US3362325A

US3362325A - Electrostatic printer employing belt type stencil thru which ions are deposited

Info

Publication number: US3362325A
Application number: US471118A
Authority: US
Inventors: William P Foster
Original assignee: Borg Warner Corp
Current assignee: Borg Warner Corp
Priority date: 1965-07-12
Filing date: 1965-07-12
Publication date: 1968-01-09
Anticipated expiration: 1985-01-09
Also published as: GB1143162A

Description

Jan. 9, 1968 s ER 3,362,325

ELECTROSTATIC PRINTER EMPLOYING BELT TYPE I STENCIL THRU WHICH IONS ARE DEPOSITED, Filed July 12, 1965 2 Sheets-Sheet 1 INVENTOR W\LL.\AM P. FOfiaTEQ.

EZW C ATTORNEY Jan. 9, 1968 w. P. FOSTER 3,362,325

ELECTROSTATIC PRINTER EMPLOYING BELT TYPE STENCIL THRU WHICH IONS ARE DEPOSITED Filed July 12, 1965 2 Sheets-Sheet 2 $1) 0v, :3 I l} [L Q 9 N o m 7 .6

INVENTOR WILUAM P. FOSTER g I I ATTORNEY United States Patent ELECTROSTATIC PRINTER EMPLGYING BELT TYPE STENCIL THRU WHICH KENS DEPOSITED William P. Foster, Paoli, Pa., assignor to Borg-Warner Corporation, Chicago, Ill., a corporation of Illinois Filed July 12, 1965, Ser. No. 471,118 19 Claims. (Cl. 101122) ABSTRACT OF THE DISCLOSURE An endless metallic stencil is journalled around a pair of

pulleys

21, 22 and constrained between upper and lower air bearings 30 as it passes between a plurality of charge heads 16 and a dielectric web to regulate the deposition of charges on the web. The pulleys are crowned (FIG. 8) to index the endless belt mask, and pulley 22 (FIG. 7) is mounted on a spring loaded base to facilitate insertion and removal of the belt mask. The other pulley 21 completes an electrical circuit (97, 96, 23, 21) to apply a bias potential to the metallic belt mask. Air under pressure is displaced into the housing (top, FIG. 6) which encloses the entire arrangement to prevent build-up of particles on the endless mask by reason of the applied bias potential.

The present invention is directed to a page printer arrangement in which information is initially represented by selectively charging discrete areas of a dielectric web, and more particularly to a novel drive arrangement for the print station of such a system in which a stencil means is displaced at high speed between the source of ions and the dielectric web.

The rapid development and improvement of the computer art has greatly accelerated the rate at which information can be processed. Unfortunately a related increase has not been attained, or even approached, in the output equipment which receives electrical output signals from the computer and punches, imprints, or otherwise marks a record means to visibly record the resultant informa tion. Recent advances in the electrostatic printing art has given promise that a substantial increase can be realized in the rate of printing information from computers and other high-speed equipment. By this technique, printing speeds significantly in excess of the rate which can be obtained by mechanical means, such as solenoid-operated hammers which require a definite time for energization displacement, and return to their at-rest positions.

One method of selectively representing information at the print station of an electrostatic printer is to dispose a series of charge heads in an array, pass the dielectric web adjacent the array as the various charge heads are selectively energized to produce clouds of ions, and move a stencil between the web and the energized discharge head to shape each particular cloud of ions in accordance with the configuration of the specific stencil aperture positioned opposite a charge head at the moment it is energized. If the stencil is driven at a high speed, as it must be to accept the high-speed information output from a computer, then a problem arises with respect to maintaining the stencil at a substantially constant distance both from the web and from the discharge heads as it passes along a predetermined path between the web and the heads. A variation of the head-to-mask spacing varies the charge density passing through the stencil apertures and thus the density of the charge impinging on the web which ultimately attracts the particles to the charged area in the developing step; such a variation affects the contrast of the resultant image. A fuzziness or de-focusing of the image is caused by changes in web position ICC which cause corresponding variations in the stencil-toweb spacing.

It is therefore a primary consideration of the present invention to provide a novel and unobvious drive station for a page printer system using electrostatic techniques in which a stencil means is maintained in precise alignment along a given path as it passes between the charge heads and the web. It is a secondary consideration to maintain the information-denoting apertures in the stencil clear and free of any dust and other debris particles to maintain excellent clarity and definition of the resultant image produced by the page printer.

The foregoing and other considerations are realized in one embodiment of a non-impact printing system in which clouds of ions are displaced toward a dielectric web. Stencil means which define a series of symbol-denoting apertures is provided, as i means for displacing the stencil along a predetermined path so that each cloud of ions must pass through, and thus be conformed with, one of the symbol-denoting apertures before striking the web. A reference means is positioned adjacent the predetermined path to constrain the stencil as it is dis-placed in its operation, to maintain the stencil at a substantially constant distance from the web. In a preferred embodiment, the reference means may be an air bearing, and an enclosure means is provided to substantially surround the components including the stencil means, stencil-displacing means, and the reference means. Also included is means for introducing air within the enclosure means, thus to obviate blockage of the symbol-denoting apertures in the stencil means by dust and minute debris particles.

To enable those skilled in the art to practice the invention, the best mode contemplated for making and using like reference numerals identify like elements, and in the invention will now be described in connection with the accompanying drawings in the several figures of which which:

FIGURE 1 is a perspective illustration showing certain components of the inventive combination;

FIGURE 2 is a partial front view taken on a scale enlarged with respect to that of FIGURE 1, showing a portion of the stencil means depicted in FIGURE 1;

FIGURE 3 is a sectional view, taken on a scale enlarged with respect to that of FIGURE 1, showing the orientation of charge heads, stencil, air bearing and the dielectric web;

FIGURE 4 is a perspective view of a portion of one element of the reference means;

FIGURE 5 is a sectional view, similar to that of FIG- URE' 3, illustrating another embodiment of the air bear- 111g;

FIGURE 6 is a front view showing the support means for the stencil, the enclosure, and the means for passing air into the enclosure;

FIGURE 7 is a partial top view of one of the pulleys depicted in FIGURE 6; and

FIGURE 8 is a partial sectional view illustrating the periphery of one pulley.

General printing station arrangement FIGURE 1 depicts the layout of a printing station at which electrically charged particles or ions are selectively deposited on a dielectric web It) to represent information. Subsequently at stations not shown in the drawings, the web or tape with the charged areas is displaced through an inker partially filled with conductive, opaque particles which are attracted to the charged areas and thereafter fixed in place to provide a permanent representation of the information. The web 19 is aligned for passage in the direction shown by arrow 11 to pass around a first guide roller 12, thence upwardly against a reference electrode or anvil 13, over another guide roller 14 and in the direction represented by arrow 15. A flexible conduit or hose 19 is coupled through a bore (not shown) within anvil 13 to define a means for reducing the pressure at the point where web 10 contacts anvil 13 opposite the charge heads, thereby obviating flutter of the web at the printing position. A plurality of charge heads 16 are spaced from reference electrode 13 and disposed in an array which is linear in this embodiment. Each of the rectangular blocks 16 may in fact comprise a multiplicity of electrode pairs which, upon selective energization by application of a high voltage pulse, provide a discharge between the respective pairs of electrodes and produce ions in the atmosphere adjacent the charge heads. With appropriate application of a bias potential to the charge heads with respect to the other points in the system the ions are deflected toward anvil 13 (but strike the web 11 before reaching the anvil).

The stencil electrode or mask 17 is provided in the form of an endless belt. In a preferred embodiment the mask 17 is electrically conductive, being constructed of stainless steel approximately three thousandths of an inch in thickness. The mask is etched or punched to define both a series of information-denoting symbols 18 (FIGURE 2), and a series of apertures 20 spaced from the information-denoting symbols in a predetermined relationship. The mask or stencil 17 is journalle-d around a driving pulley 21 and a driven pulley 22. A motor 23 is provided and coupled over a shaft 24 to pulley 25, in its turn coupled over a drive belt 26 to another pulley 27 which is coupled over shaft 28 to pulley 21. Thus when motor 23 is energized the stencil means 17 is displaced along a predetermined path past the array of charge heads 16 so that each cloud of ions generated by energization of the charge heads must pass through, and be conformed or shaped in accordance with, the one of symbol-denoting apertures in the mask 17 then positioned between web 10 and the energized charge head.

In accordance with one aspect of the invention, reference means shown as an air bearing 31 is disposed adjacent the predetermined path of stencil 17 so that a cushion of air is trapped between the air bearing and the stencil as stencil 17 is rapidly displaced in its operation. As will become evident from the subsequent explanation, the air bearing 30 constrains the metallic tape and thus maintains the stencil means at a substantially constant distance from the charge heads and from the web as the stencil is rapidly displaced.

Air bearing arrangement In FIGURE 3 one of the charge heads 16 is shown, with a central rectangular portion and an extended portion 31 which fits into an aperture defined in a support plate 32. The charge head further defines a notched portion 33 for mating engagement with one of a plurality of indexing dowel pins 34, and in the upper portion the charge head defines a cylindrical bore 35 through which a fastening screw 36 is inserted for receipt in a correspondingly tapped portion 37 of the base plate 32. The multiplicity of discharge electrodes (not shown) extend from the main portion of charge heads 16 outwardly in portion 31 and the electrodes terminate flush with surface 38 so that, upon selective energization of an electrode pair, a cloud of ions is provided for acceleration through one or more of the mask apertures onto the dielectric web Where it is drawn against anvil 13 as air is drawn through bore 19A.

The air bearing 30 referenced generally in FIGURE 1 is shown in FIGURE 3, in the upper portion of this figure, as comprising a first plate member 40 abutting base 32, a shim or a spacing member 41 disposed against plate 40, and an outer or second plate member 42 positioned against shim 41. These components are maintained in place by a fastening screw 43, the head of which is received in a recessed portion of plate 42 and the body of which extends through corresponding apertures in shim 41 and plate 40 for receipt in a correspondingly tapped portion of base 32.

In a preferred embodiment the base member 32 was comprised of commercially available gauge stock, inch in thickness, and each of plates 42 was of 4 inch gauge stock. Spacer member 41 Was of shim stock material of precisely five thousandths of an inch in thickness; accordingly the fluid-bearing area defined between

plates

40, 42 is exactly five thousandths of an inch. As noted previously, the thickness of mask electrode 17 was three thousandths of an inch in the preferred embodiment, and thus there is a clearance of one mil or one thousandth of an inch between each side of the mask and the adjacent portions of

plates

40, 42. Likewise, in the lower portion of this

drawing plates

44, 45 are disposed on either side of shim 46 and secured in place by another fastening bolt 47 in a related manner.

FIGURE 4 depicts the tapering or chamfering of the end portion of plate 40 (each of

plates

42, 44 and 45 is similarly tapered) so that the complete air bearing assembly defines a throat or funnel portion at the entrance, where the moving mask pulls the boundary layer of air between

plates

40, 42 and between

plates

44, 45. The end of each plate may be cut back, at the entrance area, five to ten thosuandths of an inch. The taper extends for about one inch, at which point the plate is again of uniform cross section as seen in FIGURE 3.

There are various ways of trapping and retaining a cushion of air between mask 17 and the adjacent reference means. By way of example, the reference means could comprise only

plates

40 and 44, with the tension of the tape being adjusted so that it tends to ride near these plates, but a layer of air is dragged along between the surface of the mask

adjacent plates

40, 44 to provide the requisite cushion and thus maintain the proper spacing. Alternatively, the reference means can comprise only the upper or the lower air bearing arrangement. That is,

plates

40, 42 and constant width shim 41 can provide an air bearing arrangement for the upper edge portion of mask 17 and, with this positive alignment of a portion of the mask, the remainder of the mask is also maintained in substantial alignment. The reference means for restraining at least a portion of the mask may also include a system for adding air under pressure into the space of the air bearing, as shown for example in FIGURE 5.

As there indicated air under pressure is passed into a conduit 50 to escape through a plurality of openings 51 in a suitably apertured spacing plate 52 (analogous to spacing member 44 in FIGURE 3) and through a like plurality of apertures 53 in the corresponding spacer plate 54. While the embodiment of FIGURE 5 is an acceptable arrangement, at present the combination of FIGURE 3 is considered the preferred embodiment by reason of the precise mask alignment obtained without the necessity of introducing air under pressure into the bearing support area.

Drive system As shown in FIGURE 6, the air bearing base 32 is supported on the drive system base plate 58. At the right end, base 32 is affixed to the upper portion of a standard or support 3? by three nut-and-bolt assemblies 39a. Support 39 extends downwardly to a horizontal portion 56, and an insulated bearing support member 57 separates portion 56 from base 58. Three separate nut-and-bolt assemblies 60, 61 and 62 respectively passing through insulating bushings 60A, 60B and 60C affix portion 56- and spacer S7 to the system base 58. The left end of air bearing support 32 is similarly afiixed to system base 58.

The top portion of motor 23 is shown, together with

pulleys

25, 27 for transmitting angular displacement to an electrically conductive shaft 28, which is fastened by a nut 61) to drive pulley 21. Shaft 28 restrained within an inner bearing arrangement 64 and an outer bearing housing 65, which is extended outwardly to rest on an annular, insulating bushing 66. A fastening screw 67 has its body portion received in another insulating bushing 68, and this combination extends through registering apertures in bushings 65 and 66 and a similar aperture or bore in base member 58. A nut 69 secures this assembly together.

When the system shown in FIGURE 6 is readied for operation by placing a metallic mask around pulleys 21, 22, a bias potential is generally applied to the mask. A bias input conductor, in the form of conductive spring 96, has one end fastened by screws 97 to an insulating support block 98. The other end of spring 96 engages the end of conductive shaft 28-, which is both electrically coupled to and mechanically engaging pulley 21. Thus a bias potential can be applied over any suitable conductor to one of screws 97, and thence over spring 96, shaft 28 and pulley 21 to the tape mask.

The mounting pulley 22 at the left side of FIGURE 6 is different from the mounting of pulley 21. As there shown, a generally U-shaped support member 70 is provided with upstanding leg portions 71, 72. The base or horizontal portion of member 70 is secured by three fastening screws 74 (only two of which are visible in this view) to the system base 58. This is not a tight, precise fastening but there is some mobility of base 70- with respect to the system base 58. Three separate leveling or jacking screws 73 (one of which is shown in this view) are provided in a tripod arrangement so that the base member 7 on which pulley 22 is supported can be precisely aligned with respect to the position of the other drive pulley 21.

A pair of

support rods

75, 76 are positioned with their extremities received in corresponding openings of the up standing leg portions 71, 72 of base 70. Rod 76 has a stop member or collar 77, and rod 75 has a similar stop 78, for restraining respective bias springs 80, 81 between the collars and the upstanding leg portion 72 of the adjustable base 70. By passing

rods

75, 76 through the related bores of leg 71, lower bearing support block 82, stop members 77 and 78, springs 80 and 81, and the related bores of leg 72, the lower bearing support block 82 is aligned in the proper position. Block '82 is secured in this position by inserting pins (not shown) through lower bearing block 82 and the

support rods

75, 76. A shaft 83 (FIGURE 6) extends through bearing black 82 upwardly and through the bearings 84, and the shaft is restrained at each end by a pair of nuts 85, 8-6. A positive displacement of pulley 22 toward the other pulley 21 is provided by rotating adjusting screw 87 until the extremity abuts lower bearing support 82, to facilitate removal of the mask electrode.

The tape 17 is flexed as it travels around the pulleys and the character-defining apertures cause high stress conditions. If pulleys 21, 22 are crowned in a conventional manner, the tape in the character area would flex in two planes, causing a further increase in the stress. Crowning is very desirable, as it has proved to be an effective guiding means which insures accurate tape tracking. A non-radial peripheral design for the pulleys has proved effective. The central surface of the pulley is made cylindrical in the region where the apertured portion of tape 17 makes contact. The marginal surface areas, from the center cylindrical portion to the top and to the bottom of the pulley, are made slightly conical to guide the tape edge portions and insure accurate tracking.

In more detail, FIGURE 8 indicates, partly in section, an edge portion of one of the

pulleys

21, 22 showing the crowned mask-engaging surface with a high central portion 90 tapering off at one side along a straight, inclined portion 91 and along the other side of a similar angled portion 92. The restraining guide edges 93, 94 of the pulley are held in place by a bolt assembly 95. The mask restrainers or guides 93, 94 are not utilized in the preferred embodiment because the crowned periphery 90-92 of the pulley provides accurate vertical alignment of mask 17 without any necessity for such restraint. It has been shown that an arcuate configuration of the pulley edge, in the region 9092 is not required. Instead accurate and positive mask guiding has been obtained in an embodiment where the total crowned width 90-92 of the pulley was 1.613 inches, and the central flat portion measured 0.375 inch. The outer edge of each portion 91, 92 was set back 0.004 inch from the positions where portions 91, 92 meet the central portion 90. This system obviates the need for

guides

93, 94 and it is manifestly simple to insert and remove a tape mask from the pulley arrangement. Likewise there is no danger of wear or damage to the edge portions of the metallic tape mask.

' As described in connection with FIGURE 6, an appropriate mask biasing potential is applied to the stencil electrode. With this constant potential on the mask, there is a tendency to pick up minute dust particles, paper debris, etc., practically invisible to the naked eye but which may build up to at least partially block the apertures in the mask over a long operating time. Accordingly a significant part of the invention resides in the provision of a system for introducing air under pressure within 'an enclosure 101 provided substantially encompassing the mask electrode drive system. In one embodiment the system includes an electric motor 102 which, on energization, drives a fan 1% to pull air from an intake 104, through a filter 105 and through conduit 1% which extends within enclosure 101, to the space adjacent the drive pulleys and the air bearing. With an air input at a low pressure, even as low as /8 inch of mercury, there is a slight but constant displacement of the air from the interior of the print station outwardly (principally in the space where the mask passes through the air bearing assembly). This constant filtering of the input air maintains the mask apertures free of dirt particles and insures that appropriate clarity and charge definition will be maintained, which will subsequently result in a corresponding high standard of clarity and definition in the printed record.

Summary The system disclosed provides an accurate and positiveoperating system for displacing a metallic stencil electrode past an array of discharge heads to selectively shape the ion clouds displaced toward the mask before the cloud of ions impinges on the dielectric web. Important to the invention is the provision of reference means such as an air bearing arrangement, adjacent the predetermined path of the mask to constrain the tape and insure accurate charge head-maskweb spacing. When the reference means is an air bearing, the entrained air provides operation similar to that of a dynamically operated fluid bearing, functioning to maintain a constant, predetermined alignment of the mask in the critical area where it passes between the charge heads and the anvil. Having thus effectively solved the problem of mask alignment when traveling at high speeds, filtered air is introduced into an enclosure substantially encompassing the tape mask drive system to insure that the mask apertures are continually cleaned and kept free from dust and other minor debris, to maintain uniform clarity and density of the ion shape deposited on the dielectric web.

Another important aspect of the invention includes the shaping of the pulley face to provide a true cylinder for journalling the apertured section of the tape mask in the center portion of the pulley face, while retaining the effect of crowning by chamfering the outer portions of the pulley face to insure proper tracking.

Although various embodiments of the invention have been described and illustrated, it is manifest that variations and modifications may be made therein. It is therefore the intention in the appended claims to cover all such variations and modifications as may fall within the true spirit and scope of the invention.

The embodiments of an invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a non-impact printing system in which clouds of ions are displaced toward a dielectric web:

stencil means defining a series of symbol-denoting aper- :tures therein;

means for displacing the stencil means along a predetermined path so that each cloud of ions must pass through, and be conformed with, one of the symboldenoting apertures before striking the Web;

reference means positioned adjacent the predetermined path for constraining the stencil means and thus maintaining the stencil means at a substantially constant distance from the web;

enclosure means for substantially surrounding the stencil means, stencil-displacing means, and reference means; and

means for passing air into said enclosure means to obviate blockage of the symbol-denoting apertures in the stencil means by dust and minute debris particles.

2. In a non-impact printing system in which clouds of ions are displaced toward a dielectric we'b;

an endless metallic tape of predetermined thickness defining a series of symbol-denoting apertures;

guide means positioned to receive and position the metallic tape for movement along a predetermined path so that each cloud of ions must pass through, and be conformed with, one of the symbol-denoting apertures before striking the web;

means for displacing said tape;

reference means positioned adjacent the predetermined path for constraining the metallic tape as the tape is. displaced in its operation, to maintain the metallic tape at a substantially constant distance from the web;

enclosure means for substantially surrounding the metallic tape, pulleys, and reference means; and

means for introducing air under pressure within said enclosure means to obviate blockage of the symboldenoting aperture in the metallic tape by dust and minute debris particles.

3. In a non-impact printing system in which clouds of ions are displaced toward a dielectric Web:

a pair of pulleys spaced from each other to receive and index the metallic tape for movement along a predetermined path so that each cloud of ions must pass through, and be conformed with, one of the symboldenoting apertures before striking the web;

means for displacing said tape; and

air bearing means positioned adjacent the predetermined path so that a cushion of air is trapped between the air bearing means and the metallic tape as the tape is displaced in its operation, to maintain the metallic tape at a substantially constant distance from the web.

4. A non-impact printing system as set forth in claim 3, in which each of said pulleys has an outer tape-engaging portion which is crowned to position the metallic tape in its displacement.

5. A non-impact printing system as set forth in claim 3 in which each of said pulleys comprises an outer periphery defined by three substantially straight portions, including a central portion substantially parallel to the axis of the pulleys and two outer portions each of which is inclined at an angle from the center portion to the side plate of the pulley to define a crowned aligning surface for the metallic tape.

6. An electrostatic printing system as set forth in claim 3 and further comprising peripheral tape guides on each of said pulleys which extend beyond the periphery of the pulleys, thereby to provide positive vertical restraint of the metallic tape.

7. A printing station for an electrostatic printing system comprising:

a system base plate;

a plurality of charge heads, disposed in a linear array for selective ener-gization to produce clouds of ions tor displacement toward a dielectric web;

air bearing means, supported on said system base plate, positioned adjacent said linear array of charge heads so that air is trapped between the air bearing means and the metallic tape as the tape is displaced along a predetermined path in system operation, thus maintaining the metallic tape at a substantially constant distance both from the charge heads and from th( web;

a first crowned pulley for supporting a given portion of the metallic tape;

bearing support means, affixed to said system base plate for supporting said first crowned pulley at a reference position;

a second crowned pulley for supporting a different portion of the metallic tape, and cooperating with said first pulley to position the tape along said predetermined path;

a pulley support base for supporting and aligning said second crowned pulley; and

means, including a plurality of adjusting screws, for incrementally regulating the position of said pulley support base with respect to said system base plate.

8. A printing station as set forth in claim 7 and further comprising bias springs mounted in said pulley support base, for accommodating displacement of said second pulley relative to the first pulley to compensate for thermal expansion of the endless metallic tape and provide a system with a non-critical center-to-center distance between said pulleys.

9. In a non-impact printing system in which clouds of ions are displaced toward a dielectric web:

an electrically conductive supporting structure;

an endless, electrically conductive metallic tape defining a series of symbol-denoting apertures;

guide means including a pair of electrically conductive pulleys spaced apart from each other to receive and position the metallic tape for movement along a predetermined path so that each cloud of ions must pass through, and be shaped by, one of the symboldenoting apertures before striking the web;

means for electrically insulating said endless metallic tape from the electrically conductive supporting structure;

means for completing an electrical circuit through at least one of said electrically conductive pulleys to said metallic tape;

means for applying an electrical bias potential to said electrical circuit to bias the electrically conductive metallic tape;

means for displacing the tape; and

reference means positioned adjacent the predetermined path for constraining the metallic tape as it is displaced, thus maintaining the metallic tape at a substantially constant distance from the web.

10. In a non-impact printing system in which clouds of ions are displaced toward a dielectric web;

an electrically conductive supporting structure;

means including a pair of electrically conductive pulleys spaced from each other to receive and position the metallic tape for movement along a predetermined path so that each cloud of ions must pass through, and be shaped by, one of the symbol-denoting apertures before striking the web;

means for applying an electrical bias potential to said endless metallic tape during system operation, including an electrically conductive shaft supporting and electrically coupled to one of said pulleys, and means, including an electrically conductive bias spring con- 9 tacting a portion of said support shaft, for applying the electrical bias potential to the tape; means for displacing the tape; and

11. In an electrostatic printing system in which a plurality of charge heads are disposed in a linear array for selective energization to produce clouds of ions for displacement through an air space toward a dielectric web:

a pair of pulleys for supporting and aligning the metallic tape for movement along a predetermined path past the linear array of charge heads so that each cloud of ions must be directed through, and thus be shaped by, one of the symbol-denoting apertures before striking the dielectric web;

means for driving one of said pulleys; and

air bearing means, comprising a first plate member positioned above said air space and a second plate member positioned below said air space and directly below said first plate member, so that a cushion of air is trapped between the endless metallic tape and said plate members as the endless metallic tape is displaced in its operation, thus to maintain the metallic tape at a substantially constant distance both from the charge heads and from the dielectric web.

12. An electrostatic printing system as set forth in claim 11 in which said plate members define a plurality of channels terminating in apertures adjacent said predetermined path, and means for introducing air into said channels to emerge from said apertures and provide air between the metallic tape and the plate members.

13. An electrostatic printing system as set forth in claim 11 in which an end portion of each of said plate members is tapered to define a throat at the entrance of the air bearing means.

14. In an electrostatic printing system in which a plurality of charge heads are disposed in a linear array for selective energization to produce clouds of ions for displacement toward a dielectric web:

an endless metallic tape of predetermined thickness, having upper and lower edge portions, and a central portion defining a series of symbol-denoting apertures;

a pair of pulleys for supporting and aligning the metallic tape for movement along a predetermined path past the linear array of charge heads so that each cloud of ions must be directed through, and thus shaped by, one of the symbol-denoting apertures thefore striking the dielectric web;

means for driving one of said pulleys; and

air bearing means, comprising a first plate member positioned adjacent one of said tape edge portions and a second plate member positioned at a distance greater than said predetermined thickness from said first plate member, so that a cushion of air is trapped between the surfaces of said one edge portion of the metallic tape and said plate members as the endless metallic tape is displaced in its operation, thus to maintain the metallic tape at a substantially constant distance both from the charge heads and from the dielectric web.

15. An electrostatic printing system as set forth in claim 14 in which said plate members define a plurality of channels terminating in openings adjacent said predetermined path, and means for introducing air into said channels to emerge from said openings and provide air between the metallic tape and the plate members.

16. An electrostatic printing system as set forth in claim 14 in which an end portion of each said plate memhers is tapered to define a throat at the entrance of the air bearing means.

17. In an electrostatic printing system in which a plurality of charge heads are disposed in a linear array for selective energization to produce clouds of ions for displacement through an air space toward a dielectric web:

an endless metallic tape of predetermined thickness,

having upper and lower edge portions, and a central portion defining a series of symbol-denoting apertures;

means for driving one of said pulleys;

first air bearing means, positioned above said air space, comprising a first plate member, a first spacing shim of a thickness greater than said predetermined thickness, and a second plate member, so that a cushion of air is trapped between the upper edge portion of the endless metallic tape and said first and second plate members as the endless metallic tape is displaced in its operation; and

second air bearing means, positioned below said air space, comprising a third plate member, a second spacing shim of the same width as said first shim, and a fourth plate member, so that a cushion of air is trapped between the lower edge portion of the endless metallic tape and said third and fourth plate members as the endless metallic tape is displaced in its operation, thus to maintain the metallic tape at a substantially constant distance both from the charge heads and from the dielectric web.

18. An electrostatic printing system as set forth in claim 17 in which said plate members define a plurality of channels terminating in openings adjacent said predetermined path, and means for introducing air into said channels to emerge from said openings and provide air between the metallic tape and the plate members.

19, An electrostatic printing system as set forth in claim 17 in which an end portion of each said plate members is tapered to define a throat at the entrance of the air bearing means.

References Cited UNITED STATES PATENTS 1,546,834 7/1925 Hanington 101129 2,612,566 9/1952 Anderson et al. 179100.2 2,726,940 12/ 1955 Buhler. 3,140,621 7/1964 Stone 74-230.5 3,142,562 7/1964 Blake. 3,148,248 9/1964 Johnson. 3,187,669 6/1965 Greenblott et al. 101-114 3,261,284 7/1966 Lynott et al 101-114 3,285,167 11/1966 Childress et al 101-1 14 FOREIGN PATENTS 81,920 9/ 1956 Denmark.

ROBERT E, PULFREY, Primary Examiner. E. S. BURR, Assistant Examiner.