CRYOCOOLER

[0001] This is a bypass continuation of International PCT Application No. PCT/JP2024/031482, filed on Sep. 2, 2024, which claims priority to Japanese Patent Application No. 2023-168494, filed on Sep. 28, 2023, which are incorporated by reference herein in their entirety.

[0002] Certain embodiments relate to a cryocooler.

[0003] For example, a cryocooler such as a Gifford-McMahon (GM) cryocooler includes a displacer that reciprocates to periodically change a volume of an expansion space of a working gas is known in the related art. A refrigeration cycle is configured in the cryocooler by causing a pressure in the expansion space to fluctuate in proper synchronization with periodic fluctuations in the volume of the expansion space. As one typical method for driving a reciprocating motion of the displacer, for example, there is a type in which a motor is mechanically coupled to the displacer via a motion conversion mechanism such as a scotch yoke mechanism. The motion conversion mechanism can convert a rotary motion output by the motor into a linear reciprocating motion of the displacer.

[0004] One or more embodiments provide a cryocooler including a cold head motor, a crank coupled to the cold head motor, a housing that accommodates the cold head motor and the crank, a bearing that supports the crank, the bearing being attached to the housing to partition the housing into two chambers together with the crank, and a pressure equalizing passage that connects the two chambers.

[0010] During an operation of the cryocooler, a periodic load corresponding to intake and exhaust of a working gas acts on the displacer, thereby resulting in a load applied to the motor. In a large cryocooler having a high cooling capacity, the load applied to the motor which results from the intake and the exhaust of the working gas is likely to increase. An excessive load may cause an abnormal operation or a failure of the motor.

[0011] It is desirable to improve long-term reliability of a cryocooler.

[0012] Hereinafter, embodiments for implementing the present invention will be described in detail with reference to drawings. The same reference numerals are assigned to the same or equivalent components, members, and processes in the description and the drawings, and repeated description will be appropriately omitted. A scale or a shape of each shown element is set for convenience in order to facilitate the description, and is not to be interpreted in a limited manner unless otherwise specified. The embodiments are merely examples, and do not limit the scope of the present invention at all. All features or combinations thereof described in the embodiments are not necessarily essential to the invention.

[0013] FIGS. 1 to 3 are views schematically showing a cryocooler 10 according to an embodiment. FIG. 1 illustrates an appearance of the cryocooler 10. FIG. 2 illustrates an internal structure of a low-temperature section of the cryocooler 10, and FIG. 3 shows an internal structure of a driver. As an example, the cryocooler 10 is a two-stage type Gifford-McMahon (GM) cryocooler.

[0014] The cryocooler 10 includes a compressor 12 and an expander 14. The compressor 12 is configured to collect a working gas of the cryocooler 10 from the expander 14, to pressurize the collected working gas, and to supply the working gas to the expander 14 again. The compressor 12 and the expander 14 form a refrigeration cycle of the cryocooler 10. In this manner, the cryocooler 10 can provide desired cryogenic cooling. The expander 14 is also often referred to as a cold head. The cold head is usually installed in a vacuum container (not shown) such that a low-temperature section is disposed inside the vacuum container and a driver is disposed in a peripheral environment (for example, a room temperature atmospheric pressure environment) outside the vacuum container, and the compressor 12 is disposed in the peripheral environment. The working gas is also called a refrigerant gas, and is usually a helium gas. However, other suitable gases may be used. In order to facilitate understanding, a flow direction of the working gas is shown by an arrow in FIG. 1.

[0015] In general, a pressure of the working gas supplied from the compressor 12 to the expander 14 and a pressure of the working gas collected from the expander 14 to the compressor 12 are considerably higher than the atmospheric pressure, and can be respectively referred to as a first high pressure and a second high pressure. For convenience of description, the first high pressure and the second high pressure are simply referred to as a high-pressure and a low-pressure, respectively. Typically, the high pressure is 2 to 3 MPa, for example. For example, the low pressure is 0.5 to 1.5 MPa, and is approximately 0.8 MPa, for example. For the sake of understanding, a flow direction of the working gas is indicated by an arrow.

[0016] The expander 14 includes a cold head cylinder 16, a displacer assembly (hereinafter, also simply referred to as a displacer) 18, and a cold head housing (hereinafter, also simply referred to as a housing) 20. The cold head cylinder 16 guides a linear reciprocating motion of the displacer 18, and forms an expansion chamber (32, 34) as an expansion space for the working gas between the displacer 18 and the cold head cylinder 16. The cold head cylinder 16 is fixed to the cold head housing 20. In this manner, a casing of the expander 14 is formed, and an airtight space for accommodating the displacer 18 is formed inside the cold head cylinder 16.

[0017] In the present specification, in order to describe a positional relationship between components of the cryocooler 10, for convenience of description, a side close to a top dead center of axial reciprocation of the displacer 18 will be referred to as “up”, and a side close to a bottom dead center will be referred to as “down”. The top dead center is a position of the displacer 18 at which a volume of an expansion space is maximized, and the bottom dead center is a position of the displacer 18 at which the volume of the expansion space is minimized. Since a temperature gradient in which a temperature decreases from an upper side to a lower side is formed during an operation of the cryocooler 10, the upper side can be called a high-temperature side, and the lower side can be called a low-temperature side.

[0018] The cold head cylinder 16 includes a first cylinder 16a and a second cylinder 16b. As an example, each of the first cylinder 16a and the second cylinder 16b is a member having a cylindrical shape, and the second cylinder 16b has a diameter smaller than the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are coaxially disposed, and a lower end of the first cylinder 16a is rigidly connected to an upper end of the second cylinder 16b.

[0019] The displacer assembly 18 includes a first displacer 18a and a second displacer 18b, which are connected to each other, and move integrally. As an example, the first displacer 18a and the second displacer 18b each are members having a cylindrical shape, and the second displacer 18b has a diameter smaller than the first displacer 18a. The first displacer 18a and the second displacer 18b are coaxially disposed.

[0020] The first displacer 18a is accommodated in the first cylinder 16a, and the second displacer 18b is accommodated in the second cylinder 16b. The first displacer 18a can reciprocate in the axial direction along the first cylinder 16a, and the second displacer 18b can reciprocate in the axial direction along the second cylinder 16b.

[0021] As illustrated in FIG. 2, the first displacer 18a accommodates a first regenerator 26. The first regenerator 26 is formed by filling a tubular main body of the first displacer 18a with a wire mesh such as copper or other appropriate first regenerator materials. An upper lid and a lower lid of the first displacer 18a may be provided as separate members from the main body of the first displacer 18a, and the upper lid and the lower lid of the first displacer 18a may be fixed to the main body by any suitable method such as fastening or welding. In this manner, the first regenerator material may be accommodated in the first displacer 18a.

[0022] Similarly, the second displacer 18b accommodates a second regenerator 28. The second regenerator 28 is formed by filling a tubular main body of the second displacer 18b for example, with a non-magnetic regenerator material such as bismuth, a magnetic regenerator material such as HoCu₂, or other appropriate second regenerator materials.

[0023] The second regenerator material may be formed in a granular shape. The upper lid and the lower lid of the second displacer 18b may be provided as separate members from the main body of the second displacer 18b, and the lower lid and the upper lid of the second displacer 18b may be fixed to the main body by any suitable method such as fastening or welding. In this manner, the second regenerator material may be accommodated in the second displacer 18b.

[0024] The displacer 18 forms an upper chamber 30, a first expansion chamber 32, and a second expansion chamber 34 inside the cold head cylinder 16. In order to exchange heat with a desired object or medium to be cooled by the cryocooler 10, the expander 14 includes a first cooling stage 33 and a second cooling stage 35. The upper chamber 30 is formed between the upper lid of the first displacer 18a and an upper portion of the first cylinder 16a. The first expansion chamber 32 is formed between the lower lid of the first displacer 18a and the first cooling stage 33. The second expansion chamber 34 is formed between the lower lid of the second displacer 18b and the second cooling stage 35. The first cooling stage 33 is fixed to the lower portion of the first cylinder 16a to surround the first expansion chamber 32, and the second cooling stage 35 is fixed to the lower portion of the second cylinder 16b to surround the second expansion chamber 34.

[0025] The first regenerator 26 is connected to the upper chamber 30 through a working gas flow path 36a formed in the upper lid of the first displacer 18a, and is connected to the first expansion chamber 32 through a working gas flow path 36b formed in the lower lid of the first displacer 18a. The second regenerator 28 is connected to the first regenerator 26 through a working gas flow path 36c formed from the lower lid of the first displacer 18a to the upper lid of the second displacer 18b. In addition, the second regenerator 28 is connected to the second expansion chamber 34 through a working gas flow path 36d formed in the lower lid of the second displacer 18b.

[0026] A first seal 38a and a second seal 38b may be provided such that a working gas flow among the first expansion chamber 32, the second expansion chamber 34, and the upper chamber 30 is guided to the first regenerator 26 and the second regenerator 28 rather than to a clearance between the cold head cylinder 16 and the displacer 18. The first seal 38a may be mounted on the upper lid of the first displacer 18a to be disposed between the first displacer 18a and the first cylinder 16a. The second seal 38b may be mounted on the upper lid of the second displacer 18b to be disposed between the second displacer 18b and the second cylinder 16b.

[0027] As illustrated in FIG. 3, the cold head housing 20 is provided with an intake port 20a, which is an inlet of the working gas to the expander 14 of the cryocooler 10, and an exhaust port 20b, which is an outlet of the working gas from the expander 14. The intake port 20a is connected to a high-pressure side of the compressor 12, and the exhaust port 20b is connected to a low-pressure side of the compressor 12. The working gas is supplied from the intake port 20a to the expander 14, and is discharged from the exhaust port 20b to the compressor 12.

[0028] A driver of the cold head includes a cold head motor 40, a rotary valve 42, and a motion conversion mechanism 43. These components of the driver are accommodated in a low-pressure gas chamber 22 defined inside the cold head housing 20. The low-pressure gas chamber 22 communicates with the low-pressure side of the compressor 12 through the exhaust port 20b. Therefore, the low-pressure gas chamber 22 is always maintained at a low pressure.

[0029] The cold head motor 40 is provided in the expander 14 as a drive source of the displacer 18 and the rotary valve 42. The cold head motor 40 may be an appropriate electromagnetic motor, and may be configured to rotate the motor rotary shaft 40a at a constant rotational speed, or may be capable of variably controlling a rotational speed of the motor rotary shaft 40a.

[0030] The rotary valve 42 alternately connects the high-pressure side and the low-pressure side of the compressor 12 to the cold head cylinder 16 (that is, the upper chamber 30, the first expansion chamber 32, and the second expansion chamber 34), and is configured to periodically switch the intake and the exhaust of the cold head cylinder 16.

[0031] The rotary valve 42 includes a valve rotor 42a and a valve stator 42b, and the valve rotor 42a is in contact with the valve stator 42b to rotate while sliding with respect to the valve stator 42b. The valve stator 42b is fixed to the cold head housing 20. An elastic body such as a spring for pressing a valve stator 42b toward a valve rotor 42a in a direction of the rotary shaft of the valve rotor 42a may be interposed between the valve stator 42b and the cold head housing 20.

[0032] The cold head housing 20 is provided with a housing internal flow path 24 that connects the rotary valve 42 to the upper chamber 30. A valve internal flow path is formed in the valve rotor 42a and the valve stator 42b of the rotary valve 42 to alternately connect the housing internal flow path 24 to the intake port 20a of the cold head housing 20 and the low-pressure gas chamber 22. Various known forms can be adopted as the valve internal flow path, and details thereof will not be described herein.

[0033] The motion conversion mechanism 43 is configured to transmit the rotation of the motor rotary shaft 40a to the rotary valve 42, to convert the rotation into the linear reciprocation of the displacer 18, and to couple the cold head motor 40 to the rotary valve 42 and the displacer 18. An example of the motion conversion mechanism 43 will be described below. One rotation of the motor rotary shaft 40a causes one reciprocation of the displacer 18 via the motion conversion mechanism 43, thereby periodically changing the volume of the expansion space for the working gas. Simultaneously, one rotation of the motor rotary shaft 40a causes one rotation of the rotary valve 42 via the motion conversion mechanism 43, thereby periodically changing the pressure of the expansion space for the working gas.

[0034] FIG. 4 is a view schematically showing an exploded perspective view of a main part of the driver of the cold head according to the embodiment. FIG. 4 illustrates the motor rotary shaft 40a and the motion conversion mechanism 43. The motion conversion mechanism 43 includes a scotch yoke mechanism in this embodiment. Therefore, as shown in FIGS. 3 and 4, the motion conversion mechanism 43 includes a crank 44 including a crank pin 44a, a scotch yoke shaft 45, and a crank pin bearing 46. The scotch yoke shaft 45 includes a scotch yoke plate 45a, an upper rod 45b, and a lower rod 45c.

[0035] The crank 44 is fixed to the motor rotary shaft 40a. In this manner, the crank 44 is coupled to the cold head motor 40. The crank 44 is accommodated in the cold head housing 20 together with the cold head motor 40. The crank 44 includes a crank pin 44a that extends toward the scotch yoke mechanism, on a side opposite to the motor rotary shaft 40a. The crank pin 44a extends parallel to the motor rotary shaft 40a at a position eccentric from the motor rotary shaft 40a.

[0036] The scotch yoke plate 45a is a rectangular plate-shaped member having a horizontally elongated window 47. The horizontally elongated window 47 extends in an axial direction of the cold head and in a direction perpendicular to the motor rotary shaft 40a. The crank pin bearing 46 is disposed to be capable of rolling in the horizontally elongated window 47. For example, the crank pin bearing 46 may be a roller bearing. An engagement hole 46a that engages with the crank pin 44a is formed at the center of the crank pin bearing 46, and the crank pin 44a penetrates the engagement hole 46a.

[0037] On a side opposite to the crank 44 with respect to the scotch yoke plate 45a, the valve rotor 42a of the rotary valve 42 is disposed such that a center axis thereof coincides with the motor rotary shaft 40a, and a tip of the crank pin 44a penetrating the engagement hole 46a is fixed to the valve rotor 42a.

[0038] The upper rod 45b extends upward from the center of an upper frame of the scotch yoke plate 45a, the lower rod 45c extends downward from the center of a lower frame of the scotch yoke plate 45a, and the rods are coaxially disposed. The scotch yoke plate 45a and the upper rod 45b are accommodated in the low-pressure gas chamber 22, and the lower rod 45c penetrates the cold head housing 20 to extend into the cold head cylinder 16. A tip of the lower rod 45c is coupled to the displacer 18 inside the cold head cylinder 16.

[0039] A first sliding bearing 48a is provided between the upper rod 45b and the cold head housing 20, and a second sliding bearing 48b is provided between the lower rod 45c and the cold head housing 20. The cold head housing 20 has a recessed portion for receiving the upper rod 45b on an upper portion thereof, and the first sliding bearing 48a is disposed in the recess to support the upper rod 45b to be slidable in the axial direction. In addition, the second sliding bearing 48b is disposed in a through-hole of the cold head housing 20 through which the lower rod 45c passes, and the lower rod 45c is supported to be slidable in the axial direction. The second sliding bearing 48b is provided with a seal portion such as a slipper seal and a clearance seal, and is configured to be airtight. Therefore, the low-pressure gas chamber 22 is isolated from the upper chamber 30. There is no direct gas circulation between the low-pressure gas chamber 22 and the upper chamber 30.

[0040] In this embodiment, as illustrated in FIG. 3, the crank 44 is rotatably supported by the crank support bearing 50 with respect to the cold head housing 20. The crank support bearing 50 may include an inner ring fixed to an outer periphery of the crank 44, an outer ring fixed to the cold head housing 20, and a rolling element disposed between the inner ring and the outer ring, and may be an appropriate bearing such as a deep groove ball bearing and an angular ball bearing. The crank support bearing 50 allows the crank 44 to rotate, and can support a load in a cold head axial direction (that is, an extending direction of the scotch yoke shaft 45), which is applied to the crank 44 from the displacer 18 due to the intake and the exhaust of the working gas.

[0041] The crank support bearing 50 is attached to the cold head housing 20 together with the crank 44 to partition the low-pressure gas chamber 22 inside the cold head housing 20 into the first chamber 22a and the second chamber 22b. The cold head motor 40 is accommodated in the first chamber 22a, and the scotch yoke shaft 45 and the rotary valve 42 are accommodated in the second chamber 22b.

[0042] As an example of attachment of the crank support bearing 50 to the cold head housing 20, a bearing retaining member 52 may be used. The bearing retaining member 52 has a ring shape to surround the crank 44 with a gap between the crank 44 and the bearing retaining member 52, and is fixed to the cold head housing 20. The bearing retaining member 52 is disposed adjacent to the crank support bearing 50 on a side of the cold head motor 40 with respect to the crank support bearing 50. A bearing retainer 53 is formed in the cold head housing 20 on a side of the scotch yoke shaft 45 with respect to the crank support bearing 50. The crank support bearing 50 is interposed between the bearing retaining member 52 and the bearing retainer 53, and is positioned in a direction of the motor rotary shaft 40a. For example, a cushioning material 54 such as an O-ring may be interposed between the bearing retaining member 52 and the crank support bearing 50.

[0043] As in the crank 44, the valve rotor 42a of the rotary valve 42 is also rotatably supported by the valve support bearing 56 with respect to the cold head housing 20. The valve support bearing 56 may be disposed symmetrically with the crank support bearing 50 with respect to the scotch yoke shaft 45. For example, the crank support bearing 50 and the valve support bearing 56 may be disposed at approximately equal distances from the scotch yoke shaft 45. In this case, an axial load acting on the displacer 18 and the scotch yoke shaft 45 due to the intake and the exhaust of the working gas can be uniformly supported by the crank support bearing 50 and the valve support bearing 56.

[0044] In addition, in this embodiment, the crank 44 includes a pressure equalizing passage 58 that connects the first chamber 22a and the second chamber 22b. The first chamber 22a is connected to the second chamber 22b through the pressure equalizing passage 58, and the second chamber 22b is connected to the exhaust port 20b. In this way, the low-pressure gas chamber 22, that is, the first chamber 22a and the second chamber 22b are connected to the exhaust port 20b.

[0045] The pressure equalizing passage 58 may be at least one through-hole formed in the crank 44, and may extend in a direction of the motor rotary shaft 40a. As illustrated in FIG. 4, the crank 44 may include a plurality of through-holes as the pressure equalizing passage 58, and the through-holes may be formed around the shaft hole 44b of the crank 44 into which the motor rotary shaft 40a is fitted.

[0046] A diameter of the pressure equalizing passage 58 may be equal to or smaller than a diameter of the shaft hole 44b, for example, 90% to 100% of the diameter of the shaft hole 44b. A hole diameter of the pressure equalizing passage 58 is aligned with the shaft hole 44b, or is slightly smaller than the shaft hole 44b. In this manner, the pressure equalizing passage 58 can be simultaneously processed in a process of processing the shaft hole 44b in the crank 44. The crank 44 can be efficiently manufactured.

[0047] The pressure equalizing passage 58 is not limited to the through-hole. For example, the pressure equalizing passage 58 may have any shape such as a groove or a cutout formed in the outer periphery of the crank 44, and that connects the first chamber 22a and the second chamber 22b across the crank support bearing 50.

[0048] In the above-described configuration of the cryocooler 10, when the cold head motor 40 is driven, the motor rotary shaft 40a is rotated. The rotation is transmitted to the rotary valve 42 and the motion conversion mechanism 43. Due to the rotation of the motor rotary shaft 40a, the crank pin bearing 46 engaged with the crank pin 44a rotates to draw a circle. In this case, the crank pin bearing 46 reciprocates in the horizontally elongated window 47 of the scotch yoke plate 45a, and the scotch yoke shaft 45 and the displacer 18 reciprocate in the axial direction. In this way, the cold head motor 40 drives the displacer 18 to reciprocate in the axial direction, and rotates the rotary valve 42 in synchronization with the reciprocation.

[0049] In this way, the synchronized volume fluctuation and pressure fluctuation are generated in the expansion space to configure the refrigeration cycle of the cryocooler 10, whereby the cryocooler 10 can provide desired cryogenic cooling. The first cooling stage 33 can be cooled to a first cooling temperature, and the second cooling stage 35 can be cooled to a second cooling temperature lower than the first cooling temperature. The first cooling temperature may be, for example, in a range of about 10 K to about 100 K or in a range of about 20 K to about 40 K. The second cooling temperature may be, for example, about 20 K or lower, or about 10 K or lower, or in a range of about 1 K to about 4 K.

[0050] During the operation of the cryocooler 10, a periodic load corresponding to the intake and the exhaust of the working gas acts on the displacer 18. When the displacer 18 moves upward, the working gas is suctioned into the expansion space (32, 34) via the first regenerator 26 and the second regenerator 28. Therefore, the displacer 18 receives a downward load from the working gas. In addition, when the displacer 18 moves downward, the working gas is exhausted from the expansion space. Therefore, the displacer 18 receives an upward load from the working gas. This load in a direction opposite to a movement direction of the displacer 18 is a load applied to the cold head motor 40. In particular, in a large cryocooler having a high cooling capacity, the load applied to the motor due to the intake and the exhaust of the working gas is likely to increase. An excessive load may cause an abnormal operation or a failure of the motor.

[0051] The support of the crank 44 by the crank support bearing 50 is helpful in reducing this load applied to the cold head motor 40. Since the crank 44 is disposed closer to the scotch yoke shaft 45 than the motor rotary shaft 40a, the crank 44 is supported by the bearing. Therefore, a shaft load received from the scotch yoke shaft 45 can be more effectively supported, compared to when the motor rotary shaft 40a is supported.

[0052] However, in the existing cryocooler, a solid crank is used. Therefore, when the crank is supported by the bearing, a space inside the cold head housing is divided into two by the crank and the bearing. The pressure of the working gas differs between the space on the motor side and the space on the scotch yoke side when the crank and the bearing are set as a boundary, and a differential pressure can be generated between the two spaces. There is a possibility that a load acting on the bearing by the differential pressure has an adverse effect on an operation or reliability of the bearing.

[0053] According to this embodiment, since the pressure equalizing passage 58 is provided in the crank 44, it is possible to avoid occurrence of the differential pressure between both sides of the crank support bearing 50, that is, the first chamber 22a and the second chamber 22b. In this manner, the load applied to the crank support bearing 50 can be reduced, and long-term reliability of the cryocooler 10 can be improved.

[0054] Hitherto, the present invention has been described based on the embodiments. The present invention is not limited to the above-described embodiments. It may be understood by those skilled in the art that various design changes can be made, various modification examples can be adopted, and the modification examples also fall within the scope of the present invention. Various features described with reference to a certain embodiment are also applicable to other embodiments. A new embodiment acquired from a combination of the embodiments compatibly achieves each advantageous effect of the combined embodiments.

[0055] FIG. 5 is a view schematically illustrating the cryocooler 10 according to another embodiment. As in FIG. 3, FIG. 5 illustrates an internal structure of the driver of the cold head. As illustrated in FIG. 5, the pressure equalizing passage 58 may be formed in the cold head housing 20. Even in this case, it is possible to avoid the occurrence of the differential pressure between both sides of the crank support bearing 50, that is, the first chamber 22a and the second chamber 22b.

[0056] Hitherto, the embodiments have been described with reference to the two-stage type GM cryocooler. The present invention is not limited thereto, and the pressure equalizing passage 58 according to the embodiment is applicable to a single-stage type or multi-stage type GM cryocooler or other cryocoolers in which the crank 44 is supported by the cold head housing 20 via the crank support bearing 50.

[0057] Hitherto, the present invention has been described based on the embodiments. The present invention is not limited to the above-described embodiments. It may be understood by those skilled in the art that various design changes can be made, various modification examples can be adopted, and the modification examples also fall within the scope of the present invention.

[0058] The present invention can be used in a field of cryocoolers.

[0059] It should be understood that the invention is not limited to the above-described embodiment, but may be modified into various forms on the basis of the spirit of the disclosure. Additionally, the modifications are included in the scope of the disclosure.