Abstract:
A colloid mill utilizes a motor-driven shaft configuration that connects to the rotor of the colloid mill to the electric motor rotor. In this way, the mill rotor shaft is directly driven. Complex gear or belt drive arrangements between a separate electric motor and the fluid processing components of the colloid mill are thus avoided. Moreover, the gap between the mill rotor and mill stator can be adjusted simply by axially translating the motor-driven shaft. Such translation is provided by a timing belt-based arrangement to limit backlash. As a result, a simple hand-operated knob or stepper motor arrangement can be used to control the gap.

Description:
RELATED APPLICATION  
       [0001]    This application is a divisional of application Ser. No. 09/315,589, filed May 20, 1999, the teachings of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Industrial-grade mixing devices are generally divided into classes based upon their ability to mix fluids. Mixing is the process of reducing the size of particles or inhomogeneous species within the fluid. One metric for the degree or thoroughness of mixing is the energy density per unit volume that the mixing device generates to disrupt the fluid particles. The classes are distinguished based on delivered energy densities. There are three classes of industrial mixers having sufficient energy density to consistently produce mixtures or emulsions with particle sizes in the range of 0 to 50 microns.  
           [0003]    Homogenization valve systems are typically classified as high energy devices. Fluid to be processed is pumped under very high pressure through a narrow-gap valve into a lower pressure environment. The pressure gradients across the valve and the resulting turbulence and cavitation act to break-up any particles in the fluid. These valve systems are most commonly used in milk homogenization and can yield average particle sizes in the 0-1 micron range.  
           [0004]    At the other end of the spectrum are high shear mixer systems, classified as low energy devices. These systems usually have paddles or fluid rotors that turn at high speed in a reservoir of fluid to be processed, which in many of the more common applications is a food product. These systems are usually used when average particle sizes of greater than 20 microns are acceptable in the processed fluid.  
           [0005]    Between high shear mixer and homogenization valve systems, in terms of the mixing energy density delivered to the fluid, are colloid mills, which are classified as intermediate energy devices. The typical colloid mill configuration includes a conical or disk rotor that is separated from a complementary, liquid-cooled stator by a closely-controlled rotor-stator gap, which is commonly between 0.001-0.40 inches. As the rotor rotates at high rates, it pumps fluid between the outer surface of the rotor and the inner surface of the stator, and shear forces generated in the gap process the fluid. Many colloid mills with proper adjustment achieve average particle sizes of 1-25 microns in the processed fluid. These capabilities render colloid mills appropriate for a variety of applications including colloid and oil/water-based emulsion processing such as that required for cosmetics, mayonnaise, or silicone/silver amalgam formation, to roofing-tar mixing.  
         SUMMARY OF THE INVENTION  
         [0006]    Existing colloid mills have suffered from a number of performance- and ease-of-use-related problems.  
           [0007]    One such problem relates mechanical complexity and stability. In the past, colloid mills have had mill housings for the rotor/stator and separate electrical motors with direct drive, reduction gear-, or belt-drive systems connecting the motors to the mill rotors. Elaborate mechanical isolation is required because both the mill rotor and the electric motor have separate bearing systems. Furthermore, the mechanisms used to enable rotor-stator gap adjustment, worm gear arrangement in one commercial device, have been mechanically complex and potentially dynamic during operation primarily due to thermal expansion effects.  
           [0008]    In the present invention, these problems are avoided by relying on a motor-driven shaft configuration. That is, the shaft that drives and connects to the rotor of the colloid mill extends to the electric motor stator of the electric motor. In this way, the mill rotor shaft is directly driven.  
           [0009]    The benefits resulting from this configuration primarily concern simplicity. Complex gear or belt drive arrangements between a separate electric motor and the fluid processing components of the colloid mill are avoided. Moreover, the gap between the mill rotor and mill stator can be adjusted simply by axially translating the motor-driven shaft. The small movements, of typically less than a 0.1 inches, have no or negligible effect on the electromagnetic field generation in the electric motor. Moreover, in this configuration, only one set of thrust bearings are required, and these are located very close to the rotor, thus minimizing any thermal expansion effects on the mill rotor-stator gap.  
           [0010]    In general, according to one aspect, the invention features a colloid mill comprising a mill stator, a mill rotor, an electric motor stator, and a motor-driven shaft. This motor-driven shaft functions as an electric motor rotor that operates in cooperation with the electric motor stator, but also extends from the electric motor stator to the mill rotor, providing a direct drive arrangement.  
           [0011]    In specific embodiments, a gap adjustment system is provided that changes a gap between the mill stator and the mill rotor by axially translating the motor-driven shaft relative to the electric motor stator. Further, the electric motor driven shaft is axially supported to counteract forces generated between the mill stator and mill rotor by at least one thrust bearing, preferably an angular contact bearing set, that is located on the side of the electric motor stator proximal to the mill rotor. As a result, mere radial support bearings are needed on the distal side of the electric motor stator relative to the mill rotor.  
           [0012]    Another problem that arises in existing colloid mill designs is related to the stability of the mill rotor-stator gap and specifically the system used to adjust the gap. One of the most common configurations utilizes a worm-gear arrangement. This system, however, is hard to calibrate and can jam or freeze in response to the forces generated between the mill rotor and stator.  
           [0013]    This problem is solved in the present invention by providing a timing belt-based arrangement for adjusting the gap. Such a timing belt system provides for no backlash. As a result, a simple hand-operated knob or stepper motor arrangement can be used to control the gap.  
           [0014]    Specifically, a thrust bearing is supported in a threaded sleeve that mates with the colloidal mill body. The timing belt engages the sleeve to rotate it relative to the body, thus adjusting the thrust bearings axially and thereby controlling the gap between the mill stator and mill rotor.  
           [0015]    In general, according to another aspect, the invention features a gap adjustment system for a colloid mill. The system comprises at least one thrust bearing that supports a shaft carrying a mill rotor in proximity to a mill stator. A threaded sleeve in turn carries the thrust bearing, its threads mating with complimentary threads of a body of the colloid mill. A timing belt, which is supported by the colloid mill body, engages the threaded sleeve to enable rotation relative to the body to thereby translate the thrust bearings, yielding axial movement of the shaft. This changes the gap between the mill stator and mill rotor.  
           [0016]    In specific embodiments, a knob is used to manually adjust the timing belt.  
           [0017]    In other embodiments, an adjustment motor, such as a stepper motor is used to adjust the timing belt under microprocessor control.  
           [0018]    Another problem that arises in existing mills concerns what happens when a customer requires a new colloid mill for a given manufacturing process to handle higher fluid processing rates. In the past, manufacturers have offered larger and smaller-sized colloid mills to meet customer demand. The problem, however, has been that typically when moving to colloid mills of a higher throughput the manufactures have simply offered larger versions of a geometrically similar mill rotor-stator configuration. Put another way, a colloid mill with a higher throughput had a rotor and stator that looked like the colloid mill with a lower throughput but were simply larger. This technique for modifying colloid mill rotor/mill stator configurations to handle higher fluid volumes yields different processing effects on those fluids. The larger colloid mills tended to process the fluid at different energy densities, typically higher than the smaller colloid mills. This was a problem to the customer since it required recalibration of the processing parameters of the fluid in order to maintain a consistent product.  
           [0019]    The present invention uses the recognition that the energy density delivered to the fluid or the characteristics that provide a uniform particle size at the output is related to the third power of the rotor speed and the second power of the rotor diameter. As a result, when scaling mill rotor/mill stator configurations to higher fluid throughput and consequently larger rotors, it is necessary to decrease the rotor speed. In order that the fluid has a consistent residence time and velocity gradient in the mill rotor-stator gap, the surface angle or rotor pitch, however, is increased with increases in the size of the rotor to counteract the effects of the slower rotor speeds. This provides kinematic similarity, or similar changes in velocity as the product traverses the mill rotor-stator gap of different sizes of the colloid mill.  
           [0020]    In general, according to another aspect, the invention features a family of colloid mills in which the rotor surface pitch angles increase with increases in colloid mill throughputs. Said another way, the mill rotor surface angles and rotor surface lengths are controlled between colloid mills having different throughput in order to standardize the energy input into the processed fluids.  
           [0021]    Another problem with existing mills has been colloid mill rotor configurations. Some mills have long slots that extend down the entire face of the mill rotor, whereas other configurations utilize relatively smooth conical- or disk-shaped rotor configurations. Each configuration has its relative advantages and disadvantages. The smooth rotor configuration tends to generate high and consistent shear forces in the processed fluid. The configuration with the long axially and radially running slots provides high fluid throughput rates, while establishing good turbulence.  
           [0022]    The present invention utilizes a largely smooth rotor configuration in order to generate uniformly high shear forces, and thus consistency with correspondingly low variance in the particle size in the processed fluid. The inventive rotor, however, adds an annular region extending around the circumference of the rotor that provides an increased mill rotor/mill stator gap between upstream and downstream, relatively smooth, processing surfaces. This region of increased gap is designed to establish a cavitation field to compliment the largely shear-based fluid processing performed by the adjacent smooth rotor surfaces.  
           [0023]    In general, according to another aspect, the invention features a colloid mill rotor that comprises a primary processing surface extending annularly around the rotor, and a secondary processing surface, also extending annularly around the rotor downstream of the primary processing surface. An intermediate, annular processing surface is located axially between the primary and secondary processing surfaces and is depressed relative to those surfaces. During operation, the relative operation of the primary and secondary processing surfaces establishes a low pressure region in the enlarged gap created by the intermediate processing surface. This establishes in many cases a cavitation field that compliments the shear processing of the fluid.  
           [0024]    In specific embodiments, radially and axially extending slots are provided in the primary processing surface to facilitate the movement of the processed fluid through the gap. These slots in the primary processing surface cooperate with slots in the associated mill stator to facilitate pre-maceration of the fluid.  
           [0025]    The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    In the accompanying drawings, like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Of the drawings:  
         [0027]    [0027]FIG. 1 is a side cross-sectional scale view of a colloid mill of the present invention;  
         [0028]    [0028]FIG. 2A is a front plan view of the inventive colloid mill;  
         [0029]    [0029]FIG. 2B is a front plan view of the inventive colloid mill according to another embodiment offering automated gap control;  
         [0030]    [0030]FIG. 3 is a side part plan and part cross-sectional view of the inventive mill rotor;  
         [0031]    [0031]FIG. 4 is a top plan view of the inventive rotor;  
         [0032]    [0032]FIG. 5 is a side cross-sectional view of the mill stator and housing proximal endplate;  
         [0033]    [0033]FIG. 6 is a partial plan view of the mill stator according to the present invention; and  
         [0034]    [0034]FIG. 7 is a schematic diagram illustrating the difference in rotor surface angles with increases in rotor size to accommodate larger fluid throughput according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]    [0035]FIG. 1 shows a colloid mill, which has been constructed according to the principles of the present invention. Generally, the colloid mill  100  comprises a body  110  forming the outer casing and structure of the mill  100 . The body  110  comprises a motor housing  112  that largely contains the electrical, motor components of the mill  100 . The body  110  also comprises a mill housing  114  in which a rotor  180  and stator  178  located, and between which the fluid passes to be processed. Connecting the motor housing  112  with the mill housing  114  is a connecting section housing  116 , which contains the mill rotor-stator gap adjustment system and sealing systems to isolate the interior of the electric motor housing  112  from the interior of the mill housing  114 .  
         [0036]    Turning first to the electric motor housing  112 , the motor housing comprises a hollow cylindrical motor jacket  118 . The distal end of the jacket  118  is sealed by a distal motor end-plate  120 , which is attached to the jacket  118  via bolts  122 . The end plate has a center bore  132  to accommodate the mounting of a motor-driven shaft  130 . The distal end of the shaft  130  is supported at the end-plate  120  via radial support bearing  128 . The radial support bearing  128  is prohibited from rotating in the inner bore  132  of the end-plate  120  by bearing gasket  134 .  
         [0037]    Within the electric motor housing, attached around the inter-surface of the jacket  118 , are stator coils  136 . These cooperate with rotor coils  138  attached to the shaft  130  to generate an electromotive force to drive the shaft  130 .  
         [0038]    The electric motor housing  112  is supported in this embodiment on a formed baseplate.  
         [0039]    The proximal end of the electric motor casing  118  is closed by a proximal endplate  142 . This end-plate has a center bore  144  to accommodate the shaft  130 . The center bore  144  has internal threads  146  that cooperate with threads  150  on a thrust bearing sleeve  148 .  
         [0040]    The thrust bearing sleeve  148  carries, in the illustrated embodiment, three thrust bearings  152 , which are preferably angular contact-bearings to provide good rigidity and limit backlash. The thrust bearings are prohibited from axial movement in the distal direction within the bearing sleeve  148  via an annular retaining ring  154  which is bolted to the distal end of the sleeve via bolts  156 , and the thrust bearings are retained from moving in the proximal axial direction by lip  158  on sleeve  148 .  
         [0041]    The shaft  130  is moved axially relative to the body  110  by rotating the bearing sleeve  148  in the proximal end-plate  142 . This adjustment allows the control of the mill rotor/stator gap. Bearing sleeve rotation is achieved by a timing belt  160 . The timing belt engages a bearing sleeve belt pulley  162  that is rigidly connected to and turns with the thrust bearing sleeve  148 . Access is provided to the belt pulley ring  162  via a partially annular slot  164  in the connecting section housing  116 . As a result of this configuration, driving the timing belt  160  causes the rotation of the bearing sleeve  148  relative to the mill body  110 . This moves the thrust bearing sleeve  148  axially via the interaction between threads  146 ,  150  to move the thrust bearings  152  and thus the shaft  130  axially. The gap between the processing surfaces of the mill rotor and mill stator is adjustable from approximately 0.001 to 0.050 inches in the preferred embodiment.  
         [0042]    [0042]FIG. 2A is a front view of the colloid mill  100  specifically showing the support system for the timing belt  160 . Specifically, a triangular-shaped support bracket  210  extends from the connecting housing  116 , being attached by a series of bolts  212 . A knob  214  is journaled to the support bracket  210 . The path of the timing belt  160  extends from the bearing sleeve belt pulley  162  to an adjustment pulley  216  connected to the knob  214 . As a result of this arrangement, manual rotation of the knob  216  rotates the bearing sleeve  148  to move it axially and thus, adjust the gap between the processing surfaces of the mill rotator  180  and mill stator  178 .  
         [0043]    [0043]FIG. 2B illustrates an alternative embodiment for effecting mill rotor/stator gap control. Instead of a knob, a stepper motor  200  is used to drive the timing belt  160 . The stepper motor  200  is controlled by computer  202  to provide automated control of the rotor-stator gap with feedback from the LVDT  161 . This automated system enables better process control since the gap is continuously monitored and adjusted when necessary, and a history of gap size for a processing run is maintained to provide for process validation. Further, it enables clean-in-place operations in which the gap is changed automatically according to a profile while a cleaning solution is passed through the mill, thus requiring limited operator supervision. Preferably, the speed of the shaft  130  is also controlled by modulating the stator and/rotor field current using the computer  202 .  
         [0044]    In alternative embodiments, the stepper motor is configured to directly turn the bearing sleeve, preferably via a gear train. This configuration is not preferred, however, because of the loss of the beneficial effects of the timing belt, such as backlash control.  
         [0045]    Returning to FIG. 1, the belt pulley ring  162  of the bearing sleeve  148  additionally has a system that cooperates with the connecting section housing  116  to indicate or provide a read-out for the mill rotor/stator gap. The pulley ring  162  has an read-out surface  163 , the angle of which preferably matches the angle of the rotor. A window  165  is formed in the connecting section housing  116 . A linearly variable distance transducer (LVDT)  161  is installed within the window  165  and detects changes in the distance to the read-out surface  163 . As a result of this arrangement, by reading-out the distance to the read-out surface  161 , the distance between the processing surfaces of the mill rotor  180  and stator  178  is determined electronically by the LVDT  161 . Alternatively, a dial indicator or a digital position indicator can be installed together with or in place of the LVDT so as to permit direct mechanical readout of the mill/rotor/stator gap.  
         [0046]    The mill housing  114  is a fluid sealed compartment. It comprises a hollow cylindrical casing  168  with a distal, end-plate  170 . The end-plate  170  of the mill housing  114  has a center bore  172  through which the shaft  130  projects into the mill housing  114 . A system of seals  174 , surrounding the shaft within the center bore  172 , prevents contamination from the motor/environment from reaching the fluid to be processed within the housing  114  and prevents processed fluid from escaping into the outside environment from within the mill housing  114 . Additionally, a proximal oil seal  166  seals the connecting section housing  116  from the motor housing  112 .  
         [0047]    The proximal end of the mill housing is sealed via a proximal mill housing endplate  176 , which also functions as the mill stator. Specifically, the proximal mill housing end-plate comprises an axial-extending tubular column  177  providing an input port  179  through which fluid to be processed enters the colloidal mill  100 . A corkscrew-shaped fluid pump  194  within the entrance port  179  draws the fluid to be processed into the mill housing  114 .  
         [0048]    The fluid progresses to the left in the illustration of FIG. 1 to the processing surface of a stator  178 , which is an integral part of the mill housing proximal end-plate  176 . Rotor  180 , which is connected to the shaft  130 , pulls the fluid to be processed between the processing surfaces of the rotor  180  and the stator  178  into processed fluid reservoir  182 , from which the fluid exits the mill housing  114  via exit tube  184  out through exit port  186 .  
         [0049]    The proximal mill end-plate  176  is sealed to the mill casing  168  via primary and secondary seals  188 ,  190 . Cooling fluid reservoir  192  in the mill housing proximal endplate carries a cooling liquid to remove heat generated by the rotor&#39;s rotation against the stator  178 .  
         [0050]    [0050]FIG. 3 is a side, partially cut-away view of a mill rotor constructed according to the principles of the present invention. In the preferred embodiment, the pitch angle of rotor  180  is approximately α=81.4 degrees.  
         [0051]    Specifically, the mill rotor  180  has an annular primary processing surface  310 . A series of radially and axially extending slots  312  are formed in the primary processing surface. The slots facilitate pre-maceration of the incoming fluid.  
         [0052]    Downstream of the primary processing surface is an intermediate processing surface  314 . This intermediate processing surface is depressed relative to the primary processing surface  310 . In the preferred embodiment, it is depressed by approximately a=0.063 inches. This depression, creates a reservoir of fluid in the gap between the intermediate processing surface  314  and the processing surface of stator  178 . In this reservoir, a low pressure field is generated which facilitates cavitation. This effect contributes to the mixing of the fluid to be processed and complements the largely shear effects created in the fluid between the primary processing surface  310  and the stator  178 . The intermediate processing surface length is c=0.688 inches in the preferred embodiment.  
         [0053]    Downstream of the intermediate processing surface  314  is a secondary processing surface  316  also extending annularly around the rotor  180 . The secondary processing surface  316  is raised above the intermediate processing surface  314  by essentially the same distance as the primary processing surface is above the intermediate processing surface. Both the intermediate and secondary processing surfaces are continuous in contrast to the primary processing surface  310  that has the slots  312 . In the preferred embodiment, the surface length of the secondary processing surface  310  is b=0.74 inches.  
         [0054]    [0054]FIG. 4 is a top plan view of the rotor  180 , showing the primary processing surface  310 , the intermediate processing surface  314  and the secondary processing surface  316 . Also shown are the array of slots  312  in the primary processing surface  310 . In the preferred embodiment,  12  slots are provided evenly spaced around the circumference of the rotor. Also as shown, the central line  318  of the slots  312  does not pass through the axis of rotation  320  of the rotor  180 . There is a distance of e=0.563 inches between the center line of slot  312  and a line extending parallel to the slot centerline  318  through the axis of rotation  320  of the rotor  180 . In the preferred embodiment, the slots are approximately d=0.125 inches wide. Additionally, the total diameter of the rotor  180  is j=5.0 inches and the center diameter is k=1.562 inches.  
         [0055]    [0055]FIG. 5 is a cross sectional view of the proximal mill housing end-plate  176 . A series of stator slots  340  are formed on the inner surface of the stator  178 . These slots are f=1.2 inches long. Downstream of the slots&#39; termini is a hardened annular section  342  of the stator  178 . Specifically, this hardened section is approximately g=1.487 inches long and is filled with STELLITE to a depth of h=0.075 inches in order to provide a long-wearing processing surface.  
         [0056]    [0056]FIG. 6 is a plan view of the stator  178  looking out through the input port  179 . This view shows that in the preferred embodiment, ten of the slots  340  are provided in the inner surface of the stator evenly spaced and extending in a radial direction.  
         [0057]    A different number of rotor slots than stator slots is used so to remove any beating and thereby minimize vibration. As a result, the slots in the rotor do not all confront a slot in the stator at the same time during rotation. Further, the rotor slots  312  are angled with respect to the stator slots  340 . This feature creates the effect of the stator slots  340  moving radially outward and downward over the rotor slots  312  as the rotor  180  turns. This generates a pressure-popping effect that facilitates mixing.  
         [0058]    [0058]FIG. 7 illustrates the relationship between colloid mill rotors for colloid mills of different throughputs, when the rotors are constructed according to the principles of the present invention.  
         [0059]    According to the present invention, the intent is to match the energy input per unit volume into the fluid across the range of colloid mills with different fluid throughput. This is achieved by maintaining the same value of the rotor speed, in revolutions per minute, to the third power, times rotor diameter to the second power (N 3 D 2 ) at the exit of the milling gap. The time over which a given volume of fluid is processed in the mills&#39; rotor/stator gaps and the change in milling intensity is standardized between different throughput mills by maintaining the same percent change in velocity of the processed fluid as it moves down the processing surface of the rotor.  
         [0060]    If bar  414  is defined as an arbitrary axial length of a potential rotor for a colloid mill of the present invention, and  416  is a point selected along the rotor&#39;s axis of rotation  320 , then where rays  410 , evenly spaced about the axis of rotation, cut through the bar defines the rotor&#39;s processing surfacing length and rotor diameter. The angle α′ between the rays defines the rotor&#39;s pitch angle. To design a rotor for a higher throughput colloid mill, rays  412  from point  416  are defined at an increased rotor pitch angle α″. Where these new rays cross bar  414 , they define the rotor processing surface length and rotor diameter. As a result, the rotor pitch angle increases with increases in the rotor diameter and thus colloid mill throughput according to the present invention. Processed fluid moves at the same velocity through the gap regardless of rotor size. The increases in pitch has the effect of exposing the fluid to increases in the centripetal force even though the net force remains the same due to the decreased speed at which the larger rotors are run.  
         [0061]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described specifically herein. Such equivalents are intended to be encompassed in the scope of the claims.