Abstract:
A self-cleaning centrifuge for clarifying contaminated liquids has a rotor assembly comprising nested first and second sets of concentric cylinders supported for rotation about an axis. The sets of concentric cylinders are rotatably connected. A motor means is connected to rotate one of the sets. Means supply contaminated liquid to the nested cylinders, and the liquid provides coupling between both of the sets so that both sets rotate about the axis. Centrifugal force causes foreign matter to collect on the circumferential surfaces of the cylinders. The resultant clarified liquid is collected for reuse. Selectively utilized brake means provides sudden braking of one of the rotating sets to purge collected foreign matter from the cylinders. The purged foreign matter is collected for disposal.

Description:
PRIOR PROVISIONAL APPLICATION  
         [0001]    This application is based on our prior U.S. Provisional Patent Application No. 60/433,525, filed Dec. 16, 2002, the disclosure of which is incorporated herein.  
         BACKGROUND OF THE INVENTION  
         [0002]    Centrifuges are widely used for clarifying contaminated liquids, a common example being liquids used for cooling or flushing the “work area” of operational machine tools such as grinders, lathes, milling machines, thread rollers, etc.  
           [0003]    Wire Electrical Discharge Machines (Wire EDM(s)) are machine tools in widespread use for precision metal cutting but present a special challenge because they require relatively large volumes of cutting liquids of high clarity while generating large volumes of metal particles which must be carried away by the cutting liquid. More specifically, high clarity de-ionized water is required for Wire EDM machining because this process utilizes a high voltage applied across a small gap to produce arcing between the metal piece being machined and a moving brass wire. The de-ionized water washes away particles (of both the metal piece and the wire) generated in the cutting process. If the de-ionizing is degraded, then the increased conductivity of the water will interfere with the arcing and significantly reduce the speed and accuracy of the cutting process. Advanced Wire EDM machines can generate up to ten pounds of steel particles in one hundred hours of coarse cutting of a steel work piece and an equal amount of brass particles from the moving EDM wire.  
           [0004]    Wire EDM also produce particles that are difficult to remove with a centrifuge. “Normal” steel particles have a density of about seven grams per cubic centimeter and are easily removed by a centrifuge where the separating force depends on the density difference between the particles and the carrying liquid. However, the fine particles generated in the Wire EDM cutting process become hydrolyzed and grow in size about seven times. Because of the inclusion of water, the density of these hydrolyzed steel particles is very low, on the order of 1.2 grams per cubic centimeter. Thus, there is a small density difference between these hydrolyzed particles and water, which, of course, has a density of 1.0 grams per cubic centimeter. This small difference in density makes the hydrolyzed particles very difficult to remove with a centrifuge. Studies have shown that effective clarification of Wire EDM liquids requires separation efficiencies 20 to 40 times greater than the efficiencies of known prior art centrifuges. Our invention provides centrifuges that more than satisfy the aforementioned requirement.  
           [0005]    Prior art centrifuges have not been satisfactory for providing an adequate supply of clarified, deionized water or other liquid for many applications. For example, while U.S. Pat. No. 3,858,793 teaches a centrifuge with a separation efficiency to yield a satisfactory water clarity for Wire EDM machining, this centrifuge has a major shortcoming because it is not self-cleaning. It uses a replaceable cartridge that must be serviced manually when the cartridge has reached its capacity. Hence, this type of centrifuge is not practical for use to support Wire EDM machining.  
           [0006]    At this time, most Wire EDMs utilize integral or associated disposable cartridge filtration systems to provide desired water clarity. None of these systems are completely satisfactory; they are expensive in terms of degrading the productivity of the machine as well as the costs of acquisition and installation of replacement filters and disposal of used filters.  
           [0007]    U.S. Pat. No. 3,861,584 teaches a self-cleaning centrifuge but has an unsatisfactory separation of foreign matter from the carrying liquid.  
         SUMMARY OF THE INVENTION  
         [0008]    While the present invention is especially advantageously applicable to the clarification of Wire EDM cutting liquids, it is not limited thereto but may be used for a wide spectrum of applications.  
           [0009]    The present invention provides a very high efficiency centrifuge which is both self-cleaning or self-purging and has a very high capacity to clarify contaminated liquid. Our invention directly provides a significant increase in the productivity of a machining operation such as a Wire EDM; our centrifuge is thus far superior to prior art units.  
           [0010]    In broad terms, our invention provides a self-purging centrifuge comprising a rotor assembly supported for rotation about an axis. The rotor assembly has nested first and second sets of concentric cylinders. Further, the said sets of concentric cylinders are rotatably connected, i.e., one or a “first” set is supported for rotation about the axis relative to the other or “second” set. The concentric cylinders have inner and outer collection surfaces. Means are provided for rotating one of said sets (the driving set) about the axis. Means are provided for supplying contaminated liquid to the axial end of said rotating sets of concentric cylinders. Through initial air drag, and subsequently through liquid drag, the other of said sets (the driven set) is driven at an angular velocity which closely approaches that of the driving set.  
           [0011]    In the preferred embodiment, the rotor assembly will be supported for rotation about a vertical axis. The contaminated liquid may be supplied in the form of a spray from nozzle means against either the top or the bottom axial end of the rotor assembly.  
           [0012]    The rotation, i.e., angular velocity, of the rotor assembly, in combination with the supply of contaminated liquid, produces a rotating vortex of the liquid and centrifugal force is thus applied to all molecules in the vortex, i.e., (1) water (or other liquid) and (2) waste or foreign matter. Foreign matter can be either of greater or lesser density than the density of the carrying liquid. Heavier foreign matter will accumulate on the inner collection surfaces of all of the rotating concentric cylinders. Lighter foreign matter (for example an oil having a relatively low density) will accumulate on the outer collection surfaces of the concentric cylinders. Concurrently with the foreign matter accumulating on the collection surfaces as aforesaid, the liquid is clarified and is collected by appropriate means into an appropriate reservoir means from which it may then be transferred for further use.  
           [0013]    Our invention further includes means for selective and sudden/quick braking the rotation of one of the sets of concentric cylinders (relative to the other set). The braking causes the accumulated foreign matter to be sheared or sloughed off from the collection surfaces to then, under gravitational force, drop downwardly into a suitable receptacle. The braking is done at appropriate intervals of time and the interruption of the primary purpose of clarifying the contaminated liquid is minimized. All of the system control steps and functions may be automatically controlled.  
           [0014]    Our concept includes (1) either set of concentric cylinders may be the “driver”, (2) either set of concentric cylinders may be braked, (3) the same set of concentric cylinders may both be the “driver” and also be braked, and (4) the “feed” of contaminated liquid may be to either of the axial ends of the rotary assembly of nested concentric cylinders. 
       
    
    
     DESCRIPTION OF THE DRAWING  
       [0015]    [0015]FIG. 1 shows a schematic diagram of our invention in a representative system application for clarifying contaminated liquid;  
         [0016]    [0016]FIG. 2 shows an exploded view of the shaft means which includes a stationary shaft and additional shaft means connected respectively to the first and second sets of concentric cylinders;  
         [0017]    [0017]FIG. 3 shows the apparatus as shown in FIG. 2 in assembled form and on an enlarged scale;  
         [0018]    [0018]FIG. 4 shows the apparatus of FIG. 3 assembled with (for this exploded view) axially spaced-apart first and second sets of concentric cylinders;  
         [0019]    [0019]FIG. 4A is a view, on an enlarged scale, of a portion of the set of lower concentric cylinders as shown in FIG. 4;  
         [0020]    [0020]FIG. 5 is an end view of the apparatus of FIG. 4 as viewed along section lines  5 - 5  of FIG. 4; FIG. 5 is thus a plan view of an end plate means or lower cylinder drive plate for the lower set of concentric cylinders as shown in FIG. 4;  
         [0021]    [0021]FIG. 6 is an exterior view of the top of the upper set of concentric cylinders;  
         [0022]    [0022]FIG. 7 is a cross-section of one embodiment of our invention;  
         [0023]    [0023]FIG. 8 is a plan view of the bottom or underside of a bowl cover plate used with the apparatus shown in and as viewed along section lines  8 - 8  of FIG. 9;  
         [0024]    [0024]FIG. 9 is a cross-sectional view of the cover of FIG. 8 as viewed along section lines  9 - 9  thereof;  
         [0025]    [0025]FIG. 10 is a cross-sectional view of a portion of the cover of FIG. 9 as viewed along section lines  10   −10  thereof;  
         [0026]    [0026]FIG. 11 is a plan view or view of the top surface of an upper cylinder mounting plate as viewed along section lines  11 - 11  of FIG. 4;  
         [0027]    [0027]FIG. 12 is an enlarged view (from FIG. 7) of a section of the top of the nested concentric cylinders in assembled relationship with the bowl cover shown in FIGS. 8,9 and  10 ;  
         [0028]    [0028]FIG. 13 is an enlarged view (from FIG. 7) of a section of the bottom of the nested concentric cylinders showing (following the braking of one of the sets of rotating concentric cylinders) the sloughed off or dislodged “foreign matter” in transit to the sludge tub ST; and  
         [0029]    [0029]FIG. 14 is a somewhat schematic cross-section of an alternate embodiment of our invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    In FIG. 1 the preferred embodiments of our invention is identified as the centrifuge AA comprising, in part, a housing H having a base or centrifuge support means h′ including legs h″ and h″′. The centrifuge AA is shown in much greater detail in FIG. 7. A motor means M is shown in FIG. 1 having a drive pulley M′ connected via a drive belt means M″ to the top of the centrifuge AA, adjacent to which is a selectively controlled brake means AB. Rotation of the motor means causes the rotation of the nested sets of concentric cylinders. Integral with the main housing H is an upper or auxiliary housing enclosure H′ (best shown in FIG. 7) to include a plate member  26  connected to a short support  24  by screw means  26 ′. The support  24  is attached by means not shown to top plate  22  of housing H.  
         [0031]    Contaminated liquid from a source such as a machine tool is delivered by a conduit a to a reservoir R′. The contaminated liquid may be selectively sprayed against the underside or bottom axial end of the rotor assembly, i.e., the rotating nested sets of concentric cylinders. The contaminated liquid is pumped by a pump means P from the reservoir R′ via conduits b and b′ to nozzle means NZ. The foreign matter in the liquid will be collected on the collection surfaces of the concentric cylinders and the liquid, now clarified, will be collected within the housing H for transfer, via a drain means  126  and a conduit c, to a clarified liquid reservoir R″. Typically, the clarified liquid in reservoir R″ would, by means including conduit means not shown, be returned to the original machine tool to be used again, and again  
         [0032]    The motor depicted is electrically energized by a controllable switch means S which in turn is controlled via a connection d by a controller CTR which also controls, via connection e, the pump P and, via connection f, a valve means V for selective actuation of the brake means AB. When the brake means is actuated to brake the rotation of one of the sets of concentric cylinders, then the accumulated foreign matter will slough off, as described above, and drop down through an opening  136  at the bottom of the centrifuge and thence into an appropriate receiving means depicted as a “sludge tub” ST.  
         [0033]    [0033]FIG. 2 shows, in broad terms, a hollow stationary shaft means  30 , a hollow driving shaft means  50 , and a solid driven shaft means  40  which, when assembled, form a unitary assembly BB as shown in FIGS. 3 and 4. These elements are coaxial with respect to the centrifuge rotational axis RA  
         [0034]    Hollow stationary shaft means  30  comprises an elongated tube  34  having an inner surface  34 ′, external thread means  34   a  at the bottom thereof, a radially extending flange  32  at the top thereof, and a recess  32   a  adapted to receive the outer race of a bearing means  36 . An internally threaded collet  38  is shown adjacent to bearing means  36  in FIG. 2 and is adapted to be screwed onto a threaded means  38 ′ on solid shaft  40  when the shafts  30 ,  40  and  50  are assembled as is shown in FIG. 3. At that time the collet  38  will bear against the inner race of the bearing means  36 . The flange  32  has a plurality of threaded bores  32 ′ for receiving the threaded ends of bolt means  26 ″ which extend through bores in the member  26  of the housing H′ as shown in FIG. 7 so as to firmly attach or connect the assembly BB to the housing H. Adjacent to the flange  32  is a circumferential surface  35   a  on the tube  34  which is sized to receive the inner race of a bearing means  35 , the outer race of which is adapted to be received by a recess  35   b  in the top end of the hollow driving shaft means  50  which, as shown, is an elongated tube  52 .  
         [0035]    The bottom annular end  53  of tube  52  has a plurality of circumferentially spaced-apart threaded bores  53 ′ extending parallel to the axis RA. A flange  54 , having a plurality of axial bores  54 ′, extends radially from tube  52  to provide a means for attaching tube  52  to the upper or first set  70  of concentric cylinders thereto as is best shown in FIG. 7. The bottom annular end of tube  52  also has a series of stepped recesses  55  and  56  for receiving, respectively, bearing means  55   a  and  56   a  as shown in FIG. 3. A collet  57   a  is adapted to hold bearing means  56   a  in recess  56  via a plurality of bolt means  57  screwed into the threaded bores  53 ′. The bearing means  55   a  also is held in the-recess  55  by collet  34   b  which has internal threaded means for engagement with the threaded end  34   a  on the bottom end of the stationary hollow shaft means  34 .  
         [0036]    The solid shaft means  40  is elongated and has a top portion  40   a  adapted to be selectively gripped by the brake means (to be described below) and a lower or bottom end  40   b  to which is connected a hub member  42  having a radially extending flange  44  and an end cap  46  sized to fit into the center opening  60 ′ of end plate  60  (to be discussed below). The flange  44  has a plurality of threaded bores  44 ′ for receiving bolt means  44 ″ when the end plate  60  is attached to the bottom of shaft  40 .  
         [0037]    Thus, a cornpound rotatable support means is provided for the rotor assembly. Our invention provides (1) a rotatable support of a first set of concentric cylinders  70  relative to the stationary hollow shaft means  34  (and thus the housing H), and (2) the rotatable support of the second set of concentric cylinders  80  relative to the first set  70 , such rotatable support being best illustrated in FIG. 3.  
         [0038]    Shaft means  34  positions the outer race of bearing means  36 , the inner race of which positions the top end  40   a  of the solid shaft  40 . The hollow shaft  50  is supported (by shaft means  34 ) in an axial sense and for rotation about axis RA by upper bearing means  35  and lower bearing means  55   a . Further, the solid shaft  40  is supported (both in a rotational and an axial sense) by the hollow shaft  50  via the bearing means  56   a , the inner race of which journals the lower portion  40   b  of the solid shaft  40 .  
         [0039]    The first and second sets of concentric cylinders  70  and  80  respectively are shown in pre-assembled axially-spaced apart relationship in FIG. 4. Each of sets  70  and  80  is shown to have seven concentric cylinders. Set  70  has cylinders  71 ,  72 ,  73 ,  74 ,  75 ,  76  and  77  with cylinder  71  having the largest diameter and with the diameters decreasing in steps to cylinder  77  the smallest diameter. Likewise, set  80  has cylinders  81 ,  82 ,  83 ,  84 ,  85 ,  86  and  87  with cylinders  81  and  87  having the largest and smallest diameters, the intermediate cylinders having diameters decreasing in uniform steps.  
         [0040]    Each of the cylinders has, near one end thereof, a plurality of circumferentially elongated slots therethrough which are used in combination with radially extending bar means to attach all of the cylinders of a set to an end plate means. Thus, the set  80  has a plurality of such shaped slots  80   a  positioned circumferentially about the lower end of the concentric cylinders  81 - 87  of the set. Elongated bars  79  are positioned in slots  80   a  of all of the concentric cylinders and thus are radially oriented. The bars  79  are sized to fit snugly in slots  80   a . In addition, the elongated bars  79  have a plurality of threaded bores  79   a  for receiving the threaded ends of bolt means  60   f , as hereinafter explained. An end plate or lower cylinder drive plate  60  shown in FIGS. 4, 4A and  5  has an annular section  60   a  with a central circular opening  60 ′ sized to receive the hub  46  of shaft  40 . Adjacent to the edge of the opening  60 ′ are a plurality of bores  60 ″ in annular section  60   a  for passage therethrough of machine screw means  44 ″ to be screwed into threaded bores  44 ′ of flange  44  to thus attach end plate  60  to flange  44  of shaft  40 . The end plate  60  further includes a spider-like plurality of radially extending spokes or fingers  60   f  each having a plurality of radially arranged, axially extending bores  60   b  for passage therethrough of a plurality of attachment means  60   f ′ which have at their upper ends thread means for engaging the aforementioned threaded bores  79   a  in the bars  79 . As shown in FIG. 5, the end plate  60  has eight spokes or fingers  60   f  with approximately 45 degrees of spacing between adjacent spokes; the number and angular spacing of spokes should match the number and be in register with the angular spacing of the bars. The number of spokes and co-acting bars can be greater or less than the eight shown. The purpose of the end plate is, in combination with the elongated bars, to provide a strong, solid connection means for attachment of the concentric cylinders to the flange  44  and thus to the solid (driven) shaft  40 . In practice, the plurality of concentric cylinders  81 - 87  are arranged in the order shown in FIG. 4; and the fastening means  60   f ′ are inserted through bores  60   b  and threaded tight into the threaded bores  79   a  of the bars  79 . The cylinders are sized in thickness and of selected material for functioning in a rotor assembly rotating at high angular velocities. We have found  16  gauge stainless steel to be satisfactory. However, it should be understood that other materials and other thicknesses may be used. In general, it is advantageous to use thin material for the cylinders to enjoy improved separation efficiencies and reduced size of the centrifuge. The spacing between the cylinders (of the nested sets) is nominally a target of 0.08 inches; this target may, of course, be varied.  
         [0041]    The upper set of concentric cylinders  70  has an associated upper cylinder mounting plate or end plate  90  (shown best in FIGS. 11 and 12) which is quite similar to end plate  60  with, again, eight spider-like spokes or fingers  90   f  extending radially from an annular center section  90   a , an exception being that the radially spaced-apart, axially extending bores  90   f ″ through the spokes  90   f  are counter-bored (see FIG. 12) so that the heads of bolt means  90   f ′, when assembled, do not project above the top surface  90   t  of the end plate  90  as is shown in FIG. 4. The bolt means  90   f ′ pass through the bores  90   f ″ and are screwed into threaded bores in elongated bar means  69  which are positioned through circumferentially elongated slots  70   a  as is clearly shown in FIG. 4. Thus the top surface  90   t  of end plate  90  (as viewed in FIGS. 4 and 7) is planar which is required because it is abutted by the bottom planar surface  102   b  of a bowl cover  102  shown in FIGS.  7 - 10 . End plate  90  has a central circular opening  90 ′ sized for receiving the driving shaft  50  and, as shown in FIGS. 4 and 7, the bottom surface  90   b  of end plate  90  is abutted by the top surface of flange  54  of shaft  50 . End plate  90  also has, adjacent to the center opening  90 ′, a plurality of bores  90 ×arranged in a circle and positioned sufficiently radially from the axis RA to permit bolts  103 ′ to pass through for threaded engagement with threaded bores  54 ′ in flange  54 .  
         [0042]    The bowl cover  102  is a flat, annular member having a central opening  102 ′, a plurality of axial bores  103  arranged in a circle adjacent to opening  102 ′, and a plurality of axial bores  104  arranged in a circle at a pre-selected distance from the axis RA. Bores  103  facilitate the attachment of the bowl cover to shaft  50 . Bores  104  are ports for the passage of clarified fluid as will be discussed below. A plurality of U-shaped channels  105  are provided on the bottom surface  102   b  of bowl cover  102  and extend radially from the bores  104  to the outer periphery of the cover as is best shown in FIGS.  8 - 10 . FIG. 8 shows eight channels  105  arranged uniformly about the axis RA. The outer periphery of the bowl cover has a flange means  102   f  through which extend a plurality of bores  106 . The number of channels  105  (and bores  104 ) must equal the number of spokes  90   f . The spokes  90   f  have a width greater than the width of the channels  105  and the end plate  90  is assembled with the bowl cover  102  with the spokes  90   f  in overlapping register with the channels  105  (see FIG. 10) to form liquid passageways for the clarified liquid as will be explained below.  
         [0043]    The bowl cover  102  is one of the component parts of a bowl B; it is connected to the top  100   a  of a vertically oriented elongated outer cylinder or bowl body  100  by bolt means  106 ′ passing through bores  106  and screwed into threaded bores around the top  100   a  as is shown in FIG. 7. The bottom  100   b  of the cylinder  100  has attached thereto by bolt means  114  an annular end plate member  110  having a large central opening  112 .  
         [0044]    A plurality of bolt means  103 ′ extend through bores  103  in bowl cover  102 , thence through bores in end plate  90  and are screwed into threaded bores  54 ′ of flange  54  as is clearly shown in FIG. 7 to thus couple the bowl B and the upper set  70  of concentric cylinders to the hollow drive shaft means  50  which, as indicated, is supported for rotation about the axis RA by the stationary shaft  30  and bearings  35  and  55   a . Concurrently, the lower set  80  of concentric cylinders is nested (spaced from and proximate to) with the upper set  70  and the shaft  40  thereof is supported by shaft  50  and with bearing  56   a  and stationary hollow shaft  30  with bearing  36  for rotation about the axis RA with respect to the upper set  70 .  
         [0045]    An annularly shaped liquid splash plate means  108 , positioned adjacent to and slightly above the ports  104  (see FIG. 7), is connected to rotate with the bowl B.  
         [0046]    A short axial portion  58  on the outer surface on the top of hollow shaft means  50  is adapted for engagement with a drive belt-type means M″ driven by motor means M. Thus, when motor M is energized, the drive belt-type means M″ will apply torque to tend to rotate shaft  50  and the upper set  70  of concentric cylinders. The “load” or torque requirements on motor M at starting are relatively low; thus, a relatively low power motor can be used with our invention. The set of concentric cylinders  70  will begin to rotate and eventually will be rotated at a pre-selected angular velocity determined by that of the motor and the diameters of M′ and  58 . The lower or driven set of concentric cylinders  80  may receive some rotational torque via air rotated by set  70 . Typically, when motor M is energized, the system operator or automatic control system would concurrently actuate pump P to pump contaminated liquid CL via conduit b′ to nozzle means NZ to spray the contaminated liquid CL against the bottom axial ends of the nested concentric cylinders. As soon as the droplets of the liquid contact the rotating cylinders of the upper set  70  the said droplets are subject to centrifugal force and tend to move outwardly toward the inside surfaces of each of the rotating cylinders and, eventually, to the inside surface of the bowl cylinder  100  to initiate the formation of a toroidal-shaped body of contaminated liquid. The liquid may move outwardly by flowing from the axial ends of the cylinders in the spaces between the spokes  60   f  and  90   f  respectively of the end plates  60  and  90  (see FIGS. 5 and 11). The introduction of the sprayed contaminated liquid CL greatly increases the coupling coefficient between the driving set  70  and the driven set  80  and, in due course, the driven set  80  will have an angular velocity very close to that of the driving set  70 . A typical angular velocity for the driving set  80  is 3600 RPM which has been found to be satisfactory. However faster or slower speeds may be used.  
         [0047]    For the case where the foreign material has a density greater than that of the carrying liquid (this is the case for most industrial contaminated liquid scenarios), the foreign material will accumulate on the inner surfaces of the concentric cylinders of both the upper and lower sets in a continuous process which results in the carrying liquid being clarified. However, if the foreign material is lighter than the carrying liquid, the foreign material will accumulate on the outer surfaces of the nested cylinders.  
         [0048]    The rotating toroidal-shaped body of clarified liquid will increase in radial depth or extent as more and more contaminated liquid is sprayed against the axial end of the nested sets, growing in size (radial extent) starting with the outer cylindrical surface thereof abutting the inner cylindrical surface of the bowl cylinder  100  and with the inner circumferential surface of the toroid growing, i.e., moving toward the rotational axis RA. The aforesaid radial growth of the toroid continues until said inner cylindrical′ surface thereof is slightly closer to the axis RA than the plurality of ports  104  in bowl cover  102  whereupon a pressure differential is created which forces the clarified liquid to flow from the top of the inner surface of bowl cylinder  100  into the passageways formed by channels  105  and spokes  90   f  (see FIG. 10) and thence radially inward in said passageways to and through and out the axially oriented bores  104 . Thus the clarified liquid is expelled under pressure out of bores  104  of bowl cover  102  whereat such liquid will tend to travel radially toward the inside cylindrical surface  20 ×of the housing H. The expelled clarified liquid may impact the splash plate means  108  as it travels toward surface  20 ×. Under the centrifugal force of the rotating assembly, the clarified liquid expelled from ports or bores  104  will be transferred to a clarified liquid collection means. For the embodiment of our invention shown in FIG. 7, the clarified liquid collection means is the annular space  124  defined between housing  20  and inner cylinder  120  with annular member  122  providing a bottom member for the collection means. Means including a fitting  126  and conduit means c permit the draining or transferring of clarified liquid from the collection means into a clarified liquid reservoir R″ shown in FIG. 1.  
         [0049]    Our invention further includes means for selectively braking the rotation of one of the sets of concentric cylinders with respect to the other set. Such braking should be as “sudden” or quickly as can be reasonably be accomplished because a quick braking maximizes the purging action. The braking is done from time to time, the frequency of braking being determined by the build up of foreign matter on the collection surfaces of all of the concentric cylinders in contact with the contaminated liquid. As indicated, the braking may be applied to either the driving or the driven set of cylinders. The arrangement shown in FIG. 7 has the braking applied to the driven set  80  of concentric cylinders. Again, it is desirable to have the rotation stopped quickly so as to maximize the purging efficiency. As depicted, the driven set  80  has less rotational inertia than that of the driving set  70 . Thus, for a given level of braking torque, the driven set  80  may be braked quicker than the driving set  70 .  
         [0050]    The sudden braking of the driven set  80  is caused by actuation of a brake means AB mounted on the top of plate  26  of the housing and fitted with a controllable shaft clamping means adjacent to the top axial portion  40   a  of the solid shaft  40  to which the driven set  70  of concentric cylinders are connected. Various braking means may be used for this purpose. The brake AB is depicted as an air operated brake. One brake which has been found to be satisfactory is Model S-800 sold by Nexen Group, Inc. Another variable or choice is to have a brake-clutch linkage (not shown) as part of the drive means between the motor M and the rotatable assembly of nested concentric cylinders.  
         [0051]    The sudden braking provides the very important function of purging the foreign matter from the collection surfaces of the nested concentric cylinders. The rapid changing of angular velocities between the driving and driven cylinders produces a shearing-like or sloughing-like force on the accumulated foreign matter to thereby separate same from all collection surfaces (except the inside surface of cylinder  77 ) following which the foreign matter falls under gravitational force down through the opening  112  and exit  136  into a sludge tub ST or equivalent as shown in FIG. 13.  
         [0052]    Another embodiment of our invention is shown, somewhat schematically, in FIG. 14 where three centrifuges (each illustrating the embodiment) I, II, and III are mounted on a frame FR in side-by-side relationship and are positioned over a common open-topped reservoir R″′ for the collection of clarified liquid. It should be noted that the three centrifuges do not have a formal or defined housing surrounding the rotating bowl assemblies B′, B″ and B″′ as was the case for the FIG. 7 embodiment. Each of the centrifuges has a shroud or umbrella means which deflects clarified liquid ejected in the manner above described out of the bores  104  of the bowl cover plate. Thus shroud or umbrella means U′, U″ and U″′ are provided above bowl assemblies B′, B″ and B″′ and function to capture and direct the clarified liquid to the clarified liquid reservoir R″′.  
         [0053]    While we have described in considerable detail two embodiments of our invention, others skilled in the art may provide modifications or variations of our apparatus. In particular it should be understood that our concept is specifically intended to cover nested first and second sets of concentric cylinders where (a) either set may be the driver with the other set being the driven set, (b) either set may be braked, (c) the same set of cylinders may be the “driver” and may be braked, and (d) the feed of contaminated liquid may be provided to either (or both) axial ends of the sets of concentric cylinders. In short, the scope of our invention should be measured only by the following claims.