US2964107A - Noise reducing means for high speed perforated surfaces - Google Patents

Description

Dec. 13, 1960 c. w. E. WALKER 2,964,107

NOISE REDUCING MEANS FOR HIGH SPEED PERFORATED SURFACES Filed Aug. 1, 1955 2 Sheets-Sheet 1 PRIOR ART INVENTOR CHARLES W. E. WALKER BY W x I HTTORNE Dec. 13, 1960 w. E. WALKER 2,964,107

NOISE REDUCING MEANS FOR HIGH SPEED PERFORATED SURFACES Filed Aug. 1, 1955 2 Sheets-Sheet 2 IN VE' NTOR CHARLES W. E. WALKER BY MW: 4

HTTORIVEYS United States Patent NOISE REDUCING MEANS FOR HIGH SPEED PERFORATED SURFACES Charles W. E. Walker, Powell River, British Columbia, Canada, assignor to MacMillan Bloedel and Powell River Limited, a corporation of British Columbia,

Canada Filed Aug. 1, 1955, Ser. No. 525,545

2 Claims. (Cl. 162371) This invention relates to means for reducing noise created by the sudden movement of gas through the holes of perforated surfaces moving at high speeds as said surfaces move past edges near and generally extending transversely of the surfaces, the gas movement taking place the instant the perforations clear the edges.

The invention may be used in connection with any perforated surface at which noise is created in the manner described, but it is primarily designed for reducing the noise produced by suction rolls such as used in paper making machines, and particularly in the rolls commonly known as couch rolls. For the sake of clarity, the invention will hereinafter be described in connection with couch rolls, but it is understood that it is not limited thereto.

In addition to the usual machine noises as are produced by gears and other mechanical actions, particularly in machines operating at very high speeds, there are in these suction rolls particularly intense noise sources which are due to a type of siren action. Throughout the following description, the term suction roll noise will be used to designate the sound produced by these particularly intense noise sources.

The intense noise may be created by a perforated surface moving very rapidly over a suction box. This situation exists at numerous points in paper making machinery, including the couch roll. A suction roll consists of a cylindrical shell which may be several feet in diameter, and which in the case of couch rolls is commonly between three and six feet. These shells are usually from one to one and one-half inches thick. Such a shell has a large number of holes drilled through it in some regular pattern which, in the case of couch rolls, is commonly in the form of straight lines of holes parallel to the axis of the cylinder.

Inside the cylindrical shell, and bordering on its inside circumference, there are one or more compartments running the length of the shell which are connected to vacuum pumps and are commonly called vacuum boxes. Along the lines of contact between these boxes and the inside of the shell and attached to the boxes are strips of material which act as sealing strips to prevent the ingress of air into the vacuum boxes along these lines of contact, when the vacuum pumps are operating.

These sealing strips are commonly made of rubber, Micarta, or of some other similar material, and where they rub against the rotating shell, they are lubricated with water.

During the operation of paper making machinery, the outside of the shell adjacent the vacuum boxes is covered with a wire screen or felt and a wet sheet of paper which sufficiently covers the outside ends of the holes or perforations in the shell to enable the air pressure in the boxes and in theshell holes to be lowered by a substantial amount. As the shell rotates, the holes therein repeatedly pass over the vacuum boxes where they are partially evacuated and then refilled with air as they emerge past the leaving sealing strip.

As each hole emerges past the leaving sealing strip a ICC 1 pulse of air will enter it thereby setting up a pressure wave which if repeated N times per second will produce sound having a fundamental frequency of N cycles per second. Thus if holes follow each other at the rate of N per second, sound having a frequency of N cycles per second will be radiated. In a standard couch, for example, the holes are drilled in parallel lines parallel to the axis of the cylindrical shell, which is also parallel to the sealing strips. Many holes therefore emerge past the leaving sealing strip simultaneously, and a line of pulses enters the shell at a time. There is thus effectively a line sound source at the leaving sealing strip having a fundamental frequency equal to the number of lines of holes passing the sealing strip per second.

Similar sound sources can clearly exist at other than the leaving sealing strip, due to pulses of air coming out of the shell holes as the ends of the holes enter the vacuum boxes. The sound will not normally, however, be radiated to the room because the sources are enclosed within the vacuum boxes and suction roll shell. Now the pulses of air entering the shell holes must each have a finite duration, and I have discovered that if the pulses are made to follow each other sufiiciently rapidly, within each small length of the sealing strip, the pulses overlap or run together, so as to produce a nearly continuous, instead of a pulsating flow of air. The intensity of the sound radiated is thereby greatly reduced. I have found that this can be achieved by suitably shaping the leaving edge of the sealing strips.

There is an alternative theoretical approach to the problem which is more amenable to calculation and enables the optimum shape of sealing strip or of drilling pattern to be estimated and provides an estimate of the sound reduction that can be expected from any particular shape or drilling pattern.

Each ring of holes around the suction roll shell can be regarded as producing a small sound source at the leaving edge of a sealing strip. If the shape of the sealing strip and the pattern of the holes in the shell is such that all the small sound sources, along the length of the suction roll, are in phase, then the sound radiated will be a maximum. This will be the case if the holes are drilled in lines parallel to the length of the suction roll, and the sealing strips have straight edges parallel to the lines of holes, as is the case with standard couch rolls. If the sealing strip is so shaped that, within each small given length of the sealing strip, the phases of the small sound sources are staggered, then the sound radiated will be less than the maximum. The invention operates in such a manner as to produce this staggering of the phases. To achieve a large reduction in the sound radiated, each given length must be small compared to the wave length in air of the sound produced by the sources. At the same time, however, there must be a sutficient number of small sound sources included in the given length to satisfactorily reduce all harmonics that are present in the sources. This is equivalent to ensuring that there are enough small sources so that the phase lag between successive ones is small enough to provide adequate overlapping of the air pulses to approximate constant air flow.

In the apparatus under consideration, there are three elements, namely:

(1) A surface element which normally travels at high speed and has perforations therein,

(2) An edge element near and generally extending transversely of the surface, and

(3) A sudden gas flow through the perforations of the surface when the perforations leave or clear the edge as the surface moves past the latter.

It is this sudden flow or pulse of gas that creates the objectionable noises.

Generally stated, this invention embodies noise reducing means comprising means'in the edge element for segregating the perforations in the surface into groups that, laterally with respect to the direction of movement, are small relative to the wavelength of the sound produced, said means also staggering and overlapping relative to each other the phase of the sound vibrations produced by gas passing through the perforations of each group. This reduces the radiated sound.

The invention is further described in connection with accompanying drawings, in which,

Figure 1 diagrammatically illustrates a suction roll,

Figure Zdiagrammatically illustrates suction equipment at a travelling belt,

Figure 3 is an enlarged fragmentary sectional view through a roll, taken on the line 33 of Figure 1,

Figure 4 shows diagrammatically a standard couch drilling pattern,

Figure 5 diagrammatically,illustrates a serrated sealing strip having an edge thereof cut to a saw tooth design,

Figure 6 diagrammatically illustrates successive positions of a saw tooth or serration of Figure 5 relative to the standard pattern of holes of Figure 4, and

Figure 7 diagrammatically illustrates the location of eight small ring sources of Figure 6 contained within one of the saw teeth or serrations.

Referring to Figure l of the drawings, 10% is a perforated suction or couch roll, having a suction box 1&1 therein. This box has sealing strips or elements 102 bearing against the inner surface of the roll. A wire screen or felt 105 travels over the roll and covers the portion thereof running over the suction box. When in operation, a wet sheet of pulp or paper 107 is carried by the screen or felt.

As this equipment is in common use and is well known in the art, it does not need any detailed description herein. During operation, gas or air is drawn out of box 101, creating suction through the perforated roll, the screen and the wet pulp or paper carried by the latter. The holes in the shell or roll are partially evacuated as they pass over the box, and then are refilled with air as they emerge past the leaving sealing strip, or edge. This is the main source of noise. However, it is obvious that as each hole passes over the sealing strip or edge at the box entrance, air is suddenly sucked through it. This source of noise is not so important since the noise is confined within the suction box, but it is to be understood that the invention may be applied to the entering strip as well as to the leaving strip. Of course, if the form of invention used is the new hole pattern, it is effective at both strips.

Figure 2 illustrates a perforated strip or surface element 110 which passes over a suction box 111, said box having sealing strips or edge elements 112. A sheet 115 of wet paper or other material is shown on the strip 110. The purpose of this figure is merely to illustrate that the invention may be used with other than suction rolls.

Figure 3 illustrates perforations or holes 120 through the shell or surface element 121 of the suction roll 1%. These are the holes hereinafter referred to. As the roll is rotated, holes 120 move over suction box 101 and its sealing strips or elements 132, see Figure l. in Figure 2, the perforations of the strip or surface element 11% pass over suction box 111 and its sealing strips or edge elements 112.

Referring to Figure 4 it should be noted that the drilling pattern spirals along the length of the shell, the pitch of the spiral being equal to the lateral pitch b between the holes. This spiraling has the effect of shifting the pattern of holes relative to the pattern of the sealing strip, with effectively a repetitive cycle which is such that the relative positions of the two go through one cycle for each rotation of the shell. For this reason it is essential to choose a sealing strip shade that is not critical to the relative positions of hole pattern and strip pattern, if maximum noise reduction is to be obtained.

It will be noted that alternate rows of holes are shifted laterally by /zl2 equal to half the lateral pitch of the holes. This has the effect of splitting the small ring sources into two, each with half the fundamental frequency; i.e., if N is the frequency with which spiral rows of holes pass over a sealing strip, the fundamental frequency of each small ring source is /zN with harmonics N, 3/2N, 2N and so on. Each such source is, however in opposite phase to the next one spaced /zb'from'it sothat the fundamental /2N and all odd harmonics 3/2N, 5/ 2N, 'etc., are almost completely cancelled.

Figure 5 illustrates a portion ofa sealing strip or edge element 102 having serrations or teeth along the edge thereof. This strip or edgeelement projects from the side of suction box 101 and is in contact with the inside surface of roll 100, extending from'end to end thereof. The serrations 125 span groups of holes in the roll surface, and are formed in the trailing edge of the strip with reference to the direction of rotation ofjthe roll. The pitch of each tooth pattern is equal to 3 /2 lateral pitches of the hole pattern of Figure 4 so that each saw tooth should include seven small ring sources. Due to the spiral shift, however, this will only be true for a small part of each shell rotation. In general there will be eight small ring sources to each saw tooth with two of them shared with neighboring saw teeth.

Referring to Figure 6, the circles 1, 2, 3, 4, 5, 6, 7, and 8 represent the holes in the shell as they appear on the inside surface .of the shell. The lines 9, 10, 11, 12, 13, 14, 15 and 16 represent successive positions of one tooth 125 relative to these holes. Position 9 corresponds to the instant when hole 1 starts to emerge past the sealing strip, position 19 to the instant when hole -2 starts to emerge and so on to position 16 which corresponds to the instant when hole 8starts to emerge. It can be seen that holes 4 and 8 are shared with neighboring teeth.

The circumferential distance by which the tooth moves, relative to theshell from line 9 toline 13 is equal to S, which is the circumferential distance between successive lines of holes on the inside of the suction roll shell, and takes a'time T equal to the fundamental period of the sound of frequency N.

Similarly the circumferential distance-from line 9 to line 10 is /zb cot 0, where 0 is half the angle-of the saw' tooth as shown in Figure 6, and therefore takes a time 1" equal to 1 5 cot 0 Writing cot 0=p gives Referring now to Figure 7, 17, 18, 19, 2t), 21, 22, 23 and 24 represent the locations of small ring sound sources along the edge of a sealing strip such that the source at 17 is produced by the ring of holes in the shell which includes hole 1 of Figure 6. The source at 18 is produced by the ring of holes which includes hole 2 of Figure 6 and so on to the source at 24 which is produced by the ring of holes which includes hole 8 of Figure 6.

The instantaneous amplitude of the sound pressure wave received at the point 25 located on the line of the sealing strip and distant x from source 17 will be a from Where n is the amplitude from one source at unit distance from that source t is time co-ordinate and A is the wavelength of the sound having period T and M and M' are the factors by which the amplitudes of sources and 24 must be considered as being reduced because of sharing with neighboring saw teeth.

Considering the point 25 to be sufficiently far from the sources so that 2b can be neglected in comparison with x and writing the total instantaneous amplitude of the sound pressure wave received at point 25 from all eight sources equals A where +M' Sin (w g- 1 For any given values of b, T, A, M, M this equation can be rewritten in the form Where 1) is a function of -r.

The sound intensity at the point 25 is therefore I Where MIL 0] having holes A inch diameter with a lateral pitch b=% inch and circumferential spacing between rows of holes S=0.46 inch as measured on the inside of the shell, minimum intensity along the couch axis should be ob tained for giving p=cot 0:.4662 and 0:65".

For a normal straight edged sealing strip -r=0 making the unsilenced sound intensity on the couch axis The reduction in sound intensity along the axis produced by the serrated strip, expressed in decibels is thus ftfl) 1 m The reduction in the sound in other directions can be expected to be greater than this because from other directions the sources will be more nearly equidistants and there will be less phase change due to the lateral spacing of the sources.

For the standard 44 inch diameter couch considered above the calculated reduction along the couch axis is 22.6 db. Reductions of suction roll noise, estimated to be close to this amount, have been achieved on operating machines.

What I claim as my invention is:

1. ln apparatus including a surface element which normally travels at high speeds and has perforations therein extending in rows generally in the direction of movement of the surface element, an edge element near and generally extending transversely of the surface element, and pulsations of gas flowing through said perforations the instant the latter clear the edge element thereby producing a plurality of small sound sources at said edge element, each source originating from one row of perforations; serrations on said edge element for reducing the noise from said sources, said serrations extending across the surface element, the width of each serration being a small fraction of the wave length of the sound produced, and being wide enough and shaped to encompass a group of small sound sources to stagger relative to each other the phases of the sound vibrations and to cause overlapping of the gas pulses within said groups.

2. Apparatus as claimed in claim 1 in which the perforations of the surface element are arranged in rows extending generally in the direction of movement with the perforations substantially equally spaced from each other in all directions and the perforations of each row staggered relative to those of adjacent rows, and each serration is approximately V-shaped and encompasses substantially 3% rows.

10 log References Cited in the file of this patent UNITED STATES PATENTS 395,444 Chapin Jan. 1, 1899 693,894 Parker Feb. 25, 1902 991,999 Kieren May 9, 1911 1,236,359 Reynolds Aug. 7, 1917 1,664,722 Yoder Apr. 3, 1928 1,723,181 Kilberry Aug. 6, 1929 1,834,470 Millspaugh Dec. 1, 1931 1,895,039 Joseph Jan. 24, 1933 2,107,812 Berry et a1. Feb. 8, 1938 2,274,641 Abbott et a1. Mar. 3, 1942 FOREIGN PATENTS 276,884 Great Britain Sept. 8, 1927 681,075 France May 9, 1930 677,526 Germany June 27, 1939

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