Abstract:
A split shaft assembly for vibration control between multiple actuator pivots in a disk storage device. A first actuator pivot is mounted on a first shaft unit and a second actuator pivot is mounted on a second shaft unit. The second shaft unit is mated to the first shaft unit in axial alignment along a common pivot axis by a separating portion of a vibration control material, which interrupts transmission of vibrational force between the first actuator pivot and the second actuator pivot.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the manufacture of disk storage devices. More particularly, the invention is directed to an apparatus and method for vibration control between multiple actuators as are commonly used in such storage devices. 
     2. Description of the Background Art 
     Disk storage devices are well known in the industry today. Many versions exist, with the most widely used probably being magnetic disk storage devices termed “hard drives” or “fixed disk drives.” In these, one or more disks coated with a magnetic storage media are rotated and data is written to and read from the media with read/write heads pivotally mounted on actuator assemblies. Optical and other types of disk storage devices are also known or possible, and it should clearly be appreciated that the present invention may improve many embodiments of these as well, but for exemplary purposes the present discussion will primarily be directed to magnetic disk storage devices. 
     Modern disk storage devices often have a number of competing design goals. Without limitation, these may include reliability, accuracy, small size, high storage density, and high data access and transfer speeds. A key portion of a disk storage device may thus be the pivot or actuator assembly or assemblies which position read/write heads over the storage media. Traditionally, single pivot assemblies have primarily been used, but multiple pivot assembly systems are also known, and in seeking to reach various of the competing design goals the industry is now turning to multiple pivot assembly systems, particularly dual pivot assembly systems. 
     Unfortunately, aside from the obvious additional mechanical complexity, multiple pivotal actuator assemblies introduce a number of additional problems for the designers of disk storage devices. Of present interest are how they create, transfer, and are effected by vibration. Before turning to a discussion of this, however, a brief summary of the state of the prior art may by useful. 
     Vibration is a problem even in single pivot assembly systems. U.S. Pat. No. 5,930,071 by Black teaches a rubber-like material to dampen vibrations at the bearings and the shaft at which the single actuator assembly pivots. Japanese Pat. No. 2-139772 by Hidehiro teaches a single pivot assembly wherein the shaft has an elastic core into which a screw extends to hold the shaft in place. And Japanese Pat. No. 1-048271 by Hiroshi teaches a vibo-elastic material on an outer circumference to reduce vibration from the device housing effecting a single carriage. 
     In notable contrast, when the industry has turned to dual pivot assembly systems it has essentially ignored the problem vibration control, or worked around it using basic design methodologies not germane to this discussion. U.S. Pat. No. 4,544,972 by Kogure et al. and Japanese Pat. No. 62-78783 also by Kogure teach dual actuator assemblies without vibration dampening or isolation. U.S. Pat. No. 5,761,007 by Price et al. also teaches multiple actuators, and it even uses a elastometric sleeve. But this sleeve is merely part of a crash stop against which an actuator stops its pivotal motion in one direction, rather than any manner of vibration control. Thus multiple pivot assembly systems with vibrations control remain something unknown in the art. 
     As described, traditional multiple actuator designs have a dual pivot with a single shaft. Unfortunately, the conventional single shaft used provides a transmission path for vibration to travel between the respective actuators. Since the use of multiple actuators is generally a straight forward extension of the principles for dual actuators, the dual actuator case will primarily be discussed herein. 
     FIG. 1 (background art) is a side broken view of bearing assemblies for dual actuators mounted on a single shaft, as might be found in the prior art. A common shaft  1  is provided which is mounted within a disk storage device housing (not shown). Respective bearing assemblies  2 , one per actuator, are mounted on the common shaft  1 , typically spaced apart by a separation maintainer  3  (e.g., a spacer or bushing) as shown in FIG.  1 . 
     The bearing assemblies  2  each include two bearings  4  which are mounted in a sleeve  5  of the actuator (also not otherwise shown). Specifically, in the embodiment shown in FIG. 1, the bearings  4  include inner races  6  and outer races  7 . The bearings  4  depicted in FIG. 1 are ball-type bearings, but roller-types and, at least in theory, other types of bearings may also be employed. 
     As can be seen in FIG. 1, the outer races  7  of the bearings  4  are fixedly mounted in the sleeves  5  of the respective actuators, and the inner races  6  of the bearings  4  are fixedly mounted on the common shaft  1 . FIG. 1 also depicts one common arrangement, wherein the inner race  6  of the lower-most bearing  4  in the bottom bearing assembly  2  abuts against a base flange  8  of the common shaft  1 . The separation maintainer  3  then abuts against the top-most inner race  6  of the bottom bearing assembly  2  as well as against the lower-most inner race  6  of the upper bearing assembly  2 . In this manner, when the media disk in a disk storage device is oriented to revolve in a horizontal plane, the actuators are horizontally pivotally mounted and vertically fixedly mounted on the common shaft  1 . 
     Unfortunately, this arrangement provides a transmission path for vibration between the respective actuators. In FIG. 1, path arrows  9  stylistically depict the paths for vibrational force from the upper actuator into the lower actuator. When vibration occurs in the upper actuator, for instance, it may travel through the upper sleeve  5  and the bearings  4  into the common shaft  1  and the separation maintainer  3  (in embodiments where one is used), and from these into the lower bearings  4  and sleeve  5  of the lower actuator. In this manner, vibration occurring in one actuator has a continuous transmission path to any other actuators mounted on the common shaft  1 . 
     In practice, since both the upper and lower actuators move separately, vibration can be generated in both and interact complexly to effect actuator-mounted device operation, such as that of data read/write heads. It should also be appreciated that vibration inherently has time and frequency related components. Vibrational energy present at a first instant in time may be stored, somewhat, and have an effect at a later second instant in time. Vibrational energy may also be generated, transferred, and absorbed differently depending upon its frequency and its relationship to the resonant and harmonic frequencies of the physical structures which are present. 
     This can cause particularly undesirable results. For example, a common use of multiple actuators is to separate track following and seeking functions in a magnetic disk storage device such as a computer hard drive. Vibrations occurring in the seeking actuator can travel to the tracking actuator and can cause heads mounted on it to go off course. Alternately, vibrations from the tracking actuator can cause an increase in the settle time for the seeking actuator. Or vibrations created in an actuator at one instant can travel outward, elsewhere into the entire storage assembly, and be reflected back at a later time to adversely effect the operation of the same actuator. 
     The preceding is not an exhaustive list of all possible vibro-mechanical interactions, but it is enough to demonstrate that disk storage devices are quite complex structures and that designers of them do not have an easy task. If disk storage device design is to continue to evolve, using increasing numbers of mechanical subassemblies operating separately and in concert at increasing speeds, systems are sorely needed for vibration control. Accordingly, an object of the present invention is to provide apparatus and method for vibration control between multiple actuators in disk storage devices. Other objects and advantages will become apparent from the following disclosure. 
     SUMMARY OF THE INVENTION 
     The present invention relates to split shaft assemblies for vibration control between multiple actuator pivots in a disk storage device. A first actuator pivot is mounted on a first shaft unit and a second actuator pivot is mounted on a second shaft unit. The second shaft unit is mated to the first shaft unit in axial alignment along a common pivot axis by a separating portion of a vibration control material, which interrupts transmission of vibrational force between the first actuator pivot and the second actuator pivot. 
     A more through disclosure of the present invention is presented in the detailed description which follows and the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and, advantages and features of the present invention will be more clearly understood by reference to the following detailed disclosure and the accompanying drawings in which: 
     FIG. 1 (background art) is a side broken view of dual actuator bearing assemblies mounted on a conventional single common shaft; 
     FIG. 2 is a side broken view of dual actuator bearing assemblies mounted on a split shaft mated together with a boss and flange, in accordance with one embodiment of the present invention; 
     FIG. 3 is a side broken view of dual actuator bearing assemblies mounted on a split shaft mated together with an internal post, in accordance with another embodiment of the present invention; 
     FIG. 4 is a side broken view of dual actuator bearing assemblies mounted on a split shaft mated together with a spacer ring, in accordance with yet another embodiment of the present invention; 
     FIG. 5 is a side broken view of dual actuator bearing assemblies mounted on a split shaft mated together with concentric bushings, in accordance with yet another embodiment of the present invention; 
     FIG. 6 is a side broken view of dual actuator bearing assemblies mounted on a split shaft mated together with adhesively connected flanges, in accordance with yet another embodiment of the present invention; 
     FIG. 7 is a side broken view of dual actuator bearing assemblies mounted on a split outer shaft units mounted on a common inner shaft, in accordance with yet another embodiment of the present invention; 
     FIGS. 8 a-c  (prior art) are performance graphs of respective arm responses for a standard common shaft pivot assembly; and 
     FIGS. 9 a-c  are performance graphs of respective arm responses for a pivot assembly according to the embodiment of the present invention depicted in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to split shaft assemblies for vibration control between multiple actuator pivots, as may be commonly used in a disk storage device. 
     A preferred embodiment of the present invention is a split shaft assembly employing a vibration control material. As illustrated in the various drawings herein, and particularly the views of FIGS. 2-7, some exemplary embodiments of the present invention are collectively depicted by the general reference character  10 . 
     Turning now to FIG. 2, a side broken view is shown of one pivot assembly  10  according to the present invention. A split shaft assembly  12  includes a bottom shaft unit  14  and a top shaft unit  16 . The bottom shaft unit  14  includes a boss  18  and the top shaft unit  16  includes a flange  20 . The bottom shaft unit  14  and the top shaft unit  16  are mated together in male-to-female manner by insertion of the boss  18  into the flange  20 , but with a layer of vibration control material  22  preventing actual physical contact between any potions of the boss  18  and the flange  20  or directly between the bottom shaft unit  14  and the top shaft unit  16 . The extent to which the boss  18  is inserted into the flange  20 , i.e., into a hollowed out interior of the top shaft unit  16 , provides desirable stiffness to the split shaft assembly  12  as a whole in the axial and bending directions. 
     In particular, the bottom shaft unit  14 , the top shaft unit  16 , and the entire split shaft assembly  12  share a common pivot axis  24  about which essentially conventional actuator assemblies (or “pivot assemblies”) may be provided. As shown in FIG. 2, a bottom actuator assembly  26  may be mounted on the bottom shaft unit  14  and a top actuator assembly  28  may be mounted on the top shaft unit  16 . These actuator assemblies  26 ,  28  each include bearings  30  having inner races  32  and outer races  34 . The bearings  30  are further mounted into sleeves  36  of the respective actuator assemblies  26 ,  28 . 
     In the particular variation shown in FIG. 2, the bottom shaft unit  14  includes a base flange  38  against which the inner race  32  of the bottom bearing  30  in the bottom actuator assembly  26  abuts. As noted, the top shaft unit  16  includes the flange  20 , and the inner race  32  of the lower bearing  30  in the top actuator assembly  28  abuts against this. In this manner, this embodiment may dispense with the separation maintainer  3  of FIG. 1 (background art). This is often highly desirable because space between the bottom actuator assembly  26  and the top actuator assembly  28  can be very constrained. The spacing shown in FIG. 2 is somewhat exaggerated compared to what will be the case in many embodiments, with the flange  20  shown for illustrative purposes as being much thicker than it typically needs to be. 
     The vibration control material  22  plays a particularly important role in all embodiments of the pivot assembly  10 . It may be selected for its ability to isolate the bottom shaft unit  14  and the top shaft unit  16  from vibrations, or it may be selected for its ability to dampen passing vibrations, or it may selected to provide varying degrees of isolation and dampening concurrently. 
     Without limitation, some representative examples of materials for use as the vibration control material  22  are urethanes, particularly moldable ones, and acrylics. The urethanes, and other synthetic “rubbers,” can be particularly stiff and have useful isolating characteristics, while acrylics can have useful dampening characteristics. 
     One interesting material is epoxy. Not all epoxies are stiff or harden to brittleness, particularly at the typical operating temperatures in disk storage devices (e.g., 65 degrees centigrade). Thus, essentially all polymers have some potential for use as the vibration control material  22 . 
     The vibration control material  22  in FIG. 2 has been described above as a “layer” and is shown filling the entire region between the boss  18  and the flange  20  portions of the bottom shaft unit  14  and the top shaft unit  16 . This will likely be the case in most embodiments, but it should be appreciated that these are not requirements. The vibration control material  22  separates the bottom shaft unit  14  and the top shaft unit  16 , but its shape and the quantity used can vary. 
     Turning now to FIG. 3, a side broken view of a different pivot assembly  10  is depicted there. A split shaft assembly  52  is provided which includes a bottom shaft unit  54 , a top shaft unit  56 , and a post  58 . Both the bottom shaft unit  54  and the top shaft unit  56  have recesses  60  suitable for receiving one of respective ends  62  of the post  58 . The post  58  is prevented from actual physical contact with the shaft units  54 ,  56  by a layer of vibration control material  64  (which may be essentially the same as that described for the embodiment in FIG.  2 ). 
     In FIG. 3 the shaft units  54 ,  56  are both depicted as being hollow. This is not a requirement but may be motivated by the same reasons that conventional single, common shafts are usually hollow, to save material and to reduce weight. Being hollow here, however, also conviently provides the recesses  60 . 
     While using the post  58  alone may serve to provide the split shaft assembly  52  with adequate stiffness, FIG. 3 also shows how optional bushings may increase these characteristics and provide other benefits. 
     A lower bushing  66  may be provided at the upper end of the bottom shaft unit  54  and an upper bushing  68  may be provided at the lower end of the top shaft unit  56 . These may be press fit on or they may be loose, and the upper bushing  68  may even be made an integral part of the top shaft unit  56 . These bushings  66 ,  68  may also abut against inner races  70  of bearings  72  in sleeves  74  of a bottom actuator assembly  76  and a top actuator assembly  78 , although this is not a requirement when bushings are used. However, as shown, vibration control material  64  is provided to separate such bushings  66 ,  68  when they are present. This vibration control material  64  may be the same as that used at the post  58  or it may be different. This is a matter of design choice. But, for example, it may be a useful way to control two particular different sets of vibration frequencies concurrently. 
     Much as was the case for FIG. 2, the vibration control material  64  can be a layer and fill entire regions or it may be shaped differently and used more sparingly. The thickness of the bushings  66 ,  68  shown in FIG. 3 is also somewhat exaggerated for illustrative purposes compared to that which is likely to be necessary. 
     In summary, the versions of this embodiment, with or without the use of the bushings  66 ,  68 , provides adequate stiffness in the axial and bending directions and facilitates maintaining a common pivot axis  80 . 
     Turning now to FIG. 4, it is a side broken view of yet a different pivot assembly  10 . A split shaft assembly  102  is provided which includes a bottom shaft unit  104 , a top shaft unit  106 , and a spacer  108  which is made of a vibration control material. 
     The bottom shaft unit  104  has an upper end  110  which extends past an inner race  112  of an upper bearing  114  in a sleeve  116  of a bottom actuator assembly  118 . The top shaft unit  106  has a lower end  120  which extends past the inner race  112  of a lower bearing  114  in the sleeve  116  of a top actuator assembly  122 . 
     The spacer  108  has a coaxial top opening  124  and bottom opening  126 . These may be part of one common bore, as shown, or they simply may separate recesses. The upper end  110  of the bottom shaft unit  104  nests into the bottom opening  126  of the spacer  108  and the lower end  120  of the top shaft unit  106  nests into the top opening  124  of the spacer  108 . The bottom of the spacer  108  abuts against the inner race  112  of the upper bearing  114  in the bottom actuator assembly  118 . The top of the spacer  108  may simply abut against the inner race  112  of the upper bearing  114  in the top actuator assembly  122 . Alternately, as shown in FIG. 4, an optional flange  128  may be provided near the lower end  120  of the top shaft unit  106  and the spacer  108  may abut against that flange  128 . 
     FIG. 5 is a side broken view of still a different pivot assembly  10 . A split shaft assembly  152  is provided which includes a bottom shaft unit  154  and a top shaft unit  156 . When assembled, the split shaft assembly  152  and many of its components share a common pivot axis  158 . A top end  160  of the bottom shaft unit  154  has a first concentric bushing  162  and a bottom end  164  of the top shaft unit  156  has a second concentric bushing  166  which is axially offset differently than the first concentric bushing  162 . The bottom shaft unit  154  and the top shaft unit  156  are assembled into the split shaft assembly  152  by inter-nestingly engaging the concentric bushings  162 ,  166  with a separating layer of vibration control material  168 . Optionally, as shown, the vibration control material  168  may also fill a gap  170  present between the top end  160  of the bottom shaft unit  154  and the bottom end  164  of the top shaft unit  156 . 
     FIG. 6 is a side broken view of another pivot assembly  10 . A split shaft assembly  202  is provided which includes a bottom shaft unit  204  and a top shaft unit  206 , all having a common pivot axis  208  when assembled. The shaft units  204 ,  206  each have a respective flange  210  at one end and may, as shown, be the same part but oriented differently when assembled. One benefit of using exactly the same part in this manner is reducing the variety of parts which must be stocked, and this the potential cost of disk storage units. 
     The inner races  212  of bearings  214  in actuator assemblies  216  abut against the flanges  210  on one side, and the opposite sides of the flanges  210  are engaged by a layer of vibration control material  218  which has adhesive properties in addition to vibration control properties. In this manner, the split shaft assembly  202  as a whole has desired stiffness in the axial and bending directions and maintains the common pivot axis  208 . 
     FIG. 7 is a side broken view of yet another pivot assembly  10 . A split shaft assembly  252  is provided here which includes a hollow bottom shaft unit  254 , a hollow top shaft unit  256 , and a common inner shaft  258 , again all having a common pivot axis  260  when assembled. The shaft units  254 ,  256  are mounted on the common inner shaft  258 , but separated from direct contact with it by a vibration control material  262 . 
     Conceptually, the embodiment of FIG. 7 may be viewed as a version of the embodiment of FIG. 3 wherein the post  58  is taken to an extreme to become the common inner shaft  258 . As was the case in FIG. 3, where optional bushings  66 ,  68  where shown, the embodiment in FIG. 7 may optionally also employ bushings  264  to yet further provide desired stiffness in the axial and bending directions and to maintain the common pivot axis  260 . 
     FIGS. 8 a-c  (prior art) and FIGS. 9 a-c  are performance graphs of arm response for both a standard common shaft pivot assembly the pivot assembly  10  of FIG.  6 . Upper and lower range peaks are particularly noted in each graph. 
     In FIGS. 8 a  and  9   a  the graphs depict the response at the top arm on a bottom actuator when the bottom actuator is the excitation source. In the lower range, the prior art system peaks at 3.49 kHz and 136.0 dB, while the inventive pivot assembly  10  peaks at 2.72 kHz and 132.0 dB. In the higher range, the prior art system peaks at 7.18 kHz and 139.8 dB, while the inventive pivot assembly  10  peaks at 7.17 kHz and 145.5 dB. Two particular conclusions can be drawn here. 
     Firstly, there are differences in the peak frequencies and these may be beneficially employed. This may not be immediately appreciated by those used to dealing with prior art systems, since peak frequencies in such are dependent on the rigid parts used and any degree of control is considerably harder to accomplish. However, in the pivot assembly  10  the choice and application of the vibration control material  218  can easily be used to specifically control peak frequencies, e.g., to avoid resonant or harmonic frequencies. 
     Secondly, the peak amplitudes of the respective systems are notably different. In the lower range, the pivot assembly  10  has a clear 4 dB advantage. In the higher range, however, the peak values taken alone can be deceptive. While the pivot assembly  10  might appear to suffer a 5.7 dB disadvantage, the graph in FIG. 9 a  is much smoother and the argument can be made that the numbers in FIG. 8 a  are not actually those of the true high range peak (if one notes the spike at 9 kHz). Based on the overall performances depicted, most designers would prefer that depicted in FIG. 9 a  over that in FIG. 8 a.    
     In FIGS. 8 b  and  9   b  the graphs depict responses at the bottom arm of the top actuator when the bottom actuator is again the excitation source. In both the lower and upper ranges, the inventive pivot assembly  10  exhibits striking 13.5 dB advantages, as well as smoother overall response curves. 
     In FIGS. 8 c  and  9   c  the graphs depict responses at the top arm of the top actuator when the bottom actuator is yet again the excitation source. In the respective lower and upper ranges, the pivot assembly  10  exhibit clear 9.9 and 3.3 dB advantages. 
     In sum, as FIGS. 8 a-c  and  9   a-c  demonstrate, the inventive pivot assembly  10  has preferable performance criteria over the prior art system and its prompt acceptance and use by the industry can be anticipated. 
     Although this invention has been described with respect to specific embodiments, the details thereof are not to be construed as limitations, for it will be apparent that various embodiments, changes and modifications may be resorted to without departing from the spirit and scope thereof; and it is understood that such equivalent embodiments are intended to be included within the scope of this invention.