Patent Publication Number: US-6986831-B2

Title: Suction roll of a paper machine

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
   This application claims priority on Finnish Application No. 20011823, filed Sep. 17, 2001, the disclosure of which is incorporated by reference herein. 
   STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   The invention relates to a suction roll of a paper machine comprising a perforated shell and a static suction box sealed against the inner surface of the shell to create a suction zone on the surface of the suction roll while the shell rotates. 
   Suction rolls provided with a suction box are used particularly in the forming section and the press section of paper and board machines at locations where a high local vacuum is needed. The suction box sealed against the inner surface of the roll shell is connected to a vacuum system, whereby a local vacuum of desired magnitude and, as a result of it, a flow of water or air into the suction roll are produced in that sector of the shell which is at the suction box each time when the roll is rotating. The holes drilled through the shell are generally provided with countersinks, or counterbores, in the outer surface of the shell, the function of which countersinks is to reduce the size of unbroken connecting portions in the outer surface of the shell and to enlarge the open area of the outer surface. In the suction rolls of prior art, the countersink surrounds the suction hole symmetrically. Both the countersink pattern and the bore pattern are generally regular, which is important in order to provide an even suction effect and to prevent marking of the web. 
   One considerable problem associated with suction rolls is the noise they generate. At the end of the suction zone, a vacuum prevails in the individual holes of the shell, which vacuum is filled with air suddenly and noisily after the seal of the trailing side. As the roll comprises thousands of holes and the phenomenon is repeated in rapid succession, the ear distinguishes only a continuous tune, whose pitch depends on the speed of the roll and on the length of the regular drill pattern. Noise peaks can additionally occur at certain multiples of rotation frequency. If the holes are in axial rows, an entire row produces simultaneously a sound impulse, with the result that the noise which is produced may become intolerable at high speeds. By arranging the drill pattern of the holes to be spiral-like, the number of the holes filled simultaneously can be made smaller at the same time as their spacing is reduced, whereby the volume of sound decreases. However, in many cases this is not sufficient to attenuate the noise to an acceptable level. 
   The aim of the invention is to reduce the noise caused by suction rolls. 
   SUMMARY OF THE INVENTION 
   The suction roll according to the invention is characterized in that the holes of the shell are situated at distances of varying magnitude from one another such that the hole pattern formed on the inner surface of the shell is irregular. 
   A suction roll which has been drilled in a traditional manner produces very powerful noise peaks at certain multiples of rotation frequency. When the position of the holes is changed at random in particular in the circumferential direction, regular periodicity disappears and the sound turns into a humming-type sound, thus being less disturbing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the invention will be described in greater detail with reference to the figures in the appended drawings, but the invention is not meant to be strictly limited to the details of the figures. 
       FIG. 1  is a cross-sectional view of a suction roll provided with a suction box. 
       FIG. 2  shows hole patterning of a suction roll in accordance with the invention as viewed from the direction of the surface of the roll. 
       FIG. 3  is a cross-sectional view of the structure of the suction roll shell in the thickness direction taken along the line A—A shown in  FIG. 2 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The suction roll shown in  FIG. 1  comprises a perforated shell  10 , which is arranged to rotate around a roll shaft in the direction of the arrow B. A static suction box  11  is placed inside the shell  10 , which suction box is connected to a vacuum source (not shown). The suction box  11  is sealed against the inner surface of the shell  10  by means of sealing strips  12  and  13 , whereby a suction zone  11   a  acting in the area remaining between the seals  12 ,  13  is formed in the surface of the roll. In the suction rolls in accordance with the state of the art, holes  14  extending through the shell have been drilled at regular intervals from one another such that they form a spiral-shaped drill pattern on the surface of the roll. In the area of the suction zone  11   a , a vacuum produced by the suction box  11  prevails in the holes  14 . After the seal  13  of the trailing side, air penetrates into the holes  14  through the interior of the roll, in which connection a powerful popping or whistling sound is produced in the holes  14 . When a large number of holes  14  is filled with air simultaneously over the entire length of the roll, the sound is multiplied. At certain multiples of rotation frequency, the sounds are further strengthened and particularly powerful noise peaks are created. 
     FIG. 2  shows, as spread out, a portion of the surface pattern of a suction roll in accordance with the invention, which pattern is formed of holes  14  extending through a shell  10  and of countersinks  15  surrounding the holes (the background grid illustrates the position of the holes  14  and the countersinks  15  with respect to one another). The countersinks  15  have been drilled at regular intervals a from one another such that there is an equal distance a from the circumference of each countersink  15  to the outer edge of six countersinks  15  closest to it. The pattern formed by the countersinks  15  may naturally also be different from the regular hexagon shown in  FIG. 2 . A curved line A—A curving rectilinearly along the outer surface of the roll can be drawn through the centers of the countersinks  15 , the angle of inclination of the curved line differing to some extent from the circumferential direction y perpendicular to the axial direction x of the roll. The holes  14  extending through the shell  10  have been placed at random such that each hole  14  is inside the countersink  15 , but the centers of the hole  14  and the countersink  15  surrounding it do not generally coincide. It follows that the distances b i  between successive holes  14  are of varying magnitude within the limits allowed by the countersinks  15  situated at regular intervals a from one another. This has been illustrated in  FIG. 2  by showing some of the distances b n−2 , b n−1 , b n , b n+1 , b n+2  between the holes  14  in the direction of the curved line A—A, which distances are unequal in magnitude as compared with one another. 
     FIG. 3  shows a portion of the curved surface of the shell  10  of the suction roll—for the sake of simplicity, as straightened and in a cross-sectional view taken along the curved line A—A shown in  FIG. 2 , in which connection the centers of the holes  14  placed in succession in the circumferential direction of the roll are situated on the same line A—A but at varying distances b i  from one another. The figure also shows a sealing strip  13  on the trailing side of the suction box, the holes  14  being filled with air after they pass the sealing strip. The distance a between two successive countersinks  15  drilled into the outer surface of the shell  10  is constant, but the distances b 1 , b 2 , b 3 , b 4  remaining between successive holes  14  extending through the shell  10  are unequal in magnitude as compared with one another. 
   When there is regular irregularity like that shown in  FIGS. 2 and 3  over the entire length of the roll in each hole row parallel to the curved line A—A, the number of the holes  14  which come simultaneously to the trailing side of the sealing strip  13  is smaller than in the conventional suction roll having a regular hole pattern. When the holes in adjacent rows A—A are filled after the sealing strip  13  at a different pace, the noise that is produced is more of the humming type than in the suction rolls of the present type, and thus noise peaks are avoided. 
   When the countersink pattern in the surface of the shell is regular and the pattern formed by the holes extending through the shell is irregular, the noise level caused by the roll can be substantially lowered without losing the even suction effect of the suction zone of the roll at the same time and without increasing the tendency to marking.