US2669396A - Hammer mill screen changing mechanism - Google Patents

Description

1954 A. G. NICKLE HAMMER MILL SCREEN CHANGING MECHANISM 7 Sheets-Sheet 1 Filed Sept. 27, 1949 IN V EN TOR.

Feb. 16, 1954 5 NlCKLE 2,669,396

HAMMER MILL SCREEN CHANGING MECHANISM Filed Sept. 27, 1949 7 Sheets-Sheet 2 I NV EN TOR.

M3 N3 %w\ a 1954 A. G. NICKLE HAMMER MILL SCREEN CHANGING MECHANISM '7 Sheets-Sheet 5 Filed Sept. 27, 1949 INVENTOR.

mm mwm mm C Feb. 16, 1954 A. G. NICKLE HAMMER MILL. SCREEN CHANGING MECHANISM 7 Sheets-Sheet 4 Filed Sept. 2'7, 1949 FIB EI I N VEN TOR.

A. G. NICKLE HAMMER MILL SCREEN CHANGING MECHANISM Feb. 16, 1954 7 Sheets-Sheet 5 Filed Sept. 27, 1949 IN V EN TOR.

Feb. 16, 1954 NlCKLE 2,669,396

HAMMER MILL SCREEN CHANGING MECHANISM Filed Sept. 27, 1949 7 Sheets-Sheet 6 IN V EN TOR.

Feb. 16, 1954 A NICKLE HAMMER MILL SCREEN CHANGING MECHANISM 7 Sheets-Shae Filed Sept. 2'7, 1949 mmm 3 A\ \ww MN I Q .1: m.

- INVENTOR. BY WW Patented Feb. 16, 1954 HAMMER MILL- SCREEN 5 GHANGING- MECHANISM Arthur G. N ickle,. Saginaw, Mich., assignor. to Nickle Engineering, Saginaw, Mich.

Application September 27, 1948, Serial No. 51,433

40 Claims.

My invention relates to a mechanism for automatically interchanging an idle screen with a working screen in a hammer type grinding mill by remote electrical control. More especially my invention is applicable to hammer mills used in custom feed-grinding plants that process ear corn and small grain to produce livestock feeds of any desired fineness. The hammer mills in general use in such plants are: installed below the operating floor, so that the operator is compell'ed to travel up and down from one floor to th'eother to manually change the Working screen when the fineness of the reduced material must be modified to suit individual customers. Such manual screen-changing operations not only entail extra work. for the operator and a. loss of valuable time, but a waste of mill power, especially so if the mill must be stopped. and started to change the screen.

This invention provides an electrically controlled mechanism for removing the working screen from a hammer-grinding mill, which screen is one of a plurality of similar screens interchangeable therewith, and automatically replacing it by any idle screen selected from the plurality. To effect the screen interchange the operator simply presses the electrical pushloutton that is associated with the. selected screen that is to be introduced into the mill housing. As the. automatic screen interchange cycle involves a time interval of less than nine seconds, it will be apparent that this invention affords a valuable means for conserving time and mill power when.

the fineness of the reduced product must be modilied.

A detailed description of this automatic screenchanging mechanism, as applied to a hammer mill grinding unit, is made with reference to the accompanying drawings in which is embodied one form of my invention.

Description of figures:

Fig. l is a side elevation of a hammer-grinding mill unit that embodies the automatic screenchanging mechanism.

Fig. 2 is a fragmentary plan view that corresponds to Fig. 1.

Fig. 3 is a fragmentary vertical transaxial sectional view through the grinding chamber of the mill which shows the grading screen clamped in working relation with the hammer rotative element and the idle screens mounted above the mill'housing, the view being indicated by line 3-4 in Fig. 2.

Fig; 4 is an enlarged detail elevational view of abellcrank employed in the screen-clamping mechanism.

Fig. 5 is an edgeview of the bellcrank shown in Fig. 4.

Fig. 6 is a fragmentary vertical sectional view showing screen-clamping and screen-translating means, the view being taken on line 6-5' of Fig. 1.

Fig. 7 is a fragmentary sectional elevation of the screen-clamping mechanism taken on line 7---'! of Fig. 6, the clamping mechanism being shown in the released position it assumes when the screen is entering or being withdrawn from the mill housing.

Fig. 8 is a fragmentary plan view showing how the upper end of one of the screens is attached to the screen-translating chains.

Fig. 9 is a side view of the chain and attachment shown in Fig. 8.

Fig. 10 is a fragmentary sectional view taken on line l0l0 of Fig. 8.

Fig. 11 is an exploded isometric view showing details of the slipclutch driving mechanism employed in delivering power to the screen-clamping and screen-translating mechanisms.

Fig. 12 is a side elevational view showing in assembly the tracks. and sprocket wheels employed in guiding the several screen-translating chains on one side of the mill housing.

Fig. 13 is an enlarged detailed sectional elevation of the track assembly taken on line l3-l3 of Fig. 12.

Fig. 14 is an enlarged sectional elevation of the track assembly taken on line M-I 4 of Fig. 12.

Fig. 15 is an enlarged sectional elevation taken on line I5-|5 of Fig. 12 showing the chain tighteners.

Fig. 16 is a fragmentary enlargement of one of the track plates shown in Fig. 14 with the addition of a screen-translating chain mounted thereon.

Fig. 17 is a vertical sectional view taken on line l l--ll of Fig. 3 showing the solenoids associated with the screen-restraining latches.

Fig; 18 is an electrical Wiring diagram showing remote controls applied to motor that operates the screen-changing mechanism.

Description of hammer-grinding mill and auxiliary equipment vates it through pipe I3 to a dust collector (not shown). The crusher-feeder H], which is driven by V-belts I4 from motor l5,reduces the unground materials, such as ear corn, to a particle size that promotes accurate flow regulation to the grinding mill, the rate of flow being regulated by turning handle it on vertical stem H, which stem projects a convenient distance above the operating floor. The lower end it of stem IT is attached to the crusher-feeder flow control mechanism at I9 (Fig. 2) The material fed to the mill is ground in usual manner by impact of hammers (Fig. 3) which are hinged to pivot bars 21 carried by radial arms 22 mounted upon shaft 23 for rotation therewith. The mill shaft 23, connected to a 3,600 R. P. M. motor (not shown), is journaled in a pair of bearings 25 (Fig. 1) attached to vertical side walls 25 of the lower section of the mill housing. The side walls 25 are tied together below the crusher-feeder by an inclined bottom plate 26 (Fig. 3) which delivers the flow of unground materials to the grinding chamber through the inlet opening 21.

With further reference to Fig. 3, the removable cover section 28 over the grinding chamber comprises a pair of semi-circular side walls 29, the periphery of which is welded to arcuate plate 353. The walls 29 are aligned vertically above side walls 25 of the bottom section of the mill housing, the upper and lower sections having flanges 3i and 32 respectively (Fig. 1) that are clamped together on parting line 33 to provide dust-tight construction around the hammer rotative element. The lowermost end 3c of arcuate plate 35 (Fig. 3) extends below the parting line 33 and is welded to cross bar 35 which bar is removably secured between side walls 25 of the lower section of the housing.

Underlying the hammer rotative element in the periphery of the grinding chamber is a rectangular opening 55 for the efilux of ground material, which opening is defined by two arcuate ribs 31 welded to side walls 25, the lower edge 38 of crossbar 35, and the lower edge 35 of inclined receiving plate 25. Removably clamped over the border elements that define the opening 36 is an arcuate working screen 4B through which the reduced product is graded as it escapes from the grinding chamber, this screen being held in working relation with the hammer rotative element by a clamping mechanism that will be subsequently described. The working screen 40 is subject to automatic removal and replacement by any one of the three similar idle screens 4], 42 and 43 shown stacked in layer formation above cover section 28 of the mill housing. Likewise, regardless of which screen is in the mill at the beginning of the screen interchange cycle, any one of the three other idle screens may be selected for direct interchange therewith.

The compartment i l, Figs. 3 and 6, which receives the efnux of ground material that is graded through the working screen, has an inclined bottom plate 45 (Figs. 1 and 3) which joins with the side walls 25 to form a gradually increasing U-shaped trough 55 (Fig. 6) which connects with the circular outlet opening 4'! in the vertical cross wall 48 of the housing, the opposite end 49 of plate 45 being flanged upwardly to join with receiving plate 26. From opening 41 the reduced material travels through the tubular element 55 (Fig. l) to inlet of fan iii. The top of the efiiuxreceiving compartment 44 is covered by plate 5! (Fig. 3) that is flanged upward at 52 to provide an open passageway 53 through which any screen may be translated into or out of compartment 44. This passageway is made dust tight, while the 4 working screen is held in operative relation with the hammer rotative element, by a closure that operates automatically in timed relation with the screen-translating mechanism, as will be subsequently described.

The several idle screens stored above the mill housing (Fig. 3) have their distal ends hanging downwardly into the passageway 53 in loose layer formation, so that any screen may be lowered into the mill housing or withdrawn therefrom without undue friction between abutting screen surfaces. Also, it will be seen that the proximal end 54 of working screen 40, and likewise any screen that may be introduced into the mill housing, projects above the distal ends of the idle screens. This overlapping relation of the working screen with respect to the idle.

screens makes it practical to move any screen upwardly or downwardly, within fixed limits, without danger of the ends of the screens fouling with each other.

Screen-clamping mechanism As shown in Fig. 3 the working screen 40, or any screen interchanged therewith, is removably held over the outlet opening 35 and against stationary ribs 3?, cross bar 35, and the lower end of receiving plate 26 by means of a power-operated toggle-clamping mechanism, which is further illustrated in Fig. 7 with the'screen removed. This clamping mechanism includes three vertical arcuate clamping bars 55 which are formed con-- centrically with respect to stationary ribs 31, their lower ends 55 being hinged to pivot bar 51 supported by side walls 25 of the housing, while the upper ends are welded to cross header 58 to form a three-legged frame (Fig. 6) that may be swung inwardly to clamp the positioned screen to the border elements that define the peripheral screen opening 36; and contrawise, as shown in Fig. '7, this three-legged frame may be swung outwardly to release the screen for removal and interchange. The horizontal header 58, as best shown in Fig. 3, is angular in cross section, the inclined leg 59 being formed with a lower marginal edge that bears upon the outer face of the screen on a line that is directly opposite cross bar 35, thereby forming a dust-tight joint between screen and elements 59 and 35 in contact therewith. The upper marginal edge of vertical leg 60 of header 58 seats against resilient rubber gasket 6!, which gasket is secured to underside of cover plate 5|, thereby providing a dust-tight closure for screen passageway 53 when the screen is clamped in working relation with the hammer rotative element. Also, it will be seen that the closing and likewise the opening of passageway 53 takes place simultaneously with respect to the clamping and releasing of the screen.

The toggle mechanism (Figs. 3 and 6) that automatically clamps and releases the working screen is actuated by shaft 62 which has its ends journaled in side walls 25, the near end projecting beyond the wall to receive power-driven means that rocks the shaft to and fro through an angle of approximately 30 as will be described. Upon shaft 62 is mounted a tubular sleeve 63 secured thereto by pin 64 or other means, this sleeve having three upwardly projecting arms 65 that are aligned to engage and support cross shaft 65 which swings therewith when shaft 62 is moved. In pivotal engagement with shaft 56 are three plain links 61 that have their upper ends pivotally engaged to cross shaft 68 which is movable therewith. The linkage which operates the screenclamping elements further includes: three" bellcranks 69, details of which areshown in Figs. 4 and 5. Theouter end of each bellcrank has an opening 76 which pivotally engages shaft 68, and an opening on the opposite endwhichpivotally engages shaft 72. carried by clamping bars 55 (Fig. 7). Formed integral with each bellcnank 69 is an upwardly projecting ear I3 which has an opening 14 that receives'the'body'of capscrew I5 in loosely fitting relation as bestshown-in Fig. 7-, the threaded end. of the capscrew being adjustably screwed into the threaded opening I5 of. boss TI on leg 59 of header 58. To make the clamping mechanism effective when closedupon screenslof different thicknesses, acoiled compression spring 18 is assembled over capscrew 15, the lower end of the spring seating against ear 13 of the bellcrank, andthe upper. endagainstboss car 73, as indicated at 19 in Fig. 3, then spring is automatically compensates for any variation in thickness of the several screens employed in the mill. In addition, spring 18 effects a sudden snap-action release of the screen when the shaft I52. is turned clockwisea few degrees. After the screen is released, the head of capscrew I5 bears against the lower face of ear 13 of bellcrank 69, as may be seen in Fig. 7.. When the screen is fully released; clamping bars 55 are swung outwardly away fromribs 3'! to form the open space 85' therebetwcen, this space. being relatively wide at the top and narrow at the bottom. The outward swing of clamping bars 55 is limited by making contact at 3] with sleeve 63 on shaft 62. Upon the projecting end. of shaft 62 is mounted the bifurcated arm 82 (Fig. l), which arm is made integral with hub 83 this hub being pinned at 84 (Fig. 6) to shaft 52, so that when arm 82 is tilted downward, as shown in Fig. 1, the screenclamping mechanism is. closed. Contrawise, when arm 82 is tilted upward, as shown in dot and dash lines, the screen is released for removal from the mill; The means employed to tilt arm 82 up and down through an angle of about will be subsequently described.

Slipclutch mechanisms Referring to Figs. 1, 2 and 11, the power that drives the screen-clamping mechanism, and likewise the screen translating mechanism, is obtained from shaft which is journaled inbearlugs 86 mounted upon cover section 28 of the grinding chamber. Shaft 85 is. driven by the reversible three-phase motor 8? which is mounted within the mill housing as shown by broken lines in Fig. 1. The power from motor 81 is transmitted to shaft 85' by any suitablespeed-reducing means, the mechanism illustrated including countershaft 88 upon which is mounted sheave 59 driven from sheave 90 on thexmot'or'byv-belts 9!, the countershaft beingsupported by'the mill housing in any suitable manner. From sprocket 92 on countershaft 88 thepower. is transmitted by chain belt 93 to the. relatively large: sprocket 94 mounted on end section 95 ofi shaft 85. The

hub. 9.6- of sprockets! lis bored: for? axialislipifitxlon shaft 85, the shaftibeingsdriven;byvaifeatherrkey (not shown) that engagesthe keyway' 91 (Fig. 11) in the shaft; Also, bored for slip fit and mounted on end section 95-of theshaft (Fig: 2) is a series of 'five'smaller sprockets 9 B te l 02* which drive chains I 03- to I01 respectively, the chain 103 being belted to a relativelylargesprocket. I08 which operates the screen-clamping mechanism and also serves-as a rotative timing element for the electricalcontrols as will be explainedi On the opposite end section IOE- of shaft are mounted four sprockets I Ill-to lfI t thatare duplicates of sprockets 99 to' I02 on-opposite' end of shaft, and to sprockets III) to H3 are belted chains H4 to Ill'respectively. The chains I IW- II4, I55-II5, IllG-I-IG and IIJT-II'I are paired to travel in unison and are'employed for translating the screens-40" to 43 respectively intoand out of themlll, as Willbe-fur-ther explaineoli To each of the driving sprockets mounted on shaft 85 is applied anindividual slipclutch mechanism that transmits a-predetermined amount of torque before slippage takes place, this torque being the same for all of the sprockets. With shaft 85 turning ineither direction, the paired sprockets associated with anyscreen slip inunison when the screen is standingstill, andunder certain conditions all of theclutchmechanisms may slip simultaneously." Inasmuch as the several clutch mechanismsare-all identical in construction and operation, only one will be described, and for the purpose'of illustration the one applied to sprocket III) has been selected. The slipclutch mechanism that drives sprocket IIIl, Fig. 11, includesapair of' fiber friction disks IIB bored for running fit on shaft85i and a pair of metal disks H9, each of which is formed with bore I20 having an inwardly projecting spline I21 which slidably engageskeyway 9-1 in the shaft, so that the disk rotates withthe shaft regardless of its axial position thereon. These disks are assembled on the shaft in symmetrical relation with respect to sprocket III], the fiber: disk'being sandwiched between the metal'disk andsprocket and in frictional contact with both, so that when the assembly is subjected; to axial. thrust from opposite directions, the sprocketiscausedlto rotate with the shaft. The amount of: torque delivered by the sprocket tothechainthereon, before slippage takes place, depends upon the amount of axial pressure applied to the assembly.

With further reference: toFig; 11; the nut" I22 in engagement with threaded end H19: of shaft 85 provides a convenient means-for regulating the amount of axial thrust that is simultaneously applied to the several slipclutch mechanisms as-' sembled in multiple upon the end section of. the shaft. The nut I22 seals against auxiliary metal disk I23, which disk is bored for slip'fit on the shaft and held in spacedrelationfrom the outer most metal disk I I9 by three or 'more coiled com pression springs I24, these springs beingheldin place by assembling one end over suitable inwardly projecting bosses I25can'ied by'enddisk I23. Thrust collar I26 (Fig. 2) takes the reaction of nut I22 on end section of shaft 85*. The sprocketdisk M on endlsection 95 of the shaft serves the same purpose as-diski 4233' on: the op:- posite end section, anddisk 94 is. held in spaced relation from adjacent splineddisk I I9 by coiled springs I24 in the same manner, so that when nut I 27 is adjustedto bear againsthub 96 of sprocket 94, the multiplev slipclutch' mechanisms: are :sub

j ected to' the necessary axial thrusuin sameimam Screen-translating mechanisms To the proximal end of each of the four screens is attached a horizontal drawbar I23, Figs. 8 and 10, which projects beyond sides of the screen to engage translating chains associated therewith. Drawbar I28 is attached to screen 40 by means of centrally disposed hinge butts I29 and I38 welded to the drawbar and screen respectively, the butts being detachably secured together by removable pivot pin I3I. Translating chains I04 and H4 associated with screen 40 each carry an attachment I32 which is formed with a rectangular opening I33 asshown in Fig. 9, this opening being adapted to receive the end of drawbar I28 in loosely fitting relation, so that when the paired chains travel in unison the screen is moved therewith. Consequently, the screen is translated into the mill housing when the chains travel in a clockwise direction, and contrawise, from the housing when the travel is counterclockwise. In like manner screens 4!, 42 and 43 are translated by paired chains I-I I5, I08I IE and I0"II I? respectively. The downward travel of the screens is limited by a pair of vertical stops I34-I34 (Fig. 3) upon which the drawbar I28 comes to rest when the screen is positioned within the mill ready for clamping. These stops also form the opposite end walls of screen passageway 53. The upward travel of the screens is also limited by the drawbars coming to rest against vertical stop plates I35--I35, which plates are welded to cover section 28 of the mill housing. After the screen has reached the limit of its travel in either direction, the slipclutch mechanism associated therewith must slip until shaft 85 comes to a standstill.

Screen-restmz'ning mechanisms When shaft 85 rotates in a clockwise direction the idle screens are all urged by their respective slipclutch mechanisms to move downward into the mill housing. As only the selected screen may be allowed to move downward into the mill, it becomes necessary to apply individual restraining means to each of the several screens, this means being subject to solenoidal release as will be explained. The restraining means (Figs. 3, 8 and 17) comprises spring-urged latches I36 to I39 for screens 40 to 43 respectively. To the upper edge of each draw bar I28 is welded a flat centrally disposed ear I40 that has an elongated opening I4I which is adapted to engage the restraining latch associated with the screen. As the latches are all similar in construction and operation, only latch I36 (Fig. 3) will be described in detail, this latch being formed as a bellcrank with body welded to cross shaft I42 which has opposite ends pivotally mounted in vertical stop plates I35 so that the latch may be tilted in a vertical plane. The upper end I43 of the latch is formed as a pointed hook that engages opening MI in drawbar ear I40 when drawbar I28 is positioned against stop plates I35. The pointed hook I43 is urged downward into opening MI by action of coiled spring I44, this spring being tensioned between arm I45 of the latch and cover plate I43 which spans stop plates I35. Thus engaged by latch I36, screen 40 is securely restrained from downward movement into the mill while shaft 85 is rotating in clockwise direction. The means employed to free the screen from the latch includes vertical pushrod I41 which is operated by a solenoid, as will be further explained, the lower end of this pushrod being pivotally attached to the outer end of arm I45 of the latch, so that when the latch is tilted counterclockwise the hook I43 is raised to clear drawbar ear I40. In this manner similar pushrods (Fig. 17) I48, I49 and I50 are employed to actuate latches I31, I38 and I39 respectively.

Solenoidal release of selected screen Referring more especially to Figs. 3 and 17, it will be seen that latches I35 to I39 are operated by individual solenoids I5I to I54 attached to vertical pushrods I41 to I50 respectively. These are conventional solenoids with an iron plunger that moves downward when energized. They are mounted upon plate I46 which is provided with an opening (not shown) below each solenoid through which the plungers connect with the vertical pushrods. Only one of the solenoids may be energized at a time, and this solenoid remains energized until the bifurcated arm 32 is tilted downward to clam the positioned screen in operative relation with the hammer rotative element. After the solenoid is deenergized, the spring-urged latch tilts clockwise to resume its normal position in readiness to engage the screen associated therewith when the screen is withdrawn from the mill. The electrical circuits and the timing mechanism employed in energizing the four solenoids will be subsequently described in connection with the wiring diagram shown in Fig. 18.

Tracks for screen-translating chains To translate the screens up and down through the screen passageway 53 without flexing, and to avoid undue friction with each other and stationary parts or the mill housing, it becomes necessary to guide the proximal end of each screen and the drawbar connected thereto over a fixed path, this path approximating the arc of a circle having its center located within the confines of the grinding chamber. This is accomplished by providing a curved or arcuate track for each of the screen translating chains to travel upon, the several tracks being generally designed as assemblies I55 and I56 which are rigidly attached to opposite side walls of cover section 28 of the mill housing as shown in Figs. 1, 2, 6, 12, 13 and 14. The assembly I55, which is mounted on near side of mill, includes spacedapart track plates I5? to I60, the upper marginal edges of these plates serving as tracks for chains I84 to I07 respectively. Likewise, assembly I56 includes track plates IEEI to I65 for chains II4 to I II respectively. Referring more especially to Figs. 12, 13 and 14, the lower ends of the curved track plates are supported in common on stud I65 welded to adjacent wall plate of cover section. With stud I55 as a pivotal point, the upper sections of the several track plates in each assembly are fanned out in fixed overlapping relation, so that the upper marginal edges of paired plates I60-I84, I58I63, I58-I62 and I5I-I6I are positioned at progressively higher levels in the order related. This arrangement of the track plates provides working clearance between the several drawbars I28 when they are stacked above the cover section. This clearance decreases as the arcuate tracks approach a common point of tangency directly over the screen passageway 53 (Fig. 3). In other words, regardless of which screen is in the mill, its drawbar will be positioned in common as shown in dot and dash lines I86, Fig. 12.

'As illustrated in'Figs. 12 "and 16, "the screentranslating chains are conventional roller chains with rollers Iii-l tied together by paired links Hill-468 and I'69-I69, the lower extremities of inside links I68 overlapping the rollers and straddling the track plate such as ldlythereby guiding the travel of the chain thereon. To further increase the stability of the chains traveling over the arcuate tracks, the periphery of each track plate, such as I51, is made with aplurality of circumferentially spaced G-shaped indentations I'1IJ into each of which is assembled a small sprocket I1I that is positioned sothatits uppermost teeth engage the chain traveling on the track plate. As best shown in Fig. 13, each sprocket is rotatively mounted on a shouldered stud I12 riveted to ear I13 which ear is Welded to the track plate. The lower end of track plate I51 is Welded to ear I14 (Fig. 12) which engages stud I65, thereby providing supporting means for the lower end of the plate. Also, mounted upon stud I65 for rotation thereon and aligned with plate IE1 is an idler sprocket 115 that is similar to sprocket I1! and which serves the same purpose. Likewise, track plates I", I59 and I65! are provided with idler sprockets HI and I15. The track assembly I56 is a duplicate of assembly I55 in all respectsexcept it is mounted upon the opposite side of cover section 28 in opposite hand relation, so that fanned-out ends of track plates Ilse-454, Ids-4E3, let-I62 and i51-I6I are paired to support the chains mounted thereon at progressively higher levels.

To provide means for individually tensioning each of thescreen-translating chains, such as chain 1%, the-lower span of the chain is carried over idler sprocket 115 as shown in Figs. 12 and 15, the sprocket being rotatably supported upon stud Iii riveted to plunger element I18, the plunger'being slidably mounted for vertical adjustment within the boxlike housing I19 attached to the adjacent side wall of mill. The plunger is raised to tension chain m4 by adjusting setscrew I 80 threaded into bottom I8I of housing Ii-e, the upper end of the setscrew bearing against the lower end of the plunger. The chains I05, Iilt and lei are tensioned in the same manner, the several plungers associated therewith being assembled in housing I19. Chains H4 to H1 on opposite side of mill are tensioned in the same manner.

Timing mechanism for screens It will now be recalled that when shaft 62,Fig.

3, is rocked in a clockwise direction by tilting Sprocket I28 is rotatively mounted upon stud shaft Hi2 which is supported by near side wall of the mill. This sprocket I08 carries an inwardly projecting crankpin I 63 that tilts arm 82 up and down, the travel of the crankpinbeing about 315 back and forth on circular are shown by dot and line I34. The-crankpin I83 is shown in cross section as a result of a fragment of sprocket 1538 being deleted. While the crankpin is moving upward in counterclockwise direction, the working :screen is released from the ginning of the screen interchangeoycle. "The selected screen is released from its tilting latch while shaft I35 is turning counter-clockwise, under which condition there is no strain on the latch as the reserve screens are being urgedaupward by the slipclutches associated therewith. The crankpin I33 is limited in its travel in'ibdth directions by electrical controls appliedtoimotor 81 which willbe presently described.

The bifurcated arm82 (Fig. 1) is'fonnedwith upper and lower forks I85 and I86 respectively, these forks being spaced apart vertically in parallel relation a distance'that'is approximatelyitwo to three times the diameter of crankpin I183. :The upper fork is made shorter than-the lower (fork, and the distal end of the lowerforkis turnedxupward to provide finger I81 which serves 5355-3, positive stop against which the crankpinrcomes to rest at the end of itsclockwisemovement, this also being the end of the screen'interchangecycle. When the crankpin seats againstfinger 48:1, the bifurcated arm 82 is tiltedto its;lowermost position, and the workingscreen .is-clamped ready for service. The open space I88 'between' flnger I81 and the end of upper fork I185 provides agap through which the crankpin, when traveling counterclockwise, escapes from the confines of the two forks, the armc82 being tilted to its=uppermost position as shown by dot anddash lines. Continuing its counterclockwiseitravel;thecrarikpin engages a spring-loaded arm Ifiilrwhich aCtuates single pole double-throw limit switch EH10 secured to nearside wall of the mill housinggits arm I89 being normally positioned to maintain. motor 8'17 energized for counterclockwise:rotation by means of holding coil I9! of'Fig. I8. iHol-izling coil IBI is released 'andopposite holdingxcoil 192 is energized when arm I89 is engaged :by the crankpin, thereby reversing the motor'for'clockwise rotation, and likewise shaft :-.8.5 and the crankpin I83. The reversal ofrshaftafifidnstantly starts the previously released screen on itsrdown- 'ward journey into the mill. Holding coil l92:con-

tinues to function until the end of "the :cycle, however, arm I89 of limit switch I returns=to its normal position as soon as the crankpin has traveled a few degrees in clockwise direction. e'In its clockwise travel the crankpin re-enters gap I88 and tilts the bifurcated arm downwardthereby closing the clamping mechanism .upon :the interchanged screen. The time interval involved in the screen clamping operation:is'relativelyilong as compared with that required to release ithe screen, which, as willbe recalled, is'a sudden snap action. This sudden release of thescreen. speeds up its removal from the-mill.

With further'reference toFigs. land 18,:before crankpin I83 reaches the limitof its clockwise travel, and likewise before bifurcated arin 82*is tilted to its lowermost position, the lower edge ofthe bifurcated arm ,engages the spring-loaded arm I93 of single-poledouble-throw limit switch I 94, :which ISWitCh is :also mounted on meat-"side of millhousing. The engagement of arm 82 with switch arm I93 releases holding coil I92 and interrupts the current to motor 81. After the motor is de-energized it continues to coast until the crankpin stops against finger I81 of the bifurcated arm. During this brief coasting period the slipclutch mechanism applied to sprockets 88 to I82 and II to H3 on shaft 85 are all slipping. The solenoid associated with the screen now in the mill was simultaneously de-energized when holding coil I92 was released to stop the motor. The electrical circuits employed in controlling the motor, solenoids, and the pilot lights will be presently described in detail.

' Pushbutton controls and signal lights Referring to the wiring diagram, Fig. 18, the three-phase motor 81 is operated from line wires I95, I96 and I91, and is energized for counterclockwise rotation at the beginning of each screen interchange cycle by manually pressing any one of the four spring loaded pushbuttons I52, I99, 200.0r 20I that is not in a depressed position, these pushbuttons being associated with screens 4!], 4|, 42 and 83 respectively. Each pushbutton operates a normally open double Z-point-make selecting switch that closes two circuits simultaneously, one of these circuits lighting a signal lamp associated with the idle screen that has been selected for interchange with the working screen in the mill. The other circuit energizes the solenoid that releases the restraining latch that is associated with the selected screen. When the operator presses any one of pushbuttons I53, I59, 225 and 2!, this simultaneously closes contacts 2&2 and 203, 204 and 205, 206 and 201, or 258 and 269 respectively. The contacts 202, 2G4, 225 and 288 are in the circuits that energize pilot lamps 2H), 2! I, H2 and 2I3, which lamps are associated. with screens 40, 4|, 42 and 43 respectively. The contacts 293, 205, 201 and 209 are in the circuits that energize solenoids I5I, I52, I53 and I54 respectively, these solenoids being associated with screens 40, 4|, 42 and 43 as previously mentioned. When motor 81 is at rest the current from line wire I95 flows through single-pole double-throw limit switch I94 as a result of its spring-loaded arm I93 being engaged with bifurcated arm 82. thereby holding blade 2M against terminal 255 from which the current flows through the signal lamp associated with the screen that is in the mill, and thence the current flows to line wire I91 to complete the circuit.

Mechanically connecting the four pushbutton switches I98, i951, 280 and 20! in common is a normally open switch blade 2I6 which is hinged on terminal 2I1 so that it may be momentarily closed against terminal 2I8 when the operator presses any pushbutton associated with an idle screen. These pushbuttons are so arranged and interlocked that when the selected pushbutton is pressed, it first releases any other pushbutton that may be in a depressed position as a result of the prior screen interchange, and then the selected pushbutton locks itself mechanically in the depressed position until subsequently released by the operator when he presses one of the other pushbuttons to effect another screen interchange. The signal lamp associated with the ex-working screen is extinguished when the depressed pushbutton is released, and simultaneously the signal lamp associated with the selected screen is ener gized as a result of the operator pressing the selected pushbutton. lighted for only an instant, or until motor 31 has However, this lamp is.

turned a'few counterclockwise revolutions to raise crankpin I83 and free bifurcated arm 32, which arm now swings upward with a snap action. This upward movement of arm 82 frees arm I93 of limit switch I94, the current to the lamp being interrupted when blade ZI-t leaves terminal NB. This same lamp circuit is again energized at the end of the screen interchange cycle, as will be later described.

Motor starting and reversing controls The only manual operation required to make the screen interchange is for the operator to press the pushbutton associated with the selected idle screen that is to be put into the mill. The pressing of this pushbutton momentarily closes the switch blade 216 against terminal 2 it! and energizes holding coil I9 I, the flow of the current bein from line wire I through closed switch 226,

contacts 2L9 which are operated by heater coilsmagnetically closes the four ganged contacts 223,

22A, 225 and 226 simultaneously. The closing of contacts 223 shunts the current around switch 2H; and keeps holding coil I9I energized after switch blade 2E6 has resumed its normally open position. The closing of contacts 22%, 225 and 225 delivers current to motor 81 from line wires I25, I95 and I91, and the motor starts in a counterclockwise direction, the crankpin I83 being driven in the same direction. After the motor has turned a few revolutions and crankpin I23 has traveled upward a short distance, the bifurcated arm 82 swings upward with a snap action as p eviously explained, thereby releasing the springloaded arm I93 of limit switch I94 and swinging blade 2M to terminal 221. This completes the solenoid circuit from line wire I95 through limit switch Hi4 and the closed pushbutton contacts to line wire I91, the solenoid associated with the selected screen now being energized and the screen freed from its restraining latch. Simultaneously, when blade 2 I4 contacts terminal 221 the wire leading therefrom to holding coil I92 becomes ready to function, and holding coil I52 is energized the instant the circuit is completed through limit switch 595, as will be presently described.

After leaving the bifurcated arm 82 the crankpin continues its counterclockwise travel until it engages the spring-loaded arm I89 of limit switch I96, during which time interval the exworking screen is withdrawn from the mill. When blade 228 leaves terminal 229 in limit switch use, this de-energizes holding coil HI, and, when the blade swings over to terminal 235, this completes the circuit that energizes holding coil I92. The de-energization of holding coil I9l releases the four ganged contacts 223, 224, 225 and 225, and the motor stops with the crankpin in engagement with arm I89 on limit switch I 56. The energizing of holding coil I92 magnetically closes ganged contacts 23I, 232, 233 and 235 simultaneously. The closing of contacts 23L 232 and 233 permits the current from line wires I85, 95 and Isl to flow through the motor in reverse phase relation, thereby energizing the motor for clockwise rotation. The closing of contacts 232 shunts the current around limit switch I98 so that holding coil I52 remains energized. After crankpin I53 has traveled upward a short distance, and has accuses disengaged :springeloaded arm L182 ofilimit switch I90, the switchblade 228-returns to:its normal position in contact withterminal 229. The motor continues to rotate in clockwise direction until crankpin I33 re-engages bifurcated arm '32 and tilts it downward to re-engage arm I33 of limit switch I94, the blade 2M swinging away from terminal 2 2 I thereby simultaneously interrupting the flow of current through holding coil .492 and the solenoid which Was energized at the beginning of the screen interchange. The daenergizati'onofholdingcoil 92 permits gangedcontacts 23I, 252, 23$.and 23d toopen and :de-energizethe motor, this action taking placea brief interval of time before crankpin 83 comes to rest -against finger It! on the bifurcated arm. The motor continues to coast until .the friction of the slipping clutches onshaft .35 brings it to a standstill.

Example illustrating scr en interchange cycle Any one of the four screens that may be in the mill and clamped ready for service is subject to replacement by any one of the other three screens by simply pressing the pushbutton associated with the selected idle screen. Therefore, the electrical controls illustrated in wiring-diagram, Fig. 18, must be designed to effect a total of twelve screen-interchange combinations, all of which operate on the same basic cycle. Consequently, it will be necessary to describe in detail only one screen interchange, and for this purpose idle screen 42 has been selected for introduction into the-mill to replace working screen 353 which is now clamped in operative relation with the hammer rotative element as shown in 3. As a result of the preceding screen interchange, pushbutton I95 is in a depressed position and the entire electrical control system is at rest with the exception of signallamp 213 which is lighted-to indicate that screens-l3 is in the mill. The current for lamp ZIil flows from line wire I35 through junction 235, lead 23-3, overload protection contacts 2N, safety switch 222, lead 233 to junctioniilfi thence through lead 239 to limit switch 534 which hasits spring-loaded blade 22d closedagainst terminal 2I5. From terminal ZIE lead Ztil conducts the current to pushbutton make-contacts 282, through lamp 22B, lead 2 H, thence to junction 242 with line wire Isl, thereby completing the circuit for lamp 2m which remains lighted. as long as pushbutton $28 is depressed.

To start the screen interchange cycle, whereby working screen ili .now in the mill is replaced by selected screen 32, the operator simply presses pushbutton 22% thereby simultaneously releasing depressed pushbutton I88 and momentarily closing switch 2% against contact .2I'8. As :pushbutton 206 remains ina depressed position, the two-point make-contacts 2dr; and'2fil areclosed and remain closed until the next screen interchange. While switch ZIE is momentarily closed against terminal 2 I8 the current from line wire I35 is carried through junction 235 with lead wire 236,norma1ly closed'switches2I9 and1222, lead 23'? through switch 2H5, lead 243, holding coil I9I, lead 254 to terminal 229 of limit switch I98 against which blade 228 is normally closed, thence through lead 245 to junction 24% .to line wire J91, thereby completing the circuit between lines I95 and I9! which energizes holding :coil I S I andmagneticallycloses the-.four gangedcontacts 223,224, 225-.and 226.

ill

When contacts #23 are .;closed this :shunts out thetmomentary contactlIS, so that when switch 2I'6 is again opened, the :holding coil lei will remain energized :to maintain ganged contacts 223, 224, 225 and226 closed. As soon as these four contacts .are'closed, the three-phase current from lines I95, I96 and I97 flows through contacts 224,;225 and 226 :respectively. From contact 22 thecurrent flows through lead 24? to heater coil 220,.and then throughlead Mil-to motor 8?. From contacts .225 the lead 249 goes to the motor. From contactsJ226the lead 250. goes to heater'coil 22Iifrom which'the current goes-to motor through lead :25I. This completes the three circuits to the "motor which is now energized for counterclockwise rotation.

.It'willnow'be recalled that crankpin E83 (Fig. 1) carried lay-sprocket I03 rotates in same direction as motor 87, consequently, the cranlrpin starts traveling counterclockwise when holding coil: I9I isenergized atLthe beginning of thescreen interchange cycle, thereby tilting the bifurcated arm 82 :upwardly to release the screen-clamping mechanism that holds screen it, which screen starts its upward journey as soon as freed from the clamping mechanism. In addition, when arm is tilted upwardly it frees the springloaded arm I93 oflimit switch It? so that blade 25 i is moved to contact terminal 22?, which, through lead 252, conducts the flow'to contacts 2%? of pushbutton 2%, these contacts being mechanically maintained in closed position, as previously explained, so that the current may iiow through lead253 to solenoid I53 associated with screen 42. From solenoid I53 the current is car ried through wire 245 to junction 2 56- and line wire Iii! to complete the circuit, thereby energizing solenoid 1153 which hits the restraining latch I33, and releases screen 42 which is now free tostart its downward journey into the mill housing at'the instant'the motor is reversed for clockwise rotation. The crankpin i8 3, continuing its counterclockwise travel, engages arm we of limit switch I 99, during which time interval screen 40 is completely withdrawn'from the mill. housing and positioned so that car I643 on drawbar I28 engages thespring-urged restraining latch Engagement of the crankpin with arm its or" the switch I swings blade 223 from contact 22$ to contact-.235). When blade 228 leaves contact 22.9,the circuit through holding coil is! is broken, therebyreleasing contacts 223, 22%, 225i and 228 and disconnecting motor 8'! from its source of energy.

Withblade 228 of limit switch tell in contact with terminal 238, as mentioned in preceding paragraph, motored! is again energized, this time thedirectionof rotation being reversed or clockwise. The reversal is effected by current from line wire I flowing through branch 2%, normally closed switches '2IS and 222, lead 23ft, junction'2'35, branch 239 thence to limit switch I24 having blade 2M normally held in. contact with'terminal 22?, then through lead 252 to June tion EMand IeadZES to holding coil I32, lead 253 t0'1i1nit switch rec having blade in contact withterminallZfifi as a result of engagement of crankpin I33 with arm I33. From blade 228 the current .fiows through lead 255 to junction 2-16 with line wire 19?, thus completing the circlosesthe fourganged contacts'flt'I, 232, 233 and 234. With" contactv 234 closed, 1 the current nowing through :holdingucoil I92 vis shunted across I from wire 256 to junction 242 with line wireIISI, thereby maintaining holding coil I92 energized after blade 228 of limit switch I92 has been moved from contact with terminal 230 to engage terminal 229, which action takes place as soon as crankpin I83 disengages the spring-urged arm I89 of switch I93.

With contacts 23I, 232, 233 and 234 closed by action of holding coil I92 the three-phase current is carried from line wire I95 to junction 251, through lead 258, closed contacts 23I, lead 259 to junction with 250, thence through heater coil 22I to motor 81. From line wire I26 the current flows through lead 260, closed contacts 232, lead 23I to junction with wire 249 which is connected to the motor. From line wire It! the current flows through lead 262, closed contacts 233, lead 263 to junction with wire 24?, and then through heater coil 22!] to motor 81. Energizing these three circuits reverses the motor for rotation in a clockwise direction. Simultaneously crankpin I83 starts on its return journey in a clockwise direction and screen 42 starts its downward journey into the mill housing, it coming to rest when its drawbar I28 engages stops I34 (Fig. 3). Immediately following the arrival of screen 42 in position to be clamped for service, the crankpin, having re-entered gap I38 of the bifurcated arm, engages the lower fork I85 of the arm and starts tilting it downwardly, this action clamping screen 42 in operative relation with the hammer rotative element as previously described. Before arm 82 is tilted to its lowermost position, lower fork I85 engages arm I93 of limit switch I94 and swings blade 2I4 from terminal 221 to 2I5, thereby interrupting the current flowing through holding coil I92 and slenoid I53, and releasing contacts 23I, 232, 233 and 234 which action simultaneously stops the motor and lights signal lamp 2E2 to indicate that screen 42 is clamped ready for grinding service. The circuit for signal light 2 I2 is same as the circuit previously described for light 2W except the current flows through pushbutton contacts 206 instead of contacts 222, and light 2I2 remains lighted until screen 42 is withdrawn from the mill housing, or the circuit is interrupted by opening switch 222.

What I claim is:

1. In a hammer mill or the like including a housing, an arcuate screen which is one of a plurality of similarinterchangeable screens removable from and insertable therein, means attached to the housing for supporting the screens exteriorly thereof, an opening in the housing through which a selected screen may be translated, a mechanism for closing the opening, a mechanism for translating the selected screen through the opening, and motor means for actuating said mechanisms in timed relation, said means including an oscillatable element.

2. In a hammer mill or the like including a housing, a screen which is one of a plurality of interchangeable grading screens removable from and insertable therein, an automatic mechanism for removing any screen of the plurality that may be in the housing and inserting a selected screen of the plurality in substitution therefor, said mechanism including a combination a pair of sprocket chains associated with each screen, a sprocket for driving each chain, a motor-driven reversible shaft upon which the said sprockets are mounted for rotation. therewith or relative thereto, said sprockets also being mounted for axial movement relative to the shaft, and a 16 spring-urged frictional driving connection: be'-- tween the shaft and each sprocket.

3. The combination as cited in claim 2 wherein the maximum torque transmitted by the driving connection is varied by adjusting means carried by the shaft and rotatable therewith.

l. The combination as cited in claim 2 wherein the driving connections associated with the paired chains slip simultaneously.

5. The combination as cited in claim 2 wherein the driving connection includes a driving disk mounted on the shaft adjacentto the sprocket for positive rotation with the shaft and axial movement relative thereto, an intermediate disk mounted between the sprocket and the driving disk for rotational and axial movement relativeto the shaft, and resilient means for forcing the driving disk and sprocket axially towards each other to cause said sprocket to rotate with the shaft.

6. In a hammer mill or the like including a housing, a screen which is one of a plurality of interchangeable screens insertable therein and removable therefrom, means associated with the housing for supporting the reserve screens thereabove in an established order, an opening in the periphery of the housing into which any selected screen may be inserted, a screen inserted in the opening, and means for maintaining the inserted screen in the established order with respect to the reserve screens thereabove, the said means being the distal ends of the reserve screens hanging loosely in overlapping relation with proximal end of the inserted screen.

7. In a hammer mill or the like including a housing, an arcuate screen which is one of a plurality of interchangeable metal screens insertable therein and removable therefrom, an opening in the housing through which any selected screen may be translated, a screen inserted in the housing, and means for supporting the reserve screens in loose-leaf formation with their proximal ends in spaced apart relation and their distal ends in metal-to-metal contact, said supporting means being the housing structure.

8. In a hammer mill or the like including a housing, a screen which is one of a plurality of interchangeable screens insertable therein and removable therefrom, an opening in the 110115- ing through which any selected screen may be translated, a reversible mechanism for translating the selected screen through the opening, said mechanism being operated by a motor, a source of electrical energy to the motor, an active screen inserted in the housing and the motor at rest, manually operated electrical controls for starting the motor in a predetermined direction to remove the inserted screen, and electrical contacts associated with the screen translating mechanism for automatically reversing the motor after removal of said screen.

9. The combination as set forth in claim 8 wherein the electrical controls include a normally closed spring-loaded limit switch, and means for opening the switch to automatically de-energizing the motor after completion of the screen interchange, said means including an oscillatable element which is actuated by said motor.

10. In a hammer mill or the like including a housing, a screen which is one of a plurality of interchangeable screens insertable therein and removable therefrom, .an opening in the housing throughv which a selected screen may be translated, a reversible motor-driven mechanism for removing the active screen and inserting the selected screen, a source of energy to the motor, manually operated electrical controls for starting the motor to eiiect a screen interchange, said controls including a switch which closes two pair of contacts simultaneously, a signal lamp associated with each of the several screens, and means for energizing the lamp associated with the inserted screen while the balance or" the electrical system is at rest, said means being one pair of the closed contacts.

11. In a grinding mill or the like including a housing, a rotative hammer element which reduces material fed thereto, a screen supported in operative relation with the rotative element, said screen being one of a plurality of similar screens interchangeable therewith and positionable upon the screen supports in like operative relation, a screen-translating mechanism for automatically removing the operative screen and positioning any selected screen of the plurality in substitution therefor and in like operative relation, a rotative element for timing the operations of the screen-translating mechanism, and reversible power-driven means in common for actuating the timing element and the screentranslating mechanism.

12. The combination as cited in claim 17.. which includes torque-responsive means for establishing a driving connection between the power driven means and the screen translating mechanism, said means being a slipclutch.

13. In a hammer-grinding mill or the like in cluding a housing into which is inserted an active screen through which the eillux of reduced material is graded, said screen being one of a plurality of similar screens interchangeable therewith and positionable in substitution therefor, means for supporting the reserve screens exteriorly of the housing, a reversible screen positioning mechanism for replacing the active screen by any selected reserve screen, power-operated means for actuating the screen positioning mechanism to effect the screen interchange, a source of electrical energy, and electrical controls associated with each of the several screens which may be manually operated to select the reserve screen prior to the interchange.

14:. The combination as cited in claim 13 wherein the power operated means is an electric motor having reversing controls, said controls being actuated automatically by rotative means driven by the motor.

15. The combination as cited in claim 13 which includes a rotative timing element associated with the screen-positioning mechanism, a reversible electric motor for actuating the timing element and the positioning mechanism, a source of energy to the motor, and electrical means for controlling the direction of motor rotation, said means comprising a pair of spring-loaded singlepole double-throw limit switches which are actuated by the rotative timing element.

16. In a hammer-grinding mill or the like in cluding a housing into which is inserted a screen through which the efilux of the reduced material is graded, said screen being one of a plurality of similar screens interchangeable therewith and positionable in substitution therefor, a motordriven screen positioning mechanism for effecting an interchange of any selected reserve screen with the active screen, a source of energy to the motor, and electrical controls for the motor, said controls including a manually operated switch for selecting the reserve screen that is to interchange with the active screen, and means in common with the manually operated switches for start ing the motor, said means being a normally open switch.

17. The combination as cited in claim 16 wherein the screen-positioning mechanism includes a slip clutch and a solenoidally operated latch associated with each of the reserve screens, the said latches being adapted to hold. the reserve screens immovable while the selected screen is being interchanged with the active screen, and means for energizing the solenoid to release the selected screen, said means being the manually operated switch.

18. In a hammer-grinding mill or the like including a housing, a screen which is one of a plurality of interchangeable grading screens insertable therein and removable therefrom, an opening in the housing through which any selected screen of the plurality may be inserted, a reversible mechanism for translating the selected screen into the housing, said mechanism includ ing an oscillatable clamping element power driven means for actuating the said mechanism, a source of electrical energy, an electrically operated indicator associated with each of the several screens for visually indicating which one of the screens is inserted in the housing, said indicators being stationed remotely from the mill, an electrical circuit for each indicator, a switch in each indicator circuit, means for closing the switch associated with the selected screen, a master switch in common with all the indicator circuits, and means for operating the master switch, said means being the cscillatable clamping element.

19. In a hammer-grinding mill or the like including a housing into which is inserted an arcuate screen through which the eiiiux of reduced material is graded, said screen being one of a plurality of similar screens interchangeable therewith and positionable in substitution therefor, means carried by the housing for supporting the reserve screens thereabove in juxtaposed nested relation, an opening in the housing into which a selected screen may be inserted and removed, a reversible mechanism for removing the active screen from the housing and replacing it with any selected reserve screen in substitution therefor, and motor means for actuating the reversible mechanism.

20. In a hammer-grinding mill of the type that employs interchangeable grading screens, a housing having a grinding chamber in which is journaled a rotative hammer element for reducing material fed thereto, a compartment which receives the reduced efilux from the grinding chamber, an opening in the periphery of the grinding compartment which communicates with the efilux-receiving compartment, a screen removably positioned over the opening in overlapping relation With the border elements that define the opening, a reversible mechanism for clamping the positioned screen to the border elements and holding said screen in operative relation with the hammer element, and an electrically operated indicant remote from the mill for visually indicating when the positioned screen is clamped in operative relation with the hammer element.

21. In a hammer-grinding mill or the like including a housing into which is journaled a hammer rotative element for reducing material fed thereto, a screen inserted in the housing through which the efiiux of reduced material is graded, said screen being one of a plurality of similar reserve screens interchangeable therewith and positionable in substitution therefor, means for supporting the reserve screens, a reversible screen-positioning mechanism for replacing the operative screen by one of the reserve screens, and an electrically operated indicant remote from the mill for indicating which one of the several screens is positioned in operative relation with the hammer element.

22. In a hammer-grinding mill or the like including a housing into which is inserted a screen through which the efiiux of ground material is graded, said screen being one of a plurality of similar screens interchangeable therewith and positionable in substitution therefor, a screenpositioning mechanism for effecting the screen interchange to selectively vary the fineness of the material in the efiiux, an indicant associated with each screen which may be seen by the operator from a station elevated above the mill housing, a normally open electrical circuit for operating each indicant, a source of energy for each circuit, and means for automatically closing the circuit which is associated with the inserted screen.

23. The combination as set forth in claim 22 wherein the indicant associated with the inserted screen includes a lamp.

24. In a machine that employs a removable screen or the like, said screen being one of a plurality of similar screens interchangeable therewith and positionable in substitution therefor, the combination that includes a mechanism for translating each screen to effect an interchange of any reserve screen with the employed screen, a rotative shaft in common for actuating the screen translating mechanisms, a, permanently engaged torque-responsive element for connecting each screen translating mechanism to the shaft, said elements being mounted on the shaft for rotation therewith or relative thereto, and means supported by the machine structure for restraining movement of the screens to cause a plurality of the torque-responsive elements to slip simultaneously.

25. In a hammer mill that employs arcuate grading screens, a housing, an opening in the housing into which a screen may be inserted, a screen supported above the housing by means attached to the mill structure, a reversible mechanism for inserting the screen into the opening, means for actuating the reversible mechanism, and pivotal means for connecting the screen to the said mechanism, said pivotal means being detachable from the screen.

26. In a hammer mill, a housing, said housing having a lower arcuate opening for discharge of material, an arcuate screen, movable into a lower position to cover said opening and into a higher reserve position exteriorly of the housing, guide means for directing travel of the screen between the two positions, motor means for moving the screen between the two positions, including a pair of endless belts attached to said screen, and. means to drive the belts in unison.

27. In a hammer mill, a housing, a screen which is one of a plurality of interchangeable screens insertable therein and removable therefrom, means attached to the housing for supporting the screens exteriorly thereof, an opening in the housing into which a screen may be inserted, a mechanism for translating each screen into the opening, motor means in common for operating the translating mechanisms, a slip clutch driving connection between the motor and each translating mechanism, said clutch being adapted to transmit a limited amount of torque, a selected screen moving into the open ing, and means for holding the reserve screens immovable against the pull of their respective slip clutches, said means being a latch in engagement with each of the reserve screens.

28. The combination as cited in claim 27 wherein the latch is disengaged from the selected screen by solenoiclally operated means, the disengagement being made prior to removal of the inserted screen.

29. The combination as cited in claim 27 which includes means for releasing the latch from the selected screen to allow movement into the mill, said means being a solenoid.

30. In a hammer mill of the type that employs interchangeable grading screens, a housing, a discharge outlet in the periphery of the housing, said outlet having border elements adapted to seat a screen, a screen positioned over the outlet, motor means including an oscillatable element for clamping the positioned screen to the border elements, said means also being adapted to release the screen, and means for timing the releasing and clamping operations, said means including the oscillatable element.

31. The combination as cited in claim 30 wherein the screen-clamping mechanism includes means for spring-urging the clamped screen against the border elements of the peripheral opening, said means being compressed when screen is clamped.

32. The combination as cited in claim 30 wherein the means for actuating the screenclamping mechanism includes a torque-responsive slipclutch, said clutch being adapted to slip when the screen is clamped.

33. In a hammer mill, a housing, said housing having an arcuate outlet for discharge of material, an arcuate screen, movable into a lower position to cover the outlet and into a higher reserve position exteriorly of the housing, an opening in the housing into which the screen may be inserted, guide means exteriorly of the housing for directing the travel of the screen in an arcuate path between the two positions, said means being supported by the housing, and motor means for moving the screen between the two positions.

34. In a hammer grinding mill, a housing, said housing having a grinding compartment, an arcuate outlet in periphery of said compartment for discharge of material, said outlet having border elements adapted to seat an arcuate screen, a compartment that receives efliux from the discharge outlet, an opening in the eiilux compartment through which a screen may be inserted and withdrawn, a screen supported exteriorly of the efliux receiving compartment by means attached to the mill structure, means for inserting the screen into the efflux receiving compartment and positioning it over the discharge outlet, means for clamping the positioned screen to the border elements of the discharge outlet, means for closing the screen opening after the screen is positioned, and motor means in common for actuating the said inserting, clamping and closing means in sequence.

35. In a hammer mill, a housing into which is inserted a grading screen, said screen being one of a plurality of similar screens interchangeable therewith and positionable in substitution therefore, motor means for positioning a selected screen in substitution for the inserted screen,

21 manually operated controls for selecting a reserve screen, a source of electrical energy for the motor, electrical controls for starting the motor, and means for operating said starting controls, said means being the manually operated selecting controls.

36. In a hammer mill, a mill structure including a housing, an arcuate screen which is one of a plurality of similar interchangeable screens insertable in the housing and removable therefrom, an opening in the housing into which a screen may be inserted, a selected screen inserted in the opening and having one edge projecting therefrom, means for supporting the reserve screens exteriorly of the housing and in overlapping relation with the projecting edge of the inserted screen, said supporting means being the mill structure, a mechanism for withdrawing the inserted screen from the opening while the reserve screens are standing still, and means for supporting said mechanism, said means being attached to the mill structure.

37 In a hammer-grinding mill or the like including a housing into which is inserted a grading screen, said screen being one of a plurality of similar reserve screens interchangeable therewith and positionable in substitution therefor, a reversible screen-positioning mechanism for interchanging a selected screen with the inserted screen, motor means for actuating the screenpositioning mechanism, a source of electrical energy for the motor, and electrical controls for simultaneously selecting the reserve screen and starting the motor, said controls including a manually operated pushb-utton switch associated with each screen and a normally open switch which is momentarily closed by the selected pushbutton to energize a holding coil that starts the motor.

38. In a hammer mill or the like including a housing, an arcuate screen which is one of a plurality of similar interchangeable screens insertable in the housing and removable therefrom, means for supporting the screens exteriorly of the housing in fixed relation with each other, said means including a stationary track, an opening in housing through which a selected screen of the plurality may be translated, a mechanism including an oscillatable element carried by mill structure for translating the selected screen into the opening while the reserve screens are standing still, said mechanism being reversible for removal of the inserted screen, and stationary means attached to the mill structure for limiting travel of the outgoing screen.

39. In a hammer mill including a housing, an arcuate screen which is one of a plurality of similar interchangeable screens removable from and insertable therein, means for supporting the screens exteriorly of the housing in a fixed order with respect to each other, said means including a stationary track, an opening in the housing into which a selected screen may be inserted, a reversible mechanism for translating the selected screen into the opening while the reserve screens are standing still, said mechanism also being adapted for removing screen, means for limiting movement of the screen-translating mechanism in both directions, said means bein stationary elements associated with the mill structure, and means for limiting travel of the screen, said means being the screen-translating mechanism.

40. In a hammer mill, a housing, .a screen which is one of a plurality of interchan eable screens insertable therein, means for supporting the reserve screens exteriorly of the housing, said means being attached to the housing, an opening in housing into which a selected screen may be inserted, a reversible mechanism including an oscillatable element for inserting the selected screen, means for supporting the reversible mechanism, said means including a stationary track, guide means for directing travel of moving screen in a fixed path, said means including the track, and means for actuating the reversible mechanism while the reserve screens are standing still.

ARTHUR G. NICKLE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,076,025 I-Iazle Apr, 6, 1937 2,076,297 Gray Apr. 6, 1937 2,227,090 Hughes Dec. 31, 1940 2,258,537 Calkins Oct. 7, 1941 2,328,170 Schutte Aug. 31, 1943 2,360,892 Rench et al Oct. 24, 1944

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