Disk storage device having a spindle driving motor

The invention relates to a spindle driving motor, in particular an external rotor motor, the main components of which are accommodated in the interior of a driving hub (4) or the back iron (5). According to the invention, in order to minimize the number of joints, both ball bearings (11, 12) are arranged directly, yet correctly as far as assembly is concerned, between the fixed shaft (10) and the back iron (5), a collar (15) having a larger diameter serving in particular to this end.

The invention relates to a spindle driving motor having the features of the 
pre-characterising part of claim 1. 
A spindle driving motor for disc memory drives of this kind is already 
known from DE-A 38 18 994. In the disc memory drive described there (see 
in particular FIGS. 3, 5 and 6), the annular back iron is cup-shaped, the 
upper section having a smaller internal diameter being supported directly 
on what is referred to as a "fixed shaft or axle" by means of a ball 
bearing, while the lower section having a larger diameter is supported on 
a ball bearing sitting on the shaft by means of a spacer ring. It is 
impossible to reduce the diameter of the annular back iron in the lower 
region as well in order to avoid the need for the spacer ring, as it would 
then be impossible to assemble the spindle driving motor, in particular 
the fixed internal stator. However, these additional joints present as a 
result of the spacer ring result in uneven, imprecise running of the 
driving hub, this leading to problems, particularly in the case of disc 
memory drives with a high storage density. Running faults include, in 
particular, non-repeatable-run-out and thermal-run-out faults. 
The object of this invention is to reduce these inaccuracies during 
operation and to increase the precision of spindle driving motors of this 
kind, particularly in the case of disc memory drives with a high storage 
density. A long service life of the bearings of the spindle driving motor 
is simultaneously aimed for. 
This problem is solved according to the invention in the case of a spindle 
driving motor of the type described at the outset by the features of the 
characterising part of claim 1. 
The invention departs from the knowledge that the entire spindle driving 
motor must have a uniform material structure, consisting of the fixed 
shaft or axle, the ball bearings and the back iron, the number of joints 
being reduced to a minimum. However, it must be borne in mind in this 
connection that the spindle driving motor must still be designed with high 
precision, yet correctly as far as assembly is concerned. 
One advantageous embodiment is characterised in that the annular back iron 
has a can-shaped narrowed portion at the second end of the driving hub and 
one of the ball bearings is arranged at this end directly between the 
narrowed portion and the shaft, and that the fixed shaft is provided with 
an integrally moulded collar on which the other larger ball bearing is 
directly arranged, on which the corresponding section of the annular back 
iron is supported. 
This design allows for simple assembly. The increase in the size of the 
(lower) ball bearing simultaneously increases the virtual distance between 
the bearings of the spindle driving motor, resulting in a higher 
load-bearing capacity and/or lengthening of the service life or allowing 
for lower preloading of the bearings with identical rigidity or allowing 
greater rigidity to be achieved with identical preloading of the bearings. 
The collar integrally moulded with the fixed shaft is preferably designed 
in the form of a shaft flange, this improving the securing possibilities 
of the shaft. In such a solution, the connecting wires of the external 
rotor motor are preferably brought out through a bore in the collar at the 
first end of the driving hub. This prevents weakening of the fixed shaft 
by bores for the passage of the connecting wires, as was the case in the 
prior art. Moreover, this facilitates assembly of the connecting wires. 
Another advantageous embodiment is characterised in that the fixed shaft 
has a section of reduced diameter between the ball bearings on either side 
in order to receive the internal stator and that the internal stator is 
composed of at least two segments which are inserted radially into this 
section and are secured, e.g. by gluing. 
This method of radial assembly of the internal stator divided into segments 
allows for freedom of the external diameter of the fixed shaft, in order 
in this manner to accommodate both ball bearings directly between the 
fixed shaft and the back iron in order to minimise the number of joints. 
The divided design of the stator and the gluing of the individual sectors 
to the shaft result in the further advantage of further vibration damping 
of the entire arrangement if an appropriate adhesive is used. 
In order to further reduce the number of joints, it is also possible for 
the inner ring of at least one of the ball bearings to be integrally 
formed by the fixed shaft (or the integral collar thereof) and/or for the 
outer ring of at least one of the ball bearings to be integrally formed by 
the back iron, i.e. for the corresponding ball races to be formed in these 
parts. 
The invention can be applied to spindle driving motors of any desired type, 
e.g. with a.c. motors or d.c. motors. However, spindle driving motors of 
this kind are preferably provided with an external rotor motor in the form 
of a brushless d.c. motor with a permanent-magnetic external rotor. 
Several embodiments of the invention will now be described with reference 
to the accompanying drawings. In the drawings, the left and right halves 
show partially different solutions, this being indicated by the suffixes 
"a" and "b".

The prior art will first be described with reference to FIG. 1a (left half 
of the drawing), although reference will be made to similar components in 
the right part of the drawing (FIG. 1b), which shows a first embodiment of 
the invention. 
A disc memory drive includes a housing 27 enclosing a clean room 17, at 
least one magnetic hard storage disk 18 located in the clean room, at 
least one read/write head 19 mounted in the clean room 17 for reading and 
writing data on the storage disk 18 and a brushless dc motor. 
An internal stator 1 having excitation coils 16 is secured to an (in the 
drawing) vertical fixed shaft 10. The entire spindle driving motor is 
secured by the fixed shaft 10 by means of flange parts (as shown in FIGS. 
1a and 1b) in a housing of a disc memory drive or the like. An external 
rotor 2 consisting essentially of rotor magnets and a back iron 5 is 
provided outside the internal stator 1. A driving hub 4 on which memory 
discs are supported in the case of a disc memory drive is secured outside 
the back iron 5. However, it is also possible to form the driving hub by 
means of the annular back iron 5 itself. 
As shown in FIG. 1a, the annular back iron 5 is can-shaped with an upper 
section 13 having a smaller diameter in the form of a narrowed portion and 
a lower section 14 having a larger diameter. The rotor magnets 3 are also 
secured in this lower section 14. The upper section 13 is supported 
directly on the external diameter of the fixed shaft 10 by means of a 
first ball bearing 11, while the lower section 14 outside the internal 
stator 1 is supported on the fixed shaft 10 by means of a second bearing 
12 with the interposition of a spacer ring 6 (left half of the drawing, 
FIG. 1a). The parts of the motor including both ball bearings 11 and 12 
are sealed off from the exterior by suitable seals 7, in this case 
magnetic fluid seals. Moreover, the connecting wires 9 are brought out 
through a central bore 8 in the interior of the shaft. The first 
embodiment of the invention will now be described with reference to the 
right part of the drawing, i.e. FIG. 1b. The upper bearing arrangement of 
the back iron 5 or the driving hub 4 is designed just as in the left half 
according to FIG. 1a, and likewise the design of the internal stator 1 and 
the external rotor 2. However, a collar 15 having a larger diameter than 
the shaft 10 on which a ball bearing 12 having a larger diameter is 
arranged is integrally moulded with the fixed shaft 10 in the lower 
region. This ball bearing 12 directly supports the lower section 14 of the 
back iron 5, i.e. without a spacer ring. This therefore means that there 
is one joint fewer present and the construction is improved from the point 
of view of a uniform material structure, resulting in smoother running of 
the spindle driving motor. Moreover, as a result of the omission of the 
spacer ring 6 (FIG. 1a), the inertial mass of the rotor is reduced, there 
is an increase in the resonance frequencies as a result of greater 
rigidity of the bearing arrangement and, finally, the larger bearing means 
a higher load-bearing capacity and less wear. 
FIG. 2 shows a similar design to that of FIG. 1b, although in this case the 
collar 15 is designed in the form of a shaft flange, allowing for simpler 
securing of the spindle driving motor and increasing the rigidity of the 
system. Components with the same reference numerals are similar to those 
according to FIG. 1b, and so they do not need to be described again. For 
the sake of clarity, the driving hub 4 is not shown in FIG. 2, although, 
as already stated, it can be formed by the external circumference of the 
back iron 5. By virtue of the fact that the collar 15 is designed as a 
shaft flange, there is no need for a central bore in the fixed shaft 10 
and instead a corresponding bore 8a can be provided laterally in the 
collar 15, out through which the connecting wires 9 can then be brought. 
This means that the shaft 10 is not weakened. 
FIG. 2 also shows in detail how the upper ball bearing 11 is arranged 
directly between the external diameter 20 of the shaft 10 and the internal 
diameter 21 of the upper section 13 of the back iron 5, while the lower 
ball bearing 12 is arranged directly between the increased diameter 22 of 
the collar 15 and the internal diameter 23 in the lower section of the 
back iron 5, so that the number of joints in the mechanical bearing 
arrangement of the back iron 5 on the shaft 10 is reduced to a minimum. 
A variant of the embodiment according to FIG. 2 will now be described with 
reference to FIG. 3b. The left half of this drawing, i.e. FIG. 3a, 
corresponds to the left half according to FIG. 2. The embodiment according 
to FIG. 3b differs from that according to FIG. 2 in that the lower ball 
bearing 12 is partially integrated into the collar 15, i.e. in that the 
inner ball race 24 is worked into the external diameter 22 of the collar 
15. The balls 25 of the lower ball bearing 12 therefore run between this 
integrated ball race 24 and a corresponding ball race 26 in the outer 
ball-bearing ring 27. 
A further embodiment according to FIG. 4b differs from that according to 
FIG. 3b in that the upper ball bearing 11 is also partially integrated 
into the fixed shaft 10 or into a corresponding shoulder having an 
external diameter 30. A ball race 34 is integrated into the outer surface 
30 of the shaft 10. The balls 35 run between this ball race 34 and the 
corresponding ball race of an outer ball-bearing ring 37. The outer 
ball-bearing ring 37 is inserted into an internal bore 31 in the upper 
section 13 of the back iron 5. The left half of the drawing, i.e. FIG. 4a, 
corresponds to the left-hand views in FIGS. 2 and 3a. 
Even if the ball races 24 or 34 (FIGS. 3b and 4b) are only worked into the 
inner surface, i.e. on the shaft 10 or the collar 15, while the outer 
surface has outer ball-bearing rings 27 or 37, it is theoretically also 
conceivable to dispense with the outer ball-bearing ring and to arrange 
corresponding ball races directly on the corresponding sections of the 
back iron 5. In this case, the surfaces must be appropriately hardened at 
these points of the back iron, although this sometimes has an adverse 
effect on the magnetic properties. Moreover, the entire assembly must of 
course be constructed correctly as far as assembly is concerned. 
In order to have more degrees of freedom in the construction, i.e. so that 
the corresponding section of the fixed shaft 10 can be selected 
substantially freely in accordance with the requirements of the bearing 
arrangement, it is proposed according to the invention that the internal 
stator 1 is not manufactured in one piece and slipped axially on to the 
shaft 10, but is composed of several segments 41, i.e. at least two 
segments, so that radial assembly can be effected. A segment 41 of this 
kind having excitation coils 46 is shown in diagrammatic form in FIG. 5. 
It can also be seen here that the laminated stator segment is designed for 
nested assembly in order to improve the magnetic transitions. However, it 
is also conceivable to allow the individual stator segments 41 to be 
butt-jointed given accurate production. The partial excitation coils 46 on 
the stator segments 41 are connected together accordingly after assembly. 
FIGS. 6a and 6b then show how another design with more degrees of freedom 
is possible using a divided internal stator according to FIG. 5. It can be 
seen that the fixed shaft 10 has three sections 10a, 10b and 10c with 
different external diameters, the central section 10b having the smallest 
diameter and receiving the segments 41 of the internal stator 1. In this 
manner, the diametrical steps of the back iron 5 or the driving hub can be 
smaller, resulting in substantially identical sizes for the upper ball 
bearing 11 and the lower ball bearing 12. A construction of this kind 
therefore has better symmetry, resulting in lower heat-sensitivity for the 
bearing arrangement. 
In FIG. 6a (right part of the drawing), both ball bearings 11 and 12 are 
designed as conventional bearings, i.e. they contain inner and outer 
ball-bearing rings which are arranged directly between the shaft 10 and 
the back iron 5. However, like the embodiment according to FIG. 4b, FIG. 
6b (left half of the drawing) shows that the ball bearings 11 and 12 are 
partially integrated into the shaft 10, i.e. that corresponding ball races 
24 and 34 are worked into corresponding sections 10c and 10a of the shaft 
10. The opposing ball races for the balls 25, 35 are situated in 
corresponding outer ball-bearing rings 27, 37.