; said support being formed with a plurality of orifices, at least one comunicating with each of said surface portions; and conduit-forming means for delivering pressure iiuid through those of said orifices communicating with said opposed one of said surface portions to the interface between said support and said sleeve, to form a uid bearing support for holding said sleeve away from frictional contact with said support against the load of said traveling element.'

9. A roll as recited in claim S, in which said support comprises an inner stationary support member having a circular cylindrical outer surface, and an outer annular support member having a circular cylindrical inner surface receiving said inner stationary member conformably therein for relative angular adjustment; said positioning means being constructed and arranged for positioning said outer member angularly about said inner stationary member; said inner stationary member being formed with at least one further orifice communicating with said inner surface of said outer member, for alignment with at least one of said first-mentioned orifices communicating with a corresponding surface portion upon positioning of said corresponding surface portion in opposition to said traveling element; said conduit-forming means communieating with said further orifice through said inner stationary member.

19. A roll as recited in claim 9, in which said inner stationary member is formed with a plurality of said further orifices arranged in three rows, two of said rows deiining between them a longitudinally-extending wedgeshaped minor arcuate sector of said inner stationary member, and the third row being located in substantial alignment with points of application of the load of said travel- References Cited UNITED STATES PATENTS 2,054,214 9/1936 Buss. 3,012,301 12/1961 Rogers et al 29-116 XR 3,094,771 6/ 1963 Robertson 29--116 BILLY J. WILHITE, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Paten-t No. 3 ,386 ,149 June 4 1968 John D. Robertson It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, line 4l, "prevent" shold read present Signed and sealed this 27th day of January 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents

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