Bearings – Rotary bearing – Fluid bearing
Reexamination Certificate
2001-03-07
2003-01-21
Schwartz, Christopher P. (Department: 3683)
Bearings
Rotary bearing
Fluid bearing
C384S107000, C384S114000
Reexamination Certificate
active
06508590
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an externally pressurized gas bearing spindle and particularly to a bearing spindle suitable for high-speed rotation.
FIG. 1
shows an externally pressurized gas bearing spindle (hereinafter referred to as an air spindle). This air spindle has bearing sleeves
2
,
2
′ mounted in a housing
1
. A motor bracket
4
is joined to the rear end of the housing
1
. A main shaft
5
is inserted into the bearing sleeves
2
,
2
′. A small-diameter portion at the rear end of the main shaft
5
is coupled to a rotor
6
of a motor
3
in the motor bracket
4
. The motor
3
has its stator
7
supported by the motor bracket
4
. A rotary encoder
8
is mounted on the rear end of the main shaft
5
, which protrudes outwardly out of the motor bracket
4
. The encoder
8
detects the number of revolutions of the main shaft
5
.
At a portion near the motor
3
, the main shaft
5
is provided with a thrust plate
11
in a flange-like manner. The thrust plate
11
is disposed between the bearing sleeve
2
and another bearing sleeve
2
′.
A gas-supply passage
12
for radial bearing is provided so as to extend in a radial direction through the housing
1
and the bearing sleeve
2
. A plurality of narrow diametric nozzles
13
are arranged annularly in four rows in an inner or radial bearing surface of the bearing sleeve
2
.
The gas-supply passage
12
communicates with the four rows of nozzles
13
. By supplying pressurized gas through the gas-supply passage
12
, radial bearings
14
,
14
′ for the main shaft
5
are formed at two places. Exhaust is made through an exhaust passage
15
extending radially and axially through the housing
1
.
A gas-supply passage
16
for a thrust bearing is provided so as to extend through the housing
1
and the bearing sleeve
2
′. A plurality of narrow axial nozzles
17
are arranged annularly in a thrust bearing surface of the bearing sleeve
2
′. The gas-supply passage
16
communicates with the nozzles
17
. By supplying pressurized gas through the gas-supply passage
16
, a thrust bearing
18
for the thrust plate
11
of the main shaft is formed. Exhaust is made through an exhaust passage
19
extending through the bearing sleeve
2
′ and the motor bracket
4
.
The motor bracket
4
has a cooling gas-supply passage
20
to air-cool the motor
3
.
The number of the nozzles
13
of each radial bearing
14
is four to six for each row. The distance between the rows, i.e. the axial distance S between the nozzles, is set at 40-60% of the bearing width W.
For an air spindle for medium to low speed, it is required to set the bearing stiffness as high as possible and to minimize the consumption of gas supplied. In order to meet these requirements, the optimum bearing clearance is set at several micrometers.
Specifically, as shown in
FIG. 4
, the bearing clearance was set at 5-8 &mgr;m (see the curve for a conventional product) to maintain the bearing stiffness as high as 0.9-1.0 kgf/&mgr;m. As will be apparent from
FIG. 4
, since the optimum range of bearing clearance is narrow in width and if the bearing clearance gets out of the range, the bearing stiffness decreases sharply, high machining accuracy is required for the bearing portions.
Also, heretofore, the ratio of the supply gas flow rate Q (liter/min) to the bearing stiffness H was set substantially at one (see the lines for conventional products in FIGS.
2
and
3
).
When the air spindle set as described above is used at high-speed rotation, the bearing friction loss increases markedly because it increases in proportion to the square of the number of revolutions. Since the load on the motor
3
also increases in proportion to the bearing friction loss, the motor current increases. Due to these factors, the temperature of the air spindle increases markedly during high-speed rotation.
When the temperature of the air spindle increases, heat is conducted to surrounding parts, causing thermal expansion, thus deteriorating the accuracy of the device. Also, if it is used in a disk examination device, if the main shaft
5
of the air spindle extends due to thermal expansion, it is possible that examination becomes impossible because the measuring range of a measuring sensor or a magnetic head cannot follow the displacement of the main shaft during examination.
Thus, conventional air spindles could be used only at a speed of 20000 r/min or under.
An object of this invention is to provide an air spindle for high-speed rotation in which heat generation is suppressed during high-speed rotation without lowering the bearing stiffness to minimize the elongation with temperature rise and influence of elongation on surrounding devices.
SUMMARY OF THE INVENTION
According to this invention, there is provided an externally pressurized gas bearing spindle comprising a housing, a bearing sleeve mounted in the housing, a main shaft supported in the bearing sleeve, a gas supply passage extending through the bearing sleeve and the housing, a plurality of narrow nozzles provided in the bearing sleeve, a radial bearing formed by supplying gas to a bearing surface between the bearing sleeve and the main shaft, the ratio of the supply gas flow rate Q to the bearing stiffness H of the radial bearing being set at 2-10.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which:
REFERENCES:
patent: 3617102 (1971-11-01), Wada et al.
patent: 3685874 (1972-08-01), Gerard
patent: 3698774 (1972-10-01), Saulgeot et al.
patent: 3785708 (1974-01-01), Miyasaki
patent: 4613288 (1986-09-01), McInerney
patent: 4869601 (1989-09-01), Scott
patent: 5274286 (1993-12-01), Yamamura
patent: 5553948 (1996-09-01), Ito
patent: 5707154 (1998-01-01), Ichiyama
patent: 6019515 (2000-02-01), Fujii et al.
patent: 6071013 (2000-06-01), Inaguma et al.
patent: 2001/0016090 (2001-08-01), Takanashi et al.
patent: 2235259 (1991-02-01), None
patent: 2246176 (1992-01-01), None
Kramer Devon
NTN Corporation
Schwartz Christopher P.
Wenderoth , Lind & Ponack, L.L.P.
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