Dry Vacuum Pump

Abstract

A dry vacuum pump (1) comprises a purge gas supply passage (9). Between a fixed-side internal end face (63) of a pump chamber (6) and a rotor end face (243) of a Roots rotor (24), a small gap portion that can maintain gas sealing is formed along an outer peripheral edge portion of the rotor end face (243), and a wide-area gap portion (94) surrounded by the small gap portion is formed. A purge gas is supplied to the gap portion (94) through the purge gas supply passage (9). The Roots rotor (24) is rotated in a state in which the purge gas is accumulated in the gap portion (94), and a suction gas near the internal end face (63) of the pump chamber (6) is diluted. Precipitation and deposition of products on the internal end face (63), and harmful effects such as corrosion of the internal end face (63) and the rotor end face (243) are reduced.

Description

[Technical field]

[0001] The present invention relates to a dry vacuum pump having a pump chamber, each of which houses a pair of Roots rotors, arranged in one or more stages, and more particularly to a dry vacuum pump capable of supplying a purge gas for diluting an intake gas such as a condensable gas or a corrosive gas. [Background technology]

[0002] In semiconductor manufacturing equipment that performs etching and CVD processes on substrates such as semiconductor wafers and liquid crystal panels, dry vacuum pumps are used to evacuate the vacuum chamber. If condensable or corrosive process gases are introduced into a dry vacuum pump, products may precipitate or accumulate on the pump chamber interior surface and the Roots rotor surface, or the pump chamber interior surface and the Roots rotor surface may corrode.

[0003] In order to avoid such problems, a purge gas (an inert gas such as nitrogen gas) is introduced into the pump to dilute the gas to be sucked that flows inside the pump chamber (see Patent Document 1).

[0004] The present inventors have proposed that, in a dry vacuum pump, only the outer peripheral edge portion of the axial end face of the Roots rotor faces the pump chamber end face with a small gap that can maintain gas sealing properties, and the remaining end face portion surrounded by this outer peripheral edge portion faces the pump chamber end face with a wide gap (see Patent Document 2). With a Roots rotor of this configuration, the area of ​​the end face portion that faces the pump chamber end face with the small gap can be made as small as possible to ensure gas sealing properties, and the remaining end face portion with a large area can be made to face the pump chamber end face with a wide gap that is unlikely to be clogged with foreign matter such as products. This makes it possible to avoid problems such as products adhering and accumulating on the pump chamber end face clogging the small gap between the pump chamber end face and the axial end face of the Roots rotor, hindering normal rotation of the Roots rotor. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2004-293466 A [Patent Document 2] International Publication No. 2020 / 234947 Summary of the Invention [Problem to be solved by the invention]

[0006] Here, it is desirable to actively introduce purge gas into the gap between the axial end face of the Roots rotor and the pump chamber end face to prevent products from adhering to and accumulating on these parts, and to prevent these parts from corroding. In particular, when the outer peripheral portion of the axial end face of the Roots rotor faces the pump chamber end face with a small gap and its inner portion faces the pump chamber end face with a wide gap, it is difficult to supply purge gas to the inner wide gap through the outer peripheral small gap that functions as a gas seal. It is desirable to be able to efficiently introduce purge gas into such a portion.

[0007] An object of the present invention is to provide a dry vacuum pump in which a purge gas for diluting suction gas such as a condensable gas or a corrosive gas can be efficiently introduced between the axial end face of the Roots rotor and the inner end face of the pump chamber. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention provides A pump room; a rotor shaft extending rotatably through a shaft hole opening on an inner end surface of the pump chamber; a Roots rotor located in the pump chamber and attached to the rotor shaft; a first gap portion for gas sealing between the interior end surface of the pump chamber and a rotor end surface of the Roots rotor, the first gap being such that an outer peripheral edge portion of the rotor end surface faces the interior end surface; a second gap portion between the inner end surface of the pump chamber and the rotor end surface, the rotor end surface facing the inner end surface with a gap wider than the first gap; In a dry vacuum pump comprising: a purge gas introduction passage extending from a purge gas introduction port opening on an outer peripheral surface of the pump casing to an inner peripheral surface opening opening on an inner peripheral surface of the shaft hole; a shaft-side communication passage formed in the rotor shaft and communicating with the inner circumferential surface opening of the shaft hole; a rotor-side communication passage formed in the rotor end surface of the root rotor and communicating between the shaft-side communication passage and the second gap portion; The present invention is characterized by the following features.

[0009] In the present invention, the rotor shaft has an outer peripheral surface portion facing an inner peripheral surface of the shaft hole, and an annular stepped end surface formed at a position adjacent to the outer peripheral surface portion, The rotor end surface of the Roots rotor may have an annular end surface that abuts the stepped end surface of the rotor shaft. In this case, the shaft side communicating passage is provided with an annular groove formed around the entire outer circumferential surface portion of the rotor shaft, and at least one communicating hole having one end communicating with the annular groove and the other end opening to the step end face. A communication groove is formed in the annular end face portion as the rotor side communication passage, and this communication groove is formed at a position corresponding to the communication hole opening into the stepped end face to communicate with the second gap portion.

[0010] This establishes a communication state between the purge gas introduction passage formed in the end wall of the pump chamber, which is the fixed side, and the shaft side communication passage formed in the rotating rotor shaft. Purge gas is supplied to the second gap surrounded by the first gap, which exhibits a gas sealing function.

[0011] Here, one axial side of the rotor shaft is the fixed side supported in a state in which axial movement is restricted, and the other side is the free side supported in a state in which axial movement is possible, and axial expansion and contraction due to thermal deformation of the rotor shaft is permitted. Hereinafter, in this specification, as necessary, of the indoor end faces on both axial sides of the pump chamber, the side of the fixed-side shaft end face will be referred to as the fixed-side indoor end face, and the opposing Roots rotor end face will be referred to as the fixed-side rotor end face.

[0012] In this case, the rotor shaft expands due to thermal expansion during pump operation, so the gap between the end face of the fixed rotor and the end face of the pump chamber is narrower when the pump is stopped than when the pump is operating. Therefore, when the pump is restarted after being stopped, the gap is narrower than when it is operating, and the product is more likely to come into contact with it. Therefore, it is sufficient to introduce purge gas at least into the second gap formed between the end face of the fixed side of the pump chamber and the rotor end face of the Roots rotor facing it.

[0013] A foreign matter scraping portion may be formed on part of the outer circumferential edge of the Roots rotor, facing the indoor end face of the pump chamber with a gap narrower than the first gap. When the Roots rotor rotates, foreign matter such as products adhering to the indoor end face of the pump chamber is scraped off by the foreign matter scraping portion facing the indoor end face of the pump chamber with a gap narrower than the first gap. Effect of the Invention

[0014] In the dry vacuum pump of the present invention, a first gap capable of maintaining gas sealing is formed along the outer periphery of the rotor end face between the interior end face of the pump chamber and the rotor end face of the Roots rotor, and a second gap having a large area is formed surrounded by the first gap. Purge gas is supplied to the second gap from the outside through a purge gas supply path. The Roots rotor rotates with purge gas accumulated in the gap. As a result, the suction gas near the interior end face of the pump chamber is diluted, and adverse effects such as product deposition and accumulation on the interior end face and corrosion of the interior end face and the rotor end face are avoided or suppressed. [Brief description of the drawings]

[0015] [Figure 1] FIG. 2A is a schematic cross-sectional view of a dry vacuum pump taken along a horizontal plane including the pump central axis, and FIG. 2B is a schematic cross-sectional view of a dry vacuum pump taken along a vertical plane including the pump central axis. [Diagram 2] (A) is an end view showing the free rotor end face of the second stage Roots rotor, (B) is an end view showing the fixed rotor end face, (C) is a longitudinal cross-sectional view of the Roots rotor cut along line CC', (D) is a side view of the Roots rotor as seen from the direction of arrow D, and (E) is a partially enlarged view showing an enlarged portion of the outer peripheral edge of the Roots rotor. [Diagram 3] 1A is an explanatory diagram showing the range in which a step surface is formed on a convex end face of a Roots rotor, and FIG. 1B is an explanatory diagram showing the range in which the step surface moves. [Figure 4] 1A is an explanatory diagram showing a purge gas supply path in a second-stage pump chamber, FIG. 1B is an explanatory diagram showing a shaft-side communicating passage thereof, and FIG. 1C is an explanatory diagram showing a rotor-side communicating passage thereof. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, an embodiment of a dry vacuum pump to which the present invention is applied will be described with reference to the drawings. The dry vacuum pump described below is a two-stage dry vacuum pump, but the present invention is similarly applicable to a one-stage dry vacuum pump and a multi-stage dry vacuum pump having a different number of stages. In addition, in the following example, a two-blade rotor is used as the Roots rotor, but the shape of the Roots rotor is not limited to the two-blade type and may be, for example, a three-blade rotor.

[0017] FIG. 1(A) is a schematic cross-sectional view of a dry vacuum pump taken along a horizontal plane including the central axis of the pump, and FIG. 1(B) is a schematic cross-sectional view of the dry vacuum pump taken along a vertical plane including the central axis of the pump.

[0018] As shown in these figures, the dry vacuum pump 1 includes a motor 2, a pump body 3, and a gear chamber 4. In the pump body 3, a first-stage pump chamber 5 and a second-stage pump chamber 6 are arranged on the free side (vacuum side) along the direction of the pump central axis 1a. The motor 2 is arranged on the side of the first-stage pump chamber 5 on the free side of the pump body 3, and the gear chamber 4 is arranged on the side of the second-stage pump chamber 6 on the fixed side.

[0019] The pump body 3 includes a free-side end plate 11 attached to the side of the motor 2, a cylindrical casing 12 having one end sealed by the end plate 11, and a fixed-side end plate 13. The inside of the casing 12 is partitioned by an inner wall 14. Inside the casing 12, a first-stage pump chamber 5 is formed between the end plate 11 and the inner wall 14, and a second-stage pump chamber 6 is formed between the inner wall 14 and the end plate 13.

[0020] An intake port 15 is formed in the casing 12, and the intake port 15 is connected to the first-stage pump chamber 5. The first-stage pump chamber 5 is connected to the second-stage pump chamber 6 via a communication passage (not shown) formed in the inner wall 14. The second-stage pump chamber 6 is connected to an exhaust port 16 formed in the end plate 13.

[0021] A driving rotor shaft 21 having a central axis coincident with the pump central axis 1a is coaxially connected to or integrally formed with the motor shaft 2a of the motor 2. The rotor shaft 21 extends through the end plate 11, the pump chamber 5, the inner wall 14, the pump chamber 6, and the end plate 13, and its shaft end 21a on the tip side is located inside the gear chamber 4. A driven rotor shaft 22 extends parallel to the rotor shaft 21 and penetrates these portions, and its shaft end 22a on the tip side is located inside the gear chamber 4. Inside the gear chamber 4, a driving transmission gear 4a attached to the shaft end 21a of the rotor shaft 21 meshes with a driven transmission gear 4b attached to the shaft end 22a of the driven rotor shaft 22, and the rotor shafts 21 and 22 rotate synchronously in opposite directions.

[0022] A first-stage two-lobe Roots rotor 23 and a second-stage two-lobe Roots rotor 24 are coaxially attached to the driving rotor shaft 21. Similarly, a first-stage two-lobe Roots rotor 23A paired with the Roots rotor 23 and a second-stage two-lobe Roots rotor 24A paired with the Roots rotor 24 are coaxially attached to the driven rotor shaft 22. The pair of Roots rotors 23, 23A are located in the first-stage pump chamber 5, and the pair of Roots rotors 24, 24A are located in the second-stage pump chamber 6.

[0023] The rotor shaft 21 has a shaft end 21a on the gear chamber 4 side supported by the end plate 13 in a freely rotatable state via a bearing 17, and the other shaft end 21b supported by the end plate 11 in a freely rotatable state via a bearing 18. The shaft end 21a of the rotor shaft 21 is a fixed-side shaft end supported by the end plate 13 via the bearing 17 so as not to move in the axial direction, and the other shaft end 21b is a free-side shaft end supported by the end plate 11 via the bearing 18 so as to be movable in the axial direction, so that axial elongation due to thermal expansion of the rotor shafts 21 and 22 during pump operation is permitted. The other rotor shaft 22 is similarly supported by the bearings 17A and 18A. As described above, for the interior end surface of the pump chamber and the rotor end surface of the Roots rotor, the side of the fixed-side shaft end in the axial direction will be called the fixed side, and the other side in the axial direction will be called the free side, as necessary.

[0024] When the motor 2 is driven, the rotor shaft 21 on the driving side is driven to rotate, and the rotor shaft 22 on the driven side also rotates synchronously in the opposite direction. Gas sucked into the first-stage pump chamber 5 from the intake port 15 is compressed by a pair of Roots rotors 23, 23A that rotate synchronously in the opposite direction while maintaining a small gap along the inner circumferential surface of the pump chamber 5, and is transferred to the second-stage pump chamber 6 through a communication passage (not shown). The compressed gas transferred to the pump chamber 6 is further compressed by a pair of Roots rotors 24, 24A that rotate synchronously in the opposite direction while maintaining a small gap along the inner circumferential surface of the pump chamber 6, and is discharged from the exhaust port 16 to the outside of the pump.

[0025] When the dry vacuum pump 1 is used to discharge process gases such as condensable gases and corrosive gases, products may adhere and accumulate inside the pump. In addition, corrosion may occur on the inner surface of the pump. In particular, in the first and second pump chambers 5 and 6, the rotor end faces (axial end faces) of the Roots rotors 23, 23A, 24, and 24A face the interior end faces of the pump chambers 5 and 6 with a small gap between them to ensure gas sealing, and the Roots rotors 23, 23A, 24, and 24A rotate in this state. If process gas enters between the interior end faces of the pump chambers 5 and 6 and the rotor end faces of the Roots rotors 23, 23A, 24, and 24A, foreign matter such as reaction by-products may adhere and accumulate, and the foreign matter may clog the gap between the interior end faces of the pump chambers 5 and 6 and the rotor end faces of the Roots rotors 23, 23A, 24, and 24A, hindering proper driving. In addition, corrosion may occur on these surfaces.

[0026] Furthermore, when the pump is in operation, the rotor shafts 21 and 22 expand due to thermal expansion, and the gap between the rotor end faces of the Roots rotors 23, 23A, 24, and 24A fixed to the rotor shafts 21 and 22 and the indoor end faces of the pump chambers also fluctuates, albeit slightly. Of the rotor end faces on both sides in the axial direction, the gap between the fixed rotor end face and the opposing indoor end faces of the fixed sides of the pump chambers 5 and 6 is narrower when the pump is stopped than when the pump is in operation. Therefore, when the pump is restarted after being stopped, the gap is narrower than when it is in operation, and the product is likely to come into contact with the fixed rotor end face and the indoor end face that face it across the narrow gap.

[0027] Therefore, in the dry vacuum pump 1 of this embodiment, the Roots rotors 23, 23A, 24, 24A are provided with an adhesion scraping function for scraping off foreign matter adhering to the interior end faces of the fixed sides of the pump chambers 5, 6. In addition, a purge gas supply mechanism 8 is provided so that a purge gas (an inert gas such as nitrogen gas) for diluting the suction gas can be supplied from outside the pump to between the interior end face of the second-stage pump chamber 6 and the rotor end faces of the Roots rotors 24, 24A. The purge gas supply mechanism 8 includes a purge gas supply source 81, an external pipe 82, and a purge gas supply path 9 formed inside the dry vacuum pump 1.

[0028] (Roots Rotor) The second-stage Roots rotor 24 will be described with reference to FIG. 2. FIG. 2(A) is an end view showing the rotor end face on the free side of the second-stage Roots rotor 24, FIG. 2(B) is an end view showing the rotor end face on the fixed side, FIG. 2(C) is a longitudinal sectional view taken along line CC', FIG. 2(D) is a side view taken from the direction of arrow D, and FIG. 2(E) is a partially enlarged view showing a part of the outer peripheral edge of the Roots rotor 24. In this example, since the pair of Roots rotors 24 and 24A arranged in the pump chamber 6 are common parts, one of the Roots rotors 24 will be described, and the description of the other Roots rotor 24A will be omitted. Note that the pair of Roots rotors 23 and 23A arranged in the first-stage pump chamber 5 are also common parts, and since their basic configurations are similar to those of the Roots rotor 24, their description will also be omitted.

[0029] The Roots rotor 24 includes a cylindrical boss 24a coaxially fixed to the rotor shaft 21, and a rotor body 24b having a bilobe-shaped profile. A rotor outer peripheral surface 241 of the Roots rotor 24 faces the inner peripheral surface of the casing 12, which is the inner peripheral surface 61 of the pump chamber 6, with a small gap. A free-side rotor end surface 242 of the Roots rotor 24 faces an indoor end surface 62 (inner wall 14) on one free side of the pump chamber 6 with a small gap from the direction of the pump central axis 1a. A rotor end surface 243 on the other fixed side also faces an indoor end surface 63 (end surface of the end plate 13) on the other fixed side of the pump chamber 6 with a small gap from the direction of the pump central axis 1a.

[0030] An annular end face 246 on the fixed side of the boss 24a of the Roots rotor 24 abuts against an annular stepped end face formed on the rotor shaft 21, and an annular end face 246C on the free side abuts against an annular end face of a cylindrical collar 25 that abuts against the first-stage Roots rotor 23 (see FIG. 1). Note that, as described later (see FIG. 4), a communication groove (rotor-side communication passage) that is part of the purge gas supply path is formed in the annular end face 246 on the fixed side of the boss 24a.

[0031] As shown in FIG. 2B, a rotor end face 243 on the fixed side of the root rotor 24 is formed with a convex end face 245 of a constant width along the outer periphery connected to the rotor outer periphery 241. The convex end face 245 merges with an annular end face 246 of the boss 24a at both sides of the minor axis line S at a position on the center side of the rotor end face 243 having a bilobe-shaped outline. In this example, the convex end face 245 and the annular end face 246 of the boss 24a are located on the same plane. In addition, the part of the rotor end face 243 surrounded by the convex end face 245 and the annular end face 246 is a concave end face 247 that is recessed from the convex end face 245. The concave end faces 247 are formed on both sides of the major axis line L of the rotor end face 243 having a bilobe-shaped outline.

[0032] As shown in FIG. 2(C), the convex end surface 245 faces the fixed side indoor end surface 63 of the pump chamber 6 with a gap equal to or smaller than a minute first gap Δ1 so as to ensure gas sealing. For example, the gap is equal to or smaller than 1 mm. A gap portion for gas sealing (first gap portion) is formed between the convex end surface 245 and the indoor end surface 63 of the pump chamber 6. In contrast, the concave end surface 247 faces the indoor end surface 63 of the pump chamber 6 with a gap Δ wider than the minute gap Δ1. A gap portion 94 (second gap portion) is formed between the concave end surface 247 and the indoor end surface 63 of the pump chamber 6.

[0033] 2(B), (D), and (E), the convex end surface 245 has a step surface 245A formed by protruding a portion of the convex end surface 245. The step surface 245A faces the interior end surface 63 of the pump chamber 6 across a second gap Δ2 narrower than the first gap Δ1. In FIG. 2(B), the area in which the step surface 245A is formed on the convex end surface 245 is shown by a shaded area. For example, the step surface 245A protrudes toward the interior end surface 63 of the pump chamber 6 by about 1 mm or less beyond the convex end surface 245. The step surface 245A functions as a foreign matter scraping portion that scrapes off foreign matter adhering to the interior end surface 63 of the pump chamber 6.

[0034] The convex end surface 245 and the step surface 245A are defined by a plane perpendicular to the pump central axis 1a, and the concave end surface 247 is also defined by a plane perpendicular to the pump central axis 1a. It is also possible to define the convex end surface 245 and the step surface 245A by a convex curved surface, and the concave end surface 247 by a concave curved surface.

[0035] Next, the rotor end face 242 on the free side of the root rotor 24 has basically the same structure as the rotor end face 243 on the fixed side. As shown in FIG. 2(A), a convex end face 245C of a constant width is formed along the outer peripheral edge connected to the rotor outer peripheral surface 241 over the entire circumference of the outer peripheral edge. The convex end face 245C merges with the annular end face 246C of the boss 24a at both sides of the minor axis line S at a position on the center side of the rotor end face 242 having a bilobe-shaped outline. In addition, the part of the rotor end face 242 surrounded by the convex end face 245C and the annular end face 246C is a concave end face 247C that is recessed from the convex end face 245C. The concave end faces 247C are formed on both sides of the major axis line L of the rotor end face 242 having a bilobe-shaped outline.

[0036] The convex end surface 245C faces the indoor end surface 62 of the pump chamber 6 with a certain first gap Δ1 so as to ensure gas sealing performance. For example, the facing is a minute gap of 1 mm or less. The concave end surface 247C faces the indoor end surface 62 of the pump chamber 6 with a gap wider than the minute gap Δ1. The convex end surface 245C is defined by a plane perpendicular to the pump central axis 1a, and the concave end surface 247C is also defined by a plane perpendicular to the pump central axis 1a. It is also possible to define the convex end surface 245C by a convex curved surface and the concave end surface 247C by a concave curved surface.

[0037] FIG. 3(A) is an explanatory diagram showing the formation range of the step surface 245A on the rotor end surface 243 on the fixed side of the Roots rotor 24, and FIG. 3(B) is an explanatory diagram showing the movement range of the step surface 245A during rotation. The formation range of the step surface 245A is set based on the tendency of the products to precipitate on the indoor end surface 63 of the pump chamber 6. In addition, the smaller the range of the step surface 245A, the smaller the contact area with the products that have adhered or precipitated when the pump is restarted after stopping, and the larger the sliding resistance, which is advantageous for restarting. In this example, in consideration of the fact that more products are precipitated in the part of the indoor end surface 63 of the pump chamber 6 that faces the part of the rotor end surface 243 of the Roots rotor 24 that is on the outer periphery side than the rotation center side, the step surface 245A is arranged so that the product deposition portion is scraped off and the area of ​​the step surface 245A is minimized.

[0038] In the convex end surface 245 formed in a band shape with a constant width along the outer periphery of the rotor end surface 243 on the fixed side of the Roots rotor 24, a step surface 245A for scraping off foreign matter extends from a position P1 (first position) of one outermost edge end intersecting with the major axis line L to a position P2 (second position) at a predetermined distance X from the rotation center O toward the rotor rotation direction R. The distance X can be set to ½ or less of the major axis r at the position of the major axis line L. As shown in FIG. 3B, the moving range (shaded area) of the step surface 245A is the region between a circle C1 described by the outermost edge (position P1) of the rotor end surface 243 of the Roots rotor 24 and a circle C2 described by the position of the distance X (position P2).

[0039] In this example, the Roots rotor 24 is a common part to the other Roots rotor 24A that rotates synchronously in the opposite direction. Therefore, as shown by the shaded areas in Fig. 3(A) , the convex end surface 245 has step surfaces 245A formed in two places so as to be symmetrical with respect to the short diameter line S.

[0040] In this manner, a convex end face 245 of a constant width is formed along the outer periphery of the fixed rotor end face 243 of the Roots rotor 24. This ensures a gas seal between the rotor end face 243 of the Roots rotor 24 and the indoor end face 63 of the fixed side of the pump chamber 6.

[0041] In addition, a part of the convex end surface 245 is a step surface 245A for scraping foreign matter, which faces the indoor end surface 63 of the pump chamber 6 with a narrower second gap Δ2. The second gap Δ2 is set as narrow as possible within a range where the step surface 245A does not contact the indoor end surface 63 of the pump chamber 6. This allows foreign matter such as products adhering to and accumulating on the indoor end surface 63 of the pump chamber 6 to be effectively scraped off by the part where the step surface 245A is formed. In the indoor end surface 63 of the pump chamber 6, products adhering to and accumulating on this surface get stuck between the convex end surface 245 of the Roots rotor 24, which faces the indoor end surface with a small first gap Δ1 to ensure gas sealing, and the indoor end surface 63, thereby preventing or suppressing problems such as impeding the rotation of the Roots rotor 24.

[0042] Furthermore, the area of ​​the convex end face 245 facing the interior end face 63 of the pump chamber 6 across the minute first gap Δ1 is narrow in order to ensure gas sealing performance, and the area of ​​the concave end face 247 facing the interior end face 63 of the pump chamber 6 across a wide gap is wide in order to avoid problems caused by the accumulation and fixation of products. Thus, a dry vacuum pump 1 can be obtained that can reliably eliminate problems caused by the accumulation and fixation of products while maintaining pumping performance.

[0043] As mentioned above, the first-stage Roots rotors 23 and 23A have the same structure as the Roots rotors 24 and 24A, but differ only in overall length, and similar effects are obtained on the first-stage pump chamber 5 side, so descriptions of these are omitted.

[0044] (Purge gas supply mechanism) The purge gas supply path 9 of the purge gas supply mechanism 8 formed in the dry vacuum pump 1 will be described with reference to Fig. 4. Fig. 4(A) is an explanatory diagram showing the purge gas supply path 9 in the second-stage pump chamber 6 of the dry vacuum pump, Fig. 4(B) is an explanatory diagram showing its shaft side communicating passage, and Fig. 4(C) is an explanatory diagram showing its rotor side communicating passage.

[0045] Purge gas supplied from a purge gas supply source 81 outside the pump via external piping 82 is supplied via a purge gas supply path 9 directly to two gaps 94 (second gaps) formed between the indoor end face 63 on the fixed side of the pump chamber 6 and the concave end face 247 on the fixed side of the Roots rotor 24. The purge gas supply path 9 includes a purge gas introduction passage 91 formed in the end plate 13, a shaft side communicating passage 92 formed in the rotor shaft 21, and a rotor side communicating passage 93 formed in the annular end face 246 on the fixed side of the boss 24a of the Roots rotor 24.

[0046] The purge gas introduction passage 91 extends in the radial direction of the housing. The purge gas introduction passage 91 is formed inside the end plate 13 and communicates with a purge gas introduction port 91a that opens on the outer circumferential surface of the casing. An external pipe 82 is connected to the purge gas introduction port 91a. The end plate 13 is formed with a shaft hole 13a through which the shaft portion 21c of the rotor shaft 21 passes in a rotatable state, and the inner end opening of the purge gas introduction passage 91 communicates with an inner circumferential surface opening formed on the inner circumferential surface of the shaft hole 13a.

[0047] 4(A) and (B), the shaft portion 21c of the rotor shaft 21 penetrating the shaft hole 13a of the end plate 13 has a diameter slightly larger than the adjacent shaft portion 21d to which the Roots rotor 24 is attached, and an annular stepped end face 21e is formed between these shaft portions 21c and 21d. An annular end face 246 of the boss 24a of the Roots rotor 24 abuts against the stepped end face 21e from the axial direction.

[0048] The shaft-side communication passage 92 formed in the rotor shaft 21 includes an annular groove 92a and a communication hole 92b. The annular groove 92a is a groove of a constant width and a constant depth formed along the outer circumferential surface of the large-diameter shaft portion 21c over the entire circumference. The groove opening of the annular groove 92a is substantially closed by the inner circumferential surface of the shaft hole 13a of the end plate 13, and communicates with the inner end opening of the purge gas introduction passage 91 that opens on the inner circumferential surface. The communication hole 92b extends in the axial direction inside the shaft portion 21c, with one end communicating with the bottom portion of the annular groove 92a and the other end opening on the annular stepped end surface 21e. In this example, four communication holes 92b are formed. Of course, the number of communication holes 92b is not limited to four.

[0049] 4(C), the rotor-side communication passages 93 in this example are four communication grooves 93a formed in the annular end face 246 of the boss 24a of the Roots rotor 24. The communication grooves 93a are grooves of a constant width and a constant depth formed at positions facing the respective communication holes 92b opening in the annular stepped end face 21e of the rotor shaft 21. The outer peripheral ends of two of the communication grooves 93a communicate with one of the gap portions 94 formed between the fixed-side indoor end face 63 of the pump chamber 6 and the fixed-side rotor end face 243 of the Roots rotor 24, and the remaining two communication grooves 93a communicate with the other of the gap portions 94.

[0050] In the pump chamber 6 of the dry vacuum pump 1, a purge gas for diluting the suction gas is supplied from the outside through a purge gas supply path 9 between the interior end face 63 of the fixed side and the rotor end face 243 of the Roots rotor 24. The purge gas is introduced into a purge gas inlet passage 91 through an external piping 82, and is guided from the purge gas inlet passage 91 through a shaft side communicating passage 92 formed in the rotor shaft 21 to a communicating groove 93a, which is a rotor side communicating passage 93 formed in the annular end face 246 of the boss 24a of the Roots rotor 24. The purge gas is supplied through the communicating groove 93a to two gaps 94 formed between the Roots rotor 24 and the interior end face 63 of the pump chamber 6.

[0051] The gap 94 is surrounded by a minute gap for gas sealing formed between the convex end face 245 of the Roots rotor 24 and the interior end face 63 of the pump chamber 6. Therefore, the Roots rotor 24 rotates with the purge gas supplied to the gap 94 accumulating in the gap 94. The suction gas that enters between the rotor end face 243 of the Roots rotor 24 and the interior end face 63 of the pump chamber 6 is diluted by the purge gas accumulating in the gap 94. In this way, the purge gas can be reliably supplied between the rotor end face 243 of the Roots rotor 24 and the interior end face 63 of the pump chamber 6, and the purge gas can be retained between them. This reliably prevents harmful effects such as the deposition and accumulation of products in these parts and the occurrence of corrosion in these parts.

[0052] In this example, the purge gas is supplied to the gap 94 between the interior end face 63 of the second-stage pump chamber 6 and the rotor end face 243 of the Roots rotor 24. It goes without saying that the purge gas can also be supplied to the gap between the interior end face 53 of the first-stage pump chamber 5 and the rotor end face 233 of the Roots rotor 23 in the same manner.

Claims

1. A pump room; a rotor shaft extending rotatably through a shaft hole opening on an inner end surface of the pump chamber; a Roots rotor located in the pump chamber and attached to the rotor shaft; a first gap portion for gas sealing between the interior end surface of the pump chamber and a rotor end surface of the Roots rotor, the first gap being formed such that an outer peripheral edge portion of the rotor end surface faces the interior end surface through a first gap; a second gap portion between the inner end surface of the pump chamber and the rotor end surface, the second gap portion being such that the rotor end surface faces the inner end surface with a gap larger than the first gap; a purge gas introduction passage extending from a purge gas introduction port opening on an outer peripheral surface of the pump casing to an inner peripheral surface opening opening on an inner peripheral surface of the shaft hole; a shaft-side communication passage formed in the rotor shaft and communicating with the inner circumferential surface opening of the shaft hole; a rotor-side communication passage formed in the rotor end surface of the Roots rotor and communicating between the shaft-side communication passage and the second gap portion; A dry vacuum pump comprising:

2. In claim 1, the rotor shaft has an outer circumferential surface portion facing an inner circumferential surface of the shaft hole, and an annular stepped end surface formed at a position adjacent to the outer circumferential surface portion, the rotor end surface of the Roots rotor has an annular end surface that abuts against the stepped end surface of the rotor shaft, the shaft-side communicating passage includes an annular groove formed around the entire circumference of the outer circumferential surface portion of the rotor shaft, and at least one communicating hole having one end communicating with the annular groove and the other end opening into the stepped end face, The rotor-side communicating passage is a communicating groove formed in the annular end face portion, and this communicating groove is formed at a position corresponding to the communicating hole opening into the stepped end face, and is connected to the second gap portion.

3. In claim 2, a rotor end face that is one of the rotor end faces on both sides in the axial direction and that is the end face on which the gap between the rotor end face and the inner end face of the pump chamber is narrower when the pump is stopped than when the pump is operating, in a dry vacuum pump.

4. In claim 3, A dry vacuum pump in which a portion of the outer peripheral edge portion of the Roots rotor is a foreign matter scraping portion that faces the inner end surface of the pump chamber with a third gap narrower than the first gap.

5. In claim 1, a rotor end face that is one of the rotor end faces on both sides in the axial direction and that is the end face on which the gap between the rotor end face and the inner end face of the pump chamber is narrower when the pump is stopped than when the pump is operating, in a dry vacuum pump.

6. In claim 1, A dry vacuum pump in which a portion of the outer peripheral edge portion of the Roots rotor is a foreign matter scraping portion that faces the inner end surface of the pump chamber with a gap narrower than the first gap.

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