CA2055251A1

CA2055251A1 - Web tensioning device

Info

Publication number: CA2055251A1
Application number: CA 2055251
Authority: CA
Inventors: Graham L. Shirley; Geoffrey W. Vernon; Geoffery W. Vernon
Original assignee: Graham L. Shirley; Geoffrey W. Vernon; Geoffery W. Vernon; Molins Plc
Current assignee: Mpac Group PLC
Priority date: 1990-11-13
Filing date: 1991-11-12
Publication date: 1992-05-14
Also published as: GB9024631D0; DE4137369A1

Abstract

Abstract of the Disclosure Web Tensioning a Device A web tensioning device comprises a reservoir (4) into which a loop (26) of the web (24) extends. The reservoir has a width which generally only slightly exceeds that of the web, and a variable pressure drop is established across the loop in the reservoir by means of a fan.
Side ports (16;16A) allow a restricted air flow from one side of the loop to the other. The air flow may vary according to the position of the loop.
In this way the pressure drop across the loop, and hence the tension in the web, varies with the position of the loop in the reservoir. The device is capable of maintaining a web at a predetermined tension or within a predetermined range of tensions and responds rapidly to external influences on tension, e.g. reversal of drive to the web.

Description

2 ~ 1 Web T0nsionin~ Device 3This invention relates to a device for applying or maintaining 4tension in a web, particularly in apparatus where the web is conveyed sthrough processing machinery, e.g. incorporating printing apparatus.
6According to a first aspect of the invention a web tensionin~
7device comprises guide means for a ioop of the web, means for 8establishing a pressure drop across the loop so as to impose a tension gon the web, and means for varying the pressure drop to control tension oin the web. The tension may be varied in accordance with position of l1the loop. The varying means may comprise air flow rneans capable of 12variable flow rates, which may be dependent on the position of the loop.
13The establishing means may comprise a fan or the like generating a 14pressure drop which is variable according to the flow rate permitted 15through said air flow means. In one arrangement the loop partially or 16totally blocks said air flow means in one position of said loop relative to 17said guide means and unblocks or exposes more of said air flow means 18in another position. The size of the loop, i.e. the length of the web in 19said loop, may vary in said different positions. The varying means may 20be arranged so that the tension imposed on the web varies linearly with 21the length of the loop.
22In a preferred construction the device includes a cham~er into 23which the loop extends so as to divide the chamber into portions on 24opposite sides of the loop. Said establishing means is preferably 25arranged to maintain different pressures in said portions of said 26chamber. In order to achieve this, the chamber preferably includes side 27walls adjacent the edges of the web in the loop so as to restrict 28undesired air flow around the sides of the loop. With this arrangement 29the varying means may comprise a variable restrictor in an air flow path, 30e.g. Ieading to a fan, and/or one or more openings in a side wall which 31allow a variable air flow so as to vary said pressure difference as the 32loop moves with respect to said opening or openings. Thus, an opening 33in a side wall may be defined by profiled edges such that as the loop 34extends further into said chamber an increased area of said opening is 35exposed thereby allowing increased air flow from the higher pressure 36side of said loop. This results in reduced pressure drop across the loop 37and hence reduced tension imposed on the web. Instead of an opening 38with profiled edges there may be a plurality of openings arranged so 2~2~

that the number of openings on each side of the loop varies as the 2 position of the loop varies. Alternatively the opening may have straight 3 parallel edges so that the air flow does not vary with the position of the 4 loop.
s In a preferred construction the device comprises a chamber 6 having spaced side walls which in use lie adjacent to the edges of the7 web forming a loop in the chamber, one or more openings in said side 8 walls being arranged to control the air flow from the chamber on one g side of the web to the chamber on the other side of the web. The o opening or openings may be arranged so that an increased area of said Il opening or openings is exposed as an increased length of loop extends 12 into said chamber. Preferably the establishing means comprises a fan 13 or the like generating a reduced pressure at or adjacent one end or side 14 of the chamber, the loop extending into the chamber from the other endor side. The chamber may have means allowing restricted flow of air 16 into the chamber at said other end, said means conveniently including a 17 removable closure. In this arrangement by selecting or adjusting the air 18 flow through said closure, in relation to the expected range of air flows 19 through said air flow means, the total flow and pressure drop may be kept within the working range of said establishing means (e.g. a fan). A
21 closure or similar means at said other end of the chamber is not 22 essential, however, and said end may be substantially open.
23 According to another aspect of the invention a web tensioning 24 device comprises means for establishing a pressure difference across aloop of the web by generating an air flow along a path, rneans for 26 restricting air flow around the sides of said loop, said restricting means 27 including one or more apertures whose total area increases with overall 28 length of the loop, said apertures being arranged in said air flow path, 29 the arrangement being such that said pressure reduces as the length ofsaid loop Is increased (and decreases as the length of said loop is 31 decreased) so that tension imposed on the web is correspondingly 32 decreased or increased respectively.
33 According to another aspect, the invention provides a chamber 34 forming a reservoir for a loop of a web, means for establishing a pressure difference on opposite sides of the loop in the chamber, said 36 establishing means including an air flow path extending from one side of 37 the loop to the other side of the loop, said path being at least partly 3\3 defined by at least one side port capable of communicating with the 2~?~
3 JCW/3~1 ~

chamber on both sides of said loop. Said port may have a width which 2 varies so that air ~low in said path varies according to the position of said 3 loop in said chamber and so that said pressure difference 4 correspondingly varies. Preferably said port is profiled so that its width varies continuously in at least a working range of paths of said loop.
6 According to a further aspect of the invention a web tensioning 7 device comprises a chamber into which a loop of a web extends in use, 8 means for establishing a pressure difference across the loop in the g chamber, and means defining an air flow path which, for at least a o working ran~e of positions of the loop in the chamber, connects the l1 chamber on one side of the loop to the chamber on the other side of the 12 loop, said path defining means being arranged to allow restricted air 13 flow so as to maintain a pressure differential in the chamber on opposite 14 sides of the loop, so as to impose a predetermined tension on the web.The predetermined tension may be within a predetermined range of 16 tensions, and preferably bears a predetermined relationship to said 17 pressure difference. The path defining means may be arranged to allow 18 increased or decreased air flow as the length of the loop respectively19 increases or decreases in said chamber, whereby the pressure difference across said loop is respectively decreased or increased and 21 tension imposed on the web correspondingly decreased or increased.
72 Preferably the chamber has a width only slightly greater than the width 23 of the web, so that the loop substantially restricts air flow from one side 24 of the loop to the other except by way of said air flow path.
The invention may be advantageously used in processing 26 machinery where it is required to provide tensioning means for a moving 27 web particularly where it is required that the tensioning means should2~ react quickly. In this respect it should be noted that the device may be 29 constructed with no moving parts (apart from the pressure difference establishing means, e.g. a fan, and the web itself) and may therefore 31 have extremely low inertia. A typical situation where the device is useful 32 iS where the web is conveyed through processing machinery at a 33 varying speed. Having very low inertia the device is capable of 3'1 responding very quickly to any slackening of tension in the web upstream or downstream of the device so as to maintain tension in the 36 web. A particular application for the device is control of tension in a 37 moving web which is moved intermittently or in a reciprocating manner.3~ For example the device is particularly suitable for control of tension in 2~2~ ~

the printing method and apparatus disclosed in European patent 2 specification No. 441~96A, the disclosure of which is hereby 3 incorporated herein in its entirety. Referring to said specification the 4 device of the present invention may replace the reservoir 63, 163 or s 1 6.3A.
6 The invention will be further described, by way of example only, 7 with reference to the accompanying diagrammatic drawings, in which:
8 Figure 1 is a part-sectional view of a web tensioning device, g Figure 2 is a sectional view of part of the device on the line ll-ll in I0 Figure 1, II Figure 3 is a sectional view on the line lll-lll in Figure 2, 12 Figure 4 is a sectional view of part of the device on the line IV-IV
13 in Figure 1, 14 Figure 5 is an exploded view of part of the device of Figure 1, IS Figure 6 is a graphical representation of a typical profile of an air 16 flow port in a web tensioning device, 17 Figure 7 is a graphical plot of web tension against air flow in a 18 web tensioning device, and lg Figure 8 is a side view of printing apparatus incorporating a web tensioning device.
21 Referring to Figures 1-5, a web tensioning device 2 comprises a 22 substantially rectangular reservoir chamber 4 having opposed side walls Z3 6, edge walls 8, a bottom wall 10, and a top closure 12. Within the 24 chamber 4 and iying immediately adjacent each side wall 6 is a flow control panel 14 having a profiled aperture 16. One of the panels 14 26 has a further round aperture 18 below the aperture 16. A corresponding27 round aperture (not shown) extends through the adjacent side wall 6 28 and the apertures are connected to a source of suction (e.g. a fan). A29 U-shaped guide 20 extends in the chamber 4 between the panels 14.
The top closure 12 is substantially T-shaped in section and rests on the 31 upper edges of the side walls 6 and panels 14 (see Figure 4). Two 32 bores 22 extend through the top closure 12. Note, however, that a top 33 closure 12 is not essential.
34 In use, a length of web 24 extends into the chamber 4. The web 24 may be stationary or intermittently or continuously moving, and may 36 for example extend from a reel to a processing unit (e.g. a printer). The 37 portion of the web 24 extending into the chamber 4 forms a loop 26 and3~3 the width of the web is such that its edges lie closely adjacent the inner 2 ~

edges of the panels 14.
2 Suction applied by way of aperture 18 generates a reduced 3 pressure in the portion of the chamber 4 beneath the loop 26 and hence 4 generates a pressure drop across the loop 26. For a range of lengths of the loop 26 a portion of the loop lies adjacent the profiled apertures 16 6 in the panels 14. Thus an air flow path around each side of the loop 26 7 and connecting those parts of the chamber above and below the loop 8 26 is formed by way of each aperture 16. It will be appreciated that g because of the shape of the aperture 16 this air flow path is more o restricted when the loop 26 intersects the aperture 16 near the upper pa~ts of the apertures than when it intersects the apertures lower down.
12 Assuming that the suction applied to the chamber 24 through the 13 aperture 18 remains reasonably constant, and ignoring secondary 14 effects such as leakage between the sides of the loop and the inner edges of the panels 14, the pressure drop across the loop depends on 16 the relative flow rates around the web of the loop by way of the 17 apertures 16 and that through the bores 22 in the top closure 12 (or 18 through the open top where no top closure is provided). Since the 19 cross-sectional area of the latter is fixed the pressure drop across the loop 26 varies as the length of the loop in the chamber 4 varies and 21 overlaps a varying part of the apertures 16. The apertures 16 are 22 profiled so as to increase in area as the length of the loop 26 increases;
23 hence the pressure drop across the loop will tend to reduce as the 24 length of the loop increases. For a workin0 range of possible tensions therefore, the loop will tend to stabilise its position in the chamber 4.
26 Note that in normal use the loop 26 will never extend below the lower 27 edge of aperture 16. If, during initial use of the device for example, the 2\3 loop occupies the region of chamber 4 above the aperture 16 it will tend 29 to be drawn into the chamber as the pressure drop across the loop is high in that region.
31 In a preferred arrangement the profile of the apertures 16 is 32 selected so that the tension imposed on l:he web 24 varies 33 approximately linearly with the depth of the loop in the chamber. In 34 Figure 6 line A is a plot of the width (in metres) of the apertures 16 at varying loop depths (also in metres) required to achieve that in a typical 36 device similar to that depicted in Figures 1-5 and havin~ a panel 14 37 thickness of about 5 mm and a web 24 width of about 12 mm. Figure 7 3~3 iS a plot of generated web tension (in newtons) against air flow (in cubic 2 ~

metres/hour) in a similar device, using a fan capable of rnaintaining a 2 total pressure drop cf about 800 Pa.
3 The shape of the apertures 16 need not be such as to provide a 4 tension which varies with the length of the loop 26. Referring to Figure 1, a modified aperture t6A having straight, paraliel sides is indicated in 6 dotted lines. The pressure drop across a loop of the web 24 in a device 7 having such modified apertures 16A would remain constant for the 8 length of the apertures. For reference, plot B in Figure 6 shows the g width (11 mm) of a typical aperture 1 6A.
o Fi~ure 8 shows apparatus including tensioning devices 2A, 2B for 11 a web 24A being conveyed past printing cylinders 46, 48. The 12 apparatus is similar to that disclosed in said European Specification No.
13 441596A, the web comprising a carrier ribbon for thin film security 14 material to be transferred to sheets such as bank notes (not shown) supplied to the lower cylinder 48. The web 24A is advanced 16 intermittently past the cylinders 46, 48 by capstans 58, ~0, drive being 17 reversed immediately after a printing operation so that the web 24A is18 retracted by tension imposed by the device 2A.
19 In the arrangement shown in Figure 8 it would be possible to locate the capstans 58, 60 in the upstream positions indicated in dotted 21 lines at 58A, 60A in Figure 8, so that the capstans generate positive 22 movement of the web 24A in the reverse direction, the tensioning device 23 2B ensuring the web remains taut during movement in the forward 24 direction, Each of the devices 2A, 2B may be the same as the device 2 of 26 Figures 1-5 (wilh apertures 16 or 16A). Either of the devices 2A, 2~ may 27 be replaced by an identical device used in an inverted position, as 28 indicated at 2C in Figure 8. Inverting the device 2 has the advantage, in 2Y the apparatus of Figure 8, of preventing contact between the lower side of the web, which side carries the thin film security material, and the 31 sides of the device.
32 In an arrangement as shown in Figure 8, or in any of the 33 arrangements shown in said European specification No. 441596A, 34 tracking of the web, i.e. control of its lateral position, may be important.
We have found that by illuminating the web from unclerneath, pref0rably 36 at an acute angle, the edges of the web become illuminated (even 37 where the web is substantially transparent) to an extent which can be 38 detected by a video camera or the like. The latter may then form part of 2 ~

a controi loop circuit for adlusting the lateral position of the web.

lY

Claims

1. A web tensioning device comprising guide means for a loop of the web, means for establishing a pressure drop across the loop so as to impose a tension on the web, and means for varying the pressure drop to control tension in the web.

2. A device as claimed in claim 1, including means for varying the pressure drop in accordance with the position of the loop.

3. A device as claimed in claim 2, wherein the varying means comprises air flow means capable of variable flow rates.

4. A device as claimed in claim 3, wherein the establishing means comprises means for generating a pressure drop which is variable according to the flow rate permitted through said air flow means.

5. A device as claimed in any of claims 2-4, wherein the air flow means is arranged so that the loop partially or totally blocks said air flow means in one position of the loop relative to the guide means and unblocks or exposes more of said air flow means in another position.

6. A device as claimed in claim 1, including a chamber into which the loop extends so as to divide the chamber into portions on opposite sides of the loop.

7. A device as claimed in claim 6, wherein said establishing means is arranged to maintain different pressures in said portions of said chamber.

8. A device as claimed in claim 7, wherein the chamber has spaced side walls which in use lie adjacent to the edges of a web forming a loop in the chamber, one or openings in said side walls being arranged to control the air flow from the chamber on one side of the web to the chamber on the other side of the web.

9 JCW/3616 9. A device as claimed in claim 8, wherein the opening or openings are arranged so that an increased area of said opening or openings is exposed as an increased length of loop extends into said chamber.

10. A device as claimed in claim 8, wherein the establishing means is arranged to generate a reduced pressure at or adjacent one end or side of the chamber, the loop extending into the chamber from the other end or side.

11. A device as claimed in claim 10, wherein the chamber has means allowing restricted flow of air into the chamber at said other end.

12. A device as claimed in any of claims 8-11, including an opening in a side wall defined by profiled edges such that as the loop extends further into said chamber an increased area of said opening is exposed thereby allowing increased air flow between opposite sides of the loop.

13. A device as claimed in any of claims 8-11, wherein at least one of said openings has parallel edges for at least part of its length, whereby air flow between opposite sides of the loop may remain substantially constant while the loop remains in such position that it intersects said part of said openings.

14. A web tensioning device comprising means for establishing air pressure difference across a loop of the web by generating an air flow along a path, means for restricting air flow around the sides of the loop, said restricting means including one or more apertures whose total area increases with overall length of the loop, said apertures being arranged in said air flow path, the arrangement being such that said pressure reduces as the length of said loop is increased (and decreases as the length of said loop is decreased) so that tension imposed on the web is correspondingly decreased or increased respectively.

15. A web tensioning device comprising a chamber forming a reservoir for a loop of a web, means for establishing a pressure difference on opposite sides of the loop in the chamber, said establishing means including an air flow path extending from one side of the loop to the other side of the loop, said path being at least partly defined by at least one side port capable of communicating with the chamber on both sides of said loop.

16. A device as claimed in claim 15, wherein said port has a width which varies so that air flow in said path varies according to the position of said loop in said chamber and so that said pressure difference correspondingly varies.

17. A device as claimed in claim 16, wherein said port is profiled so that its width varies continuously in at least a working range of positions of said loop.

18. A device as claimed in claim 15, wherein said port has a substantially constant width in a working range of positions of said loops, so that air flow in said path remains substantially constant while the position of said loop remains in said working range.

19. A web tensioning device comprising a chamber into which a loop of a web extends in use, means for establishing a pressure difference across the loop in the chamber, and means defining an air flow path which, for at least a working range of positions of the loop in the chamber, connects the chamber on one side of the loop to the chamber on the other side of the loop, said path defining means being arranged to allow restricted air flow so as to maintain a pressure differential in the chamber on opposite sides of the loop, so as to impose a predetermined tension on the web.

20. A device as claimed in claim 19, wherein the path defining means is arranged to allow increased or decreased air flow as the length of the loop respectively increases or decreases in said chamber, whereby the pressure difference across said loop is respectively decreased or increased and tension imposed on the web correspondingly decreased or increased.

21. A device as claimed in claim 19 or claim 20, wherein the chamber has a width only slightly greater than the width of the web, so that the loop substantially restricts air flow from one side of the loop to the other except by way of said air flow path.