Post for guiding tape

Abstract: A magnetic tape drive 20 includes a tape path 21 having first and second reels 22 and 23 for a tape 1, a transducer head 24 and guides 10a, 10b and 10c for the tape. The guides 10a, 10b and 10c are pivotally mounted on axles 15a, 15b and 15c and the operation of the drive is controlled by a controller 25. Fach of the guides is a constrained pivot compound radius tape post. The post is mounted on an axle which allows it to pivot when the tape changes direction. The pivoting action serves to peel away all tape which has been in stationary contact with the post, thereby reducing startup stiction. Above a certain pivotation angle the post is prevented from further pivotation either by a fixed stop or by torque exerted on the post by the tape which is under tension.

Description: This invention relates to posts for guiding tape in a magnetic tape drive. Web and tape guiding apparatus are well known. Perhaps the most common use of such an apparatus is for guiding tapes in magnetic tape drives over a magnetic read/write head. Guiding of the tape over the head is critical to the writing of data to and the reading of data from the magnetic tape. A typical magnetic tape drive includes several elements in the tape path to ensure proper alignment and operation of the tape and head. For example, the tape path of the IBM 3480/3490 tape drives includes a supply reel inside a tape cartridge, a vibration decoupler, a cleaner blade, an arcuate supply side air bearing device, a magnetic read/write head, an arcuate storage side air bearing device, a tension transducer, a storage reel, and tape edge guides are located along the arcuate bearing surfaces because the tape in such proximity can support a larger guiding force without collapsing than can a freely suspended tape. The tape edge guides physically align the lateral position (i.e. in the direction of the tape width) of the tape relative to the read/write head. Lateral positioning maintains proper alignment between the read/write elements of the head and the data tracks on the magnetic tape. The tape guides also support and guide the tape in its easy direction of bending (i.e. in the direction of tape travel). US-A-4575023, used as the basis of the pre-characterising part of claim 1, and FR-A-2224830 disclose guides for audio cassettes designed to stop tape loops forming. In US 4575023 the guide comprises a cylindrical barrel plus protrusion, the protrusion engaging the tape to prevent loop formation. In FR-A-2224830 the guide is mounted on an axis for rotation, the orientation of the guide being determined by the relative torque exerted by the tape on either side of the axis, and slowly changing to accommodate the variation in tape trajectory as the tape empties or fills one reel. GB-A-1002180 describes a tape guide having a continuously changing curvature to avoid any points of inflexion in the tape path, thereby reducing vibrations in the tape. Three basic types of guides are known for easy direction tape guiding. The first such guide is a fixed cylindrical post. The use of this type of guide is based on the theory that a hydrodynamic film of air forms between the tape and the post when the tape is in motion, thereby reducing friction and wear. However, in practice, such posts fail to produce an air film of sufficient thickness to reduce friction and wear to a significant extent. Also, when the tape is stopped and rests on the post under tension it sticks to the post. Such "stiction" makes the initiation of tape movement difficult and may result in damage to the tape. Another type of easy direction tape guide is a roller. As compared to posts, rollers reduce friction with a tape in motion and stiction with a tape at rest. However, rollers introduce another source of vibration into the tape path (which may disrupt the head-tape interface). Also, rollers store energy that must be positively controlled to maintain adequate closed loop tension control. Finally, rollers and posts do not steer tape in the same manner, and rollers often require grooves or other surface geometries to prevent the tape from flying over them. The third type of easy direction tape guide is an externally pressurised air bearing. Although an air bearing can dramatically reduce tape friction and stiction, it is more complex and expensive than a post or a roller. An air bearing is complex because it requires the space and parts for air to be provided under controlled pressure. An air bearing is expensive because of such complexity, and because a source of compressed air must be provided. Finally, more than one type of easy direction guide can be used in a tape path to form a variety of tape path configurations. US-A-4,276,575 discloses the use of a plastics lubricant on a tape guide to reduce friction. However, none of these guides is a simple, low cost, low friction/stiction tape guide which also minimises vibration and accommodates changes in tape direction (i.e. easy direction bending). This invention seeks to provide an to improved easy direction tape guide. This invention also seeks to reduce friction, stiction, and vibration in an easy direction tape guide. This invention further seeks to provide such an improved guide at a minimum of additional expense and complexity, yet accommodating changes in tape direction. Accordingly this invention provides a guide for a flexible tape comprising an axle and a post which is allowed to pivot about the axle but which is constrained from completing a full revolution, the post including a surface to engage the tape, whereby in operation, a change in direction of said tape causes the guide to pivot between a first position and a second position, and characterised by the post surface having a longitudinal contour with a compound radius of curvature having first and second portions with a relatively large radius of curvature and a third portion with a relatively small radius of curvature between said first and second portions, such that when the tape changes direction at least a portion of the tape disengages from the guide by a peeling action. Thus the constrained pivot includes a compound radius post, mounted on an axle which allows it to pivot when the tape changes direction, but which is constrained from a full revolution. The pivoting action serves to peel away all tape which has been in stationary contact with the post, thereby reducing startup stiction. Beyond a certain pivot angle, the post is prevented from further pivoting either by a fixed stop or by torque exerted on the post by the tape which is under tension. The post includes a tape engaging surface with a compound radius of curvature. The longitudinal contour reduces friction by maximising the thickness of the air film between the tape and the post. The term "tape" in this specification is to be taken as including webs and the like, capable of longitudinal translation. The scope of the invention is defined by the appended claims; and how it can be carried into effect is hereinafter particularly described with reference to the accompanying drawings in which:- Figs. 1a and 1b are plan views of a tape guide according to the invention in different positions; Fig. 2 is a view similar to Fig. la of an alternative embodiment of guide; Fig. 3 is a lateral cross-section of the tape guide of Fig. la; Fig. 3b is a view similar to Fig. 3a of an alternative embodiment guide; Fig. 4 is a schematic diagram of a magnetic tape drive incorporating guides according to the invention. In the application of the invention to a magnetic tape drive, a magnetic tape 1 (Figs. 1a and 1b) is bent in its easy direction around a tape guide 10. Guide 10 is a post mounted to pivot about an axle 15 and including a tape engaging surface 11. The guide 10 is not a conventional cylindrical post, but has a radius of curvature that changes along the length of tape engaging surface 11. Tape 1 may travel in either direction. In Fig. 1a, tape 1 is moving clockwise in the direction shown by the arrow at the end thereof. In Fig. 1b, tape 1 is moving counterclockwise in the direction shown by the arrow at the end thereof. Otherwise, the tape 1 may be at rest. It is known that the hydrodynamic film of air which separates a tape from a tape engaging surface is largely determined by the curvature at the entrance to the tape engaging surface, and is thereafter relatively constant along its length. The entrance to a guide is the portion of the guide which is first approached by a tape as it moves past the guide. For example, in Fig. 1A the entrance to guide 10 is at the lower left portion of tape engaging surface 11. For a cylinder of radius R, the thickness t of the uniform air film is given approximately by: t = .643R(6uU/T)2/3 where u is the viscosity of air, U is the tape velocity, and T is the tape tension per unit width. For further description of equation (1), see Gross et al, Fluid Film Lubrication, John Wiley and Sons, Inc., NY, 1980, p. 493. As used herein, tape 1 and guide 10 are in "close proximity" when they are at least close enough to apply equation (1). In view of equation (1), it is therefore known that the air film between a tape and a guide can be made very large by increasing R. It is also known that a thick air film can reduce contact between a tape and a guide, thereby reducing friction. Thus, a simple cylindrical guide with a very large radius of curvature would achieve a thick air film and low friction. However, at least three other factors must be considered. First, a large radius of curvature is incompatible with the trend towards small form factor tape drives. Second, if the tape is laterally guided by exerting force on its edges, a small radius of curvature is required to prevent the tape from buckling. Third, when there is no tape motion the tape will rest against the guide. The larger the radius of curvature the more contact there will be between the tape and the tape engaging surface, resulting in increased stiction. Friction and stiction are minimised in the guide 10 (Figs. 1a and 1b) by tape engaging surface 11 having a longitudinal contour (i.e. along the direction of tape movement) with a compound radius of curvature. In the preferred embodiment shown, the longitudinal contour includes five adjacent portions. Two outer portions have relatively large radii 12 and 14 of curvature and therefore establish a thick air film that will lubricate the other portions. The central portion is of a relatively small radius 13 of curvature to allow for any desired bending of the tape in its easy direction. Two blending portions join the outer portions with the central portion to provide a smooth transition between the distinct radii. The blending portions minimise adverse effects of tape stiffness by ensuring a continuous second derivative along the contour. This also aids in maintaining the large air film established through the central portion. There is a variety of ways to accomplish the transition between the outer and central portions. They can be joined for example by an elliptical contour, or a contour whose radius of curvature changes smoothly as a function of distance along the bearing, or simply by a polishing process that removes the discontinuity between the curvatures. In the preferred embodiment, the blending portion is described by: 1 R(s) = ksx + 1 R1 where s is the distance along the contour, R(s) is the radius of curvature at any point along the contour, R1 is the radius of curvature in the outer portion, 0.5 < X <2 and k is a constant. Magnetic tape heads having a compound radius of contour are known, from US-A-4,479,158, but are designed to achieve a very low flying height of the tape (or actual contact of the tape with the head) to maintain the signal-to-noise ratio and are thus not designed as previously described. Such heads use outer portions of a small radius of curvature to achieve a very low flying height. In the preferred embodiment, radius 12 of curvature is greater than the radius 13 of curvature and approximately the same as radius 14 of curvature to accommodate tape travel in both directions equally. Preferably, radii 12 and 14 of curvature are at least five times greater than radius 13 of curvature. A suitable guide has radii 12 and 14 of curvature of approximately 20.0 mm and a radius 13 of curvature of approximately 3.2 mm for a tape approximately 8.0 mm wide and 12 µm (microns) thick. The central portion of tape engaging surface 11 having radius 13 of curvature extends over an arc of approximately 60 degrees, and the outer portions of surface 11 having radii 12 and 14 of curvature extend over arcs of up to 50 degrees. The entrances to guide 10 provided by the outer portions of tape engaging surface 11 having radii 12 and 14 of curvature should have an arc of at least 6 degrees to ensure that equation (1) applies. The blending portions are created using surface polishing. The optimal radii of curvature, arcs (i.e. length of each portion), and other dimensions of guide 10 depend upon the actual application of the guide. Thus, in alternative embodiments, the longitudinal contour of guide 10 may be changed to accommodate particular needs. For example, radius 12 of curvature may differ from radius 14 of curvature in applications in which the tape travels in a single direction only. Also, material can be removed from areas of the post which do not contact the tape bearing surface, to reduce the mass and inertia of the post. Guide 10 is mounted to pivot about an axle 15 which is a simple shaft that is held at the base and protrudes through the compound-contour post. The post pivots on the axle by virtue of the slip-fit between the axle diameter and a throughhole in the post. The pivoting action occurs as follows: Assume that guide 10 is at rest in the position shown in Fig. 1a after tape 1 has last moved in the clockwise direction to the extreme right in Fig. 1a. At the start of tape movement in the opposite direction (i.e. toward the lower left portion of Fig. 1a), tape 1 is in contact with guide 10 which pivots counterclockwise as a result of stiction between the tape and guide. As the guide pivots, the tape which has adhered to the guide is peeled away, and a hydrodynamic air bearing is formed between the tape and the guide as the tape velocity is increased to its nominal operating value. The pivoting action results in the position shown in Fig. 1b. As peeling stiction for a flexible web is much smaller than sliding stiction, the stiction and resulting wear is greatly reduced with the pivoting post, compared to that with a stationary post. At the nominal operating tape velocity, the post ceases to pivot either because of contact with a mechanical stop, or in view of the compound contour post, because the tape tension prevents the post from pivoting into a position where it deflects the tape far beyond its original path. This "self-acting" stop occurs when the drag between the tape and the post equals the restoring torque exerted by the tape under tension. A mechanical stop can be used as well to ensure that the post never pivots beyond a certain arc over the lifetime of the device. If the direction of tape movement reverses, guide 10 pivots clockwise from the position of Fig. 1b to that of Fig. 1a. Guide 10 thus has two stable positions and is not a simple roller, but a post which acts like a roller through a limited angle of pivotation. The position of guide 10 depends upon the direction of tape movement (or in the case of a motionless tape, upon the last direction of tape movement). The pivoting action ensures that the tape engages the bearing over the large radius portions. In the preferred embodiment, axle 15 is located such that the guide 10 is pivoted about the centre of radius 13 of curvature. Other locations may be chosen, however, to influence where the tape first engages the outer portions of tape engaging surface 11. For example, the axle location can be moved back, away from the tape engaging surface, along the bisector of the post as shown in Fig. 2. As a result, the pivoting post has two new bistable positions which are determined by a balance of torques on the post exerted by the tape under tension. The offset axle location also causes the tape to engage a larger arc of the outer portions of tape engaging surface 11. Which of the two positions assumed by the post is determined by the direction of tape motion. Friction is also reduced by proper lateral contouring of guide 10. It has been shown that enough air leaks from beneath the sides of a tape flying over a simple cylindrical post to produce a lateral tape contour such that the edges of the tape sag or curl (see Deckert et al, Dynamic Response of Self-acting Foil Bearings, IBM Journal of Research of Development, November, 1974, pp. 513-520). Such sagging of the tape edges tends to increase the area of contact between the tape and the guide, thereby increasing friction. By designing the lateral contour of the tape engaging surface to match the lateral profile of the tape, such friction is minimised. In the embodiment illustrated in Fig. 3a, the tape 1 flies above guide 10, the lateral profile of whose tape engaging surface 11 is convex and crowned to match the lateral profile of tape 1. Suitable crowned guides have lateral radii of curvature of 300 mm upwards for a tape approximately 8.0 mm wide and 12 µm (microns) thick. In the embodiment illustrated in Fig. 3b the width of guide 10 is reduced and the convex lateral contour is created by edge rounding. The optimum lateral contour for an application will vary depending upon the stiffness of the tape, longitudinal contour of the guide, and operating parameters of the application. Such guides may be used in a magnetic tape drive 20 (Fig. 4) which includes a tape path 21, which is that portion of drive 20 in contact or close proximity with magnetic recording tape 1. Tape 1 may be any flexible magnetic recording tape; the composition of the tape is not relevant to the subject invention, but a suitable tape is disclosed in US-A-4,467,411. Tape 1 is wound at one end upon a first tape reel 22 and wound at the other end upon a second tape reel 23. Reels 22 and 23 are mechanically driven to rotate in either direction, as required to position the desired portion of tape 1 in close proximity adjacent to a magnetic tape head 24. Head 24 includes one or more magnetic transducers capable of magnetically writing data to and/or reading data from tape 1. The type of head is not relevant to the subject invention, but a suitable head is disclosed in US-A-4,685,005. Data (including analog or digitally encoded audio, visual or any computer related data) is recorded in one or more tracks on tape 1 using any available recording format. Guides 10a, 10b, and 10c similar to that described in relation to Fig. 1 or Fig. 2, maintain the position of tape 1 and pivot about axles 15a, 15b, and 15c respectively. The pivotal positions of guides 10a, 10b, and 10c depend upon the direction of tape movement. Guides 10a to 10c pivot counterclockwise to the positions shown in Fig. 4, being the first pairs of stable positions during tape movement from one reel to the other reel. Guides 10a to 10c, pivot clockwise to their respective other stable positions, during tape movement in the reverse direction. The stable positions of guides 10a to 10c adjust gradually as the amount of tape 1 wrapped upon the respective reels 22 and 23 changes during operation of drive 20. Because there are still only two stable positions of guides 10a to 10c depending upon the direction of tape movement, the guides are considered to be "bistable". Guides 10a to 10c are positioned to maximise their utility in any particular application. Because the amount of tape 1 wound upon a reel changes over time, the outer portion of the tape engaging surface (of each guide) is able to accommodate a varying angle of tape approach. Again, the optimal design of the guides will depend upon the characteristics of the application. The friction produced in tape path 21 can be tested for optimisation by using a simple strain gauge. By mounting a guide on an axle, and coupling the axle to the strain gauge, the gauge monitors the tendency of the guide to rotate and hence the friction between the tape and the guide. Many tape paths include the transducing head on the same side of the tape as the tape guides. In addition, one or both of the reels may be packaged into a tape cartridge. In the embodiment shown in Fig. 4, head 24 is on the opposite side of tape 1 from guides 10a to 10c, and reels 22 and 23 can be packaged into a single tape cartridge. Head 24 is not part of the cartridge, but is mounted in drive 20 and mated with tape 1 upon insertion of the cartridge. In another embodiment, head 24 may be located on the same side of tape 1 as the guides and the tape cartridge include only one of the reels. The other reel, head 24, and the guides would not be part of the cartridge, but would be mounted in drive 20. Upon insertion of the cartridge into drive 20, the end of tape 1 is removed from the cartridge reel and threaded through the tape path and onto to the reel in drive 20. A tape cartridge and threading apparatus suitable for such an embodiment is shown in US-A-4,334,656. If the guides are part of a tape cartridge, they are preferably moulded of plastics material to reduce weight and cost. If the guides are part of drive 20, they are preferably cast or machined of stainless steel or other wear resistant materials. In either case, the guides are simple in shape and therefore inexpensive to manufacture. The operation of reels 22 and 23 and head 24 is managed by a controller 25 to write data to and/or read data from tape 1. The electrical and mechanical connections to controller 25, the operation of controller 25, and additional components in path 21 are not relevant to the invention, but may be derived from US-A-4,467,411, 4,406,425, and US-A-4,389,600. While the invention has been described with respect to a preferred embodiment thereof, it will he understood by those skilled in the art that various changes in detail may be made therein without departing from the scope of the invention. For example, the guide described herein could be used for flexible web guiding applications other than magnetic tape drives. Such applications include optical tape drives and film and fabric winding. Also, a tape path can include any number of tape guides as shown herein, as required by the application. The guides, reels, and other path components can be arranged in various configurations.

Claims: A guide for a flexible tape comprising an axle (15) and a post (10) which is allowed to pivot about the axle but which is constrained from completing a full revolution, the post including a surface (11) to engage the tape, whereby in operation, a change in direction of said tape causes the guide to pivot between a first position and a second position, and characterised by the post surface having a longitudinal contour with a compound radius of curvature having first and second portions with a relatively large radius of curvature (12,14) and a third portion with a relatively small radius of curvature (13) between said first and second portions, such that when the tape changes direction at least a portion of the tape disengages from the guide by a peeling action. The guide of claim 1, wherein pivoting of the post (10) is limited by the torque exerted on it by the tape under tension. The guide of claim 1 or claim 2, further including mechanical stop means to limit the rotation of the post. The guide of any preceding claim, wherein the axle (15) is located at the centre of curvature of said third portion of the post surface. The guide of any of claims 1 to 4, wherein the axle (15) is offset from the centre of curvature of said third portion of the post surface, the axle being further away from said third portion than the centre of curvature. The guide of any preceding claim, wherein the guide is bistable at normal tape operating velocities such that the tape is in contact with either said first or second portion of the post surface, depending on the direction of movement of the tape. A tape cartridge for insertion into a tape drive, the tape cartridge including a first reel, with a second reel being included either in the tape cartridge or the tape drive, whereby a tape is transferred from the first to the second reel or vice versa past a transducing head by rotation of the first and second reels, the tape cartridge further including a tape guide according to any preceding claim. A tape cartridge as in claim 7, further including a second tape guide according to any of claims 1 to 6, such that with respect to the tape path the transducing head is located between the two tape guides. A tape drive for receiving a tape cartridge, the tape cartridge including a first reel, with a second reel being included either in the tape cartridge or the tape drive, whereby a tape is transferred from the first to the second reel or vice versa past a transducing head by rotation of the first and second reels, the tape drive further including a tape guide according to any of claims 1 to 6. A tape drive as in claim 9, further including a second tape guide according to any of claims 1 to 6, such that with respect to the tape path the transducing head is located between the two tape guides.

Inventors: EATON JAMES HOWARD, SPONG JACQUELIN KETNER, EATON, JAMES HOWARD, SPONG, JACQUELIN KETNER
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1992
Main classification: G11B15
IPC codes: G11B15, G11B23