Apparatus and method for testing thrust bearings

Measuring and testing – Frictional resistance – coefficient or characteristics

Reexamination Certificate

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Reexamination Certificate

active

06199425

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates in general to hydrodynamic bearings and in particular to an apparatus and method for testing a hydrodynamic thrust bearing in a disk drive spindle motor.
2. Background Art
In hydrodynamic lubrication theory, it is well known that the coefficient of friction is velocity dependent. This relationship is known as the Stribeck curve (FIG.
1
). For example, when the rotational speed of a fluid hydrodynamic bearing (FDB) is low, the friction torque is high due to solid contact between the elements of the bearing. This is the “boundary lubrication” area indicated in FIG.
1
. However, as the rotational speed is increased, a fluid film begins to form which decreases friction. Since the fluid film is not fully developed at this stage, the contact between the components is intermittent. This second stage is known as “mixed lubrication.” In the final stage, the bearing has a critical or “take off” speed where the fluid film becomes fully developed and friction is at a minimum. This last stage is known as “full fluid lubrication.” Thereafter, friction gradually increases as a linear function of viscous drag.
The take off speed
10
of one type of journal bearing in a disk drive spindle motor is illustrated in FIG.
2
. Take off speed is a function of bearing design, surface roughness, sufficient oil, excessive tilt, and external load. If the take off speed for a particular bearing is higher than its design level, a defect of some sort is present in the assembly. Thus, bearings may be tested in this manner in the assembly line to assure their quality prior to assembly into the end product.
In the prior art, an apparatus for testing hydrodynamic rotary or journal bearings has been developed. In this apparatus (FIG.
3
), an object such as a disk drive spindle motor
11
containing journal bearings
13
and a thrust bearing
15
, has a coaxial drive shaft
17
that is rigidly mounted to a stationary torsion transducer
21
for rotation therewith. A load motor
23
rotates the motor
11
relative to the shaft
17
by applying a transverse or side load to spindle motor
11
through a flexible belt
25
. Load motor
23
is mounted to a linear stage
27
which generates selected radial load forces from load motor
23
to spindle motor
11
. A load cell
29
monitors the load exerted by linear stage
27
. Unfortunately, this method is only capable of testing the take off speed of journal bearings
13
, not that of the thrust bearing
15
. Thus, a method and apparatus for testing the take off speed of a thrust bearing is needed.
SUMMARY OF THE INVENTION
A device for testing the take off speed of a thrust bearing in a disk drive spindle motor is disclosed. The device has a stationary torsion transducer that is used to support a drive shaft of the spindle motor. A load motor located adjacent to the transducer rotates the motor about the shaft while applying a radial load to the spindle motor through a belt. The load motor is attached to a linear stage which generates selected radial loads. In one version, the device also uses a ring magnet to attract the housing of spindle motor. The magnetic attraction imposes an axial force on the spindle motor and an equal reactive load on the thrust bearing. The axial load is such that the take off speed for the thrust bearing is much greater than that for the journal bearings. Under these conditions, the friction torque is dominated by the thrust bearing so that an accurate assessment of its quality may be assessed. As an alternative to the magnet, air jets may be used to apply a similar axial load to the thrust bearing.


REFERENCES:
patent: 3023604 (1962-03-01), Gordon et al.
patent: 3685342 (1972-08-01), Gordon
patent: 4493514 (1985-01-01), Henry, IV
patent: 4763508 (1988-08-01), Buck
patent: 5311763 (1994-05-01), Gibbs, Jr. et al.
patent: 1515088 (1989-10-01), None

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