Patent Document

[0001]    This application claims priority from (Japanese) Patent Application No. 2002-011529 filed on Jan. 21, 2002.  
         BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to a method and a device for cleaning fluid dynamic pressure bearings.  
           [0003]    In order to satisfy the increasing information density and the increasing rotational speeds of the magnetic disks used by computer hard disk drives, there exists a need for high rotational accuracy, low friction, low noise and long life bearings for the spindle motors that drive the magnetic disks. Accordingly fluid dynamic bearings have been developed for use with such spindle motors.  
           [0004]    Fluid dynamic bearings operate by generating a dynamic pressure in a fluid contained between the spindle motor&#39;s shaft and the sleeve that supports the shaft. This pressure is generated by dynamic pressure generating grooves, which are formed on one or more or the bearing surfaces such as the sleeve or the shaft. These dynamic pressure generating grooves are shaped so as to generate a dynamic pressure when the shaft or the sleeve rotates and they have a depth of several microns. The dynamic pressure generated by the fluid allows the shaft to be supported in a non-contact state without mechanical friction. Accordingly, fluid dynamic bearings are able to achieve high rotational speed, low friction, low noise and long life.  
           [0005]    Although other materials can be used, the shaft and the sleeve of fluid dynamic pressure bearings are often made from stainless steel. Finishing work on the bearing, cutting and polishing, creates a mirror-like bearing surfaces on the sleeve and the shaft. Dynamic pressure generating grooves are then be formed on the bearing surfaces of the sleeve or the shaft by electrochemical machining (ECM).  
           [0006]    As a result of the electrochemical machining used to form the dynamic pressure grooves on the fluid dynamic pressure bearing, a hard oxide film is generated on the surface of the electrochemical machined grooves portion of the bearing. Since the clearance between the sleeve and the shaft is very small, if this oxide film comes loose from the sleeve during use, it can become wedged into the clearance between the sleeve and the shaft, the friction caused thereby can cause rotational irregularities and deficiencies due to adhesion and the like.  
           [0007]    Conventionally, the oxide film generated by ECM is removed prior to the bearing&#39;s use by having an operator manually rub the bearing surface with a brush to which an abrasive material such as toothpaste or the like has been applied, or it is removed with an electric drill or the like furnished with a brush to which an abrasive material has been applied. Accordingly, removing the oxide film is an extremely difficult, inconsistent, labor intensive process that substantially impacts upon the quality of fluid dynamic bearings. Additionally, after the oxide film has been removed, it is necessary to wash off the abrasive material resulting in an additional manufacturing cost. Furthermore, the work must be done with scrupulous care to avoid damaging the bearing surface, since irregularities or the like can easily occur during the brushing and or the washing process.  
         SUMMARY OF THE INVENTION  
         [0008]    It is therefore an object of the present invention to provide a cleaning method and a cleaning device for fluid dynamic pressure bearings that allows the easy and efficient removal of the oxide film that is generated by electrochemical machining of the dynamic pressure generating grooves on a fluid dynamic pressure bearing.  
           [0009]    In order to solve the aforementioned problems, one aspect of the present invention is a cleaning method designed to remove the oxide film generated when electrochemical machining of dynamic pressure grooves on the bearing surface of a fluid dynamic pressure bearing is conducted. This method is characterized by removing the oxide film from the electrochemical machined grooves portion of the surface by spraying a high-pressured liquid jet onto the bearing surface. This method makes it possible to remove the oxide film produced by electrochemical machining without damaging the bearing surface. Additionally, this method pre-removes undesirable protrusions and burrs, which are caused by mechanical machining.  
           [0010]    Another aspect of the present invention is to use deionized pure water for the high-pressured liquid jet. The use of deionized pure water eliminates the need for subsequently washing off the abrasives or cleaning agents, which are used in brush polishing. Moreover, when deionized pure water is used, there is no occurrence of scaling, corrosion or blockages in the high-pressure liquid jet nozzle and the high-pressure tubing resulting from the various impurities contained in tap water or the like.  
           [0011]    Another aspect of the present invention is to use water mixed with abrasives and surfactants for the high-pressure liquid jet. The use of abrasives and surfactants makes it possible to effectively remove the protrusion, burrs and other byproducts of mechanical machining.  
           [0012]    Another aspect of the present invention is a cleaning device designed to remove the oxide film generated when electrochemical machining of the dynamic pressure grooves on the bearing surface of the fluid dynamic pressure bearing is conducted. This device removes the oxide film by spraying a high-pressured liquid jet onto the bearing surface. Additionally, this device makes it possible to remove other foreign matter such as protrusions and burrs produced by mechanical machining. The cleaning device can be used with either deionized pure water or it can be used with water and an abrasive or surfacant. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The invention is illustrated by way of example and not limitation and the figures of the accompanying drawings in which like references denote like or corresponding parts, and in which:  
         [0014]    [0014]FIG. 1 is an explanatory view showing the cleaning method of the first embodiment of this invention.  
         [0015]    [0015]FIG. 2 is an explanatory view showing the cleaning method of the second embodiment of this invention.  
         [0016]    [0016]FIG. 3 is a longitudinal cross-sectional view of a conventional spindle motor provided with a fluid dynamic pressure bearing.  
         [0017]    [0017]FIG. 4 is an enlarged view of a sleeve composing the fluid dynamic pressure bearing in the conventional spindle motor shown in FIG. 3.  
         [0018]    [0018]FIG. 5 is a bottom view of the sleeve shown in FIG. 4.  
         [0019]    [0019]FIG. 6 is a layout view showing the third embodiment of this invention.  
         [0020]    [0020]FIG. 7 is a plane view of the jig that holds the sleeves that are cleaned by the device shown in FIG. 6.  
         [0021]    [0021]FIG. 8 is a plane view of the jig that supports the counter-plates that are cleaned by the device shown in FIG. 6. 
     
    
     DETAILED DESCRIPTION  
       [0022]    Two embodiments of the present invention are shown in FIGS. 1 and 2, and they are described hereunder by reference to FIGS.  1 - 5 .  
         [0023]    [0023]FIG. 3 shows a spindle motor containing a fluid dynamic bearing. The fluid dynamic bearing is comprised of shaft  3 , sleeve  5 , thrust-plate  13 , which is affixed to the shaft, counter-plate  15 , which is affixed to the sleeve, and pressure generating grooves  17 ,  18 ,  19 , and  20 . As shown in FIGS. 3, 4, and  5 , the pressure generating grooves can be placed on either the shaft components or the sleeve components of the fluid dynamic bearing.  
         [0024]    Shaft  3 , sleeve  5 , thrust-plate  13 , and counter-plate  15  are generally made from stainless steel. Bearing surfaces are formed on shaft  3 , sleeve  5 , thrust-plate  13 , and counter-plate  15  by cutting and polishing the stainless steel to create a mirrored finish. Pressure generating grooves  17 ,  18 ,  19 , and  20  are then formed by electrochemical machining of the mirror finished bearing surface.  
         [0025]    A cleaning method according to the first embodiment of the present invention is shown in FIG. 1. A high pressure fluid is provided from fluid pump  22  to nozzle  21 . Nozzle  21  sprays the high pressure fluid onto pressure generating grooves  19 , which are formed on thrust-plate  13 . The fluid spray from nozzle  21  has a diameter d equal to the injection mouth diameter of nozzle  21 . The high pressure fluid spray removes the oxide film from the grooved surface of thrust plate  19 .  
         [0026]    The nozzle  21  is initially directed to spray the high pressure fluid at the far outer edge of thrust-plate  13 , which is a distance R 0  from the axis of thrust-plate  13 , where R 0  equals one half the diameter of thrust-plate  13  (R 0 =½D). Nozzle  21  then revolves around thrust-plate  13  at a rotational speed A, which is measured in revolutions per second (rps). The radius of revolution R(t) is initially equal to R 0  (the distance from the axis of thrust-plate  13  to the far outer edge of thrust-plate  13 ). The radius of revolution R(t) then decreases at a constant rate F (“the moving speed”) until R(t) is equal to the radius of the inner edge of thrust-plate  13  (R(t)=R 1 ). At this point, the cleaning can stop or the nozzle  21  can continue revolving and spraying high pressure fluid onto the grooved surface of thrust-plate  13  with the radius R(t) increasing at a constant rate F until R(t) is equal to R 0 .  
         [0027]    As described above, nozzle  21  has two cleaning cycles. During cycle  1 , the radius of revolution R(t) is equal to the outer radius of thrust plate  13  R 0  minus the moving speed F multiplied by the time from the beginning of the cycle t (R(t)=R 0 −F*t). During cycle  2 , the radius of revolution R(t) is equal to the inner radius of thrust plate  13  R 1  plus the moving speed F multiplied by the time from the beginning of the cycle t (R(t)=R 1 +F*t). The cleaning cycles can be repeated continuously as many times as desired.  
         [0028]    Provided that the rotational speed A (RPS), the moving speed F (mm/s), and the injection mouth diameter d (mm) of nozzle  21  meet the relationship F÷A≦d then the entire surface of thrust-plate  13  will be cleaned without any gaps.  
         [0029]    In this embodiment, deionized pure water is used for the high-pressure liquid jet, and the injection mouth diameter of the nozzle  21  is set at 0.25 mm, injection pressure is set to 700 kg/cm 2 , and the distance between the nozzle  21  and the work W is set at 20-40 mm. The rotational speed A is 1000 rpm (16.67 rps) and the moving speed F is 2 to 5 mm/s. Nozzle  21  may be reciprocated one or more times. By this means, one can efficiently remove the oxide film. This method can also be used to clean the bearing surface on counter-plate  15 , which contains grooves  20 , and other similar bearing surfaces.  
         [0030]    A cleaning method according to a second embodiment of the present invention is shown in FIG. 2. A high pressure fluid is provided from fluid pump  24  to nozzle  23 . Nozzle  23  sprays the high pressure fluid onto the bearing surface of sleeve  5 , which has pressure generating grooves  17  and  18  formed thereon. Nozzle  23  includes a pair of injection mouths  25 , which are diametrically opposed to each other. The fluid spray from nozzle  23  has a diameter d equal to the diameter of injection mouths  25  of nozzle  23 . The high pressure fluid spray removes the oxide film from the grooved surface of sleeve  5 .  
         [0031]    Nozzle  23  is positioned in the axial center of sleeve  5  and it rotates at a constant rotational speed B. Nozzle  23  moves up and down at a rate G (“the moving speed”) within sleeve  5 . Provided that the rotational speed B (rps), the moving speed G (mm/s), and the diameter d (mm) of injection mouths  25  meet the relationship G÷2B≦d then the entire bearing surface of sleeve  5  will be cleaned without any gaps. However, if the process is reciprocated, the oxide film can be effectively removed even if there are gaps.  
         [0032]    In this second embodiment, deionized pure water is used for the high-pressure liquid jet, and the diameter d of the injection mouth  25  of the nozzle  23  is set at 0.25 mm, the injection pressure is set to 700 kg/cm2, and the distance between the nozzle  23  and the work W is set at 0.5-1.0 mm. The rotational speed B is 3000 rpm (50 rps) and the moving speed G is 5 to 9 mm/s. Nozzle  23  may be reciprocated one or more times. By this means, one can efficiently remove the oxide film.  
         [0033]    As shown in the first and second embodiment, by injecting a high-pressure liquid jet onto the bearing surface, the oxide film produced in electrochemical machining can be unfailingly removed without injury to the finished surface, and other minute foreign matter such as processed chips, bearded needles, and burrs, which are produced at the time of machining, can simultaneously be completely removed. Moreover, by using deionized pure water for the high-pressure liquid jet, there is no need to wash off any cleaning agents, abrasive agents, etc., and one can simplify the cleaning process. Consequently, the above embodiments make it possible to automate the cleaning process and greatly raise productivity by automation.  
         [0034]    In addition to deionized pure water, one can also use municipal tap water, RO (reverse osmosis) water and the like for the high-pressure liquid jet, but it is preferable to use deionized deionized pure water from the standpoint of preventing blockages of impurities, scaling, corrosion, chemical reactions and the like due to the small diameter of the injection mouth of the nozzle. On the other hand, using water mixed with a surfactant and abrasive agents such as CRB ceramic powder, calcium carbonate, silicon, and high polymer resin granules as the high-pressure liquid jet aids in removing the fins, burrs and the like generated during machining.  
         [0035]    The third embodiment of the present invention is described by reference to FIGS.  6 - 8 . As shown in FIG. 6, the cleaning device  26  is provided with a sleeve-cleaning device  27  for the purpose of cleaning the sleeve  5 , a plate-cleaning device  28  for the purpose of cleaning the counter-plate  15  or the thrust-plate  13 , and a deionized pure water-recycling device  29  for the purpose of recycling the deionized pure water supplied to sleeve-cleaning device  27  and plate-cleaning device  28 .  
         [0036]    As shown in FIG. 6, cleaning device  27  includes platform  30 , on which are arranged input part  31 , flat cleaning part  32 , internal cleaning part  33 , draining part  34 , and discharge part  35 . A conveyance device  37  is provided to sequentially move jig  36 , which supports a plurality of works W (bearing surfaces that are to be cleaned), from the input part  31  to the flat cleaning part  32 , to the internal cleaning part  33 , to the draining part  34  and to the discharge part  35 .  
         [0037]    As shown in FIG. 7, the jig  36  is a roughly rectangular aluminum block, and it has multiple support holes  38  vertically formed in its support surface where the sleeves  5  (work W) are inserted and supported while attached to the base  4 . The support holes  38  are arranged in two parallel rows with six holes each at equal intervals in a rectilinear manner, for a total of twelve holes. In the jig  36  are formed an engagement depression  39  for purposes of engaging the conveyance device  37  and a positioning depression  40  for purposes of positioning.  
         [0038]    In the flat cleaning part  32 , four rotary guns  39  are attached to the feed bar  38 . The four rotary guns  39  are arranged at the front, back, right and left on the support surface of the jig  36  that is moved from the input part  31  and positioned by the conveyance device  37 . Each rotary gun  39  corresponds to the three works W on the jig  36 . Each rotary gun  39  is equipped with a nozzle (not illustrated) facing the work W, and this nozzle is connected to a high-pressure generating device (not illustrated) that supplies deionized pure water at high pressure. The rotary guns  39  are rotated at the specified rotational speed via the belt  41  by the motor  40 , and the feed bar  38  is moved at the specified feeding speed, with the result that a high-pressure liquid jet (deionized pure water) is injected by the method shown in FIG. 1 onto the work W while the nozzle is moving.  
         [0039]    The internal cleaning part  33  is provided with  12  rotary guns  42  facing the work W on the jig  36  that is moved from the flat cleaning part  32  and positioned by the conveyance device  37 . Each rotary gun  42  is equipped with a nozzle (not illustrated) possessing an injection mouth on the side face that is inserted into the work W (sleeve  5 ), and this nozzle is connected to a high-pressure generating device (not illustrated) that supplies deionized pure water at high pressure. With regard to the rotary guns  42 , three guns neighboring one another are rotated by one motor  43  via the belt  44  so that four motors  43  rotate the total of twelve rotary guns  42 . The rotary guns  42  are attached to the feed bar (not illustrated), and can be moved in the axial direction. The rotary guns  42  are rotated at the specified rotational speed, and are moved in the axial direction at the specified feeding speed, with the result that a high-pressure liquid jet (deionized pure water) is injected by the method shown in FIG. 2 onto the work W while the nozzle is moving.  
         [0040]    The draining part  34  conducts draining and accelerates drying by air-blowing the work W on the jig  36  that has been moved from the internal cleaning part  33  by the conveyance device  37 . The discharge part  35  sends the jig  36 , which has been moved from the draining part  34  by the conveyance device  37 , to the roller conveyor  45  and discharges it to the specified position by the roller conveyor  45 .  
         [0041]    The deionized pure water recycling device  29  recycles the cleaning water that has been drained from the flat cleaning part  32 , the internal cleaning part  33  and the draining part  34  after cleaning of the work, regenerates the deionized pure water level by activated carbon, ionic exchange and the like, and recirculates it to the high pressure generating device.  
         [0042]    As shown in FIG. 6 centrifugal mist treatment device  46  attracts and conducts purification treatment of the spray, mist and the like of the cleaning water produced by the flat cleaning part  32 , internal cleaning part  33  and draining part  34 . Reference number  47  is the control panel that serves to control the flat cleaning part  32 , internal cleaning part  33 , draining part  34 , conveyance device  37 , deionized pure water recycling device  29  and mist treatment device  46 .  
         [0043]    When the work W is put on the jig  36  and introduced to the input part  31 , the jig  36  is sequentially moved by the conveyance device  37 . The end face of the large bore part  14  of the sleeve  5  is cleaned by the flat cleaning part  32 , the inner periphery of the sleeve  5  is cleaned by the internal cleaning part  33 , and, after draining by the draining part  34 , discharge of the jig  36  is conducted from the discharge part  35 . In this manner, the oxide film and foreign matter produced by electrochemical machining on the bearing surface of the sleeve  5  can be automatically removed and cleaned and productivity can be greatly improved.  
         [0044]    Plate cleaning device  28  includes platform  48 , on which are arranged input part  49 , cleaning part  50 , draining part  51  and discharge part  52  in this order. The jig  53 , which holds the work W that is introduced at the input part  49 , is sequentially moved to the cleaning part  50 , draining part  51 , and discharge part  52  by the conveyance device (not illustrated).  
         [0045]    As shown in FIG. 8, jig  53  includes twenty-five support holes  54  (only the two holes at the two ends are illustrated) arranged on the support surface in one row at equal intervals into which the counter-plate is inserted and supported.  
         [0046]    The cleaning part  50  is preferably provided with only one rotary gun  55 , which is similar to the cleaning part  32  of the sleeve-cleaning device  27 . The jig  53  is moved at the specified feeding speed by the conveyance device and the nozzle (not illustrated) is rotated at the specified rotational speed. Accordingly, a high-pressure liquid jet (deionized pure water) is injected by the method shown in FIG. 2 onto the work W, which is held by jig  53  while the nozzle is moving.  
         [0047]    The draining part  51  conducts draining and accelerates drying by air-blowing the work W, which is held on the jig  53 , that has been moved from the cleaning part  50  by the conveyance device.  
         [0048]    The discharge part  52  sends the jig  53  that has been moved from the draining part  51  by the conveyance device to the roller conveyor  56  and it discharges the jig to the specified position by the roller conveyor  56 .  
         [0049]    The plate cleaning device  28  is provided with a centrifugal mist treatment device  57  that attracts and purifies the spray, mist and the like produced by the cleaning part  50  and draining part  51 . Moreover, the cleaning water that is discharged from the cleaning part  50  and draining part  51  is recycled and treated by the deionized pure water-recycling device  29  in the same way as the aforementioned sleeve-cleaning device  27 .  
         [0050]    In this third embodiment, a work W is put on jig  53  at the input part  49 , the jig  53  is sequentially moved by the conveyance device, is cleaned by the cleaning part  50 , and drained by the draining part  51 , after which discharge is conducted from the discharge part  52 . In this manner, the oxide film and foreign matter produced by electrochemical machining on the bearing surface of the counter-plate  15  can be automatically removed and cleaned.  
         [0051]    Through the use of this third embodiment, it is simultaneously possible to remove the oxide film produced by electrochemical machining of the dynamic pressure grooves without injury to the bearing surface, and also to unfailingly remove foreign matter such as fins and burrs caused by machining, thereby allowing obtainment of a clean bearing surface. By this means of applying a high-pressure liquid jet, it becomes possible to automate the cleaning process for dynamic pressure bearings, and to greatly improve productivity.  
         [0052]    Furthermore, by using deionized pure water for the high-pressure liquid jet, not only is the oxide film and foreign matter such as fins and burrs removed by the high-pressure liquid jet, but also as adequate cleaning is able to be conducted with the deionized pure water and there is no need for subsequently washing off abrasives, cleaning agents or the like, thereby, simplifying the cleaning process. Moreover, by using deionized pure water, there is no occurrence of scaling, corrosion or blockages in the high-pressure liquid jet nozzle and the high-pressure tubing resulting from the various impurities contained in municipal tap water or the like.  
         [0053]    On the other hand, it is possible to effectively remove the fins, burrs and the like produced by mechanical machining through the use of water mixed with abrasives and surfactants for the high-pressure liquid jet.  
         [0054]    For the convenience of the reader, the above description has focused on a representative sample of all possible embodiments, a sample that teaches the principles of the invention and conveys the best mode contemplated for carrying it out. The description has not attempted to exhaustively enumerate all possible variations. Other undescribed variations or modifications may be possible. For example, where multiple alternative embodiments are described, in many cases it will be possible to combine elements of different embodiments, or, to combine elements of the embodiments described here with other modifications or variations that are not expressly described. Many of those undescribed variations, modifications and variations are within the literal scope of the following claims, and others are equivalent.

Technology Category: f