Air-floating support capable of reducing jet impact and design method for air inlet hole pressure-bearing cavity

Abstract

The present invention discloses an air-floating support capable of reducing jet impact and a design method for the pressure-bearing cavity of the air intake hole. The air-floating support includes an air-floating platform, and an air intake hole composed of an air inlet, a throttle hole, and a pressure-bearing cavity is arranged through the air-floating platform. The ratio of the depth z of the pressure-bearing cavity to the diameter d of the throttle hole satisfies; the design method first determines the diameter of the throttle hole, then calculates the depth of the pressure-bearing cavity based on the depth-to-diameter ratio of the pressure-bearing cavity and the diameter of the throttle hole, and finally calculates the diameter of the pressure-bearing cavity based on the depth of the pressure-bearing cavity and the width of the jet boundary discontinuity surface. By appropriately setting the depth-to-diameter ratio of the pressure-bearing cavity, the present invention can effectively inhibit the backflow phenomenon at the bottom of the air film and improve the stability of the floating plate.

Description

Technical Field

[0001] The present invention relates to the field of aerostatic bearings, and specifically to an aerostatic bearing capable of reducing jet impact and a design method for the air inlet hole pressure-bearing cavity thereof. Background Technique

[0002] Due to the characteristics of high motion accuracy, durability, and no pollution to the environment of aerostatic bearings, they have been widely used. However, with the development of precision and ultra-precision technologies, there are higher requirements for the stiffness and stability of aerostatic bearings, and the stability control of aerostatic bearings is one of the technical difficulties.

[0003] In the application of existing aerostatic bearing technologies, porous throttle aerostatic bearings have the advantage of good stability, but have high usage requirements and high costs; the small-hole throttle technology, as a widely used and mature technology, has the advantages of easy manufacturing and low costs. However, from the perspective of the stability of the aerostatic bearing system, the small-hole throttle is not as stable as the porous throttle. Due to the huge advantages of the small-hole throttle aerostatic bearing in terms of usage cost, if its stability accuracy can be further improved, it is of great significance for expanding its application range.

[0004] The principle of a common aerostatic bearing system is as Figure 1 shown: The aerostatic bearing system includes an aerostatic platform 02 and an air inlet hole vertically penetrating the aerostatic platform 02. A floating plate 01 to be supported is arranged above the aerostatic platform 02. The air inlet hole sequentially includes an air inlet 06, a throttle hole 04, and a pressure-bearing cavity 05 from bottom to top. The air inlet 06 is used to install a gas supply nozzle 03. The gas with a constant pressure enters the pressure-bearing cavity 05 through the throttle hole 04 via the gas supply nozzle 03, and then flows out through the gap between the floating plate 01 and the aerostatic platform 02. During this process, the gas forms an air film with a certain bearing capacity between the floating plate 01 and the aerostatic platform 02 to float the floating plate 01, achieving the effect of the air film supporting the floating plate. In the aerostatic bearing system, the gas directly impacts the floating plate 01 after flowing out of the throttle hole 04, thus constituting an impact on the floating plate 01 and making the stability of the floating plate 01 worse. Therefore, the jet impact of the air flow on the floating plate is an important factor leading to the deterioration of the floating plate stability.

[0005] The jet model after the gas flows out of the throttle hole is as Figure 2As shown in the figure: After the gas exits from the throttling orifice 04 at a certain initial velocity u0, a discontinuous surface 21 with discontinuous velocity is formed with the surrounding static gas. According to the theory of turbulent mechanics, the velocity discontinuous surface 21 is unstable and will surely generate fluctuations and develop into vortices, thus causing turbulence. In this way, the originally static fluid in the surrounding will be entrained into the jet flow. As the turbulence develops, the gas entrained and moving together with the jet flow continuously increases, the jet boundary 22 gradually expands to both sides, and the flow rate increases along the way. Due to the mixing of the surrounding static fluid and the jet flow, a corresponding resistance to the jet flow is generated, which reduces the velocity of the edge part of the jet flow and makes it difficult to maintain the original initial velocity. Therefore, the velocity distribution on the jet cross-section shows uneven characteristics. The velocity in the central part of the jet is the largest, and the gas flow velocity gradually decreases away from the center. Moreover, the average gas flow velocity on the jet cross-section gradually decreases. The mixing of the jet flow and the surrounding fluid develops from the edge to the center gradually. After a certain distance, it develops to the center of the jet. Since then, the entire cross-section of the jet develops into turbulence. The mixing zone that expands inward and outward from the orifice boundary is called the mixing zone 23. The central part that is not affected by the mixing and still maintains the area of the original outlet flow velocity is called the core zone 24. The section from the orifice to the end of the core zone is called the starting section 25 of the jet. The jet after the full development of turbulence is called the main section 27 of the jet (there is a very short transition section 26 before the starting section and the main section, which is generally not considered in the analysis).

[0006] In Figure 1 In the jet model shown, the ratio d of the depth z of the pressure-bearing cavity 05 to the diameter of the throttling orifice 04 is called the depth-diameter ratio of the pressure-bearing cavity, and the ratio of the diameter d of the throttling orifice 04 to the diameter D of the pressure-bearing cavity 05 is called the orifice diameter ratio. The uneven velocity distribution on the jet cross-section is related to the depth-diameter ratio of the pressure-bearing cavity. The larger the depth-diameter ratio of the pressure-bearing cavity, the lower the average velocity on the jet cross-section, the more uniform the distribution, the smaller the impact on the floating plate, and the better the stability of the floating plate, otherwise vice versa.

[0007] Figure 3 It is a schematic diagram that the jet boundary 22 is directly sprayed onto the floating plate 01 without being restricted by the pressure-bearing cavity 05. In this case, there is a reflux gas 32 in the bottom layer of the gas film that refluxes to the bottom 31 of the pressure-bearing cavity and forms a swirl, bringing low-energy fluid into the mainstream at the bottom of the pressure-bearing cavity 05 and entraining the mainstream fluid to the wall of the pressure-bearing cavity 05 on the upper side of the pressure-bearing cavity 05, resulting in the disorder of the jet boundary layer. In addition, during the generation and development of the swirl, pressure pulsation is formed inside the gas film and acts on the lower surface of the floating plate 01 in the form of pressure waves, and the system stability is poor.

[0008] In summary, the depth-diameter ratio of the pressure-bearing cavity and the orifice diameter ratio are important factors for forming the jet, and they have a greater impact on the stability of the gas film formed by the air-floating support. Summary of the Invention

[0009] The object of the present invention is to provide an air floating support capable of reducing jet impact and a design method for the pressure-bearing cavity of the air inlet hole, so as to solve the problem of poor air film stability of the existing air floating support due to the jet effect.

[0010] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0011] An air floating support capable of reducing jet impact includes an air floating platform, and a plurality of air inlet holes are provided through the air floating platform. The air inlet holes sequentially include an air inlet, a throttle hole, and a pressure-bearing cavity along the axial direction. The outlet of the pressure-bearing cavity is located on the table surface of the air floating platform. The characteristic is that the ratio of the depth z of the pressure-bearing cavity to the diameter d of the throttle hole satisfies , so that the jet boundary formed by the pressure-bearing cavity is tangent to the outlet of the pressure-bearing cavity, and further makes the jet at the outlet of the pressure-bearing cavity reach the self-similar state, so as to reduce the impact of the jet on the floating plate.

[0012] Further, the ratio of the depth z of the pressure-bearing cavity to the diameter d of the throttle hole satisfies .

[0013] A design method for the pressure-bearing cavity of the air inlet hole of an air floating support capable of reducing jet impact includes the following steps:

[0014] Step 1: Determine the aperture d of the throttle hole;

[0015] Step 2: Let the ratio of the depth z of the pressure-bearing cavity to the aperture d of the throttle hole take any value greater than 40, and calculate the depth z of the pressure-bearing cavity based on the aperture of the throttle hole;

[0016] Step 3: Based on the depth z of the pressure-bearing cavity obtained in Step 2, calculate the width b of the discontinuous surface corresponding to the contact point between the jet boundary formed by the pressure-bearing cavity and the pressure-bearing cavity e ;

[0017] Step 4: Based on the width b of the jet discontinuous surface obtained in Step 3 e , calculate the diameter D of the pressure-bearing cavity = 2×b e ;

[0018] Step 5: Based on the depth z of the pressure-bearing cavity obtained in Step 2 and the diameter D of the pressure-bearing cavity obtained in Step 4, process the pressure-bearing cavity of the air inlet hole of the air floating support;

[0019] In Step 3, the width b of the jet discontinuous surface e = 0.114×z.

[0020] By appropriately setting the depth-diameter ratio of the pressure-bearing cavity, the present invention avoids the jet flowing out of the pressure-bearing cavity through the throttle hole and directly spraying onto the floating plate without restraint, and forms a uniform pipe flow at the outlet of the pressure-bearing cavity, which can not only effectively inhibit the backflow phenomenon at the bottom of the pressure-bearing cavity 05, but also improve the stability of the floating plate. Description of the Drawings

[0021] Figure 1 is a structural diagram of an air-floating support in the prior art.

[0022] Figure 2 is a schematic diagram of the circular turbulent flow characteristics of an air-floating support in the prior art.

[0023] Figure 3 is a schematic diagram of a free jet of an air-floating support in the prior art.

[0024] Figure 4 is a schematic diagram of a tangential jet in an embodiment of the present invention.

[0025] Figure 5 is a schematic diagram of a stable jet in an embodiment of the present invention. Detailed Embodiment

[0026] The present invention will be further described below with reference to the drawings and embodiments.

[0027] As Figure 4 shown, in this embodiment, by appropriately setting the depth-diameter ratio of the pressure-bearing cavity 05, the jet boundary 22 is exactly tangent to the outlet of the pressure-bearing cavity. Figure 4 The middle jet is tangent to the side wall of the pressure-bearing cavity to form a tangent point 41. At this time, due to the existence of a low-pressure area 42 at the bottom of the pressure-bearing cavity, the jet boundary is concave and deformed. As Figure 5 shown, at this time, the contact point of the jet boundary and the pressure-bearing cavity moves downward, and the width of the jet discontinuity surface corresponding to the contact point is b e , the outer-ring jet generates reflection after hitting the inner wall of the pressure-bearing cavity. The pressure-bearing cavity 05 plays a certain role in restricting the pipe flow of the air flow ejected from the throttle hole 04. At the same time, the backflow phenomenon at the bottom of the pressure-bearing cavity 05 is suppressed, so that the jet impact intensity acting on the floating plate is reduced, the unstable phenomenon of the air film caused by the backflow at the bottom of the pressure-bearing cavity 05 is suppressed, and the stability of the floating plate is improved.

[0028] The flow characteristics of the circular turbulent jet are as Figure 2 shown. The flow field is distributed according to the static pressure. According to the conservation of the momentum flux of each cross-section of the jet, the relationship between the axial velocity of the circular turbulent jet and the depth-diameter ratio of the pressure-bearing cavity can be obtained as shown in formula (1):

[0029] (1),

[0030] This formula shows that the axial velocity of the circular turbulent jet changes with the depth-diameter ratio of the pressure-bearing cavity. The larger the depth-diameter ratio of the pressure-bearing cavity, the smaller the axial velocity of the circular turbulent jet reaching the floating plate, and the better the stability of the floating plate. In formula (1), z is the depth of the pressure-bearing cavity 05, d is the diameter of the throttle hole 04, u m represents the axial velocity, and u0 represents the inlet velocity.

[0031] Let \(u\) m \(= u_0\), the initial length \(L_0\) of the circular turbulent jet can be obtained as shown in Equation (2):

[0032] \(L_0 = 6.2d\) (2),

[0033] Therefore, in order to reduce the direct impact of the impinging jet on the floating plate and improve the stability of the floating plate, the depth-diameter ratio of the pressure-bearing cavity should be at least 6.2. This can only reduce the impact of the initial section of the jet on the floating plate and generally cannot meet the engineering requirements for stability. Therefore, it is necessary to further increase the depth-diameter ratio of the pressure-bearing cavity.

[0034] As Figures 3 to 5 shown, the width \(b\) of the jet discontinuity surface e is related to the size of the diameter \(D\) of the pressure-bearing cavity 05. According to the relevant jet theory, it can be known that:

[0035] \(b\) e \(= 0.114×z\) (3),

[0036] In terms of achieving the balance of turbulent stress, generally, it is difficult for a turbulent jet to reach the self-similar state when the depth-diameter ratio of the pressure-bearing cavity is less than 40. When the depth-diameter ratio of the pressure-bearing cavity is greater than 40, the turbulent intensity of the jet is basically a constant, and there are obvious similarities in the distribution of turbulent shear stress and turbulent intensity. Therefore, in this embodiment, the depth-diameter ratio of the pressure-bearing cavity is designed to be a value greater than 40.

[0037] The following is the quantitative design method of this embodiment, including the following steps:

[0038] S1. Determine the diameter of the throttle hole 04: The diameter of the throttle hole 04 should be as small as possible. Limited by the processing technology, a throttle hole 04 with a diameter of 0.2 mm is selected.

[0039] S2. Determine the depth of the pressure-bearing cavity 05: Since when the depth-diameter ratio of the pressure-bearing cavity 05 is greater than 40, the turbulent intensity of the jet is basically a constant. In this embodiment, the depth-diameter ratio of the pressure-bearing cavity 05 is taken as

[0040] \(= 50\), then the depth \(z\) of the pressure-bearing cavity 05 is 10 mm.

[0041] According to Equation (3), it can be known that the thickness \(b\) of the flow field e \(= 0.114×z\), and \(b\) e \(= 0.912\) mm.

[0042] S4. Based on the width b of the jet discontinuity surface obtained in step 3 e , combined with the geometric relationship D = 2b as shown in Figure 5 , the diameter D of the pressure-bearing cavity can be calculated as D = 2 e b = 1.824 mm. e = 1.824 mm.

[0043] S5. Based on the depth z of the pressure-bearing cavity 05 calculated in step S2 being 10 mm and the diameter D of the pressure-bearing cavity calculated in step S4 being 1.824 mm, the pressure-bearing cavity 05 is machined on the air-floating support.

[0044] By measuring the stability of the air-floating platform machined according to the above method for designing the depth-diameter ratio of the pressure-bearing cavity using jet theory, the requirements of industrial production can be met.

[0045] As shown in Figure 5 , in the air-floating support obtained based on the design method of this embodiment, the intake holes penetrating through the air-floating platform 02 sequentially include an air inlet 06, a throttle hole 04, and a pressure-bearing cavity 05 along the axial direction. Among them, the aperture d of the throttle hole 04 is 0.2 mm, the depth z of the pressure-bearing cavity 05 is 10 mm, and the diameter D of the pressure-bearing cavity 05 is 1.824 mm. That is, the ratio of the depth z of the pressure-bearing cavity 05 to the diameter d of the throttle hole 04 satisfies = 50, thereby making the jet at the outlet of the pressure-bearing cavity reach the self-similar state and reducing the impact of the jet on the floating plate.

[0046] The embodiments described in the present invention are only descriptions of the preferred embodiments of the present invention, and do not limit the concept and scope of the present invention. Without departing from the design idea of the present invention, various modifications and improvements made by those skilled in the art to the technical solutions of the present invention should fall within the protection scope of the present invention. The technical content claimed by the present invention has been fully recorded in the claims.

Claims

1. An air-floating support capable of reducing jet impact, comprising an air-floating platform, wherein a plurality of air inlet holes are provided through the air-floating platform, wherein the air inlet holes sequentially comprise an air inlet, a throttle hole, and a pressure-bearing cavity along the axial direction, wherein the outlet of the pressure-bearing cavity is located on the table of the air-floating platform, characterized in that: The ratio of the depth z of the pressure chamber to the diameter d of the throttle hole satisfies , thereby making the jet at the outlet of the pressure chamber reach a self-molding state, so as to reduce the impact of the jet on the floating plate; The method for designing a pressure-bearing cavity of an air-floating support air inlet that can reduce jet impact comprises the following steps: Step 1, determine the aperture d of the throttle hole; Step 2: setting the ratio of the depth z of the pressure chamber to the diameter d of the throttle hole to any value greater than 40, and calculating the depth z of the pressure chamber based on the diameter of the throttle hole; Step 3: Based on the depth z of the pressure chamber obtained in step 2, calculate the width b of the discontinuity corresponding to the contact point between the jet boundary formed by the pressure chamber and the pressure chamber. e ; Step 4: Based on the width b of the jet discontinuity obtained in step 3 e , calculate the diameter of the pressure chamber D = 2 × b e ; Step 5: Based on the depth z of the pressure-bearing cavity obtained in step 2 and the diameter D of the pressure-bearing cavity obtained in step 4, the pressure-bearing cavity of the air-floating support air inlet hole is processed; In step 3, the width b of the jet discontinuity e =0.114×z.

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