Vacuum pump, degassing device, and liquid chromatograph

Abstract

A vacuum pump according to the present invention includes one or more pump units, wherein: the one or more pump units each include a pump head that has a pump chamber formation surface for forming a pump chamber, a diaphragm that is provided so as to cover the pump chamber formation surface of the pump head, and a rod that is driven in the axial direction in a state in which the tip end surface thereof faces the pump chamber formation surface and that holds the diaphragm at the tip end surface; the internal capacity of the pump chamber formed between the pump chamber formation surface and the diaphragm changes when the rod is driven in the axial direction and the pump chamber thereby suctions and discharges a fluid; the one or more pump units are each provided, inside the pump head, with a duckbill type check valve that opens only when the fluid is discharged from the pump chamber; the pump head is provided with a first flow path that allows for the fluid communication between the pump chamber and the inlet of the check valve and a second flow path that allows for the fluid communication between the outlet of the check valve and the outside; and the second flow path has an inhibitory structure that inhibits a whirlpool from being formed by the fluid flowed out from the outlet of the check valve.

Description

Vacuum pump, degassing device, and liquid chromatograph

[0001] The present invention relates to a vacuum pump, a degassing device, and a liquid chromatograph.

[0002] In a high-performance liquid chromatograph (hereinafter referred to as HPLC), a degassing device for removing dissolved oxygen and bubbles contained in the mobile phase fed by a liquid feed pump is provided. The degassing device is a device in which a gas-permeable tube that allows gas to pass through and does not allow liquid to pass through is provided in a vacuum chamber, and by reducing the pressure in the vacuum chamber with a vacuum pump, gas components are extracted from the liquid flowing in the gas-permeable tube and discharged to the outside of the vacuum chamber (see Patent Document 1).

[0003] A diaphragm-type vacuum pump is often used as the vacuum pump of the degassing device for HPLC. The diaphragm-type vacuum pump is provided with a diaphragm so as to cover the pump chamber forming surface provided on the pump head, and a pump chamber is formed between the pump chamber forming surface and the diaphragm. The diaphragm is held at the tip of the rod, and when the rod is driven in the axial direction, the volume of the pump chamber changes, and gas is inhaled and discharged into the pump chamber.

[0004] Japanese Patent No. 6991592

[0005] An object of the present invention is to improve the quietness of the above-described diaphragm-type vacuum pump.

[0006] The vacuum pump according to the present invention is a vacuum pump comprising at least one pump unit, each of the at least one pump unit comprising: a pump head having a pump chamber forming surface that forms a pump chamber; a diaphragm provided so as to cover the pump chamber forming surface of the pump head; and a rod that is driven axially with its tip facing the pump chamber forming surface and holds the diaphragm with its tip, wherein the internal volume of the pump chamber formed between the pump chamber forming surface and the diaphragm changes when the rod is driven axially, thereby causing fluid to be drawn into the pump chamber and fluid to be discharged from the pump chamber, and each of the at least one pump unit is provided inside the pump head with a duckbill type check valve that opens only when fluid is pushed out of the pump chamber, and the pump head comprises a first flow path that provides fluid communication between the pump chamber and the inlet of the check valve, and a second flow path that provides fluid communication between the outlet of the check valve and the outside. The second flow path has an obstruction structure that inhibits the formation of vortices caused by the fluid flowing out from the outlet of the check valve.

[0007] The degassing apparatus according to the present invention comprises a degassing channel made of a gas-permeable tube that allows gas to pass through but not liquid, a vacuum chamber that houses the degassing channel inside, and a vacuum pump according to the present invention connected to the vacuum chamber for reducing the pressure inside the vacuum chamber, and performs degassing of the mobile phase flowing in the degassing channel.

[0008] The liquid chromatograph according to the present invention comprises: a liquid delivery pump for delivering a mobile phase; a degassing device according to the present invention for degassing the mobile phase delivered by the liquid delivery pump; an autosampler fluidly connected downstream of the liquid delivery pump for injecting a sample into the mobile phase; a separation column fluidly connected downstream of the autosampler for separating a plurality of components in the sample injected into the mobile phase by the autosampler from each other; and a detector fluidly connected downstream of the separation column for detecting each component separated by the separation column.

[0009] According to the vacuum pump of the present invention, the second flow path provided in the pump head for fluid communication between the outlet of the check valve and the outside has an obstruction structure that inhibits the formation of vortices by the fluid flowing out from the outlet of the check valve. As a result, the formation of vortices by the fluid coming out of the outlet of the check valve is suppressed, and the generation of whistling noise near the check valve is suppressed. This provides a vacuum pump with high noise reduction.

[0010] According to the degassing device of the present invention, since a highly quiet vacuum pump of the present invention is used, quiet operation during degassing of the mobile phase is ensured.

[0011] According to the liquid chromatograph of the present invention, since a highly quiet degassing device of the present invention is used, quiet operation during analysis is ensured.

[0012] This is a cross-sectional view of a pump unit showing an embodiment of a vacuum pump, showing the state when the rod is at the bottom dead center. This is a cross-sectional view showing the state when the rod is at the top dead center in the same embodiment. This is a cross-sectional view showing an example of an eccentric cam mechanism that drives the rod in the same embodiment. This is a schematic configuration diagram showing an embodiment of a degassing device and a liquid chromatograph.

[0013] The vacuum pump, degassing device, and liquid chromatograph according to the present invention will be described below with reference to the drawings.

[0014] A vacuum pump is configured to drive one or more pump units with a single motor. First, the structure of one pump unit of a vacuum pump will be explained using Figures 1 and 2. Figures 1 and 2 are cross-sectional views of pump unit 1, with Figure 1 showing the rod 8 at its bottom dead center and Figure 2 showing the rod 8 at its top dead center.

[0015] The pump unit 1 mainly comprises a housing 2, a pump head 4, a diaphragm 6, and a rod 8.

[0016] The pump head 4 is attached to the housing 2 by bolts 38. The pump head 4 has a pump chamber forming surface 18 on the housing 2 side. The diaphragm 6 is made of a fluororesin such as PTFE. The diaphragm 6 is positioned to cover the pump chamber forming surface 18 of the pump head 6, and its peripheral edge is sandwiched and held between the housing 2 and the pump head 4. A pump chamber 21 is formed between the pump chamber forming surface 18 and the diaphragm 6.

[0017] The rod 8 is positioned on the opposite side of the pump chamber forming surface 18 from the diaphragm 6, with its tip facing the pump chamber forming surface 18 side (upward in the figure) of the pump head 4. The rod 8 is reciprocated axially (vertically) by an eccentric cam mechanism (see Figure 3), which will be described later, to change the distance between its tip and the pump chamber forming surface 18. A retaining member 10 is attached to the tip of the rod 8 by a fixing member, such as a bolt.

[0018] The diaphragm 6 is held at the tip of the rod 8 by having its central portion sandwiched between the tip of the rod 8 and the fixing member 10, and changes the internal volume of the pump chamber 21 formed between the diaphragm and the pump chamber forming surface 18 as the rod 8 moves in the axial direction.

[0019] A cushioning sheet 12 made of an elastic material such as rubber is sandwiched between the diaphragm 6 and the tip surface of the rod 8. The cushioning sheet 12 is interposed between the diaphragm 6 and the peripheral edge of the tip surface of the rod 8 when the rod 8 reaches its top dead center, and is intended to prevent the diaphragm 6 from wearing down due to direct contact with the rod 8.

[0020] The pump head 4 is provided with a fluid inlet (not shown in the figure) for drawing fluid into the pump chamber 21 from outside the vacuum pump 1, and a fluid outlet 32 ​​for discharging fluid from the pump chamber 21 to the outside of the vacuum pump 1. The fluid inlet and the fluid outlet 32, respectively, are connected to the pump chamber 21 formed between the pump chamber forming surface 18 and the diaphragm 6 via an inlet check valve (not shown) and an outlet check valve 24 provided inside the pump head 4. The inlet check valve opens during the suction operation when the rod 8 moves from top dead center to bottom dead center to increase the internal volume of the pump chamber 21, and closes during the discharge operation when the rod 8 moves from bottom dead center to top dead center to decrease the internal volume of the pump chamber 21. The outlet check valve 24 closes during the suction operation and opens during the discharge operation.

[0021] The outlet check valve 24 is a duckbill-type check valve and is located in a space 22 provided within the pump head 4. A recess 20 is provided in the center of the pump chamber forming surface 18 of the pump head 4, and a first flow path 30 is provided at the bottom of the recess 20, leading to the opening end (lower end in the figure) which is the inlet of the outlet check valve 24 in the space 22. The space on the outlet 25 side (upper side in the figure) of the outlet check valve 24 of the space 22 and the outlet flow path 32 constitute a second flow path for guiding the fluid that has flowed out from the outlet 25 of the outlet check valve 24 to the outside of the pump unit 1. The second flow path has a wall surface 23 near the outlet 25 of the outlet check valve 24 that is substantially perpendicular to the direction of fluid outflow from the outlet 25 of the outlet check valve 24 (upward direction in the figure), and is configured to guide the fluid that has come out of the outlet 25 of the outlet check valve 24 in a substantially right-angle direction near the outlet of the outlet check valve 24. This structure of the second flow path acts as an obstruction structure that inhibits the formation of vortices caused by the fluid flowing out of the outlet 25 of the outlet check valve 24. By inhibiting the formation of vortices caused by the fluid flowing out of the outlet 25 of the outlet check valve 24 by the wall surface 23, the generation of whistling noise caused by the formation of vortices by the fluid flowing out of the outlet check valve 24 is suppressed.

[0022] The pump head 4 comprises a first member 14 and a second member 16. The first member 14 is positioned on the housing 2 side, and the second member 16 is positioned on the opposite side of the housing 2, with the first member 14 in between. The first member 14 and the second member 16 are fixed to the housing 2 by bolts 38 in a state where they are overlapping each other.

[0023] The first member 14 of the pump head 4 is provided with a pump chamber forming surface 18 and a first flow path 30, and the second member 16 is provided with a space 22 and a fluid outlet 32. The space 22 of the second member is open on the side of the first member 14 (the lower side in the figure), and an outlet check valve 24 is fitted into the space 22 through this opening. The opening of the space 22 is closed on the side of the pump chamber forming surface 18 of the first member 14 that is opposite to the bottom surface of the recess 20 (the upper surface in the figure). The inlet end of the outlet check valve 24 (the lower end in the figure) is supported on the side of the pump chamber forming surface 18 of the first member 14 that is opposite to the bottom surface of the recess 20, thereby holding the outlet check valve 24 in the space 22.

[0024] A cylindrical projection 28 is provided on the surface of the first member 14 facing the second member 16. The projection 28 is located inside the opening of the space 22 of the second flow path 16, and the first flow path 30 penetrates the inside of the projection 28. The projection 28 is fitted into the inlet of the outlet check valve 22. In this embodiment, the inlet of the outlet check valve 22 has a hollow cylindrical shape, and the inner diameter of the inlet of the outlet check valve 22 is approximately the same as the outer diameter of the projection 28. However, the present invention is not limited thereto, and the inlet of the outlet check valve 22 does not necessarily have to be an injection cylindrical shape, nor does the projection 28 necessarily have to be cylindrical.

[0025] To increase the vacuum level of the pump unit 1, it is preferable to reduce the volume of fluid remaining inside the pump unit 1 in a compressed state when the rod 8 of the pump unit reaches top dead center (referred to as the compressed volume). However, this compressed volume also includes the internal volume of the outlet check valve 24. Therefore, simply reducing the internal volume of the pump chamber 21 when the rod 8 reaches top dead center has limitations in improving the vacuum level of the pump unit 1. In this embodiment, since the projection 28 of the first member 14 is fitted into the inlet of the outlet check valve 22, the volume of the internal space 26 of the outlet check valve 24 is reduced, thereby enabling a high vacuum level of the pump unit 1.

[0026] Incidentally, since the mobile phase of the HPLC is transported through narrow pipes with low heat capacity, it is easily affected by the ambient temperature. The gaseous components, including the vapor of the solvent extracted by the degassing device, flow into the pump chamber 21 of the vacuum pump's pump unit 1 at a temperature close to that of the ambient air. At this time, if the temperature difference between the pump head 4 of the pump unit 1, particularly the pump chamber forming surface 18, and the ambient air is large, condensation may occur inside the vacuum pump. Therefore, it is preferable to bring the temperature inside the pump unit 1 closer to that of the ambient air.

[0027] To address the above issues, in this embodiment, a recess 34 is provided on the side of the first member 14 of the pump head 4 opposite to the pump chamber forming surface 18. As a result, the heat capacity of the first member 14 is reduced, and the back surface of the pump chamber forming surface 18 is exposed to the air. Furthermore, a recess 36 is also provided on the side of the second member 16 of the pump head 4 that faces the first member 14. As a result, the back side of the pump chamber forming surface 18 inside the pump head 4 is hollow. Although not shown in the figure, openings are provided on the sides of the first member 14 and / or the second member 16. Outside air flows into the hollow inside the pump head 4 formed by the recess 34 of the first member 14 and the recess 36 of the second member 16, and the back surface of the pump chamber forming surface 18 of the first member 14 comes into contact with the outside air. This reduces the heat capacity of the pump head 4, and the back surface of the pump chamber forming surface 18 is constantly in contact with the outside air, making it easier for the temperature of the pump chamber forming surface 18 to approach the ambient temperature. This structure suppresses the occurrence of condensation in the pump unit 1.

[0028] Next, an embodiment of a vacuum pump 100 equipped with the pump unit 1 described above will be explained with reference to Figure 3.

[0029] Figure 3 is a cross-sectional view showing the structure of the eccentric cam mechanism of the vacuum pump 100. The vacuum pump 100 is configured to drive two pump units 1 with a motor 102. In this embodiment, the rod 8 of one of the two pump units 1 is denoted as "8a", and the rod 8 of the other pump unit 1 is denoted as "8b".

[0030] An eccentric shaft 106 is fixed to the drive shaft 104 of the motor 102, and the eccentric shaft 106 rotates in conjunction with the rotation of the drive shaft 104. The base end of a rod 8a is connected to the eccentric shaft 104 via a bearing 108a at a first position close to the motor 102, and the base end of a rod 8b is connected to the eccentric shaft 104 via a bearing 108b at a second position further from the motor 102 than the first position. When the eccentric shaft 104 rotates, the rods 108a and 108b move in a direction perpendicular to the axial direction of the drive shaft 104 (up and down in the figure). 110 is a spacer inserted between the bearings 108a and 108b.

[0031] The outer diameter R1 of the first position on the eccentric shaft 106 to which the rod 8a is connected (i.e., the inner diameter of the bearing 108a) is larger than the outer diameter R2 of the second position to which the rod 8b is connected (i.e., the inner diameter of the bearing 108b).

[0032] When rods 8a and 8b are connected to the eccentric shaft 104, bearing 108a must be pressed into the eccentric shaft 106 from the tip side (right side in the figure) and fixed to the first position of the eccentric shaft 106, and then bearing 108b must be pressed into the eccentric shaft 106 to fix to the second position of the eccentric shaft 106. If the inner diameters of bearing 108a and bearing 108b are the same, when bearing 108a is inserted from the tip side of the eccentric shaft 106 to fix to the first position of the eccentric shaft 106, the outer surface of the second position of the eccentric shaft 106 and the inner surface of bearing 108a may rub against each other, potentially damaging the inner surface of bearing 108 and the outer surface of the second position, which could lead to unstable fixing of bearing 108a and / or bearing 108b to the eccentric shaft 106. If the fixing of bearings 108a and / or 108b to the eccentric shaft 106 becomes unstable, it can lead to a deterioration in the accuracy of the movement of rods 8a and / or rod 8b, and the generation of abnormal noises.

[0033] In the vacuum pump 100 of this embodiment, the inner diameter R1 of bearing 108a is larger than the inner diameter R2 of bearing 10b. Therefore, when mounting bearing 108a to the eccentric shaft 106, it is possible to prevent the inner surface of bearing 108a from contacting the outer surface of the second position on the eccentric shaft 106 where bearing 108b is mounted, thus preventing the above-mentioned problems from occurring.

[0034] Next, a liquid chromatograph 200 equipped with a degassing device 204 using the vacuum pump 100 described above will be explained.

[0035] The liquid chromatograph 200 comprises a liquid delivery pump 202, a degasser 204, an autosampler 206, a separation column 208, and a detector 210. The liquid delivery pump 202 delivers the mobile phase to the separation column 208. The degasser 204 is located upstream of the liquid delivery pump 202 and degasss the mobile phase. The autosampler 206 is fluid-connected downstream of the liquid delivery pump 202 and injects the sample into the mobile phase. The separation column 208 is fluid-connected downstream of the liquid delivery pump 202 and separates the multiple components contained in the sample injected into the mobile phase by the autosampler 206. The detector 210 is fluid-connected downstream of the separation column 208 and detects each of the multiple components separated from each other by the separation column 208.

[0036] The degassing device 204 includes a degassing channel 212 made of a gas-permeable tube that allows gas to pass through but not liquid, a vacuum chamber 214 that houses the degassing channel 212, and a vacuum pump 100 that reduces the pressure inside the vacuum chamber 214 by discharging gas from the vacuum chamber 214 to the outside. The vacuum pump 100 has the structure of the embodiment described above.

[0037] The embodiments described above are merely examples of the vacuum pump, degassing device, and liquid chromatograph according to the present invention. Embodiments of the vacuum pump, degassing device, and liquid chromatograph according to the present invention are as follows.

[0038] One embodiment of the vacuum pump according to the present invention is a vacuum pump comprising at least one pump unit, each of the at least one pump unit comprising: a pump head having a pump chamber forming surface that forms a pump chamber; a diaphragm provided so as to cover the pump chamber forming surface of the pump head; and a rod that is driven axially with its tip facing the pump chamber forming surface and holds the diaphragm with its tip, wherein the internal volume of the pump chamber formed between the pump chamber forming surface and the diaphragm changes when the rod is driven axially, thereby causing fluid to be drawn into the pump chamber and fluid to be discharged from the pump chamber, and each of the at least one pump unit is provided inside the pump head with a duckbill type check valve that opens only when fluid is pushed out of the pump chamber, and the pump head comprises a first flow path that provides fluid communication between the pump chamber and the inlet of the check valve, and a second flow path that provides fluid communication between the outlet of the check valve and the outside. The second flow path has an obstruction structure that inhibits the formation of vortices caused by the fluid flowing out from the outlet of the check valve.

[0039] In the first embodiment of the above-described vacuum pump, the second flow path has a wall surface that is substantially perpendicular to the direction of fluid flow near the outlet of the check valve, as the obstruction structure.

[0040] In a second embodiment of the above-described embodiment of the vacuum pump, the pump head comprises a first member having the pump chamber forming surface and the first flow path, and a second member mounted on the side of the first member opposite to the pump chamber forming surface and having the second flow path. This second embodiment can be combined with the first embodiment.

[0041] In the second aspect described above, the check valve has an open end that contacts a surface of the first member opposite to the pump chamber forming surface, a protrusion is provided on the surface of the first member opposite to the pump chamber forming surface, a terminal end portion of the first flow path on the check valve side is located at a tip surface of the protrusion, and the protrusion may be inserted from the open end of the check valve into the inside of the check valve, so that the volume of the internal space of the check valve may be reduced even if so.

[0042] In the above structure, the check valve may be a cylindrical portion having a hollow cylindrical shape on the open end side, and the protrusion of the first member may have a cylindrical shape having an outer diameter substantially the same as the inner diameter of the cylindrical portion.

[0043] Also, in the second aspect described above, the pump head may have a structure that brings outside air into contact with a surface on the back side of the pump chamber forming surface of the first member.

[0044] In the third aspect of the above embodiment of the vacuum pump, each of the at least one pump unit further includes an elastic buffer sheet provided so as to be interposed between a peripheral portion of the tip surface of the rod and the diaphragm. This third aspect can be combined with the first aspect and / or the second aspect.

[0045] In a fourth aspect of the above embodiment of the vacuum pump, the at least one pump unit includes a first pump unit and a second pump unit, the rod of the first pump unit and the rod of the second pump unit are driven axially by a common eccentric cam mechanism, the eccentric cam mechanism comprises a motor, an eccentric shaft rotated by the motor, a first bearing mounted at a first position on the eccentric shaft, and a second bearing mounted at a second position on the eccentric shaft further from the motor than the first position, the rod of the first pump unit is connected to the eccentric shaft via the first bearing, the rod of the second pump unit is connected to the eccentric shaft via the second bearing, the outer diameter of the eccentric shaft at the first position is greater than the outer diameter at the second position, and the inner diameter of the first bearing is greater than the inner diameter of the second bearing. This fourth aspect can be combined with the first, second, and / or third aspects.

[0046] In one embodiment of the degassing apparatus according to the present invention, the apparatus comprises a degassing channel made of a gas-permeable tube that allows gas to pass through but not liquid, a vacuum chamber that houses the degassing channel inside, and a vacuum pump according to the present invention, as described above, connected to the vacuum chamber for reducing the pressure inside the vacuum chamber, and performs degassing of the mobile phase flowing in the degassing channel.

[0047] In one embodiment of the liquid chromatograph according to the present invention, the apparatus includes: a liquid delivery pump for delivering a mobile phase; a degassing device according to the present invention described above for degassing the mobile phase delivered by the liquid delivery pump; an autosampler fluidly connected downstream of the liquid delivery pump for injecting a sample into the mobile phase; a separation column fluidly connected downstream of the autosampler for separating a plurality of components in the sample injected into the mobile phase by the autosampler from each other; and a detector fluidly connected downstream of the separation column for detecting each component separated by the separation column.

[0048] 1 Pump unit 2 Housing 4 Pump head 6 Diaphragm 8 Rod 10 Retaining member 12 Cushioning sheet 14 First member 16 Second member 18 Pump chamber forming surface 20 Recess in pump chamber forming surface 21 Pump chamber 22 Space 23 Wall surface in space (obstruction structure) 24 Outlet check valve 25 Outlet of outlet check valve 26 Internal space of outlet check valve 28 Projection 30 First flow path 32 Fluid outlet 34 Recess in first member 36 Recess in second member 38 Bolt 100 Vacuum pump 102 Motor 104 Motor drive shaft 106 Eccentric shaft 108a, 108b Bearing 110 Spacer 200 Liquid chromatograph 202 Liquid transfer pump 204 Degassing device 206 Autosampler 208 Separation column 210 Detector 212 Degassing channel 214 Vacuum chamber

Claims

1. A vacuum pump comprising at least one pump unit, each of the at least one pump unit comprising: a pump head having a pump chamber forming surface that forms a pump chamber; a diaphragm provided so as to cover the pump chamber forming surface of the pump head; and a rod that is driven axially with its tip facing the pump chamber forming surface and holds the diaphragm with its tip, wherein the internal volume of the pump chamber formed between the pump chamber forming surface and the diaphragm changes when the rod is driven axially, thereby causing fluid to be drawn into the pump chamber and fluid to be discharged from the pump chamber, and each of the at least one pump unit is provided with a duckbill type check valve inside the pump head that opens only when fluid is pushed out of the pump chamber, and the pump head comprises a first flow path that provides fluid communication between the pump chamber and the inlet of the check valve, and a second flow path that provides fluid communication between the outlet of the check valve and the outside. The second flow path has an obstruction structure that inhibits the formation of vortices caused by the fluid flowing out from the outlet of the check valve, in a vacuum pump.

2. The vacuum pump according to claim 1, wherein the second flow path has a wall surface substantially perpendicular to the direction of fluid flow near the outlet of the check valve as the obstruction structure.

3. The vacuum pump according to claim 1, wherein the pump head comprises a first member having the pump chamber forming surface and the first flow path, and a second member mounted on the side of the first member opposite to the pump chamber forming surface and having the second flow path.

4. The vacuum pump according to claim 3, wherein the check valve has an open end in contact with the surface of the first member opposite to the surface forming the pump chamber, a projection is provided on the surface of the first member opposite to the surface forming the pump chamber, the end of the first flow path on the check valve side is located on the tip surface of the projection, the projection is inserted into the interior of the check valve from the open end of the check valve, thereby reducing the volume of the internal space of the check valve.

5. The vacuum pump according to claim 4, wherein the check valve has a cylindrical portion with a hollow cylindrical shape at its open end, and the projection of the first member has a cylindrical shape with an outer diameter substantially the same as the inner diameter of the cylindrical portion.

6. The vacuum pump according to claim 3, wherein the pump head has a structure that brings the back surface of the pump chamber forming surface of the first member into contact with the outside air.

7. The vacuum pump according to claim 1, wherein each of the at least one pump units further comprises an elastic cushioning sheet interposed between the peripheral edge of the tip surface of the rod and the diaphragm.

8. The vacuum pump according to claim 1, wherein the at least one pump unit includes a first pump unit and a second pump unit, the rod of the first pump unit and the rod of the second pump unit are driven axially by a common eccentric cam mechanism, the eccentric cam mechanism comprises a motor, an eccentric shaft rotated by the motor, a first bearing mounted at a first position on the eccentric shaft, and a second bearing mounted at a second position on the eccentric shaft further from the motor than the first position, the rod of the first pump unit is connected to the eccentric shaft via the first bearing, the rod of the second pump unit is connected to the eccentric shaft via the second bearing, the outer diameter of the eccentric shaft at the first position is greater than the outer diameter at the second position, and the inner diameter of the first bearing is greater than the inner diameter of the second bearing.

9. A degassing apparatus comprising: a degassing channel made of a gas-permeable tube that allows gas to pass through but not liquid; a vacuum chamber housing the degassing channel inside; and a vacuum pump according to claim 1 connected to the vacuum chamber for reducing the pressure inside the vacuum chamber, wherein the apparatus degasses a mobile phase flowing through the degassing channel.

10. A liquid chromatograph comprising: a liquid delivery pump for delivering a mobile phase; a degassing device according to claim 9 for degassing the mobile phase delivered by the liquid delivery pump; an autosampler fluidly connected downstream of the liquid delivery pump for injecting a sample into the mobile phase; a separation column fluidly connected downstream of the autosampler for separating a plurality of components in the sample injected into the mobile phase by the autosampler from each other; and a detector fluidly connected downstream of the separation column for detecting each component separated by the separation column.

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