US2682812A

US2682812A - Pulp screen

Info

Publication number: US2682812A
Application number: US288820A
Authority: US
Inventors: Wilfred F Mathewson
Original assignee: Mathewson Machine Works Inc
Current assignee: Mathewson Machine Works Inc
Priority date: 1952-05-20
Filing date: 1952-05-20
Publication date: 1954-07-06
Anticipated expiration: 1971-07-06

Description

July 6, 1954 w. F. MATHEwsoN PULP SCREEN Filed May 20, 1952 '7 Sheets-Sheet 1 July 6, 1954 w. F. MATHEwsoN PULP SCREEN '7 Sheets-Sheet 2 Filed May 20, 1952 July 6, 1954 w. F. MATHEwsoN yPULP SCREEN 7 Sheets-Sheet 3 Filed May 20, 1952 Inversion- 'ifwed Mawsof,

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July 6, 1954 w. F. MATHEwsoN PULP SCREEN 7 Sheets-Sheet 4 Filed May 20, 1952 July 6, 1954 w. F. MATHEwsoN 2,682,812

PULP SCREEN Filed May 20, 1952 7 Sheets-Sheet 6 Ieweow: Wz'fued E'Maewsow,

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7 Sheets-Sheet 7 W. F. MATHEWSJON PULP SCREEN wzfved Elfman, y @234ML c. .8mm

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Patented July 6, 19.54

usarla.

BULP SCREEN Wilfred F. ,Malthewsvom Boston, Mas s,, .assignorto Mat.liewsonl` Machine Wol-ke` Inc.,

Quincy,

This invention relates to Ypulp screens, and especially to pulp screens` of*A the type=shoivn in Patents ifi-2,450,85583 October 5, 19.48, and #b2-15253101; October 10; 1950.

The'pulpl screen show-nin each of f said` pate ents comprises two -concentrically arrangedi cylindrical screening elementsformingi between therrr anannular clearance'space of: a size4 to permit4A the -acceptable'portion of l unscreened Ipulp to passtherethrough; but ofA insuiii'cient sizev to allow the unacceptable pcrtionoffsuch pulpzto passY therethrough; One off the twoscreening velements-'isY providedwitha plurality of-` slots which extendparallel'to the axis 'oflsaidielement and provide passages fromthel outside off the Said screening--elernentis also provided- With'- a plurality of grooves=which are parallel vto -theslots and' which are open tothen clearance space.k Preferably l the slots'- and f grooves are alternately arranged.

The screening operation involves thepassage oftheunscreened'` pulprthrough the slotstinto ableportion'ofI said pulp-through the clearance space-into thegrooves Which have commun-icaing` elements is rotated aboutC its 1 airis-V during the vpulp-screening. operation;

When the-slots aretaperedf as shownlin-Patent #2,525,7013'they-are liabletobec'orne clogged Y by either coarse, unacceptable-.portions of Sthe pulp stock'or--oeks-resulting from flocculation Yof] the-pulp- `which enterthe louterv wide-inletl edge of1a-slotfbutW-hich aretoo largeto passthrough nplurality Qffradial slotslwhich egtendparallel to] the axis offsaidf screenelementand provide .passageways through which unscree-rredy pulp in vthe-l casing limaviiow, into. the ,Clearance I space S.

the narrow inner.v discharge edge of the slot which isopenlto theclearance space.'

One of" the objects of' ther present' invention is to provide novel-meansfor periodically clearing eachtapered slotynotonly` ofanyv coarse `or unacceptable portionv offthepulp stock; but also of any occulatedrpulpwhich may bcomelodged the screening elements. is maintained constantly with the proper dimension.

Othery obj ects ofthe? invention are/to improve pulp screens in varousparticularswhich willbe hereinafter ypointed f out. Y

In Athe drawingsl wherein I have illustrated a selected z embodiment ofV my invention;

Fig,- 1 is a sectional view takenon substantiallythe,v line. I-I,*Fig. 2, thesGreeningunits being shownliri; plan View;

Eig.v 2.zisa section on the line 2 2, Fig. 1;

Eigf, 3 iisfasection onrtheline-B, Eig. 2;

Fig, 4f is.,a,vertical sectional view through the lower portionfof one-*of thescreening units;

Figlyzis as ection ontheline--SLEig. 6;

Eig-.- 6i is arienlarged section through i 0.11 of the,1 screening.. units (mille lille 1li-, Eig. 5.

Fig. 7 is a section on the line 1 1, Fig'. 6.

They pulp. screen-1 hereinl illustrated .comprises a1 casing I- having. an inletop eningv 2 to receive unscreened. pulpJ and` ai; plurality. ofk screening units loeatedwithin said easing. Each screenina unitcomprisesf an, inner cylindrical Screeningvelement fleuri. an; Outer cylindrical Screening element;v 5.@ which is concentricwiih. the inner screening; elementi saidscreening elements providmg; hetweenthem an annularfclearan space v6: which., is` of a.k size.. t0 Permit. the acceptable yportion ctunscreerlednulp io pass therethrough,

hutf which, .is insuiient. in Size. to; Ylfeltnfli any coarse. or, unacceptable... portion -0f Such., pulp. i0

passitheretlfirough.k 1

The: outer screenelement 5 is provided-With a zThe-.outer screenz element 54 isalso provided withla plurality of` grooves 8`1Which are opento 'thelclearance space andJ which extend parallel vtothef slotsf- 1. As ,in the. devices vof the p patents above referredtdthe screening of the pulp r e sultslfrom-the flow of.L the puipthrough the slots .1: intorthelclearance spacel, and through said clearance.; spacelinto the groei/est.v The dimensionfof the clearance space is .such that .only the acceptable portion of. thevunscreened stock will passztherethrough into the. grooves 8, andhence the1pulp'whichflows into the grooves will` be properly; screened.

The grooves rcommuncate at their upper ends withan upper chamber Bawith which the casing,r Iiisiprovided, and the lower` ends ofthe grooves 81 communicate1 with a lower chamber.A I0. also locatedwithin theN casing l.. The tvvochambers 9; and. l0 communicatewith a discharge duct Il which leadstoa discharge opening l2:

Eachloffthe screeningfunits'wthin the casing 3 has the construction above set forth, and the upper end of the discharge grooves 8 of all of the screening units 3 deliver the screen pulp into the upper chamber 9 and the lower ends of said discharge grooves 8 deliver the screened pulp into the lower chamber III.

All of the screening units are thus supplied with unscreened stock through the inlet opening 2, and the screened stock or pulp from all of the screening units is delivered through the discharge opening I2.

In the device shown in the drawings, the outer screening element 5 of each screening unit is mounted for rotation about its axis, and similarly the inner screening element 4 of each screening unit is mounted for rotation.

The upper end of the outer screening element I5 of each screening element has a bearing I4 in the partition member I3 which divides the upper chamber 9 from the interior of the casing I in which the screening units operate (see Figs. 5 and 7). The lower end of each screening element 5 is connected to a depending bearing sleeve I5 which is mounted in the portion I6 of the supporting frame on which the casing I rests, said bearing sleeve having a gear I1 fast thereon by which it can be turned. The bearing sleeve I5 is shown as being connected to the outer screening element 5 by means of a pin connection I8, shown in Fig. '1, the upper end of the bearing sleeve having the flange I9 which carries the pin I8, and the lower end of the outer screening element 5 having a socket 2G in which the pin is received.

The inner screening element 4 has a bearing 2| in the upper end of the outer screen element, each inner screen element having a closed upper end 22. The lower end of each inner screen element 4 is connected by a pin connection with a hollow shaft 23 which fits and has a bearing in the sleeve I5, as shown in Fig. 2. The lower end of the hollow shaft 23 has a bearing in the portion 24 of the supporting framework. Mounted on the hollow shaft 23 below the bearing sleeve I5 is a gear 25 by which the inner screen member 4 may be rotated. The pin connection between the hollow shaft 23 and the inner screen member 4 is formed by one or more pins 26 which are mounted in the upper end of the hollow shaft 23 and extend into a socket 21 with which the inner 1 screen member 4 is provided, as shown in Fig. 7 In the operation of a pulp screen of this type it is important that the clearance space 6 should be maintained with a constant dimension, and to provide for this not only the upper end of the inner screen member 4 but also the central portion and the lower end thereof have a bearing in the outer screen member 5. As shown in Fig. 7, the lower end 28 of the inner screen member is rotatably mounted in the portion 29 of the outer screen member 5, and a bearing B is provided between the inner and outer screen members at the central portion thereof. Hence said inner and outer screen members are compelled to turn accurately about the same axis, and the width of the clearance space will remain constant.

The lower end of the outer screen member is mounted in a floating bearing which includes a ring 30 of yielding material that surrounds and is retained in a groove formed in the lower portion 11 of the outer screening element l5 and is received between two retaining rings 18 rigid with the bottom of the casing I. The lower portion 11 ofthe screening element 5 is spaced slightly' from the inner edges of the retaining ring 18 and the resilient character of the ring 30 permits a slight radial movement of the lower ends of the screening elements. This construction, together with the pin connections I8 and 26, provide for free rotative movement of the screening elements even if the axis about which said elements are rotating is not precisely in line with the axis of the shafts 23 and I5.

In the operation of the4 device the inner and

outer elements

4 and 5 of each screening unit rotate at slightly diiferent speeds. The inner screen element 4 is hollow and is provided with a plurality of openings 3I leading from the clearance space 6 into the interior of said screen element. During the relative rotation of the inner and outer screen elements each opening 3I will be brought into register with the tapered slots 1 successively, and if any flock or unacceptable portion of the pulp stock begins to accumulate in any slot such material may be flushed through an opening 3| when it comes into registry with the clogged slot, and thus the material which was clogging the slot will be delivered into the interior of the inner screen member 4. This unacceptable material or tailings will now through the hollow shaft 23 into a lower chamber 32 in which the tailings from all of the lvarious screening units 3 are delivered, and such tailings will be delivered from the chamber 32 through a suitable discharge outlet 33.

Any suitable means may be employed for rotating the inner and outer screen elements of each screening unit. As shown in Figs. 2 and 3, the gear I1 by which the outer screen element 5 of each screening unit is rotated meshes and is driven by a central gear 34 mounted on a shaft 35 that is supported in suitable bearings 36. The gear 34 is driven by a pinion 35a carried on a vertical shaft 36a mounted in the supporting frame on which the casing I is carried. Said shaft 36a has at its lower end a sprocket wheel 31 around which passes a sprocket chain 38 that is driven by a sprocket on a shaft 39. The shaft 39 carries a large pulley 4U which is driven by a `belt 4I from the shaft 42 by a suitable motor 43.

Each gear 25 by which the inner screen element of each screening unit is rotated meshes with and is driven by a second large gear 44 which is mounted for rotation on the shaft 35. The shaft 39 carries a pinion 45 which meshes with and drives the gear 44 and by this means the two

gears

44, 34 will be driven at different speeds.

While the flushing openings 3l with which the inner screen element 4 of each screening unit is provided are useful in flushing the inlet slots 1 in case any slot begins to clog, yet these flushing openings are not always completely successful in keeping the slots 1 free, and I have, therefore, provided a mechanical means in the form of a blade which periodically is thrust into each slot 'I of each of the screening units, and thereby forcibly clears from the slot any material which may be clogging said slot. The slot-clearing movement of the clearing blade is preferably so timed with relation to the rotation of the inner screen element 4 that during the time said blade occupies any slot, an opening 3I in said inner screen element will move past and thus come into direct communication with the discharge edge of said slot, so that any material which is clogging the slot will be forcibly thrust through the slot and into and through the flushing opening 3l by the blade.

In the construction herein shown, the shaft 35 extends upwardly through the casing and the ace-asie upper. endl supportedlina bearing 361' carried by. the.. partition. it. There. are two clearing blades 45carried. by. the..jupp,er portionof' the shaft` 3 5, ,said blades` being. .'oppositely disposed.' Each blade` is mounted 1 ina.. blade-supporting member 46...and.tlie. tWo=..blade.-supporting members .46.. arecarried.: by, arms, 41 that. are secured to,and extend from the.-,sh'aft. 35.A

In. orderv not.. to. unduly, weaken theouter screening membe1g5, .the slots.1 therein aremade in sections. 1a, ,the circumferential ,portion 48 of theouter. screen member. between each twoad# jacent seriesof slot -sectionsfld being. solid.

TheV blades 45..` are provided" with operative blade sections 45a, each Y, blade. section' having, a length-corresponding. toa. slot sectiOnJa. Each clearing.bladel.is,mounte.d in alhead49. which is received in a vertical recess 50.with whichfthe bladefsupporting.member. Miv is provided. The head .49? is .formed with. a .plurality of ."stems 5| whichextend through the supporting member Samir-each of'which. is..screw threaded at its outer. end.r to recevecheck-.nuts52l Thehead 49 fis actednpon by. springs A53., which encircle the stemsf 5| and yieldingly` hold. thehead 49l with its blade A-,inoperative position. I f during the operationrof the machine ablade 45'. enters a clogged -fslotflata time .when there Ais no opening ,3L in. communication. therewith, the material cloggingY the v slot.wil1 .be forced inwardly. toward the` inner. edge of theslot `but cannot be pushed therethrough. Insuch-casethe springs. 53.pro vide-for. a backward yielding movementof the blade-and'ithus prevents injury to the blade or the; screening unit.

It=.will be noted that-,the screening units 3 .are arranged-in an arc within the casing l andthe shaft-.35.lis locatedvatthe 'center of said circle Vandis thus equidistantfrom allfof ,the screening units...

The rotative movement of theshaft 35. and

`that ofthe outer screeningmemben 5.,.. of each screen unit have sucha relation thatas said shaft rotates each blade 45 will enter a.slot..1 of. the

outerscreen members'of the various screening units.in-succession, each blade section 45a .enterblade` sections a'.- rotate, said.bladefsections-will Y mesh'withthe slot sections in much the same .way-as thefteeth of'` al pair of gea-rs'mesh with each other.

. As aeresult, the'rotary` movementofthe outer` Ascreeningelements-of the Various screening .units andthatl of theshaft 35 will cause each blade sectionz45aty to enter and clear a slot sectionv'la clear theslot sections-otadifferentslot Y'l ofr each screening: unit, and4 thus as-the-rotation ofthe screening elementsf 5f and the-shaft 35; continues each oneffoffthe slots-1 of :each screening-unit will 1n turn. have received: ablade'45andfwill thus be: freed; from any f' unacceptable material or' occulatedpulp ywhich may yhavebecomelodged therein;

Because; of the. different speeds-at which the innerf'a-ndouterscreeningelements;` 4, ,5,- of each screening :unit: 3;: rotate;- ani opening 3l of each;

inner element will move periodically into communication with each slotl foi the ycorresponding outer screening element 5, ,andgby properly timingY therelation between .the'rotarymotionof the shaft 35v carrying.. the bla-des 45 and-therotary motion of the innerscreen member ll, anl opening 3| .of` said inner. member will `be movedintocommunication with each slot?! of'I the corresponding outer screen member while said'slot is4 being clearedby. a..blade.

lclaim:

1. .A-pulp. screen comprisingntwo concentrically arranged screening4 elements' forming between them an annularclearance: space of'a size to per@ mit., acceptable` portions of," pulp stock'to" pass therethrough but of insufficient size to. permit unacceptable, portionsof said Astock to pass'therethrough,.the outerscreenng element having a plurality, of, inletslts leading to vsaid clearance space through ,which pulpto be screenedis delivered thereto., .and also.v having aplurality 'of discharge grooves.-extending. parallel to the slotsand open to saidclearance space, ,whereby acceptable portions` ofthe. pulp stockmaypass through the inlet .slots to .the discharge, groove through the clearanceA space, achamber communicating with all Y of said discharge. grooves. and vintowhich .the screenedstock-is delivered through said grooves,

aslot clearerhaving a blade, andblade-actuating meansfto-move the bladeperiodically vinto each inlet sloty to .remove therefrom any accumulated pulpl stock which-is clogging said slot.

2. A pulp screen comprising two concentrically arranged screening elements. forming between them -an annular'clearancespace. ofja size to permit acceptable portions l.of.unscreened pulpv stock to pass therethroughbutcf insucient size to permit unacceptableportions of said stock to pass therethrough, the outer. screening element having a plurality. of. inletslots leading to said clearance space,yand.also havinga plurality of discharge grooves extending, parallel to said 'slots and open tothe clearance space, whereby acceptable portions of the pulpstock may pass from the inlet slots tothe discharge grooves through the clearanceaspacaa chamber communicating with all. of :the discharge vrgrooves. into, which. the

.screenedstock isdelivered, meansgto rotate said .outer screening. element, alfslotclearing blade, and f blade:actuatingY meansl to move Y said'. blade Q periedically| into each v slot as the. outer screen element rotates, thereby to remove rom, the slot any accumulated pulp stock. Y

3. A pulp screenas defined in claim 2 and in rwhich the blade-actuating. means moves said blade into thevarious slots in a predetermined order, thereby to clear said slots of any accumuk lated pulp stock which tends to clog them.

4. A pulp screen as defined in claim 2 which includes means mounting the slot-clearingblade for. rotation about an axis parallel to that of the outer. screen element and so situated that at each rotation of the blade it is movednto and then withdrawn from one of the inlet slots, and means to rotate the blade about its axis in timed relation with the rotation of the screening. element.

5. A. pulp screen comprising two concentric cylindrical screening elements forming between them a clearance space of a size to permit'acceptable portions of unscreened pulp stock to pass 1therethrough but insufcient in size to permit unacceptable portions of..said stock topass therethrough, the outer screening element having inlet slots .extending therethrough from its outer face to the clearance space, .and alsohaving discharge grooves extending parallel to the slots and'open to the clearance space, whereby the acceptable portions of the pulp stock may pass from the inlet slots into the discharge grooves through the clearance space, a chamber communicating with all the discharge grooves and into which said screened pulp is delivered, means to rotate the outer screening element about its axis, a shaft situated exterior to the outer screening element and parallel to its axis, a plurality of slot-clearing blades carried by said shaft, each blade ,adapted upon each rotation of the shaft to enter and then be withdrawn from a slot, thereby clearing said slot from any pulp stock accumulated therein, and means to rotate said shaft in timed relation with the rotation of the outer screening element.

6. A pulp screen comprising two concentric cylindrical screening elements forming between them an annular clearance space of a size to permit acceptable portions of unscreened stock to pass therethrough but insufficient in size to permit unacceptable portions of such stock to pass therethrough, the outer screening element having inlet slots extending therethrough and leading to the clearance space and also having a plurality of discharge grooves extending parallel to the slots and open to the clearance space, a chamber communicating with all the discharge grooves and into which the screened stock received by the grooves is delivered, said inner screening element having an interior chamber communicating with a tailings outlet and also having a flushing opening leading from the clearance space to said chamber, means to rotate the outer and inner screening elements at different speeds whereby the slots are brought successively into communication with said flushing opening, a slot-clearing blade, and blade actuating means to move said blade periodically into each slot at a time when said slot is in communication with said flushing opening, whereby any accumulated pulp stock which has become lodged in said slot is forced therethrough and into and through said flushing opening.

7. A pulp screen as dened in claim 6 in which the inner screening element has a plurality of iiushing openings leading from the clearance space to said interior chamber, and which includes blade-actuating means to move the slotclearing blade into each slot at a time when said Vslot is in communication with one of the flushing openings.

8. A pulp screen as defined in claim 6 in which the inner screening element is provided with a plurality of flushing openings leading from the clearance space to said interior chamber, and which includes a plurality of slot-clearing blades and blade-actuating means to move each blade into an inlet slot at a time when said slot is in communication with a iiushing opening.

9. A pulp screen comprising two concentrically arranged cylindrical screening elements forming between them an annular clearance space of a size to permit acceptable portions of unscreened pulp stock to pass therethrough but insufcient in size to permit unacceptable portions of said stock to pass therethrough, the outer screening element having inlet `slots extending therethrough to the clearance space, and also having discharge grooves extending parallel to the slots and open to said clearance space, a chamber communieating with all the discharge grooves and into which the screened pulp stock is delivered, said inner screening element having an interior chamber communicating with a tailings outlet,

and also having a plurality of flushing openings leading from the clearance space to said chamber, means to rotate the outer and inner screening elements at different speeds whereby each slot is brought into communication with the flushing openings successively, a rotary blade-supporting shaft parallel to the axis of the screening elements, means to rotate said shaft, a slot-clearing blade mounted on said shaft and adapted during the rotation thereof to move into and out of each inlet slot periodically at the time said slot is in communication with a flushing opening.

l0. A pulp screen as dened in claim 9, and which includes a plurality of slot-clearing blades carried by the shaft and each adapted during the rotation of the shaft and the inner and outer screen elements to be moved into and out of each inlet slot while it is in communication with a ushing opening.

l1. A pulp screen comprising a casing having an inlet opening through which unscreened pulp stock is delivered, la plurality of individual screening units situated within said casing and arranged in circular formation and all located at the same distance from a common center, each screening unit comprising two concentrically arranged cylindrical screening elements forming between them a clearance space of a size to permit acceptable portions of the pulp stock to pass therethrough but insuicient in size to permit unacceptable portions of said stock to pass, the outer screening element of each screening unit having a plurality of inlet slots extending therethrough and communicating with the clearance space thereof, and also having a plurality of discharge grooves open to the clearance space, said casing having a chamber with which all of the discharge grooves of all of the screening units have communication and into which the screened pulp from all of the screening units is delivered, means to rotate the outer screening element of each screening unit, a slot-clearing blade, and blade-actuating means to move said blade into and out of a slot in each of the screening units in succession.

12. A pulp screen as dened in claim ll and which includes a rotary shaft located at the center of the circularly arranged screening units, two oppositely disposed slot-clearing blades mounted on said shaft and adapted during the rotation of the shaft to enter and to be withdrawn from a slot in each rotating slotted screening element,and means to rotate the shaft in timed relation with the rotary movement of the slotted screening element.

13. A casing having an inlet opening through which unscreened pulp stock is delivered thereto, and also having a tailings outlet, a plurality of screening units situated within said casing and arranged in an arc about a common center, each screening unit comprising two concentrically arranged cylindrical elements forming between them a clearance space, the outer screening element of each screening unit having a plurality of inlet slots extending therethrough and communicating with the clearance space, and also having a plurality of discharge grooves which are open to the clearance space, said casing having a chamber with which all of the discharge grooves of all of the screening units have communication and into which the screened pulp from all of the said grooves is delivered, the inner screening element of each screening unit having an interior chamber and a flushing opening forming a communication between the clearance space Y and said chamber, means to rotate the inner and outer screening elements of each screening unit at different speeds, whereby the ushing opening of each inner screening element is brought into direct communication with the slots of the corresponding outer element successively, a shaft situated coaxial with said common center, means to rotate said shaft, "a slot-clearing blade mounted on said shaft and adapted during the rotation of the shaft to enter and to be withdrawn periodically from each slot in each rotating slotted screening element at a time when said slot is in communication with a iiushing opening, and means providing a communication between the interior chamber of each inner screening element and said tailings outlet.

14. A pulp screen comprising a casing having an inlet opening through which unscreened pulp stock is delivered, a plurality of separate screening units situated within said casing and arranged in circular formation, each screening unit comprising two concentrically arranged cylindrical screening elements forming between thema clearance space of a size to permit acceptable portions of the pulp stock to pass therethrough, but insuincient in size to permit unacceptable portions of such stock to pass therethrough, the outer screening element of each screening unit having a plurality of inlet slots extending therethrough and communicating with the clearance space thereof, and also having a plurality of discharge grooves open to the clearance space, said casing having a chamber with which all of the discharge grooves of all of the screening units have communication and 1nto which the screened pulp from all of the screening units is delivered, the inner screening element of each screening unit having an interior chamber and also having a flushing opening leading from the clearance space to said chamber, said casing having a tailings outlet with which the interior chambers of al1 of the screening units communicate, means to rotate the outer and inner screening element of each screening unit at different speeds, whereby the slots in the outer screening element of each screening unit are brought successively into communication with the flushing opening of said unit, and means common to all of the screening units to clear each slot of the outer screening element of each screening unit at a time when said slot is in communication with the ilushing opening of said unit.

` outer screening element of each screening unit yhaving a plurality of inlet slots extending therethrough and communicating with the clearance space and also having a plurality of discharge grooves Which are open to the clearance space,

said casing having a chamber with which all,

the discharge grooves of all the screening units have communication and into which the screened pulp from all of said grooves is delivered, means to rotate each outer screening element, and meanscommon to all the screening units to clear periodically each inletrslot of the outer screeningelement of each screening unit.

16. A casing having an inlet opening through which unscreened pulp stock is delivered thereto, a plurality of screening units situated within said casing and arranged in an arc about a common center, each screening unit comprising two concentrically arranged cylindrical elements forming between them a clearance space,

the outer screening element of each screeningv unit having a plurality of inlet slots extending therethrough and communicating with the clearance space and also having a plurality of discharge grooves which are open to the clearance space, said casing having a chamber with which allthe discharge grooves of all the screening units have communication and into which the screened pulp 4from all of said grooves is delivered, means to rotate each outer screening element, and mechanical means common to all the screening units to clear periodically each inlet slot of the outer screening element of each screening unit, and to clear in succession an nlet slot in each of the outer screening elements.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,270,422 Lindquist June 25, 1918 2,368,876 Terradas Feb. 6, 1945 2,450,838 Mathewson Oct. 5, 1948 2,525,701 Mathewson Oct. 10, 1950