Segmented gas supply surface throttling gas floating spindle bearing

Abstract

The application discloses a surface throttling type air floating main shaft bearing with segmented air supply, which comprises a main shaft rotor, a front air floating bearing and a rear air floating bearing, wherein the main shaft rotor is a stepped shaft with a ring-shaped flange arranged in the middle; the front air floating bearing and the rear air floating bearing are sleeved on the main shaft rotor; the lower part of the front air floating bearing is provided with a ring-shaped groove, and the ring-shaped flange is located in the ring-shaped groove; ring-shaped apertures are arranged between the main shaft rotor and the shaft sleeve, and the adjacent apertures are communicated with each other, the longitudinal section of the ring-shaped apertures is in a stepped structure, and the shaft sleeve is provided with air supply flow channels and air outlet flow channels in sections. The segmented air supply method can avoid the circumferential flow caused by the change of the pressure gradient of the fluid flowing in the circumferential direction, improve the carrying capacity of the bearing, the stepped surface throttling can make the rotor work more stably, realize higher rotation precision, and the additional throttler is not needed, so that the assembly efficiency and reliability are greatly improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of ultra-precision equipment manufacturing, and relates to a surface throttling hydrostatic bearing, in particular to a segmented gas supply surface throttling air floating spindle bearing. BACKGROUND

[0002] The hydrostatic sliding bearing adopts fluid as a lubricant, and external pressurized gas enters the pore through a throttler to generate a pressure lubrication film in the pore to float the spindle rotor. The lubrication film of the hydrostatic sliding bearing has an error homogenization effect, which can improve the rotation accuracy of the spindle. Compared with the dynamic pressure sliding bearing, the lubrication film of the hydrostatic sliding bearing spindle rotor can remain complete in the low-speed or high-speed running state, the work is more stable, the service life is long, and the processing is convenient. In the technical field of ultra-precision equipment manufacturing, a gas lubricant is usually used. The gas film has compressibility, the spindle rotation accuracy is higher, and the gas film friction is small, so that low heat and low-speed non-climbing can be realized. The main throttling methods of the gas hydrostatic sliding bearing include small hole throttling, slit throttling, surface throttling and porous throttling. When the spindle size is large, the small hole throttling needs to install a throttler additionally, and the large number of throttlers will increase the assembly difficulty and affect the rotation accuracy; the porous throttling has a slow response and the pore is easy to be blocked; the slit throttling has high processing accuracy requirement and large processing difficulty; the surface throttling sets a fine groove on one side of the bearing gas film surface for throttling. Since there is no throttling hole, the gas film thickness can be reduced to a certain extent, the stiffness can be improved, and the gas flow can be reduced. The surface throttling is widely used in precision shaft systems.

[0003] The longitudinal section of the fine groove of the surface throttling presents a ladder shape, and the fluid has a pressure gradient from high to low along the ladder to achieve the throttling effect. The flat plate step lubrication film belongs to an ideal model, the fluid pressure changes only in one dimension, and the performance of the bearing is in an extreme state. The actual use of the hydrostatic sliding bearing is a cylindrical body. After the load is applied, the oil film gap is uneven, and the fluid will spread from the high pressure area to the low pressure area in the axial and circumferential directions. The circumferential spread of the fluid will produce a ring flow to affect the bearing carrying capacity and gas film stiffness. Therefore, suppressing the ring flow through some methods can improve the performance of the hydrostatic sliding bearing. The conclusion shows that the smaller the length-diameter ratio, the smaller the circumferential proportion, and the closer the flow field to the ideal state of one-dimensional flow. When the fluid compressibility is not considered, the dimensionless carrying capacity, the dimensionless stiffness and the flow of the bearing are only related to the lubrication film gap thickness ratio and the step length to the lubrication film full length ratio. Therefore, after the spindle rotor is designed separately according to the requirements, the segmented gas supply method is used to reduce the length-diameter ratio to achieve the effect of suppressing the ring flow. SUMMARY

[0004] The present application aims to provide a segmented gas supply surface throttling type gas floating main shaft bearing with simple structure, good stability, high rotation accuracy and convenient processing.

[0005] The present application aims to provide a segmented gas supply surface throttling type gas floating main shaft bearing with simple structure, good stability, high rotation accuracy and convenient processing.

[0006] A segmented gas supply surface throttling type gas floating main shaft bearing, comprising a main shaft rotor, a front gas floating bearing and a rear gas floating bearing, wherein:

[0007] The main shaft rotor is a stepped shaft with an annular flange arranged in the middle;

[0008] The front gas floating bearing and the rear gas floating bearing are sleeved on the main shaft rotor;

[0009] The lower part of the front gas floating bearing is provided with an annular groove, and the annular flange is located in the annular groove;

[0010] The outer side wall of the front shaft sleeve of the front gas floating bearing is segmented and provided with a gas supply port I, a gas outlet port I, a gas supply port II and a gas outlet port II, the outer side wall of the cylindrical structure of the upper part of the annular flange of the main shaft rotor is provided with annular slits I, annular slits II and annular slits III between the front shaft sleeve, the adjacent slits are communicated with each other, the gas supply port I is communicated with the junction of the annular slits I and the annular slits II through a flow channel, the gas outlet port I is communicated with the junction of the annular slits II and the annular slits III through a flow channel, the upper part of the annular flange of the main shaft rotor is provided with an annular slit IV between the front shaft sleeve, the gas supply port II is communicated with the junction of the annular slits III and the annular slits IV through a flow channel, the outer side wall of the annular flange of the main shaft rotor is provided with an annular slit V between the front shaft sleeve, the upper end of the annular slit V is communicated with the annular slit IV, and the gas outlet port II is communicated with the middle part of the annular slit V through a flow channel;

[0011] The outer side wall of the rear shaft sleeve of the rear gas floating bearing is segmented and provided with a gas supply port III, a gas outlet port III and a gas supply port IV, the outer side wall of the cylindrical structure of the lower part of the annular flange of the main shaft rotor is provided with annular slits VII, annular slits VIII and annular slits IX between the rear shaft sleeve, the adjacent slits are communicated with each other, the gas supply port IV is communicated with the junction of the annular slits VIII and the annular slits IX through a flow channel, the gas outlet port III is communicated with the junction of the annular slits VII and the annular slits VIII through a flow channel, the lower part of the annular flange of the main shaft rotor is provided with an annular slit VI between the rear shaft sleeve, the gas supply port III is communicated with the junction of the annular slit VI and the annular slit VII through a flow channel, and the lower end of the annular slit V is communicated with the annular slit VI.

[0012] In this invention, the air-bearing spindle bearing further includes a rotary table, which is located above the spindle rotor and the front air-bearing bearing and is fixed on the spindle rotor.

[0013] In this invention, the air-bearing spindle is connected to a motor and an encoder to achieve closed-loop servo control of the air-bearing spindle. The motor is located below the rear air-bearing bearing and fixed on the rear air-bearing bearing. The encoder includes a resolver sensor and a grating system. The resolver sensor is located below the motor and fixed on the motor, and the grating system is located below the resolver sensor and fixed on the resolver sensor.

[0014] In this invention, the air-bearing main shaft also includes a rear end cover, which is located below the grating system and fixed to the grating system.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] 1. Compared with traditional surface throttling hydrostatic bearings, this invention improves the annular orifice of the traditional surface throttling hydrostatic bearing with a coaxial diameter to an annular orifice with a radial stepped structure, and then opens the air inlet and outlet in sections on the bushing, with a rectangular chamfer transition between the air inlet flow channel and the annular orifice.

[0017] 2. In traditional surface-throttling hydrostatic bearing structures, the spindle becomes eccentric under pressure after high-pressure gas is introduced, resulting in uneven gas film clearance. The gas spreads axially and circumferentially from the high-pressure area to the low-pressure area within the gaps, forming a circumferential flow that affects the bearing's load-bearing capacity and gas film stiffness. This invention uses a segmented gas supply method to reduce the length-to-diameter ratio of each segment, i.e., to reduce the circumferential proportion, making the flow field closer to the ideal state of one-dimensional flow. This suppresses the generation of circumferential flow, thereby improving the performance of the hydrostatic sliding bearing.

[0018] 3. Existing surface throttling hydrostatic bearings typically use a single air supply source and require an additional throttling device. The surface throttling hydrostatic bearing of this invention uses a stepped throttling method, eliminating the need for a throttling device. The bearing has a simple structure, fewer parts, is easy to process, convenient to assemble, and is more stable and reliable during operation. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the axial cross-sectional structure of the segmented air supply type air bearing of the present invention.

[0020] Figure 2 This is a schematic diagram of the fluid flow direction of the segmented air supply front air bearing of the present invention.

[0021] Figure 3 This is a schematic diagram of the fluid flow direction of the segmented air supply rear air bearing of the present invention.

[0022] Figure 4 This is a three-dimensional structural diagram of the segmented air supply surface-throttling air bearing of the present invention.

[0023] In the diagram: 1. Main spindle rotor; 2. Front air bearing; 21. Front bushing; 22. Air outlet I; 23. Air outlet II; 24. Air supply I; 25. Air supply II; 3. Rear air bearing; 31. Rear bushing; 32. Air outlet III; 33. Air supply III; 34. Air supply IV; 4. Motor; 41. Motor rotor mounting shaft; 42. Motor rotor; 43. Motor stator; 44. Motor stator mounting base; 5. Grating system; 51. Grating mounting shaft; 52. Circular grating; 53. Reading head; 54. Reading head mounting base; 6. Rear end cover; 7. Resolver sensor; 71. Resolver clamping nut; 72. Resolver rotor; 73. Resolver stator; 74. Resolver stator mounting base; 75. Resolver rotor mounting shaft; 8. Rotary table. Detailed Implementation

[0024] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the present invention. In the description of the present invention, it should be understood that terms such as "upper," "lower," "horizontal," and "longitudinal," which indicate orientation, are only for the convenience of describing the present invention and do not indicate or imply that the referred element must have a specific orientation, and therefore should not be construed as a specific limitation of the present invention. In the present invention, the shell is a cylindrical surface, the generatrix direction of the cylindrical surface is called the axial direction, and the radial direction on the same circular cross-section is called the radial direction.

[0025] This invention provides a segmented air supply type throttling air-bearing spindle bearing, such as... Figure 1 and Figure 4 As shown, the throttling type air-bearing main shaft bearing includes a main shaft rotor 1, a front air-bearing bearing 2, and a rear air-bearing bearing 3, wherein:

[0026] The main shaft rotor 1 is a stepped shaft with an annular flange in the middle;

[0027] The front air bearing 2 is a cylindrical structure with an annular groove at the bottom;

[0028] The rear air bearing 3 has a cylindrical structure;

[0029] The front air bearing 2 and the rear air bearing 3 are sleeved on the main shaft rotor 1, and the annular flange is located in the annular groove.

[0030] The front air bearing 2 uses a segmented air supply surface throttling method to support the main shaft rotor 1. The outer wall of the front bushing 21 is segmented with air supply port I 24, air outlet I 22, air supply port II 25, and air outlet II 23. The outer wall of the cylindrical structure on the upper part of the annular flange of the main shaft rotor 1 is provided with annular slot I, annular slot II, and annular slot III between it and the front bushing 21, and the adjacent slots are interconnected. The annular slot I, annular slot II, and annular slot III have the same structure and their longitudinal sections are all stepped. The air supply port I 24 is connected to the junction of annular slot I and annular slot II through a flow channel. The air outlet I22 is connected to the junction of the annular slot II and annular slot III through a flow channel. An annular slot IV is provided between the upper part of the annular flange of the main shaft rotor 1 and the front bushing 21. The annular slot IV and annular slot III have the same structure and both have a stepped structure in longitudinal section. The air supply port II25 is connected to the junction of the annular slot III and annular slot IV through a flow channel. An annular slot V is provided between the outer wall of the annular flange of the main shaft rotor 1 and the front bushing 21. The upper end of the annular slot V is connected to the annular slot IV. The air outlet II23 is connected to the middle part of the annular slot V through a flow channel.

[0031] The rear air bearing 3 uses a segmented air supply surface throttling method to support the main shaft rotor 1. The outer wall of the rear bushing 31 is segmented with air supply port III 33, air outlet III 32, and air supply port IV 34. The outer wall of the cylindrical structure at the lower part of the annular flange of the main shaft rotor 1 is provided with annular slots VII, VIII, and IX between it and the rear bushing 31, and the adjacent slots are interconnected. The annular slots VII, VIII, and IX have the same structure and all have a stepped structure in longitudinal section. IV34 is connected to the junction of annular slot VIII and annular slot IX through a flow channel. The outlet III32 is connected to the junction of annular slot VII and annular slot VIII through a flow channel. Annular slot VI is provided between the lower part of the annular flange of the main shaft rotor 1 and the rear bushing 31. Annular slot VI and annular slot VII have the same structure and both have a stepped structure in longitudinal section. The air supply port III33 is connected to the junction of annular slot VI and annular slot VII through a flow channel. The lower end of annular slot V is connected to annular slot VI.

[0032] In this invention, the annular slots I, II, and III are all located on the inner wall of the front bushing 21 or on the outer wall of the main shaft rotor 1; the annular slots VII, VIII, and IX are all located on the inner wall of the rear bushing 31 or on the outer wall of the main shaft rotor 1; the annular slots IV and VI are respectively located on the upper and lower surfaces of the annular flange or on the upper and lower walls of the annular groove.

[0033] In this invention, a rectangular chamfer I is provided at the junction of the annular slot I and the annular slot II, and a rectangular chamfer II is provided at the junction of the annular slot III and the annular slot IV. The annular slot I, annular slot II, annular slot III and annular slot IV extend from the rectangular chamfer to both sides in a stepped manner from high to low. The air inlet I is connected to the rectangular chamfer I through a flow channel, and the air inlet II is connected to the rectangular chamfer II through a flow channel.

[0034] In this invention, a rectangular chamfer Ⅲ is provided at the junction of the annular slot VIII and the annular slot IX, and a rectangular chamfer Ⅳ is provided at the junction of the annular slot VI and the annular slot VII. The annular slots VI, VII, VIII and IX extend from the rectangular chamfer to both sides in a stepped manner from high to low. The air inlet Ⅲ is connected to the rectangular chamfer Ⅲ through a flow channel, and the air inlet Ⅳ is connected to the rectangular chamfer Ⅳ through a flow channel.

[0035] In this invention, the air-bearing spindle bearing further includes a rotary table 8, which is located above the spindle rotor 1 and the front air-bearing bearing 2 and is fixed to the spindle rotor 1 by bolts.

[0036] In this invention, the air-bearing spindle can also be connected to a motor 4 and an encoder to achieve closed-loop servo control of the air-bearing spindle. The motor 4 includes a motor rotor mounting shaft 41, a motor rotor 42, a motor stator 43, and a motor stator mounting base 44. The motor rotor mounting shaft 41 is cold-fitted onto the spindle rotor 1, the motor rotor 42 is cold-fitted onto the motor rotor mounting shaft 41, and the motor stator 43 is cold-fitted onto the inner wall of the motor stator mounting base 44. The motor stator mounting base 44 is located below the rear air-bearing bearing 3 and is fixed to the rear air-bearing bearing 3 by bolts.

[0037] In this invention, the encoder includes a resolver sensor 7 and a grating system 5. The resolver sensor 7 includes a resolver rotor mounting shaft 75, a resolver rotor 72, a resolver stator 73, and a resolver stator mounting base 74. The resolver rotor mounting shaft 75 is cold-fitted onto the main shaft rotor 1. The resolver rotor 72 is fitted onto the resolver rotor mounting shaft 75 and its end is fixed with a resolver clamping nut 71. The resolver stator 73 is cold-fitted onto the resolver stator mounting base 74. The resolver stator mounting base 74 is located below the motor 4 and is fixed to the motor 4 with bolts.

[0038] In this invention, the grating system 5 includes a grating mounting shaft 51, a circular grating 52, a reading head 53, and a reading head mounting base 54. The grating mounting shaft 51 is located below the resolver sensor 7 and is fixed to the resolver rotor mounting shaft 75 with bolts. The circular grating 52 is sleeved on the end of the grating mounting shaft 51 and fixed with bolts. The reading head 53 is fixed on the reading head mounting base 54. The reading head mounting base 54 is located below the resolver sensor 7 and is fixed to the resolver sensor 7 with bolts.

[0039] In this invention, the air-bearing main shaft also includes a rear end cover 6, which is located below the grating system 5 and is fixed to the grating system 5 by bolts.

[0040] In this invention, the schematic diagram of the fluid flow direction of the segmented air supply throttling type front air bearing is shown below. Figure 2 As shown, the lubricant introduced through air supply port I24 flows into rectangular chamfer I through the flow channel, then into annular slot I and annular slot II, and flows out through the upper section of the front axle sleeve 21 and air outlet I22; the lubricant introduced through air supply port II25 flows into rectangular chamfer II through the flow channel, then into annular slot III, annular slot IV and annular slot V, and flows out through air outlet I22 and air outlet II23.

[0041] In this invention, the schematic diagram of the fluid flow direction of the segmented air supply throttling type rear air bearing is shown below. Figure 3 As shown, the lubricant introduced through air supply port Ⅲ33 flows into rectangular chamfer Ⅲ through the flow channel, then into annular slots Ⅴ, Ⅵ and Ⅶ, and flows out through air outlet Ⅱ23 and air outlet Ⅲ32; the lubricant introduced through air supply port Ⅳ34 flows into rectangular chamfer Ⅳ through the flow channel, then into annular slots Ⅷ and Ⅸ, and flows out through air outlet Ⅲ32 and the lower section of rear axle sleeve 31.

[0042] This invention employs a segmented gas supply method, avoiding the uneven gas film gap caused by the introduction of high-pressure gas. This prevents the circumferential flow formed during the circumferential flow of the gas from affecting the load-bearing capacity and gas film stiffness of the air bearing. By distributing the length-to-diameter ratio of each segment, i.e. reducing the proportion in the circumferential direction, the flow field is made closer to the ideal state of one-dimensional flow, which can suppress the generation of circumferential flow and thus improve the performance of the hydrostatic sliding bearing.

[0043] The surface-throttling hydrostatic bearing of this invention employs a stepped throttling method. After high-pressure lubricant is introduced through the air supply port, the point pressure source transforms into a linear pressure source at the rectangular chamfer, forming a uniformly pressurized lubricating film between the main shaft rotor 1 and the front and rear bushings 2 and 3, thus lifting the main shaft rotor 1. The stepped annular slots I, II, III, IV, V, VI, VII, VIII, and IX transform the lubricating film into a stepped shape, achieving a throttling effect. No additional throttling device is required, resulting in a simple bearing structure, fewer parts, easy processing, convenient assembly, greater stability during operation, high reliability, and ensuring high rotational accuracy of the rotary table 8.

Claims

1. A segmented air supply type air-float main shaft bearing, characterized in that... The throttling type air-bearing main shaft bearing includes a main shaft rotor, a front air-bearing bearing, and a rear air-bearing bearing, wherein: The main shaft rotor is a stepped shaft with an annular flange in the middle; The front and rear air bearings are mounted on the main shaft rotor. The lower part of the front air bearing is provided with an annular groove, and the annular flange is located inside the annular groove; The outer wall of the front axle sleeve of the front air bearing is provided with an air supply port I, an air outlet I, an air supply port II, and an air outlet II in sections. The outer wall of the cylindrical structure on the upper part of the annular flange of the main shaft rotor is provided with annular slots I, II, and III between the front axle sleeve and the main shaft sleeve, and the adjacent slots are interconnected. The air supply port I is connected to the junction of annular slots I and II through a flow channel. The air outlet I is connected to the junction of annular slots II and III through a flow channel. The upper part of the annular flange of the main shaft rotor is provided with annular slot IV between the front axle sleeve and the main shaft sleeve. The air supply port II is connected to the junction of annular slots III and IV through a flow channel. The outer wall of the annular flange of the main shaft rotor is provided with annular slot V between the front axle sleeve. The upper end of annular slot V is connected to annular slot IV. The air outlet II is connected to the middle part of annular slot V through a flow channel. The rear air bearing has three sections on its outer side wall: an air supply port III, an air outlet III, and an air supply port IV. The outer side wall of the cylindrical structure at the lower part of the annular flange of the main shaft rotor has three annular slots VII, VIII, and IX connected to the rear shaft sleeve. Air supply port IV is connected to the junction of annular slots VIII and IX via a flow channel. Air outlet III is connected to the junction of annular slots VII and VIII via a flow channel. Annular slot VI is provided between the lower part of the annular flange of the main shaft rotor and the rear shaft sleeve. Air supply port III is connected to the junction of annular slots VI and VII via a flow channel. The lower end of annular slot V is connected to annular slot VI.

2. The segmented air supply type air-float main shaft bearing according to claim 1, characterized in that... Both the front and rear air bearings are cylindrical structures.

3. The segmented air supply type air-float main shaft bearing according to claim 1, characterized in that... The annular slots I, II, III, and IV have the same structure and their longitudinal sections are all stepped. The annular slots VI, VII, VIII, and IX also have the same structure and their longitudinal sections are all stepped.

4. The segmented air supply type air-float main shaft bearing according to claim 1 or 3, characterized in that... The annular slots I, II, and III are all located on the inner wall of the front bushing or on the outer wall of the main shaft rotor; the annular slots VII, VIII, and IX are all located on the inner wall of the rear bushing or on the outer wall of the main shaft rotor; the annular slots IV and VI are located on the upper and lower surfaces of the annular flange or on the upper and lower walls of the annular groove, respectively.

5. The segmented air supply type air-float main shaft bearing according to claim 4, characterized in that... A rectangular chamfer I is provided at the junction of the annular slot I and the annular slot II, and a rectangular chamfer II is provided at the junction of the annular slot III and the annular slot IV. The steps from the annular slot I, annular slot II, annular slot III and annular slot IV extend to both sides of the rectangular chamfer in a step-like manner from high to low. The air inlet I is connected to the rectangular chamfer I through a flow channel, and the air inlet II is connected to the rectangular chamfer II through a flow channel.

6. The segmented air supply type air-float main shaft bearing according to claim 4, characterized in that... A rectangular chamfer Ⅲ is provided at the junction of the annular slot VIII and the annular slot IX, and a rectangular chamfer Ⅳ is provided at the junction of the annular slot VI and the annular slot VII. The annular slots VI, VII, VIII and IX extend from the rectangular chamfer to both sides in a stepped manner from high to low. The air inlet Ⅲ is connected to the rectangular chamfer Ⅲ through a flow channel, and the air inlet Ⅳ is connected to the rectangular chamfer Ⅳ through a flow channel.

7. The segmented air supply type air-float main shaft bearing according to claim 1, characterized in that... The air-bearing spindle bearing also includes a rotary table, which is located above the spindle rotor and the front air-bearing bearing and is fixed on the spindle rotor.

8. The segmented air supply type air-float main shaft bearing according to claim 1 or 7, characterized in that... The air-bearing spindle is connected to a motor and an encoder to achieve closed-loop servo control of the air-bearing spindle. The motor is located below the rear air-bearing bearing and is fixed on the rear air-bearing bearing. The encoder includes a resolver sensor and a grating system. The resolver sensor is located below the motor and is fixed on the motor. The grating system is located below the resolver sensor and is fixed on the resolver sensor.

9. The segmented air supply type air-float main shaft bearing according to claim 8, characterized in that... The motor includes a motor rotor mounting shaft, a motor rotor, a motor stator, and a motor stator mounting base. The motor rotor mounting shaft is cold-fitted onto the main shaft rotor, the motor rotor is cold-fitted onto the motor rotor mounting shaft, and the motor stator is cold-fitted onto the inner wall of the motor stator mounting base. The motor stator mounting base is located below and fixed to the rear air bearing. The resolver sensor includes a resolver rotor mounting shaft, a resolver rotor, a resolver stator, and a resolver stator mounting base. The resolver rotor mounting shaft is cold-fitted onto the main shaft rotor. The resolver rotor is mounted on the resolver rotor mounting shaft, and the resolver stator is cold-fitted onto the resolver stator mounting base. The resolver stator mounting base is located below the motor and fixed to the motor. The grating system includes a grating mounting shaft, a circular grating, a reading head, and a reading head mounting base. The grating mounting shaft is located below the resolver sensor and is fixed to the resolver rotor mounting shaft with bolts. The circular grating is mounted on the end of the grating mounting shaft. The reading head is fixed to the reading head mounting base, which is located below the resolver sensor and fixed to the resolver sensor.

10. The segmented air supply type air-float main shaft bearing according to claim 8, characterized in that... The air-bearing main shaft also includes a rear end cover, which is located below the grating system and fixed to the grating system.

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