US2556641A

US2556641A - Grinder for pigments and other materials

Info

Publication number: US2556641A
Application number: US28607A
Authority: US
Inventors: Harding F Bakewell
Original assignee: Individual
Current assignee: Individual
Priority date: 1948-05-22
Filing date: 1948-05-22
Publication date: 1951-06-12
Anticipated expiration: 1968-06-12

Description

June l2, 19.51 H, F, BAKEWELL 2,556,641

GRINDER FOR PIGMENTS AND OTHER MATERIALS June 12, 19451 H, F BAKEWELL 2,556,641

GRINDER FOR PIGMENTS AND OTHER MATERIALS Filed May 22, 1948 3 Sheets-Sheet 2 IN VEN TOR. Hfen/NG /BAKEWELL BY W y @nu TTOQ vs.

June 12, 1951 H. F. BAKEWELL 2,556,641

GRINDER FOR PIGMENTS AND OTHER MATERIALS Filed May 22, 194e s Vshees-snem 3 INVEN TOR.

Meo/N6 EBA/EWELL QT-ro Nes/s.

Patented June 12, T951 UNI-Taio ,STATE-s PATENT vorties '2,556,641 GRNDR FOR PIGME'S AD TIR MATERIALS Harding' F. Bakewell, Sanr4 Marino, Gali Application Mayv 22, 194,'seial` Nasa-607 .5 Claims. (-Cl. 24ll5) The present invention relates generally to grinders or mills, and is more particularly concerned with grinders' of the type which areadap'ted to the iine `grinding of pigments and other similar materials. However'it Will-be understood that the material lground in the device -consti tutes 4nolirnitation upon nity-invention, since the constructions and principles vof operation described herein may be readily adaptedfto grinders for many differentkindsof materials.

Grinders vof the type with which We are-here concerned generally comprise'a pair-f cooperating -grinding'elements, of 'which one is stationary y and the'cther rotates. rIhese elements are typicallyfcarborundum stones and so arereferred to herein-as stones', but other substances may be used for `one orboth-of the `vgrinding fir-crushing elements, I as may" be best adapted to the vmaterial being ground. In general, devices of this -character have one stone whichrotates about axed axis with respect-to the other stone-*the material being ground passing outwardly'between the two stones so that there normally exists a thin lm of the pigment or other material vbetvleenthe two stones.

However, there occur times in grinding when the supply to-the grinder is interrupted; or for some other reason-the iilin of pigment or other material between the'stones fails or becomes discontinuous. Under i the'selconditions, 'it has hap`` pened that thegrind-ing' elements come into contact :with each other and, being very abrasive,` they rapidly Ysobre eachother. At times, ythe grinding elements so deeply abrade eachother that they are entirely ruined ibefore the grinder can be stopped or the. nlm of pigment restored, so that the grinding elements must beat .least refaced andin some instances replaced.

Even if the Vfilm of pigmentor thezlike ismaintained, the two `grinding elements may tend to abradeone another. If a high spot'develops on one stone, it wears a groove intheother, with the result that thecoacting grindingsurfaceson the tvvo stones often become scored. `.This situation results from the condition that any point on one stone continuously retraces'the same. path 'on the other stone.

Another .diiculty encountered 1with known types of grinders is that of establishingproper separation or spacing between the grinding velements in order to controll the size of the ground product. In order to 'produce a vclosely sized product, it is necessary that thisfspacingbe estab-V lished With cer'tainty and within'nar'row limits, and also that it b'e'- quiekiy and easily adjustable' andcapable of being rnaintained.A Otherwise, the product maybe of other than expectedv size.

'In/conventional grinders of this general type, considerable difficulty vhas been experienced in obtaining A'proper dynamic balance of the rotor assembly. As a resiilt, the main bearings are subjected -to relatively high impact loadings `as the oilt-of-balance grinding -element is rotated at high speed arid-these lbearin'gs vfrequently .fail and need replacement. It is impractical to obtain perfect dynamic balance of `the rotating stone; and in any event Wear is frequently uneven soi'tl'iatlthestonerbeoornes unbalancedinuse.

it'beeomesa general object vo'frmy inven tion to provide a grinding device of the character described Vin which the relativelymovablegrinding element hasravmotion' such. that one particle n the moving stone does not continuouslyretrace the same path on thexstationary stone, ibut rather. follows. a variable path so'that the Acoacting surfaces are maintainediin the best possible conditionior grinding.

It iszalsona primarylobject of my inventionto provideza grinding device .of the character described in which the motion of the grinding elements aids incrusningthe pigment or other-mater-iai being Vgroundand feeding itpast theglrincb ing :surfaces in order. tov maintain a continuous filmof ypigment between the grinding surfaces.

Itisa further object of my Vinventionto provide' a grinding devioelnaving means Vfor' initially establishing and maintaining the proper.spacing betweenthe grinding elements in order that v'the size of the productc'an beclosely established and maintained.

It is'also anobjec't of my invention to provide a grinder Vin 'which the bearings are protected against `shocksresulting from rotation of an unbalanced grinding element.

'The aboveand other objects of my invention areA attained by providing a grinding device having la pair of cooperating grinding elements of which one is stationary and the' other is rotatable,andmeans -for mounting the rota-table element in-s'uch yfashion that it has a movement which may be described as being an oscillatory rotat-ionalmovement. In the preferred embodiment of ymy 'invention described herein, the two grind` ing-elementshave spherically contoured coacting surfaces, and the shaft carrying the rotating ele-A nient is resiliently or flexibly mounted at or near one end so that itsiongitudinal axis, which-isthe axis of rotation of the shaft and the stone, can be deiiected and caused to move over a predeter mined -deiinite path.A Thus, in addition tothe strict rotational movement, the stone has irnparted to it a second motion which causes the stone, when viewed from one side to appear to oscillate across the stationary stone. The resultant motion is thus a compound rotational and oscillatory motion which produces a novel radial component to the relative motion of the two elements in addition to the normal circumferential component.

The stationary stone is mounted for closely controlled limited movement toward and away from the moving stone, this motion being obtained by providing two threadedly interengaging members one of which is held against rotation and the other of which is held against movement of translation, so that rotation of the latter moves the former toward or away from the rotating grinding element. These two members are enclosed in a fluid-tight chamber so that fluid pressure can be applied to the end of the translatable member to establish a predetermined grinding pressure at the coacting grinding surfaces.

How the above objects and advantages of my invention, as well as others not specifically referred to herein, are attained will be more readily understood by reference to the following description and to the annexed drawings, in which:

Fig. 1 is a vertical median section through a grinding device constructed according to my invention;

Fig. 2 is a plan view as indicated by line 2-2 in Fig. l;

Fig. 3 is a horizontal transverse section taken on line 3-3 of Fig. 1 showing the eccentric mounting for the upper shaft bearing;

Fig. 4 is a plan view of the rotatable grinding element removed from the remainder of the device;

Fig. 5 is a fragmentary vertical transverse section similar to Fig. 1 showing a modified form of grinding elements; and

Fig. 6 is a horizontal transverse section on line 3-3 of Fig. 1 showing a planetary gear train used to obtain the eccentric motion of the rotating shaft.

Referring now to the drawings, it will be seen that the grinder is enclosed in a housing which is generally upwardly extending and is circular in cross section, or approximately so. I prefer that the rotor assembly turn about a vertical axis as shown and later described; but my invention is not necessarily so limited as an inclined or horizontally extending axis may be used if desired. This housing comprises a head section I which is pivotally mounted at II onto the next lower section I 2 which forms the enclosure for the grinding chamber I5. A discharge spout I4 is located at one side of the housing for removal of the ground products from grinding chamber I5. The bottom section I6 of the housing provides a pedestal or stand for the device and also encloses the driving mechanism, drive motor II being enclosed in section I 6a of the housing.

The grinding means is located within grinding chamber I and comprises a pair of cooperating, relatively movable grinding elements. The upper element 2li is stationary while the lower element 2I is mounted for a compound rotational motion as will be described. The two grinding elements are usually stones, either natural or articial, but may equally well be metal or other substances which are suited to the type of grinding and the kind of material being handled by the device.

. Both grinding elements 20 and 2l are annular 4 in configuration, and define an annular grinding zone at the coacting annular grinding surfaces of the two elements indicated at 23. Grinding surfaces 23 are spherically contoured, as will be explained more fully. The lower stone 2l is mounted upon back plate 22 which is preferably lightened by providing openings 22a inwardly of the position of the stone. The grinding element is held in place by disc 24 which engages a lip or projecting rim on the stone and is secured by nut 25 to the upper end of drive shaft 21.

YDrive shaft 2'1 is journaled at a point intermediate. its ends, and preferably just below the hub of plate 22, in bearing 29 to rotate about its longitudinal axis 28. This bearing may be of any suitable type but is here shown as being a ball bearing of which the outer race is held in a composite steel-and-rubber bushing indicated generally at 33. A layer of steel in this bushing is next to the outer race of bearing 29 and is bonded to a surrounding rubber layer which in turn is bonded to an outer steel shell.

Bushing 29 fits within a circular bore 3l open to the upper end of sleeve 33. Sleeve 33 is journaled at its upper end in ball bearing 34 and its lower end in a similar bearing 35 so that it is free to revolve about axis 36 with respect to intermediate housing section I2 which encloses and supports bearings 34 and 35. Bore 3| is eccentric with respect to the axis 36 of revolution of sleeve 33, as may be seen in Figs. 1 and 3, the bore being oiset to the left of axis 36 in Fig. l. Cover plate 38 is bolted to the upper end of sleeve 33 to rotate therewith, and holds bushing 30 in place in bore 3 I. Cover 33 ts over the stationary parts of housing I2 with small clearance in order to protect the bearings to a greater or less extent against the entry of material from grinding chamber I5.

At the lower end of drive shaft 27, there is located driving pulley 4I! which is rotatably mounted in two spaced bearings indicated at 4I so that the pulley turns freely within housing section I6 about axis 33. Centrally located within pulley 4G is a thrust bearing block 42 which receives the end thrust on shaft 21. The lower end of drive shaft 27 is rounded at 43 in the shape of a hemisphere with its center at 44, as shown in Fig. 1, and this lower end of the shaft, which is opposite to the end. carrying stone 2 I, is seated in thrust bearing 42. Because of the hemispherical shapes of the end of the shaft and the cooperating seat in the thrust bearing, shaft 2'I is able to rocl; slightly in the bearing and yet the vertically applied loads do not change in their point of application so that the reaction forces in the bearing do not shift substantially in position or direction. Center 44 is located on the longitudinal axis 28 of shaft 2'I and is also the center of the spherical grinding surfaces 23, permitting these surfaces to move relative to each other when stone 2l swings about center 44, without disengaging the grinding surfaces. Since center point 44 lies on both axes 28 and 3B, these two axes intersect at 44.

Drive shaft 2l is drivingly connected to pulley 43 by a flexible connection indicated generally at 45, which may be another composite steel-andrubber member that allows shaft 2l to rock slightly in seat 43 yet continues to deliver driving torque to the shaft. Connection 45 comprises an inner cylindrical metal sleeve fastened to drive shaft 2l by splines 4l and also connected to the surrounding exible or elastic portion 45a which fits within bore 46 opening at the top side of '5 drive pulley 40. Bore 46 is concentric with pulley 40 and axis 36.

Drive pulley 40 is driven by a plurality of V- belts 48 which .pass over a pulley 4.9on the end of the shaft of motor l1.

Sleeve 33 is driven from drive pulley 40| by means of a planetary gear train which is shown in both Figs. 1 and 6. This gear train comprises an internal ring gear i attached to the bottom edge of sleeve 33, and a similar internal ring gear 52 which is fastened to the walls of housing section l2 and so is stationary. Ring gears 5| and 52 are of slightly different diameters in order that one ring gear revolveswith respect to the other and so drives sleeve 33. This difference in diameters may be varied according vto the relative speeds which are desired; but the rotational speed of sleeve 33 need not be very high. Ring gear 52, which is the lower one,l is here shown as being of larger diameter. Inside the two ring gears, is a plural-ity of pinions, asv seen best inFig. 6. The upper pinions AEll mesh with ring gear 5l, while the lower Ypinions 55 mesh with the other ring gear 52. The pinion gears -are arranged in pairs of an upper and a lower gear which are drivingly connected by -a sleeve 56 journaled on a vertically extending pin 51. v.'Ihere are three such pairs of pinions, although the number may be increased or' decreased as desired, and a corresponding number of .pins 5l'. The lower ends of pins 5l are held in sockets in plate 58 which is fastened to the upper end of pulley` ed. Plate 58 also serves to close bore de and hold bushing 45 securely in place. rIfhe upper ends of pins 5l are held in position by ring 59.

It will be noted that lower pinions 55 are of larger diameter than upper pinions '5d in order properly to mesh with the larger ring gear 52.

Going now to the upper end of the grinder, it will be noted that stationary grinding element 20 is mounted upon the lower end of an annular carrier member 65 which is mounted for sliding movement within head lf of the housing. Member `(i5 is held against rotation with-in the housing by key 63 in keyway Gelso that the only movement of which the carrier is capable is one of translation toward and away from the lower grinding element 2i.. Keyway V513 is preferably parallel to axis 3S; and translation of member 65 then moves the upper grinding element also toward and away from the lower rotatable grinding element. Carrier member 65 threadedly engages at E6 sleeve 6l which surrounds and is journaled upon an inner fixed sleeve $8 which is fastened at its upper end to housing section EB. The threads at te are preferably a loose nt, having a predetermined clearance of a few thousandths of an inch, for reasons explained later. Sleeve 68 provides an inlet passage for the feed to the grinding chamber and is arranged concentrically with the upper annular 'grinding elem ment 2S'.

Sleeve 6'! is confined at both ends against axial movement on inner sleeve GS, so that the outer sleeve 6l is capable only of rotational movement. lThis rotational movement is imparted by pinion l@ which meshes with gear H which may very conveniently be made as an integral portion of the upper end of sleeve 61. Pin-ion 'le is mounted on the lower end of shaft 'l2 which is journaled in housing lita mounted on the upper end of head section le. Shaft T2 is turned by handle 'l5' at the upper end of the shaft. Thus rotation of handle 'f5 eifects corresponding rotational movement of shaft 12 and gear 'lllv which produces a smaller 61 rotational movement of sleeve 61. By virtue of the threaded interengagement at 66 between sleeve S'l and carrier G5 and key 63 engaging the wall of the housing, such rotation of sleeve 61.

produces linear movement of stationary stone 2i] and its carrier 65.

Handle 'l5 is preferably adjustable relative to shaft 12, and it is also desirable to provide some type of detent mechanism to hold the handle and shaft in selected rotational positions. To obtain adjusting movement of the handle, it is journaled at its inner end on shaft 12, as shown in Fig. 1. The handle carries on its upper side a slide 'M which can be moved longitudinally of the handle. Slide 'I4 carries at its inner end a driving dog 'Md which, when the slide is advanced inwardly toward shaft '52, engages the teeth of cog-wheel 13, in the manner shown in Fig. 2. Cog-whee1 13 is mounted on the squared upper end of shaft 'I2 so that rotation of the cog-wheel rotates the pinion shaft. Cog-wheel 'E3 is held in place by lock nut 'E8 on the end of shaft 'E2 which is covered by cover plate 19.

In order to hold handle l5 in any selected rotational position, there is provided a detent plate l@ which has a number of radially extending depressions or notches 'Eea in its upper surface. A small projection 'ld on the under side of handle 'l5 is adapted to drop into any one of 'these radial depressions lea to hold the handle frictionally-in that position.

I prefer that the depressions 15a be so spaced angularly, and the gear ratio between pinion l@ and the driven gear Il be so calculated, that the angular movement of handle 'l5 required to advance ther handle from one depression 16a to the next one results in a known and predetermined movement of grinding element 25. It is convenient if this movement of the handle moves the grinding element by either .001 or .6005 for each notch in the detent plate, and the detent plate is marked accordingly; but of course it will be understood that the plate may 'be graduated in any other convenient unit of movement, as may be desired. Depressions 'l'a may be numbered, with Zero at one end of plate 1E, or other indexing means may be used, if desired, in order to indicate the movement of stationary element 2li obtained by any given rotation of handle l5.

The upper portion of the interior of housing lli is designed to be a fluid-tight chamber, so that when fluid under pressure is admitted through inlet pipe 5l) the upper radial surface of carrier member has fluid pressure applied to it and the member acts as a piston in a cylinder. Line 8@ is preferably provided with an air chamber 9| as indicated for reasons to be mentioned later; but such chamber is optional. In order to hold this fluid pressure, members 6l and 65 may be provided with O-rin'gs'or other sealing means as indicated. Additional sealing members may be used as required to vhold adequately the applied uid pressure. It will be noted that this fluid pressure does not interfere with the rotational movement of sleeve 6l but that it effects axial movement of the translatable member 65.

The assembly of parts for mounting and adjustably positioning the upper grinding element 20 are carried inside head section l@ which is pivotally connected at il to the intermediate housing section l2. In order that housing sections lil and I2 are kept in tight engagement to close the upper end of grinding lchamber I5',

aegee-11 latch means is provided on housing section I2 at a point opposite pivot Il. This latch means comprises plug 82 having on its under side an inclined cam surface which engages a similarly inclined surface on the upper face of lug 33 on housing section I ll. Plug 82 has a Vstem 88 which is threadedly engaged by an internal thread in hand screw 85 so that stem 84 and plug 82 are advanced or retracted by manually rotating screw 85. As the plug 82 is advanced, it engages the upper surface on lug 83 and presses head section l downwardly into tight engagement with the opening into the grinding chamber within intermediate section l2.

The shape of the grinding elements is shown in Figs. 1 and 4. The upper grinding element 2i) is an annulus gripped at its upper end for support and having at its lower end a spherically contoured grinding surface. The lower grinding element 2l is also generally annular in shape and is provided at its lower end with a lip which is engaged by disc 24 to hold the stone securely against back plate 22. The upper face of this stone is also spherically contoured to establish a grinding face which coacts with the similarly shaped face of upper stone 20. As shown in both Figs. 1 and 4, I preferably provide on the lower grinding element a plurality of pockets 2m in the inner face of the annular wall of the grinding element. These pockets are open to the spherical grinding surfaces and are located to receive the material to be ground from the feed Zone at the center of the stones. The pockets taper down outwardly and crush any large pieces of pigment before feeding the pigment to the grinding zone. The pockets improve the continuity of feed of material to the grinding zone and so reduce the possibility of a discontinuity in the pigment iilm between the stones.

I prefer to place pockets Zia on the rotating element since in this way the centrifugal force of the rotating element is most effective to throw material into the pockets and feed it out of the pockets into the space between the two grinding surfaces at 23. However, as an alternate construction, I show in Fig. 5 a pair of modified grinding elements 8l and 88. 'Ihe elements 8l and 88 correspond in general shape and function to the grinding elements 20 and 2i respectively shown in Fig. 1, except that pockets have been omitted in the rotating member and there has been substituted therefor a plurality of pockets 87a in the inner cylindrical wall of the stationary grinding element. These pockets also operate to collect and feed material to be ground to the grinding Zone, but are located in the other grinding element than in the first embodiment of my invention. It is anticipated that under some circumstances this variational form may be preferred.

Bearing in mind the foregoing description of the construction of a grinding machine according to my invention, I shall now describe the operation thereof. Material to be ground is supplied to the grinder through an inlet 90 which communicates with the upper end of sleeve 68, as shown in Fig. l. Inlet duct 90 may be a part of any suitable container or conveyor duct. Since the construction of this member is no essential part of the present invention, it is not shown in detail but it will be understood that the duct is so constructed by means not shown in the drawings, that it is movable for connection and disconnection to the upper end of sleeve 68. This is done since duct 90 must be removed before head section i0 can be swung about pivot Il. The material to be ground may be fed to the grinder either as a wet slurry or in a dry granular state. In either condition, the material is fed by gravity at a rate suitable for the grinding capacity of this machine.

The initial step in using the machine is to adjust the spacing between the spherical grinding surfaces of

elements

20 and 2| to produce the desired particle size. This is accomplished by first applying sulcient iiuid pressure through conduit to the upper face of member 65-to cause the latter to move slightly downwardly relative to sleeve 6l. This takes up all the clearance between the threads at 66 so that no further relative translational movement takes place between members 65 and 6l.

Next, sleeve 61 is rotated by means of handle 'l5 and pinion 'l0 until the upper grinding element is in contact with the lower grinding element. During this time, fluid pressure is maintained on the upper side of member 65. Now then an exact spacing between the grinding surfaces is obtained by reversing the direction of movement of handle 15 to back off the upper grinding element by the desired amount.

If the handle T5 has not already been zeroed, this step is taken at the time that the grinding elements are first brought into contact. It is done by moving slide 14 radially outward to disengage dog 'Ma` from the teeth of cog-wheel 13. The handle, now free to turn about shaft l2, is brought to the zero marking on depressions '16a on the detent plate. Then slide 'I4 is advanced to re-engage cog-wheel 13. Now, rotation of handle I5 through a given arc separates

stones

20 and 2| by the amount shown by the graduations on plate '16. The slide is locked in advanced position by tightening down screw 17.

All of this time, an adequate fluid pressure has been maintained on the upper end of member 65 to take up all clearance in the threads at 66 and cause the two members E5 and 61 to move as a unit toward or away from the rotating grinding element. Now that the proper grinding spacing has been established, a higher fluid pressure may be applied to conduit 80 as may be required in order to carry out the grinding operation. If relatively large particles of the material being ground lodge in between the two grinding elements and the inter-face pressure exceeds the fluid pressure, the upper stone can move upwardly away from the rotating element against the applied fluid pressure because the air chamber at 8| makes the uid elasticf Air chamber 8| acts as a pressure regulator and can be replaced by a conventional pressure relief valve, but has the advantage that it does not bleed fluid out of the system. This upward movement of the upper stone is limited by the amount of clearance in thread 66, and is against the uid pressure above member 65 which continually tends to restore and maintain the initial spacing of the grinding elements. However, should the feed of pigment to the grinding Zone fail for any cause for a sufficient length of time to break the continuity of the film of pigment between the grinding surfaces, the two stones are kept from dropping into contact with each other by mutual engagement of the threads at 66. Thus the stones may revolve in a dry condition, that is without any pigment between them, without harmful effects since the predetermined spacing between them cannot be decreased by failure of the pigment film. This construction permits applica- 9 tion of a given grinding pressure to the grinding elements yet insures that the clearance between them cannot drop below an established minimum.

The fluid pressure and the thread clearance can be omitted if desired. With a tight thread the spacing between stones is relatively constant and grinding pressure is applied through the threads at 66 rather than by fluid pressure on carrier member 65. Spacing may be established as described. y

After the grinding spacing has been established, but beforefeedof materialto theV grinderv is commenced, the lower grinding element is 'placedin motion by. starting upmotor I1. Motor l1 drives pulley 49 which, through the agency of belts 48, drives pulley 40. Shaft 21 being drivinglyconnected to pulley 40 through the agency of splines '47 and the flexible connection 45. The shaft turns with the pulley at the same speed and rotates the lower grinding element 2l.

If bearing 29. were concentric with the axis 36 of revolutionw of drive pulley 40, then the longitudinal axisof shaft 21, which is the axis of revolution of the shaft, would remain fixed in f space and be concentric with axis 36. Under these conditions, the motion of lower grinding element 2| would be purely a rotational oneand entirely conventional in every respect. However, there has been added to this rotational motion an oscillatory motion so'tliat the actual motion of the lower grinding element is a resultant of i these two motions.

The oscillatory motion isv introduced by the fact that bore 3iis eccentric with respect to vthe axis of revolution of sleeve 33 as established by bearings 3ll-and 35, thislatter'axis beingaxis 36 and concentric with the axis of drive pulley 40. 'Ihe eccentricity may be seen in Fig. 3 at 21a, and is normally of the general magnitude of M3, but may be more or less as desired. As drive pulley 40 turns, it carries with it plate 53 and bearing pins 51. The relative movement of the bearing pins with respect to stationary ring gear 52 causes large pinions 55 to rotate about pins 51. Small pinions e rotate at the same speed as pinions 55, but having a smaller number of teeth they cause ring gear 5l to be rotated in the opposite direction to shaft 2'! and at a considerably lower speed. The absolute speed of sleeve 33 may be less than 0.1 the speed of the drive shaft so that the moving stone 2l oscillates across the stationary stone at a relatively low speed. Typically, if the drive pulley is turning at 1200 R. P. M. sleeve 33 is revolved at 100 R. P. M. or thereabouts.

Rotating sleeve 33 carries with it the eccenltrically mounted bushing and bearing 29,

causing axis 28 to move positively over a definite path. Since axis 28 passes through xed point 44, to intersect axis 325, axis 28 is normally canted or deflected slightly with respect to axis 36, as shown in Fig. l. Bearing 2s is likewise canted slightly; but, viewed endwise axially of axis 36, it moves in aacircular path lying in a plane normal to axis and substantially so with respect to axis 28. Thus at any transverse cross section, shaft 2 may be considered as having a circular movement about axis 3S in a plane generally perpendicular to the longitudinal axis of the shaft. The radius of the circle of motion at bearing 2Q equals the eccentricity Z'a referred to above.

From these considerations it is obvious that axis 28 follows a conical path with its apex at io 44j and4 viewed from the side oscillates between the

positions

28 and 28a of Fig. 1. The shaft is positively deflected from alignment with the vaxis of pulley 40 by movement of bearing 29,

this deflection being permitted by the spherical conformation of the lower end of drive shaft 2T in thrust bearing 42. This oscillating movement of shaft 2, causes lower grinding element 2| to oscillate from side to side across stone 20 with the result that no point on the moving stone continuously retraces the same path over the stationary stone.

Drive shaft '2l is mounted near both ends in bushings which comprise flexible or elastic sections. 'Ihese bushing sections are preferably made of rubber so that they act as shock absorbers to take up shock or vibrationsresulting from imperfect dynamic balance of the rotor assembly. By interposing these elastic members in the bearing assembly, the main bearings are protected against excessive wear and their life is greatly extended. A

From the foregoing it will be apparent that the fine grinding may be accompanied by more or less crushing action, or that crushing may be a major part of the action ofthe elements 20 and 2l. I refer to them as grinding elements; but do so with the understandingA that I am not limiting my invention by so doing to any specific type of grinding or crushing action between surfaces 23.

Having described a preferred embodiment of my invention and certain lmodifications thereof,

it will be understood that various changes may be made in the design and construction of a grinder of the character described without departing from the spirit and scope of my invention; and consequently I4 wish theV foregoing description to be considered as illustrative of, rather than limitative upon, the appended claims.

I claim:

1. In grinding apparatus, the combination comprising: a pair of cooperating grinding elements having spherically contoured coacting grinding surfaces, one of said elements being rotatable; a shaft rotatably mounted and carrying at one end said rotatable grinding element, the other end of said shaft being held in a resilient mounting permitting limited movement of said shaft about the center of said spherically contoured surfaces; means for moving the longitudinal axis of the shaft in a path which is circular in a plane generally perpendicular to the longitudinal axis of said shaft; means for moving the other of said grinding elements toward or away from the rotatable element to adjust the grinding clearance between the two elements, said means comprising a pair of loosely threadedly interengaging members capable of limited nonrotational axial movement relative to each other, one of which carries the stationary grinding element and is held against rotation while the other is held against translation, and means for rotating said other member to move the one member toward or away from the rotatable grinding element; and means for applying uid pressure to the translatable one of the interengaging members to establish a predetermined grinding pressure at the grinding surface.

2. In grinding apparatus including a pair of cooperating grinding elements having coacting grinding surfaces of which one element is stationary and the other element is rotatable, means for adjusting the grinding clearance between the two elements, comprising: a pair of loosely threadedly interengaging members capable of limited non-rotational axial movement relative to each other, one of which carries the stationary grinding element and is held against rotation and the other of said interengaging members is held against translation; means for rotating said other one of the interengaging members a predetermined amount to move said one member toward or away from the rotatable grinding element; and means for applying fluid pressure to the translatable one of the interengaging members to establish a predetermined grinding pressure at the grinding surfaces.

3. In grinding apparatus, the combination comprising: a pair of cooperating grinding elements having spherically contoured coacting grinding surfaces, one of said elements being rotatable; a shaft rotatably mounted and carrying at the upper end said rotatable grinding element; driving pulley means mounted to rotate about a xed axis; a concentric thrust bearing carried on the drive pulley to rotate therewith and engaging the lower end of the shaft to resist axial thrust on the shaft, the bearing permitting rocking movement of the shaft relative thereto; a resilient driving member connected to the drive pulley means and to the shaft to transmit power for rotating the shaft, the resilient member permitting limited deflection of the shaft axis from the pulley axis; and secondary drive means engaging the shaft intermediate its ends to move the rotatable grinding element in a path around the pulley axis.

4. Grinding apparatus as in claim 3 which also includes a planetary gear train drivingly interconnecting the drive pulley means and the secondary drive means.

5. Grinding apparatus as inclaim 3 in which the secondary drive means includes a rotatable sleeve; a bearing for the shaft mounted on the lsleeve eccentrically with respect to the axis of rotation of the sleeve; and a resilient driving member interposed between the last mentioned bearing and the sleeve to permit the bearing to tilt with respect to the sleeve as the bearing is moved through a path around the pulley axis.

HARDING F. BAKEWELL.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 29,033 Briggs July 3, 1860 209,792 Axe Nov. 12, 1878 464,083 Raymond Dec. 1, 1891 653,679 Kimble July 17, 1900 835,286 Menier Nov. 6, 1906 1,129,142 Torrance et al Feb. 23, 1915 1,226,275 Symons May 15, 1917 1,628,619 Sprado May 10, 1927 1,712,369 Walker May 7, 1929 1,993,900 Rumpel Mar. 12, 1935 2,121,275 Zober et at June 2l, 1938 2,147,833 Fehrenwald Feb. 21, 1939 FOREIGN PATENTS Number Country Date 1,559 Germany Dec. 13, 1877 102,631 Great Britain Dec. 21, 1916 318,392 Great Britain Sept. 5, 1929 428,742 Great Britain May 17, 1935 333,635 Great Britain Aug. 21, 1930