The present invention relates to an externally pressurized gas bearing spindle assembly that allows highly accurate rotation of the spindle in a vacuum, reduced pressure or other special atmosphere.
The spindle assembly has a rotary shaft rotatably mounted in a housing through bearing gaps. During operation, compressed air or gas is introduced into the bearing gaps to support the rotary shaft while keeping it out of contact with the housing. The gas bearing structure thus allows high-precision rotation of the rotary shaft and ensures high endurance of the spindle assembly. Such a spindle assembly is therefore especially useful in high-precision machining, or in semiconductor processing.
Since e.g. optical disc masters are expected to be processed in a vacuum for higher accuracy and recording density, a gas bearing spindle assembly must be placed in a vacuum chamber. Thus, it is necessary to provide the spindle assembly with means for keeping compressed air used for the bearing from leaking into the vacuum chamber and decreasing the degree of vacuum.
As such means, a spindle assembly with a non-contact seal arrangement is proposed in Japanese patent publication 63-174802 to keep the rotational accuracy of the spindle in a vacuum or under reduced pressure while minimizing leakage of gas from the gas bearing into the vacuum chamber.
This type of non-contact seal arrangement includes a plurality of exhaust grooves formed in the inner surface of the bearing housing near its end. In order to positively prevent gas from leaking into the vacuum chamber, a plurality of exhaust pumps are connected to the respective exhaust grooves to draw gas that has flowed from the bearing gap into the respective exhaust grooves, to the open air outside the vacuum chamber.
However, in order to increase the degree of vacuum with a conventional spindle assembly, a plurality of exhaust pumps are needed to discharge the gas used for bearings. This increases the required installation space and the running cost. The spindle assembly is often mounted on a linear positioning table. In such a case, a plurality of tubes connecting the exhaust pumps to the respective exhaust grooves are needed and they would become a major obstacle in positioning the spindle. The more the tubes, the greater the resistance offered by the tubes and the more the positioning accuracy decreases.
The narrower the non-contact seal gap, the larger the flow resistance it imposes and the more effectively it can reduce leakage of gas into the vacuum chamber. But forming a narrow seal gap requires extremely high dimensional accuracy of the parts and thus incurs higher cost.
An object of the present invention is therefore to provide an externally pressurized gas bearing spindle assembly that can minimize leakage of gas using a minimum number of exhaust pumps and without the need to narrow the non-contact seal gap so extremely.
According to this invention, in an externally pressurized gas bearing spindle assembly wherein a rotary shaft is provided in a housing through bearing gaps, and wherein compressed gas is introduced into the bearing gaps to support the rotary shaft in a non-contact manner relative to the housing, an exhaust gas suction device for sucking gas discharged from the bearing is provided in the housing.
Since compressed gas leaking from the bearing gap toward the outside of the housing is sucked by the exhaust gas suction device, it is possible to minimize the leakage of gas into the vacuum chamber in a non-contact manner.
As another embodiment, the exhaust gas suction device may be provided in the rotary shaft.
As the exhaust gas suction device, a negative pressure generator, which generates negative pressure by the flow of compressed gas, may be used. By communicating the gas supply passage and the exhaust passage for the negative pressure generator with a compressed gas supply passage for introducing compressed gas into the bearing gap and a discharge passage for guiding exhaust gas from the bearing gap to the outside, respectively, no additional passageway is necessary for gas supply and exhaust for the negative pressure generator. Thus an externally pressurized gas bearing spindle assembly is obtained which is more compact and accurate and can be used in a vacuum.
The negative pressure generated by the negative pressure generator will be sufficient if only exhaust gas from the bearing gap can be effectively sucked.
According to this invention, there is provided a spindle assembly further comprising a plurality of non-contact seal gaps provided between the rotary shaft and the stationary portion so as to communicate with a gas discharge end of the bearing gap, a first exhaust groove formed between the gas discharge end of the bearing gap and the non-contact seal gap, and a second exhaust groove formed between the adjacent non-contact seal gaps to form a labyrinth seal between the rotary shaft and the stationary member. The labyrinth seal structure effectively suppresses leakage of gas into the chamber in which is placed the spindle assembly.
The first exhaust groove may communicate with the outside of the vacuum chamber with at least one of the other exhaust grooves communicating with the exhaust gas suction device, or alternatively the first exhaust groove may communicate with the exhaust gas suction device. This arrangement effectively minimizes leakage of gas into the chamber in which is placed the spindle assembly.
The other of the exhaust grooves may communicate with an exhaust pump provided outside of the vacuum chamber to forcibly discharge the exhaust gas from the seal gap. This makes it possible to minimize the leakage of gas into the vacuum chamber.
Further, as another means for solving the above problems, according to this invention, there is provided a spindle assembly wherein two externally pressurized gas bearings for supporting the rotary shaft at least in an axial direction in a non-contact manner are provided in the housing, wherein among the two bearings, the gap of the one nearer to the outside of said housing is smaller than that of the other, and wherein a non-contact seal gap is formed close to the gas bearing near the outside of the housing to prevent exhaust gas from the gas bearings from leaking into the outside of the housing. In this case, externally pressurized gas bearings for radial support of the rotary shaft may be provided as necessary.
The rotary shaft is rotated with high accuracy by e.g. an electric motor while kept out of contact with a stationary part by externally pressurized gas bearings with one end thereof exposed to a special atmosphere such as a vacuum.
A seal gap provided close to the gas bearing at the side exposed to the special atmosphere may be formed between flat surfaces perpendicular to the rotary shaft or between conical surfaces coaxial with the rotary shaft. As the rotary shaft moves axially, the seal gap changes.
Among the two bearings for axial support of the rotary shaft, the one nearer to the seal gap has a smaller gap and so larger stiffness than the other by adjusting the supply pressure and various bearing specifications. With this arrangement, any dimensional variation due to machining and assembling errors or thermal deformation is absorbed by a change in the size of the bearing gap of the gas bearing having a smaller stiffness. Thus, the bearing gap of the gas bearing near the seal gap is scarcely influenced. Thus, even though the seal gap is narrower than conventional ones, it is maintained stably.