CA1182560A - Web aligning system - Google Patents

Abstract

Abstract A web guide system wherein misalignment of take-up and supply rolls relative to a reference surface may be compensated without tape distortion by a crown pulley and an adjacent concave web guide. The concave web guide is a fixed, flangeless post about which the web is parially wrapped. The web tends to be self-centering in the concave curved portion of the guide. Tape is fed from a roll to the concave guide and then to the crown pulley of minimum radius which is mounted for axial as well as rotational motion. Pulley axial motion promotes equal edge tension and completes web self-alignment. Final axial alignment without edge pressure is accomplished by fixed tape edge-guides in the region of the head.

Description

The invention relates to web transport systems and in particular to apparatus for guiding webs past a transducer.
In tape or film (web) drive systems such as magnetic tape and film recorders, the web is usually drawn from a supply roll, over a transducing system, and thence onto a take-up roll. In such systems the lateral regis-tration of the web with respect to the transducer is an important factor in determining the ultimate capability of the reproducing transducer to resolve the data previously stored on the web, both on an instantaneous and long-term basis.
Lateral registration is usually accomplished by web edge guides, whose locations are established by the predetermined dimensional relationships and alignment between the transducer and each of the web rolls. In one typical guide system, fixed flanged posts are interposed in the paths between web rolls and a transducer. The flanges are either spaced apart a dimension equal to or greater than the widest web, in which event the web may wander between the flanges an amount equal to or greater than its width tolerance, or one of the flanges may be spring loaded against one edge so as to urge the other edge against the fixed flange.
A disadvantage of the fixed-post fixed-edge guide system is that the 20 ~ ability of the web to slide or register axially along the post is dependent upon the presence of an adequate air film to ensure web-to-post lubrication. The collapse of the air film from high tension or slow speed may cause web edge damage if the web is delivered into the guide with substantial misalignment.
~ven with air-film lubrication, the drag on the web will normally be higher than the corresponding drag of a roller, because the bearing surface drag of the roller acts at a reduced radius with respect to the roller surface. I`he = 1 -drag will result in energy loss and wear in the guide system and in the tape.
In another typical guide system, fixed posts are used without flanges.
Such posts are exemplified in United States patent 4,191,345. United States patent 3,474,9~1 shows a stationary bar having a straight central section and two opposed end sections that are gradually curved upward so that the edges of the record medium are tensioned slightly greater than the central portion of the record medium. This type of guide tends to keep a web centered during wind-ing. ~lowever, the lateral registration is inexact, providing poor "tracking" of the head on the tape between the recording and reproduction processes.
~n alternate guide approach is to use rotating cylindrical rollers, rather than fixed guides, in combination with rotating or fixed web edge-guides.
One of the disadvantages of a rotating gulde system is that once the web is wrapped around the idler, its air-film is lost and no axial movement may occur;
therefore the web edge is engaged and registered while it is still unsupported as a plane; the unit pressure on the edge may become high if the web is delivered into the guide with substantial misalignment. This may result in the elastic limit of the web being exceeded, with permanent deformation in the web edge resulting. -~ith either fixed or rotating edge-guides, edge drag may cause long-term and short-term variations in edge velocities with respect to the web center-line, known as static and dynamic skew, respectively. Static skew may be mini-mi~ed by use of rotating flanges. However, scraping of such a flange agains~
the web edge may cause rapid longitudinal vibrations in the web, known as flutter, and may increase dynamic skew. Flutter causes absolute timing errors in the recovered data; skew causes relative timing errors in data recovered from parallel tracks.
For systems in which tape cassettes are used, the planes of the take-up and supply rolls are sometimes misaligned with respect to a horizontal reference surface9 such as cassette walls that support the shaft on which the take-up and supply rolls are mounted. Any slight misalignment of the planes of the tape rolls may cause a substantial misregistration between the tape, the tape guides, and the transducer. In high-speed applications, such as those involved with digital computers and instrumentation typ~ of analog recorders, such misregistrations, even though ~small, limit the number of tracks and bit-packing densities that may be recorded on the tape. Moreover, even slight mis-alignments can cause uneven tape tension laterally across the width since the velocity of one tape edge is different from the velocity oE the other tape edge.
The lateral unevenness in tape tension maq cause tape edge wear and perhaps flutter and skew, as mentioned above.
An attempt to mînîmize the above-described transverse tension and edge force problems is taught by H. E. Haas in United States patent 3,393,849, in which a free~floating crown pulley is employed between two fixed planar edge guides to equalize the transverse tension. In practice, this method suffers from three problems. The first problem is that for web edge force to be reduced to zero, the planes of the edge guides must either be reduced to a single point of contact with the web, or that failing, must at once be parallel with the planes of the tape-rolls and the crown pulley or the tape will be forced to warp as its centerline departs from the tilted roll plane to the idler plane which is normal to the head face.
The qecond problem is that in recognition of the first problem~ the patent shows the guides located in the immediate region of the crown pulley, requiring the pulleq to float between them. In order to float, the pulley is shown to be narrower than the tape9 leaving the tape overhanging the pulley edgeæ with no edge tension or sti~fening support. Such an edge will be readil~
damaged by the application of edge forces. Further, iE in attempting to correct planar misalignment, the pulley seeks a position too far off the centerline of the tape, the tape edge with the largest overhang will be dra~n toward the pulley center by the unopposed tape edge tension, developing a transverse force which cannot be overcome by the pulley. The pulley will thus often become "clamped"
to one end or the other of its travel.
The third problem is that, to be effective, the crown pulley must generate substantially greater stretch, i.e. tension, at the centerline than at the edges of the web. This will cause the pressure between head and tape to vary greatly from edges to center, causing excessive susceptibility to data dropouts at the edge tracks. ~ large radius crown will require excessive axial movement, and will be unstable. The problems described are particularly acute in cassette systems where the spans between tape rolls~ guides and heads are of necessity very short.
A~ object of the inyention is to proyide a web guide system which compensates for substantial misalignments of the planes of the take-up and supply rolls even in short-span tape-drive systems, while maintaining uniform transverse web tension and velocity. Another object is to provide a web guide system which avoids high edge contact pressure, minimizing "coining" and pre-mature edge wear.
The above objects are achieved by a web guide system having the com-bination of at least one fixed concave web guide feeding the web to an axially moving convex crown pulley9 followed by an edge guide. The concave guide serves two ilnportant functions. The first is to stabilize the axial position of an axially floating crown pulley without imposing tape edge restraint in the region 32~

of the pulley, thus allowing vertical registration of the web to be accomplished after, and only after, the tap has passed around the pulley, where i~s center-line can be aligned with the plane normal to the head face without tape distor-tiOII and allowing the crown pulley to be wider than the tape, to give itcontinuous edge-to-edge support. The second is to equalize net transverse tape tension in the web delivered to the head by complementing the loss in edge tensions caused by the crown.
Neither the concave guide, the convex crown pulley, nor the fixed edge guide, working alone, can prov:ide the guiding through the plane changes into proper registration with the head, without tape distortion. Ilowever, with the concave guide providing stability and edge tension e~ualization, the crown pulley will correct for tape roll plane misalignment without edge contact, and the edge guide will thereafter provide vertical registration.
In a preferred embodiment, a concave web guide is positioned in the supply and take-up spans near a convex crown pulley on each side of a trans-ducer. In another embodiment a convex web guide and a concave crown pulley is used. This enables misalignment compensation from either web roll, since either can be the supply or take-up roll in many tape systems.
In accordance with one aspect of this invention, there is provided 2Q a web guide system comprising, a horizontal reference surface over which a web path is defined between a transducer and a web source having a fixed width web with opposite edges, a first web guide member between the transducer and the source, the guide partially defining said web path by said web being partially wrapped about said guide, the guide having an upright axis relative to the reference surface and having a web contact surface exceeding the width of the web and having a s~nmetrical arcuate profile, shallow enough so that web contact is maintained across the surface when the web travels past the guide but great ~2S~

enough to exert axial self-alignment forces on the part of the web in contact with said web contact surface of the guide, without edge guide contact with the web, a crown member with a symmetrical arcuate profile positioned between the transducer and the source, spaced from the first web guide me~ber, the member partially defining the web path proximate to said web guide by said web being partially wrapped about said member, said member having a width at least equal to the web and having an upright axis generally parallel to the web guide, said member adapted for axial motion, one of the pro:Eiles of the web guide and crown member being concave, the other convex whereby sa.id proximate first web guide urges a moving web toward said crown member in such a manner as to tend to positioll said member toward the center of its range o:E axial movement, thus stabilizing the axial motion of the member, without edge guide contact with the web, and whereby said cro~n member seeks an axial position such that the path lengths of the web edges from transducer to the source are substantially equal, and a 1anged edge guide partially defining the web path between said transducer and said crown member or said web guide whereby said flanged edge guide registers said web transversely with respect -to said transducer.
In accordance with another aspect of this invention, there is provid-ed in a cartridge of the type having a transducer, web source and take-up rolls mounted on a reference surface and a web path defined between the rolls and past the transducer, including a ~langed edge guide for web registration Wit}l respect to the transducer, the improvement comprising: a fixed web guide member aetween the transducer and the source, the guide partially de~ining said web path by said web being partially wrapped about said guide, thc guide having an upright axis relative to the reference surface and having a web contact surface exceeding the width o$ the web and having a s~lmetrical arcuate profile, shallow enollgh so that web contact is maintained across the surface when the web travels 32~

past the guide but great enough to exert vertical self-alignment forces on the part of the web in contact with said web contact surface of the guide, without edge guide contact with the web, a crown member with a symmetrical arcuate profile positioned between the transducer and the source, spaced from the first web guide member, the member partially defining the web path proximate to said web guide by said web being partially wrapped about said member, said member having a width at least equal to the web and having an upright axis generally parallel to the web guide, said member adapted for axial motion, one of the profiles of ~he web guide and crown member being concave, the other convex whereby said proximate first web guide urges a moving web toward said crown member in such a manner as to tend to position said member toward the center of its range of axial movement, thus stabilizing the axial motion of the member, without edge guide contact with the web and whereby said crown member seeks an axial position such that the path lengths of the web edges from transducer to the source are substantially equal.
This invention is described with reference to the following drawings:
Figure 1 is a top view of a tape cassette having a web path guided by the web guide system of the present invention.
Figure 2 is a side view of a concave fixed web guide of the present invention.
Figure 2a is a top view of the guide of Figure 2.
Figure 3 is a side view of a crown pulley of the present invention.
Figure 4 is a top view of the web guide system of the present in-vention, shown in a greatly enlarged detail view.
Figure 5 is a perspective view of a preferred embodiment of the web guide detail illustrated in Figure 4.
Figure 6 is a perspective view of another embodiment of the web guide ~2~

system illustrated in Flgure 4.
Referring now to the drawings, Figure 1 shows a sample tape cassette configuration especially suited for electronic data processing applications using magnetic tape. The invention is not restricted to magnetic tape, but is applicable to other webs being wound and unwound. Similarly, the invention is described relative to tape cassettes, but the invention is applicable to the other containers as well, or even to open reels mounted on supports.
A suitable cassette for use in this invention is described in prior United States Patent 4,172,569, assigned to the assignee of th:is invention. A
short description of thi.s cassette follows. While this casset~e is pre:Eerable because of high tape speeds which can be attained, cassettes, cartridges and other containers with other constructions may be used.
A cassette 10 includes a wall 14 with a head aperture or port 16, a rotary drive and belt guide member 18, and a tape guard 20, which is mounted for rotation about a pin 22 fixed to wall 14 in order to render the cartridge compatible with existing commercially available equipment.
First and second hubs 52, 54 act as tape supply and take-up hubs and are rotatably mounted with parallel axes of rotation and in a generally coplanar relationship on the lower wall 14 by means of a pair of axial shafts 56, 5~. A length of magnetic tape 50 having a nominally 0.0025 cm backing thickness, or thinner, the backing typically made of ~ylar, is distributed with-in the cassette 10, having one end portion or roll 60 wound on the first hub 52, an opposite end portion or roll 62 wound on the second hub 54, and an inter-mcdiate portion 64 extending between the hubs from a peripheral point 74 on tape roll 60 to a peripheral point 76 on tape roll 62. ~n operation the tape 50 passes between the rolls 60, 62 in a path parallel to the wall 14. The tape backing has a coating of magnetic material completely across the wid~h of the 3%~6~

tape, the width being typically 0.635 cm.
The intermediate portion 64 of the tape is trained over a pair of Eixcd tape cleaning guides 31, 33. The tape cleaning guides have an edge for removing oxide particles from the tape. Another pair of fixed guides 35, 37 may be provided for balancing tape tension. The fixed concave guides 41, 43 are positioned adjacent to the crown pulleys 45, 47, respectively. The concave guides and the crown pulley are described more fully below.
Two other fixed guides 49, 51 provide proper alignment of the tape 50 with the aperture 16 in order to maintain a proper operational relationship with a magnetic recor~ing head which may be part of a tape drive, not shown, approaching the tape at the magnetic head position 86 when in operative place-ment with the tape drive. A tape support pin 88 fixed to the lower wall 14 prevents the tape 50 from touching the drive and belt guide member lS whil0 the head is in operational contact with recording tape 50.
The drive member 18 is rotatably mounted on a shaft to the wall 14 at a centered frontal position on the cassette 10 so as to come into contact with a drive capstan 94 when the cassette 10 is in operative placement in a record/playback apparatus, not shown.
The pair of similar rotary belt guide members, or idlers 36, 38 are rotatably mounted on a pair of parallel shafts fixed to the lower wall 14. The idlers 36, 38, drive member 18 are all disposed on axes parallel to the axes of the hubs 52, 54 and are vertically aligned. A tensioning belt 46 is trained about the rotary drive member 18 and idlers 36, 38. Drive member 18 is engaged by capstan 94 for rotary motion.
An endless loop drive belt 11 of the present invention having a fixed length is trained about the drive member 18 and the idlers 36, 38. The tension-ing belt 46 is at a higher elevation than the drive belt 11 so as to clear the _ g ~

~z~

tape rolls.
The tape rolls 60 and 62 have planes defined by an extension of the tape center line radially outwardly from the rolls. By mowlting the axial shafts 56 and 58 perpendicular to wall 1~ and by placing the tape rolls at the same elevation, the planes defined by the outwardly extended center lines should be parallel to the reference wall 14. However, in assembling tape car-tridges, the axial shafts 56 and 58 may be slightly misaligned such that the planes defined by the center-lines of the tape rolls are not coplanar, nor are they normal to the engaging face of the head 86. This creates the misalignment problem previously clescribed. If the Inisalignment is not corrected, the tape will be vertically tilted with respect to the head. Such tilt implies that one edge of the tape delivered from such tilted roll is travelling at a greater velocity than the opposite edge of the tape. This causes transverse tension within the tapeJ as previously described.
The problem described above may be partially corrected by warping the tape, such as by passage over a crown pulley. However, a crown pulley does not provide adequate axial registration of the tape with respect to the head, and even a crown pulley which axially floats, combined with fixed tape edge guides, as described in United States Patent 3,393,894 will not be adequate to correct substantial misalignment. This is because the guides must surround the idler and the tape can drive the floating roller to an extreme position and then suffer edge damage in a crash against a wall or other surface.
In the present invention, we have discovered that the combination of a fixed concave guide in combination with a crown p~llley followed by a single edge guide removed from the pulley~region, will stabilize the crown pulley and allow proper compensation for tape roll plane misalignment and edge registration, each in independent and optlmum ~ashion.

- 10 r~

The tape guides 41 and 43 are identical and are illustrated and ex-plained wi~h reference to Figure 2. In Figure 2, the tape guide 41 is shown to be a generally upright cylinder having a lower threaded portion 51, a cen-tral concave portion 53 and an upper portion 55.
The threaded portion 51 extends into the lower wall of the cassette which may be by way of example approximately 0.254 cm in extent. The position of the concave guide 41 may be vertically adjusted by means of the amount the threaded portion 51 extends into a mating bore within wall 14. The overall length of the tape guide from one end to the other is 1.481 cm, and its major diameter is 0.318 cm. The central concave portion 53 is gently curved and has a dimension greater than the width of the tape which it guides. For example, tape section 64 is shown to be wrapped around the guide and centered within the concave portion. The arrows 57 and 59 indicate a dimension on either side of the tape which allows the tape to "walk" on the guide. The dimension indicated by the arrows 57 and 59 is approximately 0.064 cm, although a slightly greater dimension could be provided.
The web contact portion of the guide exceeds the width of the web in the range of 0.025 cm to 0.254 cm on each side of 0.635 cm webs and proportion-ate thereto for other web widths. The maximum depression of the guide is in the range 0.013 to 0.127 cm for 0.635 cm webs and proportionate thereto for other web widths. The preferred concave surface is one having a circular arc of radius R extending from an origin 0. Radius R has an extent of 1.109 cm.
Although a circular concave arc is preferred, other symmetric concave shapes can be used. The function of the concave shape is to cause tape warping and to complement the crown pulley effect. Such warping allows the edge tension from planar misalignment of the tape to be corrected by creating forces across the width of the tape wherein the tape is urged to a new lateral position.

The upper portion 55 of the tape guide has an extent so that the tape will not touch yoke 67. As a matter of convenience, the upper end 61 of section 55 is adapted with a groGve so that a screw driver can advance the guide into a mating bore. In the top view of Figure 2a the groove 63 in the guide 41 may be seen. Alignment of the guide with the head face can be accom-plished by supportillg end 55 in a hole in yoke 67.
The concave guide 41 is positioned so that tape will be wrapped around the guide su~ficiently so that the tape is warped. The desired angle of wrap depends upon tape width, idler crown radius and wrap angle. The tape should maintaill contact with the guide and crown pulley at all times without wrinkling, and the curvature of the guide and crown or their angles of wrap may be adjusted within that limit in order that the tape be subjected to a suffi-cient extent of corrective force. The extent of corrective force would depend upon the amount of expected tape roll misalignment, the tape thickness and its tension. In the above examp]e, a 19 tape wrap around the concave guide will effectively complement the edge-to~edge tension variation caused by a 32C tape wrap around a crown pulley with radius of curvature of 5.08 cm, without wrink-ling the tape. This will correct a full-roll edge-to-edge tilt of at least ~
0.0~4 cm while maintaining the crown pulley within its axial range. Tape which is misaligned in the concave section will experience a greater path length at an edge which is furthest from the center of the curved section. The same mis-alignment without the concave guide will cause that same tape edge to go a shorter distance to reach the head. Thus, as tape slides over the concave tape guide, the two edges tend to travel the same distance, leaving only "fine"
correction to the crown pulley of ~igure 3. With reference to Figure 3, the pulley 45 is shown, which is identical to pulley 47 in Figure 1. The function of the pulley is to change the tape direction in an angle approximately equal to 12 _ go as well as assisting in providing tape alignment by making a final ad-justment in the tape-edge path lengths. Pulley 45 is made of a non-resilient material, such as a hard plastic and has a central bore allowing the axiai shaft 65 retained in yoke 67 to pass therethrough, or may be mounted on a live shaft which rotates in bearings in the yoke. Pulley 45 is free to move axially as well as to rotate freely. The crown pulley 45 has a convex shaped outer surface over which ~ape 64 will tend to be centered over the highest point of the roller. In the preferred embodiment, the contour of the arc may complement the radius of the arc of the concave guide of Figure 2, but is not limited to this curvature. Since the crown pulley 45 can move axially, the pulley can chase a web which is out of alignment, as the web delivered from the roll moves transversely. However, crown pulley 45 will tend to be stabilized at a central position on shaft 65 by the concave tape guide 41.
Yoke 67 is mounted to wall 14 adjacent to the concave tape guide 41.
The concave warping o~ the tape provided by the fixed guide 41 is immediately countered by convex warping provided by the crown pulley 45. In each instance the tape center line will be adjusted relative to the extended center line of the tape rolls, but neither the concave tape guide, nor the crown pulley, by itself, will provide adequate or stable correction. However, when both work to-gether, a new result is achieved and tape is shi-fted exactly to a desired elevation without distortion or substantial edge pressure. Once at a desired elevation, fixed guides, with flanges, such as guides 49 and 51 feed the tape in a tape span which faces a transducer at the head position 86.
~ile the convex crown pulley of this patent application has been shown to have an arc shape, it will be appreciated that trapezoidal crowns could also be used. In the event that a convex crown pulley is used, then the concave fixed guide should also be trapezoidal, with a trapezoidal shape which complements the trapezoidal shape of the convex pulley.

, ~ 13 -With reference to Figure 4J a tape span 64 is shol~m to be partially wrapped about tape cleaning guide 33 which guides tape toward the fixed guide 43. Tape 64 is partially wrapped about this guide and is then directed to the crown pulley 47 where the direction of the tape is substantially changed. Last-ly, the tape passes between the flanges of the fixed guide 51 where the tape is provided with a proper vertical alignment with respect to a magnetic head.
Each of the guides, as well as the crown pulley, is mounted with a parallel axis perpendicular to base 1~. In cassette systems, opposed ends of the guide and pulley may be mounted in opposite cassette walls. Alternatively, the yoke 67 may be used to provide support for a free end of each oE the guides and the pulley shaft.
~lith reference to Figure 5, tape 64 is shown passing through a pre-ferred embodiment of the web guide system of the present invention. The tape cleaning guide 33 is optional and is used not only to clean tape, but to feed tape at a desired angle to the fixed concave guide 43. From there the tape passes to the crown member 47 which floats axially above base 14. From there, tape is guided to the fixed Elange edge guide 51, as previously described.
Ir most tape cassettes, two tape guiding systems of the type illus-trated in Figure 5 or 6 would be used, one on either side of the head. Ilowever, in other web devices, it may be desirable to guide the web only in one direction.
In such an instance, only one web guiding system of the present invention would be used.
Figure 6 is similar to Figure 5, except that the concave-convex relationship between the fixed guide 43 and the crown member 47 of Figure 5 is interchanged. In ~igure 6, the Eixed web guide '1!3' is convex and feeds tape 64 to a concave crown member 47'. The stabilization between the crown member 47' and the fixed guide 43' is essentially the same as previously described. It 25~
should be noted that the flanged edge guide 51 provides transverse registration of the web with respect to a transducer, such as magnetic head 86 of Figure 1.
In other words, the flanged edge guide 51 sets the eleva-tion of the tape with respect to a transducer. While a fixed guide is shown to be to the right of the crown member in Figures 5 and 6, it could also be on the left side. The fixed guide may precede the crown member, or vice-versa.
In this patent application, the crown member 47 has been described as a pulley. Tt will be realized by those skilled in the art that axial motion o member 47 is the principal component of its motion, not rotary motion. There-fore, if rotary motion is prevented, such as by keying member 47 so that it may slide axially, but not rotate, the crown member is still within the spirit and scope of the present invention. Crown member 47 may be convex or concave as selected. The curvature of the crown must complement the curvature of the web guide.
We have found it practical to guide webs as thin as 0.0005 cm in the cartridge described at speeds as high as 240 ips in such a guide system, with edge track bit-packing densities as high as 6~00 bpi without error after over 100,000 passes.
While this invention has been described with re-ference to a tape cartridge, it is applicable to other web guiding systems where close web align-ment is needed.

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A web guide system comprising, a horizontal reference surface over which a web path is defined be-tween a transducer and a web source having a fixed width web with opposite edges, a first web guide member between the transducer and the source, the guide partially defining said web path by said web being partially wrapped about said guide, the guide having an upright axis relative to the reference surface and having a web contact surface exceeding the width of the web and having a symmetrical arcuate profile, shallow enough so that web contact is maintained across the surface when the web travels past the guide but great enough to exert axial self-alignment forces on the part of the web in contact with said web contact surface of the guide, without edge guide contact with the web, a crown member with a symmetrical arcuate profile positioned between the transducer and the source, spaced from the first web guide member, the mem-ber partially defining the web path proximate to said web guide by said web being partially wrapped about said member, said member having a width at least equal to the web and having an upright axis generally parallel to the web guide, said member adapted for axial motion, one of the profiles of the web guide and crown member being concave, the other convex whereby said proximate first web guide urges a moving web toward said crown member in such a manner as to tend to position said member toward the center of its range of axial movement, thus stabilizing the axial motion of the member, without edge guide contact with the web, and whereby said crown member seeks an axial position such that the path lengths of the web edges from transducer to the source are substantially equal, and a flanged edge guide partially defining the web path between said transducer and said crown member or said web guide whereby said flanged edge guide registers said web transversely with respect to said transducer.

2. The web guide system of Claim 1 wherein said crown member has a crown of radius small enough so that web contact is just maintained across its width.

3. The web guide system of Claim 1 wherein the wrap angle and profile of the web guide substantially complements the effect of the curvature and wrap angle of the crown member upon transverse tension across the width of the web.

4. The web guide system of Claim 3 wherein the said complementing effect of the wrap angle and profile of the concave web guide is substantially complete at the edges and the center-line of the web.

5. The web guide system of Claim 1 wherein said crown member has a width greater than the tape by an amount at least equal to its range of axial motion.

6. The web guide system of Claim 1 wherein said concave web guide pre-cedes said crown member.

7. The web guide system of Claim 1 wherein said crown member precedes said concave web guide.

8. The web guide system of Claim 1 wherein said web contact portion of the web guide exceeds the width of the web in the range of 0.025 cm to 0.254 cm on each side of the tape.

9. The web guide system of Claim 1 wherein said maximum depression of said profile of the concave web guide is in the range of 0.013 to 0.127 cm for 0.635 cm webs, and proportionate thereto for other web widths.

10. The web guide system of Claim 1 wherein said crown member exceeds the width of the web in the range of 0.025 to 0.254 cm for 0.635 cm webs, and proportionate thereto for other web widths.

11. The web guide system of Claim 1 wherein said crown member is a crown pulley.

12. The web guide system of Claim 1 wherein said crown member has a con-vex profile and said web guide has a concave profile.

13. In a cartridge of the type having a transducer, web source and take-up rolls mounted on a reference surface and a web path defined between the rolls and past the transducer, including a flanged edge guide for web registration with respect to the transducer, the improvement comprising:
a fixed web guide member between the transducer and the source, the guide partially defining said web path by said web being partially wrapped about said guide, the guide having an upright axis relative to the reference surface and having a web contact surface exceeding the width of the web and having a symmetrical arcuate profile, shallow enough so that web contact is maintained across the surface when the web travels past the guide but great enough to exert vertical self-alignment forces on the part of the web in contact with said web contact surface of the guide, without edge guide contact with the web, a crown member with a symmetrical arcuate profile positioned between the transducer and the source, spaced from the first web guide member, the member partially defining the web path proximate to said web guide by said web being partially wrapped about said member, said member having a width at least equal to the web and having an upright axis generally parallel to the web guide, said member adapted for axial motion, one of the profiles of the web guide and crown member being concave, the other convex whereby said proximate first web guide urges a moving web toward said crown member in such a manner as to tend to position said member toward the center of its range of axial movement, thus stabilizing the axial motion of the member, without edge guide contact with the web and whereby said crown member seeks an axial position such that the path lengths of the web edges from transducer to the source are substantially equal.