JP7746864B2 - vacuum pump - Google Patents

Description

The present invention relates to a vacuum pump.

In fields such as semiconductor manufacturing equipment, vacuum pumps are used to create a high vacuum atmosphere (see, for example, Patent Document 1).

The vacuum pump shown in Patent Document 1 has a turbine pump section located on the intake side and a drag pump section located on the exhaust side.

When using a turbomolecular pump for exhaust, one of the performance indicators is the intake pressure relative to the gas flow rate, and the intake pressure increases as the gas flow rate increases.

Another performance indicator used when using a turbomolecular pump for exhaust is the backpressure characteristic. Backpressure characteristics are the results of measuring the change in intake port pressure when the pressure at the exhaust port of the turbomolecular pump is increased while a constant amount of gas is introduced. As the exhaust port pressure increases, the number of gas molecules that flow back from the exhaust side to the intake side increases, causing the pressure on the intake port side to increase. The higher the exhaust port pressure at which the intake port pressure begins to rise, the more effectively the gas molecules that flow back can be reduced, and the better the backpressure characteristic is considered to be.

Japanese Patent Application Laid-Open No. 2021-102926

On the other hand, in recent years, the gas flow rate flowing into turbomolecular pumps has tended to increase, creating a need to reduce the intake port pressure when exhausting large flow rates.

In order to reduce the intake port pressure during high-flow exhaust, it is effective to design the drag pump section to have a larger conductance. However, with this design, not only do more gas molecules move from the intake side to the exhaust side, but more gas molecules flow back from the exhaust side to the intake side, resulting in poor backpressure characteristics.

The objective of the present invention is to provide a vacuum pump that can handle large-flow exhaust and has improved back pressure characteristics.

A vacuum pump according to one aspect of the present invention comprises a housing, a rotor cylindrical portion, and a stator cylindrical portion. The housing has an intake port for drawing in gas and an exhaust port for discharging the drawn gas. The rotor cylindrical portion is housed in the housing. The stator cylindrical portion is housed in the housing and is disposed opposite the rotor cylindrical portion. A thread groove is formed on one of the opposing surfaces of the stator cylindrical portion and the rotor cylindrical portion. The groove depth is smaller at the exhaust side end than at the intake side end. The rate of decrease in groove depth is greater on the intake side than on the exhaust side.

The above-described aspects of the present invention make it possible to provide a vacuum pump that can handle high-flow exhaust and has improved back pressure characteristics.

1 is an external view of a vacuum pump according to a first embodiment. 2 is a perspective view of a stator cylindrical portion of the vacuum pump according to the first embodiment, as viewed from the intake side. FIG. 3 is a perspective view of a stator cylindrical portion of the vacuum pump according to the first embodiment, viewed from the opposite side to that of FIG. 2. FIG. FIG. 1A is a view from the inside of the threads near the intake end of the stator cylindrical portion of the vacuum pump according to the first embodiment; FIG. 1B is a view from the inside of the threads near the exhaust end of the stator cylindrical portion of the vacuum pump according to the first embodiment. 3 is a cross-sectional view perpendicular to the screw angle of the stator cylindrical portion of the vacuum pump according to the first embodiment. FIG. FIG. 4 is a diagram showing a change in the thread groove depth of the cylindrical stator portion of the vacuum pump according to the first embodiment. FIG. 10 is a diagram showing the change in thread groove depth in a comparative example. FIG. 1 is a graph showing changes in intake-side pressure relative to exhaust-side pressure of a drag pump section in Example 1 and Comparative Example 1. FIG. 1 is a graph showing changes in intake-side pressure relative to exhaust-side pressure of a drag pump section in Example 1 and Comparative Example 1. FIG. 10 is a graph showing the change in intake-side pressure relative to the exhaust-side pressure of the drag pump section in Example 2 and Comparative Example 2. FIG. 10 is a graph showing the change in intake-side pressure relative to the exhaust-side pressure of the drag pump section in Example 2 and Comparative Example 2. FIG. 10 is a graph showing the change in intake-side pressure relative to the exhaust-side pressure of the drag pump section in Examples 3 and 4 and Comparative Example 2. FIG. 10 is a graph showing the change in intake-side pressure relative to the exhaust-side pressure of the drag pump section in Examples 3 and 4 and Comparative Example 2. FIG. 10 is a diagram showing a change in the depth of the thread groove of the cylindrical stator portion of the vacuum pump according to the second embodiment. FIG. 10 is a diagram showing a change in the depth of the thread groove of the cylindrical stator portion of the vacuum pump according to the second embodiment. FIG. 10 is a diagram showing a change in the depth of the thread groove of the cylindrical stator portion of the vacuum pump according to the second embodiment. FIG. 10 is a diagram showing a change in the depth of the thread groove of the cylindrical stator portion of the vacuum pump according to the second embodiment. FIG. 10 is a graph showing the change in intake-side pressure relative to the exhaust-side pressure of the drag pump section in Examples 5 to 7 and Comparative Example 4. FIG. 10 is a graph showing the change in intake-side pressure relative to the exhaust-side pressure of the drag pump section in Examples 5 to 7 and Comparative Example 4. FIG. 10 is a graph showing the change in intake-side pressure relative to the exhaust-side pressure of the drag pump section in Examples 8 to 10 and Comparative Example 5. FIG. 10 is a graph showing the change in intake-side pressure relative to the exhaust-side pressure of the drag pump section in Examples 8 to 10 and Comparative Example 5. FIG. 10 is a graph showing the change in intake-side pressure relative to the exhaust-side pressure of the drag pump section in Examples 11 and 12 and Comparative Example 5. FIG. 10 is a graph showing the change in intake-side pressure relative to the exhaust-side pressure of the drag pump section in Examples 11 and 12 and Comparative Example 5. FIG. 10 is a graph showing the change in intake-side pressure relative to the exhaust-side pressure of the drag pump section in Examples 13 to 15 and Comparative Example 5. FIG. 10 is a graph showing the change in intake-side pressure relative to the exhaust-side pressure of the drag pump section in Examples 13 to 15 and Comparative Example 5. FIG. 10 is a diagram showing a change in thread groove depth in a modified example of the embodiment.

The vacuum pump according to an embodiment of the present disclosure will be described below with reference to the drawings.

(Embodiment 1)
The vacuum pump of the first embodiment will be described below.

(Outline of vacuum pump 1)
FIG. 1 is a cross-sectional view of a vacuum pump 1 according to an embodiment.

The vacuum pump 1 includes a turbine section P1 and a drag pump section P2. The turbine section P1 constitutes a turbomolecular pump. The drag pump section P2 constitutes a threaded pump. The vacuum pump 1 is connected to an evacuation target device that includes a space to be evacuated. Gas from the space to be evacuated is evacuated by the turbine section P1, then evacuated by the drag pump section P2, and then exhausted outside the vacuum pump 1.

As shown in FIG. 1, the vacuum pump 1 has a housing 2 (an example of a housing), a rotor 3, a motor 4, multiple stator vane units 5, and a stator cylindrical portion 6. The housing 2 houses the rotor 3, the motor 4, multiple stator vane units 5, and the stator cylindrical portion 6.

(Housing 2)
The housing 2 has a casing 8, a base 9, and a fixing flange 10. The housing 2 is formed from a metal such as an aluminum alloy or iron. The casing 8 is a cylindrical member having the fixing flange 10 at one end.

The casing 8 houses multiple stator blade units 5 and multiple stages of rotor blade units 22 provided on the rotor 3. The casing 8 has a first end 11, a second end 12, and a side surface 13.

The first end 11 is attached to the exhaust target device. An intake port 14 is provided at the first end 11. The second end 12 is located on the opposite side of the fixing flange 10 in the axial direction A1 of the rotor 3. The second end 12 is connected to the base 9. The side portion 13 connects the first end 11 and the second end 12. A first internal space S1 is formed inside the casing 8.

The base 9 is positioned to close the opening on the second end 12 side of the casing 8. The base 9 houses the stator cylindrical portion 6 and the rotor cylindrical portion 23 provided on the rotor 3. The base 9 has a base end portion 15 and an exhaust port 16. The base end portion 15 is connected to the second end portion 12 of the casing 8. A second internal space S2 is formed inside the base 9. The second internal space S2 is in communication with the first internal space S1. The exhaust port 16 is in communication with the second internal space S2.

The fixing flange 10 is connected to the casing 8. The fixing flange 10 protrudes from the casing 8. The fixing flange 10 is fixed to the exhaust target device with bolts 20. Note that "connection" includes the joining of separate members. Also, "connection" includes the joining of separate parts of a single member.

(Rotor 3)
The rotor 3 has a shaft 21 , multiple stages of rotor blade units 22 , and a rotor cylindrical portion 23 .

The shaft 21 extends in the axial direction A1 of the rotor 3. In the following description, the direction from the casing 8 toward the base 9 in the axial direction A1 is defined as downward, and the opposite direction is defined as upward.

The vacuum pump 1 includes a protective bearing 29 and multiple bearings 24A-24C. The protective bearing 29 functions as a touchdown bearing that limits radial runout on the upper side of the shaft 21. The protective bearing 29 is attached to the base 9. When the shaft 21 is rotating steadily, there is no contact between the shaft 21 and the protective bearing 29. However, when a large disturbance is applied or when the whirling of the shaft 21 increases during acceleration or deceleration of rotation, the shaft 21 comes into contact with the inner surface of the inner ring of the protective bearing 29. The protective bearing 29 can be, for example, a ball bearing.

The multiple bearings 24A-24C rotatably support the rotor 3. The multiple bearings 24A-24C are attached to the base 9. The multiple bearings 24A-24C include, for example, magnetic bearings. However, the multiple bearings 24A-24C may also include other types of bearings, such as ball bearings.

The multiple stages of rotor blade units 22 are each connected to the shaft 21. The multiple stages of rotor blade units 22 are arranged at intervals in the axial direction A1. Each rotor blade unit 22 includes multiple rotor blades 25, which, although not shown, extend radially from the shaft 21. Note that in the drawings, only one of the multiple stages of rotor blade units 22 and one of the multiple rotor blades 25 is labeled with a reference number; the other rotor blade units 22 and other rotor blades 25 are not labeled with a reference number.

The rotor cylindrical portion 23 is connected to the shaft 21. The rotor cylindrical portion 23 is arranged below the rotor blade unit 22. The rotor cylindrical portion 23 is cylindrical and extends in the axial direction A1. The rotor cylindrical portion 23 is arranged on the outer periphery of the shaft 21 so as to surround the shaft 21. The outer periphery 23s of the rotor cylindrical portion 23 is a cylindrically curved surface.

(Motor 4)
The motor 4 drives the rotor 3 to rotate. For example, a DC brushless motor is used as the motor 4. The motor 4 has a motor rotor 26 and a motor stator 27. The motor rotor 26 is attached to the shaft 21. The motor stator 27 is attached to the base 9. The motor stator 27 is disposed opposite the motor rotor 26.

(Multiple stages of stator blade units 5)
The multiple stages of stator blade units 5 are connected to the inner surface of the casing 8. The multiple stages of stator blade units 5 are arranged at intervals from one another in the axial direction A1. Each of the multiple stages of stator blade units 5 is arranged between the multiple stages of rotor blade units 22. Each of the stator blade units 5 includes a plurality of stator blades 28. Although not shown in the figure, the multiple stator blades 28 each extend radially from the shaft 21 as a center.

The multiple stages of rotor blade units 22 and the multiple stages of stator blade units 5 constitute the turbine section P1 (turbomolecular pump). Note that in the drawings, only one of the multiple stator blade units 5 and one of the multiple stator blades 28 are labeled with a reference number, and the reference numbers for the other stator blade units 5 and other stator blades 28 are omitted.

(Stator cylindrical portion 6)
The stator cylindrical portion 6 is disposed radially outside the rotor cylindrical portion 23. The stator cylindrical portion 6 is connected to the base 9. The stator cylindrical portion 6 is disposed facing the rotor cylindrical portion 23 in the radial direction of the rotor cylindrical portion 23.

A helical thread groove 60 (described below) is provided on the inner peripheral surface 6s (an example of an opposing surface) of the stator cylindrical portion 6. The rotor cylindrical portion 23 and the stator cylindrical portion 6 constitute a drag pump portion P2 (thread groove pump). Note that in Figure 1, the outward direction of the radial direction B is indicated by B1, and the inward direction of the radial direction B is indicated by B2. Also shown is the end 6a of the stator cylindrical portion 6 on the intake port 14 side, and the end 6b on the exhaust port 16 side.

Figure 2 is a perspective view of the stator cylindrical portion 6 viewed from the intake side. Figure 3 is a perspective view of the stator cylindrical portion 6 viewed from the exhaust side. The intake side refers to the upper side where the intake port 14 is located, and the exhaust side refers to the lower side where the exhaust port 16 is located.

The stator cylindrical portion 6 has a thread groove 60 on its inner peripheral surface 6s. The stator cylindrical portion 6 has a cylindrical portion main body 61 and multiple threads 62. The multiple threads 62 protrude from the inner peripheral surface 61s inward in a direction B2 (see Figure 1). The multiple threads 62 are formed in a spiral shape from end 6a to end 6b. The tip of the thread 62 protruding inward in a direction B2 is indicated as 62a. The tip 62a is formed parallel to the axial direction A1.

The thread groove 60 on the inner peripheral surface 6s is formed by the threads 62 and the inner peripheral surface 61s between the threads 62. The following describes the factors involved in designing the thread groove 60 on the stator cylindrical portion 6: thread angle, groove depth, thread inner diameter, number of threads, and groove width ratio.

Figure 4(a) is a view of end 6a of thread 62 as seen from the inside. Figure 4(b) is a view of end 6b of thread 62 as seen from the inside.

The screw angle is the angle with respect to a plane perpendicular to the rotation axis (axial direction A1). This plane is shown by the dashed double-dashed line labeled F in Figures 4(a) and 4(b).

In Figure 4(a), end 6a and plane F coincide, and the angle α formed between end 6a and direction C of thread 62 is the thread angle on the intake side. Also, in Figure 4(b), end 6b and plane F coincide, and the angle β formed between end 6b and direction C of thread 62 is the thread angle on the exhaust side.

Figure 5 shows a cross section perpendicular to the direction in which the helical thread 62 is formed (thread angle).

The space between adjacent threads 62 forms a groove portion 63, and the inner circumferential surface 61s corresponds to the groove bottom of the groove portion 63. The groove depth D is the length from the inner tip 62a of the thread 62 to the inner circumferential surface 61s of the cylindrical main body 61. The groove depth D of the thread groove 60 will be explained in detail using Figure 6 below, but it becomes shallower from end 6a to end 6b. It can also be said that the height from the inner circumferential surface 61s to the tip of the thread 62 becomes lower from end 6a to end 6b.

The inner diameter of the screw is the diameter inside the tip 62a of the screw thread 62. The inner diameter of the screw is determined by the outer diameter of the rotor cylindrical portion 23 and the gap between the rotor cylindrical portion 23 and the tip 62a of the screw thread 62. In Figure 4, the radius of the inner diameter of the screw is shown as r/2.

The number of threads refers to the number of threads 62 arranged in a 360-degree circumferential direction. In this embodiment, as shown in Figures 2 and 3, six threads 62 are formed as an example. The six threads 62 are arranged at 60-degree intervals in a cross section perpendicular to the axial direction A1.

The groove width ratio is the ratio of the width W1 of the groove portion 63 to the sum of the width W1 of the groove portion 63 and the width W2 of the thread 62 when cut in a cross section perpendicular to the direction in which the thread 62 is formed. In other words, the groove width ratio is expressed as W1/(W1+W2), and is a value greater than 0 and less than 1.

Figure 6 shows the change in groove depth D in the axial direction A1. In Figure 6, the upper side indicates the intake side, and the lower side indicates the exhaust side. The groove depth D at end 6a is shown as intake side groove depth Da. The groove depth D at end 6b is shown as exhaust side groove depth Db.

The exhaust side groove depth Db is smaller than the intake side groove depth Da. The groove depth D decreases from end 6a to end 6b.

The inner circumferential surface 61s of the cylindrical main body 61 has an intake side portion 71 (an example of a first portion) and an exhaust side portion 72 (an example of a second portion). The intake side portion 71 is a portion where the groove depth D decreases at a constant rate of decrease Ea. The exhaust side portion 72 is located closer to end 6b (the exhaust side) than the intake side portion 71. The exhaust side portion 72 is a portion where the groove depth D decreases at a constant rate of decrease Eb. The rate of decrease Eb of the exhaust side portion 72 is smaller than the rate of decrease Ea of the intake side portion 71, and is set to Ea > Eb. The rate of decrease is calculated by dividing the amount of decrease in groove depth D when moving a predetermined distance from end 6a toward end 6b along the axial direction A1 by a predetermined amount.

The end of the intake side portion 71 on the side of end 6b is connected to the end of the exhaust side portion 72 on the side of end 6a. Because the rate of decrease is constant, the intake side portion 71 and the exhaust side portion 72 are shown as a straight line in Figure 6. The portion where the intake side portion 71 and the exhaust side portion 72 join is shown as a transition portion 73, where the rate of decrease changes. The groove depth D in this transition portion 73 is shown as the transition portion groove depth Dc. As shown in Figures 2 and 3, the transition portion 73 is formed along the circumference on a plane perpendicular to the axial direction A1. Although a step is formed at the tip 62a of the thread 62b on this circumference in Figures 2 and 3, this step need not be formed. Note that Figure 6, as well as Figures 7, 14 to 17, and 26 described below, show a state where no step is formed at the tip 62a.

It is preferable that the intake side groove depth Da, exhaust side groove depth Db, and transition portion groove depth Dc satisfy the following (Equation 1).

Db≦Dc<(Da+Db)×0.5...(Formula 1)
Furthermore, it is preferable that the intake side groove depth Da, the exhaust side groove depth Db, and the transition portion groove depth Dc further satisfy the following (Equation 2).

1.5≦Da/Db... (Formula 2)
Furthermore, it is preferable that the intake side groove depth Da satisfies the following (Equation 3).

8mm≦Da...(Formula 3)
Furthermore, if the length along the axial direction A1 from the end 6a to the transition portion 73 is La, and the length along the axial direction A1 from the end 6a to the end 6b is Lb, it is preferable to satisfy the following (Equation 4).

0<La<Lb×2/3 (Equation 4)
With the above configuration, by making the rate of decrease in groove depth D larger on the intake port 14 side than on the exhaust port 16 side, it becomes easier to send gas molecules to the intake port 14 side of the thread groove 60 toward the exhaust port 16. Also, by making the rate of decrease in groove depth D smaller on the exhaust port 16 side than on the intake port 14 side, it is possible to prevent gas molecules from flowing back from the exhaust port 16 side of the thread groove 60 toward the intake port 14 side.

As a result, even if the conductance on the intake side of the drag pump section P2, which is composed of the rotor cylindrical section 23 and the stator cylindrical section 6, is increased, the back pressure characteristics can be improved. The conductance of the drag pump section P2 can be increased by increasing the gas flow path. That is, the conductance of the drag pump section P2 can be improved by increasing the flow path of the groove portion 63 of the screw groove 60. Increasing the flow path of the groove portion 63 can be achieved by increasing the groove depth D, reducing the number of threads 62 that form the screw groove 60, or increasing the groove width ratio W1/(W1+W2).

(Example)
The vacuum pump 1 of this embodiment will be further described below using examples.

In Examples 1 to 3, performance calculations were performed by changing each element of the drag pump section P2 as shown in Tables 1 to 3 for each example.

For each example, performance calculations for a comparative example were also performed at the same time. Figure 7 shows the change in groove depth D in the cylindrical body 1061 and thread 1062 of the comparative example. The groove depth D from the tip 1062a of the thread 1062 to the inner surface 1061s of the cylindrical body 1061 decreases at a constant rate from end 6a to end 6b, and changes linearly. In other words, unlike the examples, the comparative example does not have a section where the groove depth changes.

Example 1
The elements of the drag pump section P2 were set to the values shown in Table 1, and the performance calculation results are shown in FIGS.

(Table 1)
The thread groove 60 of the first embodiment has a transition portion 73 at a position that is 50% of Lb (La=Lb/2), where the groove depth Dc is 7 mm.

Figure 8 shows the relationship between the intake-side pressure and exhaust-side pressure of the drag pump section P2 when N2 was pumped at 2000 sccm in Example 1 and Comparative Example 1. Figure 9 shows the relationship between the intake-side pressure and exhaust-side pressure of the drag pump section P2 when N2 was pumped at 3000 sccm in Example 1 and Comparative Example 1. In Figures 8 and 9, the horizontal axis represents the exhaust-side pressure, and the vertical axis represents the intake-side pressure ratio. This is the ratio when the intake-side pressure of the drag pump section P2 when the exhaust-side pressure of the drag pump section P2 is 20 Pa is used as the reference. In Figures 8 and 9, Example 1 is shown by a solid line, and Comparative Example 1 is shown by a two-dot chain line.

As shown in Figures 8 and 9, even when large volumes of exhaust, such as 2000 sccm or 3000 sccm, are exhausted, the increase in intake side pressure due to the increase in exhaust side pressure is suppressed in Example 1 compared to Comparative Example 1. Therefore, it can be seen that the back pressure characteristics are improved in Example 1 compared to Comparative Example 1.

Example 2
The elements of the drag pump section P2 were set to the values shown in Table 2, and performance calculations were carried out. The results are shown in FIGS. 10 and 11.

(Table 2)

The thread groove 60 of the second embodiment has a transition portion 73 at a position that is 50% of Lb (La=Lb/2), where the groove depth Dc is 4 mm.

Figure 10 shows the relationship between the intake-side pressure and exhaust-side pressure of the drag pump section P2 when N2 was pumped at 2000 sccm in Example 2 and Comparative Example 2. Figure 11 shows the relationship between the intake-side pressure and exhaust-side pressure of the drag pump section P2 when N2 was pumped at 3000 sccm in Example 2 and Comparative Example 2. In Figures 10 and 11, the horizontal axis represents the exhaust-side pressure, and the vertical axis represents the intake-side pressure ratio. This is the ratio when the intake-side pressure of the drag pump section P2 when the exhaust-side pressure of the drag pump section P2 is 20 Pa is used as the reference. In Figures 10 and 11, Example 2 is shown by a solid line, and Comparative Example 2 is shown by a two-dot chain line.

As shown in Figures 10 and 11, even when large volumes of exhaust, such as 2000 sccm or 3000 sccm, are exhausted, the increase in intake side pressure due to the increase in exhaust side pressure is suppressed in Example 2 compared to Comparative Example 2. Therefore, it can be seen that the back pressure characteristics of Example 2 are improved compared to Comparative Example 2.

(Examples 3 and 4)
The elements of the drag pump section P2 were set to the values shown in Table 3, and the performance calculation results are shown in FIGS.

(Table 3)
The thread groove 60 of Example 3 has a transition portion 73 at a position that is 33% of Lb (La=0.33×Lb) where the groove depth Dc is 12 mm.

The thread groove 60 in Example 4 has a transition portion 73 at a position that is 66% of Lb (La = 0.66 x Lb), where the groove depth Dc is 12 mm.

Figure 12 shows the relationship between the intake-side pressure and the exhaust-side pressure of the drag pump unit P2 when N2 was pumped at 2000 sccm in Examples 3 and 4 and Comparative Example 3. Figure 13 shows the relationship between the intake-side pressure and the exhaust-side pressure of the drag pump unit P2 when N2 was pumped at 3000 sccm in Examples 3 and 4 and Comparative Example 3. In Figures 12 and 13, the horizontal axis represents the exhaust-side pressure, and the vertical axis represents the intake-side pressure ratio. This is the ratio when the intake-side pressure of the drag pump unit P2 when the exhaust-side pressure of the drag pump unit P2 is 20 Pa is used as the reference. In Figures 12 and 13, Example 3 is shown by a solid line, Example 4 is shown by a dashed line, and Comparative Example 3 is shown by a two-dot dashed line.

As shown in Figures 12 and 13, even when large volumes of exhaust, such as 2000 sccm or 3000 sccm, are exhausted, the increase in intake side pressure due to the increase in exhaust side pressure is suppressed in Examples 3 and 4 compared to Comparative Example 3. Therefore, it can be seen that the back pressure characteristics are improved in Examples 3 and 4 compared to Comparative Example 3.

(Embodiment 2)
The following describes a vacuum pump of embodiment 2. The vacuum pump of embodiment 2 differs from the vacuum pump of embodiment 1 in the change in groove depth D of the thread groove 60 of the stator cylindrical portion 6. In embodiment 2, the differences from the first embodiment will be mainly described.

In the stator cylindrical portion 6 of the first embodiment, there is one transition portion between the end 6a on the intake side and the end 6b on the exhaust side, where the rate of decrease in the groove depth D changes. However, in the stator cylindrical portion of the second embodiment, there are multiple transition portions.

Figure 14 shows the change in groove depth D in the axial direction A1 of the stator cylindrical portion 6 in this embodiment 2.

In Figure 14, the upper side indicates the intake side and the lower side indicates the exhaust side. The groove depth D at end 6a is shown as the intake side groove depth Da. The groove depth D at end 6b is shown as the exhaust side groove depth Db. The exhaust side groove depth Db is smaller than the intake side groove depth Da.

The inner circumferential surface 61s of the cylindrical main body 61 in embodiment 2 has a first reduced portion 171, a second reduced portion 172, and a third reduced portion 173, arranged in this order from the intake side to the exhaust side. The first reduced portion 171 is a portion where the groove depth D decreases at a constant reduced rate E1. The second reduced portion 172 is a portion where the groove depth D decreases at a constant reduced rate E2. The third reduced portion 173 is a portion where the groove depth D decreases at a constant reduced rate E3. The reduced rate is calculated by dividing the amount of reduction in groove depth D when moving a predetermined distance from end 6a toward end 6b along the axial direction A1 by a predetermined amount. Because the reduced rate is constant, the first reduced portion 171, second reduced portion 172, and third reduced portion 173 are shown as straight lines in FIG. 14.

The first decreasing portion 171 is formed from the end 6a toward the exhaust side. The exhaust side end of the first decreasing portion 171 is connected to the intake side end of the second decreasing portion 172. The portion where the first decreasing portion 171 and the second decreasing portion 172 are connected is illustrated as the first changing portion 175, where the rate of decrease changes. The groove depth D in this first changing portion 175 is shown as the changing portion groove depth Dd. The first changing portion 175 is formed along the circumference on a plane perpendicular to the axial direction A1.

The third reduced portion 173 is formed from end 6b toward the intake side. The intake side end of the third reduced portion 173 is connected to the exhaust side end of the second reduced portion 172. The portion where the second reduced portion 172 and the third reduced portion 173 are connected is illustrated as a second varying portion 176 where the reduction rate changes. The groove depth D in this second varying portion 176 is shown as the varying portion groove depth De. The second varying portion 176 is formed along the circumference on a plane perpendicular to the axial direction A1.

In the stator cylindrical portion shown in Figure 14, the reduction rate E1 of the first reduction portion 171 is set to be greater than or equal to the reduction rate E2 of the second reduction portion 172 and greater than or equal to the reduction rate E3 of the third reduction portion 173.

The configuration is not limited to that shown in Figure 14, and any configuration in which the groove depth Db at the exhaust side end 6b is smaller than the groove depth Da at the intake side end 6a, and the rate of decrease in groove depth on the intake side is greater than that on the exhaust side, may be used. For example, the configuration of the stator cylindrical portion shown in Figure 15 or Figure 16 may also be used.

In the stator cylindrical portion 6 shown in Figure 15, compared to the stator cylindrical portion shown in Figure 14, the reduction rate E1 of the first reduction portion 171 is set to be greater than or equal to the reduction rate E3 of the third reduction portion 173 and greater than or equal to the reduction rate E2 of the second reduction portion 172.

In the stator cylindrical portion 6 shown in Figure 16, compared to the stator cylindrical portion shown in Figure 14, the reduction rate E2 of the second reduction portion 172 is set to be greater than or equal to the reduction rate E1 of the first reduction portion 171 and greater than or equal to the reduction rate E3 of the third reduction portion 173.

Furthermore, while the stator cylindrical portion shown in Figures 14 to 16 has two change sections where the reduction rate changes, three may also be provided. Figure 17 shows the change in groove depth D in the axial direction A1 of a stator cylindrical portion with three change sections where the reduction rate of the groove depth changes.

In the configuration shown in Figure 17, a fourth decreased portion 174 is further provided between the third decreased portion 173 and end 6b described in Figure 14. The portion where the third decreased portion 173 and the fourth decreased portion 174 are connected is shown as a third changing portion 177. The fourth decreased portion 174 is a portion where the groove depth D decreases at a constant rate of decrease E4. In the configuration shown in Figure 14, the decrease rate E1 of the first decreased portion 171 ≥ the decrease rate E2 of the second decreased portion 172 ≥ the decrease rate E3 of the third decreased portion 173 ≥ the decrease rate E4 of the fourth decreased portion 174 is set. In addition, the groove depth in the third changed portion 177 is shown as the changing portion groove depth Df.

In this way, the stator cylindrical portion may have four sections with different constant reduction rates and three change sections with varying reduction rates.

In the configuration shown in FIG. 17, if the groove depth Db at the exhaust-side end 6b is smaller than the groove depth Da at the intake-side end 6a, and the groove depth reduction rate on the intake side is greater than that on the exhaust side, the reduction rates may be different between the reduction portions. Although not shown in the figures, as in the case of a configuration with two change portions (FIGS. 14 to 16), the reduction rate E1 of the first reduction portion 171 may be set greater than the reduction rate E2 of the second reduction portion 172 greater than the reduction rate E4 of the fourth reduction portion 174 greater than the reduction rate E3 of the third reduction portion 173. As shown in Examples 8 to 15 below, the reduction rate E1 of the first reduction portion 171, the reduction rate E2 of the second reduction portion 172, the reduction rate E3 of the third reduction portion 173, and the reduction rate E4 of the fourth reduction portion 174 may be set greater than the reduction rate E3 of the third reduction portion 173.

(Example)
The following describes the embodiment in detail using examples.

(Examples 5 to 7)
In the following Examples 5 to 7 and Comparative Example 4, the elements of the drag pump section P2 were set to the values shown in Table 4.

(Table 4)
Furthermore, the length Lc from the intake side end 6a of the stator cylindrical portion 6 to the first transition portion 175 was set to 40 mm, and the length Ld from the end 6a to the second transition portion 176 was set to 70 mm.

The stator cylindrical portion in Example 5 has the shape shown in Figure 14, and the reduction rate E1 of the first reduced portion 171 is set to be greater than or equal to the reduction rate E2 of the second reduced portion 172 and greater than or equal to the reduction rate E3 of the third reduced portion 173.

The stator cylindrical portion in Example 6 has the shape shown in Figure 15, and the reduction rate E1 of the first reduced portion 171 is set to be greater than or equal to the reduction rate E3 of the third reduced portion 173 and greater than or equal to the reduction rate E2 of the second reduced portion 172.

The stator cylindrical portion in Example 7 has the shape shown in Figure 16, and the reduction rate E2 of the second reduced portion 172 is set to be greater than or equal to the reduction rate E1 of the first reduced portion 171 and greater than or equal to the reduction rate E3 of the third reduced portion 173.

The stator cylindrical portion of Comparative Example 4 has the shape shown in Figure 7 above, with the groove depth D decreasing linearly at a constant rate from end 6a to end 6b.

The dimensions of the stator cylindrical portion in Comparative Example 4 and Examples 5 to 7 are shown in Table 5 below.
(Table 5)
The elements of the drag pump section P2 were set to the values shown in Table 4 and Table 5, and performance calculations were performed. The results are shown in FIGS. 18 and 19.

Figure 18 shows the relationship between the intake-side pressure and the exhaust-side pressure of the drag pump unit P2 when N2 was pumped at 2000 sccm in Examples 5 to 7 and Comparative Example 4. Figure 19 shows the relationship between the intake-side pressure and the exhaust-side pressure of the drag pump unit P2 when N2 was pumped at 3000 sccm in Examples 5 to 7 and Comparative Example 4. In Figures 18 and 19, the horizontal axis represents the exhaust-side pressure, and the vertical axis represents the intake-side pressure ratio. This is the ratio when the intake-side pressure of the drag pump unit P2 when the exhaust-side pressure of the drag pump unit P2 is 10 Pa is used as the reference. In Figures 18 and 19, Example 5 is shown by a solid line, Example 6 is shown by a dotted line, Example 7 is shown by a dashed line, and Comparative Example 4 is shown by a two-dot dash line.

As shown in Figures 18 and 19, even when large volumes of exhaust, such as 2000 sccm or 3000 sccm, are exhausted, the increase in intake side pressure due to the increase in exhaust side pressure is suppressed in Examples 5 to 7 compared to Comparative Example 4. Therefore, it can be seen that Examples 5 to 7 have improved back pressure characteristics compared to Comparative Example 4. This shows that it is preferable for the part with the greatest reduction in groove depth to be located closer to the intake side than the part with the smallest reduction in groove depth.

Furthermore, by comparing Example 5 with Example 7, it can be seen that it is preferable to position the portion with the greatest groove depth reduction rate closest to the intake side, as this will further improve back pressure characteristics. Furthermore, by comparing Example 5 with Example 6, it can be seen that it is preferable to position the portion with the smallest groove depth reduction rate closest to the exhaust side, as this will further improve back pressure characteristics.

(Examples 8 to 15)
In the following Examples 8 to 15, the elements of the drug pump section P2 were set to the values shown in Table 6.

(Table 6)

As shown in FIG. 17, the length Lc from the intake side end 6a of the stator cylindrical portion 6 to the first change portion 175 was set to 25 mm, the length Ld from the end 6a to the second change portion 176 was set to 50 mm, and the length Le from the end 6a to the third change portion 177 was set to 75 mm.

The stator cylindrical portion in Example 8 has the shape shown in Figure 17, and shows an example where the reduction rate E1 of the first reduction portion 171 is set to be greater than or equal to the reduction rate E2 of the second reduction portion 172, greater than or equal to the reduction rate E3 of the third reduction portion 173, and greater than or equal to the reduction rate E4 of the fourth reduction portion 174.

The stator cylindrical portion in Example 9 shows an example where the reduction rate E1 of the first reduction portion 171 is set to be greater than or equal to the reduction rate E2 of the second reduction portion 172, greater than or equal to the reduction rate E4 of the fourth reduction portion 174, and greater than or equal to the reduction rate E3 of the third reduction portion 173.

The stator cylindrical portion in Example 10 shows an example where the reduction rate E1 of the first reduction portion 171 is set to be greater than or equal to the reduction rate E3 of the third reduction portion 173, greater than or equal to the reduction rate E2 of the second reduction portion 172, and greater than or equal to the reduction rate E4 of the fourth reduction portion 174.

The stator cylindrical portion in Example 11 shows an example where the reduction rate E2 of the second reduction portion 172 is set to be greater than or equal to the reduction rate E1 of the first reduction portion 171, greater than or equal to the reduction rate E3 of the third reduction portion 173, and greater than or equal to the reduction rate E4 of the fourth reduction portion 174.

The stator cylindrical portion in Example 12 shows an example where the reduction rate E2 of the second reduction portion 172 is set to be greater than or equal to the reduction rate E1 of the first reduction portion 171, greater than or equal to the reduction rate E4 of the fourth reduction portion 174, and greater than or equal to the reduction rate E3 of the third reduction portion 173.

The stator cylindrical portion in Example 13 shows an example where the reduction rate E3 of the third reduction portion 173 is set to be greater than or equal to the reduction rate E1 of the first reduction portion 171, greater than or equal to the reduction rate E2 of the second reduction portion 172, and greater than or equal to the reduction rate E4 of the fourth reduction portion 174.

The stator cylindrical portion in Example 14 shows an example where the reduction rate E2 of the second reduction portion 172 is set to be greater than or equal to the reduction rate E3 of the third reduction portion 173, greater than or equal to the reduction rate E1 of the first reduction portion 171, and greater than or equal to the reduction rate E4 of the fourth reduction portion 174.

The stator cylindrical portion in Example 15 shows an example where the reduction rate E3 of the third reduction portion 173 is set to be greater than or equal to the reduction rate E2 of the second reduction portion 172, greater than or equal to the reduction rate E1 of the first reduction portion 171, and greater than or equal to the reduction rate E4 of the fourth reduction portion 174.

The stator cylindrical portion of Comparative Example 5 has the shape shown in Figure 7 above, with the groove depth D decreasing linearly at a constant rate from end 6a to end 6b.

(Table 7)
The elements of the drag pump section P2 were set to the values shown in Table 6 and Table 7, and performance calculations were performed. The results are shown in FIGS. 20 and 21.

Figure 20 shows the relationship between the intake-side pressure and the exhaust-side pressure of the drag pump section P2 when N2 was pumped at 2000 sccm in Examples 8 to 10 and Comparative Example 5. Figure 21 shows the relationship between the intake-side pressure and the exhaust-side pressure of the drag pump section P2 when N2 was pumped at 3000 sccm in Examples 8 to 10 and Comparative Example 5. In Figures 20 and 21, the horizontal axis represents the exhaust-side pressure, and the vertical axis represents the intake-side pressure ratio. This is the ratio when the intake-side pressure of the drag pump section P2 when the exhaust-side pressure of the drag pump section P2 is 10 Pa is used as the reference. In Figures 20 and 21, Example 8 is shown by a solid line, Example 9 is shown by a dotted line, Example 10 is shown by a dashed line, and Comparative Example 5 is shown by a two-dot chain line.

(Table 8)
The elements of the drag pump section P2 were set to the values shown in Table 6 and Table 8, and performance calculations were performed. The results are shown in FIGS. 22 and 23.

22 shows the relationship between the intake-side pressure and the exhaust-side pressure of the drag pump unit P2 when N2 was pumped at 2000 sccm in Examples 11 and 12 and Comparative Example 5. FIG. 23 shows the relationship between the intake-side pressure and the exhaust-side pressure of the drag pump unit P2 when N2 was pumped at 3000 sccm in Examples 11 and 12 and Comparative Example 5. In FIGS. 22 and 23, the horizontal axis represents the exhaust-side pressure, and the vertical axis represents the intake-side pressure ratio. This ratio is based on the intake-side pressure of the drag pump unit P2 when the exhaust-side pressure of the drag pump unit P2 is 10 Pa. In FIGS. 22 and 23, Example 11 is shown by a solid line, Example 12 is shown by a dotted line, and Comparative Example 5 is shown by a two-dot chain line.

(Table 9)
The elements of the drag pump section P2 were set to the values shown in Table 6 and Table 9, and performance calculations were performed. The results are shown in FIGS. 24 and 25.

Figure 24 shows the relationship between the intake-side pressure and the exhaust-side pressure of the drag pump section P2 when N2 was pumped at 2000 sccm in Examples 13 to 15 and Comparative Example 5. Figure 25 shows the relationship between the intake-side pressure and the exhaust-side pressure of the drag pump section P2 when N2 was pumped at 3000 sccm in Examples 13 to 15 and Comparative Example 5. In Figures 24 and 25, the horizontal axis represents the exhaust-side pressure, and the vertical axis represents the intake-side pressure ratio. This is the ratio when the intake-side pressure of the drag pump section P2 when the exhaust-side pressure of the drag pump section P2 is 10 Pa is used as the reference. In Figures 24 and 25, Example 13 is shown by a solid line, Example 14 is shown by a dotted line, Example 15 is shown by a dashed line, and Comparative Example 5 is shown by a two-dot dash line.

As shown in Figures 20 to 25, even when large volumes of exhaust, such as 2000 sccm or 3000 sccm, are exhausted, Examples 8 to 15 suppress the increase in intake side pressure due to the increase in exhaust side pressure compared to Comparative Example 5. Therefore, it can be seen that Examples 8 to 15 have improved back pressure characteristics compared to Comparative Example 5. This shows that it is preferable for the part with the greatest reduction in groove depth to be located closer to the intake side than the part with the smallest reduction in groove depth.

Furthermore, by comparing Example 8 with Examples 13 and 14, for example, it can be seen that it is preferable for the portion with the greatest rate of reduction in groove depth to be located closest to the intake side. Furthermore, by comparing Example 8 with Example 9, for example, it can be seen that it is preferable for the portion with the smallest rate of reduction in groove depth to be located closest to the exhaust side, as this will further improve back pressure characteristics.

(Other embodiments)
Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the gist of the invention.

In the above embodiment, the tip 62a is formed in a straight line because the rate of decrease is constant in the intake side portion 71 and the exhaust side portion 72, but it may also be formed in a curved line so that it passes through the transition portion 73. In other words, as long as the rate of decrease in the intake side portion 71 is greater than the rate of decrease in the exhaust side portion 72, the rate of decrease does not have to be constant.

In the above embodiment, thread grooves are formed on the inner peripheral surface 6s of the stator cylindrical portion 6, but thread grooves may also be formed on the outer peripheral surface of the rotor cylindrical portion 23. Furthermore, the stator cylindrical portion 6 may be integrated with the base 9.

In the above embodiment, five to eight threads 62 are formed as an example, but the number may be five or less, or eight or more.

In the above embodiment, the intake-side end of the intake-side portion 71, which is an example of the first portion, coincides with end 6a, and the exhaust-side end of the exhaust-side portion 72 coincides with end 6b. However, a cylindrical body and a threaded portion may also be provided on the intake side of the intake-side portion 71. Furthermore, a cylindrical body and a threaded portion may also be provided on the exhaust side of the exhaust-side portion 72. Figure 26 shows the change in groove depth D from the tip 62a' of the threaded portion 62' to the inner circumferential surface 61s' of the cylindrical body 61' in a modified example. On the inner circumferential surface 61s' of the cylindrical body 61', an end portion 74 is provided between the intake-side portion 71 and end 6a, and an end portion 75 is provided between the exhaust-side portion 72 and end 6b. In the modified example shown in Figure 26, the groove depth D of the end portion 74 is formed at a constant height equal to the groove depth Da of the end of the intake-side portion 71 on the intake port 14 side. In the modified example shown in Figure 26, the groove depth D of the end portion 75 is formed to be constant and the same height as the groove depth Db of the end of the exhaust side portion 72 on the exhaust port 16 side. Note that the end portions 74, 75 may be formed so that the groove depth D decreases from end 6a to end 6b.

(Aspect)
It will be appreciated by those skilled in the art that the exemplary embodiments described above are examples of the following aspects.

(First aspect) The vacuum pump comprises a housing, a rotor cylindrical portion, and a stator cylindrical portion. The housing has an intake port for sucking in gas and an exhaust port for discharging the sucked gas. The rotor cylindrical portion is housed in the housing. The stator cylindrical portion is housed in the housing and is positioned opposite the rotor cylindrical portion. A thread groove is formed on one of the opposing surfaces of the stator cylindrical portion and the rotor cylindrical portion. The groove depth is smaller at the exhaust side end than at the intake side end. The rate of decrease in groove depth is greater on the intake side than on the exhaust side.

In the vacuum pump according to the first aspect, the groove depth of the thread groove is smaller at the exhaust end than at the intake end, and the rate of decrease in groove depth is set to be greater on the intake side than on the exhaust side. In this way, by making the rate of decrease in groove depth greater on the intake side, it becomes easier to send gas molecules from the intake side of the thread groove to the exhaust side. Furthermore, by making the rate of decrease in groove depth smaller on the exhaust side, it is possible to prevent gas molecules from flowing back from the exhaust side of the thread groove to the intake side.

As a result, back pressure characteristics can be improved even when the conductance of the drag pump section, which is made up of the rotor cylindrical section and the stator cylindrical section, is increased.

In addition, by improving the back pressure characteristics, the increase in pressure on the intake side can be reduced even with a higher exhaust pressure, making it possible to select a smaller auxiliary pump, which is a factor that leads to an increase in pressure on the exhaust port side.This allows for greater design freedom, reduces costs, and improves maintainability.

(Second Aspect) In the vacuum pump according to the first aspect, the thread groove has a first portion and a second portion. The first portion has a constant rate of decrease in groove depth. The second portion is located closer to the exhaust side than the first portion and also has a constant rate of decrease in groove depth. The rate of decrease in the first portion is greater than the rate of decrease in the second portion.

In the vacuum pump according to the second aspect, gas molecules are easily sent to the exhaust side in the first section, and gas molecules are less likely to flow back in the second section. Therefore, back pressure characteristics can be improved even when the conductance of the drag pump section formed by the rotor cylindrical section and the stator cylindrical section is increased.

(Third Aspect) In the vacuum pump according to the second aspect, the first section and the second section are connected by a transition section. If the groove depth at the intake end of the first section is Da, the groove depth at the exhaust end of the second section is Db, and the groove depth at the transition section is Dc, then the relationship Db≦Dc≦(Da+Db)×0.5 is satisfied.

In the vacuum pump according to the third aspect, by satisfying Db≦Dc≦(Da+Db)×0.5, back pressure characteristics can be improved even when the conductance of the drag pump section is increased.

(Fourth Aspect) In the vacuum pump according to the second aspect, if the length along the axial direction of the rotor cylindrical portion from the intake end of the first section to the transition section is La, and the length along the axial direction from the intake end of the first section to the exhaust end of the second section is Lb, then the relationship 0 < La < Lb x 2/3 is satisfied.

In the vacuum pump according to the fourth aspect, by satisfying the relationship 0 < La < Lb × 2/3, it is possible to improve the back pressure characteristics even when the conductance of the drag pump section is increased.

(Fifth Aspect) The vacuum pump according to the third or fourth aspect further satisfies 1.5≦Da / Db.

In the vacuum pump according to the fifth aspect, by satisfying 1.5≦Da / Db, it is possible to improve the back pressure characteristics even when the conductance of the drag pump section is increased.

(Sixth Aspect) In the vacuum pump according to the first aspect, the thread groove has multiple portions with different rates of reduction in groove depth. The portion with the greatest rate of reduction in groove depth is located closer to the intake side than the portion with the smallest rate of reduction in groove depth.

In the vacuum pump of the sixth aspect, by positioning the portion with the greatest rate of decrease in groove depth closer to the intake side than the portion with the smallest rate of decrease in groove depth, it becomes easier to send gas molecules from the intake side of the thread groove to the exhaust side, and gas molecules can be prevented from flowing back from the exhaust side of the thread groove to the intake side.

As a result, back pressure characteristics can be improved even when the conductance of the drag pump section is increased.

(Seventh Aspect) In the vacuum pump according to the sixth aspect, the portion with the greatest rate of reduction in groove depth is located closest to the intake side of the multiple portions.

In the vacuum pump according to the seventh aspect, it becomes easier to send gas molecules from the intake side of the screw groove to the exhaust side.

(Eighth aspect) In the vacuum pump according to the sixth or seventh aspect, the portion with the smallest rate of reduction in groove depth is located closest to the exhaust side among the multiple portions.

The vacuum pump according to the eighth aspect can better prevent gas molecules from flowing back from the exhaust side to the intake side of the screw groove.

1: vacuum pump, 2: housing, 3: rotor, 4: motor, 5: stator blade unit, 6: stator cylindrical portion, 6a: end, 6b: end, 6s: inner peripheral surface, 8: casing, 9: base, 10: fixing flange, 11: first end, 12: second end, 13: side portion, 14: intake port, 15: base end, 16: exhaust port, 20: bolt, 21: shaft, 22: rotor blade unit, 23: rotor cylindrical portion, 23s: outer peripheral surface, 24A-24C: bearings , 25: rotor blade, 26: motor rotor, 27: motor stator, 28: stator blade, 29: protective bearing, 60: thread groove, 61: cylindrical body, 61s: inner peripheral surface, 62: thread, 62a: tip, 62': thread, 63: groove portion, 71: intake side portion, 72: exhaust side portion, 73: transition portion, 74: end portion, 75: end portion, 1062: thread, P1: turbine portion, P2: drag pump portion, S1: first internal space, S2: second internal space

Claims (6)

a housing having an intake port for drawing in gas and an exhaust port for discharging the drawn gas;
a rotor cylindrical portion accommodated in the housing;
a stator cylindrical portion accommodated in the housing and disposed opposite the rotor cylindrical portion,
a thread groove is formed on one of the opposing surfaces of the stator cylindrical portion and the rotor cylindrical portion,
The groove depth of the screw groove is smaller at the exhaust side end than at the intake side end,
the rate of decrease in the groove depth is greater on the intake side than on the exhaust side,
The screw groove is
a first portion in which the groove depth decreases at a constant decreasing rate;
a second portion disposed closer to the exhaust side than the first portion, in which the groove depth decreases at a constant decreasing rate;
The reduction rate of the first portion is greater than the reduction rate of the second portion;
the first portion and the second portion are connected by a portion where the rate of decrease changes,
the groove depth at the intake side end of the first portion is Da,
the groove depth at the exhaust side end of the second portion is Db;
When the groove depth in the transition portion is Dc,
Db≦Dc≦(Da+Db)×0.5 is satisfied.
Vacuum pump. a length along the axial direction of the rotor cylindrical portion from the intake side end of the first portion to the transition portion is defined as La;
When the length along the axial direction from the intake side end of the first portion to the exhaust side end of the second portion is Lb,
0<La<Lb×2/3 is satisfied.
2. The vacuum pump according to claim 1 . Further satisfying 1.5≦Da/Db,
3. A vacuum pump according to claim 1 or 2 . The thread groove has a plurality of portions where the groove depth reduction rate is different,
The portion where the groove depth reduction rate is greatest is located closer to the intake side than the portion where the groove depth reduction rate is smallest.
2. The vacuum pump according to claim 1. The portion with the largest rate of reduction in groove depth is disposed closest to the intake side among the plurality of portions.
5. A vacuum pump according to claim 4 . the portion with the smallest rate of decrease in groove depth is disposed closest to the exhaust side among the plurality of portions;
6. A vacuum pump according to claim 4 or 5 .