GB2117354A - Magnetic tape guiding system - Google Patents

Abstract

A tape guide system wherein misalignment of take-up and supply rolls 60, 62 relative to a reference surface may be compensated without tape distortion comprises a rotatable and axially movable roller, 47 an adjacent, fixed, flangeless tape guide, 43, one of the roller or the guide having a convex tape engaging surface and the other having a concave tape engaging surface, and a fixed flanged guide 51. Tape 64 tends to be self-centring in the concave curved portion of the guide 43, and the axial motion of the roller promotes equal edge tension and completes web self-alignment. Final axial alignment without edge pressure is accomplished by a fixed tape edge- guide 51. The guide system may be included within a tape cassette of the kind in which the supply roll 60 and the take-up roll 62 are driven by an endless drive belt 11 which contacts their peripheries. <IMAGE>

Description

SPECIFICATION Web aligning system This invention relates to web aligning 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 registration 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, 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 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.

Even 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. The 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 U.S. Patent 4 191 345. Furthermore, U.S.

Patent 3 474 981 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 winding. However, the lateral registration is inexact, providing poor "tracking" of the head on the tape between the recording and reproduction processes.

An alternate guide approach is to use rotating cylindrical rollers, rather than fixed guides, in combination with rotating or fixed web edgeguides. One of the disadvantages of a rotating guide system is that once a web is wrapped around an 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. With either fixed or rotating edge-guides, edge drag may cause long-term and short-term variations in edge velocities with respect to the web centre line, known as static and dynamic skew, respectively. Static skew may be minimized by use of rotating flanges.However, scraping of such a flange against 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 surface, 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 highspeed application, such as those involved with digital computers and instrumentation type of analog recorders, such misregistrations, even though small, limit the number of tracks and bitpacking densities that may be recorded on the tape. Moreover, even slight misalignments can cause uneven tape tension laterally across the width since the velocity of one tape edge is different from the velocity of the other tape edge.

The lateral unevenness in tape tension may cause tape edge wear and perhaps flutter and skew, as mentioned above.

An attempt to minimize the above-described transverse tension and edge force problems is taught by H. E. Haas in U.S. 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 failing that, must be parallel with both planes of the tape-rolls and the crown pulley or the tape will be forced to warp as its centre line departs from the tilted roll plane to the idler plane which is normal to the head face.

The second 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 pulley to float between them.

In order to float, the pulley is shown to be narrower than the tape, leaving the tape overhanging the pulley edges with no edge tension or stiffening support. Such an edge will be readily damaged by the application of edge forces. Further, if in attempting to correct planar misalignment, the pulley seeks a position too far off the centre line of the tape, the tape edge with the largest overhang will be drawn toward the pulley centre 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, vhe crown pulley must generate substantially greater stretch i.e. tension, at the centre line than at the edges of the web. This will cause the pressure between head and tape to vary greatly from edges to centre, causing excessive susceptibility to data dropouts at the edge tracks. A 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.

An object of the present invention is to provide 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 premature edge wear.

With these objects in view, the present invention provides 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 fixed web guide between the transducer and the source, which guide partially defines 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, said 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 web guide urges a moving web toward said crown member in such a manner as to tend to position said member toward the centre 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 the 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.

Preferably the web guide system has a combination of at least one fixed concave web guide feeding the web to an axially moving convex crown pulley, followed by an edge guide.

The concave guide serves two important functions. The first is to stabilize the axial position of an axially floating crown pulley without imposing web edge restraint in the region of the pulley. This allows vertical registration of the web to be accomplished after, and only after, the web has passed around the pulley, where its centre line can be aligned with the plane normal to the head face without web distortion and also allows the crown pulley to be wider than the web, to give it continuous edge-to-edge support. The second important function is to equalize net transverse web 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 provide the guiding through the plane changes into proper registration with the head, without tape distortion. However, with the concave guide providing stability and edge tension equalization, the crown pulley will correct for web 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 transducer. In another embodiment a convex web guide and a concave crown pulley are used.

This enables misalignment compensation from either web roll, since either can be the supply or take-up roll in many web systems.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a plan view of a tape cassette having a web path guided by an embodiment of the web guide system of the present invention; Fig. 2 is a side view of a concave fixed web guide of a preferred embodiment of the present invention; Fig. 2a is a plan view of the guide shown in Fig. 2; Fig. 3 is a side view of a crown pulley of the preferred embodiment of the invention; Fig. 4 is a plan view of the preferred embodiment of the web guide system of the present invention, shown in greatly enlarged detail; Fig. 5 is a perspective view of the web guide detail illustrated in Fig. 4; Fig. 6 is a perspective view of another embodiment of the web guide system of the invention.

Referring now to the drawings, Fig. 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 other containers as well, or even to open reels mounted on supports.

A suitable cassette for use in this invention is described in prior U.S. Patent 4 172 569 assigned to the assignee of this invention. A short description of this cassette follows. While this cassette is preferable because high tape speeds can be attained, cassettes, cartridges and other containers with other constructions may be used.

A cassette 10 includes a horizontal reference surface in the form of 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 the wall 14 in order to render the cassette 10 compatible with existing commercially available equipment.

First and second hubs 52 and 54 act as tape supply and take-up hubs respectively 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. 58. A length of magnetic tape 50 having, for example, backing thickness of 0.0025 cm or less, the backing typically made of Mylar, is distributed within 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 intermediate portion 64 extending between the hubs 52 and 54 from a peripheral point 74 on the tape roll 60 to a peripheral point 76 on the tape roll 62. In 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 across the entire width of the tape 50, the width being typically 0.635 cm.

The intermediate portion 64 of the tape 50 is trained over a pair of fixed tape cleaning guides 31 and 33. The tape cleaning guides 31 and 33 each have an edge for removing oxide particles from the tape 50. Another pair of fixed guides (not shown) may be provided for balancing tape tension. A pair of web guides in the form of fixed concave guides 41 and 43 are positioned adjacent to a pair of crown members in the form of crown pulleys 45, 47, respectively. The concave guides 41 and 43 and the crown pulleys 45 and 47 are described more fully below.

Two other flanged edge guides in the form of fixed guides 49 and 51 provide proper alignment of the tape 50 with the aperture 1 6 in order to maintain a proper operational relationship with a magnetic recording head which may be part of a tape drive (not shown). Such a magnetic recording head would approach the tape 50 at a magnetic head position 86 when the cassette 10 was in operative placement 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 18 while the magnetic recording head is in operational contact with the recording tape 50.

The drive and belt guide member 18 is rotatably mounted on a shaft extending from the wall 14 at a central position at the front of 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).

A pair of similar rotary belt guide members, or idlers 36 and 38 are rotatably mounted on a pair of parallel shafts fixed to the lower wall 14. The idlers 36 and 38 and the rotary drive and belt guide 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 and belt guide member 18 and the idlers 36 and 38. The rotary drive and belt guide member 18 is engaged by the capstan 94 for rotary motion.

An endless loop drive belt 11 of the present invention having a fixed length is also trained about the rotary drive and belt guide member 18 and the idlers 36 and 38. The tensioning belt 46 is at a higher elevation than the drive belt 11 so as to clear the tape rolls 60 and 62.

The tape rolls 60 and 62 have planes defined by outward radial extension of the tape centre line from the rolls. By mounting the axial shafts 56 and 58 perpendicular to the wall 14 and by placing the tape rolls 60 and 62 at equal elevation, the planes defined by the outwardly extended centre lines should be parallel to the reference wall 14. However, in assembling tape cassettes, the axial shafts 56 and 58 may be slightly misaligned such that the planes defined by the centre lines of the tape rolls 60 and 62 are not coplanar, nor are they normal to the engaging face of the magnetic recording head positioned at 86. This creates the misalignment problem previously described. If the misalignment is not corrected, the tape 50 will be vertically tilted with respect to the head.Such tilt implies that one edge of the tape 50 delivered from such tilted roll is travelling at a greater velocity than the opposite edge of the tape 50. This causes transverse tension within the tape 50, as previously described.

The problem described above may be partially corrected by warping the tape 50, for example by passage over a crown pulley. However, a crown pulley does not provide adequate axial registration of the tape 50 with respect to the head, and even a crown pulley which axially floats, combined with fixed tape edge guides, as described in U.S. Patent 3 393 894 will not be adequate to correct substantial misalignment.

This is because the guides must surround the idlers 36 and 38 and the tape 50 may drive the floating pulley 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 41, 43 in combination with a crown pulley 45,47 followed by a single edge guide 49, 51 removed from the pulley region, will stabilize the crown pulley 45, 47 and allow proper compensation for tape roll plane misalignment and edge registration, each in independent and optimum fashion.

The tape guides 41 and 43 are identical and are illustrated and explained with reference to Fig.

2. In Fig. 2 the tape guide 41 is shown to be a generally upright cylinder having a lower threaded portion 40, a central concave portion 53 and an upper portion 55.

The threaded portion 40 extends into the lower wall 14 of the cassette which may be, for 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 40 extends into a mating bore within the wall 14. The overall length of the tape guide 41 from one end to the other is approximately 1.481 cm, and its maximum diameter is approximately 0.318 cm. The central concave portion 53 is gently curved and is longer than the width of the tape 50 which it guides. For example, tape section 64 is shown to be wrapped around the guide 41 and centred within the concave portion 53. The arrows 57 and 59 indicate a dimension on either side of the tape which allows the tape section 64 to "walk" on the guide 41.The dimensions indicated by the arrows 57 and 59 are each approximately 0.064 cm, although a slightly greater dimension could be provided.

The web contact portion of a concave guide according to the present invention 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 proportionate thereto for other web widths. The maximum depression of the guide is in the range of 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 41 has an extent so that the tape section 64 will not touch a yoke 67. As a matter of convenience, the upper end 61 of section 55 is adapted with a groove 63 so that a screw driver may be used to advancd the guide 41 into a mating bore. In the plan view, Fig. 2a, the groove 63 in the guide 41 is clearly illustrated. Alignment of the guide 41 with the head face may be accomplished by supporting end 55 in a hole in the yoke 67.

The concave guide 41 is positioned so that tape section 64 will be wrapped around the guide sufficiently so that the tape 50 is warped. The desired angle of warp depends upon tape width, idler crown radius and warp angle. The tape section 64 should maintain contact with the guide 41 and the crown pulley 45 at all times times without wrinkling, and the curvature of the guide 41 and the crown pulley 45 or their angles of wrap may be adjusted within that limit in order that the tape section 64 be subjected to a sufficient 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 example, a 70 tape wrap around the concave guide 41 will effectively complement the edge-to-edge tension variation caused by a 320 tape wrap around the crown pulley 45 with a radius of curvature of 5.08 cm, without wrinkling the tape section 64. This will correct a full-roll edge-to-edge tilt of at least +0.064 cm while maintaining the crown pulley 45 within its axial range. Tape which is misaligned in the concave section 53 will experience a greater path length at an edge which is furthest from the centre of the concave section 53. The same misalignment without the concave guide 41 will cause that same tape edge to go a shorter distance to reach the head. Thus, as the tape section 64 slides over the concave tape guide 41, the two edges tend to travel the same distance, leaving only "fine" correction to the crown pulley 45, shown in Fig. 3.Fig. 3 shows the crown pulley 45 which is identical to the crown pulley 47. Both pulleys 45 and 47 are shown in Fig. 1. The function of each pulley 45 and 47 is to change the tape direction in an angle approximately equal to 320, as well as assisting in providing the tape alignment by making a final adjustment ill the tape-edge path lengths. The crown pulley 45 is made of a non-resilient material, such as a hard plastics material and has a central bore allowing an axial shaft 65 retained in the yoke 67 to pass there-through.

Alternatively, the crown pulley 45 may be mounted on a live shaft which rotates in bearings in the yoke 67. The crown 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 the tape section 64 will tend to be centred over the highest point of the surface. In the preferred embodiment, the contour of the arc of the convex surface complements the radius of the arc of the concave guide 41 of Fig. 2. However, in further embodiments the convex surface is not limited to this curvature. Since the crown pulley 45 can move axially, said pulley 45 can chase tape section 64 which is out of alignment, as the tape section 64 delivered from the roll 60 moves transversely. However, the crown pulley 45 will tend to be stabilized at a central position on the shaft 65 by the concave tape guide 41.

The yoke 67 is mounted on the wall 14 adjacent to the concave tape guide 41. The concave warping of the tape section 64 provided by the fixed concave guide 41 is immediately countered by convex warping provided by the crown pulley 45. In each instance the tape centre line will be adjusted relative to the extended centre line of the tape rolls 60 and 62 but neither the concave tape guide 41, nor the crown pulley 45 by itself, will provide adequate or stable correction. However, when both work together, a new result is achieved and the tape section 64 is shifted exactly to a desired elevation without distortion or substantial edge pressure. Once at a desired elevation the fixed guides 49 and 51 which have flanges feed the tape section 64 in a tape span which faces a transducer at the head position 86.

While the convex crown pulley of this patent application has been described as having an arc shape, it will be appreciated that a trapezoidal crown pulley could also be used. In the event that a trapezoidal convex crown pulley is used, the concave fixed guide should also have a trapezoidal shape which complements the trapezoidal shape of the convex pulley.

With reference to Fig. 4, the tape section 64 is shown to be partially wrapped about the tape cleaning guide 33 which guides tape toward the fixed concave guide 43. The tape section 64 is partially wrapped about this guide 43 and is then directed to the crown pulley 47 where the direction of the tape is substantially changed.

Lastly, 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 33, 43 and 51, as well as the crown pulley 47, is mounted with a parallel axis perpendicular to the wall 14. In cassette systems, opposed ends of the guides and pulleys may be mounted in opposite cassette walls.

Alternatively, the yoke 67 may be used to provide support for a free end of each of the guides 33, 43 and 51 and the pulley shaft, as indicated in Fig. 4.

With reference to Fig. 5, the tape section 64 is shown passing through a preferred embodiment of the web guide system of the present invention.

The tape cieaning guide 33 is optional and is used not cnly to clean the tape, but also to feed the tape 64 at a desired angle to the fixed concave guide 43. From there the tape 64 passes to the crown pulley 47 which floats axially above the wall 14. From there, the tape 64 is guided to the fixed flange edge guide 51, as previously described.

In most tape cassettes, two tape guiding systems of the type illustrated in Fig. 5 or 6 would be used, one on either side of the head, as shown in Fig. 1. However, 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. Fig. 6 is similar to Fig. 5, except that the concave-convex relationship between the fixed guide 43 and the crown pulley 47 of Fig. 5 is interchanged. In Fig.

6, the fixed web guide 43' is convex and feeds the tape 64 to a concave crown pulley 47'. The stabilization between the crown pulley 47' and the fixed guide 43' is essentially the same as previously described. It should be noted that the flanged edge guide 51 provides transverse registration of the tape 64 with respect to a transducer, such as a magnetic head at position 86 of Fig. 1. In other words, the flanged edge guide 51 sets the elevation of the tape 64 with respect to the transducer. While fixed guides 43, 43' are shown to be to the right of the crown pulleys 47, 47' in Figs. 5 and 6, in other embodiments they could also be on the left side.

In other words the fixed guide may precede the crown pulley or vice-versa.

In this patent application, the crown member has been described as a pulley 47, 47'. It will be realized by those skilled in the art that axial motion of the crown member is the principal component of its motion, not rotary motion.

Therefore, if rotary motion is prevented, such as by keying the crown member so that it may slide axially, but not rotate, the crown member is still within the scope of the present invention. The crown member may be convex or concave as selected. The curvature of the crown member must complement the curvature of the fixed web guide.

We have found it practical to guide webs as thin as 0.0005 cm in the cassette described at speeds as high as 240 ips (approx 600 cmps) in such a guide system, with edge track bit-packing densities as high as 6400 bpi (approx. 16,000 bpcm) without error after over 100,000 passes.

While this invention has been described with reference to a tape cassette it is applicable to other web guiding systems where close web alignment is needed.

Claims (14)

Claims

1. 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 fixed web guide between the transducer and the source, which guide partially defines said web path by said path 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 enought 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, said 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 web guide urges a moving web toward said crown member in such a manner as to tend to position said member toward the centre 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 the 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. A web guide system as claimed in claim 1 wherein said crown member has a crown of radius small enough so that web contact is just maintained across its width.

3. A web guide system as claimed in claim 1 or 2, wherein the profile of the web guide and the angle at which the web is wrapped about said web guide, i.e. the wrap angle, substantially complement the effect of the curvature and wrap angle of the crown member upon transverse tension across the width of the web.

4. A web guide system as claimed in claim 3 wherein said complementing effect of the wrap angle and profile of the web guide is substantailly complete at the edges and the centre-line of the web.

5. A web guide system as claimed in any preceding claim wherein said crown member has a width greater than the web by an amount at least equal to its range of axial motion.

6. A web guide system as claimed in any preceding claim wherein said web guide precedes said crown member.

7. A web guide system as claimed in any claims 1 to 5 wherein said crown member precedes said web guide.

8. A web guide system as claimed in any preceding claim wherein said web contact surface 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 web.

9. A web guide system as claimed in any preceding claim wherein the web guide has a concave profile and the crown has a convex profile.

10. A web guide system as claimed in claim 9 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.

11. A web guide system as claimed in any preceding claim 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.

12. A web guide system as claimed in any preceding claim wherein said crown member is a crown pulley.

13. In a cartridge of the type having a transducer, a web source roll and a take-up roll 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, an improved web guiding system comprising a fixed web guide between the transducer and the source, which guide partially defines 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, said 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 web guide urges a moving web toward said crown member in such a manner as to tend to position said member toward the centre 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.

14. A web guide system substantially as hereinbefore described with reference to and as illustrated by Figs. 1 to 5 or Fig. 6.