US2291815A - Hammer mill - Google Patents

Description

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H. E.` KORUM Aug. 4, 1942.

HAMMER MILL- Filed Sept. 27, 1937 [N VEN TOR l HENRY E. KOR/M .ruhr/fair.......::::..:.....:1....: .F.,FII I .l A

B Y Y Amal 'M' y A TTORNE Ys H. E. KORUM HAMMER MILL Aug. 4, 1942.

Filed Sept. 27, 1937 3 Sheets-Sheet 3 /N VEN TOR HENRY E. K/PUM ma ,UW

www/M A TTORNE Ys Patented Aug. 4, 1942 HAMMER MILL Henry E. Korum, Moline, Ill., assignor to Deere dav Company, Moline, lll., a corporation of Illinois Application September 27, 1937, Serial No. 165,857

Claims.

The present invention relates generally to grinding machinery and the like and is more particularly concerned with certain new and useful improvements in hammer mills.

Heretofore, mills of this type have required the expenditure of considerable power in order to secure as much as even a moderate capacity, and the principal object of the present invention is the provision of a hammer mill inwhich the capacity is materially increased and the power requirements materially reduced.

An additional object of the present invention is the provision of a hammer mill having rotatable hammers and a screen disposed about a portion of the rotor but in which the hammers are spaced far enough from the screen so that the only grinding action that takes place occurs by the hammers striking the material in mid-air and without rubbing or grinding the material between the hammers and the screen. As a result of this arrangement, there is no excessive wear on the screen nor is there any great amount of friction between the material being ground and the screen, so that there is no tendency for the screens to heat and power losses due to this cause are thereby avoided. A further object of the present invention is the provision of an improved breaker bar construction against which grain and other material is projected by the operation of the hammers. This feature of the invention particularly cooperates with the last mentioned feature, relating to the spacing of the hammers from the screen, and according to the present invention the grinding is done by the hammers striking and cutting the material in mid-air, throwing it against the breaker bars and back against the hammers until the material is broken down enough to pass through the screen. Thus, the provision of the breaker bars and the spacing cf the screen provide an arrangement wherein the screen serves only the purpose for which it was intended, namely, to determine the flneness of grinding and not necessarily to enter into the operation of reducing the material.

Still further', another object of the present invention is the provision of improved means securing the screen in position below the rotor, and particularly it is an important object of the present invention to provide a combined screen lock and breaker bar which not only accommodates various kinds of screens but cooperates with screens of various sizes and holds each in proper position for the best operation. Another object of the present invention in this connection is the vision of new and improved baile for the hoodA of the mill which not only protects thehood from wear but which can be adjusted to accommodate the feeding of different kinds of materials into the mill. 'Specically, it is an object of the present invention to provide an adjustable baiile which can be shifted relative to the breaker bar to accommodate the feeding of small materials, such as grain and the like, or stalk and fibrous material, such as fodder, into the mill.

Still another object of the present invention is to provide a hammer mill which can be converted from a mill adapted particularly -to grind small grains into a mill especially arranged to handle fodder or other stalk or other fibrous ma-V terials.

An additional object of the present invention is the provision of a hammer mill in which the chamber receiving the ground material isA so constructed and arranged that there is no tendency for the screen underneath the rotor to become clogged and reduce the capacity of the mill. It is also an object of the invention to provide a suitable auxiliary or cross current of air within the under side of the screen clear of any material that might tend to cling thereto. This is of special importance when grinding fine. It is also an object of the present invention toprovide an exhaust fan for drawing the ground material from the chamber and delivering it to a feed collector or other point, the exhaust fan being connected with the chamber by an intake duct that is so constructed that the ilow of material from the chamber to the exhaust fan is distributed evenly, whereby any concentrated wearl on the intake ductfis eliminated.

These and other objects and advantages of the present invention will be apparent to those skilled in the art after a consideration of the following detailed description, taken in conjunction with the accompanying drawings illustrating the preferred form of the invention.

In the drawings:

Figure 1 is a longitudinal vertical section taken through a hammer mill constructed according to the principles of the present invention;

Figure 2 is a vertical section taken substantially along the line 2-2 of Figure 1;

Figure 3 is an enlarged fragmentary section provision of a yielding breaker bar construction taken on substantially the same plane as the I hammer mill.

bar means that cooperates with .the rotor;

Figure 4 is a' perspective view of the exhaust fan casing and .the particular form of the intake duct connecting the exhaust fan casing with the casing of the hammer mill;

Figure is a fragmentary view, taken along the line 5-5 of Figure 2 and illustrating the auxiliary air inlet for the lower portion of the ground material chamber; and

Figure 6 is a view taken along the line 6-6 of Figure 1 and illustrating the auxiliary air inlet that leads into the ground material chamber from a point above the rotor screen.

Referring now more particularly to Figures 1 and 2, the reference numeral I8 indicates the casing of the mill, the easing being formed of two end plates II and I2 and two side plates I3 and I4. Ilf'he side plates and end plates of the mill are preferably formed of boiler plate and are securely welded and bolted together,

thereby providing a rugged frame for supporting the rotor shaft, its bearings and other cooperating parts, and to this end, each of the side plates I3 and I4 is provided with end flanges I5 and I6 to receive bolts I1 but secure the end plates II and I2 to the flanged portions of the side plates I3 and I4. The end plates II and- I2 are flanged, as at I8, at their lower portions, the bottom edges of the side plates I3 and I4 also being flanged, as indicated at I9, the flanges I8 and I9 thus serving as a base for the` mill. Preferably the body or casing I8 of the mill is bolted to wood skids 2| to provide ample support for the mill and to facilitate its movement from one building to another. The upper end of the casing I8 is closed by a hood 25, which will be referred to in detail later, the hood having an open side or end 26 that is pivoted on bolts 21 which, in turn, are carried at the upper ends of a pair of strap members 28. The fixed supporting strap members 28 are bolted to the side plates I3 and I4 by any suitable means, such as 2,291,815 section in Figure '1, showing the second breaker the edges of the rtriangular plates 18, and three banks or rows of hammers 12 carried by the rotor. The plates 18 and spacers are slidable on the rotor shaft 58, and lock nuts 61 and 68 are disposed on the outside of the assembled triangular plates and castings 18 and 1I so that when the nuts 61 and 68 are tightened the castings and plates are fixed firmly and rigidly to the rotor shaft 58 to rotate therewith.

Each hammer 12 in a row or bank is pivotally mounted on a single through pin 16 extended through registering holes formed in the aligned corners of the triangular plates 18. The hammers 12 are each thinner than the associated circular or cylindrical spacer casting 1 I, and adjacent each hammer the pin 16 carries suitable spacing washers 11. The thickness of each washer 11 is substantially equal to the thicknessof a hammer, and in the preferred form of construction, each spacer casting 1I is equal in thickness to a hammer and two of the associated spacing washers 11. Any suitable means, such as cotter keys 88, may be employed for maintaining thev hammer supporting pins 16 in position. A drive pulley 82 is fixed to one end of the rotor shaft 58 while to the other end of the rotor shaft is bolts 38. The upper side of the hood 25 slopes tached and which is pivoted to a suitably formedl bracket 43 that is bolted, as at 44, to the side 0f the mill casing I8. Preferably, the knob 48 is threaded onto the eye bolt 42 so as to provide for firmly clamping the hood in its closed position.

'I'he side plates I3 and I4 of the body I8 carry suitable support angles 5I land 52 bolted, as at 53, to the casing sides, and the angles 5I and 52 support in any suitable manner a pair of main bearing housings 55 and 56. Bearings 51 support the shaft 56 of the rotor, indicated in its entirety by the reference numeral 68, of the The rotor shaft 58 has suitable shouldered sections 6I and 62 that receive the inner races of the bearings 51, and the central enlarged portion of the rotor shaft 58 is threaded, as at 64, to receive right and left hand lock nuts 61 and 68. The rotor proper is disclosed in a copending application, Serial No. 355,977, filed September 9, 1940. Briefly, it consists of a plurality of triangular plates.18, a plurality of spacers 1I, which are generally circular castings of a diameter such that the spacers 1I are flush with fixed an exhaust fan which will be described later. Normally, the hammers are not clamped rigidly between the washers 11 but are free to swing on the rods 16, the amount of swinging action being determined by the amount of clearance between the butts 84 of the hammers 12 and the spacer castings 1I, as best shown in Figure 1. The slight swing of the hammers which the spacing between the latter and the spacing collars 1I provides is sufficient to accommodate the passage of foreign materials, such as bolts, etc., through the mill without serious damage thereto.

As mentioned above, the hammers 12 are arranged in three rows or banks, and to facilitate this arrangement the plates 18 are in the form of equilateral triangles, with thehammer supporting pins 16 disposed at the apices of`the triangular plates. 'I'his construction is of material importance in increasing the capacity of the mill, for as clearly shown in Figure 1 there is a rela- `tivelylarge space between each two adjacent banks of hammers to receive the grain or other material to be ground and to permit the hammers to have a full contact with the material and affording all of the cutting edges a full opportunity to act upon the material. The cylindrical spacing collars 1I are flush with the sides of the triangular plates 18 so that there are no projecting parts within the space between adjacent banks of hammers to interfere with or otherwise disturb the action of the latter in striking and reducing the material to be ground.

A generally semi-.circular screen I88 is disposed just below the rotor 68, and the screen |88 is spaced from the ends of the hammers 12 to a suicient distance (preferably in the order of radially so that no grinding takes place between the radially outer ends of the hammers and the screen itself. The grinding is done by the hammers striking and cutting the material in mid-air, and not by rubbing or forcing the material against or through the screen. The screen |88 consists of a generally senil-circular metal plate having perforations IOI punched or otherwise formed therein, and to receive the screen and to make it possible to remove one screen and substitute another screen having different sized openings the sides I3 and I4 of the mill casing I8 are equipped with pairs of inner and outer ring members |03 and |04 riveted or otherwise xed to the mill casing in spaced apart relation, thereby providing a screen receiving groove |05 at each side of the hammer mill housing. A plate is disposed horizontally at the upper portion of the hammer mill case I0 adjacent the pivots 21 for the hood 25, the horizontal plate ||0 being fastened in position by any suitable means, such as a pair of angles and ||2. The inner edge of the horizontal plate ||0 is extended into -substantial contact with the inner screen ring |03, as best shown in Figures 1 and 3, the latter being an enlarged view of this portion of the hammer mill. The trailing edge of the screen |00 is adapted to be disposed against the under side of the adjacent edge of the horizontal plate ||0 when the screen |00 is disposed in position underneath the rotor and in the grooves |05. This edge of plate ||0, indicated by the reference numeral ||0 in Figure 3, serves as a ledger plate or breaker bar when screens of 4certain sizes are to be used, as will be referred to later.

The screen |00 is locked in place by a combination screen lock and breaker bar that isv best shown in Figure l. At the ends of the screen receiving rings |03 andr|04 opposite the horizontal plate I0 is a breaker bar ||5 which is arranged to be removably supported in a position overlying and engaging the adjacent edge of a screen |00. The breaker bar ||5 consists of a member that is provided with two grooves ||6 and |1 of different depths, one of which receives the adjacent edge of the screen |00 when the breaker bar ||5 is inserted with its ends resting on the adjacent ends of the outer ring |04. A pair of shiftable blocks |20, each mounted on a side wall of the hammer mill housing by bolts |2| disposed in slots |22 in the block, serves to hold the breaker bar 5 in such a position in the casing that the breaker bar itself holds the screen ||0 in position between the rings |03 and |04 and with its opposite edge up against the horizontal plate ||0 which thus serves as a screen stop and support.

Material is fed to the rotor by suitable means preferably carried on the pivoted hood 25. Adjacent tlie pivots 21 the hood carries a circular baille plate '|25 that is held in place and clamped between the side walls 26 of the hood by a pair of tie-bolts |26 and |21, the latter extending through brackets |28 that are fixed in any suitable manner to the baille plate |25. A feed chute |33 is xed in any suitable manner to the hood, preferably bysuitable braces |34, and the inner end of the feed chute is bolt-ed to a feed chute adapter |36 which is fixed, as by bolts |31, to the side walls 26 of the rotor hood 25. The inner end of the feed chute adapter is disposed at the upper edge of the rear baille plate |25 and delivers the material directly into the relatively wide spaces between the banks of hammers. The feed opening |40 that is defined by the hood 25 and the feed-chute unit |33, |36 is controlled by a gate |4| which includes a sheet metal member |42 that is secured in any manner to a U- shaped bracket or clip |43 that is pivoted, as at |44, to the side walls of the rotor hood 25. A small feed control gate or slide |41 is adjustably fixed, as by a set screw and nut |50, so as to cooperate with the main gate member |42 in securing the desired rate of flow of material toward therotor 60. A safety ilap or inner baffle |54 is pivoted, at |55, to the hood 25 and acts as a shield to prevent material from being thrown backwardly out of the feed opening |40. A front baille |51 is xed by bolts |58 at its upper edge to the top wall 33 of the rotor hood 25 and curves downwardly toward the breaker bar ||5, being held in place in the hood 25 by a pair of tie rods |59. As best shown in Figure 1, the lower edge of the bale plate |51-engages they upper groove in the breaker bar ||5 when the hood is closed, so that when the hood is locked in place by the locking members 4| (Figure 4) the lower edge of the front baille |51 engages the breaker bar I5 so as to cause the latter to hold the screen |00 in lplace with its rear edge up against the screen stop plate ||0. 'I'he lower edge of the rear baffle |25 engages the upper surface of the plate 0 so as to prevent the screen from forcing the plate ||0 out of position, thereby cooperating with the angle bars and ||2. A tie rod |62 is disposed labove the plate ||0 at about the point where the rear baille |25 engages the same when the rotor hood 25 is closed and locked. The front baille |51 protects the rotor hood and, yin addition, forms a plenum chamber which will bev ref ferred to later.

The material that is fed into the mill andreduced by the hammers is drawn through the screen |00 and out of the mill casing by an exhaust fan unit indicated in its entirety by the reference numeral |15 in Figures 2 and 4. The exhaust fan unit |15 includes a fan casing |16, the inner wall |11 of which carries an angle member |18 that is bolted or otherwise fastened in any suitable manner to the angle 5| that supports the bearing housing 55 for the end of the rotor shaft 58 opposite the pulley 92. 'Ihis end of the shaft carries a blower fan |80 which has a hub |8| xed to the fan end of the rotor shaft 58 in any suitable manner, the blower fan being disposed within the fan casing |16. The fan casing |16 has a blower conduit |83, and the side wall |84 opposite the wall |11 of the fan casing is provided with an intake opening |85 over which the discharge end member |86 of a curved intake conduit pipe |81 is bolted, as at |88. The opposite end of the curved intake conduit |81 includes an elongated section |90 that is bolted at |9| over an opening |92 formed in the lower portion of the wall I4 of the hammer mill casing I0. As best indicated in Figure 4, the curved section of the intake conduit |81 is square'in cross-section but the intake and discharge ends of the members |90 and |86, respectively, are circular, the members |86 and |90 being particularly formed to take care of the transition from the square cross-section of the curved member to the circular cross-section of the intake and discharge ends where the conduit unit |81 is connected to the hammer mill casing and the fan casing. One particular advantage in having the member |81 f of square cross-section is that wear on the member |81 is appreciably reduced.

The operation of the fan serves to draw air down through the feed opening |40 and the mill casing, thereby drawing the ground material through the openings |0| in the screen |00 below the rotor 60. According to the present inl vention, the hammer mill is so constructed that chamber 200 consists of a pair of downwardly. inclined plate sections 20| and 202 which are bolted, as at 203 and 204, respectively, to the side plates |3 and- |4 of the hammer mill casing below the screen |00. The lower converging edges of the plates 20| and 202 engage and support a with respect to a vertical line passing through the axis of the rotor shaft 53, the extent of offset being indicated at a in Figure l. Both of the downwardly converging sheets 20| and 202 are spaced a considerable distance at the upper portions from the screen |00, the distance'b between the left hand sheet 20| and the screen |00 being somewhat smaller than the distance c between the right hand sheet 202 and the screen 00. Both distances b and c are, however, suiilcient to prevent any tendency for the ground material to clog between the screen and theplates 20| and 202. The `plate or sheet 202, by virtue of the above mentioned offset, is disposed more nearly vertical than the opposite sheet 20|. The reason for this construction is that by having a steep slope to the Wall 202 adjacent the side of the screen where the hammers are moving upwardly, that is, in a direction opposite to the direction in which the air is drawn downwardly through the mill, a somewhat greater opportunity is provided for the material above the rotor to pass through the screen and slide down into the bottom trough 201 than at the other side where the hammers. move downwardly, in the same direction that the air is drawn through the screen, and the passage of the ground material through the screen at this side is facilitated by the downward pressure of the rotating hammers. Hence the spacing b need not be quite as much as the spacing c for the opposite .plate 20|. However, the spacing of the wall 20| from the screen at the leftl side of the mill (Figure 1) is great enough to prevent any tendency for the material to clog at this point.

The prevention of clogging below the rotor` screen is further aided by the provision of a plenum chamber P between the baille plate |51 and the adjacent portions of the hood 25, such chamber communicating with the ground material chamber at the space between the left end of the screen and the left or front end wall of the hammer mill case. Provision is made for the admission vof limited amounts of air into the chamber P and from there into the ground material chamber along the lower surface of the left be moved to variouspositions.

of the bottom trough 201 is .offset to the right into the plenum chamber P. As will be clear from Figure l, during the operation of the mill, the auxiliary air that is drawn in through the opening 2|5 sweeps across the under surface of the screen |00 and effectively keeps this portion of the ground material chamber clean and pre- .vents clogging at this point.

A similar auxiliary air opening is provided for the ground material chamber 200. Referring now to Figures 2 and 5, the side wall plate |3 and the hammer mill casing has the circular. opening "200, mentioned above, disposed in alinement with the opening |92 in the opposite side wall of the casing. A shield 224 is fixed by bolts 225 to the side wall plate I3 and serves to deflect material coming from the screen away from the opening 200. An adjustable damper 221 is disposed on the outside of the plate Il and is provided with a slot 220 to receive an adjusting bolt 220 that is threaded into the plate Il below -the trough 201. The damper 221 is provided with a rolled edge 221'A that facilitates manipulation of the damper to exactly the right position over the opening 200. In fine grinding the air intake 200 may be closed or partially closed, in-= suring a greater volume-of air through the feed opening |40 `and the screen |00, and for coarse grinding it is usually desired to introduce more air at the opening 200 so as to facilitate the carrying away of the relatively heavier material by the draft of air that moves 'across the lower portion of the ground material chamber and into the intake duct |81.

The operation of the hammer mill described' above is substantially as follows:

The material to be ground is placed in the feed chute unit |33, |30 and the gate |4| adjusted to deliver the same uniformly'to the rotor. Any suitable source of power is connected with the drive pulley 82 to drive the rotor 50 and the fan that is carried directly on the end of the rotor shaft, as indicated in Figure 2. The relatively wide spaces between the adjacent rows or banks of hammers make it possible to introduce the material into. the rotor in such a manner that the hammers strike the material while it is still in mid-air, reducing the same whileat the same time projecting the material against the baille |51 or the breaker bar sections ||0l and ||5, from which the material rebounds back into the path of the rotating hammers. The spacing between the radially outer ends of the hammers and the screen |00 is great enough to prevent any tendency for the hammers themselves to rub or force material along through the screen. Instead, practically all of the grinding or reducing .action takes place by the hammers striking the material in mid-air, some of the grinding taking place due to the material being projected against the ledger the lower surface of the screen and keeps thelatter cleanand also prevents clogging at this point. At this time the auxiliary air inlet 208 is closed or partially closed since it is not necessary to have a very strong blast of air move across the ground material chamber since the ground `material is relatively light.

The mill can be adjusted to grind feed to practically any fineness desired. Each mill is usually equipped with two or more screens which are interchangeable by simply loosening the fastening units 4|, swinging the rotor hood back. which makes it possible to lift out the breaker bar |5 and then slide the screen out and insert another one in its place having a desired size of openings therein. 'I'he hood 25 is then swung back into closed'position after replacing the breaker bar H5, and the lower edge of the baille plate |51 engages the breaker bar and acts through the latter to hold the newly inserted screen in position for operation. In this connection it will be observed from Figure 9 that the groove |05, that is formed by the screen receiving rings |03 and |04 is wide enough to accommodate screens of various thicknesses. Usually, tlZscreens are perforated while in the form of flat plates, the plates while at all being of the'same length. Screens having smaller openings are necessarily thinner than those having large openings, since it is not practical to punch very small perforations in thick screens. All sizes of screens, however, are cut in the same lengths while flat, and then are shaped into cylindrical form. Therefore, when a thin screen is installed in a mill, it lies against the outer ring 4| and the end of the screen adjacent the breaker bar ||5 does not extend upwardly as far as when a thick screen is installed in the mill, since the median line between the two surfaces of a thin screen is thus disposed at a greater radius than is the median line of the thick screen. Hence, the longer notch or groove air will be drawn down through the feed opening |40 and along with the incoming grain to prevent carry it into the fan. On the other hand, theA coarse screens used when grinding roughage do not have any appreciable tendency to clog and. therefore, do not require 'a strong current of air along the under surface of the screen to keep ||1 is placed downwardly against the screen when a thick screen is used, as is illustrated in Figure 1, but when a thin screen is used, the bar ||5 is reversed and the opposite notch or groove ||6 is placed against the edge of the screen. Thus, in each case, the screen is locked in place and forced out against the outer rings |04 and with its trailing edge up against the stop plate ||0 when the rotor hood 25 is locked in closed position.

The breaker bars I I0a (Figure 3) and l5 are of especial advantage in reducing ne materials but are not necessary in grinding the larger sized material, and especially when using the mill as a roughage mill. According to the present invention, the breaker bars are so disposed that when thin screens are used, a considerable extent of the rear breaker bar is exposed to the action of the upwardly moving hammers in projecting the material against the cutting edge of the part ||0. However, when thicker screens are used, less of the section ||0a is exposed, so that when only a small, if any, breaker bar action is desired, only a small portion of the breaker bar section is exposed. When operating as a roughage mill, such as for grinding fodder and similar fibrous or stalk-like material, the gate |4| may be thrown back out of position to facilitate feeding the material to the rotor. At the same time, the tie bolts 59 may be loosened and the baille plate |51 shifted inwardly past the upper edge of the bar I5, thus making it easy to shove stalks and the like into the feed opening |40 of the mill without getting the stalk ends caught on the upper edge of the breaker bar ||5. Also, it will be noted that this adjustment of the baffle |51 reduces the action of the screen lock bar ||5 as a breaker bar.

When using the mill as a roughage mill, it is usually desired to adjust the plate 22`| (Figure 5) so that there is a considerable sweep of air across the ground material chamber 200 to facilitate lifting the heavier and larger particles and moving them into the intake duct |81 by the blast of air drawn through themill casing by the exhaust fan |80. However, when mill is used to grind grain, it is desirable to at least partially close the air opening 208 so that more the latter clear.

As mentioned above, the breaker bar retaining blocks |20 at each end of the breaker bar ||5 are fastened to the side walls of the hammer mill housing l0 by bolts disposed in slots, and by virtue of this construction, in the event that a foreign object, such asta stone or bolt, gets into the mill and is caught between a hammer and a breaker bar I5, the latter will be pushed back away from the hammer until the lower edge of the breaker bar clears the upper end of the screen retaining ring |04, thus allowing the breaker bar ||5 to drop downwardly and release the foreign object over the top of the breaker bar. The bolts holding the blocks |20 are preferably formed of relatively soft material, such as brass or aluminum, so that in case a large object gets into the mill, the bolts will'shear 0E and drop the breaker bar downwardly, thereby providing more room for the object to drop into the ground material chamber 200. Thus, about the only part of the mill that couldbe damaged, other than the bolts I2 would be the screen |00. A

While I have shown and described above the proposed embodiment of my invention, it is to be understood that the invention is not`to be limited to the specific details shown and described but that, in fact, widely di'erent means may be employed in the practice of the broader aspects of my invention.

What I claim, therefore, and desire tofsecure by Letters Patent is: y

1. In a hammer mill having a rotor and screen receiving grooves dimensioned to receive anyone of a set of screens of different thicknesses, the combination of a breaker bar, means `on the hammer mill receiving said breaker bar in a position to hold a screen in position in said grooves,

and having recesses in opposite edges of saidv bar adapted to receive said opposite end of said screen, said breaker bar being reversible and said recesses being of diierent depths for receiving screens extending diiferent distances from said fixed stop in said screen supporting means, respectively.

3. A hammer mill comprising a casing, a rotor journaled therein, a rotor hood pivoted to said casing and adapted to close the upper end thereof about said rotor, `grooved screen receiving means on said casing, a iixed screen stop at one end of said groove means, a removable generally semi-circular screen slidably supported by said n -screen receiving means and against said stop, a

baille wall carried by said hood and extending to a point adjacent the edge of the screen opposite said screen stop when the hood is closed, and a breaker bar carried by said casing and having a grooved portion receiving said one edge of said screen, the breaker bar having another grooved portion receiving .the lower edge of said hood baille when the hood is closed, whereby the hood acts through the breaker bar to hold .the screen in position, said grooves being of different depths -and said breaker bar being reversible end for end,

whereby screens extending different distances in said groove screen receiving means are accommodated. f

4. In a hammer mill comprising a casing and a rotor therein, means on each side of the casing forming screen receiving grooves thatl are generally arcuate about the axis of rotation of said rotor, a screen adapted to be inserted in place by sliding the samealong the grooves into position adjacent said rotor, a generally horizontal plate secured to and forming a' part of the hammer mill case, said horizontal plate overlying the grooves at one side of the rotor serving as a stop for the trailing end of said screen and terminating in an edge substantially in alignment with the inner sides of said grooves, so that said edge of the horizontal plate projects inwardly over the end of a relatively thin screen thereby serving as a breaker bar for fine grinding, but lying substantially ilush with the inside surface of a sci-een of maximum thickness, thereby shielding the breaker edge for coarse grinding, a breaker bar at the other side of the rotor and adapted to overlie the other end of the screen when the latter is disposed in position about the lower portion of the rotor and in said grooves, and means on the sides of the casing for removably receiving said breaker bar. v

5. A hammer mill comprising a casing includ ing upper and lower portions hingedly connected together, a rotor journaled therein, means forming grooves on opposite side walls of the lower portion of said casing, said grooves being adapted to receive any one of a number of screens of different thicknesses, means for holding a screen in said grooves in contact with the -radially outer portions thereof, and a breaker bar disposed at the trailing edge of a screen disposed in, said grooves, said breaker bar comprising a generally horizontal plate xed to said lower portion oi' the casing, serving as a top'wall thereof. upon which

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