US2798675A - Compound movement rotary chamber mills - Google Patents

Description

July 9, 1957 COMPOUND MOVEMENT ROTARY CHAMBER MILLS F. LIMB Filed Dec. 8, 1953 BQW.

2,798,675 COMPOUND MOVEMENT ROTARY CHAMBER MILLS Frank Limb, Scarcroft, near Leeds, England Application December 8, 1953, Serial No. 396,928

Claims priority, application Great Britain December 11, 1952 s cums. (Cl. 241-175 This invention relates to rotary chamber mills, such as ball or like mills, for grinding or similarly treating material, for example for dispersing one material in another, such as a pigment in a paintvehicle.

By the term mill is meant an apparatus in which material is acted upon by coacting surfaces, in the present case the surface of the chamber and the surfacesof balls or other grinding media in the chamber, and apparatus such as a tumbling or mixing machine, in which material is merely agitated in a drum or other chamber, is not included.

I have found that the efiiciency of any mill, in terms of the time taken to achieve a given result, such as reduction of particle size to a given fineness, depends upon the total contact area of the surfaces at which the milling action takes place, the rate of relative movement of the surfaces and the pressure between them. In the case of a rotary chamber mill containing a charge consisting of the. material to be treated and grinding media such as balls, the surfaces at which milling action takes place comprise the charge-contacting surface or surfaces of the mill chamber, the surfaces of any balls or other grinding media and the surfaces of any solids in the treated material in the charge.

Consequently it is desirable in a ball mill, or like rotary chamber mill, to cause the charge to sweep as large an area as possible of the chamber surface, to effect such sweep at as high a speed and contact pressure as are commensurate with other factors, such as generation of heat, wear and economy of working,1and to maintain a high rate of internal movement, that is to say relative movement of the components in the mass of the charge.

It has already been proposed to provide a ball mill comprising a spherical chamber mounted and driven about two axes of rotation, one vertical and the other horizontal, intersecting perpendicularly at the centre of the chamber. In such a mill, the swept area may be relatively large and the mill can be operated at a high speed without encountering a critical speed at which the charge moves bodily with the chamber, which is a limiting factor in single-axis mills. However, with axes of rotation intersecting perpendicularly, the charge components at the polar areas of the chamber on the two axes are liable to become relatively static and a considerable proportion of milling action is there lost.

There have been other proposals for mills having chambers with other than a simple rotary motion so as to promote relative movement of and within the charge at high speeds. For example eflicient results can be obtained with high speed planetary motion of vertical-axis mill chambers. Planetary rnotion mills however have limitations such as the relatively small capacity of the chambers compared with the overall size of the whole apparatus.

The object of the present invention is to provide a rotary chamber mill having a capacity commensurate with its overall size, a large area swept at a pressure which may be as high as required and a high rate of relative movement of and within the charge.

In the present invention, a rotary chamber mill has a spherical, or similarly continuously curved, closed chamber surface, the chamber is arranged to be driven about one axis of rotation relatively to a chamber-carrying member which is itself arranged to be driven simultaneously about a second axis of rotation relatively to a fixed support and, according to the invention, the two axes of rotation cross obliquely within the chamber.

In a mill as provided by the present invention, the chamber is given a gyratory motion, causing the charge to sweep the chamber surface continuously under a centrifugal force which is not constant, and the charge has no relatively static polar areas. 7

In the preferred arrangement, the two axes intersect at the centre of the chamber and, in the preferred construction, the mill chamber is a sphere mounted to rotate about an axis inclined to the vertical in ,a carrier, for example a ring itself mounted to rotate about a vertical axis in a stationary support. Conveniently the chamber is mounted by trunnion journals and one journal is geared to the frame about the ring axis of rotation so that rotation of the ring causes simultaneous rotation of the chamber relatively to the ring. V n

A mill according tothe invention is illustrated, somewhat diagrammatically by way of example, on the accompanying drawing, in which: i n

Fig. 1 is a side elevation, partly in section, and

Fig. 2 is a plan. V i

The mill as shown comprises a base 1 and a stationary frame 2, bothmade of flanged metal bars braced and welded together, in which a ring 3 mounted by trun' nion journals 4 to rotate about avertical axis A-B in a lower step bearing 5 and an upper bearing 6. The axis A -B need not be vertical. As can be seen in Fig. 2, the ring 3 is a double ring with cross-members 3 The lower bearing 5 of the ring 3 is housed in a stationary bevel-gear crown wheel 7 fixed to the base 1. Meshing with the crown wheel '7 is a pinion 8 fast on the lower trunnion journal 9 of a spherical mill chamber 10 mounted to rotate about an axis I- D, at 45 to the axis AB, in the ring 3 by that lower journal 9 and an upper trunnion journal 11 carried by bearings 12 and 13 respectively in cross-members 3 of the ring 3. The upper journal of the ring 3 carries fast a pulley 14 for a driving belt from a motor (not shown). It will be apparent that rotation of the pulley 14 will rotate the ring 3 about the vertical axis AB and that the consequent driving of the pinion 8 around the stationary crown wheel 7 will rotate the mill'chamber 10 relatively to the ring 3 about the inclined axis C D.

The mill chamber trunnion journals are hollow and are closed respectively by a plug 15 in the upper journal 11 through which the chamber 10 can be charged, with material to be treated and any required grinding media, and by a plug 16 in the lower journal 9 through which the chamber can be discharged.

It will be appreciated that since the present invention resides in the geometrical form and rotational arrange ment of a mill chamber, the accompanying drawing is somewhat diagrammatic and' does not show constructional detail, such as the actual mill chamber construction and lining, andthe charging and discharging arrangements, all of which can follow standard mill practice according to the material to be treated.

When a spherical mill chamber as shown, containing a charge, for example of material such as paint and grinding media such as balls or pebbles, is driven about the obliquely intersecting axes AB and CD, relative ii atented July 9, 1957 movement takes place continuously, between the charge as a whole and the surface of the chamber and between the components of the charge whatever the speeds of IO- tation.

This relative movement can be eifected under high pressure, by driving the mill appropriately at high speed, and thus is produced in the charge a shear eifect which is desirable in milling highly viscous materials.

For any given size and shape of mill chamber, the rates and mode of relative movement of the charge as a whole and of the particles in the charge depend mainly upon the obliquity of the angle between the two axes and the speeds and relative directions of the rotations about the axes. The movement is also afiected by the nature of the charge material as regards its density, particle size and liability to pack and by the viscosity and adhesive character of any liquid component of the charge.

Stroboscopic observation of a transparent mill chamber having an internal diameter of 12 inches, arranged as shown on the drawing and driven at a speed of between 200 and 300 R. P. M. with a 1:1 ratio between the gears 7 and 8, reveals that a charge, for example of paint and pebbles, assumes an envelope shape which is an annulus, like an equatorial belt, the axis of which lies between the two axes of rotation, as indicated on the drawing.

It has also been determined that with an oblique angle between the axes of rotation, for each revolution of the chamber the amplitude of movement of the chamber surface relatively to the charge as a whole is twice the angle between the axes. A further observed effect, to which great importance is attached, is that the radial thickness of .the belt shape which the charge assumes has a cyclical variation. This is presumably a consequence of the fact that since the components of the charge have no constant radius of resultant movement, the rotational axis CD of the chamber having a locus represented by two coaxial cones point to point, the components are subject to a cyclically varying centrifugal force. This produces an alternating increase and relaxation of pressure between the charge components equivalent to a rhythmical squeezing of the charge. Such a rhythmical squeezing accompanied by the relative movement producing shear is an ideal type of action to elfect grinding and dispersing.

It will be apparent that many variations may be made in the absolute and relative speeds about the two axes of rotation producing diiferent modes of movement of and within the charge. An analysis of the possible variations will not be attempted but attention is drawn to the fact that the direction of rotation about the two axes need not be the same as is produced by the direct engagement of the gears '7 and 8 as illustrated.

By interposing an idler pinion between the gears 7 and 8, or by providing any other suitable train of gears or a variable gear, the direction of rotation about the axis A-B can be opposed to the direction of rotation about the axis CD and any desired ratio of speeds can be obtained. This enables special effects to be obtained. For example, with a small included angle and opposite rotation about the two axes, the movement of the chamber surface relatively to the charge as a whole will have a frequency equivalent to the speed of rotation about the ring axis AB, but the rotational speed of the charge will depend upon the difference between the speeds about the two axes. The smaller this difference, whatever the individual speeds of rotation are, the smaller will be the centrifugal thrust. This makes it possible to have a high frequency of movement with a low centrifugal thrust and this is an important advantage in milling some materials with which high contact pressure should be avoided.

There are obvious advantages in the use of a spherical mill chamber, such as maximum capacity and mechanical balance, but another shape of closed chamber with a continuously curved surface, such as an oblate spheroid, so that there is no abrupt change in the path of the charge sweeping the chamber surface, could be used.

Also there are advantages, from the aspects of balance and simplicity of the driving arrangement, in arranging the two axes of rotation of the mill to intersect at the centre of the chamber but the advantages of continuous relative movement of and within a charge over a large chamber area swept at optimum pressure could be achieved with rotational axes crossing obliquely but not intersecting. For instance the two axes could cross by extending relatively obliquely in parallel planes passing through the chamber. i

It will be apparent to those familiar with ball mill practice that the present mill is suitable for any use in the same way as a ball mill. For instance the mill chamber could be jacketted for cooling, using the trunnions for circulatingv coolant, and the hollow trunnions could be used forfeeding material in and out for continuous operation. Viscous material could be discharged centrifugally during running of the mill and collected for example in an annular trough surrounding the locus of the discharge outlet. For material requiring impacting, lifter bars could be provided in the mill chamber to interrupt the otherwise smooth sweep of the charge.

I claim:

1. A rotary chamber mill comprising a stationary sup port, a mill chamber carrier rotatably mounted in said support about a vertical axis, a closed mill chamber, having the form of a unitary sphere with a continuous spherical inner surface, rotatably mounted in said carrier about an inclined axis intersecting said vertical axis at the centre of said sphere, and means for driving said carrier and said chamber simultaneously about their respective axes of rotation, said means comprising a stationary gear Wheel, fixed to said support coaxially about said vertical axis, and a pinion fast with said chamber coaxially with said inclined axis and geared to said gear wheel so that rotation of said carrier about said vertical axis causes simultaneous rotation of said chamber about said inclined axis.

2. A rotary chamber mill comprising a stationary support, a mill chamber carrier rotatably mounted in said support about a first axis, a closed mill chamber having the form of a unitary sphere with a continuous spherical inner surface rotatably mounted in said carrier about an oblique second inclined axis intersecting said first axis at the center of said sphere, and means for driving said carrier and said chamber simultaneously about their respective axes in rotation.

3. A rotary chamber mill as claimed by claim 2, in which said axes intersect at 45.

References Cited in the file of this patent UNITED STATES PATENTS 706,102 Pendleton Aug. 5, 1902 721,649 Pendleton Feb. 24, 1903 FOREIGN PATENTS 3,597 Great Britain of 1900 259,895 Germany May 15, 1913 873,100 France Mar. 9, 1942 877,245 Germany May 21, 1953

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