Squeeze film damping for a hard disc drive

Details Squeeze film damping for a hard disc drive Squeeze film damping for a hard disc drive

1996-10-08

2001-05-29

Letscher, George J. (Department: 2754)

Dynamic magnetic information storage or retrieval

Record transport with head stationary during transducing

Disk record

Reexamination Certificate

active

06239943

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to a hard disc drive support system, and more specifically to means for damping out vibrations in a rotating disc utilized in a hard disc drive.
The present invention relates to mounting of a rotating discs or disc so that the disc rotates in a very stable fashion without vibration or the like.
BACKGROUND OF THE INVENTION
With the development of hard disc drives having a high storage density, demand has been created for improvements in the performance of the spindle motors and mounting for the disc. High storage density is achieved with reduced width and spacing of the circular tracks holding the stored information. Any vibration, wobbling or deformation of the disc as it rotates will create a temporary misalignment between the read/write transducer and the circular track of stored information. Such misalignment may lead to read/write errors. Hence, high storage density depends, among other characteristics, on damping of any disc movement and the stiffness of the mounting which positions the rotating disc relative to the transducer.
Most current disc drive designs have very small vibrational damping characteristics. This is a problem because vibration can easily be induced into a mechanical system. Any mechanical system will have a series of natural frequencies, i.e. frequencies at which the system prefers to resonate. Each of these frequencies has a corresponding mode shape. In standard disc drives there are series of these natural frequencies related to the vibration of the discs. These frequencies, and their corresponding mode shapes can be predicted by mathematical modeling; the existence of these resonance modes has been verified by experimental testing. For example,
FIGS. 1A and 1B
are plots of the vibration measured on a spinning disc as a function of frequency with a white noise excitation. These plots will be discussed in further detail below. However, these plots clearly show both forward and backward gyro mode (see FIG.
1
A), that is frequencies at which the discs vibrate back and forth like a seesaw with one rotating diameter as a stationary node. These plots also show an axial vibration of the disc (see FIG.
1
B). At this frequency the discs vibrate up and down in an axisymmetric umbrella shape.
Vibration in either of these modes would obviously have the potential for causing significant misalignment between datatrack and transducer. The problem has become more significant with the use of thinner discs. Thinner discs are less stiff and result in lower resonance frequencies as well as in increased amplitude of deformation or distortion of the disc.
Some disc drive designs in the past have utilized a dampening material in the base or top cover to help improve acoustics. However, this sort of damping, while improving acoustics, does very little to stabilize the rotor dynamic system of motor and discs. Moreover, it is well known that the ball bearings used to mount most spindle motors have very low damping characteristics.
If sufficient damping could be introduced into the vibrational system of motor and discs, several problems could be improved: operational vibration, shock resistance, rotor dynamic stability problems, and parametric stability problems (e.g. stiffness dependent on angular position of rotor related to ball bearing misalignment).
SUMMARY OF THE INVENTION
In view of the above described circumstances, it is an object of the present invention to provide a spindle motor which is designed so that the rotational stability during high speed rotation is enhanced, and vibrations are significantly damped out.
To this end, the present invention provides a spindle motor having a motor stator, a motor rotor, the motor being energized to cause rotation of the rotor supporting a single disc or a stack of discs which is mounted in such a way that the rotating bottom or top (or both) disc surface is closely adjacent to a stationary disc drive casting surface. The squeeze film action in the remaining air gap provides significant damping of the disc vibration. Typical implementation use air gaps of 0.004″-0.006″ for 2½″ drives and 0.006″-0.010″ for 3½″ drives.
It should be noted that squeeze film damping has been used in industrial turbo machines as well as aircraft engines in the past. However, in these applications the damper is incorporated into the bearing support as shown in the following
FIGS. 2A
,
2
B,
2
C.
FIG. 2A
illustrates a squeeze film bearing damper as applied to a rolling element bearing in an aircraft turbojet engine. A clearance space (typically 0.005-0.0010″) is provided around the outer race and supplied with oil. The outer race is pinned or keyed to prevent rotation but is allowed to orbit.
FIG. 2B
illustrates a similar hydrodynamic bearing where oil separates a shaft which is free to whirl and rotate inside a housing.
FIG. 2C
illustrates an alternative damper, where the outer race of a rolling element bearing or the bearing segment of a hydrodynamic bearing is mounted in the inner element of the damper. A fluid oil film separates the inner and outer damper elements. Dampers are generally supplied with oil at the center either through holes or through a central supply groove. Exit leakage at the ends is controlled by end seals which can be either an o-ring or piston ring design. Thus, all of these design approaches are distinctly different from the approach to be disclosed and claimed herein. Due to the small size of disc drives, squeeze film dampers of the type shown in these figures is not possible. Even if a squeeze film damper could be mounted at the bearing support, the level of damping it would provide would be small. In order for a damper to work efficiently it needs to be placed in a location where the operational levels would otherwise be large. In a disc drive, this location is on the disc surface themselves. Thus, the method of the present invention is to introduce damping directly to the disc surface. In addition, typical squeeze film dampers of the type shown in
FIGS. 2A
,
2
B and
2
C are supplied with oil; this concept for disc drives utilizes air.


REFERENCES:
patent: 4268878 (1981-05-01), Kearns
patent: 4581668 (1986-04-01), Campbell
patent: 4583213 (1986-04-01), Bracken et al.
patent: 5235482 (1993-08-01), Schmitz
patent: 5282100 (1994-01-01), Tacklind et al.
patent: 54-154310 (1979-12-01), None
patent: 56-137559 (1981-10-01), None
patent: 4289577 (1992-10-01), None
patent: 5-234327 (1993-09-01), None

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