Abstract:
The invention is relates a web cleaning apparatus including: a pair of operatively rotating cleaning rollers operatively disposed on opposite sides of the web; a housing surrounding a major part of the surface of each roller; a vacuum chamber substantially co-extensive with each roller; a longitudinal slot extending parallel to each roller and substantially co-extensive therewith, the slot having an inlet proximate the roller surface and an outlet in communication with the vacuum chamber and operatively providing an air flow path from adjacent the roller into the vacuum chamber, wherein the air flow speed at each inlet is not less than the surface speed of the rollers, and a method of using such apparatus. Another apparatus of the invention includes a housing surrounding the roller and spaced therefrom with a gap of about 0.5 mm to about 5 mm. Further embodiments of the invention provide a shutter for controlling the air flow around the rollers in a cleaning step. Further embodiments of the invention provide a valve controlled bypass line whereby a given portion of a web cleaning apparatus can communicate with the atmosphere such that it is not under vacuum.

Description:
BENEFIT CLAIMS  
       [0001]     This application claims the benefit of British Application No. GB 0423467.0, filed 22 Oct. 2004.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to apparatus and methods for cleaning moving webs of material, and in particular for cleaning webs of printable material, in particular paper, prior to printing in a printing press such as a lithographic, flexographic or gravure printing press or the like.  
       BACKGROUND OF THE INVENTION  
       [0003]     The requirement for cleaning webs of printing substrate (paper) is well known in the art. The surface of the paper may carry dust and dirt particles, lint, loose paper fibres and loose paper coating particles which can all be transferred to the printing apparatus itself where they accumulate and degrade the quality of the resultant printing. This necessitates cleaning of the printing apparatus on a frequent basis which is time consuming and expensive. The cleaning process itself for printing plates, blankets and associated components can shorten their useful life.  
         [0004]     Various methods of cleaning the web of printable material have been used in the prior art. One example of a successful web cleaning device is described in U.S. Pat. No. 6,598,261.  FIG. 1  shows a perspective view of a typical apparatus as described in U.S. Pat. No. 6,598,261 and  FIG. 2  shows a cross section through the apparatus. Referring to  FIGS. 1 and 2  it can be seen that a web of paper  500  passes through the cleaning apparatus  502 . The web  500  is driven by a conventional drive roller arrangement (not shown) along a path which may include one or more idler rolls  501 . The direction of movement of the web through the apparatus  502  is indicated by arrows  500 A.  
         [0005]     The rollers  504  most preferably have a soft outer surface, usually comprising a fabric-tufted buffing material. The rollers  504  are disposed in a fixed location on either side of web  500  and rotate in a direction opposite to the direction of movement  500 A of the web  500 . That is, the tangential direction of any point on the surface of each roller  504  when closest to the web is opposed to the direction of movement  500 A of the web  500 . The direction of rotation of the rollers  504  is indicated by arrows  504 A,  504 B.  
         [0006]     Each roller  504  is surrounded by a housing  506  which is connected by means of ducts  508  to a vacuum source (not shown). Mounted within the housing  506  is an “air separating bar”  510  the leading edge  510 E of which is disposed adjacent the roller  504 . An air flow path is defined from the general region of the moving web  500  and the rollers  504  through a region  512  between the air separating bar and a wall of the housing  506 , into a chamber  514  and out through an outlet  516  located at the side of the chamber  514 .  
         [0007]     U.S. Pat. No. 6,598,261 states that the rollers  504  are mounted “in close proximity” to the web  500 . This is interpreted as meaning that the rollers  504  are close to, but do not touch, the web  500 . A spacing of 0.001 inch (0.0254 mm) to 0.01 inch (0.254 mm) between the respective rollers  504  and the web  500  is suggested. U.S. Pat. No. 6,598,261 also suggests that the peripheral speed of each roller  504  is at least 20% greater than the surface speed of the web  500  and also the each roller  504  has a peripheral speed at least double the speed of the moving web  500 .  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention seeks to provide an improved web cleaner and, more specifically, an improved web cleaner generally of the type described in U.S. Pat. No. 6,598,261.  
         [0009]     According to a first aspect of the present invention, there is provided a web cleaning apparatus comprising:  
         [0010]     a pair of operatively rotating cleaning rollers operatively disposed on opposite sides of the web;  
         [0011]     a housing surrounding a major part of the surface of each roller;  
         [0012]     a vacuum chamber substantially co-extensive with each roller;  
         [0013]     a longitudinal slot extending parallel to each roller and substantially co-extensive therewith, the slot having an inlet proximate the roller surface and an outlet in communication with the vacuum chamber and operatively providing an air flow path from adjacent the roller into the vacuum chamber,  
         [0014]     wherein the air flow speed at each inlet is not less than the surface speed of the rollers.  
         [0015]     Preferably the air flow speed at each inlet is at least 1% greater than the surface speed of the rollers, more preferably at least 10% greater than the surface speed of the rollers and especially at least 50% greater than the surface speed of the rollers.  
         [0016]     Preferably the rollers are so spaced apart in use as to provide a clearance gap between each roller and the respective face of the web. That is, the gap between the roller and the respective face of the web should be as small as possible.  
         [0017]     Preferably the rollers are so spaced apart in use as to provide a gap of not more than 6 mm, preferably not more than 3 mm and especially not more than 1 mm between each roller and the respective face of the web.  
         [0018]     In preferred embodiments the rollers rotate in a direction opposite to the direction of movement of the web.  
         [0019]     In particularly preferred embodiments of the invention the inlet of the slot is arranged on the upstream side of the roller with respect to the direction of movement of the web.  
         [0020]     Preferably the differential between the surface speed of each roller and the speed of the web is from about 5 ms −1  to about 30 ms −1 , more preferably from about 6 ms −1  to about 12 ms −1  and especially about 10 ms −1 .  
         [0021]     According to a second aspect of the invention there is provided a method of cleaning a moving web comprising:  
         [0022]     providing a an apparatus comprising  
         [0023]     a pair of rotating cleaning rollers disposed on opposite sides of the web;  
         [0024]     a housing surrounding a major part of the surface of each roller;  
         [0025]     a vacuum chamber substantially co-extensive with each roller and connected to a vacuum source;  
         [0026]     a longitudinal slot extending parallel to each roller and substantially co-extensive therewith, the slot having an inlet proximate the roller surface and an outlet in communication with the vacuum chamber and operatively providing an air flow path from adjacent the roller into the vacuum chamber,  
         [0027]     the method comprising operating the rollers and the vacuum source such that the air flow speed at each inlet is not less than the surface speed of the rollers.  
         [0028]     Preferably the air flow speed at each inlet is at least 1% greater than the surface speed of the rollers, more preferably at least 10% greater than the surface speed of the rollers and especially at least 50% greater than the surface speed of the rollers.  
         [0029]     In particularly preferred embodiments, the rollers rotate in a direction opposite to the direction of movement of the web.  
         [0030]     Most preferably the slot is arranged on the upstream side of the roller with respect to the direction of movement of the web.  
         [0031]     Preferably in this embodiment the differential between the surface speed of each roller and the speed of the web is from about 5 ms −1  to about 30 ms −1 , more preferably from about 6 ms −1  to about 12 ms −1  and especially about 10 ms −1 .  
         [0032]     Preferably the rotational speed of the rollers is from about 50 to 100 revs −1 .  
         [0033]     According to a third aspect of the invention there is provided a web cleaning apparatus comprising:  
         [0034]     a pair of operatively rotating cleaning rollers operatively disposed on opposite sides of the web;  
         [0035]     a vacuum chamber substantially co-extensive with each roller;  
         [0036]     a longitudinal slot extending parallel to each roller and substantially co-extensive therewith, the slot having an inlet proximate the roller surface and an outlet in communication with the vacuum chamber and operatively providing an air flow path from adjacent the roller into the vacuum chamber,  
         [0037]     a housing surrounding a major part of the surface of each roller,  
         [0038]     wherein the housing has a part-cylindrical internal surface opposed to the external surface of the roller and spaced therefrom with a gap of about 0.5 mm to about 5 mm.  
         [0039]     Preferably said gap is from about 1 mm to 3 mm.  
         [0040]     Preferably in this embodiment the internal surface of the housing extends from a first side proximate the web on the downstream side of the roller to the to a second side proximate the slot inlet. Typically, and preferably, the internal surface extends through about 180° to 270° 
         [0041]     Most preferably the air flow speed at each inlet is not less than the surface speed of the rollers.  
         [0042]     According to a fourth aspect of the invention there is provided a web cleaning apparatus comprising:  
         [0043]     a pair of operatively rotating cleaning rollers operatively disposed on opposite sides of the web;  
         [0044]     a vacuum chamber substantially co-extensive with each roller;  
         [0045]     a longitudinal slot extending parallel to each roller and substantially co-extensive therewith, the slot having an inlet proximate the roller surface and an outlet in communication with the vacuum chamber and operatively providing an air flow path from adjacent the roller into the vacuum chamber,  
         [0046]     a housing surrounding a major part of the surface of each roller,  
         [0047]     a shutter moveable, in the absence of the web, from an open position to a closed position, in which closed position the shutter is located between the respective slots, whereby air can flow into the slots substantially only along a path extending from the downstream side (with respect to the operative movement direction of the web) of the rollers and between the rollers.  
         [0048]     Preferably the shutter comprises a flap moveable about an axis between the open and closed positions. More preferably the flap is moveable about an axis substantially parallel to the plane of the web and perpendicular to the direction of movement of the web.  
         [0049]     According to a fifth aspect of the present invention there is provided a method of cleaning the rollers of a web cleaning apparatus, the apparatus comprising:  
         [0050]     a pair of operatively rotating cleaning rollers operatively disposed on opposite sides of the web;  
         [0051]     a vacuum chamber substantially co-extensive with each roller;  
         [0052]     a longitudinal slot extending parallel to each roller and substantially co-extensive therewith, the slot having an inlet proximate the roller surface and an outlet in communication with the vacuum chamber and operatively providing an air flow path from adjacent the roller into the vacuum chamber,  
         [0053]     a housing surrounding a major part of the surface of each roller,  
         [0054]     a shutter moveable, in the absence of the web, from an open position to a closed position, in which closed position the shutter is located between the respective slots thereby substantially preventing the flow of air from the upstream side (with respect to the operative movement direction of the web) of the rollers into the slots,  
         [0055]     the method comprising  
         [0056]     removing the web, if present  
         [0057]     moving the shutter from the open position to the closed position, and rotating the rollers while maintaining a vacuum in the vacuum chamber  
         [0058]     whereby air can flows into the slots substantially only along a path extending from the downstream side of the rollers and between the rollers.  
         [0059]     Preferably the rollers are rotated in a sequence of respective high speed and lower speed rotations.  
         [0060]     Preferably the rollers are rotated in the forward and reverse directions; 
        Preferably the shutter comprises a flap moveable about an axis between the open and closed positions.        
 
         [0062]     Preferably the flap is moveable about an axis substantially parallel to the plane of the web and perpendicular to the direction of movement of the web.  
         [0063]     According to a sixth embodiment of the invention there is provided web cleaning apparatus comprising:  
         [0064]     a pair of operatively rotating cleaning rollers operatively disposed on opposite sides of the web;  
         [0065]     a housing surrounding a major part of the surface of each roller;  
         [0066]     a vacuum chamber substantially co-extensive with each roller;  
         [0067]     a longitudinal slot extending parallel to each roller and substantially co-extensive therewith, the slot having an inlet proximate the roller surface and an outlet in communication with the vacuum chamber and operatively providing an air flow path from adjacent the roller into the vacuum chamber;  
         [0068]     a vacuum source in communication with each vacuum chamber;  
         [0069]     a vacuum line extending between the vacuum source and the vacuum chamber; and  
         [0070]     a bypass line connected at its first end to the vacuum line and communicating at its second end with the atmosphere, the bypass line including a valve having open and closed positions, the closed position preventing the flow of air through the bypass line and the open position permitting such flow.  
         [0071]     Preferably the apparatus further comprises at least one pressure sensor disposed in the respective longitudinal slots.  
         [0072]     Preferably the apparatus further comprises a valve in the vacuum line disposed upstream (with respect to the air flow direction) of the bypass line and operable to close the vacuum line to the flow of air.  
         [0073]     Preferably the apparatus further comprises first and second branch vacuum lines extending respectively from the vacuum line to each vacuum chamber, each branch vacuum line including a valve operable to close the respective branch vacuum line to the flow of air.  
         [0074]     According to a seventh aspect of the invention there is provided method of operating a web cleaning apparatus according to the sixth aspect of the invention, the method comprising  
         [0075]     providing an apparatus as defined in the sixth aspect of the invention,  
         [0076]     detecting an abnormality of pressure by means of said pressure sensor, and, if such an abnormality is detected,  
         [0077]     opening the valve in said bypass line.  
         [0078]     According to an eighth aspect of the invention there is provided a web cleaning arrangement comprising:  
         [0079]     a common vacuum source  
         [0080]     a common vacuum line extending from the vacuum source; and  
         [0081]     a plurality of web cleaning apparatus each comprising:  
         [0082]     a pair of operatively rotating cleaning rollers operatively disposed on opposite sides of a respective web;  
         [0083]     a housing surrounding a major part of the surface of each roller;  
         [0084]     a vacuum chamber substantially co-extensive with each roller;  
         [0085]     a longitudinal slot extending parallel to each roller and substantially co-extensive therewith, the slot having an inlet proximate the roller surface and an outlet in communication with the vacuum chamber and operatively providing an air flow path from adjacent the roller into the vacuum chamber;  
         [0086]     a first vacuum lines extending between the common vacuum line the respective web cleaning apparatus; and  
         [0087]     a bypass line connected at its respective first end to the first vacuum lines and communicating at its second end with the atmosphere, each bypass line including a valve having open and closed positions, the closed position preventing the flow of air through the bypass line and the open position permitting such flow.  
         [0088]     Preferably the arrangement further comprises at least one pressure sensor disposed in the respective longitudinal slots of the web cleaning apparatus.  
         [0089]     Preferably each web cleaning apparatus further comprises a valve in the first vacuum line disposed upstream (with respect to the air flow direction) of the bypass line and operable to close the first vacuum line to the flow of air.  
         [0090]     Preferably the apparatus further comprises first and second branch vacuum lines extending respectively from the first vacuum line to each vacuum chamber, each branch vacuum line including a valve operable to close the respective branch vacuum line to the flow of air.  
         [0091]     According to a ninth aspect of the invention there is provided a method of operating a web cleaning arrangement as defined in the eighth aspect of the invention, the method comprising:  
         [0092]     providing an arrangement as defined in the eight aspect of the invention,  
         [0093]     detecting any abnormality of pressure by means of said pressure sensor, and, where such an abnormality is detected  
         [0094]     opening the valve in said bypass line of the apparatus where the abnormality was detected. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0095]     For a better understanding of the invention and to show how the same may be carried into effect, reference will be made by way of example only to the following drawings, in which:  
         [0096]      FIG. 1  is a perspective view of a prior art web cleaning apparatus according to U.S. Pat. No. 6,598,261;  
         [0097]      FIG. 2  is a cross section through the prior art apparatus of  FIG. 1 ;  
         [0098]      FIGS. 3A and 3B  are views of a vacuum chamber of a first modified shape for an apparatus according to the invention;  
         [0099]      FIG. 4  is a side view of a vacuum chamber of an alternative shape for an apparatus according to the invention;  
         [0100]      FIG. 5  is a schematic end view of an apparatus having a vacuum chamber similar to that of  FIGS. 3A and 3B ;  
         [0101]      FIG. 6  is a schematic section through a part of an apparatus of the invention including a vacuum chamber similar to that of  FIGS. 3A and 3B ;  
         [0102]      FIG. 7  is a representation of a web cleaning system employing a plurality of web cleaners attached to a single vacuum source;  
         [0103]      FIG. 8  is a representation of a valve for use with the web cleaner of the invention;  
         [0104]      FIG. 9  is a representation of the arrangement of valves and vacuum lines for a single web cleaner according to the invention; and  
         [0105]      FIG. 10  is a schematic section through a part of an apparatus of the invention showing a closeable flap for use in a cleaning step. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0106]     Referring now in particular to FIGS.  1  to  4 , the prior art apparatus  502  of  FIGS. 1 and 2  has a vacuum chamber housing  506  of uniform shape with respect to the whole length of the roller  504 . In other words, the lateral cross-section (with respect to the axis of rotation of the roller  504 ) has the same shape at whichever point the cross-section is taken. The outlet  516 , to which the vacuum pump is connected via ducts  508 , is arranged at the end of chamber  514  defined in housing  506 . The inventor has appreciated that this has the important consequence of providing a non-uniform pressure with respect to the axial length of the roller  504 . That is, the pressure is least (degree of vacuum is greatest) at the end of the chamber  514  nearest the outlet  516 . Thus, the vacuum applied must be relatively greater so that the actual vacuum achieved at the end of the roller  504  distant from the outlet  516  is sufficient (which carries the danger that the degree of vacuum at the end of the roller  504  nearest the outlet  516  is too great), or the vacuum at the end of the roller  504  distant from the outlet  516  is insufficient, which compromises the cleaning efficiency of the apparatus. The inventor has further appreciated that in the prior art apparatus where the degree of vacuum is insufficient the speed of the air moving through the housing  506  is consequently insufficient, which results in deposition of particles carried in the air stream.  
         [0107]     To alleviate this problem, in embodiments of the invention, as illustrated in  FIGS. 3A, 3B ,  4 ,  5  and  6  the housing is shaped to define a vacuum chamber the cross-sectional area of which reduces in a direction away from the outlet.  
         [0108]      FIG. 5  shows a web cleaning apparatus  10 . The passage of the web  12  through the apparatus is indicated by line and arrow A. The apparatus  10  includes a pair of cleaning rollers  14  which rotate in the direction shown by arrows B. That is, the direction of movement of any point on the surface of the rollers  14  when closest to the web  12  is opposite to the direction of movement of the web  12 . The rollers  14  are driven by any suitable drive means which in the illustrated example is a motor  16  and belt  18  drive arrangement. The majority of the perimeter of each roller  14  is surrounding by a housing  20 . A vacuum chamber  22  is disposed immediately adjacent each cleaning roller  14  and adjacent a side of housing  20 . The vacuum chamber  22  includes an inlet slot  24  or like orifice which is approximately co-extensive with the length of the roller  14 . The vacuum chamber  22  extends parallel to the roller  14 . Ideally, as much of the surface of the roller  14  as possible is surrounded by the housing  21 . In practice it is preferred that the housing covers the roller  14  in an arc from the web  12  to the inlet slot  24  of at least 180° to about 270°. The inlet slot  24  is preferably disposed as far away from the web  12  as practicable, so that the air flow through the slot does not disturb the passage of the web. A gap  21 , which is preferably of constant dimension, is defined between the housing  20  and the cleaning roller  14 . Preferably the gap  21  is made as small as possible, that is, a clearance gap. In this way, the possibility of carrying particles removed from the web, entrained in the roller or in the air around the roller, into the gap  21  between the roller and the housing is minimised. The rollers  14  are arranged in close proximity to the web  12 , but in the most preferred embodiments of the invention, the rollers  14  do not touch the web  12 .  
         [0109]     The vacuum chamber  22  is of non-uniform shape. Typical shapes of vacuum chamber  22  are illustrated in  FIGS. 3A, 3B  and  4  in which the vacuum chamber is identified as  22 ′ ( FIGS. 3A and 3B ) or  22 ′″ ( FIG. 4 ). In each case, the vacuum chamber  22 ,  22 ′,  22 ″ includes an outlet  26 ′,  26 ″ which is connected (via suitable ducting  28 ) to a vacuum pump (not shown). A flow of air is thus created through the vacuum chamber  22 ,  22 ′,  22 ″. The vacuum chamber  22 ,  22 ′,  22 ″ is in each case shaped such that the cross-sectional area of the chamber generally decreases in the direction away from the outlet  26 ′,  26 ″.  FIGS. 5 and 6  show the cross-sectional area of the vacuum chamber  22  decreasing from a maximum in region  32  to a minimum in region  34 . As can be seen in particular from  FIG. 4 , it is not necessary for the cross-sectional area to decrease along the whole length of the chamber  22 ,  22 ′,  22 ″.  FIG. 4  shows regions  30 A and  30 B wherein the cross-sectional area of the chamber  22 ″ is substantially constant. The objective in providing the vacuum chamber  22 ,  22 ′,  22 ″ with a decreasing cross-sectional area in the direction away from the outlet  26 ′,  26 ″ is to achieve a substantially uniform pressure along the whole length of the inlet  24 . A substantially uniform air speed at any point of the slot  24  is thereby achieved. The cleaning efficiency of the web cleaner  10  is not compromised by having a portion of the inlet slot  24  experiencing an excessively high or excessively low pressure, since the pressure does not significantly vary between any two regions of the inlet slot  24 . The vacuum chamber  22  is also shaped to provide a relatively turbulent air flow within the chamber  22 , to minimise the deposition of particulate debris on the internal walls of the chamber. In contrast, the air flow in the slot  24  should be as smooth as possible, to maximise the air speed in the slot. The slot  24  should be made as long as reasonably possible (with respect to the dimension from its inlet to its outlet). As noted above, the air in the vacuum chamber  22  is preferably turbulent. Thus, by maximizing the length of the slot  24 , the effect of that turbulence within and beyond the slot  24  is minimised. In other words, a longer slot  24  minimises any turbulence at the inlet end.  
         [0110]     With the above objective in mind, the particular construction of the vacuum chamber including its size, the maximum cross-sectional area, the minimum cross-sectional area, the rate of change between said maximum and minimum, and any sections which have substantially constant area, is a matter for selection by the person skilled in the art. These can be determined by the person skilled in the art in accordance with the pre-selected, pre-determined or required properties of the apparatus such as the size and speed of rotation of the cleaning rollers  14 , the speed of movement of the web  12  and the particular characteristics of the vacuum source used, such that the required substantially uniform pressure at the inlet  24  is achieved.  
         [0111]     A further advantage, as indicated above, of the apparatus of the invention can be seen from  FIGS. 5 and 6 , in that inner face  20   a  of housing  20  is shaped to be part cylindrical so that it conforms to outer surface  14   a  of cleaning roller  14  with only a small clearance gap  21  remaining therebetween. Most preferably, gap  21  is sufficient such that the outer surface  14   a  of roller  14  never contacts the inner surface  20   a  of the housing  20 . Typically gap  21  is of the order of about 0.5 mm to about 5 mm, preferably about 1 mm to about 3 mm and most preferably about 2 mm. This construction overcomes a serious deficiency of the prior art in which the angular construction of the housing  506  and the relatively large and variable gap  521  ( FIG. 2 ) between the cleaning roller  504  and the housing  506  permits significant differences in pressure and air speed to develop around the roller  504 . In regions of relatively low air speed or eddy currents, particles removed from the web by the action of the rollers  504  may be deposited on the inner face of the housing  506 . Typical such regions are shown in  FIG. 2  at  506 ′,  506 ″ and  506 ′″.  
         [0112]     Deposition of cleaned particulates within the housing  506  is detrimental to the cleaning efficiency of the apparatus  502 . The apparatus  10  of the invention obviates this problem by providing the relatively narrow and uniform gap  21  in which the air flow speed is substantially constant and relatively fast, and by means of which eddy currents and the like are avoided. Thus, deposition of air-borne particulates carried away from the surface of the web  12  is significantly reduced or eliminated.  
         [0113]     In the most preferred embodiments of the invention, the rollers  14  are arranged so that they do not contact the web  12 . Most preferably, the gap between the rollers  14  and web  12  is as small as possible. In an ideal arrangement, for example, the rollers  14  might initially be set in a position such that the outer surface of the rollers  14  just touches the adjacent surface of the web  12 , after which the ideal gap is obtained by running the rollers and the web until the outer surface of the rollers wears to an extent sufficient to provide a gap between the rollers  14  and the web. In any case, the gap between the outer surface of the rollers wears to an extent sufficient to provide a gap between the rollers  14  and the web. In any case, the gap between the outer surface of the rollers  14  and the web  12  is preferably less than 6 mm, more preferably less than 3 mm and most preferably less than 1 mm.  
         [0114]     In further preferred arrangements of the invention, the differential between the surface speed of the roller  14  and the speed of the web  12  is at least 5 m/s and may be as high as 25-30 m/s. Speed differentials in excess of 25-30 m/s may provide the desired cleaning effect in accordance with the invention, but no advantage is achieved with such higher speeds. Preferably, the speed differential is about 6 to 12 m/s, most preferably are about 10 m/s. However, the optimum speed differential also depends on the size of the gap between the surface of the rollers  14  and the web  12 . A speed differential of about 5-10 m/s is most appropriate for a gap of 1 mm or less. For gaps of the order of 5-6 mm, a speed differential of 25-30 mls is generally required. As noted above, smaller gaps between the rollers  14  and the web  12  of the order of 1 mm or less are preferred. Typically, the rotational speed of the rollers  14 , is in the range of from about 50 to 100 rev/sec, more preferably 65 to 70 rev/sec. Of course, the required rotational speed of the rollers  14  is determined by the speed of the web  12  and the required differential speed between the web  12  and the rollers  14 . The diameter of the rollers  14  is typically around 120 mm. Expressed differently, where the speed of the rollers  14  (expressed as the surface speed) is Z, the speed of the web is Y and the required speed differential is X, then 
 
 Z=Y+X  
 
         [0115]     Thus Z is always great than Y (and in the opposite direction, since the roller rotates in the opposite direction to the direction of the web).  
         [0116]     In another preferred embodiment of the invention the vacuum applied to the vacuum chamber  22 ,  22 ′,  22 ″ and the inlet slot  24  is such that the speed of the air entering the inlet slot  24  is equal to or greater than the surface speed of the roller. Preferably the speed of the air entering the slot is at least 1% greater, more preferably at least 10% greater and especially at least 50% greater than the surface speed of the rollers  14 . Bearing in mind that the rotation of the rollers  14  causes movement of the air immediately adjacent the rollers (which movement is exploited in the present invention for the removal of particulate matter from the web  12 ), if the speed of the air at the inlet slot  24  (arising because of the vacuum in the vacuum chamber  22 ,  22 ′,  22 ″), is lower than the speed of the air approaching the vacuum slot as a result of the rotation of rollers  14 , then the air in which the particulate matter is entrained will not enter the vacuum slot  24 .  
         [0117]     In many applications the rollers  14 , the housing  20  and the vacuum chamber  22  are so sized that their length (the dimension along the axis of the roller  14 ) is substantially the same as the width of the web which is to be cleaned. However, this is not necessarily so. For example, the width of the web may be less than the length of the roller  14 . Typical widths in this respect may be nominally half or quarter widths with respect to the width of a standard web.  
         [0118]     In another particularly preferred feature of the present invention, the vacuum chamber  22 , and more especially the inlet slot  24 , is provided with a plurality of sensors which are responsive to the speed of the air in the inlet slot  24 . Pressure sensors are preferred, but air speed sensors can also be used Suitable pressure sensors are available from suppliers such as Honeywell and Sensor Technics. The pressure sensors are most preferably substantially evenly spaced along the length of the inlet slot  24 . The provision of such sensors has numerous advantages.  
         [0119]     Where a web having a width less than the length of the roller  14  (or vacuum chamber  22  is cleaned, the use of the pressure sensors enables information to be gathered relating to the particular zone or region of the vacuum chamber  22  where the web is located. For example, if, as sometimes happens, tension in the web is lost briefly, the web may be sucked towards one or other of the vacuum chambers  22  and held by the vacuum. This will be detected by a pressure sensor local to the web as a local increase in vacuum/decrease in pressure. On detection of the pressure change action can be taken by an operator to correct the fault. Alternatively, the pressure sensors may be connected to a control arrangement of web cleaner which, on detection of a pressure reduction may temporarily release the vacuum (e.g. by shutting off the vacuum source, or opening a valve located between the vacuum chamber  22  and the vacuum source so that the web is released.  
         [0120]     Another advantage which results from the provision of pressure sensors occurs with webs of any width. Sometimes during the running of a printing apparatus loose pieces or strips of paper may be formed, for example, when webs are spliced together, typically when a first roll of printing paper expires and is joined (spliced) to a new roll to form a continuous web. These pieces of paper may be sucked into the web cleaner by the vacuum applied to the vacuum chamber  22 , potentially causing a blockage of the slot  24  and a consequently loss of vacuum as applied to the web. Clearly in these circumstances the effectiveness of the web cleaner is much reduced. The provision of the pressure sensors enables not only the occurrence of such a blockage to be detected, but also its location. Remedial action can thus be taken rapidly and easily, since an operator will know immediately where to look for the blockage.  
         [0121]     Where a plurality of pressure sensors is provided, it is very advantageous to provide the slot  24  with a plurality of lateral walls extending across the width of slot  24 . Such walls have the effect of dividing the slot into discrete sections, without significantly hindering the flow of air through the slot. A pressure sensor can then be located in each such section, so that the pressure/vacuum in each section can be independently determined. In a convenient form of the invention, one such discrete section and its associated sensor is provided per conventional web width. A conventional web width may depend on the particular application of the printing press, but a typical example is the width of standard newsprint paper used for printing newspapers. In this respect a single web cleaner according to the invention can be used to simultaneously clean a number of webs running in parallel, and one sensor is provided per such web. Similarly, the web cleaner may be used with a printing press which prints paper having a width which is say, four conventional web widths. Again, a single web cleaner according to the invention is used with four discrete sections in the slot and respectively four pressure sensors. Thus, in the case of a problem such as the blockage of a slot, the location of the problem is easily determined.  
         [0122]     Another advantageous application of pressure sensors occurs where a single vacuum source is coupled to a plurality of web cleaners. For reasons similar to those outlined above, from time to time the web passing through a given web cleaner may be sucked against the slot  24  of the web cleaner, or the slot of a given web cleaner can become blocked by a piece of paper or the like. A pressure sensor mounted on the particular web cleaner, more particularly mounted in the slot  24 , will detect the increase in vacuum (reduction in pressure) in the vacuum chamber where the problem has occurred. Means may then be provided to disconnect the vacuum source from the web cleaner where the problem has occurred, so that the operation of the other web cleaners which depend on the single vacuum source can continue.  
         [0123]     As will be readily understood, where a plurality of web cleaners is connected to a single vacuum source and one of the web cleaners becomes (partially) blocked, action is required to remove the problem. This action is required for two reasons. Firstly, to restore the blocked web cleaner to its fully operational state and secondly because the blockage of one web cleaner will, at least partially, affect the vacuum experienced by the other web cleaners connected to the same vacuum source. In particular, the vacuum experienced by these other web cleaners may increase, which may undesirably reduce their cleaning effectiveness. For these reasons, each web cleaner may be provided with one or more valves to control the air flow from the web cleaner. A typical arrangement is schematically illustrated in  FIG. 7 .  
         [0124]      FIG. 7  shows schematically a common vacuum source  112  which is linked to a plurality of (in this case, four) web cleaning apparatus  110 A,  110 B,  110 C,  110 D by way of a main vacuum line  114  and respective branch vacuum lines  116 A,  116 B,  116 C and  116 D. Other arrangements for connecting the web cleaning apparatus  110 A- 110 D to the vacuum source  112  are possible. Each web cleaning apparatus  110 A- 110 D includes respective upper and lower halves, shown schematically as  128 A,  130 A;  128 B,  130 B;  128 C,  130 C and  128 D,  130 D arranged on respective sides of the web to be cleaned, as described in relation to  FIG. 4 . The respective halves of the web cleaners are connected to vacuum branch lines  116 A- 116 D by, respectively, vacuum lines  124 A- 124 D and  126 A- 126 D.  
         [0125]     The arrangement of the web cleaning apparatus  110 A will be described in more detail, it being understood that some description applies to the other web cleaning apparatus  110 B,  110 C and  110 D, subject to the substitution of the suffix “A” in the reference numbers by the relevant suffix “B”, “C” or “D”.  
         [0126]     Vacuum line  116 A is provided with a main valve  118 A by means of which the vacuum line  116 A can be closed, that is, so that the web cleaning apparatus is no longer subject to any vacuum, as may be required for routine maintenance or to rectify any problem or fault which may occur. As noted above, shutting the valve  118 A may have an undesirable effect on the degree of vacuum experienced by the apparatus  110 B,  110 C and  110 D. This could, in principle, be compensated for by adjustment of the vacuum source  112 , but such adjustment could be complex and uncertain. The present invention provides an alternative in that the apparatus  110 A is provided with a bypass line  120 A having a valve  122 A. The bypass  120 A line is open to the atmosphere at its end  120 A′, optionally via a silencer (not shown). The valve  122 A is closed in normal use. However, when the valve  118 A is closed, to shut off the web cleaning apparatus  110 A, valve  122 A is opened. This provides a flow of air into vacuum line  116 A via line  120 A which compensates for the flow of air to line  116 A which, but for the closure of valve  118 A, would have occurred through the apparatus  110 A. Thus, opening of the valve  122 A compensates for the closure of valve  118 A so that the degree of vacuum experienced by the web cleaning apparatus is not adversely affected.  
         [0127]     The degree of vacuum experienced by, and hence the flow of air through, each half  128 A,  130 A of the web cleaning apparatus  110 A is controllable according to the present invention by means of respective valves  132 A,  134 A. The valves  132 A,  134 A are preferably gate or shutter type valves which are provided with means for fine adjustment of their setting. A typical such valve is shown in  FIG. 8  in an open condition. The valve  200  (which might be any of the valves  132 A-D or  134 A-D) is mounted in a vacuum line  202  and includes a housing  204  to which the vacuum line  202  is attached. The housing  204  defines a flow path for the passage of air through the valve  200  when the valve  200  is in a fully or partially open position. The valve  200  further includes a shutter  206  which is slidably mounted in housing  204 . The shutter  206  is raised and lowered to open and close the valve. As will be readily apparent, the shutter  206  closes the valve  200  by obstructing the air flow path though the valve. Shutter  206  is provided with an adjustment mechanism  208  which enables an operator to precisely set its location and thus to precisely control the flow of air through the valve  200 . In the preferred embodiment as illustrated, the adjustment mechanism comprises a threaded bolt  210  which is held captive on the shutter  206  and which cooperates with an internally threaded bore  212  arranged on the housing  204 . Thus, by means of valves  132 A- 132 D and  134 A to  134 D, the degree of vacuum experienced by each half of each web cleaning apparatus may be set with great accuracy so that an optimum web cleaning efficiency is achieved.  
         [0128]     The use of valves  200  is not confined only to the case of multiple web cleaning apparatus having a common vacuum source.  FIG. 9  illustrates a corresponding arrangement employing only a single web cleaning apparatus  100 E. It can be seen that the arrangement is essentially the same as that of apparatus  110 A in  FIG. 8 , with corresponding parts having corresponding reference numbers, with the substitution of the suffix “E”. The bypass line  120 E and bypass valve  122 E are in this case optional. However, their provision can be advantageous in avoiding the need to adjust or shut down the vacuum source  112  when the valve  118 E is closed. The valves  132 E and  134 E again function to ensure that a correct degree of vacuum is applied to the respective halves  128 E,  130 E of the web cleaning apparatus  11 E, and in particular to ensure that the respective halves  128 E,  130 E are in balance, with regard to the applied vacuum, so that the web is not drawn towards one half  128 E,  130 E in preference to the other half  130 E,  128 E.  
         [0129]     Valves  118 A- 118 E preferably are also gate or shutter type valves having a construction generally similar to valves  200  (i.e. valves  132 A to  132 E and  134 A to  134 E. Optionally, and advantageously, valves  118 A- 118 E may be operated pneumatically, rather than manually, under the control of an overall control means for the apparatus. Likewise, valves  122 A- 122 E may be pneumatically controlled, if desired. The valves  122 A- 122 E are preferably of similar construction to valves  118 A- 118 E.  
         [0130]     With particular reference to  FIG. 10 , a further advantageous embodiment of the invention provides means for cleaning the rollers  14  of any debris which may have become entrained in or on their outer surface. The cleaning of the rollers  14  is implemented when the web cleaner  10  is not in normal operation, that is, in the absence of web  12 . In  FIG. 10 , the normal path and direction of the web  12 —were it present—is indicated by arrow A. For cleaning the rollers  14 , a hinged flap  36  is provided. The flap is mounted on a hinge  38  so that it can move through an arc  40  from an open position  36 A to a closed position  36 B (where the flap is shown in part only). In the open position  36 A the flap  36  rests in a position which does not obstruct the path of web  12 . In the closed position  36 B the flap  36  closes the gap  42  between the respective vacuum chambers  22 . With the flap  36  in the closed position  36 B it is not possible for any significant amount of air to flow towards the housings  20  in the direction indicated by arrow A. Thus, when a vacuum is applied to vacuum chambers  22 , air is drawn through slot  24 , but the major air path is between and through rollers  14 , as indicated by arrow C ( FIG. 5 ). Thus, the air passing between and through the rollers  14  strips the rollers of particulate debris which they may be carrying and the debris thus entrained in the air flow is transferred through the slot  24  to the vacuum chamber  22  and then suitably disposed of via ducting  28 . During the cleaning process, the rollers  14  are preferably rotated. The rollers  14  may be rotated in a cleaning cycle which includes periods of fast rotation and slower rotation, and rotation in the forward and reverse directions. Typically the cleaning procedure is carried out about once per week.