KR101437640B1 - Aerostatic air bearing, assembling method thereof and aerostatic lead screw actuator using the same - Google Patents

Abstract

Air static pressure air bearings are provided. An exemplary air static pressure air bearing of the present invention includes an air static nut and a lead screw coupled to the air static nut, wherein the air static nut includes a housing having an air inlet for introducing air into the air static nut, A helical plenum chamber having a plurality of openings disposed in a spiral pattern and fluidly interconnected with the air inlet and the plurality of openings and a plurality of porous media disposed in each of the plurality of openings, Wherein the leadscrew comprises a threaded surface spaced from each of the plurality of porous media, each of the plurality of openings comprising a sheet on which a plurality of porous media is seated.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an air-tight air bearing, an assembly method of the same, and an air-aligned lead screw actuator using the same.

The present invention relates to an air static pressure air bearing, a method of assembling the same, and an air-aligned lead screw actuator using the same.

Miniaturization technology faces new challenges in the field of manufacturing. Micro-scale machine tools (MMTs) are emerging as practical and economical options to overcome these challenges.

However, difficulties have arisen in the development of machine tools for micro-scale machine tools. For example, it is a challenge for micromachined machine tools that it is difficult to rotate the titanium, stainless steel, and large metal glass strongly and process them in micrometers.

These applications require increased static / dynamic stiffness and high damping in actuators used in micro machine tools. Conventional solutions for linear positioning, ballscrews, and the like have generally failed to achieve the necessary positioning for micro-machine tools.

Hydraulic leadscrew actuators and air-pressure leadscrew actuators have been developed for use in micro-machine tools. Hydraulic screws are dirty and require critical supporting equipment.

Aerostatic lead screw actuators (ALSAs) have the potential to provide the necessary rigidity as well as to provide extremely high positioning accuracy. Air pressure leadscrews use air films instead of balls to transfer loads from the nuts to the screws. Air pressure leadscrews also appear to be particularly suitable for high performance micro-unit machine tools because they eliminate backlash and stick-slip friction. However, a major challenge in the manufacture of air-pressure leadscrews is to maintain a precise air gap between the screw and nut on the overall helical screw surface.

Prior art air pressure leadscrew designs include orifice-restricted and porous-limited air bearings. For example, Tachikawa et al., "Ultra Precision Positioning Using Air Bearing Lead Screw," Transactions of the Japan Society of Mechanical Engineers, Vol. 66, No. 645, pp. 1559-1566, 1997 discloses a porous-limited air-pressure lead screw actuator that achieves positioning accuracy of 10 nm. However, such lead screw actuator designs provide relatively low stiffness (e.g., 30 N / micron), require eight thread rotations and threaded fastening, thereby significantly increasing manufacturing costs .

A manufacturing method and design for a grooved orifice-restricted air static pressure leadscrew is disclosed by Slocum in U.S. Patent No. 4,836,042. Due to the special nature of the thread design in the '042 patent, the inaccuracy in the thread profile doubles the error seen in the void.

 U.S. Patent No. 5,090,265 to Slocum et al. Discloses the use of a hydraulic leadscrew actuator that does not require precise air regulation. The manufacturing process is based on machining both threads and nuts on the same machine tool to maximize the repeatability of the cutting operation. The '265 patent describes a fluid-based orifice-limited lead screw actuator design and method of manufacture wherein high pressure air, oil, and other fluids provide a working fluid. Leadscrew actuators focus on orifice limitation as the primary method for controlling airflow. The orifices provide a groove for distributing air to the overall threaded side of the leadscrew to form a gap between the nut and the screw.

However, focusing on orifice limitations to control airflow as such is very sensitive to instability and vibration. Also, in the '265 patent, leadscrew actuators are limited to rectangular, non-industry standard threaded configurations. Also, using the manufacturing method shown in the '265 patent doubles the inaccuracy in the thread profile at the locations where voids are considered. For example, if the threaded side has a slight angle that creates an error of 0.5 microns in outer diameter, it will produce an error of 1 micron in the air gap. This characteristic substantially forces the tolerance on the screw made in the above manner to be half, resulting in an increase in the manufacturing cost. In addition, the use of other fluids in place of air in the '265 patent requires considerable fluid support equipment, which is less desirable in machine tool applications, such as micro-machine tool applications.

An embodiment of the present invention seeks to provide an air static pressure lead screw actuator that evenly distributes the air pressure along the entire air bearing surface and a method of manufacturing the same.

In addition, one embodiment of the present invention is to provide an airtight lead screw actuator that provides frictionless operation to increase the stability of an air bearing and a method of manufacturing the same.

According to an aspect of the present invention, there is provided an air static pressure air bearing comprising: an air static nut and a lead screw coupled to the air static nut, the air static nut including a housing having an air inlet for introducing air into the air static nut, A helical plenum chamber disposed in the housing and having a plurality of apertures disposed in a spiral pattern and fluidly interconnected with the air inlet and the plurality of apertures and a plurality of porous media disposed in the plurality of apertures, Wherein the leadscrew comprises a threaded surface spaced from each of the plurality of porous media, each of the plurality of openings comprising a sheet on which the plurality of porous media is seated.

At this time, the plurality of openings may be formed in pairs on the opposed faces of the air static nut.

At this time, each of the plurality of porous media may be formed into a disk shape.

At this time, each of the plurality of openings may have a circular shape.

At this time, each of the plurality of porous media may include a graphite material.

At this time, the sheet may include a flange formed inside the opening.

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At this time, the flange can be flexible.

At this time, the plate spring may be further disposed between the flange and the porous medium.

At this time, the thread profile of the lead screw may be trapezoidal.

According to another aspect of the present invention there is provided an air static pressure lead screw actuator comprising: a base; a leadscrew rotatably coupled to the base and including a threaded surface; a lead screw coupled to the base, The first and second guide rails include first and second guide rails on which the leadscrew axes of the leadscrews and the axes of the first and second guide rails are positioned on the same plane, And a motor coupled to the leadscrew to drive the leadscrew; And a lapping module mounted on the leadscrew to provide uniform lapping along the surface of the plurality of porous media of the air pressure nut.

At this time, the first and second air bushings are fixedly coupled to the air pressure nut housing, and the first and second guide rails can extend through the first and second air bushes, respectively.

Wherein the spreader bar includes a set of spreader bars for supporting two portions of the axis of the leadscrew, the spreader bar aligning the guide rails such that the axis of the guide rail and the axis of the leadscrew are in the same plane As shown in FIG.

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The outrigger may further include two outriggers for attaching the housing of the air pressure nut to the guide rails. The outriggers may restrict the tilting and the yaw of the guide rails during the lapping operation.

According to another aspect of the present invention there is provided a method of assembling an air static pressure air bearing comprising a housing having an air inlet for introducing air into the housing, a plurality of Providing an air hydrostatic nut having an opening and having a helical plenum chamber fluidly interconnected with the air inlet and the plurality of openings; providing a lead screw comprising a threaded surface; Providing a plurality of porous media to the plurality of openings; first wrapping each of the plurality of porous media against the threaded surface to fit the plurality of porous media into a helical threaded shape; , Providing a surface-limiting layer on the surface of the first wrapped porous media step; A second method of assembling an air static pressure air bearing comprising lapping each of the porous media is provided.

At this time, in the first lapping step, the nut is freely slid on the lead screw while the rough polishing compound is applied to the lead screw, so that the porous medium is roughly wrapped, and in the second lapping step, The nuts can be freely slid on the lead screw with the abrasive compound being denser than the abrasive compound used in the first lapping step so that the porous medium can be precisely lapped.

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The air pressure lead screw actuator according to an embodiment of the present invention can evenly distribute air pressure along the entire air bearing surface.

An air static pressure lead screw actuator according to an embodiment of the present invention can provide frictionless operation, thereby increasing stability.

The individual air bearing porous media inserts in the air static pressure lead screw actuators according to one embodiment of the present invention remove stick-slip friction and backlash in the air bearings and provide sufficient stiffness to be performed in micro-unit machine tools, Make the manufacturing tolerance more loosened.

1 is a cross-sectional view illustrating an example of an air-matched air bearing according to an embodiment of the present invention.
2 is a view showing an air static nut housing according to an embodiment of the present invention.
FIG. 3 is a view showing an example of an air static pressure lead screw actuator having a wrapping module according to an embodiment of the present invention.
FIG. 4 is a view showing a part of an air static pressure lead screw actuator having a wrapping module according to an embodiment of the present invention.
Fig. 5 is a view showing a part of the air-static pressure lead screw actuator of Fig. 3;
6 is a diagram illustrating an example of a trapezoidal thread profile modeled for a lead screw according to an embodiment of the present invention.
7 is a flowchart of a method of assembling an air static pressure air bearing according to an embodiment of the present invention.
FIG. 8 is a diagram illustrating an exemplary coarse-wrapping graphite disk insert according to an embodiment of the present invention. FIG.
9 is an exemplary diagram of potting a graphite disk insert after grooved lapping in accordance with an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, which will be readily apparent to those skilled in the art to which the present invention pertains. The present invention may be embodied in many different forms and is not limited to the embodiments described herein. In order to clearly illustrate the present invention, parts not related to the description are omitted, and the same or similar components are denoted by the same reference numerals throughout the specification.

 One embodiment of the present invention is directed to an air static pressure lead screw actuator (ALSA) assembly that has high rigidity and little friction-free motion. An exemplary air-pressure leadscrew actuator is a nearly frictionless device that converts the rotational motion of the leadscrew into a linear motion of the air-pressure nut. In this system, almost friction free movement is achieved by providing all moving connections through a series of air bearings in a noncontact manner. Air bearings are operated by pressurized air to generate lift to form a gap between the two machine bodies. An exemplary air-pressure leadscrew actuator substantially eliminates stick-slip friction and backlash.

One embodiment of the present invention provides a porous-restriction air bearing. This type of restriction constitutes a more stable air bearing as compared to the orifice-limited air bearing.

Exemplary manufacturing processes are also simplified using insertable porous materials. In contrast to the manufacture of air pressure nuts using solid blocks of porous material, the insertable porous material (porous media insert) allows the porous media to easily integrate with the spiral nut geometry.

In addition, exemplary integrated lapping modules and exemplary leaf springs for porous media inserts compensate for errors within the voids and allow lower tolerances to be set on the fabricated elements, thereby saving cost.

1 to 4, 8 and 9, an apparatus according to an exemplary embodiment of the present invention provides an air static nut 10 for an air static pressure lead screw, wherein a fluid (e.g., For example, pressurized air, such as air, may be supplied to form a contactless interface between the air pressure nut 10 and the threaded side of the leadscrew 4.

Air tightening nut 10 may include a housing 12 having a series of openings 16 disposed therein in a spiral pattern. In addition, the housing 12 has an inlet 14 for a fluid, such as air, and an inlet 14 and a series of openings, respectively, fluidly interconnected. These fluid interconnections or fluid interconnections preferably allow a substantially uniform fluid (e.g., air) pressure to be supplied through each of the series of openings 16 through the inlet 14. [ In an exemplary air tightening nut 10, this is provided by a helical plenum chamber 20 disposed within the housing 12.

The apertures 16 are disposed on the surface of the helical plenum chamber 20 in a spiral pattern. The helical plenum chamber 20 is formed in fluid communication with the opening 16 with the inlet 14 so that uniform or substantially uniform air pressure is provided through each opening.

Each opening 16 is covered by an insertable porous medium 30 as shown in Figure 1 and the other figures to provide a restriction on the air bearing.

In one embodiment, the porous medium 30 is disk-shaped, and the openings 16 have a generally circular shape, but the disk and / or the openings can have a variety of other shapes.

Non-limiting exemplary porous media 30 are graphite porous media. The porous media 30 can be inserted into the opening 16 in any suitable manner and can be joined using methods that are not limited to attachment.

An O-ring 34 is provided on the outer circumferential surface of the porous medium 30 to prevent air from passing through the outer surface of the porous medium 30 when the porous medium 30 is installed in the opening 16. [

The porous media 30 can be formed through one or more lapping processes, an example of which is the process for forming the porous media 30 on the threaded surface 5 of the leadscrew 4, Media and the lapping modules of Figs. 3-4 and is described below.

The leadscrew 4 preferably includes a seat on which the insert is disposed, in order to take into account variations in manufacture in the threaded surface 5. In a non-limiting exemplary air tightening nut, this seat may be provided with a flange formed into the housing as in an opening.

An exemplary air pressure lead screw actuator (ALSA) 1 includes an air pressure nut, such as the air pressure nut 10 described herein, and a lead screw 4 coupled to the air pressure nut.

The leadscrew 4 is secured to the air pressure nut 10 by means of a gap 22 provided between the aperture 16 and the porous media disk insert 32 when the leadscrew 4 is coupled to the air pressure nut 10. [ And a surface 5 separating from the lead screw thread. In a non-limiting embodiment, the lead screw 4 thread profile is substantially trapezoidal.

A suitable drive device, which is not limited to the motor 6, may be provided for rotating the leadscrew 4 and for driving the leadscrew actuator 1.

3 to 5, an exemplary air-static pressure lead screw actuator 1 includes a base 2, a lead screw 4 rotatably coupled to the base 2, a lead screw 4 connected to the lead screw 4, For example, the axis of the lead screw 4 passes through the opening of the air pressure nut 10,

The leadscrew 4 comprises a threaded surface 5 which is separated from the air pressure nut 10 by means of a gap 22. An actuator, not limited to the motor 6, is coupled to the base 2 to drive (e.g., rotate) the leadscrew 4.

In the exemplary assembly, first and second guide rails 62 and 64 are coupled to the base 2 and disposed on opposite sides of the leadscrew 4 to increase stiffness.

Referring to FIG. 5, the air-fitting nut 10 is formed by a nut housing 40 having a hexahedron shape so as to surround an outer side surface in a radial direction with respect to a rotation axis of the air-fitting nut 10. A nut housing air injection part 42 is formed at one side of the nut housing 40 so that air is injected into the plenum chamber 20 of the air-fitting nut 10.

An extension, for example, an outrigger, is fixedly coupled to either side of the nut housing 40. An air bushing (48) is coupled to the outrigger (60). An opening is provided in the center of each air bushing (48). The first and second guide rails 62 and 64 pass through the openings of the respective extensions such as the outriggers 60 so that the first and second guide rails 62 and 64 The axes of the lead screws 4 are placed on the same plane or substantially coplanar.

The leadscrew 4 may be disposed in a suitable bushing (support bushing 66, thrust bushing 68) coupled to the base 2 for rotation.

The air pressure nut 10 for the exemplary leadscrew actuator 1 generally includes a housing 12, a porous graphite disk insert 32, a spiral plenum chamber 20, and a flange 18, .

The porous graphite disk insert 32 makes it possible to integrate the porous material in a spiral pattern while avoiding the difficulty of forming a solid porous body in a helical threaded configuration.

The spiral plenum chamber 20 of the present embodiment includes all porous graphite disk inserts 32 at the same inlet pressure (e. G., ≪ RTI ID = 0.0 & Interconnecting the openings fluidly interconnected).

The airtight leadscrew actuator 1 according to the exemplary embodiment has a housing (not shown) of the air pressure nut 10 with respect to (to) a trapezoidal thread of the leadscrew 4 to form a plurality of air bearing surfaces simultaneously 12 arranged in a spiral pattern.

The housing 12 of the air pressure nut 10 supports the spiral plenum chamber 20 and the operation of the porous graphite disk insert 32 interconnected with all the porous graphite disk inserts 32 at the same air supply pressure. And a flange 18 for fastening.

The air bearing surface creates an air film between the air tightening nut 10 and the lead screw (4) threads to create lift and provide a substantially frictionless operation and high axial stiffness.

The porous media disc insert 32 is preferably disposed on the flange 18 in each opening 16 to compensate for manufacturing defects in the leadscrew 4. [ A leaf spring 36 is provided corresponding to the high and low positions on the side of the lead screw to maintain a more precisely controlled gap.

A precision lapping module 50 is also incorporated within the exemplary assembly design to enable continuous lapping of the graphite disk insert 32 during manufacture of the actuator. The exemplary air pressure nut 10 has a significant improvement advantage over existing air static pressure lead screw actuators.

According to the present exemplary embodiment, it is possible to obtain a relatively high rigidity. As an example, 50 N / micron is achieved when fastened with a single threaded rotation. Linear systems that combine with more than one air pressure nut can also achieve higher stiffness values.

Exemplary air-pressure leadscrew actuators have an accuracy of sub-micron accuracy, rigidity of 50 N / m (at 120 psi supply pressure), low inlet air pressure (e.g., below 827 kPa) Lt; / RTI > In order to economically manufacture an airtight leadscrew actuator, a static stiffness of at least 50 N / m is provided with a positioning accuracy of less than a micron over the entire length of travel, and in particular the exemplary airtight leadscrew actuator has a precision ground leadscrew Lt; RTI ID = 0.0 > a < / RTI > porous-limited graphite air bearing.

Characteristics of an exemplary air-static pressure lead screw actuator will be described below. The four key evaluation parameters of air-pressure lead screw actuators include friction torque, stiffness, air pressure and travel distance. The evaluation parameters of such air pressure lead screw actuators are often evaluated based on the desire to accommodate high dynamic cutting forces while maintaining the accuracy of sub-micron actuators.

Friction torque: The positioning accuracy of the air pressure lead screw actuator is highly dependent on the inherent friction of the actuator. Tolerances in micro-unit parts generally range from 1-3 microns. For an air pressure lead screw actuator according to an embodiment of the present invention, the target accuracy is provided in the sub-micron range. In order to achieve such micro range accuracy, the stick-slip friction should be substantially eliminated in the air static pressure lead screw actuator according to the present embodiment. If the torque of 0.1 N-m is able to overcome all inherent friction in the actuator, the finish can be considered negligible.

Stiffness: In the operation of a micro machine tool, deformation significantly affects surface roughness and component accuracy. Ellicott et al., "Achainability Investigation of Micro-Scale Hard Turning of 52100 Steel," Transactions of NAMRI / SME, Vol. 37, pp. 143-150, 2009, a cutting force with a size between 5-12N was observed during a strong rotation of the small bearing (HRC 65). The stiffness of 50 N / 탆 is important to ensure that deformation occurs below 200 nm under most cutting conditions.

Air Pressure: At normal factory settings, the air compressor has a peak at approximately 827 kPa (120 PSI). This provides an exemplary limiting factor for the inflow pressure that can be supplied to the exemplary air pressure lead screw actuator.

Travel distance: The travel distance for a micro machine tool is much smaller than the travel distance of a typical microscale machine tool. In an exemplary air-pressure lead screw actuator application, a travel distance of 50 nm is selected.

1 is a cross-sectional view of a porous air bearing of an exemplary air-static pressure lead screw actuator 1 according to an embodiment of the present invention.

Within the air pressure nut 10, air enters the plenum chamber 20, which plenum chamber 20 is coupled (e.g., connected) to a porous medium 30, such as a porous graphite disk insert 32 . This porous medium 30 serves as a constraint to control the air flow.

The porous graphite disk insert 32 is mechanically coupled to the flange 18. At this time, in order to maintain a sustained downward force on the insert 32, a relatively rigid mechanical leaf spring 36 in this exemplary system is positioned between the flange 18 and the porous medium 30 . It is also possible that the leaf spring 36 is not located between the flange 18 and the porous medium 30 and the flange 18 forms a flexible bent portion to replace the leaf spring.

The air flow through the porous medium 30 (e.g., graphite disk insert 32) reaches the air bearing surface and forms a gap 22 between the porous medium 30 and the leadscrew surface 5 Generate lift. (E. G. Graphite disk insert 32) material, the air flow is uniformly distributed along the entire air bearing surface.

The balance between the downward force provided by the leaf spring 36 and the lifting force of the air flow forms a cavity 22 that provides the stiffness required for this exemplary airtight lead screw actuator 1.

Figure 2 illustrates a housing 12 of an exemplary air pressure nut 10 according to an exemplary embodiment of the present invention in which a graphite porous medium 30 (or another suitable porous medium 30) ). ≪ / RTI >

As a non-limiting example, the housing 12 of the air-fitting nut 10 is made of aluminum, steel or stainless steel.

In order to obtain bi-directional stiffness, disc inserts 32 are provided in pairs on the opposite side in this exemplary air pressure nut 10.

In the exemplary air tightening nut 10 of Fig. 2, nine pairs of disk inserts 32, which are provided as porous media 30, are located, and in this embodiment, on each side of the air static nut 10 (I.e., two openings on the opposite side of the nut).

These pairs of disk inserts 32 are supported against the side of the lead screw thread and act as a medium through which the air moves to form a gap 22 between the lead screw 4 and the air pressure nut 10 .

Each disc insert 32 is supported by a flange 18. A leaf spring (36) is positioned between the flange (18) and the porous medium (30). The leaf spring 36, which can be supported by the flange 18 formed in the nut housing 12, is deformed corresponding to the high / low point on the leadscrew 4 and increases the resistance of the cavity 22.

In the air pressure nut 10 of the present embodiment, the porous medium 30 includes a helical plenum chamber 30 which supplies the same (or substantially the same, e.g., within the range of 402 to 420 kPa) inflow pressure to each porous medium 30, (20).

The exemplary air pressure nut 10 design shown in FIG. 2 makes it possible for the porous disc insert 32 (e.g., a graphite disc insert) to be assembled to follow the profile of the thread profile.

To make the surface of the porous graphite disk insert 32 fit into the spiral surface 5 of the leadscrew 4 so that the void 22 remains within a range of a few microns, the exemplary embodiment includes an airtight lead screw actuator 1). ≪ / RTI >

3 illustrates an exemplary air-static leadscrew actuator design including an exemplary integral wrapping module 50 included in the air-pressure leadscrew actuator 1 of the present embodiment to provide uniform lapping along all graphite disk surfaces Lt; / RTI > 4, certain elements of the exemplary lapping module 50 are shown more clearly. Almost friction free operation can be achieved because all moving contacts are supported by air bearings in this exemplary design.

In the exemplary lapping module 50, the first and second guide rails 62, 64 are held by a set of spreader bars 52 that support the two shaft portions of the leadscrew 4.

The spreader bar 52 aligns the first and second guide rails 62 and 64 so that the axes of the first and second guide rails 62 and 64 and the axis of the lead screw 4 are placed on the same plane .

When aligned, the nut housing 40 of the air pressure nut 10 is attached to the guide rail through two lapping outriggers 60 that limit tilt and yaw during the lapping operation . After manufacture, the first and second guide rails 62,64 also provide additional radial stiffness to the exemplary actuator in normal operation.

The leadscrew 4 is provided with a support bushing 66 and a thrust bushing 68 for rotatably supporting the leadscrew 4.

In a non-limiting exemplary air pressure leadscrew design, the motor is an Aerotech frameless torque motor and all bushings are New Way air bushing, which keeps the actuator in a noncontact state do.

The assembly of the exemplary air-pressure lead screw actuator preferably includes the following steps.

1. The outrigger 60 is initially slid on the first and second guide rails 62 and 64 and the first and second guide rails 62 and 64 are compressed and installed toward the spreader bar 52. This subassembly is located on the top of the airtight leadscrew actuator using a portion of the leadscrew 4 axis as a reference surface.

2. Then, the air pressure nut 10 is slid on the lead screw 4. Then,

3. A precision spacer 54 is used to offset the spreader bar 52 so that the axes of the first and second guide rails 62 and 64 are aligned with the axis of the leadscrew 4. (Points A and B in Fig. 4)

4. When the lapping module 50 is properly aligned, the spreader bar 52 is secured to the base 2 and the precision spacer 54 is removed.

5. The air pressure nut 10 is then brought into contact with the outrigger 60 through a dowel pin which is pressed into the hole after alignment and assembly of the entire air pressure lead screw actuator (refer to FIG. 5 )

The geometry and profile tolerances of the threads are two important design parameters for the leadscrew 4 of the present embodiment. Since the air bearing of this embodiment operates on the side of the screw, the thread angle of the screw affects both radial and axial stiffness. For an exemplary actuator, axial stiffness is an important parameter because it directly affects the positioning accuracy of the system. The radial stiffness can also be obtained in other ways, for example by providing a set of guide rails. Ideally, square threads should be applied to achieve maximum axial stiffness in air pressure lead screw actuators. However, a thread angle of 90 degrees makes the grinding process very expensive. Thus, in an exemplary embodiment, a trapezoidal shaped thread is provided having a pitch angle (P) of 25 mm and a thread angle (alpha) of 30 degrees. The geometry of the thread profile of this embodiment is shown in Fig. However, the thread profile is not limited to the specific thread profile shown. For example, a thread profile can be made of any profile such that the air pressure nut can be fitted between the threads. Non-limiting examples include acme, trapezoid, square, and the like.

It is desirable to maximize the space between the threads so that the porous graphite disk insert 32 can be included in the housing 12 of the air pressure nut 10. [ This is achieved by increasing the distance between the threads in the exemplary profile shown in Fig.

Air Flow Cycling: When using porous graphite as an exemplary air flow limiting medium, there are two main parameters that can be controlled to affect air bearing performance, i.e., inlet pressure and permeability. The inflow pressure is the pressure applied to each of the porous graphite disk inserts. Permeability is a measure of the performance of the porous material that transports the fluid. These two parameters affect the overall stiffness of the air pressure nut. During exemplary operation, the air pressure is maintained at 120 PSI or less at typical parameter settings.

In order to calculate the inflow pressure and permeability that yield the static stiffness required for an exemplary air-pressure lead screw actuator, the air flow through the porous material and the air flow through the air gap are first determined. Air flow analysis is performed on a single porous graphite disk insert. This enables generation of lift and stiffness plots for each individual porous disk in an exemplary air pressure nut.

The following model (Darcy) is used to calculate the flow through the porous material.

…………(1)

... ... ... ... (One)

은 질량 유량,

는 단면적,

는 투과 상수, P2 및 P1은 각각의 유입 및 유출 압력,

는 공기의 점도,

는 공기에 대한 가스 상수, T는 대기 온도(300K), 그리고 H는 다공성 매체의 두께이다. At this time,

Mass flow rate,

Sectional area,

P2 < / RTI > and P1 are the inlet and outlet pressures, respectively,

Is the viscosity of air,

Is the gas constant for the air, T is the atmospheric temperature (300K), and H is the thickness of the porous medium.

Since the mass flow rate through the porous medium can be determined using the above equation (1), it is possible to calculate the mass flow rate through the porous medium according to the method described in Plante et al., &Quot; Design Model for Circular Porous Air Bearings Using the 1D Generalized Flow Model, 29, No. 3, pp. 336-346, 2005 is used to calculate the lift for a given pore. The model developed by Plante et al. Is based on differential shell elements as a model flow characteristic in a gap between a guide surface and a porous medium for a given flight height / gap. When the lift is calculated, the stiffness can be determined by the following equation.

…………(2)

... ... ... ... (2)

At this time, F is the lift and h is the flight altitude.

The model can be used to study the effect of inflow pressure on lift and stiffness by parameters.

General Manufacturing Process: For an exemplary air-pressure lead screw actuator using porous air bearings, a typical and exemplary manufacturing process includes the following steps. (See Fig. 7)

Step 1: Manufacture of a basic component comprising a housing 12 of an air pressure nut 10, a precision ground lead screw 4 and a porous medium 30 with an appropriate profile tolerance for air static pressure application. (S10, S20, S30)

Step 2: The coarse lapping operation is performed on the porous medium 30 facing the lead screw to align the graphite disk insert 32, which is the porous medium 30, with the helical thread shape. (S40)

Step 3: Attach the graphite disk insert 32 together with the leaf spring 36 to the opening 16 of the air-fitting nut 10 to ensure a firm fit when sliding the air-fitting nut 10 on the leadscrew 4, (S50).

Step 4: Apply a surface limiting layer on the roughly wrapped graphite disk insert 32 surface. (S60)

Step 5: Perform the final lapping operation using the appropriate lapping compound. (S70)

Considerations for manufacturing the housing 12 of the air pressure nut 10 and the ground lead screw 4 have been described above. Additional exemplary manufacturing steps are described in more detail below. The lapping modules shown in Figs. 3-5 may also be used during the manufacture of an exemplary airtight lead screw actuator.

Coarse Lapping Operation (S40): The coarse lapping operation on the graphite disk insert 32 ensures that the air bearing surface is roughly aligned with the geometric profile of the leadscrew (4) threads. Since the exemplary threaded spiral shape follows the trajectory, the surface of the graphite disk insert 32 must have a curved surface portion 33 as shown in Fig. Coarse lapping is used in the exemplary manufacturing operation because initially the graphite disk insert 32 will not slide on the threads because their flat surface does not fit the curved surface of the threads. Thus, during rough lapping, only one side of the nut is wrapped at a time.

During the rough lapping operation, the lead screw 4 is coated with a polishing compound. At this time, the abrasive compound may be a garnet having a rough grit size, for example, 4 um.

At this time, the nut 10 is freely slid on the lead screw 4 while the graphite disk insert is assembled in the nut housing 12, so that the graphite disk insert is roughly wrapped. The graphite disk insert 32 is placed in the positioning aperture 16 located in the housing 12 of the exemplary air pressure nut 10 after a rough lapping operation (e.g., as shown in FIG. 9).

When the exemplary air-static leadscrew 4 actuator is disassembled, each graphite disk insert 32 is matched with its corresponding positioning aperture 16 and aligned using an alignment mark 56. However, the measurement of the material removed during the coarse lapping operation is difficult due to the curved surface of the graphite disk insert 32 and the absence of the reference surface.

During the exemplary coarse lapping operation, when the graphite disk insert 32 is over-wrapped, excessive backlash occurs within the air pressure nut 10. Thereby a leaf spring 36 is placed between the back of the graphite disk insert 32 and the flange 18 to remove the backlash and to correct the tight fit in the air pressure nut 10 and the entire graphite disk insert (32).

9 shows the shape of the housing 12 of the exemplary air-fit nut 10 before the graphite disk inserts 32 are placed in their respective openings. The graphite disk insert 32 is in place in an exemplary manner, such as by gluing the graphite disk insert 32 to the flange using, for example, epoxy. (S50)

Surface Layer Restriction (S60): The permeability of the porous graphite disk insert is an important parameter that affects the stiffness of the exemplary air pressure lead screw actuator. It is helpful to control this parameter to ensure correct volume air flow into the air pressure leadscrew. Permeability has a direct correlation with porosity limitations of the graphite material used. In air bearing applications, the limitation of air flow is important to ensure stable bearing operation. Thus, an exemplary porous medium acts as a damper to prevent too much air from entering the crevice when the air bearing encounters a defect at the guide surface.

 In an exemplary air-pressure leadscrew actuator, three general limitations-particle contamination, burr due to lapping, resin impregnation-can be encountered. Particle contamination in porous media appears to be inevitable (due to, for example, atmospheric conditions). Ultrasonic cleaning of porous graphite disk inserts can eliminate most of the clogging caused by contamination. Another limitation may be encountered during the lapping operation, after the lapping, the graphite debris and the burr may be folded to leave stains in the holes and prevent air flow through the porous graphite disk inserts.

With respect to limitations caused by particle contamination and lapping, Rasnik et al., &Quot; Porous Graphite Air-Bearing Components as Applied to Machine Tools, "Society of Manufacturing Engineers, Technical Report MRR74-02, Lt; RTI ID = 0.0 > 1974. < / RTI > In this technique, the resin dissolved in the solvent is applied to the air bearing surface and pushed into the hole by vacuum to hinder air flow. Depending on whether the air flow is measured and the measured value is greater or less than the target air flow, more solvent is supplied or solvent is used to wash out some of the pre-existing lacquer already attached to the surface. This method is repeated until the desired flow rate is achieved. By manipulating these restrictions, the porous medium will act as a damper to prevent too much air from flowing into the pores when the air bearing meets the coupling at the guide surface.

Final lapping operation (S70): After the nut 10 is slid into the leadscrew 4, the final lapping is performed and the stage is fully assembled with the exemplary precision lapping module.

In the final lapping step, the lead screw 4 is coated with a polishing compound such as iron oxide having a denser thickness than that of the polishing compound used in the rough lapping step, for example, 3 um of a grit size or 1 um of red roses, The nut 10 freely slides on the screw 4 and the porous medium 30 is precisely wrapped.

The last lapping ensures that both sides of the air pressure nut 10 have an exact clearance with the fitting surface profile and leadscrew side. When the friction torque requirement of 0.1 N-m is achieved, the stage is disassembled and cleaned before final assembly.

After the air static pressure lead screw actuator is completed, the exemplary lapping module 50 is preferably part of the last actuator to act as an axial guide rail to increase the rigidity of the overall actuator.

In accordance with an exemplary embodiment of the present invention, a porous-constrained air bearing configuration and method are provided and incorporated with an air static leadscrew actuator design.

Exemplary air-pressure leadscrew actuators, including porous media disk inserts, spiral plenum chambers, and flange examples within an exemplary air-tightening nut, create multiple simultaneous air bearing surfaces to provide frictionless operation.

Porous limiting air bearings feature a particularly uniform distribution along the entire air bearing surface, which increases the stability of the air bearing.

The inflow pressure, the number of graphite disks, and the graphite disk permeability can be formed in an exemplary embodiment to compensate for proper stiffness requirements and stable operation.

The plurality of individual air bearing porous media inserts substantially eliminate stick-slip friction and backlash and provide sufficient stiffness to be performed in a micro-unit machine tool, making manufacturing tolerances more loose.

The generalized manufacturing process of this embodiment for a cost effective air static pressure lead screw actuator includes coarse lapping, porous media disk insert installation, surface layer limitation and last lapping process.

The precision lapping module is incorporated into the exemplary air static lead screw actuator to provide uniform lapping along all porous graphite surfaces.

The guide rails in the exemplary wrapping module also provide additional axial stiffness to the actuator during normal operation.

The exemplary lapping process compensates for deformation during manufacture and allows for easier control of the air bearing gap and loosens manufacturing tolerances. The exemplary air-pressure leadscrew actuator design is particularly suited for high-performance machine tools.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

1 Air pressure lead screw Actuator 2 Base
4 Lead Screw 6 Motor
10 air pressure nut 12 housing
14 inlet 16 opening
18 Flange 20 Plenum chamber
22 Pore 30 Porous media
32 Disc insert 34 O-ring
36 Plate spring 40 Nut housing
42 Nut housing Air inlet 48 Bushing
50 Lapping module 52 spreader bar
54 Precision Spacer 56 Alignment Mark
60 outrigger 62 first guide rail
64 second guide rail

Claims (18)

As an air static pressure air bearing,
Air pressure nut and
And a lead screw coupled to the air pressure nut,
The air pressure-
A housing having an air inlet for introducing air into the housing;
A helical plenum chamber disposed within the housing and having a plurality of apertures disposed in a spiral pattern and fluidly interconnected with the air inlet and the plurality of apertures,
A plurality of porous media each disposed in the plurality of openings,
The leadscrew comprising a threaded surface spaced from each of the plurality of porous media,
Wherein each of the plurality of openings includes a sheet on which the plurality of porous media is seated,
Air static air bearing. The method according to claim 1,
Wherein said plurality of openings are formed in pairs on opposite sides of said air pressure nut. The method according to claim 1,
Wherein each of said plurality of porous media is in the form of a disk. The method according to claim 1,
Wherein each of the plurality of openings is in a circular shape. The method according to claim 1,
Wherein each of the plurality of porous media comprises a graphite material. delete The method according to claim 1,
Wherein the seat comprises a flange formed within the opening. 8. The method of claim 7,
The flange is a flexible air static pressure air bearing. 8. The method of claim 7,
And a leaf spring positioned between the flange and the porous medium. The method according to claim 1,
Wherein the thread profile of the lead screw is a trapezoid. As an air-static pressure lead screw actuator,
Base;
A lead screw rotatably coupled to the base and including a threaded surface;
First and second guide rails coupled to the base and respectively disposed on the base on opposite sides of the lead screw, wherein the lead screw shaft of the lead screw and the axes of the first and second guide rails are flush with each other First and second guide rails disposed on the first and second guide rails;
An air pressure nut according to any one of claims 1 to 5, 7 to 10,
A motor coupled to the lead screw to drive the lead screw; And
And a lapping module mounted on the leadscrew to provide uniform lapping along the surface of the plurality of porous media of the air pressure nut.
Air static pressure lead screw actuator. 12. The method of claim 11,
And first and second air bushings fixedly coupled to the air pressure nut housing,
And the first and second guide rails extend through the first and second air bushings, respectively. delete 12. The method of claim 11,
The wrapping module
And a set of spreader bars for supporting two portions of the axis of the lead screw,
Wherein the spreader bar aligns the guide rails so that the axes of the guide rails and the axes of the lead screws lie on the same plane. 12. The method of claim 11,
Further comprising two lapping outriggers for attaching the housing of the air pressure-tightening nut to the guide rails,
Wherein the wrapping outrigger limits the tilt and yaw of the guide rail during a lapping operation. A method of assembling an air static pressure air bearing,
A housing having an air inlet for introducing air into the housing, a plurality of openings disposed in the housing and arranged in a spiral pattern and including a seat, the air inlet and the plurality of openings being fluidly interconnected Providing an air hydrostatic nut with a helical plenum chamber;
Providing a leadscrew comprising a threaded surface;
Providing a plurality of porous media;
First wrapping each of the plurality of porous media against the threaded surface such that the plurality of porous media conforms to a helical threaded configuration;
Inserting the porous medium into each of the openings;
Providing a surface limiting layer on the surface of the first wrapped porous media; And
And second wrapping each of the porous media
How to assemble air static pressure air bearings. 17. The method of claim 16,
In the first lapping step, the nut is freely slid on the lead screw while the rough polishing compound is applied to the lead screw, the porous medium is roughly wrapped,
In the second lapping step, the nut is freely slid on the leadscrew so that the porous medium is precisely wrapped on the leadscrew in a state where a denser polishing compound is applied to the lead screw than the abrasive compound used in the first lapping step How to assemble air static pressure air bearings. delete

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