Note: Descriptions are shown in the official language in which they were submitted.
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
-- 5 --
~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
-- 6 --
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
7 -
~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
-- 8 --
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
- 14 -
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.
