CA1037984A - Web support with castered and gimballed roller - Google Patents

Abstract

WEB SUPPORT WITH CASTERED AND GIMBALLED ROLLER

ABSTRACT OF THE DISCLOSURE

Web support for engaging a fully constrained moving web, having a cylindrical roller which imposes no lateral constraint on the entering portion of the moving web and which angularly decouples the exiting portion of the moving web. An improved mounting mechanism dynamically supports the roller solely at its midpoint along a fixed shaft. A constraining member reduces the movement of the roller to rotation about the longitudinal axis, pivotal movement about a gimbal axis which is parallel to the plane of the entering web portion of the moving web and perpendicular to and intersects the longi-tudinal axis at the midpoint of the roller, pivotal movement about a castering axis which is substantially perpendicular to the plan of the entering web portion of the moving web and intersects the gimbal axis at a point upstream from the mid-point of the roller, and translational movement along the fixed shaft as required for pivotal movement about the castering axis.

Description

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CROSS REFERENCES TO RELATED APPLICATIONS
Reference is hereby made to commonly a~s~gned and copending Canad~an Patent ~ppllcation Serial No, 234,174, entitled WEB TRACKING APPARATUS, filad on even date here~
with in the names of Thaddeu~ Swanke, Michael Samuel Montalto, and John Edwin Morse. Reference i al~o mad~
to commonly as~igned and copending Canadian Patcnt A~plica-tion Serial No. 234,680, entitled POSITIONALLY CONSTRAINING
WEB SUPPORT, filed on even date herewith, in ~he names of Thaddeus Swanke and Richard Thomas O'MarraO
BACKGROUND OF THE INVENTION
Field of the Invention This invention relates generally to a web handl-ing device, and more specifically to a mounting mechani~m for a web engaging roller in a web support which imposes no la~eral conctraint to the entering web portion and which angularly decouples the exiting web portion. ~ ~:
De~cription of the Prior Art Web tracking apparatus for tracking flexible, uni-directionally mov~ng webs on hard surfaced, cylindrical web supports can be con~idered functionally as compri~in~
basically two types of web ~upports, The linearly m~ing web approaching a web 3upport "sees" the support, relative to a fixed frame, either as (1) a laterally constrain~ng ~;
eUpport or (2) a la~erally non constraining support. A
laterally con~training ~upport may be further ubdivided into (a) an angular lateral con~traint in which the enterlng web i8 con~trained aga~n~t changlng its lateral poeition, except a~ it~ angular position change~, snd B

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(b) a positional lateral cons~raint in which the entering web is constrained against changing its spatial lateral position, while remaining free to change its angular position. The web entering a non constraining supportg on the other hand, is free to change either its angular or its spatial lateral position without experiencing substantial lateral forces.
Whether a particular web support is a laterally - constraining or laterally non constraining support depends as much on its function in the tracking apparatus as on its structureO For example, a rotating fixed-axis cylindrisal roller, such as an idler roller or a drive roller in a tracking apparatus, is structurally an angular lateral constraint capable of constraining the moving web against change in its lateral position. To perform functionally as an angular lateral constraint, howe~Ter, the entering web has to be capable of tracking on the rotating cylindrical surface until the moving web and the rotating surface are in alignment; i.e. until the - longitudinal axis of the rotating surface is perpendicular ~o the direction of travel of the web. This tracking phenomenon is due to frictional forces developed between the linearly moving web and the rotating surface, which in turn are a function of, among other variables, wrap angle, web tension, and the upstream web-span to web-width ratio. Thus, if the wrap angle~ for example, is insufficient to create the frictional forces necessary for tracking, the entering web is free to change its angular position and/or its lateral spatial position, - wlthout experienclng substantial lateral forces resulting in a web support that is ~unctionally non constraining, although .. , . . .. . . . . , .. ~ . . .

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structurally an angular lateral constraint.
Although the above-noted variables upon whlch track-ing depend are usually parameters which are governed by the design of the web tracking apparatus, some generalities can be stated that cover a significant number of situations. Thusg for a flexible web supported by hard surface cylindrical supports, the upstream web-span to web-width ratio should be - somewhat equal to or greater than one, and the wrap angle should range between approximately 30 and 135g depending on the coefficient of friction of the surfaces in contact, and on web tension~ If otherwise, the web could be prevented from - - tracking, either because of not enough, or too much contact with the web support.
To facilitate the discussion to follow, it will be convQnient to refer to a laterally non constraining support as an "N" support, and to refer to a laterally constraining support as a "P" support if it is functionally a positional lateral constraint and as an "A" support if it is functionally an angular lateral constraint. -- 20 In designing a closed loop web tracking apparatus of the type discussed above, one o the primary considerations of the design is lateral stability of the linearly moving web.
- Generally, stability of the linearly moving web is achieved if the tracking apparatus has at least two laterally constraining supports, at least one of which is further restricted to be a P support; the remaining web supports, if any, in the tracking apparatus can be either laterally constraining supports (P and A) or non constraining supports (N) as dictated by design considcrations~

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~0379~34 Although the stability principle stated above will ensure linear stability of the moving web, lt does not, without more9 ensure uniformity of tension ln the ving web. Non-uniformity in tension ordinarily results from imperfections in the manufacture of webs and web supports, and from the lack of perfect parallelism in the longitudinal axes of the mounted web supports. It follows that if manufacturing tolerances are - minimized and the supports are mounted with a high degree of parallelism, a degree of uniformity of tension will be achieved.
However, such considerations are independent of the stability principle.
If a high degree of uniformity of tension of the web is a requisite of the tracking apparatus design, Lt can be achieved with little regard to manufacturing or mounting tolerances by conformin~ the web tracking apparatus to what will be referred to as the uniformity of tension principle. This -second web tracking principle dictates that the moving web exiting from a first laterally constraining support must be given freedom, once and only once, to change direction before ~ 20 entering a second laterally constraining support. This freedom is given to the exiting web by "gimballing" the web support; i.eO, by mounting the web support, whether of the constraining type or Qf-the non-constraining type, for pivotal movement about a gimbal axis which is parallel to the direction of linear move-ment of the entering web, and which intersects the longitudinal -axis of the support at the midpoint of the support.

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The gi~bal action of the web support, i.e.~ the capability of the exiting web to change directiong enables the exiting web to compensate for non-uniormi~y of tension of thc web in the downstream web:span. The resultant force of the non-uniform tension across the exiting web is at some perpen-dicular distance from the centerline of the moving web; the component of that resultant force which is perpendicular to the gimbal axis creates a moment about the gimbal axis which varies with the sine of the wrap angle, since the magnitude of the force component perpendicular to the gimbal axis varies with the sine of the wrap angle. For example, ~or wrap angles ~ ;~
approaching zero or 180 the magnitude of the force component approaches zero and therefore, the exiting web is not free to change direction~
It is clear from .he abGve relationship .hat the magnitude of the force component perpendicular to the gimbal axis is greatest for a wrap angle of 90; moreover, as the wrap angle increases appreciably from 180 the exi~ing web behaves as if the wrap angle were appreciably greater than zero. While the gimbal action may not be appreciably inhibited by large wrap angles (e.g., those appreciably less than 180 and especially --those appreciably greater than 180), such large wrap angles may inhibit the tracking action of a web support, thereby possibly producing an unstable tracking apparatus. -The "once and only once" requirmeent of the uniformity of tension principle can be illustrated by theorizing a tracking apparatus in which the web exiting from a first laterally constraining support encounters two N supports before entering .
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a second laterally constraining support. The ~once and only onc~" requirement provides that only one of the three supports, - l.e.~ the first laterally constrainîng P or A suppor~, the first N support, or the second N support, be gimballed; the ~ther two must prevent the exiting web from changing direction.
~or reasons noted above, gimballing one of the supports provides uniformity of tension in the downstream web without affecting lateral stability. However, if more than one support is gim-balled before the web enters a second lateral constraint, the lateral position of the web at the second and any subsequent non-constraining gimballed support, becomes unstable and indeterminate. The result could be lateral instability of the web span between the first gimballed support and the seco~d constraining web support, and possible edge dàmage to the moving -~eb due to such instability. Thus~ the 1'once and only once"
requirement ensures lateral stability in the moving web when N -~
supports are utilized in a tracking apparatus, while providing uniformity of tension.
Theoretically, the above principles would not be violated by a two-support, closed loop web tracking-apparatus.
However, technical problems such as the gimballing of a drive ~
roller to me`et the "once and only once" requirement, and wrap ;
angle considerations upon which the gimballing action depends, as well as practical problems such as utility for such a two-support apparatus, could make such an apparatus commercially -~
-unattractive. The introduction of additional supports to a closed loop web traclcing apparatus, however, eliminates such technical and practical problems if the combination of supports -7~

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conforms to the two tracking principles outlined above, and thelr location relative to each other is such that the respective wrap angles the moving web makes with the three or more supports are within the limits previously discussed. In - particular, a gimballed N support coiuld be located downstream from the drive roller of a web tracking apparabus (an A support) to decouple the angularly constrained web exiting from the drive roller, thereby providing a web tracking apparatus which - satisfies both web tracking principles: stability and uniformity of tension in the moving web.
Non-constraining N supports disclosed by the art - include low friction cylindrical non-rotating surfaces, and axially compliant rotating web supports. Such disclosed web supports, however, are not gimballed and, therefore, do not impart angular freedom to the exiting web. The art also dis~
closes web supports in which the rotating cylindrical surface of the web support is mounted for pivotal movement about caster and gimbal axes. For example, U.S. Patent No. 3,596,817 - ~
discloses a web support having a castered and gimballed roiler ; -in which the pivotal movement about the caster and gimbal axes is achieved by mounting the roller on a multiplicity of flexture arms~ This type of mounting, although adequate for -~
some purposes, can be impracticable for othersO For example, the available space and load requirements may necessitate a different mounting. Also known are web supports having cylindrical rollers which are rotatably mounted for pivotal movement about a gimbal axis. An example is U.S. Patent No

3,608,796 in which the rotating cylindrical roller is mounted . . . - .

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for pivotal movement about its midpoint. Such n~dpoint pivotal movement is reduced to pivotal movement about a gimbal axis by - a constraining member. However, unlike one that is castered and ' gimballed, such a web support angularly constrains the entering web; i.e., functionally, it is a gimballed A support~' '' ' Accordingly, it is an object of the presenc învention to provide a web support naving a castered and gimballed web engaging roller mounted on a fixed axis support~
It is another object of the invention to provide a web support in which the castering and gimballing pivotal movement of a web engaging roller is controlled by one external '' member only.

SUMM~RY OF THE INVENTI~N
These and other objects are accomplished' according to the preferred embodiment of the present invention by rotatably mounting a cylindrical web engaging roller on a fixed ~upp~rt "
for pivotal movement of the roller about its midpoint and for -translational movement of the roller along the fixed support ~ -while statically and'dynamically supporting the roller solely '' at its midpoint. A single constraining member reduces t~e move~
ment of the roller to pivotal movement about a gimbal axis and ' '~
about a castering axis, and to-translational movement along the fixed support as required for'pivotal movement about the castering axis. The gimbal axis is parallel to the plane of ~he entering web portion of the moving web and is perpendicular to and intersects the longitudinal axis of the roller. The caster ing axis is perpendicular to the plane of the entering web s g_ .

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at a point upstream from the midpoint of the roller.
In the p~eferred embodiment, a hollow cylindrical roller is supported on a fixed rigid shaft of uniform cross-section by a self-aligning radial ball bearing upon which the roller is mounted with their respective midpoints coincident.
The radial ball bearing enables the roller to rotate about its --longitudinal axis and pivot about its midpoint, and is fixedly - supported by a bushing mounted about the rigid shaft, thus enabling the roller to translate on the rigid shaft. Alter-natively, the roller may be journalled on a bushing through a plurality of flexturally mounted members between the outer surface of the bushing and the inner surface of the roller to achiev~ the same result.
The pivotal and translational movement in either embodiment is controlled by a constraining arm, def~ning a line of action, one end of which is moùnted to the rolle~ through an outboard bearing for pivotal movement about a pivotal axis. The opposite end of the constraining arm is mounted to a fixed frame for pivotal movement about a pivot point. The line of action `
corresponds to the centerline of the constraining arm and passes through the pivot point, intersecting the longitudinal axis of the roller at an oblique angle and the gimbal axis at a point upstream from the midpoint of the roller. The pivotal axis is defined by the intersection of and is mutually perpendicular to the line of action of the constraining arm and the longitudinal ~`
axis of the roller. A counter~eight is mounted at the end of the roller opposite the end upon which is mounted the outboard bearing. The counterweight counterbalances the weight o~ the .

~, - . , . . : - -- . la3~s~4 outboard bearing and the constr~ining arm so that the roller is statically and dynamically supported on the fixed shaft solely at its midpoint.
The invention and its objects and advantages will become more apparent from the detailed description of the preferred embodiment presented below.
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BRIEF DESCRIPTION OF THE DRAWINGS
In the detailed description of the preferred - embodiments of the invention presented below, reference is made to the acco~panying drawings, in which:
- Figure 1 is an isometric view of the web support of - the invention showing the mounting mechanism for the web trans-porting roller, which is partly broken away, and the relative location of the various axes; . .
Figure 2a is a longitudinal cross-sectional view of ~
the web support showing the mounting of the roller; .: ~ .
Figure 2b is a partial side view of the web support - :
with its constraining arm, . - Figure 2c illustrates another embodiment of the web support of the invention and shows a roller~supporting bushing : which is flexurally mounted to the roller at its midpoint for achieving translation along the shaft and pivotal movement about ~
the center of the roller; .
Figure 3a illustrates sti.ll another embodiment of the - web support of the invention and shows a constraining arm having ~.
a yoke at one end and a ball and socket arrangement at the other end; and ~ . . .
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-^ Figure 3b is a ~ide view o~ the embodiment of Figure 3a and ~how~ the constraining arm pivotally unted ~-to the roller ou~board bearing for pivotal movement about an axi~ which inters~cts the lo~gitudinal aæi~ of the roller, DESCR_PTION OF THE
Referring now to the drawings wherein like reference numeral9 have been u~ed in several views and figures for like elements, Fig. l illustrates a web ~upport ,, 10 having a ca~tered and gimballed roller ll, in the form 10 a hollow cylindrical drum, mounted according to the inven- ~ ;
tion. As clearly seen ~n Figures 1 and 2a, web support l0 :~
further comprises a self aligning radial ball bearing 55 mounted on a ball bushing 56, an outboard bearing 80 flxedly connected to constraining arm 60 through outer ring 82,and "`
a balancing counterweight 52.
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Ball bushing 56 i9 31idably mounted on fi~ed shaft Sl which i9 supported by yoke 71, which, in turn, i8 mounted to fixed frame 53. Bearing 55 is centrally mounted on bush~
ing 56 and roller ll, and ie ixed in position to the bu~h~ng and the roller, respectively, by locking rings 14 and locking -.
annulus 12 ~see Fig. 2a~. The radial ball bearing in combina-.: . .
tion with ~he ball bushing rotatably supports roller ll solely at its midpoint while ~imultaneously enabling ~he ~`
roller to pi~ot about its midpoint and translate along shaf~
51. The pivotal and ~ransla~ional mo~ement o~ ro~ler ll i8 ~.` "
relative to :E~ced frame 53 whi h may be the ~ram2 of a web "
tracking apparatus ~uch as that disclosed in copending C~nadi~n App:LLcatLon Ser1~1 No. 234,171.

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-12_ ` ~37~31134 A constraining arm 60, mechanically connecting roller 11 to yoke 719 reduces the degrees of freedom of movement of roller 11 to pivotal movement about gimbal axis 20 and castering axis 30 without affecting the rotational movement of the roller ~ -about longitudinal axis 40. Constraining arm 60 comprises a stiff member 63.to which is mounted at one end a resilient wire 61 and at the opposite end a semi-rigid plate 62~ Resilient wire 61 is mechanically connected to yoke 71 by attaching the . ;
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free end to a member 70 which is adjustably positionable in .

yoke 71 by screw assembly 72. Plate 62 of constraining arm 60 . .
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is bent along a line-64 (Figure 2b) which is perpendicular to and intersects the line of action 66 of constraining arm 60. i,.--The portion of plate 62 between bend line 64 and rigid member ~ .
63 makes an oblique angle with longitudinal axis 40 of roller .:
11, whereas the portion between bend line 64 and the end opposite that connected to rigid member 63- is perpendicular to the longitudinal axis of the roller. The significance of the.
oblique angle will become apparent in the discussion to followO
Further, inspection of Figure 2b will reveal that other sideO
The portion.of plate 62 which is perpendicular to -axis of rotation 40 is mounted to outer ring 82 of outboard bear- :
ing 80 by screws 67. It is readily apparent from viewing out-board bearing 80 in cross-section in Figure 2a that roller 11 is free to rotate about its longitudinal axis 40 (which in Figure 2a is coincident with axis 50 of shaft 51) while the inner ring 81 of outboard.bearing 80 remains stationary. To offset the added weight of outboard bearing 80 and constrainin~ arm 60 at one end of roller 11, counterweight 52 is added to the opposite -13- .

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end of the roller so that web support 10 is statically and dynamically balanced about the midpoint of roller 11.
From t~e foregoing-it will be seen that constraining arm 60 is free to pivot in any direction at the end comprising resilient wire 61, and free to pivot: about pivotal axis 65 only at the end comprising plate 62. The unting of constraining arm 60 on outboard bearing 80 is such that the line of action 66 of constraining arm 60, pivotal axis 65, and longitudinal axis 40 of roller 11, intersect at a common point. Moreover~
pivotal axis 65 is substantially perpendicular to the imaginary plane 45 formed by line of action 66 and longitudinal axis 40 :
(see Figure 1) whichg for convenience, will be referred to as -the entrance planeO
The resultant freedom of movement of roller 11, due to the various mechanical parts of web support 10 described above, is (1) pivotal movement about a gimbal axis 20, ly~ng in the entrance plane 45, which is perpendicular to and intersects.
longitudinal axis 40 at the midpoint of roller 11; and (2~
arcuate movement, having a radius 35, about a castering axis 30 20 ~Figso 1 and 2b) which is substantially perpendicular to and .
intersects entrance plane 45 at the intersection between gimbal axis 20 and line of action 66 of constraining arm 60. It is .
clear that the magnitude of radius 35 is dependent on the oblique .
angle line of action 66 makes with horizontal axis 40.
Inspection of the geometrics of Figures l and 2a will reveal that arcuate radius 35 varies according to the amount of translation of the midpoint of the roller along shat 51. If translation of roller 10 is toward flexure arm 60, the castering - . . - . , .
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radius becomes longer. If, on the other handg the translation is ~way from flexure arm 60, the castering radius becomes shorter~ However, for small translations from nominal, the ..
castering radius remains relatively constant to a close ap-proximationO Similarly, the gimbal axis 20 varies slightly .
from its nominal position. As seen in Figure 1, and more.
clearly in Figure 2b, the ends of roller 10 ha~e arcuate motion indicated by arrow 68 rather than straight line motion along pivotal axis 65. This is due to the pivotal action of flexure 10 arms 60. As with variations in arcuate radius 35, for small ~ :
pivotal movement of roller 10 about axis 20 the ends have .
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linear motion rather than arcuate motiong to a close approxima-. tionO These variations from nominal are indicated solely for clari~y; they do not limit the function of the web support .
in any significant way.
In àn apparatus incorporating the preferred embodimen~ `
of the invention illustrated in Figures 1-2b, the surface of :
roller 11 is polished aluminum, the web in contact with roller 11 is polyethylene tenephthalate and has a thickness in the order of seven mils, the wrap angle of web W around roller 11 is in the order.of 120, the ratio of upstream web span to web width is approximately one, and the web tension of the web W is approximately one half ounce per inch. I.t should be noted.at this point that web support 10 is unidirectional, i.e.~ it must be assembled in relation to the entering plane.of the web and its direction of travel. As seen in Figures 1 and 2b the plane of entering web portion 15 of thc web is substantial1y parallel to entrance plane 45, while the direction of travel of the web ~15- .

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ls such that the castering axis 30, which is substantially perpendicular to the plane oE entering web portion 15 of moving web W is upstream o web support lOo, ' ' . ' In operation, the fully constrained entering web portion 15 of web W, moving in the direction indicated by arrow 5, does not "see'~ web support 10 as a lateral constraint `
sin oe roller 11 will pivot about castèring axis 30 until longitudinal axis 40 is perpendicular to the direction of .
travel of entering web portion 15~ That is, through the phenomenon of tracking discussed above, roller 11 will align itself to the entering web portion 15 by pivoting about axis 30 until its longitudinal axis is perpendicular to the direction of travel of the entering web portion. Any pivotal resistance about castering axis 30 imposed by frictional or mechanical ;`~
orces WhiCIl would prevent roller 11 from fully aligning itself to the direction of travel of the entering web portion lS
(thus imposing a small lateral constraint on the web~, is ---compensated by pr~viding roller 11 with a low-friction, polished aluminum surface which promotes slippage between the surface of -20 roller 11 and the web in contact with such surface. -~
Tt is noted for illustrative purposes, that the align~
ment of roller 11 to the fully constrained entèring web portion ~
lS of the moving web illustrated by the appratus of the instant ~-invention is the reverse of what occurs when a moving web,not fully constrained, enters an angularly constraining web support such as a fixed axis, cylindrical drum. In this latter situation, it is the entering web which aligns itself to the web support so that its direction of travel becomes perpendicular to the fixed , . .. .

~ 4 longitudinal axis of the rotating cylindrical drum.
Ex~ting web portion 16 of moving web W is ~iven , '~ freedom to change its angular direc:tion, thereby angularly , ' decoupling the fully constrained entering web portion 1~ of moving web W. This freedom in exiting web portion 16 to change angular directio,n without affecting the lateral,spatial and/or '' angular position of the upstream entering web portion ]5 is due to the capability of roller 11 to pivot about gimbal axis 20 in response to downstream conditions. Although from viewing -, 10- the arcuate move~ent of the end of roller ll'indicated by - arrow 68 in Figure 2b it may appear that movement of roller 11 ~', about gimbal axis 20 will produce a change in the perpendicu- `
larity of iongitudinal axis 40 and the direction of'travel of entering web portion 15 (which would affect the lateral position - of entering web portion 15~ closer examination will reveal that roller 11 will automatlcally compensate for any changes in perpendicularity by simply pivoting about castering axis 300 , Thus, exiting web portion 16 is free to change angular direction ~ -without affecting the lateral position of the entering,web portion 150 ' ' ' As noted earlier, the surface of roller ll'-is polished - ,~
aluminum and the wrap angle of the web about roller 11 is approximately 120 which is in proper range for tracking and for gimballing. Since it is an N typç support, lateral slippage - -~
between roller 11 and the web in contact with roller will be beneficial to 'the function of the web support, which function ,-is to present no lateral resistance to the entcring web.

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Flgures 2c through 3b illustrate other embodiments of varlous parts of the invention~ Figure 2c illustrates a bushing which is flexurally mounted to the roller to provlde the center pivot feature. That is, the pivotal movement of roller ll' about lts midpoint is achieved through flexure member 90 rather than self-aligning bearing 55 as shown in Figure 2a. Figures 3a and 3b illustrate the use of a yoke 92 -~
mounted on a ball-and-socket arrangement 85, rather than flexu~re arm 60 as illustrated in Figure 2a. Ball and socket ~
85 allows yoke 92 to pivot in any direction. Yoke 92 is ~ `
pivotally mounted to outboard bearing 80l by pins 93. The centerline of pins 93 intersects the axis of rotation of roller ll'c As in the preferred embodiment, the castering radius and the gimbal axis vary slightly from nominal. However, as noted earlier5 this places no apparent restrictions on the function`
of the web support. - ~;
The lnvention has been described in detail with ~ -particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be `
effected withln the spirit and scope OT the in,ention. ~ ~ ;

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Claims (5)

I CLAIM:

1. In a web support for engaging a fully constrained moving web having an entering portion and an exiting portion relative to such web support, said web support being of the type having a web engaging cylindrical roller which presents no lateral resistance to such entering web portion and which angularly decouples such exiting web portion, the improvement which comprises:
a) a fixed support defining a fixed axis;
b) means for mounting said roller on said fixed support to statically and dynamically support said roller solely inter-mediate the roller ends within the interior of said roller, and to provide for rotational movement of said roller about a longi-tudinal axis, pivotal movement of said roller about said midpoint, and translational movement of said roller along said fixed axis;
and c) means for constraining said pivotal and translational movement of said roller to pivotal movement about a gimbal axis perpendicular to and intersecting said longitudinal axis of said roller and lying in a plane substantially parallel to such entering web portion, pivotal movement about a castering axis being substantially perpendicular to such entering web portion and intersecting said gimbal axis at a point upstream from said midpoint of said roller, and translational movement along said fixed axis as required for pivotal movement about said castering axis.

2. The invention of Claim 1 wherein said fixed support is a rigid shaft of uniform cross section.

3. The invention of Claim 1 wherein said roller mounting means includes a radial ball bearing for enabling said roller to pivot about a midpoint, and a bushing about said rigid shaft for fixedly supporting said radial ball bearing, and for enabling said roller to translate along said shaft.

4. The invention of Claim 1 wherein said roller mounting means includes a flexure member for enabling said roller to pivot about its midpoint.

5. The invention of Claim 1 wherein said constraining means includes:
a) an outboard bearing mounted at one end of said roller;
b) a constraining arm, defining a line of action, one end of which is mounted to said roller through said out-board bearing for pivotal movement about a pivotal axis, the end opposite said one end of which is mounted to a fixed frame for pivotal movement about a pivot point, said line of action passing through said pivot point and intersecting said longitudinal axis at an oblique angle and said gimbal axis at an upstream location, said pivotal axis being defined by the intersection of and being mutually perpendicular to said line of action and said longitudinal axis; and c) a counterweight mounted at the end opposite said one end of said roller for counterbalancing said outboard bearing and said constraining arm to statically and dynamically balance said roller solely at its midpoint.