Dynamic magnetic information storage or retrieval – Record transport with head stationary during transducing – Disk record
Reexamination Certificate
1998-09-22
2002-06-18
Klimowicz, William J (Department: 2652)
Dynamic magnetic information storage or retrieval
Record transport with head stationary during transducing
Disk record
Reexamination Certificate
active
06407879
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a cover for a hard disk drive which dampens rocking motion of the spindle motor, reduces the motion of the recording head relative to the disk, decreases the transmission of mechanical vibration from the spindle motor to the top cover and reduces acoustical emissions.
BACKGROUND OF THE INVENTION
Hard disk drives are used in personal computer applications for the high volume storage of data. These drives contain a disk assembly and a head arrangement for transferring data to and from tracks disposed concentrically on one or more disk surfaces. The disks are mounted to a bearing spindle hub which is rotatable around an inner stationary shaft. A motor is typically mounted within or beneath the hub and rotates the disks and hub.
Three modes of vibration may occur in a spinning disk and hub assembly. The first mode of vibration for the disk and hub is in a radial direction relative to the spindle shaft. The second mode of vibration for the disk and hub is in an axial direction relative to the spindle shaft. The third mode of vibration is a rocking displacement of the disk and hub relative to the spindle shaft. Consequently, vibrational energy transmitted to the spindle-disk assembly may cause servo systems errors and track misregistration, thereby decreasing drive performance.
The vibrations occur for several reasons. A first reason is a result of spindle generated vibrations from ball bearing defects as the bearing spins during operation. A ball bearing is not perfectly spherical and generally contains some defect such as a flat spot, crevice or the like. Consequently, the movement of the spindle as the bearing passes each defect produces an excitation which generates vibration in a spindle. With several bearings defect frequencies associated with each spindle speed, a multitude of ball bearing excitation frequencies will produce vibrations in any given spindle design. A second reason is a result of environmental vibrations or shock. Sources of environmental induced vibrations include but are not limited to physical jarring of the disk drive installed in a computer, or any movement to a computer. A third reason is a result of vertical diaphragm vibrations of the head-disk assembly transferred to the spindle-disk assembly. The diaphragm mode vibrations are vertical drum-like deformations of the top cover and bottom base plate of a head disk assembly enclosure.
In addition to the foregoing described vibrations, every given spindle structure inherently has an upper and lower rocking mode as a result of its design in combination with manufacturing tolerances among its component parts, including the structural stiffness of the disk drive housing. Thus, any given spindle will exhibit specific upper and lower natural rocking resonances, which resonance or frequency will change depending upon the number of disks supported by the spindle, as well as the rotational speed of the disks. Moreover, the rocking resonance can also be excited by the bearing defect frequency (e.g., the number of cycles or number of regular passes of a defected bearing portion in a given amount of time) if the frequency of the natural rocking mode and the bearing defect frequency are close to each other. Just like every manufactured spindle is unique, every bearing design has a unique set of bearing defect frequencies based on the geometry of the bearing and the speed at which the bearings spin. As a result, an abnormally large radial vibration of the spindle will be produced when the bearing defect frequency is the same as or close to the natural rocking mode or frequency of the spindle and disk combination, i.e., the upper and lower rocking modes.
The rocking resonance of the spindle and the overall vibration induced into the spindle-disk combination is further affected by the stiffness of the disk drive housing. Typically, the spindle is positioned between the base plate and cover of the disk drive housing. The stiffness and damping of the base plate and cover can alter and/or dampen the natural rocking resonance of the spindle and the overall vibration of the spindle-disk combination. Disk drives with rigid shafts mounted to rigid housings offer minimum damping to attenuate the effects of spindle rocking mode and vertical diaphragm mode resonances caused by environmental shocks and vibrations, and spindle generated excitations from bearing defects, nor does such rigidity shift or alter the natural rocking resonance away from the bearing defect frequency.
The spindle-disk assembly structure also has a significant effect on the amplitude of vibrations resulting from spindle rocking mode and vertical diaphragm mode resonant frequencies. The amplitude of these vibrations is directly associated with drive performance. Undamped structures exhibit vibrations of higher amplitude at their resonant frequencies, compared to equivalent structures which contain damping, and thus are more likely to effect servo positioning and track registration. Consequently, for a given vibrational input from a source such as spindle bearing defects, an undamped and rigid disk drive housing containing the spindle-disk assembly will produce larger amplitude vibrations in the spindle at its resonant frequency than an equivalent disk drive housing containing damping.
Interaction between the rocking mode frequency and the bearing defect frequency creates a large non repetitive run out (e.g., movement of the spindle-disk in the radial direction) in the spindle causing large amounts of repetitive run out if it occurs during servo write, and large amounts of non repetitive run out if it occurs when the drive is in operation. These results will cause increased position error between the recording head and the data tracks resulting in reduced track following capability. This, in turn, can lead to servo system failure resulting in increased track misregistration, perhaps to the point of producing a non-functional drive, and/or, at a minimum generation of acoustic noise.
Where a common spindle structure is utilized for drives containing, for example, from one to four disks, and operated at two different speeds, there are 16 upper and lower dynamic rocking mode frequencies, and two distinct sets of defect frequencies, creating a large number of interactions between the rocking mode and bearing defect frequencies which need to be avoided. With different spindle vendors using different bearings with different geometries, there exists numerous additional opportunities for the bearing defect frequency and rocking mode frequency interactions to occur. Thus, given the variety of bearing geometries and spindle designs available, and the need to keep the structural stiffliess of the drive housing high for external shock and vibration considerations, it is virtually impossible to avoid all frequency interactions with a single common spindle design. Thus, disk drive manufacturers are required to inventory a large variety of different spindles, to accommodate multiple disk drive platforms and performance standards in their overall product line.
It would be useful to find a way to modify the structural stiffness of a spindle to avoid or dampen generating vibrations due to natural rocking resonance and/or interactions of bearing defect frequencies, to shift the frequencies exhibited by natural rocking modes to prevent overlap with bearing defect frequencies, to accommodate differing effects resulting from varying the number of disks supported by a spindle, variations in the rotational speed of a spindle and various types of bearings, and to avoid and/or dampen the affect of vibration overall. One way is to modify the internal construction of the spindle to change the stiffness and thereby alter the rocking mode resonance to a non-interacting frequency. U.S. Pat. No. 5,483,397 to Gifford et al. discloses first and second viscoelastic dampers effective in attenuating vibrations during operation of a disk drive for improved disk drive performance. The first viscoelastic damper is inserted on top of the spindle s
Fruge′ Tave J.
Hudson Andrew J.
Sandor Mathew J.
Schneider Kris D.
Klimowicz William J
Maxtor Corporation
Sigmond David M.
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