CA1058008A

CA1058008A - Feeder for pellet mills

Info

Publication number: CA1058008A
Application number: CA244,272A
Authority: CA
Inventors: Johannes A. Vink
Original assignee: California Pellet Mill Co
Current assignee: California Pellet Mill Co
Priority date: 1975-01-28
Filing date: 1976-01-27
Publication date: 1979-07-10
Also published as: GB1492161A; DE2600648A1; NL7515267A; FR2299153A1; US3932091A; BR7600492A; CH594494A5; JPS51101181A; FR2299153B1; DE2600648C2; IT1057071B

Abstract

ABSTRACT OF THE DISCLOSURE
A pellet mill for making food pellets from a flour product comprising a vertically arranged driven annular rotary die having radial die openings and a number of extrusion rollers inside this annular die and cooperating with the inner cylindrical wall of the die, which rollers are mounted for rotation in a common roller frame carried by a shaft which is held stationary by shear pins under normal operating conditions. The roller frame supports a number of feed tubes, one for each extrusion roller, which tubes each extend axially through the interior of the annular die between the rollers and near the inner die surface and the flour product is fed to these tubes by screw conveyors adjoining the open ends of the feed tubes with minimum clearance, the other, inner ends of the tubes being closed. Each feed tube has a side opening having the same axial length as the associated roller and having a circumferential width which gradually increases in-wardly whereby the flour product discharged through the side opening of each feed tube is evenly distributed over the length of the roller and the effective width of the annular die.

Description

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The invention relates to a pellet mill for the production of extruded pellets, in particular animal food pellets from a flour -~ product. The pellet mill may form part of an assembly comprising a feeding section to which the flour product is supplied, a condit- I
ioning section for blending the flour product with additives, such L
~ as molasses and/or for heating and moistening the mixture, and a 4 .,.
press section or pellet mill for extruding the prepared flour product in the form of hard pellets of, f.i. cylindrical cross . section. An assembly of this type is disclosed in U.S.Patent u 3,828,661.
Conventional pellet mills of the present type usually comprise a rotary annular die having a Oreat number of radial die openings and enclosine an interior die space which is closed on one side and open or, the opposite feed side. At its closed side the die i8 ~upported on one end of ~ horizontally mounted hollow drive shaft.
A number of extrusion rollers, f.i. two such rollers are rotatably mounted in a common roller frame arrnnged in the interior of the die, the extrusion rollers cooperating -~ith the inner cylindrical surface of the annular die in pressing the flour product fed to the die radially outwardly through the die openin6s. The roller frame is secured on one end of a second shaft extendin~ through and rotatably `;s mounted in the hollow drive shaft, shear pin means or the likeconnecting the opposite end of this second shaft to ths frame of the mill ~or holding the second shaft and thus the roller frame stationary ~ during normal operation of the mill. Feed lueans are ~rranged opposite ; the open side of the rotary annular die for reedinO the flour produc~ ~
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~` to be pressed to the interior Or the die.
During the operation of a pellet mill 0r the above-descri~ed type the feed means should preferably feed the flour product to the die of rot~ticn -/ 30 forw~rdly of the e~tru3ion rollcrs tas seen in the direction~o~ the i~ die) allowin6 these rollers to press the material through the die openings , " - 2 - ~`~
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'i ;" ' ~hereby bar-shaped pellets are extruded which are cut to length by cutters engaging the cylindrical outer surface Or the die. For a proper and efficient operation of the pellet mill it is obviously of importance that the ~aterial to be pressed is not only fed in e~ual portions to the several extrusion rollers but is also distributed evenly over the axial length of each :.oller and thus also over the effective width of the annular die in order to make full use of` the capaci-ty of the pellet mill and to avoid uneven wear of the rollers and cf the die. However, for obtaining such an even distribution it is generally not allowable to use feed means uhich extend from outside the die into the die interior.
The reason f`or this is that the extrusion rollers and the die must be protected again;t possible dama~e caused by o~erloaQing or by the occurence ..
of f`oreign hard matter such as a piece of iron or stone, in the supplied lour product. If such a foreign hard body is clamped between a roll~r and the die and consequently the driven die exert~ a rotation~l force f''` j on the roller frsme, the shear pins normally holding the shaft of the roller frame stntionary will break whereby the roller frame can rotate ` together with the die before the hard body can cause rupture of the ~`~ die or other da~age. Rotation of the central shaft supporting the roller rame causes the drive motor of the mill to be switclled of~. Since thus ,.. . .
;~ the roller ~rame must be ~ree to rota~e in cases of emergency, there should be no stationary parts, such as feed members, extending into the interior of the die. For this reason, in the convcntional pellet mills ',i.,'J of the present type the flour product is supplied to -the open fronJside Or the die ro~ which -the m~terial must spread out across the die under .',`~ .
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~ centri~ual action and sometimes with the help of suitable blades attached -. ~
~1 to the rotating die or by means of scraper e~ements moulttea on the ., .; . .
stationary central shaft. GenerallyJthe material i6 fed at a single ~ ;~ location at the lower side of the die independently of the nutttber of the ",.
extrusion rollers and it uill be clear that in this ~anner the above-`~ mentioned even ~istribution of the supplied material over the several rollers and over the w;dth of the die is rather illusor~. A further , , ,, ~
consequence of thissituation is that the die and the extrusion rollers can only have a limited axial dimension. Furthermore in these conven-, .. . .
tional feeding systems the distribution of the material in the die depends to a groat extent on the properties of the flour product, such as the bulk density, the particle size and the fluiaity thereof while also the rotational speed of the die has its influence.

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Summary of the invention.
.~ The object of the invention is to provide a pellet mill which obviates the above-discussed draw backs o* the conventioltal pellet mills and which provides an indivldual feed means for each of the ext~t-:'S~
sion rollers assuring an even distribu~ion Or the ~lour product over ~> the axial length of these rollers.

.:~`'j,t According to the invention the pellet mill thus is provided :' 1 ~I with a feed assembly for feeding the flour pro~uct to the interior of ; 1 the annular die which feed assembly has a plurality of seperate feed _i means one associ~ted ~ritn each of the sever~l extrusiQn rollers. Each ~. .
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L05~8 feed means comprises a product conveying means~ such as a screw conveyor, havine a discharge end situated just outwar(Uy of the die and a *eed :............. .
tube supported by the roller fra~te and extending axially through the înterior of the die spaced from the inner cylindrical die surface and, ~ as seen in the direction of rotation of the d;e, forwardly of the associ-;;, ated roller. Each feed tube is closed at its in~ardly facing end and has -~ an open inlet end situated with a minim~ clearance directly opposite ... .
~` the conveying means discharge end. Each feed tube is further pro~ided , ~ 1 ' with a side opening having an axial leng~h corresponding to the axial length of the associated roller and having a circumferential width which gradually increases inwardly whereby the flour product supplied to the feed tube by the conveying means and discharged through the side opening of the tube is evenly distributed over the axial length of the associated extrusion roller and over the effective width of the annular die.
The flour product to be pressed J.S thus not merely supplied to the open side o~ the die but is led by the feed tubes into the ;nterior of the die. Since each extrusion roller has its own associated feed tube there is no problem in suppl~ving the several rollers with eaual quanti- -ies o~P flour product per unit of ti~e. The side openin~s of the feed tubes then talce care of an even distribution of the discharged material ~engthwise of the rollers. Because the feed tubes are secured to the roller fra~.e they will rotate together with this frame ~Jhen the shear : ~r ~` pins break as a result of extreme oYerloading or of the presence of a hard body between. a roller and the die. The arrangement Or the invention thus m~kes it possible ~o obtain a uniform feed independen~ly of the , ;...
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properties of the flour product and of the rotational speed of the ~ die whereby the mill can be loaded to its full capacity and noun-- even wear will occur. At the same time, in designing the mill one is not restricted in the choice o~ the width of the die by feed problems so that dies of relatively great width can be used ` if the capacity of the mill is to be increased.
:; :-~ Preferably, each feed tube has a circular cross ;.:.
sectlon, the side opening of the tube being bounded by two cir-cumferentially extending lateral edges and two longitudinal edges, one situated higher than the other in which the higher , ~ ~
~;` longitudinal edge extends in the axial direction of the tube ~; and the lower longitudinal edge follows a substantially helical .~:..:
edge course which as seen in the circumferential direction may ` extend through an angle of fui. 70 - 90~. The feed tube may be so arranged that the vertical plane through the axis of the tube intersects the lower helical edge of the side opening. As a result of this shape of the lower edqe of the side opening of each s feed tube and the gradually inwardly increasing circumferential ~, .
width of this opening following therefrom the material supplied ; 20 to the feed tube will be gradually and progressively discharged through the side opening, part of this material only falling -i : . .
through this opening on reaching the closed inner end of the tube.
In this manner, the desired even distribution of the material ;:;.' . .
i~, over the axial length of the associated extrusion roller can be ,: :
, obtained. Preferably each feed tube is mounted on the roller ~, ,.
frame for angular adjustment about its axis whereby the position ~; of the said lower helical edge of the side opening can be varied :~ depending on the properties of the material to be , :
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The drawings show a preferred embodiment of the pellet mill according to the invention.
Fig. 1 is an elevation and partly a vertical axial section of !'' ` the pellet mill;
fig. 2 is on an enlarged scale a sectio~ on the line II-II of ~`. , .
'`'` fig. 3 of the arnular die, the extrusion rollers therein and the ~eed means o~ the pellet mill of ~ie. 1;
fig. 3 is an end vieu taken on the line III-III of fig. 2 o~
the annular die, the extrusion roller assembly ànd the feed tubes; a~d fig. ~ shows schematically an end view of the ~eed assembly.

;~1 Pre~erred embodiment of the i~ventionO
- -- --. .. . _ _ _ _ Referring to the drawings and particularly to fig. 1 thereof, the illustrated pelle-t mill is intended for pressing flour products and similar material, partic~llarly animal food, to pellets. The pellet mill .,~.,~, .
has a box-shaped frame 1 forming ai the same ti~e a gear case and ~';;`~ rotatably supporting, by means of roller bearings ~ a~d 3, a hollo-~ sha~t ecured on the hollow shaf'c 4 is a drive gear 5 which throueh suit-able tr~lsmission means can be driven by an electric motor in a conven-tional manner not shown in the drawings. At its forward end the shafJc . ~.! ~ . .
is provided wnth sn integral ~lange 6 suppolting an an~ular die 7 ~' which die is l~eleasably secured to -the flange 6 by means Or a split '~ locking collar~in a conventional manner (see also fig. 2). The annular dic 7 is circumferentially provided with a plurality of die openings 9 .~ ~
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.}` o~ circular cross sect;on extending radially through the whole thickness ~ of the die.
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. A central solid shaft 12 extends coaxially through the hollow .; drive shaft 4 and is rotatably mounted therein by means of roller besrings . ~ 10 and a bearing bush 11. A sleeve 13 is fixedly mounted on the rear end , .. .
of the shaft 12 which sleeve has a conical flsnge 14 and supports the .~ bearing bush 11~ A conical flsnge member 15 is fixedly mounted at the .l rear side of the frame 1 which conical ~ember 15 encloses the conical ~ lange 1~. Conical parts 1~ and 15 have corresponding holes receiving two ,~;", shear pins 16 preventing rotation of the central shaft 12 during the .... . . .
~, normal operation of the mill.
~i, A sleeve 18 having a radial flanOe 17 is fixedly mounted on-the ,~ forward end of the central shaft 12 and ~ seal ring 19 is arranged .. i . ' : - ~ ' bet~een the flange portion 6 of the hollow shaft ~ and this sleeve 18. ~ :
~r'~ The flange 17 supports two extrusion rollers 20 situated diametrically .. . opposite one another inside the interior space o~ the annular die 7 for .
',~, cooperation with the inner cylindrical surface of the die. ~t the other ,' side the extrusion rollers 22 are supported by a substsntially diamond-.j; . . ..
shaped ~ront plate 21 which i.s connected ~o the central shaft 12 by : means of a central bolt 22 passing through the ~ont plate 21 and threaded-; 1~ received in an axial screw hole in the end face of the shaft 12. The ~ :~
~ fl~nge 17, front plate 21 and bolt 22 thus form a roller fra~e for the li extrusion rollers 20 flxedl~ secured to the central sha~t 12. T~e extru-sion rollers 20 are each in a con~entional manner mount~od for free rota-tlon on a roller sh&rt having an excentric stud portion formed at each .

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' end thereof which stud portions are adjustably received in corresponding .,~' bores in the flange 17 and the front plate 21, respectively. Only the ~, forward stud portion 23 of this roller shaft structure is visible in ~' ' .
fig. 3. Adjusting means cooperate with the stud portion 23 oP each roller shaft which means comprise a part 24 secured to the stud portion 23 and having two abutments 25 engaged by the bol~ heads of two adjus-ting bol~s 27 which are recei~ed in threaded sleeves 26 secured to the front plate 21. The bolts 27 thus main'ain the part 24 in a fixed angular position. ~y aajustment of the adjusting bolbs27 the angular position o~ the stud portion 23 of the roller sha~t can be changed for' a readJustment of the extrusion rollers 20 ~ith respect to the inner v ~'~ surface of'the annular die 7 if this is necessary f.i. on account of ~i wear o~ these rollers and/or of the die.

In the operation of the pellet mill the driven hollow shaft 4 rotates the annular die 7 in the direction of the arrow of fi~. 3 in r,l,.~, ~i which the roller frame carried by the central sha~t 12 remains stationary and the two extrusion rollers 20 supported in this roller ~-, *rame are set in motion by the rotating die rolling on the inner cylin-~`'' drical surface thereof. The flour product to be processed is fed to the ' die 7 in a manner yet to be described'and is pressed by the extrusion ~ .
rollers 20 into the die openings ~ to be extruded at the outer circum-ference of the die as co.~pressed bar-shaped bodies which are cut to length .
by cutting means (not shol~n) in a known manner. The die 7 is enclosed by a housin3 28 (fi~. 1! releasably supported on the ~rane 1 and h3~ving a discharge opening 29 at its lower side, the cut-o~f pellets fa1ling ' doT~n~ardly through this opening 2g onto suitable conveyor means or the : :.
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~wo scraper means 30 supported by the ~ront plate 21 engage the ; inner wall of the die 7 forwardly of the die openings g which scraper means~
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as seen in the direction of rotation of the die, are situated in front of ,r, , ' the respective extrusion rollers 20.
~ The pellet mill as far as described in the foregoing is of conven-; ,............... .
tional design and structure and for this reason a more detailed descrip-tion of the afore-mentioned pars of the mill may be omitted. However, ~ the feed assembly for supplylng the flour product or other material to ,~ be processed to the die 7 has a novel design and ~Jill now be discussed.
As can be seen in fig. 3 the two extrusion rollers ?0 are arranged ~ diagonally with respect to the vertic~l plane through the die axis in .~ which, as seen in fig. 3, the roller on the left side of this vertical .. ..
plane has i~ higher position than the -oller to the right side of this plane. Each extrusion roller 20 has its own product feed means associated ; therewith which; QS seen in the direction of rota-tion of the annular die ~; 7,is situated forwardly of the roller. Each of these two feed means .;,,; .
~' comprises a feed tube 31 and 31', respectively, ~rhich feed tubes fittin~-."~.i .
i~ ly extend through openings in the front plate 2' into the interior of the ~ die 7 and are secured to the fl~nge 17 at their inner ends. T*.e two feed . . .
' tubes are each others mirror image so th~t a description of the ~eed tuoe ;,~ 319 shown in cross section in fig. 2,m~y su~fice.
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; - The feed tube 31 has a for~ard end projecting outside the front ~, .
~ plate 21 and a rear end which is closed by an end disc 32 fixedly secured .. : . .
therein. ~1e disc 32 has a cen-tral circul~r bGss 33 fittingly received ; .
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in a recess 34 in the flange 17. The disc 32 is secured to -the flan~e 17 by an axial bolt 35 passing -throuyh a central hole in the disc 32 and the boss 33 and screwed into the flange 17 whereby -~ on loosenings the bolt 35 the feed tube may be angularly adjusted ~- about its axis and again be secured in any desired angular posi-.~ tion. The feed tube 31 is provided with a retaining spring ~ clip 36 mounted in a circumferential groove of the tube, the front , . . .
plate 21 abut~ing this spring clip when the feed tube is properly .`~ mounted.
'. 10 A side or discharge opening 37 is provided in the , `. cylindrical side wall of the feed tube 31 which side opening is bounded by two longitudinal edges 38and 39 and two circumferen tially extending lateral edges 40 and 41. The axial length of the ~:~ side opening 37 between the forward lateral edge 40 and the rear ;. lateral edge 41, which coincides with the front face of the end disc 32, is equal to and covers the axial length of the extrusion , rollers 20 and the width oE the effective perforated portion of the die 7. The upper edge 38 of the side opening 37 extends axially close to the highest point of the feed tube 31. The lower edge 39 of the side opening 37 follows a helical line from its for-, ward point of intersection 42 with the lateral edge 40 to its rear , point of intersection 43 with the lateral edge 41 in such a manner ,. that the circumferential width of the side opening 37 gradually .~ increases inwardly from the lateral edge 40 to the lateral edge --~
~ 41. In the normal position of the feed tube 31 the lower helical .;
`~, edge 39 intersects the vertical plane through the axis of the feed tube, the arrangement being such that the side openings 37 of . the ~: 30 .',' ~
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two feed tubes 31 and 31' from each others mirror image with respect to the vertical plane throu~h the die axis and are turned outwardly towards the inner surface of the die 7 (see also Fig. 3).
The total area of each side opening is at least approximately `~ equal to the cross sectional area of each of the feed -tubes 31 and ,~ 31'.
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The feed assembly further comprises two hoppers 44 and 44' (Fig.l and 4), one for each feed tube which hoppers form a ;i .j ~
~' single structure and have a common front wall 45. The two . 10 hoppers 44 and 44' have immediately adjoining inlet openings at their upper ends which are surrounded by a common flange 46 for ~-the connection of the hoppers to a common supply duct for the ;'' flour product. The hoppers 44 and 44' are removably mounted in a conventional manner not shown and are provided with a common circular connecting plate 47 which fits against a ring member 48 ;
; mounted in a circular opening in the front side of the housing 28.
' A ring member 49 of angular cross sectional shape is secured by ,~ bolts 50 against the end face of the die 7 which ring member 49 has a tapering forward edge situated with a minimum clearance opposite a corresponding tapering surface of the ring member 48 to prevent ~'~; leakage of the supplied flour product to a minimum.
Each hopper 44, 44' terminates at its lower side into a - ..:
~, screw housing or trough 51 having the same inner diameter as the ~-' feed tubes 31, 31' and extending through the plate 28 coaxially with the feed tubes 31 and 31', respectively. The inwardly directed open discharge ends of the screw housings 51 meet the outwardly facing open forward ends of the feed tubes 31 and 31' . with a minimum clearance so that the feed tubes are free i~ .
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~ to move past the ends of the screw housings 51 on rotation of the flange . . .17 and central shaft 12. Each screw housing 51 contains a screw conveyor 52 having a hollow tubular shaft 53 which tapers in the direction of feed. The tubular shaft 53 is closed at its free end by a plug member'5 forming a nozzle with a bore 55 directed at an angle to the coinciding - .
axes of the feecl tube 31 and the conveyor screw 53. ~he tubular shaft 53 of the conveyor screw 52 is supported on the free end of a rotatàbly mountea hollow drive shaft 56 or 56', respectivel~, sealingly extending through the outer wall of the screw housing 51. Shea~es 57 and 57' on the drive sha~;s56 and 56', respectivel~r,are coupled by means of a rope 58 to a sheave 59 which is driven in a manner not shown by a suitable variable-speed drivinr motor or motor variator preferably through a slipping clutch for limiting the mdxi~u~ t~jraue Swivel connections 60 mounted on the outer ends Qf the hollo~r shafts 56, 56' serve to connect lexible tubes 6l leading to a source of compressed air (not sho~rn).
The flour product to be processed can be supplied in equal amounts to the two hoppers 4~ and ~4' from any suitable source,for instance from a flour conditioning apparatus which may be mounted on the pellet mill in . . ~ .
;~'; a conventional manner. When the pellet mill is operating,the scre~ con-:,, .
veyors in the screw housings 51 and 51' ~eed the flour to the ~eed tubes ~i 31 and 31'. As a result of the taperin~ shape of the hollow screw sha~ts .... .
53 the flour is propelled in a unifor~ flo~r from the hoppers into the feed tubes, the flour lea~ing these tubes through the side openin~ 37 ;~; thereof. Because Or the helical form of the lo~,rer e~ge 39 of the side ~ 37 ,~. - opening~that part of the flour material which is r.ot pushed over the hi~h~r f ' ' ~ - 13 .,; .

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i~: first part of this edge 39 moves further inwardl~, finally leaving the . ~ .
feed tube at the inner end thereof adjacen~ the end disc 32. By properly choosing the helical shape of the edge 39 it is possible to àttain that the material_evenly discharged over the full length of the side opening . . - .
37 for uniform dist-ibution across the axial length of the extrusion roller 20 and Or the effective width of the die 7 For each type of flour optimum distribution can be obtained by proper angular adjvstment of the feed tubes 31 and 31'. To that effect t:he bolt 35 and also the bolt 22 are loosened and, after adjustment of the feed tubes,again ti~htened.
The flour discharged through the side openil1~ 37 of the feed tube 31 falls onto the inner cylindrical surface of the rotating an~ular die 7 and is carried by this moving surface to the associated extrusion :.,.1 . .
;~ roller 20 (on the ieft hand side of fig. 3) situated above this feed tube 31 while the material discharged fro~ the feed tube 31' falls onto .~... . .
~;~s - the associated extrusion roller 20 (on the right side of fig. 3) situ-ated therebelow and is moved by this rotating roller to.~ards the nip -¦ between this roller and the inner die face. The t~o extrusion rollers are thus fed individually and i~ e~ual measure and the material supplied --,:
to each roller is evenly distrib-~ed over the axial length thereof.

ne feed eubes 31 and 31' ca~ be provided with a suitable coating at their inner side to pre~ent adhesio~ of the flour thereto. If never-~; theless some flour ~aterial still adheres to the feed tubes, remo~al : ~ .
~, thereof is possible by periodically feeding compressed air to the hollo~r . ~ .
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~ shafts 56 and 56', the hollow screw conveyor shafts 52 and the ~ozzles .~ 54 at the end thereof whereby these rotatin~ nozzles direct ~ air jetagainst the inner wall of tlle feed tubes. ~y sdjustmeDt of the speed of the conveyor screws 52 it is possiblc to regulate the r~te of feed . independently o~ the rotætional speed of the annular die 7 and thus to adapt-this rate Or feed -to the type Or the flour mæterial to be ;
processed.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A pellet mill for making pellets from flour products or similar material, comprising a frame, a first horizontally extending hollow drive shaft mounted for rotation in said frame and having two ends, an annular die member mounted coaxially on said first drive shaft at one of said ends thereof, said annular die having an inner cylindrical surface and a plurality of radial die openings extending through the annular body of said die, said annular die enclosing an interior die space open at the side of the die removed from said drive shaft, an extrusion roller assembly including a plurality of extrusion rollers arranged in spaced relationship in the interior of said annular die for cooperation with said cylindrical inner surface thereof, and a common roller frame supporting each of said rollers for free rotation, a second shaft extending through said first hollow drive shaft and having two ends, said roller frame being secured to one end of said second shaft, bearing means rotatably supporting said second shaft, shear pin means connecting said second shaft to said frame for holding said second shaft stationary during normal operating conditions of the mill, and, a feed assembly for feeding the material to be pelleted to the interior of said annular die, said feed assembly including a plurality of separate feed means one associated with each of said extrusion rollers, each of said feed means comprising conveying means for said material having a discharge end situated opposite said open side of said annular die, and a feed tube supported by said roller frame and extending axially through the interior of said die spaced from said inner cylindrical die surface, said feed tube having an open inlet end situated with a minimum clearance directly opposite said discharge end of the associated conveying means, means closing said feed tube at its end opposite of said inlet end, said feed tube further having an opening in its side wall, said side opening having an axial length corresponding to and coinciding with the axial length of said associated extrusion roller and said side opening having a circumferential width gradually increasing inwardly towards said closed feed tube end whereby the material supplied to the feed tube by said conveying means and discharged through said side opening is substantially evenly distributed over the axial length of said associated extrusion roller and over the correspond-ing effective width of said annular die.

2. The pellet mill of Claim 1, in which the area of said side opening of each of said feed tubes is at least approximately equal to the cross-sectional area of the feed tube.

3. The pellet mill of Claim 1, in which each of said feed tubes have a circular cross-section and in which the side opening of each feed tube has two generally longitudinally extending edges, one of said edges situated higher than the other edge, the higher one of said two edges extending in the axial direction of the feed tube and the lower one of said two edges following a substantially helical course.

4. The pellet mill of Claim 3, in which said helical lower edge of said side opening extends through an angle of approximately 70-90° circumferentially of the feed tube.

5. The pellet mill of Claim 3, in which said lower helical edge of said side opening intersects a vertical plane passing through the axis of the feed tube.

6. The pellet mill of Claim 1, further comprising means for adjustably securing each of said feed tubes for angular adjustment about its axis.

7. The pellet mill of Claim 6, in which said roller frame comprises a radial flange on said one end of said second shaft, a front plate spaced from said flange and a central bolt member securing said front plate to said one end of said second shaft, said extrusion rollers being supported between said flange and said front plate, said front plate having a plurality of openings therein, each of said feed tubes being fittingly received in one of said openings to extend through said front plate, said closing means of each of said feed tubes being secured to said flange.

8. The pellet mill of claim 7 further comprising bolt means releasably and adjustably securing said closing means of each of said feed tubes to said flange.

9. The pellet mill of claim 1 in which each of said feed means further comprises individual hopper means leading to said conveying means associated with the feed tube, said hoppers of said several feed means having inlet openings at their upper sides adapted for connection to a common supply means for said material.

10. The pellet mill according to claim 1 in which said conveying means each comprises a screw conveyor having a shaft tapering in the direction of feed.

11. The pellet mill of claim 1 in which said conveying means of each of said feed means comprises a screw conveyor having a hollow tubular shaft, a source of compressed air connectable to said hollow shaft and nozzle means arranged in the end of said hollow shaft opposite said feed tube for directing an air jet against the interior surface of said feed tube.

12. The pellet mill of claim 1 further comprising drive means for said conveying means and means for adjusting the speed of said drive means independently of the rotational speed of said annular die.