Stepped variable-diameter variable-pitch screw vacuum pump and rotor structure design method thereof

Abstract

The invention relates to a stepped variable-diameter variable-pitch screw vacuum pump and a rotor structure design method thereof.The screw vacuum pump comprises two stepped variable-pitch variable-diameter rotors meshed with each other, and each stepped variable-pitch variable-diameter rotor is divided into two parts including a high-outer-diameter high-pitch rotor part, a high-outer-diameter high-pitch rotor part and a high-outer-diameter high-pitch rotor part, the low-outer-diameter low-pitch rotor part is used for reducing a tooth crest leakage line while reducing a rotor working cavity and reducing the length of the tooth crest leakage line while realizing internal compression; the molded lines of the two parts adopt different tooth width angles to enhance the air suction capacity of the high-outer-diameter and high-pitch rotor part and the tooth crest sealing capacity of the low-outer-diameter and low-pitch rotor part; the two rotor molded lines have the same center distance, the molded line tooth crest radius of the high-outer-diameter and high-pitch rotor part is larger than that of the low-outer-diameter and low-pitch rotor part, and a set distance is formed between the end faces of the axial structures of the two rotors. The space utilization rate of the rotor structure can be optimized, and the machining efficiency and machining precision of the rotor are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of dry screw vacuum pumps, and particularly relates to a stepped variable-diameter and variable-pitch screw vacuum pump and a rotor structure design method thereof. Background Art

[0002] Dry screw vacuum pumps are widely used in the fields of chemical industry, pharmaceuticals, and semiconductors. Compared with vacuum pumps, they have the characteristics of cleanliness, reliable operation, compatibility with particles, and high operating efficiency. Their core component is a pair of meshing rotors. By changing the axial or radial dimensions of the rotors, the volume of the working chamber of the screw vacuum pump is reduced, thereby realizing the internal compression process, which can effectively reduce the power consumption and exhaust temperature of the whole machine. However, at present, the change of the axial or radial dimensions of the screw vacuum pump rotor often relies on continuously changing pitches or outer diameters. This method allows the machining of screw rotors only by turning and milling methods and does not allow the application of forming machining methods, thereby restricting the improvement of rotor machining efficiency and accuracy, resulting in high machining difficulty and cost of existing dry screw vacuum pump rotors. The two-stage stepped variable-pitch rotor can use the forming machining method for the machining of screw rotors, but there is inevitably an interference area, resulting in a large leakage channel in the meshing area of the screw rotor, affecting the pumping speed, energy consumption, and ultimate vacuum of the dry pump. At the same time, the outer diameters of the two-stage screw rotors need to be the same to ensure good sealing performance. Therefore, the existing stepped variable-pitch rotor structure has incomplete space utilization and still has a large room for performance optimization. Summary of the Invention

[0003] The purpose of the present invention is to address the above problems in the prior art and provide a stepped variable-diameter and variable-pitch screw vacuum pump and a rotor structure design method thereof, which can greatly improve the space utilization rate of the rotor structure and the formability of the structure while realizing the effective internal compression structure of the rotor, and at the same time improve the machining efficiency and accuracy of the rotor, as well as the sealing performance of the first-stage screw rotor, and optimize the pumping speed, energy consumption, and ultimate vacuum performance of the dry screw vacuum pump.

[0004] To achieve the above object, the present invention has the following technical solutions:

[0005] A stepped variable-diameter and variable-pitch screw vacuum pump includes two stepped variable-pitch and variable-diameter rotors that mesh with each other. The stepped variable-pitch and variable-diameter rotor is divided into two parts, including a high outer-diameter and high-pitch rotor part on one side for realizing the suction volume increase of the rotor working chamber, and a low outer-diameter and low-pitch rotor part on the other side for realizing the reduction of the rotor working chamber while reducing the tooth tip leakage line and realizing internal compression while reducing the length of the tooth tip leakage line; different tooth width angles are adopted for the profiles of the high outer-diameter and high-pitch rotor part and the low outer-diameter and low-pitch rotor part to enhance the suction capacity of the high outer-diameter and high-pitch rotor part and the tooth tip sealing capacity of the low outer-diameter and low-pitch rotor part; the two parts of the profile of the stepped variable-pitch and variable-diameter rotor have the same center distance, the tooth tip radius of the profile of the high outer-diameter and high-pitch rotor part is greater than that of the low outer-diameter and low-pitch rotor part, and there is a set spacing between the axial structural end faces of the two stepped variable-pitch and variable-diameter rotors.

[0006] As a preferred solution, the two stepped variable-pitch and variable-diameter rotors are arranged inside a sealed vacuum pump housing. The vacuum pump housing includes an upper housing and a lower housing that are split vertically, and a first end cover and a second end cover installed on the left and right sides of the upper housing and the lower housing; suction and exhaust holes are correspondingly arranged on the upper housing and the lower housing, as well as a first eight-shaped outer hole structure and a partition structure with a flow hole that cooperate with the high outer-diameter and high-pitch rotor part, and a second eight-shaped outer hole structure that cooperates with the low outer-diameter and low-pitch rotor part; gas flows through the suction hole, is compressed and transported through the high outer-diameter and high-pitch rotor part to the flow hole of the partition structure, and then is compressed and transported through the low outer-diameter and low-pitch rotor part to the exhaust hole.

[0007] As a preferred solution, through holes for the ends of the two stepped variable-pitch and variable-diameter rotors to pass through are opened on the first end cover and the second end cover, and bearings and sealing components are installed in the through holes; one of the stepped variable-pitch and variable-diameter rotors is connected to a motor, and one side ends of the two stepped variable-pitch and variable-diameter rotors are connected by a synchronous gear.

[0008] As a preferred solution, a first oil tank housing and a second oil tank housing are respectively installed on the outer sides of the first end cover and the second end cover. The synchronous gear is placed inside the first oil tank housing, and the motor passes through the first oil tank housing and is connected to the stepped variable-pitch and variable-diameter rotor; the first oil tank housing and the second oil tank housing are designed according to the oil quantity and cooling requirements.

[0009] As a preferred solution, the rotor length L0 of the high outer-diameter and high-pitch rotor part is 1 to 3 times the pitch P0 of the high outer-diameter and high-pitch rotor part; the rotor length L1 of the low outer-diameter and low-pitch rotor part is 3 to 6 times the pitch P1 of the low outer-diameter and low-pitch rotor part.

[0010] As a preferred solution, the profile tooth width angle γ0 of the high outer diameter and high pitch rotor part is 30° to 120°, and the profile tooth width angle γ1 of the low outer diameter and low pitch rotor part is 100° to 160°;

[0011] The profile tooth tip radius R0 of the high outer diameter and high pitch rotor part is 1.1 to 2 times that of the profile tooth tip radius R1 of the low outer diameter and low pitch rotor part; there is a distance of 0.1P0 to 0.5P0 between the axial structure end faces of the two stepped variable pitch and variable diameter rotors.

[0012] A rotor structure design method for a stepped variable diameter and variable pitch screw vacuum pump includes the following steps:

[0013] According to the exhaust volume requirement, design the center distance d between the profiles of the high outer diameter and high pitch rotor part and the stepped variable pitch and variable diameter rotor. The tooth tip radius of the high outer diameter and high pitch rotor part is taken as R0, the pitch is taken as P0, and determine the profile tooth width angle γ0 and the axial length L0 of the high outer diameter and high pitch rotor part according to the sealing requirement;

[0014] According to the retraction space requirement, design the distance between the axial structure end faces of the two stepped variable pitch and variable diameter rotors to be 0.1 to 0.5P0;

[0015] According to the energy consumption requirement, design the low outer diameter and low pitch rotor part. The tooth tip radius of the low outer diameter and low pitch rotor part is taken as R1, the pitch is taken as P1, and determine the profile tooth width angle γ1 and the axial length L1 of the low outer diameter and low pitch rotor part according to the sealing and heat transfer requirements of the low outer diameter and low pitch rotor part;

[0016] Based on the above design parameters, solve the profiles of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part respectively to obtain two mutually meshing stepped variable pitch and variable diameter rotors.

[0017] As a preferred solution, for the step of solving the profiles of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part, the position vector of the tooth tip arc segment AE in the coordinate system O1xy is:

[0018] r AE =[R t cost R t sint], π - γ ≤ t ≤ π

[0019] In the formula, r AE represents the position vector of the tooth tip arc segment AE in the coordinate system O1xy, R t is the designed tooth tip radius, t is the curve parameter variable, and γ is the central rotation angle corresponding to the tooth tip and tooth root arc segments.

[0020] As a preferred solution, in the step of solving the profiles of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part, the position vector of the curve segment AB in the coordinate system O1xy is:

[0021] r AB =[x AB (t)y AB (t)]

[0022] In the formula, r AB represents the position vector of the curve segment AB in the coordinate system O1xy, x AB and y AB respectively represent the abscissa and ordinate of the curve segment AB in the coordinate system O1xy. The curve segment AB is any one curve or a combination of multiple curves among an involute, an arc, and a cycloid;

[0023] The curve segment BC is the conjugate curve of the curve segment AB, and its position vector in the coordinate system O1xy is:

[0024]

[0025] In the formula, r BC represents the position vector of a point on the curve CD, φ is the intermediate rotation angle variable parameter, and the relationship between the intermediate rotation angle variable parameter φ and the parameter t is obtained by the following formula:

[0026]

[0027] In the formula, and respectively represent the abscissa and ordinate of the tangential vector of the curve AB in the coordinate system O1xy.

[0028] As a preferred solution, in the step of solving the profiles of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part, the position vector of the root circular arc segment CD in the coordinate system O1xy is:

[0029] r CD =[R b cost R b sint],π+γ≤t≤π

[0030] In the formula, r CD represents the position vector of the circular arc segment CD in the coordinate system O1xy, R b is the defined root radius;

[0031] The position vector of the point engagement cycloid segment DE in the coordinate system O1xy is:

[0032]

[0033] In the formula, r DEIt represents the position vector of the point-meshing cycloid DE in the coordinate system O1xy.

[0034] Compared with the prior art, the present invention has at least the following beneficial effects:

[0035] In a screw vacuum pump, two stepped variable pitch and variable diameter rotors that mesh with each other are designed. The stepped variable pitch and variable diameter rotors are equal pitch and equal diameter rotors with two different pitches and different outer diameters. The high outer diameter and high pitch rotor part is used to increase the suction volume of the rotor working chamber. The low outer diameter and low pitch rotor part is used to reduce the rotor working chamber while reducing the tooth tip leakage line, and while realizing internal compression, reducing the length of the tooth tip leakage line. Different tooth width angles are adopted for the two parts of the rotor profile to enhance the suction capacity of the high outer diameter and high pitch rotor part and the tooth tip sealing capacity of the low outer diameter and low pitch rotor part. The stepped variable diameter and variable pitch screw vacuum pump of the present invention realizes the effective internal compression structure of the rotor while realizing the formable processing of the rotor, thereby improving the processing efficiency and processing accuracy of the rotor, and significantly reducing the processing cost and improving the processing efficiency of the rotor of the existing dry screw vacuum pump. At the same time, the two-stage variable pitch and variable diameter rotor structure can ensure good sealing of the screw, and at the same time make full use of space to improve the integration of the screw vacuum pump. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0037] Figure 1 Exploded schematic diagram of the overall structure of the stepped variable diameter and variable pitch screw vacuum pump according to the embodiment of the present invention;

[0038] Figure 2 Cross-sectional schematic diagram of the overall structure of the stepped variable diameter and variable pitch screw vacuum pump according to the embodiment of the present invention;

[0039] Figure 3 Schematic diagram of the rotor structure design method of the stepped variable diameter and variable pitch screw vacuum pump according to the embodiment of the present invention;

[0040] FIG. 4(a) Schematic diagram of the profile meshing of the high outer diameter and high pitch rotor part according to the embodiment of the present invention;

[0041] FIG. 4(b) Schematic diagram of the profile meshing of the low outer diameter and low pitch rotor part according to the embodiment of the present invention;

[0042] Figure 5 Schematic diagram of the profile structure of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part according to the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0043] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, those of ordinary skill in the art can also obtain other embodiments without creative efforts.

[0044] It should be noted that in the description of the embodiments of the present invention, the orientation or positional relationship indicated by terms such as "upper", "lower", "left", "right", "inner", "outer", "front end", "backend", "head", "tail", etc. is based on the orientation or positional relationship shown in the accompanying drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation to the present invention.

[0045] Please refer to Figure 1 and Figure 2 , an embodiment of the present invention provides a stepped variable diameter and variable pitch screw vacuum pump, which includes two stepped variable pitch and variable diameter rotors (a first stepped variable pitch and variable diameter rotor 11 and a second stepped variable pitch and variable diameter rotor 12) that mesh with each other. The stepped variable pitch and variable diameter rotor is divided into two parts, including a high outer diameter and high pitch rotor part on one side for realizing the suction volume increase of the rotor working chamber, and a low outer diameter and low pitch rotor part on the other side for realizing the reduction of the rotor working chamber while reducing the tooth tip leakage line and realizing internal compression while reducing the length of the tooth tip leakage line; different tooth width angles are adopted for the profiles of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part to enhance the suction capacity of the high outer diameter and high pitch rotor part and the tooth tip sealing capacity of the low outer diameter and low pitch rotor part; the two parts of the profile of the stepped variable pitch and variable diameter rotor have the same center distance, the tooth tip radius of the profile of the high outer diameter and high pitch rotor part is greater than that of the low outer diameter and low pitch rotor part, and there is a set distance between the axial structural end faces of the two stepped variable pitch and variable diameter rotors. In a possible implementation manner, the two stepped variable pitch and variable diameter rotors are arranged inside a sealed vacuum pump housing. The vacuum pump housing includes an upper housing 21 and a lower housing 22 that are split up and down, and a first end cover 31 and a second end cover 32 installed on the left and right sides of the upper housing 21 and the lower housing 22; suction and exhaust holes (a suction hole 210 and an exhaust hole 224) are correspondingly arranged on the upper housing 21 and the lower housing 22, as well as a first eight-shaped outer hole structure 220 and a partition structure 221 with a flow hole 222 that cooperate with the high outer diameter and high pitch rotor part, and a second eight-shaped outer hole structure 223 that cooperates with the low outer diameter and low pitch rotor part; gas flows through the suction hole 210, is compressed and transported through the high outer diameter and high pitch rotor part to the flow hole 222 of the partition structure 221, and then is compressed and transported through the low outer diameter and low pitch rotor part to the exhaust hole 224.

[0046] In a possible implementation, through holes for the ends of two stepped variable pitch and variable diameter rotors to pass through are provided on the first end cover 31 and the second end cover 32, and bearings (the first rotor end bearing 43 and the second rotor end bearing 44) and a sealing assembly 50 are installed in the through holes; one of the stepped variable pitch and variable diameter rotors (the second stepped variable pitch and variable diameter rotor 12 in the embodiment of the present invention) is connected to the rotating shaft of the motor 41, the motor 41 is installed in the motor housing (40), and one side ends of the two stepped variable pitch and variable diameter rotors are connected by a synchronous gear 42.

[0047] In a possible implementation, a first oil tank housing 45 and a second oil tank housing 46 are respectively installed on the outer sides of the first end cover 31 and the second end cover 32. During the high-speed rotation of moving parts such as the rotors, gears, and bearings of the screw vacuum pump, friction will be generated, and lubricating oil is required to reduce wear and lower frictional heat. The oil tank housing provides a storage space for the lubricating oil and transports the lubricating oil to each lubrication point through an oil circuit system to ensure the long-term stable operation of the equipment. Heat is generated during the compression of gas by the screw vacuum pump. The lubricating oil not only plays a lubricating role but also absorbs and takes away part of the heat to prevent the equipment from overheating. The screw vacuum pump needs to maintain an internal vacuum environment. The oil tank housing can prevent external impurities such as dust and moisture from entering the pump, avoiding contaminating the lubricating oil or damaging internal parts. At the same time, oil seals or seals in the oil tank can further prevent the leakage of lubricating oil. In the embodiment of the present invention, the synchronous gear 42 is placed in the first oil tank housing 45, and the motor 41 passes through the first oil tank housing 45 and is connected to the stepped variable pitch and variable diameter rotor; the first oil tank housing 45 and the second oil tank housing 46 are designed according to the oil quantity and cooling requirements.

[0048] Please refer to Figure 3 , in a possible implementation, the rotor length L0 of the high outer diameter and high pitch rotor part in the embodiment of the present invention is 1 to 3 times the pitch P0 of the high outer diameter and high pitch rotor part, and the rotor length L1 of the low outer diameter and low pitch rotor part is 3 to 6 times the pitch P1 of the low outer diameter and low pitch rotor part. The profile tooth width angle γ0 of the high outer diameter and high pitch rotor part is 30° to 120°, and the profile tooth width angle γ1 of the low outer diameter and low pitch rotor part is 100° to 160°; the two rotor profiles have the same center distance d, and the profile tooth tip radius R0 of the high outer diameter and high pitch rotor part is 1.1 to 2 times the profile tooth tip radius R1 of the low outer diameter and low pitch rotor part; there is a distance of 0.1P0 to 0.5P0 between the axial structural end faces of the two stepped variable pitch and variable diameter rotors.

[0049] Please refer to FIGS. 4(a) and 4(b) and Figure 5 , another embodiment of the present invention also proposes a rotor structure design method for the stepped variable diameter and variable pitch screw vacuum pump, including the following steps:

[0050] According to the exhaust volume requirement, design the center distance d between the two profile lines of the high outer diameter and high pitch rotor part and the stepped variable pitch and variable diameter rotor. The tip radius of the high outer diameter and high pitch rotor part is taken as R0, the pitch is taken as P0, and the profile tooth width angle γ0 and the axial length L0 of the high outer diameter and high pitch rotor part are determined according to the sealing requirement;

[0051] According to the retraction space requirement of the forming tool edge shape, design the distance between the axial structure end faces of the two stepped variable pitch and variable diameter rotors to be 0.1 - 0.5P0;

[0052] According to the energy consumption requirement, design the low outer diameter and low pitch rotor part. The tip radius of the low outer diameter and low pitch rotor part is taken as R1, the pitch is taken as P1, and the profile tooth width angle γ1 and the axial length L1 of the low outer diameter and low pitch rotor part are determined according to the sealing and heat transfer requirements of the low outer diameter and low pitch rotor part;

[0053] Based on the above design parameters, solve the profiles of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part respectively to obtain two meshing stepped variable pitch and variable diameter rotors.

[0054] The profiles of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part are exactly the same, and the axial structures are the same but the helix directions are opposite.

[0055] In a possible implementation manner, in the step of solving the profiles of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part, the position vector of the tip arc segment AE in the coordinate system O1xy is:

[0056] r AE =[R t cost R t sint], π - γ ≤ t ≤ π

[0057] In the formula, r AE represents the position vector of the tip arc segment AE in the coordinate system O1xy, R t is the designed tip radius, t is the curve parameter variable, and γ is the central rotation angle corresponding to the tip and root arc segments.

[0058] The position vector of the curve segment AB in the coordinate system O1xy is:

[0059] r AB =[x AB (t)y AB (t)]

[0060] In the formula, r AB represents the position vector of the curve segment AB in the coordinate system O1xy, x AB and y ABrespectively represent the horizontal and vertical coordinates of the curve segment AB in the coordinate system O1xy, and the curve segment AB is any one of an involute, an arc, a cycloid, or a combination of multiple curves;

[0061] The curve segment BC is the conjugate curve of the curve segment AB, and its position vector in the coordinate system O1xy is:

[0062]

[0063] In the formula, r BC represents the position vector of a point on the curve CD, φ is the intermediate rotation angle variable parameter, and the relationship between the intermediate rotation angle variable parameter φ and the parameter t is obtained by the following formula:

[0064]

[0065] In the formula, and respectively represent the horizontal and vertical coordinates of the tangential vector of the curve AB in the coordinate system O1xy.

[0066] The position vector of the root circular arc segment CD in the coordinate system O1xy is:

[0067] r CD = [R b cost R b sint], π + γ ≤ t ≤ π

[0068] In the formula, r CD represents the position vector of the circular arc segment CD in the coordinate system O1xy, and R b is the defined root radius;

[0069] The position vector of the point-engagement cycloid segment DE in the coordinate system O1xy is:

[0070]

[0071] In the formula, r DE represents the position vector of the point-engagement cycloid segment DE in the coordinate system O1xy.

[0072] Finally, according to the designed rotor structure, the overall structure of the screw vacuum pump is arranged. The upper and lower split housings are provided with an air inlet port 210 and an air outlet port 224 that are reasonably designed according to the exhaust volume, as well as a first eight-shaped outer hole structure 220 that matches the high outer diameter and high pitch rotor part, a flow hole 222 designed according to the rotor profile and the compression process, and a second eight-shaped outer hole structure 223 that matches the low outer diameter and low pitch rotor part.

[0073] The structures of the motor 41 and the synchronous gear 42 are designed specifically according to the displacement and functions. The first rotor end bearing 43 and the second rotor end bearing 44 select reasonable bearing types and inner and outer diameters according to the rotor structure.

[0074] The sealing assembly 50 can adopt forms such as double lip seals, labyrinth seals, mechanical seals, and combined seals.

[0075] The first end cover 31 and the second end cover 32 have the functions of bearing the whole machine and installing bearings and seals.

[0076] The first oil tank housing 45 and the second oil tank housing 46 can be designed specifically according to the oil volume and cooling requirements.

[0077] By designing an equal pitch and equal diameter rotor with two different pitches and different outer diameters, the present invention realizes the effective internal compression structure of the rotor while enabling the formable processing of the rotor, thereby improving the processing efficiency and precision of the rotor, significantly reducing the processing cost, and enhancing the processing efficiency of the existing dry screw vacuum pump rotor. At the same time, the two-stage variable pitch and variable diameter rotor structure can ensure good sealing performance of the screw and make full use of space to improve the integration of the screw vacuum pump.

[0078] For those skilled in the art, it is obvious that the present invention is not limited to the details described in the above embodiments, and can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention, and any reference signs in the claims should not be regarded as limiting the scope of protection involved.

[0079] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A stepped variable diameter variable pitch screw vacuum pump, characterized in that: The invention comprises two step-variable pitch and diameter-changing rotors meshing with each other, wherein the step-variable pitch and diameter-changing rotors are divided into two parts, including a high outer diameter and high pitch rotor part on one side for realizing an increase in the suction volume of the rotor working chamber, and a low outer diameter and low pitch rotor part on the other side for realizing a reduction in the rotor working chamber while reducing the tooth tip leakage line and realizing internal compression while reducing the length of the tooth tip leakage line; the profiles of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part adopt different tooth width angles to enhance the suction capacity of the high outer diameter and high pitch rotor part and the tooth tip sealing capacity of the low outer diameter and low pitch rotor part; the profiles of the two parts of the step-variable pitch and diameter-changing rotor have the same center distance, the profile tooth tip radius of the high outer diameter and high pitch rotor part is larger than that of the low outer diameter and low pitch rotor part, and the axial structural end faces of the two step-variable pitch and diameter-changing rotors have a set spacing.

2. The stepped variable diameter variable pitch screw vacuum pump according to claim 1, characterized in that: The two stepped variable pitch and variable diameter rotors are arranged inside a sealed vacuum pump housing, the vacuum pump housing comprising an upper housing (21) and a lower housing (22) divided into upper and lower parts, and a first end cover (31) and a second end cover (32) installed on the left and right sides of the upper housing (21) and the lower housing (22); the upper housing (21) and the lower housing (22) are provided with suction and exhaust ports, a first figure eight outer hole structure (220) and a partition structure (221) with a flow hole (222) matching with a high outer diameter and high pitch rotor part, and a second figure eight outer hole structure (223) matching with a low outer diameter and low pitch rotor part; The gas flows through the intake port (210), is compressed and transported to the flow hole (222) of the partition structure (221) by the high outer diameter and high pitch rotor part, and is then compressed and transported to the exhaust port (224) by the low outer diameter and low pitch rotor part.

3. The stepped variable diameter variable pitch screw vacuum pump according to claim 2, characterized in that: The first end cover (31) and the second end cover (32) are provided with through holes for the ends of two step-variable pitch and diameter-changing rotors to pass through, and bearings and sealing components are installed in the through holes; one of the step-variable pitch and diameter-changing rotors is connected to the motor (41), and one side ends of the two step-variable pitch and diameter-changing rotors are connected via a synchronous gear (42).

4. The stepped variable diameter variable pitch screw vacuum pump according to claim 3, characterized in that: A first oil tank shell (45) and a second oil tank shell (46) are respectively installed on the outer sides of the first end cover (31) and the second end cover (32); the synchronous gear (42) is placed in the first oil tank shell (45); the motor (41) passes through the first oil tank shell (45) and is connected to the stepped variable pitch and variable diameter rotor; the first oil tank shell (45) and the second oil tank shell (46) are designed according to the oil volume and cooling requirements.

5. The stepped variable diameter variable pitch screw vacuum pump according to claim 1, characterized in that: The rotor length L0 of the high outer diameter and high pitch rotor part is 1 to 3 times the pitch P0 of the high outer diameter and high pitch rotor part; the rotor length L1 of the low outer diameter and low pitch rotor part is 3 to 6 times the pitch P1 of the low outer diameter and low pitch rotor part.

6. The stepped variable diameter variable pitch screw vacuum pump according to claim 5, characterized in that: The profile tooth width angle γ0 of the high outer diameter and high pitch rotor part is 30° to 120°, and the profile tooth width angle γ1 of the low outer diameter and low pitch rotor part is 100° to 160°; The profile tooth top radius R0 of the high outer diameter and high pitch rotor part is 1.1 to 2 times the profile tooth top radius R1 of the low outer diameter and low pitch rotor part; the axial structure end faces of the two stepped variable pitch and variable diameter rotors have a distance of 0.1P0 to 0.5P0.

7. A method for designing a rotor structure of a stepped variable diameter variable pitch screw vacuum pump as claimed in any one of claims 1 to 6, characterized in that: The following steps are involved: According to the exhaust volume requirements, the center distance d between the high outer diameter and high pitch rotor part and the stepped variable pitch and variable diameter rotor is designed. The tooth top radius of the high outer diameter and high pitch rotor part is taken as R0, and the pitch is taken as P0. According to the sealing requirements, the profile tooth width angle γ0 and the axial length L0 of the high outer diameter and high pitch rotor part are determined; According to the requirements of the tool withdrawal space, the spacing between the end faces of the axial structure of the two stepped variable pitch and variable diameter rotors is designed to be 0.1~0.5P0; According to the energy consumption requirements, a low outer diameter and low pitch rotor part is designed. The tooth top radius of the low outer diameter and low pitch rotor part is taken as R1, and the pitch is taken as P1. According to the sealing and heat exchange requirements of the low outer diameter and low pitch rotor part, the profile tooth width angle γ1 and the axial length L1 of the low outer diameter and low pitch rotor part are determined; According to the above design parameters, the profiles of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part are solved respectively, and two step-variable pitch and variable diameter rotors that mesh with each other are obtained.

8. The rotor structure design method according to claim 7, characterized in that: In the step of solving the profile of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part, the position vector of the tooth tip arc segment AE in the coordinate system O1xy is: r AE =[R t cost R t sint],π-γ≤t≤π In the formula, r AE Represents the position vector of the tooth tip arc segment AE in the coordinate system O1xy, R t is the designed tooth top radius, t is the curve parameter, and γ is the central rotation angle corresponding to the arc segment of the tooth top and tooth root.

9. The rotor structure design method according to claim 8, characterized in that: In the step of solving the profile of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part, the position vector of the curve segment AB in the coordinate system O1xy is: r AB =[x AB (t)y AB (t)] In the formula, r AB Represents the position vector of curve segment AB in coordinate system O1xy, x AB With y AB They represent the horizontal and vertical coordinates of the curve segment AB in the coordinate system O1xy, respectively. The curve segment AB is any one of an involute, a circular arc, and a cycloid, or a combination of multiple thereof; Curve segment BC is the conjugate curve of curve segment AB, and its position vector in coordinate system O1xy is: In the formula, r BC represents the position vector of the point on the curve CD, φ is the intermediate angle variable parameter, and the relationship between the intermediate angle variable parameter φ and the parameter t is obtained by the following formula: In the formula, and They respectively represent the horizontal and vertical coordinates of the tangent vector of curve AB in the coordinate system O1xy.

10. The rotor structure design method according to claim 9, characterized in that: In the step of solving the profile of the high outer diameter and high pitch rotor part and the low outer diameter and low pitch rotor part, the position vector of the tooth root arc segment CD in the coordinate system O1xy is: r CD =[R b cost R b sint],π+γ≤t≤π In the formula, r CD Represents the position vector of arc segment CD in coordinate system O1xy, R b is the defined tooth root radius; The position vector of the point meshing cycloid segment DE in the coordinate system O1xy is: In the formula, r DE Represents the position vector of the point-meshing cycloid segment DE in the coordinate system O1xy.

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