Air guide for tape transports having air jets at tangent points

An apparatus for guiding a span of high-speed magnetic recording tape past a recording/transducing head. Low volume pressurized air is utilized to lift the tape off of the guide assembly thereby eliminating sliding friction between the apparatus and the tape. The periphery of the guide is provided with a means to facilitate the formation of an air seal with the edges of the tape thereby substantially isolating the pressurized air from the atmosphere. Air jets are provided at the tangent points where the tape enters and departs from the air guide so as to prevent the tape from touching any portion of the guide during the transport operation. The pressurized air emitted from the air jets located at the tangent point where the tape enters the guide forms a supporting boundary layer of air along the surface of the tape which is carried along by the tape across the guide.

INTRODUCTION 
The present invention relates generally to an apparatus for guiding a span 
of recording tape past a magnetic recording head and, more particularly, 
to a method and apparatus for reducing sliding friction between the guide 
apparatus and tape by providing a means for emitting pressurized air at 
the tangent points where the tape enters and departs from the air guide. 
BACKGROUND OF THE INVENTION 
Although many variations of high speed recording tape guides, including air 
guides, are known and used in the magnetic recording industry, it has been 
found that a number of serious problems relating to friction and tape 
alignment have been associated with their use. These problems become 
particularly acute when processing video signals due to the high tape 
speeds involved and, consequently, the multiplicity of tracks of extremely 
narrow width that must be utilized in order to provide record and/or 
playback capability for programs of reasonable duration on a tape of 
manageable length. 
For example, in a video system which operates at a tape speed of 120 inches 
per second, 36,000 feet of tape pass the transducing head each hour. Due 
to physical and cost limitations in reel size, the data is generally 
recorded on a multiplicity of parallel tracks, and the shorter length of 
tape which results is passed repeatedly past the transducer head, each 
time reading information from a different track. In this manner an 1800 
foot reel of tape having 30 tracks can be used to record or playback a 90 
minute program utilizing the above-described system. 
However, the requirement that the tape pass through the transport 30 times 
per program has resulted in serious wear considerations which it has been 
found may be greatly overcome through utilization of an air guide type of 
tape transport. Such prior guides utilizing a film of air as a lubricant 
have had serious disadvantages associated therewith such as unequal air 
pressure distribution across the width of the tape which resulted in 
undesirable deformation of the tape and the requirement for relatively 
large quantities of air flow under high pressure in order to support the 
guided span of tape. These problems were eliminated by the air guide 
designed in accordance with the invention disclosed in my prior U.S. Pat. 
No. 3,979,037, the disclosure of which is incorporated herein by 
reference. 
Likewise, due to the requirement that 30 different informational tracks be 
placed in parallel relation on a tape of reasonable and economical width 
for consumer use, typically 1/4 inch, a related problem concerning 
alignment of the extremely narrow tracks on the tape with a corresponding 
transducer head has arisen. This problem is compounded by the fact that 
commercially available magnetic tape is manufactured to width tolerances 
which approach the individual track width required (typically about 6 mil 
with 2 mil spacing between tracks) for utilization of 30 tracks on a 1/4 
inch tape format. Such alignment problems have been solved by the air 
guide designed in accordance with the invention disclosed in my prior U.S. 
Pat. No. 4,071,177, the disclosure of which is also incorporated herein by 
reference. 
Although the above-discussed prior inventions provided acceptable results 
in many high-speed tape transport systems, it has been found that due to 
the fact that the tape must come into contact with the tape guide at the 
tangent points where the tape enters and departs from the air guide so as 
to provide an adequate air seal, both undesirable amounts of scrape 
flutter and contamination build-up on the guide surfaces at these points 
has resulted which degrade the overall performance of the video system. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention eliminates the above-described scrape flutter and 
contamination build-up problems found with prior tape transport designs by 
providing air jets at the tangent points where the tape enters and depart 
from the air guide. The pressurized air emitted from the air jets located 
at the tangent point where the tape enters the guide forms a supporting 
boundary layer of air of substantially constant pressure along the surface 
of the tape which is carried along by the tape across the guide. In this 
manner the tape is prevented from touching any portion of the air guide 
during the transport operation. 
It has been found that although a perfect mechanical air seal is not 
maintained between the tape and guide trough at the tangent points due to 
utilization of the present air jet system, in operation significant air 
pressure and flow problems have not arisen and satisfactory tape fly over 
the guide assembly may be obtained utilizing inexpensive air pumps which 
produce only in the range of 0.7 psi of static air pressure and 0.025 cfm 
of air flow. 
Furthermore, it has been found that the mechanical design of the air guide 
assembly may be significantly simplified by introducing air to the system 
only at the air jets located at the tangent points. In such circumstances, 
since the tape flys at only about 1 mil over the bottom surface of the 
guide trough, it is desirable to form an additional channel in this 
surface so as to allow better distribution of air over the entire guide 
assembly as it is carried along by the tape.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates an embodiment of a self-centering air guide 10 
constructed in accordance with the present invention. A pair of 
corresponding guides 20 define the path of a span of recording tape 30 as 
it is transported past transducer head 40. The positioning of guides 20 is 
symmetrical with respect to head 40, which is mounted therebetween, in 
order to permit bidirectional transport of the tape. In the preferred 
embodiment the longitudinal dimension of guides 20 is formed in the shape 
of a segment of a cylinder having a 1-inch radius and the guides are 
mounted in the apparatus with a 1/2-inch gap between their inner edges 
wherein head 40 is mounted. 
Referring now to FIGS. 2 and 3, air jets 22 are formed in support surface 
24 at the tangent points 32,34 where the tape respectively enters and 
departs from guides 20 on its way to or from the take-up and supply reels 
(not shown) or over head 40. Although the air providing means is shown as 
multiple air jets 22 in the preferred embodiment, it is noted that it may 
also comprise either single or multiple slots, holes or even a porous 
material section. 
Likewise, FIGS. 2 and 3 show a single pair of air jets 22 located at the 
entry and departure tangent points 32,34 as being the sole means for 
providing air under pressure along the entire air guide apparatus. It is 
noted that although additional air providing means may be located along 
the support surface 24, the design of the preferred embodiment simplifies 
construction of the guide apparatus by reducing the complexity of the air 
delivery system. However, since such an arrangement requires that the 
supporting boundary layer of air which forms on the underside of the tape 
be carried along by the tape from the entry tangent point air jet across 
the downstream guide surface, a channel 25 is formed in support surface 24 
so as to aid in air distribution. 
Channel 25, which is best shown in FIGS. 3 and 4, is formed in the 
preferred embodiment to a depth of 5 to 6 mils and extends substantially 
across the entire width of support surface 24 between the entry and 
departure sets of air jets 22. 
A pair of flanges 26 are provided in the preferred embodiment on opposite 
sides of the path followed by the tape as it is transported over support 
surface 24. These flanges are spaced apart at their base 27 a distance 
less than the minimum width of the tape to be utilized in the system. 
Although it has been found that optimal results are obtained when beveled 
flanges 26 of the configuration shown in FIGS. 3 and 4 are utilized as a 
means for both centering and forming an air seal with the edges of the 
tape, it is noted that numerous other suitable configurations may be 
utilized to accomplish these functions either separately or in 
combination. 
In operation, a compressed air supply (not shown) is connected to air jets 
22 in guides 20 and a film of pressurized air of substantially constant 
pressure is generated along the air chamber formed by tape 30, support 
surface 24, flanges 26 and the tangent points 32,34 where the tape enters 
and exists from the guide in close proximity to the support surface as the 
boundary layer of air is carried along channel 25 to lift the tape off of 
the support surface to a point where its width substantially equals the 
horizontal distance between the beveled flanges. In practice it has been 
found that pressure in the range of from about 14 to 20 inches of water is 
sufficient to lift the tape, under 2 ounces of tension, to the 
above-described level. This operation is best illustrated by FIG. 4 where 
tape 30 is shown supported on a film of pressurized air 50 between beveled 
flanges 26. When supported in this manner, tape 30 will rise or drop along 
the surface of flanges 26 as variations in tape width are experienced 
during the transport operation, thereby maintaining the longitudinal 
centerline of the tape in a precise centered relationship with respect to 
the longitudinal centerline of support surface 24 and guide 20. Hence, 
lateral movement of the tape is eliminated irrespective of manufacturing 
variations in tape width and precision alignment of the playback head with 
the individual track recorded on the tape is maintained at all times. 
In addition, since sufficient air pressure may be provided so as to allow 
for a pressure drop between the edges of the tape and the adjacent surface 
of the flanges so as to insure that the tape will seek the proper level 
between the flanges, and since the air jets are provided at the tape entry 
and departure tangent points of the support surface, scrape flutter and 
contamination are virtually eliminated since the tape tends to be pushed 
away from all surfaces of the air guide thereby minimizing physical 
contact and resulting friction therebetween. 
While several particular embodiments of the present invention have been 
shown and described in detail, it should be understood that various 
obvious changes and modifications thereto may be made, and it is therefore 
intended in the following claims to include all such modifications and 
changes as may fall within the spirit and scope of this invention.