US2149582A - Pulverizer mill - Google Patents

Description

H. R. COLLINS PULVERIZER MILL March 7, 1939.

nFiled Oct. 2l, 1936 Ummm III

Patented Mar. 7, 1939 UNITED S'I'ATE'S yPnLvlmlznnMILL .-Harry R."Cllns, Allentown,'Pa.; "MayI-LCollins administratrx `v of said Harry R. Collins, de-

Application ctober..21, 1936, vSerial No. .106,906

19 Claims.

invention relates -to .pulverizer' .mills especially intended `for the .reduction of non-,me- .tallic vmaterials -such -as coal, limestones and cementaclinker, and in .its more specific aspects .pertains to improvements rin grinding assemblies thereforiof the typegpreferably comprisinga plurality ofannular sseries 'of `,grinding elements, such as balls, which cooperate withlgrinding surfaces, at least one of which is rotatably driven in a vhorizontal plane to actuate the balls.

.-Among fthe `objects of the Tpresent :invent-ion may be inoted the provision of ,an improved grinding'assembly for pulverizing mills, by which reduction of the-materialunder treatment is accomplished -in `a plurality of stages, the inven- -tion contemplatingfan arrangement in which the grinding assembly of -thestated character-is symmetrically disposed about .a Vertical axis which constitutes the axis of the drive shaft `for the rotating grinding surface or surfaces of ,the yassembly, lthereby to reduce twisting and/or torsienalstrains to safe limits. Preferablyalso, the grinding vassembly/is arranged asian independent unitbelow the ndriving mechanism for thedrive J shaft, with the -grinding assembly being rcapable of assembly yexteriorly of the mill casing Vfor lowering :thereinto asa unit. There -is `also-provided -a construction in which, rupon removal of the driving mechanism, vthe 'grinding vassembly vmaybe -readily lifted from the Ymill casing-for replacement vof .worn parts as is -or may be necessary.

A furtherobject of the .presenti-invention is-the provision yof a grinding assembly in which the weight of certain parts thereof is carried by a series of the `grinding aelements, thus -to apply grinding pressure thereto, and in which .the :grinding pressures maybe adjusted-to the material under treatment in a ,simple manner and vby readily .accessible means, with provision being made'for the regulationof the `degree of neness of the product to -satisfy .varying requirements.

lA :further object of theinvention ,is kthe ,provision of -an improved grinding .assembly .of ,the

l .stated character, which is self-centering, and in which vprovision is madeior .compensation of VWearoccurring `on Ythe `grinding halls and/.or the grinding surfaces with which .the latter v.cooperate, `and which is further .so designed that tramp material may pass vthrough the .mill with- `out .injury .tothe grinding-surfaces `or elements. To this end the rotatable grinding surface or ,surfaces are each resiliently supported from a rotating part of the grinding assembly .in independent manner, Mso that vstrains encountered Aby Aonemay be suitably `taken Vup without transmis- `sion to ,the other. tAmong the advantages of this .latterarrangement may be .noted enhancediiex- .ibility, .reduced cost, and lesser wear, V,all yby rea- -lson Iof the fact .that ordinary .grinding stresses,

Las well .as stresses occurring .by reason of the `existence of tramp particles in the material being pulverized, Lare 4substantially reduced by reason Aof Athe fact that each .rotary grinding surface `is capable .or substantially .so of A taking .up .through .its vunique .mounting :stresses encountered by it.

Other objects will .be .in part obvious and in part rhereinafter .pointed out inlconnection with the ,accompanying drawings illustrating Yin detail certain embodiments of the invention.

VIn these drawings, wherein like Areference .characters designate like .parts throughout ,the several views,-

Eig l is `an elevation, ,partly in section along lines `I-I, `Fig. 2, of va .pulverizing mill embodyling the improved grinding assembly, certain ,parts -being -omitted -for the .sake `of clearness;

Eig. A2 is a partial plan view o'f the grinding assembly;

Fig. V3 is y-a section vgenerally along lines 3-3 of Fig. 2; Y

vFig-fl lis Va broken-away rdetail in section of a :modied construction. y Referring 4to the Adrawings `and more particuvlarly to Figs. 1, 2 and-3, reference character Ill rdesignates generally a cylindrical `casing constituted by a lower casing section I.I provided with -a :bottom annular ange l2 which may serve as a .bed plate for theapparatusgand an upper sec-k -tion -I3. Between the casing sections II and I3 i-s arranged'a main bearing-support I4 with bolts 'or equivalent securing means I5 removably se- .curing l:the bearing suport I4 to facing flanges of .the aforesaid casing sections. The upper casing section I3 is Iprovided. with a horizontal top plate :serving -to close the top of the casing I.0 as formed by the casing sections aforesaid, and arranged on the .topplate and suitably secured.

' thereto is a vertically disposed motor I6, the

motor shaft (not shown) of .which extends into the l,chamber or compartment formed by the casing section I3 and the main bearing plate I4. rArranged in said-compartment is a speed reduction mechanism, o'f which the vcircular supporting plate .20 vof a ringvgear (not shown) is indicated, the reduction gearing being driven by the motor .shaft aforesaid. The supporting plate 2% of the ring gear issecured as by bolts 22 to a flange 'formed on rthe upper end of a main driving shaft 23, the upper portion of which is circular in cross section. Motor i6 through speed reduction gearing thus provides a drive for the main driving shaft 23.

The main bearing plate i4 is provided with a central, elongated hub 25 disposed about the circular portion of the shaft 23, there being provided suitable bearings between said hub and shaft, provision being made for lubricating said bearings, as well as the speed reduction gear as aforesaid. These elements are not shown and described in detail as they form no part of the present invention except in so far as they may enter into certain broad combinations hereinafter set forth.

The lower end of the main drive shaft 23 extends downwardly into the mill casing section Il and is of square or other angular section. The squared lower end of said driving shaft extends through central openings in upper and lower rotary grinding rings designated 30 land 3|, respectively, and through upper and lower cage rings 32 and 33, respectively, the openings in said grinding and cage rings being of the same cross section as the lower end of the driving shaft, but beingr slightly larger whereby the shaft may be withdrawn from the rings and to permit the rings free vertical movement as well as some horizontal movement, thus to maintain alignment and to accommodate for possible inaccuracies in manufacture or normal wear of the grinding elements. It will'be understood that the squared end of the driving shaft 23 provides a driving connection between said shaft and the grinding and cage rings as aforesaid.

Between the upper and lower grinding rings 33 and 3| is an intermediate fixed ring 35 supported on an inwardly extending shelf or ledge 36 formed on the inner wall of the casing section I I. On the upper face of the xed ring 35 is arranged a circumferentially channeled raceway 38, suitably secured to the fixed ring, for an upper annular series of grinding balls 40, on the upper portions of which engage a circumferentially channeled raceway 4l secured to the under face of the rim portion of the upper rotating ring 3B. By this arrangement the weight of the upper ring 3D and of the grinding balls 40 is carried by the fixed ring 35, with the grinding balls 4D and the grinding surfaces provided by the raceways 38 and 4I thus forming an upper grinding stage.

On the under face of the xed ring 35 is suitably secured a circumferentially channeled raceway 43 against which engages a lower annular series of grinding balls 44 carried by a circumferentially channeled raceway 45 secured to the upper surface of the rim portion of the lower rotating ring 3l. The said grinding balls 44, in conjunction with the raceways 43 and 45, constitute the lower grinding stage of the grinding assembly. Y

Referring now to Fig. 3, upper and lower cage rings 32 and 33 are secured to each other by a plurality of through bolts 5l), the lower ends of which are preferably headed, the upper ends each being suitably threaded to receive a spacing nut 5i and a locking nut 52, between which latter the upper cage ring 32 is disposed. Said through bolts pass through suitable apertures 53, 54, provided in the rotating grinding rings 3Q, 3|, respectively, the bolts passing freely through said apertures and being devoid of connection with said grinding rings. By the aforesaid arrangement, cage rings 32, 33 and through bolts 50 form a cage which is driven by the shaft 23 to rotate with the grinding rings 3U, 3l, the spacing between the cage rings being relatively fixed by suitable manipulation of the spacing and lock nuts 5|, 52, and the cage being positioned vertically by engagement of cage ring 32 with a shoulder 55 formed on driving shaft 23 intermediate its upper circular and lower squared portions, as will be described hereinafter.

Grinding pressure in excess of that afforded by the weight of the upper grinding ring 30 acting on the grinding balls 40 may be supplemented by a series of compression springs 60 (one being shown in Fig. 1), which react between the under face of the cage ring 32 and the top surface of the grinding ring 30. The said springs 60 are seated in upwardly opening, spaced spring pockets 6| in the upper grinding ring 30, their upper ends bearing against spring heads 62 with which cooperate compression bolts 64 extending through the top cage ring 32. By adjusting the bolts 64 to vary the compression of springs 60, grinding pressures may be varied. Lock nuts 63 are provided whereby compression of springs 60 may be set at a predetermined pressure.

The lower cage ring 33 is disposed below the hub and intermediate portion of the lower grinding ring 3l and is preferably provided with a horizontal rim portion, between the upper face of which and the under surface of the lower grinding ring 30 is arranged a plurality of compression springs 65 seating in ring pockets 66 and providingV a resilient support-for the lower grinding ring. The compression of the springs 65 may be adjusted by means of compression bolts 61 and associated lock nuts 68.

By the above described arrangement, compression springs 60 exert a resilient, downward pressure on the grinding balls 40 of the upper grinding stage, and compression springs 65 exert an upward grinding pressure on the grinding balls 44 of Vthe lower grinding stage. Preferably, springs 60 and 65 are arranged coaxially and spaced uniformly about the axis of the drive shaft 23 on the same circle on which through bolts are arranged, the sets of upper and lower springs alternating with the through bolts, as generally indicated in Fig. 2. While in the construction illustrated four sets of compression springs spaced at an angle of 90 and alternating with through bolts in the manner specified are contemplated, the number of the springs and/or bolts and their relative arrangement may vary in accordance with design requirements.

It will further be noted that the circumferential channels of the raceways 38, 4l, and 43, 45, have substantially the same curvature as the grinding balls 40, 44, and that the several grinding surfaces and the ballsare vertically aligned. Accordingly grinding pressures are at all times exerted in a vertical direction.

In order to direct appropriately the ow of material through the mill, the upper grinding ring 30 is provided with a vertically disposed flange 10, to which is bolted or otherwise secured aV circular upwardly extending dam 1|. Secured to the peripheral edge of the ring 30 is an outwardly and downwardly inclined bafe 'I2 which terminates short of a reversely inclined baffle 13 secured, for example, to the peripheral edge of the lower raceway 33 of the upper grinding stage. Between the baiiies 12 and 13 is a gap of suitable dimension as to pass material to the relatively outer side of the grinding balls 4U of the 'charge ofthe pulverized material until it has upper stage. The inner peripheral edge of the lower raceway 38 nof the upper grinding stage may carry an inclined dam 14 over which material reduced in the upper grinding stage passes and drops downwardly between the inner peripheral edge of the xed ring 35 and the intermediate, inclined upper surface of the lower rotating ring 3 l. Carried on the peripheral edge of the lower ring 3l and outwardly of the grinding balls of the lower stage is an adjustable height dam 15 carrying along its top edge a screen 16.

By the aforesaid arrangement, material spouted into the mill casingV through the inlet opening 11 thereof is directed on to the upper rim surface of the rotating grinding ring 30.

Centrifugal force, due Vto rotation of said ring, thereupon causes such material to be thrown outwardly whereupon it drops on the baille 12 and is thence directed to the outer side of the grinding balls 40 of the upper stage by the lower baille 13, the flow of material being inwardly through the .annular series of grinding balls l. The inner dam 'I4 prevents discharge of material from the upper stage until it has been suitably reduced whereupon material flows over the edge of the dam 14 and is directed to the inner side of the grinding balls of the lower grinding stageythe f flowv of material being outwardly through the annular series of grinding balls 44.

The adjustable heightdam 15- controls disbeen reduced to the desired degree of neness whereupon the product of final size passes through the screen 16 and, falling downwardly, is picked up by air entering a forwarding fan (not shown) through air inlet openings 18 formed inthe lower casing section I I.

' A grinding assembly as described in the foregoing constitutes a unitary assembly which may be assembled outside of the mill casing on an assembling shaft which is a counterpart of the squared lower end of driven shaft 23 and thereupon lowered into the casing as a unit. The assembly is supportedwithin the casing by the ledge or shelf 36 ooacting with the fixed grinding ring35, bolts 8B being provided to maintain the xed ring 35 seated on the shelf ring 36;

Upon withdrawal of the driving mechanism, including the main driving shaft 23, the grinding assembly as aforesaid may be removed as a unit from the casing, in which event holding bolts may be -unthreaded and raising bolts 8| may be manipulated to'disturb the seating relationV of the fixed ring 35 whereupon the grinding assembly may be lifted from the casing.

The provision as aforesaid of upper compression springs 60 acting on the upper rotating grinding ring and of lower compression springs 65 acting on the lower rotating grinding ring permits independent adjustment of the grinding pressures in the upper and llower stages and at the same time permits each of the rotating grinding rings to take up stresses which it individually encounters without transmitting such stresses relatively to the other ring. Accordingly, relatively light compression springs may be utilized and there results an economy in manufacture and a greater flexibility in use.

In the construction so far described, the cage structure formed by the rings`32, 33 and through bolts 50 is rotatably driven with the-grinding rings 3l), 3l, from the drive shaft 23, and while the cage is capable of vertical movement, it is limited in its upward movement by engagement of the upper ring 32 against the drive shaft shoulder 55. The parts are so proportioned and arranged and the strength and compression of springs 60 with respect to springs 65 are so selected that in normal operation cage ring 32 always bears against shoulder 55, with the result -5 that springs 60 and 65 have in effect fixed points of reaction. l

In Fig. 4, wherein is illustrated a modified construction, the lower grinding ring 3IA is resiliently supported from a spring abutment ring 33A which in turn is carried by 4a collar 33B threaded on to the circular threaded end of the driving shaft 23A, and locked thereto as by cap screws 33C which are preferably wired for safety.

Compression springs 65A react against the abut- 15 ment ring 33A and exert adjustable grinding pressure on the lower grinding balls in the manner described. An upper abutment ring generally'similar to the top cage ring 32, and the upper rotary grinding ring as described in rela- :T240

tion toFigs. 1-3, are provided. However, in the modification described, the through bolts 50 previously described are eliminated, temporary assembly bolts (not shown) which extend betweenv top and bottom spring .abutment rings being uti- Tg5 lized in their place during assembly and placement of the grinding assembly in the casing. Upon connection of the collar 33B to shaft 23A as stated, such temporary holding bolts may bev removed, and the related sets of upper andlower -f30 lower collar 33B is disconnected from the drivi235 ing shaft, access being had through the lower end of the casing il); Upon removal of the driving mechanism including the driving shaft as aforesaid, the modified grinding assembly may be bodily lifted from the casing for repair or 9&0

replacement of its parts.

As many changes could be made in carrying out the above constructions without departing from the scope of the invention, it is intended .that all matter contained in the above descripl"45 tion, or shown inthe accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. In a pulverizer mill, the combination of up- `5 0 per and lower rotating grinding rings, an intermediate fixed grinding ring, upper and lower annular series of grinding elements disposed between the upper and intermediate rings and be- .Y tween the intermediate and lower rings, the up-"'55 per and lcwergrinding rings being each provided with a central shaft opening, a driving shaft extending through saidopenings and operative to drive the rotating rings, spring means for exerting yielding downward pressure on the grindingq 60 elements of the upper series, spring means for exerting yielding upward pressure on the grinding elements of the lower series, each said spring means including a plurality of springs and a., member against which said springs react, and a65 drive connection between the driving shaft and said members, whereby both said spring means rotate with the rotating grinding rings.

2. In a pulverizer mill, the combination of up? per and lower rotating grinding rings, an inter-TO mediate fixed grinding ring, upper and lower annular series of grinding elements disposed between the upper and intermedaterings and between the intermediate and lower rings, the upper and lower grinding rings being each provided "`7`5 with a central shaft opening, a driving shaft extending through said openings and operative to drive the rotating rings, spring means for exerting yielding downward pressure on the grinding elements of the upper series, spring means for exerting yielding upward pressure on the grinding elements of the lower series, each said spring means including a plurality of springs, a member against which said springsreact, and means carried by said member for adjusting said springs to the desired pressure, and a drive connection between the drive shaft and said members, wherebyY both said spring means are rotatable with the rotating grinding rings.

3. In a pulverizer mill, the combination of upper and lower rotating grinding rings, an intermediate i'lxed grinding ring, upper and lower annular series of grinding elements disposed between the upper and intermediate rings and between the intermediate and lower rings, the upper and lower grinding rings'being each provided with a central shaft opening, a spring reacting ring arranged above said upper grinding ring, a spring reacting ring arranged below the lower` grinding ring, independently .operable sets of springs disposed between said spring rings and the lupper and lower grinding rings respectively, a driving shaft extending through the central openings Aof the rotating grinding rings, and means providing a driving connection between the drive shaft and the spring rings.

4. In a pulverizer mill, the combination of upper and lower rotating grinding rings, an intermediate xed grinding ring, `upper and lower annular series of grinding elements disposed between the upper and intermediate rings and between the intermediate and lower rings,`the upper and lower grinding rings being each provided with a central shaft opening, a springreacting ring arranged above said upper grinding ring, a spring reacting ring arranged below the lower grinding ring, said spring rings each having a central shaft opening, a` driving shaft extending through said openings and operative to drive the rotating'grinding rings and the spring rings, and independently operable sets of springs arranged between the spring Vrings and the upper and lower grinding rings respectively.

5. In a pulverizer mill, the combination of upper and lower rotating grinding rings, an intermediate ixed grinding ring, upper and lower annular series of grinding elements disposed between the upper and intermediate rings and between the intermediate and lower rings, the upper and lower grinding rings being each provided with a central shaft opening, a rotating cagestructure supported from the grinding elements of the upper series inoperative relation to said rotating grinding rings, means carried byksaid cage structure for supporting the lower grindingY ring and for exerting independent Vyielding pressure on the upper and lower grinding rings, and a driving shaft extending through the central openings of the grinding rings and having a driving connection with said cage structure.

6. In a pulverizer mill, the combination of upper and lower rotating grinding` rings, an intermediate Xed grinding ring, upper and lower annular series of grinding elements disposed between the upper and intermediate rings and between the intermediate and lower rings, the upper and lower grinding rings being each provided withl a central shaft opening, an upper cage ring operatively related to said upper rotating grinding ring, a lower cage ring supported from the upper cagering in operative relation to the lower rotating grinding ring, the cage rings having central shaft openings, a driving shaft extending through the grinding and cage ring openings, and means operative between the upper and lower cage rings and the upper and lower rotating grinding rings, respectively, for exerting resilient pressure on said rotating grinding rings.

7. In a pulverizer mill, the combination of upper and lower rotating grinding rings, an intermediate xed grinding ring, upper and lower annular series of grinding elements disposed betweenv the upper and intermediate rings and between the intermediate and lower rings, the upper and lower grinding rings being each provided with a central shaft opening, spaced cage rings, the upper of which is operatively related to the upper rotating grinding ring and the lower is supported from .the upper cage ring in operative relation to the lower rotating grinding ring, the

cage rings having central shaft openings, a driving shaft extending, through the grinding and cageV ring openings, and means operative between t'he upper and lower cagerrings and the upper and lower rotating grinding rings, respec-,l tively, for exerting yielding pressure on the ro` tating grinding rings.

8.1In a Vpulverizer mill, Vthe combination of upper and Ylower rotating grinding rings, an intermediate fixed grinding ring, upper and lower annular series of grinding elements disposed between the upper and intermediate rings and Vbetween the intermediate and lower rings, the upper and lower grinding rings being each provided with a central shaft opening, upper and lower cage rings operatively related to the upper and lower rotating grinding rings, respectively, and being each provided with a central shaft opening, means securing said cage rings in spaced relation, aplurality of springs operative between the upper cage ring and the upper grinding ring, a plurality of springs operative between the lower cage ring and the lower grinding ring, and a driving shaft extending through the shaft openings of the rotating grinding and cage rings.

,9. In a pulverizer mill, the combination of i upper and lower rotating grinding rings, an intermediate fixed grinding ring, upper and lower annular series of grinding elements disposed between the upper and intermediate rings and between the intermediate and lower rings, the upper Y*and lower grindingV rings being each provided Awith a central shaft opening, upper and lower cage rings operatively related to the upper and lower rotating grinding rings, respectively, and

being each provided with a central shaft open" ing, Vthrough bolts extending between said cage rings andV operative to space the same, a plurality of springs operative between the upper cage ring and the upper rotating grinding ring, a plurality of springs operative between the lower cage ring and the lower rotating grinding ring, and a driving shaft extending through the shaft openings of the rotating grinding and cage rings,

whereby the cage rings and the springs are rotatedV with the rotating grinding rings.

10. In a pulverizer mill, the combination of upper and lower rotating grinding rings, an intermediate fixed grinding ring, upper and lower annular series of grinding elements disposed between the upper and intermediate rings and between the intermediate and lower rings, the upper and lower grinding rings being each provided with a central shaft opening, an upper cage ring operatively related to the upper grinding ring,

amata a` lower cage ring operatively related to.- the. lower grindingringthe cage rings being each. provided with a central shaft opening, throughbolts extending betweenY said; cage rings` andz through the upper and lower grinding rings` forspacing the cage` rings, upper compression springsA operativeV between. the upper-cage `and grindingv rings, lowerA compression springs operative between thelower grinding and cage rings, and a driving shaft extending through, the shaft openings of the rotating grinding and cage rings, whereby the cage rings and the springs are rotated with the rotating grinding rings.

11. In a pulverizer mill, the combination of upper and lower rotating grinding rings, an intermediate fixed grinding ring, upper and lower annular series of grinding elements disposed between the upper and intermediate rings and between the intermediate and lower rings, the upper and lower grinding rings being each provided with a central shaft opening, an upper cage ring operatively related to the upper grinding ring, a lower cage ring operatively related to the lower grinding ring, the cage rings being each provided with a central shaft opening, through bolts eX- tending between said cage rings and through the upper and lower grinding rings for spacing the cage rings, upper compression springs operative between the upper cage and grinding rings, lower compression springs operative between the lower grinding and cage rings, a driving shaft extending through the shaft openings of. the rotating grinding and cage rings, whereby the cage rings and the springs are rotated with the rotating grinding rings, and means for adjusting the compression of the upper and lower springs independently of each other.

12. In a pulverizer mill, the combination of upper and lower rotating grinding rings, an intermediate fixed grinding ring, upper and lower annular series of grinding elements between the upper and intermediate rings and between the intermediate and lower rings, a driving shaft, a driving connection between the shaft and the rotating grinding rings, the upper rotating ring resting on the upper annular series of grinding elements, a suppo-rting ring member effectively disposed below the lower rotating grinding ring, a driving connection between the driving shaft and the supporting ring member, and means supporting the lower grinding ring from the supporting ring member.

13. In a pulverizer mill, the combination of upper and lower rotating grinding rings, an intermediate fixed grinding ring, upper and lower annular series of grinding elements between the upper and intermediate rings and between the intermediate and lower rings, a driving shaft, a driving connection between the shaft and the rotating grinding rings, the upper rotating ring resting on the upper annular series of grinding elements, a supporting ring member effectively disposed below the lower rotating grinding ring, a driving connection between the driving shaft and the supporting ring member, and means supporting the lower grinding ring from the supporting ring member, said means including springs for exerting yielding pressure on the lower grinding ring.

14. In a pulverizer mill, the combination of upper and lower rotating grinding rings, an intermediate fixed grinding ring, upper and lower annular series of grinding elements between the upper and intermediate rings and between the intermediate and lower rings, a driving shaft, a

d riying connection between the shaft andl the rotating grinding rings, the upper rotating ring resting.4 on. theupper annular series of grinding elements, a, supporting ring member effectively disposed below the lower rotating grinding ring, a driving connection between the driving shaft and, the supporting ring member, and a pluralit;T of, springs seating on. the supporting ring membei; and operative therebetween and the lower grinding ring.. y

l5. In a Ypulverizer mill, the combination of upper and lower rotating grinding rings, an intermediate fixed grinding ring, upper and lower annular series of grinding elements between the Lipper and intermediate rings and between the intermediate and lower rings, a driving shaft, a driving connection between the shaft and the rotating grinding rings, the upper rotating ring resting on the upper annular series of grinding elements, means rotating with the upper grinding ring for exerting yielding pressure thereon, a collar carried by the driving shaft and rotating therewith, and means yieldingly supporting the lower driving ring from said collar.

16. In a pulverizer mill, the combination of upper and lower rotating grinding rings, an intermediate fixed grinding ring, upper and lower annular series of grinding elements between the upper and intermediate rings and between the intermediate and lower rings, a driving shaft, a driving connection between the shaft and the rotating grinding rings, the upper rotating ring resting on the upper annular series of grinding elements, means rotating with the upper grinding ring for exerting yielding pressure thereon, a supporting collar carried by the driving shaft and disposed generally below the lower grinding ring, and a plurality of springs carried by the collar and operative between the collar and the lower grinding ring to exert yielding pressure on the lower grinding ring.

17. In a pulverizer mill, the combination of a casing, a fixed grinding ring carried thereby, an upper annular series of grinding elements carried by the fixed grinding ring, an upper rotating grinding ring carried by said elements, a rotating ring element arranged above the rotating grinding ring driving means, means including said driving means for limiting upward movement, of said ring element, means including said ring element for exerting yielding pressure on the upper ring, a lower rotating grinding ring, a lower annular series of grinding elements carried thereby and engaging the fixed grinding ring, and means carried by said driving means for supporting the lower grinding ring.

18. In a pulverizer mill, the combination of a casing, a fixed grinding ring carried thereby, an upper annular series of grinding elements carried by the fixed grinding ring, an upper rotating grinding ring carried by said elementsa rotating ring element arranged above the rotating grinding ring, springs arranged between said ring element and the upper grinding ring operative to exert yielding pressure on the upper ring, a lower grinding ring, an annular series of rotating grinding elements carried thereby and engaging the fixed grinding ring, the upper and lower grinding rings and the ring element being provided with a shaft opening, a driving shaft extending through said opening, means on the shaft limiting upward movement of the ring element, and means carried by said driving shaft for supporting the lower grinding ring.

19. In a pulverizer mill, the combination of up- 10 rotating grinding rings, means carried by said cage structure for supporting the lower grinding ring and for exerting independent yielding pressure on the upper and lower grinding rings, a driving shaft extending through the central openings of the rotating grinding rings and having a driving connection with said cage structure, and means for positioning vertically the cage structure with reference to the driving shaft.`

HARRY R. COLLINS.

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