Vacuum conveyor

Abstract: Vacuum conveyor (20), which is suitable for guiding a running web (9), e.g. paper web, in particular a lead strip (9a) or "threading tail" (e.g. during "threading" of a paper or board web into a machine for the production or finishing or processing of such a web), with the following characteristics: a) an air-pervious endless conveyor belt is tensioned over at least two rolls with a suction box (21) being located in the loop of the conveyor belt and being connected to a source of vacuum (22); b) at least one of the rolls can be driven, to allow the endless conveyor belt to travel over the rolls and over the suction box, whereby the vacuum propagates through the run of the conveyor belt running in the direction of web travel, in order to draw the web to be guided onto the conveyor belt by suction; c) a vacuum control is adapted to establish a fast decrease of the vacuum degree in the suction box; d) the vacuum control comprises a movable element positioned at an inner surface of a wall of the suction box where said wall has an aperture (23) which may be closed by said movable element; e) an actuator being connected to the movable element which actuator, in an active state, causes the closing of said aperture by said movable element.

Description: FIELD OF THE INVENTION The invention relates to a vacuum conveyor having the characteristics stated in the preamble of claim 1. Such a vacuum conveyor is used for transporting a web, preferably a lead strip of a paper web, from a section of a paper-making or paper-finishing machine to a following section of that machine. DESCRIPTION OF PRIOR ART A vacuum conveyor is known from DE 26 36 887 (preamble of claim 1) which is similar to US 4,022,366 (File PD10778). Conveyors of this kind have proven successful in operation. In some applications, however, a paper tail must be transferred from a vacuum conveyor to a rope nip (of a following machine section) at a very high speed (over 1500 meters/min). This requires a very rapid change of degree of the vacuum applied to the conveyor. For this purpose it is known to use a pneumatic operated damper at the vacuum source or in the vacuum line. Recently, however, the operating speed of paper-making or paper-finishing machines is being further increased, namely up to about 2200 meters/min. Unfortunately, in such extreme high speed systems, the conventional dampers do not operate in a reliable manner. Therefore, the invention is based on the problem of further developing the known vacuum conveyor and its vacuum control in such a way that the transfer of a tail into the ropes of a following machine section is improved, in particular at extremely high operating speeds. This problem is solved by a novel design of the vacuum control as described in claim 1. SUMMARY OF THE INVENTION According to the present invention, a wall of the suction box has an aperture which may be closed by a movable element placed at the inner side of said wall. An actuator is connected to the movable element for pressing the same onto the inner side of said wall, if one desires to close said aperture; i.e. if the vacuum conveyor is in its normal operating state. In this state, the atmospheric pressure is working onto the outer side of the movable element and wants to remove the movable element from the wall (whereby the aperture would be opened). However, onto the inner side of the movable element, two forces A and B are effective against the atmosperic pressure for maintaining the aperture closed. Force A is a variable control force or "actuator force" created by said actuator and force B is depending form the internal pressure existing in the interior of the suction box; said internal pressure is the difference of the atmospheric pressure minus the actual vacuum degree. The higher the vacuum degree is, the smaller is force B. The vacuum control according to the present invention will operate in the following way: If e.g. during a start-up of a paper-making machine the tail of a lead strip of the paper web arrives at the vacuum conveyor for further transferring the lead strip into a rope system of a following machine section, the lead strip is now covering the suction openings (e.g. slots) of the conveyor. As a result of that the internal pressure in the suction box is decreasing severely; thereby the lead strip would be held too firmly to the conveyor. However, the decrease of the internal pressure immediately results in a decrease of said force B, whereby the aperture is quickly opened, if force A had been adjusted to a relative low set value. Now the vacuum decreases (i.e. the internal pressure increases), then the movable element again closes the aperture; thus a steady and relatively low vacuum degree (depending from the set value of force A) is created within an extremely short period of time. Such a rapid change of the vacuum degree (in the vacuum conveyor) will improve the transfer of the lead strip, in particular when the tail of the lead strip must be transferred into a rope system of the following machine section (e.g. calender or coating machine or reel) of a high-speed paper-making or -finishing machine. BRIEF DESCRIPTION OF THE DRAWING In the drawing which illustrates an embodiment of the invention, Fig. 1 shows schematically two sections of a paper-making machine, with some vacuum conveyors being disposed therebetween, Fig. 2 and 3 show details of the vacuum control system of one of the vacuum conveyors of Fig. 1. DESCRIPTION OF PREFERRED EMBODIMENT According to Fig. 1, a paper web 9 is travelling through the final sections of a paper-making machine. The web leaves the last roll 10 or cylinder of a preceding section and is guided by paper rolls 11, 12 and 13 to the first roll nip 14 of a calender 15. At the paper roll 13, which is positioned close to roll nip 14, two ropes 16 and 17 form a rope nip in a manner known in the art. During start-up of the machine or after an interruption of the paper-making process, the paper web must be threaded from section to section of the machine and through each of the sections, e.g. through the calender 15. For that purpose, at first a narrow edge strip or lead strip (separated from the web by a cutting device) is transferred by means of vacuum belt conveyors 18, 19, 20 along a path 9a (illustrated by a dotted line) into the rope nip at roll 13. Thereafter the ropes 16, 17 transfer the lead strip through the calender. Then, in a known manner, the lead strip is widened up to the full width of the web. Each of the vacuum conveyors 18, 19, 20 comprises an endless perforated belt travelling over two rolls or pulleys. Between these pulleys, there is a suction box 21 connected to a vacuum source 22. One of the pulleys can be driven by a conventional motor. For the control of the vacuum degree within the suction box 21, control elements as shown in Figs. 2 and 3 are disposed at an aperture 23 of a side wall 24 of the suction box. The external surface of side wall 24 is designated as 25, while the internal surface of side wall 24 is designated as 26. Close to the internal surface 26 of side wall 24, a movable element 27 (e.g. in form of a plate) is installed which covers the aperture 23 and is therefore designed to close the same. A sealing 28 may be arranged between plate 27 and side wall 24. A support 30 (e.g. in form of a bow) is fixed to the internal surface 26 of side wall 24. To this support 30, one end of an actuator 31 (e.g. in form of an air bag) is connected. The other end of that actuator 31 supports said movable element 27. The interior of air bag 31 is connected via line 32, pressure control valve 33 and line 34 to an air pressure supply 35, whereby air bag 31 creates a variable control force or "actuator force A". Valve 33 holds said actuator force A at an adjustable set value. Valve 33 is (e.g.) self-relieving if the set value is decreased as symbolically illustrated by an arrow 36. In operation, vacuum source 22 creates in box 21 an internal pressure which is lower than the atmospheric pressure. Plate 27 closes the aperture 23 as shown in Fig. 2, if the actuator force A plus a force B (depending from said internal pressure) is greater than a force C (depending from the atmospheric pressure) being effective onto the outer side of plate 27, with the forces A and B acting onto the inner side of plate 27. If (as described above) force B suddenly decreases, then force C will exceed the sum of forces A and B and will therefore open the aperture 23 (as shown in Fig. 3), whereby the internal pressure will immediately increase again (i.e. the vacuum degree will immediately decrease). As a result of that, plate 27 will again close the aperture 23. To completely purge vacuum from conveyor box 21 by completely deactivating the actuator 31, a solenoid valve 37 is energised whereby the interior of actuator 31 is connected to the atmosphere. This action is applied so there is no vacuum acting on the paper web when the ropes have total control of the paper web.

Claims: Vacuum conveyor (20), which is suitable for guiding a running web (9), e.g. paper web, in particular a lead strip (9a) or "threading tail" (e.g. during "threading" of a paper or board web into a machine for the production or finishing or processing of such a web), with the following characteristics: a) an air-pervious endless conveyor belt is tensioned over at least two rolls with a suction box (21) being located in the loop of the conveyor belt and being connected to a source of vacuum (22); b) at least one of the rolls can be driven, to allow the endless conveyor belt to travel over the rolls and over the suction box (21), whereby the vacuum propagates through the run of the conveyor belt running in the direction of web travel, in order to draw the web to be guided onto the conveyor belt by suction; characterized in that c) a vacuum control is adapted to establish a fast decrease of the vacuum degree in the suction box (21); d) the vacuum control comprises a movable element (27) positioned at an inner surface (26) of a wall of the suction box (21) where said wall has an aperture (23) which may be closed by said movable element (27); e) an actuator (31) being connected to the movable element (27) which actuator, in an active state, causes the closing of said aperture (23) by said movable element. Vacuum conveyor as claimed in claim 1, characterized by the following features: a) the actuator (31) is an air bag, arranged between a stationary element (30) and said movable element (27); b) the interior of the air bag (31) is connected to a pressure source (35) via a control line (32, 34) wherein a control valve (33) is arranged to provide in the air bag a constant pressure of adjustable degree. Vacuum conveyor as claimed in claim 2, characterized in that the movable element (27) is formed as a plate which is connected to the air bag (31) and which is adapted to contact the inner surface (26) of the suction box (21) in order to close said aperture (23). Vacuum conveyor as claimed in claim 2 or 3, characterized in that said control valve (33) is self-relieving. Vacuum conveyor as claimed in one of claims 1 to 4, characterized in that the outer side of the movable element (27) is exposed to the atmosphere, while onto its inner side, when the aperture (23) is closed, two forces A and B are effective, wherein force A is created by said actuator (31) and force B depends from the vacuum degree prevailing within the suction box (21). Vacuum conveyor as claimed in claim 2, characterized in that an additional valve (37) is arranged in said control line (32) which valve connects the actuator (31) either with the pressure source (35) or with the atmosphere.

Inventors: DADD JAMES, DADD, JAMES
Applicants: VOITH PAPER PATENT GMBH
Application year: 2000
Main classification: D21G9
IPC codes: D21G9, B65H20