Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-05-18
2004-06-22
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S254100, C310S256000, C310S090000, C029S596000
Reexamination Certificate
active
06753636
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
In the computer industry, hard disk drives have been used for many years for the permanent storage of data.
2. Description of Related Art
The need and thus the demand for more memory capacity and shorter access times on less memory space will necessarily result in a higher track density at a respectively higher speed. Higher track densities, in turn, can be achieved only with smaller track pitches and narrower data tracks.
With a track density of 25,000 tracks per inch (corresponding to a track pitch of approx. 1 &mgr;m) with exponentially increasing tendency and a number of revolutions of 7,200 with linear increasing tendency, the requirements for the running accuracy of the spindle motors will also increase.
The drive motor, comprising a rotor with permanent magnet, a stator package with windings disposed on a base plate, a shaft, which is firmly connected with the rotor or the base plate, and a bearing system with at least one set of rolling bodies, causes the memory disk(s) disposed on the rotor to rotate.
In order to keep the effect of the unavoidable defects of form and component tolerances of the rolling bodies, the inner raceways and outer rings of the bearings on the running accuracy of the motor as low as possible it is common practice to brace the sets of rolling bearings against each other.
As a result, the bearing components rotating relative to each other are elastically deformed at their common places of contact. Consequently, each individual rolling body ball can be viewed as a system of two successively connected equivalent springs.
External forces in this spring-mass system lead the rotor to make evading motions or to wobble. As a result of an unfavorable frequency spectrum of the external forces the system can be caused to oscillate and to oscillate at a natural frequency. Depending on the system, the attenuation in this type of system is very low, so that the amplitudes of such oscillation may reach unacceptable values which could then lead to read/write errors.
Therefore, in order to further increase the running accuracy the external forces acting on the rotating system have to be minimized.
Said external forces are caused by electromagnetic interactions between the multipolar magnetized permanent magnet mounted in the rotor and the stator package. A sequential supply of current to the windings disposed on the stator package produces a magnetic traveling field that generates the torque as a result of the interaction with the individual poles of the ring-shaped permanent magnet, which torque drives the rotor. At the same time, forces acting radially on the rotor are generated.
Defects of concentricity and form can disturb the symmetry of said radial forces so that rotating or stationary residual forces can be generated which could cause the rotating system to oscillate and thus impair the running accuracy of the motor.
As a result of defects of form or position of the multipolar magnetized permanent magnet, which is manufactured by means of pressing and/or sintering and which is preferably plastic bonded and mounted in the rotor, and by positional errors (coaxiality errors, concentricity errors) of the stator package which is accommodated and positioned on the inside diameter via a collar-shaped projection in the base plate, the operating air gap of the motor is neither constant with regard to the width of the gap nor is it oriented concentric or coaxial relative to the axis of rotation. As a result the width of the gap also changes with the relative angular position between stator and rotor.
However, in order for the geometric sum of all the radial force components from the electromagnetic circuit acting on the rotor to be zero the width of the gap must be constant over the full circumference and independent of the angular position between rotor and stator.
In other words: the higher the positional errors of the stator and the higher the fluctuations in the gap width of the operating air gap that act on the circumference, the higher are the radial force components acting on the rotor in practical application.
This is where the invention comes into play with the objective of reducing both the deviation in the concentricity or coaxiality of the operating air gap with regard to the axis of rotation and the fluctuations in the gap width of the operating air gap over the circumference so as to minimize the resulting radial components of the external forces acting on the rotor.
Based on JP 10-248223 (1998), it is already known to center the stator with a mounting sleeve where the stator is pressed into the mounting sleeve and the mounting sleeve, in turn, is mounted on the stationary shaft. Said mounting is also achieved either with a press fit or by adhesive bonding.
This known embodiment, however, does not ensure that the required coaxiality of the operating air gap is achieved because the connection of the stator with the mounting sleeve is highly subject to tolerances which may cause a slanting position, for example, and the operating air gap is then no longer coaxial.
Another disadvantage of the above embodiment is that because the mounting sleeve is manufactured separately, the respective fitting surfaces of the mounting sleeve cannot be made with adequate accuracy and that a respective installation play will cause a mismatch when the individual parts of the spindle motor are assembled. As a result, the width of the operating air gap will vary in size and it is also dependent on the relative angular position between rotor and stator.
The same criticism applies to U.S. Pat. No. 5,925,946 which also provides a separately manufactured mounting sleeve which is connected with the stator so as not to rotate, which mounting sleeve, in turn, abuts on a vertically formed collar of the stationary base plate. A further disadvantage is that the centering does not take place on the actual relevant part, i.e. the stationary shaft, but only indirectly on the base plate which is connected with the stationary shaft.
Again, the potential mismatch could impair the coaxiality of the operating air gap.
BRIEF SUMMARY OF THE INVENTION
Therefore, to solve the above problem the invention according to a first embodiment provides that the sheet iron package or the complete stator, consisting of sheet iron package and winding, in accordance with the invention is now encompassed by means of injecting a centering casing, which centering casing consists of plastic and where the centering casing either—in case of a stationary shaft—is centered directly on the stationary shaft, or—in case of a rotating shaft—it is centered on the outer ball bearing ring of the rolling bearing.
With the given technical theory of using a sheet metal package or a stator encompassed by means of injecting plastic which forms a centering casing that directly joins the stationary part thus offers the substantial advantage of minimizing the mismatch of the operating air gap, i.e. reducing it to a constant very low value. Normally, the size of such air gaps is 0.1 to 0.3 mm and applying the measures of the invention it is now possible for the first time to substantially minimize the tolerances in relation to the width of the air gap, which were previously viewed as restricting the function, so as to optimize the constancy of the air gap. It was possible to achieve a reduction of up to 3% in the mismatch which was caused previously according to the state of the art by dimensional tolerances.
The reason for this success, among others, is an injected centering casing which encompasses the sheet iron package and/or the stator. Using such an injected part has the advantage that the injection mould can be manufactured with high precision resulting in highly accurate, concentric and exact sleeve, supporting and positioning surfaces where the stator can be joined, centered and encompassed by injection which was not the case with the above mentioned separate mounting sleeves pertaining to the state of the art.
It is important that the sheet metal package, or the comple
Hoffmann Jörg
Rehm Thilo
Baker & Daniels
Elkassabgi Heba
Mullins Burton S.
Precision Motors Deutsche Minebea GmbH
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