EP0899228A2 - Air cushion guiding device - Google Patents

Abstract

The air cushion guide supports a paper sheet or web on a bearing air cushion. It has at least one guide body with jet aperture blowing air between the guide body (413) and the paper (401). The air volume flow flowing out of the jets, and/or the flow rate between the guide body and the paper can be set independently of each other, so that proportionality between these two values need not be observed.

Description

The invention relates to an air cushion guide for arched or web-shaped Materials, especially for printed sheets of paper in a printing press, at which is the guided sheet or sheet material on an air cushion supported by at least one guide body with nozzle openings, through the air is blown between the guide bodies and the guided material. Such Air cushion guides are for example in DE 44 27 448 A1 and DE 42 42 730 A1. They come in all kinds of shapes and configurations among other things used to make the freshly printed and still damp Sheets of paper in the delivery of, for example, offset printing machines are non-contact from the printing unit to the delivery stack or in sheet turning devices Sheet transfer drums or similar to the sheet between two printing cylinders transport. Here the problem arises that depending on the print job very much Different types of paper, some of which are printed on both sides, are safe, i.e. without lubricate, must be transported. However, this can be done through a Air cushion guide with a fixed characteristic, which is determined by the number and shape of the Nozzles and the amount of air blown through the nozzles is determined, not for all Ensure paper types equally well.

In general, one would assume that the risk of greasing is all the less, ever greater the hovering height of the arch above the guide bodies. However, that is not quite true. Because with air cushion guides that work on the principle of hydrodynamic paradoxes depend on the stability of the guide Height of the air cushion. Thicker air cushions are more unstable, i.e. there they are Restoring forces from the air cushion to the guided sheet Distance changes are exercised, much less than at Floating guides with a small distance between the guide body and the arch, at those due to a high flow rate from the nozzles escaping air, the guided arc is stable, i.e. with high restoring forces to be led. The latter is particularly the case for thin, resilient papers more optimal solution, while with rigid, thick paper qualities a too small one Distance to the baffles is problematic.

das Gebläse aufdreht" und somit den Bogen aufgrund der größeren in das Luftpolster eingeblasene Luftmenge anhebt. Dies wird anhand des Diagramms nach Figur 4 deutlich. Dort sind die Betriebszustände einer Luftpolsterführung nach dem Stand der Technik dargestellt, und zwar zeigen dort der Graph c die Abhängigkeit der Geschwindigkeit c des durch die Düsen in das Luftpolster eingeblasenen Stroms und der Graph Q den Volumenstrom Q, jeweils abhängig vom Vordruck P_v der die Düsen speisenden Kammer des Leitkörpers. Bei einer Druckerhöhung ändern sich beide Größen ungefähr im gleichen Ausmaße, d.h. etwa proportional zueinander. Hingegen bleibt die Schwebehöhe, wie der Graph h zeigt, bei Druckänderungen über einen relativ großen Bereich zwischen 0,5 Millibar und 10 Millibar nahezu gleich.It would therefore be optimal if one could implement an air cushion guide that combines a large distance between the guided arc and, at the same time, a high degree of stability as a result of a high flow velocity of the blown air under the arc. However, it is a fallacy to assume that you can do this with an air cushion that works according to the aerodynamic paradox by simply

the fan turns on "and thus raises the arc due to the larger amount of air blown into the air cushion. This is clear from the diagram according to FIG. 4. There the operating states of an air cushion guide according to the prior art are shown, namely that the graph c shows the dependency the velocity c of the current blown into the air cushion through the nozzles and the graph Q the volume flow Q, in each case depending on the admission pressure P _v of the chamber of the guide body feeding the nozzles. When the pressure increases, both quantities change approximately to the same extent, ie approximately in proportion to one another On the other hand, as the graph h shows, the levitation height remains almost the same for pressure changes over a relatively large range between 0.5 millibars and 10 millibars.

Since it was therefore not possible to adjust the hovering height of the guided arches different paper qualities by controlling the for the air cushion guidance used compressed air generator, others have been in the past Treaded paths. For example, the above-mentioned DE 42 42 730 A1 teaches to arrange the air openings or nozzles in exchangeable exchangeable cassettes, i.e. the adjustment of the air cushion guide to the guided material is carried out by a Cassette exchange accomplished. However, this is during the operation of the Printing machine is not possible and cannot be automated.

DE 42 09 067 A1 describes a sheet guiding device in which the Floating height of the arch in the middle section of the arch by additional Blowing nozzles are enlarged, their blowing jets perpendicular to the arc surface hit and this through the impulse effect of these additional air jets in the Raise the middle. In this way, the hovering height can possibly be above Homogenize the width of the arch, a change in the total hover height however does not result.

A combination of nozzles that work according to the hydrodynamic paradox, and blowing nozzles directed vertically onto the guided paper web for the purpose of elevation and homogenization of the levitation height of the guided train is already in the DE 17 74 126 described for sheet-like materials. But there is none there either Possibility given to the levitation height when operating the device adapt different material qualities.

From DE 20 20 430 it is known for the guidance of web-shaped Materials used, formed in the manner of a wing wing Mechanically switch air cushion guides so that there are at least two stable zones for the distance between the guided path and guide body result. The characteristic of the guide body is changed so that it once as Air cushion nozzle and the other wing nozzle, i.e. after this hydrodynamic paradox works. However, the higher distance of the guided part, which results from the air cushion characteristic, by the lower Purchased the stability of the air cushion created by this type of nozzle.

It is the object of the present invention to provide an air cushion guide which also adapt to the different properties of the led materials can be adjusted so that in particular the Levitation height of the guided sheet or the guided path over simple Automation processes can change, as well as a suitable process for this specify. This object is achieved with those specified in claims 1 and 18, respectively Features resolved.

The invention is based on the knowledge that the levitation height at a only according to the hydrodynamic paradox air cushion guide can change significantly if the proportionality between that from the nozzles emerging air volume flow and the flow velocity of the air is lifted between the guide body and the guided material. To do that the air volume flow and / or the flow velocity the air is set independently of one another and thus the quotient between the changed both sizes mentioned.

This measure not only allows the suspended height of the guided part while maintaining the principle of the hydrodynamic paradox Set the given stability to different values. Through targeted non-reactive changes in air volume flow and kinetic energy of the air cushion, the air cushion guide can also be optimally attached to others Adapt influences that occur on printing presses during operation, such as Subject and degree of moisture absorption of the printed sheet, centrifugal forces occurring, Turbulence, air jets from hot air dryers etc. The printer receives one Another possibility to influence the guided bow and the result optimize the printing process.

A possibility of independent adjustment of the two sizes mentioned from each other is the number of materials covered by the led to change effective nozzle openings. For example, that the guide body contains several groups of nozzles, each group of Nozzles can be switched on and off separately with blown air. So the groups can from nozzles via switching valves to a common blower air generator be connected, or each group of nozzles to a separate one Blow air generator must be connected. By machine-controlled actuation of the Switching valves or activation of the blower air generators can then Increase the total air volume flow under the guided material or sheet, without changing the flow velocity of the air. In this way the hovering height of the guided material or sheet increases without Loss of leadership stability.

In an analogous manner, it is possible to determine the effective cross sections of individual Nozzle openings, individual groups of nozzle openings or all nozzle openings to change, for example by electrically operated flaps, slides or the like. So the nozzles can have flap-shaped, flexible tongues, which are deformed, for example, by means of electrically actuated actuating gears. At suitable cross-section of the tongues as a bimetallic strip Change nozzle opening also via the temperature of the air flow, or at lighter Flexibility of the tongues under the influence of those forming on the nozzles Pressure difference.

According to a particularly advantageous embodiment of the invention, the guide bodies have at least two groups of nozzles and the groups have different consumer characteristics, ie different dependencies of the volume flow let through the nozzle openings on the supply pressure p _v applied to the nozzle openings. If the consumer characteristic curves differ by at least a factor of 2, for example, by controlling the ratio of the pressures p _v 1, p _v 2 of the two nozzle groups, the quotient of the volume flow Q which is blown into the air cushion as a whole, with the flow velocity which is effectively formed under the material being guided c affect the air flow and thus also the hover height h. The degree of influence is, of course, the greater, the more different the consumer characteristics of both groups of nozzles are, so that it is then also expedient to operate the groups of nozzles by means of different blowing air generators, such as gas blowers, ejectors or axial fans, which are inherent provide different high admission pressures and volume flows.

The embodiment of the invention can be automated particularly well because for a control of the levitation height, for example, only the speeds of the Blow air generators, which supply the two groups of nozzles, independently need to be controlled from each other.

In the sense of automation that is as simple as possible, it is also appropriate to use one electrical control device to provide the level of suspension of which Air cushion led material or the degree of change in the levitation as Setpoint can be entered and the subsequent manipulated variables for changing the from Air volume flowing out of the nozzle openings and / or the Flow velocity between the guide body and the guided material determined. Here, the determination of the manipulated variables can be based on previously stored one or two-dimensional characteristic fields.

Figur 1

eine vereinfachte Prinzipskizze, die den Ausleger einer Bogendruckmaschine in einem Vertikalschnitt zeigt;

Figur 2

eine im Vergleich zu Figur 1 vergrößerte Darstellung der Bogenführung aus Figur 1 im Bereich des Greifers 2d;

Figur 3

ein Diagramm, in dem typische Kennlinien eines Lufterzeugers (Gebläse) und eines Verbrauchers dargestellt sind:

Figur 4

ein Diagramm, in dem für eine typische Luftpolsterführung die Abhängigkeit der Störungsgeschwindigkeit c, des Volumenstroms Q und der Schwebehöhe h des geführten Bogens vom Vordruck p_v an den Blasdüsen dargestellt ist:

Figur 5

eine Prinzipskizze eines ersten Ausführungsbeispiels der Erfindung

Figur 6

eine Prinzipskizze eines zweiten Ausführungsbeispiels der Erfindung

Figuren 7a und 7b

Prinzipskizzen eines dritten Ausführungsbeispiels der Erfindung, wobei Figur 7a einen Teil der Figur 7b in vergrößerter Darstellung zeigt,

Figuren 8a und 8b

Skizzen von im Vergleich zu Figur 7a und 7b alternativen Ausführungsformen für die Änderung des Querschnitts der Düsen des dritten Ausführungsbeispiels;

Figur 9

die Prinzipskizze eines vierten Ausführungsbeispiels der Erfindung

Figur 10

ein Diagramm, in dem zwei unterschiedliche Paare von Erzeuger- bzw. Verbraucherkennlinien der Luftpolsterführung nach Figur 9 dargestellt sind.

Figur 11

ein Diagramm, das die Schwebehöhe h des von der Luftpolsterführung nach Figur 9 geführten Bogens in zweidimensionaler Darstellung abhängig von den Vordrucken p_v1 und p_v2 in den Kammern 116a/c bzw. 116b zeigt,

Figur 12a

eine vereinfache Prinzipskizze, die eine Luftpolsterführung gemäß der Erfindung im Bereich einer Bogenumführtrommel zwischen zwei Druckzylindern einer Bogendruckmaschine zeigt,

Figur 12b

die Aufsicht auf das Bogenleitblech 513b in Figur 12a.

Further advantages of the invention result from the following description of exemplary embodiments with reference to FIGS. 1 to 12b of the accompanying drawings. Here are:

Figure 1

a simplified schematic diagram showing the boom of a sheet printing machine in a vertical section;

Figure 2

an enlarged representation of the sheet guide of Figure 1 in the region of the gripper 2d compared to Figure 1;

Figure 3

a diagram showing typical characteristics of an air generator (blower) and a consumer:

Figure 4

a diagram in which the dependence of the perturbation speed c, the volume flow Q and the suspension height h of the guided sheet on the pre-pressure p _v at the blow nozzles is shown for a typical air cushion guide:

Figure 5

a schematic diagram of a first embodiment of the invention

Figure 6

a schematic diagram of a second embodiment of the invention

Figures 7a and 7b

Schematic diagrams of a third exemplary embodiment of the invention, FIG. 7a showing part of FIG. 7b in an enlarged view,

Figures 8a and 8b

Sketches of alternative embodiments compared to FIGS. 7a and 7b for changing the cross section of the nozzles of the third embodiment;

Figure 9

the schematic diagram of a fourth embodiment of the invention

Figure 10

a diagram in which two different pairs of generator or consumer characteristics of the air cushion guide according to Figure 9 are shown.

Figure 11

9 shows a diagram which shows the levitation height h of the sheet guided by the air cushion guide according to FIG. 9 in a two-dimensional representation depending on the forms p _v 1 and p _v 2 in the chambers 116a / c and 116b,

Figure 12a

1 shows a simplified schematic diagram which shows an air cushion guide according to the invention in the area of a sheet bypass drum between two printing cylinders of a sheet printing machine,

Figure 12b

the view of the sheet guide plate 513b in Figure 12a.

In Figure 1, the delivery 1 of a sheet printing machine is shown schematically. There, the sheets of paper 4 gripped on gripper bridges 2 on the front side are conveyed by the last printing unit, not shown in the figure, via a so-called guide body 3 in the direction of the delivery stack 6. As can be seen from the enlarged sketch according to FIG. 2, the guide body 3 is designed as a box, in the hollow interior of which a blower 7 blows air via a connecting line 8 and thus generates an overpressure p _v which escapes through a plurality of nozzle openings 5 and thus enters Air cushion builds between the air body 3 or its surface and the underside of the sheet 4 guided above.

The diagram of FIG. 3 shows an example of the characteristic curves of the two essential components of an air cushion guide, namely that of a typical air generator, ie a blower (Graph B) and that of a consumer, ie a number of nozzle openings in a guide body (Graph A). The volume flow Q generated by the generator or the volume flow through the consumer as a function of the pressure p _v , ie the pressure at the consumer acting as a throttle or the pressure difference resulting from the generator due to the throttling effect at the consumer, is respectively plotted. The characteristic curve of the consumer with the nozzle openings acting as a throttle (graph A) begins at the zero point and then rises. The characteristic of the air generator (Graph B), on the other hand, reaches its maximum pressure when the volume flow drops to zero due to the throttling effect of the consumer, ie in the case that all nozzle openings are closed. The operating point p _{1 of} the air cushion guide is the intersection between the characteristic curves B of the air generator A and the air consumer.

If the output of the generator is changed, for example by changing the speed of the fan, by throttling the supply line to the consumer or by bypassing the consumer line, the generator characteristic curve changes, as shown by the dashed graphs B 'and B''in FIG. 1 . However, the points of intersection with the unchanged consumer characteristic result as operating points for the air cushion guide. Accordingly, by changing the power of the blower, as shown by the bold part of graph A in FIG. 1, the volume flow can be changed approximately in proportion to the pressure applied to the nozzles. Since the flow rate in turn depends on the pressure, as is shown in the graph according to FIG. 4, it is consequently not possible to set the flow rate independently of the volume flow by changing the power of the fan with a fixed consumer characteristic. This has the consequences already mentioned at the outset for the levitation height of the arch guided over the air cushion guide. The excess pressure at the nozzle openings of the guide body is converted into flow velocity, assuming lossless flow, according to the following equation: c = 2 x κ κ-1 x p v p 0 x 1 - p 0 p v κ-1 κ xc 0 2nd

Due to the shape of the nozzles, the accelerated air spreads evenly between the guide body containing the nozzle openings and the sheet guided above it. According to the continuity equation Q 1 = Q 2nd According to which the volume flow through the nozzle openings corresponds to the volume flow Q ₂ under the bend due to the chamber pressure p _v applied to it, this results for a cross section perpendicular to the flow under the bend Q = wxhxc and thus for the hovering height

The following apply to the specified relationships

Designations:

κ

= Isentropic exponent

p _v

= Density of the air under the pre-pressure of the chamber

p ₀

= Pressure of the air flow under the arch (corresponds to atmospheric pressure)

p _v

= Pre-pressure in the chamber

c ₀

= Air speed in the box

Q

= Volume flow

b

= Width of the cross section

H

= Hovering height

c

= Flow velocity

From this dependence it can be seen that the levitation height of a Air cushion guidance can only be changed when the quotient is off Volume flow and flow velocity is changed, i.e. if indeed the Volume flow, but not at the same time the flow rate increases or if the volume flow is significantly disproportionate to Flow rate is increased.

Since, as already mentioned, the flow rate depends on the pressure p _v at the nozzle openings, in order to achieve the object of the invention, this should be able to be changed independently or not in proportion to the volume flow through the nozzles. According to the exemplary embodiment of the invention illustrated in FIG. 5, this is made possible in that the number of effective nozzle openings covered by the guided material can be changed. For this purpose, the chamber 13 under the guide plate 13b, which together form the guide body of an air cushion guide, consists of several individual chambers 16a, b ... in whose air supply lines there is an electrically controllable valve on the underside. The air supply lines opening into the base plate 13a of the guide body are designated by way of example with 14 a, b and c and the associated valves with 15 a, b and c. The corresponding chambers 16 a, b and c of the guide body are each assigned groups of nozzles in the guide plate, of which each group is only symbolically represented as individual nozzle 12 a to c for the sake of clarity. The chambers 16a, 16c are supplied together by a speed-controllable fan 17. The fan 17, like the valves 15a, 15b, 15c ..., is connected to an electronic control unit 19 which, from the central control computer of the printing press, to which the delivery arm shown in FIG. 1 belongs, feeds the hovering height h _{should of} the non-contact sheet as a setpoint becomes.

The control works as follows:
If the levitation height is to be changed towards smaller values, the controller 19 switches off some of the valves 15 and thus reduces the number of effective nozzle openings. In this way, the volume flow of the gas blown into the air cushion is reduced, while at the same time, due to the higher throttling action of the nozzle arrangement, the pressure generated by the blower 17 increases and thus also the flow velocity of the air flow emerging from the nozzles 12. In this way, for the air cushion guide in the diagram according to FIG. 3, an operating point is obtained at the point denoted by P2. This corresponds to the intersection of a flatter consumer characteristic curve A 'with the unchanged generator characteristic curve B. If the flow speed under the bend is to be adjusted to the previous value at the same time, the blower is also regulated to a lower speed, so that the generator characteristic curve B''and thus results in a working point at the point designated P3 in the diagram according to FIG. 2.

It is clear that the division of the guide body into individual chambers 6 is done in this way must that there are no disruptive inhomogeneities in the levitation of the Arches.

While in the previous exemplary embodiment individual groups of nozzles and thus a discrete number of nozzle openings in the nozzle guide plate are supplied with blown air and thus 6, the change in the number of effective nozzle openings is realized by a continuously variable transmission in the next exemplary embodiment according to FIG. 6. The same parts are provided with a reference number that is 100 higher and are not explained again here. Here, the chamber 113 of the guide body divided into three sections 116a, b, c by two inserted webs 115a, b, of which the central section 116c is connected to a blower 117 via a hose line 118. The two webs 115a and 115b can be turned in opposite directions with the aid of threaded spindles 114a and 114b by means of a motor 120 and toothed belt 110 connected to the controller 119. In this way, the size of the two chambers 116a and 116b which are not connected to the blower 117 and thus deactivate the nozzles above them which are assigned to them can be changed.

To the volume flow and thus the levitation height of the above the baffle 113b conveyed sheet, the chambers 116a and 116c reduced by moving the webs 115a and 115b and vice versa.

In the next exemplary embodiment according to FIGS. 7a and b, the Volume flow of the air cushion guide independent of the Change flow rate by changing the cross sections of individual Nozzle openings or groups of nozzle openings or all nozzle openings are changeable. For this purpose, in the guide plate 213b, over which the bend 201 floats in a manner known per se through nozzle openings Embossing / punching process introduced so that it is inside the guide body protruding tongues 212a, b, c .... over which the air flow of the Overpressure in the chamber 213 of the guide body under the guided bend 201 can reach. On the underside of the tongues 212a, b, c .... from, for example Steel sheets are made with metal strips 214a of another material such as Aluminum glued on, so that in the area of the movable tongues 212 Nozzles gives a bimetal property. At the same time in the feed pipe 218, that connects the chamber 213 to the controllable fan 217, the heating coil 216 introduced a controllable heater 215. By changing the Heating power can thus be the temperature of the air supplied to the chamber 213 change, whereby the tongues 212/214 of the nozzles in the sheet guide plate 213b deform and reduce the cross section of the nozzle openings accordingly or enlarge. The heating power and the Speed of the fan 217 set so that the Flow velocity c of the air flow emerging from the nozzles and the air volume flow Q independently of that for the bend 201 have the optimal conditions adjusted.

Controlled bending of the resilient tongues of the nozzle openings leaves however, can also be accomplished in a different way, as outlined in FIG. 8a is. There the tongues 312 of the nozzles in the guide plate 313b of the guide body are around assigned a shaft 315 rotating eccentric discs 316, with the help of which flexible tongues 312 can be partially closed. The shaft 315 is for example connected to a stepper motor, not shown here, the is in turn connected to a control system via which the Angular position of the shaft 315 and thus the cross-sectional area of the nozzles and If necessary, have the power of the compressed air supplier set.

Of course, the cross-sectional area of the nozzle openings can also be through electrically actuated slide 318, flaps, or the like change, such as which is shown in Figure 8b, without the nozzle tongues themselves resilient must be trained and deformed.

FIG. 9 shows a particularly preferred embodiment of the invention. Here, the guide body 413, over which the sheet 401 is guided, is divided into three parallel chambers 416a, 416b and 416c, which are adjacent to one another and which are each connected to their own air supplier.
The chambers 416a and 416c are each connected to a gas blower 417a or 417c, with which relatively high admission pressures p ₀ can be achieved with small volume flows. The middle chamber 416b is supplied by an axial fan 417b, which delivers high air volume flows even at low pressure differences. The boundaries of the three chambers do not have to run in a straight line as shown in FIG. 10, but can also be jagged in order to mix as well as possible the different air flows emerging from the nozzles 417 of the guide plate 413b under the guided arc 401.

The chambers 416a and 416c on the one hand and the chamber 416b on the other now very different consumer characteristics. Accordingly, step through the associated nozzles 412a and 412c on the one hand and 412b on the other hand very much different large volume flows with widely differing Flow velocities. By mixing the air flows a mean value of the flow velocity for the added volume flows. By deliberately changing the parameters in the Chambers 416a and 416c or 416b, on the other hand, it is now possible to apply the effective one the air flow acting on the arc in relation to the flow velocity c and set the volume flow Q independently of each other. This is illustrated below with the aid of the diagram in FIG. 10:

The consumer characteristic A1 of the middle chamber 416b is relatively steep. At low pressure, a high volume flow is already achieved. Hence a large volume of air with low flow speed under the bend 401 flushed in. This is achieved by a large number of nozzles in the Sheet baffle 413a, that is, by a high nozzle density, or by using nozzles correspondingly large opening cross sections.

The consumer characteristics A2 of the two outer chambers 416a and 416c run relatively flat. To push enough air through the nozzles, a high pressure difference is created. Since the form in the chambers 416a and 416c is thus high, the air flows out of them at high speed Chamber assigned nozzles. This is achieved through a low one Nozzle density or through nozzles with very narrow throttle cross sections. In the diagram 10 shows the characteristics of the chambers 416 and the blowers 417 drawn together. For the two different chamber types then in connection with the two different blowers or Fan types the working points designated P1 and P2.

The air flowing out of the middle chamber 416b is obtained by the arrangement the nozzles are directed towards the two outer chambers 416a and c Flow direction. The large volume of air in this chamber flows at a lower rate Speed between the baffle 413a and the bend 401. This middle Chamber 416b is relatively narrow.

The air emerging from the first chamber 416b mixes with the air from the chambers 416a and 416c above the two outer chambers 416a and 416c. The volume flows add up and the speeds mix weighted according to the proportions of the volume flows. If the consumer characteristics of the chambers are chosen very differently, a wide range of operating points can be obtained by mixing the two air flows. In this way, the speed of the gas blowers 417a and c or the axial fan 417b, the volume flow blown into the air cushion and the average flow rate which is established according to the mixing rule can now be controlled simply by appropriate control c set independently of one another and thus also select the levitation height of the arch 401 over the guide plate 413b within wide limits as required. The extent to which the levitation height can be changed naturally depends on the ratio of the areas of the nozzle openings of the chambers 416a and c on the one hand and 416b on the other hand. It should be about a factor of 2 to 20.

a) der Druck in der mittleren Kammer 416b wird verändert. Hierdurch ändert sich der Volumenstrom der gesamten Luftpolsterführung sehr stark, während die mittlere Geschwindigkeit des Luftstroms nahezu unverändert bleibt. Die Schwebehöhe ändert sich hierbei etwa proportional der Änderung des Volumenstroms, der von der mittleren Kammer 416b ausgeht.

b) Stabilität: Beobachtungen zeigen, daß der Widerstand, den die Luftpolsterführung dem in das Luftpolster eintauchenden Bogen und somit einer Annäherung an das Bogenleitblech 413b entgegenbringt, vor allem von der Strömungsgeschwindigkeit abhängt. Je größer die Strömungsgeschwindigkeit um so stärker ist das Beharrungsvermögen der Luftströmung und somit auch die Reaktionskraft gegen Störungen des Luftpolsters. Um die Stabilität der Führung zu steigern, kann daher der Druck der äußeren Kammern 416a und 416c gesteigert werden.

The hovering height of the arch which arises on the basis of this formula is shown in the three-dimensional diagram according to FIG. The pressures in the chambers 416a, c and 416b are plotted there in the two ordinates and the levitation height in the coordinate. It is assumed that the air generators and air consumers have the characteristics shown in the diagram in FIG. 10. It can be seen from the diagram that the hovering height of the arc can be changed by a factor of about 3 by varying the pressures in the chambers 416, by changing the corresponding blowers 417 by the controller 419, for example by specifying the desired hovering height. The adjustment can be made according to the following parameters:

a) the pressure in the middle chamber 416b is changed. As a result, the volume flow of the entire air cushion guide changes very significantly, while the average speed of the air flow remains almost unchanged. The levitation height changes here approximately in proportion to the change in the volume flow which emanates from the middle chamber 416b.

b) Stability: Observations show that the resistance which the air cushion guide presents to the arc immersed in the air cushion and thus to an approach to the arc guide plate 413b depends primarily on the flow velocity. The greater the flow velocity, the greater the persistence of the air flow and thus the reaction force against disturbances in the air cushion. In order to increase the stability of the guide, the pressure of the outer chambers 416a and 416c can therefore be increased.

Thin papers react to high flow velocities through high-frequency vibrations. Here the pressure in both outer chambers can be reduced to one to effect gentler "guidance at about the same level of levitation.
Another preferred exemplary embodiment of the invention is shown in FIGS. 12a and b. There the sheet 501 to be printed on the back, for example, is conveyed by a transfer drum 502 between two printing cylinders 503 and 504. In order to ensure reliable and lubrication-free sheet travel between the printing units, an air cushion guide, which is semi-circular in section, is arranged below the transfer drum at a short distance from the circular arc described by the grippers of the drum 502. The top view of the likewise semicircular arch guide plate of this air cushion guide is shown in FIG. 12b. There it is indicated by the dashed lines that the chamber 513 below the nozzle openings is divided into different areas 513a to 513h, each sub-chamber being assigned an air generator 517a-h. The air generators 513a, b and 513 g, h are gas blowers which supply a few nozzles arranged on the outer edges of the sheet guide with a relatively high pre-pressure, whereas the four middle areas 513 c, d, e and f arranged one behind the other in the direction of sheet travel or the nozzles assigned to them, arranged on the sheet guide plate close to each other by four axial fans 417c-d, which generate a high volume flow. The levitation height h of the arch 501 above the arch guide plate is set similarly to that described in the exemplary embodiment according to FIG. 9.

Claims (20)

Air cushion guide for sheet or sheet materials, in particular for printed paper sheets in a printing press, in which the sheet or sheet material guided is supported on an air cushion over at least one guide body with nozzle openings, through which air is blown between the guide body and the guided material,
characterized,
that the air volume flow (Q) emerging from the nozzles and / or the flow velocity (c) between the guide body (3; 13; 113; 213; 313b; 413a, b, c; 513) and the guided material (4; 201; 301; 401 ; 501) can be set independently of one another in such a way that the proportionality between the two variables is eliminated. Air cushion guide according to claim 1,
characterized,
that the number of effective nozzle openings (5; 12a-c; 112a; 212a-c; 312; 412a-c; 312; 412a-c; 512) covered by the guided material can be changed. Air cushion guide according to claim 2,
characterized,
that the guide body contains several groups (12a-c) of nozzles, each group of nozzles being supplied with blown air that can be switched on and off separately. Air cushion guide according to claim 3,
wherein the groups (12a-c) of nozzles via switching valves (15a-c) are connected to a common blown air generator (17). Air cushion guide according to claim 3,
each group of nozzles is connected to a separate blower air generator. Air cushion guide according to claim 1,
characterized,
that the effective cross sections of individual nozzle openings, individual groups of nozzle openings or all nozzle openings (212a-c; 312; 322) can be changed. Air cushion guide according to claim 6,
wherein the cross sections of the nozzle openings can be changed by means of electrically operated flaps, slides (318a) or the like. Air cushion guide according to claim 6,
the nozzles having flap-shaped resilient tongues (312) which are deformable in a controlled manner. Air cushion guide according to claim 8,
marked by
electrically operated actuating gear (315; 316) for deforming the tongues. Air cushion guide according to claim 8,
wherein the tongues (212/214) are designed as bimetallic strips and a heating device (215/216) is provided for the air flow. Air cushion guide according to claim 8,
the tongues deform under the influence of the pressure difference (p _v - p ₀ ) which forms at the nozzles. Air cushion guide according to claim 1,
characterized,
that the guide body contains at least two groups (412a / c; 412b) of nozzles and the groups have different consumer characteristics (A1, A2), and that the pressures (P _v 1, P _v 2) for supplying the groups of nozzles can be regulated independently of one another are. Air cushion guide according to claim 12,
the sum of the throttle areas of the nozzles of both groups (412a / c, 412b) differs by at least a factor of 2. Air cushion guide according to claim 12,
the individual groups (412a / c, 412b) of nozzles being connected to different blown air generators (blowers 417a / c, ejector, axial fan (417b)). Air cushion guide according to one of claims 1-14,
marked by
an electrical control device (19; 119; 219; 419) which can enter the floating height (h _soll ) of the material (201; 401) guided by the air cushion or the extent of its change as a setpoint and which can then be used to control the change in the Air openings (Q) and / or the flow velocity (c) between the guide body and the guided material are determined. Sheetfed offset printing machine with an air cushion guide (3) for the printed Bends (4) according to any one of claims 1-15 in the boom (1) of the machine. Sheetfed offset printing machine with an air cushion guide (513) for the printed Bends according to one of claims 1-15 in the area of the sheet transfer (501) or turn between two pressure cylinders of the machine. Method for adjusting the levitation height of arched or web-shaped Materials in an air cushion guide, in which the guided material is on supports an air cushion over at least one guide body and over Air is blown into the nozzle under the guided material, whereby the quotient between the air volume flow blown through the nozzles (Q) and the flow velocity (c) of the air between the guide body and led material is changed. Method according to claim 18,
the guide body contains at least two groups (412a / c, 412b; 512a / b / g / h, 512 c / d / e / f) of nozzles with consumer characteristics of the groups that differ by at least a factor of 2 and the ratio of the air pressures (P _v 1, P _v 2) to each other is changed, with which the groups of nozzles are supplied. Method according to claim 18,
the effective cross sections of the nozzles or individual groups of nozzles are changed.

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