Special Issue; Products for Industrial Machinery - NTN

Development of Aerostatic Bearing Spindle for Precision Machine Tools
Journal bearings
Thrust bearings
AC servo motor
Rotary encoder
Rotary shaft
Non-contact seal
for vacuum chuck
Thrust plate
Fig. 1 Structure of air spindle
3. Specifications of Prototype Spindle
The specifications (target values) for NTN’s
prototype aerostatic bearing spindle for machine tools
are summarized in Table 1.
Previously, the maximum running speed with this
type of spindle was approximately 10,000 r/min. In
contrast, the maximum targeted running speed with
NTN’s spindle is 20,000 r/min. To ensure good
machining accuracy, it has been determined that it
should have an NRRO (non-repetitive run-out)
comparable to that of an aerostatic spindle for
inspection devices and the axial displacement of the
chuck face during steady running of the spindle should
be limited to 1 m or smaller.
Max. running speed
Air feed pressure to bearing
Radial rigidity
Axial rigidity
NRRO
Radial
Axial
Displacement on chuck face
Table 1 Specifications
20,000 rmin
0.49 MPa
45 N/m
227 N/m
0.010m or less
0.010m or less
1m or less during steady running
4. Structure of the Aerostatic Spindle
for Machine Tools
1) Realization of greater rigidity and higher accuracy
The structure of the authors’ aerostatic bearing
spindle is illustrated in Fig. 2. A total of four journal
bearings are placed in the front and rear sides of the
thrust bearings to enhance the rigidity of the aerostatic
bearing spindle. Furthermore, seeking higher
accuracy, the layout for the air feed holes on the
journal bearings have been optimized in order to
inhibit high order vibration that would otherwise occur
due to the interaction between the form errors on the
rotary shaft and the layout of the air feed holes, as
well as to improve the dynamic rigidity of the bearings.
2) Realization of higher torque
To attain both a higher torque on the driving section
and a smaller spindle outside diameter, two motors
are coupled in tandem, thereby the authors’ aerostatic
bearing spindle features a torque twice as high
compared to ordinary aerostatic bearing spindle
designs.
3) Control of chips and coolant
For the vacuum chuck mechanism that secures
workpieces, to prevent the coolant entering the chuck
from reaching the bearing section, the suction holes
are situated further apart from the bearings compared
to conventional aerostatic bearing spindle
arrangements. To avoid ingress of chips and coolant
into the bearings, a non-contact air-seal has been
incorporated and a narrow gap is provided between
the rotary shaft and the fixed section (housing),
thereby compressed air is allowed to gush out through
this gap to prevent chips and coolant from reaching
the bearings.
4) Control of heat buildup
Aerostatic bearings are known to generate less
heat. However, when any aerostatic bearing is run at
a higher speed, the heat buildup resulting from the
shearing of gaseous matter in the bearing clearance
cannot be ignored. In addition, the magnitude of heat
generated from the motor arrangement of NTN’s
prototype spindle was expected to be greater than that
of similar conventional arrangements. To solve this
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