Cryocooler

Abstract

This cryocooler (10) comprises: a cold head motor (40); a crank (44) that is connected to the cold head motor (40); a cold head housing (20) that accommodates the cold head motor (40) and the crank (44); a crank-supporting bearing (50) that supports the crank (44) and is attached to the cold head housing (20) so as to divide, together with the crank (44), the cold head housing (20) into a first chamber (22a) and a second chamber (22b); and a pressure equalizing passage (58) that connects the first chamber (22a) and the second chamber (22b). The pressure equalizing passage (58) may be formed in the crank (44) or may be formed in the cold head housing (20).

Description

cryogenic refrigerator

[0001] The present invention relates to a cryogenic refrigerator.

[0002] Some cryogenic refrigerators, such as the Gifford-McMahon (GM) refrigerator, have a reciprocating displacer to periodically change the volume of the expansion space for the working gas. A refrigeration cycle is achieved in the cryogenic refrigerator by varying the pressure in the expansion space in proper synchronization with the periodic volume fluctuations in the expansion space. One typical method for driving the reciprocating motion of the displacer is to mechanically couple a motor to the displacer via a motion conversion mechanism, such as a Scotch yoke mechanism. The motion conversion mechanism can convert the rotational motion of the motor into linear reciprocating motion of the displacer.

[0003] Japanese Patent Application Laid-Open No. 2016-57025

[0004] During operation of a cryogenic refrigerator, a cyclic load corresponding to the intake and exhaust of the working gas acts on the displacer, which places a load on the motor. In large cryogenic refrigerators with high refrigeration capacity, the load on the motor caused by the intake and exhaust of the working gas tends to be large. Excessive load can cause the motor to malfunction or break down.

[0005] One exemplary object of certain aspects of the present invention is to improve the long-term reliability of cryogenic refrigerators.

[0006] According to one aspect of the present invention, a cryogenic refrigerator includes a cold head motor, a crank connected to the cold head motor, a housing that accommodates the cold head motor and the crank, a bearing that supports the crank and is attached to the housing so as to divide the housing together with the crank into two chambers, and a pressure equalization passage that connects the two chambers.

[0007] According to the present invention, the long-term reliability of a cryogenic refrigerator can be improved.

[0008] Fig. 1 is a diagram schematically showing a cryogenic refrigerator according to an embodiment. Fig. 2 is a diagram schematically showing a cryogenic refrigerator according to an embodiment. Fig. 3 is a diagram schematically showing a cryogenic refrigerator according to an embodiment. Fig. 4 is a diagram schematically showing an exploded perspective view of a main part of a drive unit of a cold head according to an embodiment. Fig. 5 is a diagram schematically showing a cryogenic refrigerator according to another embodiment.

[0009] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the description and drawings, identical or equivalent components, parts, and processes are designated by the same reference numerals, and redundant explanations will be omitted as appropriate. The scale and shape of each part shown in the drawings are set for convenience to facilitate explanation, and should not be interpreted as limiting unless otherwise specified. The embodiments are merely examples and do not limit the scope of the present invention in any way. All features and combinations thereof described in the embodiments are not necessarily essential to the invention.

[0010] Figures 1 to 3 are diagrams that schematically show a cryogenic refrigerator 10 according to an embodiment. Figure 1 shows the external appearance of the cryogenic refrigerator 10. Figure 2 shows the internal structure of the low-temperature section of the cryogenic refrigerator 10, and Figure 3 shows the internal structure of the drive section. The cryogenic refrigerator 10 is, for example, a two-stage Gifford-McMahon (GM) refrigerator.

[0011] The cryogenic refrigerator 10 includes a compressor 12 and an expander 14. The compressor 12 is configured to recover the working gas of the cryogenic refrigerator 10 from the expander 14, increase the pressure of the recovered working gas, and then supply the working gas to the expander 14 again. The compressor 12 and the expander 14 form a refrigeration cycle of the cryogenic refrigerator 10, thereby enabling the cryogenic refrigerator 10 to provide the desired cryogenic cooling. The expander 14 is often referred to as a cold head. The cold head is typically installed in a vacuum vessel (not shown) with a low-temperature section disposed within the vacuum vessel and a drive section disposed in an ambient environment (e.g., room temperature and atmospheric pressure) outside the vacuum vessel, and the compressor 12 is disposed in the ambient environment. The working gas, also referred to as a refrigerant gas, is typically helium gas, although other suitable gases may be used. For ease of understanding, the flow direction of the working gas is indicated by arrows in FIG. 1.

[0012] Generally, the pressure of the working gas supplied from the compressor 12 to the expander 14 and the pressure of the working gas recovered from the expander 14 to the compressor 12 are both significantly higher than atmospheric pressure, and can be referred to as a first high pressure and a second high pressure, respectively. For ease of explanation, the first high pressure and the second high pressure are also simply referred to as a high pressure and a low pressure, respectively. Typically, the high pressure is, for example, 2 to 3 MPa. The low pressure is, for example, 0.5 to 1.5 MPa, e.g., approximately 0.8 MPa. For ease of understanding, the flow direction of the working gas is indicated by arrows.

[0013] The expander 14 includes a cold head cylinder 16, a displacer assembly (hereinafter sometimes simply referred to as a displacer) 18, and a cold head housing (hereinafter sometimes simply referred to as a housing) 20. The cold head cylinder 16 guides the linear reciprocating motion of the displacer 18, and forms expansion chambers (32, 34) between the cold head cylinder 16 and the displacer 18 as expansion spaces for the working gas. The cold head cylinder 16 is fixed to the cold head housing 20, thereby forming a housing for the expander 14, and an airtight space for accommodating the displacer 18 is formed within the cold head cylinder 16.

[0014] In this document, for convenience in explaining the positional relationship between the components of the cryogenic refrigerator 10, the side closer to the top dead center of the axial reciprocation of the displacer 18 will be referred to as "top" and the side closer to the bottom dead center will be referred to as "bottom." The top dead center is the position of the displacer 18 where the volume of the expansion space is maximum, and the bottom dead center is the position of the displacer 18 where the volume of the expansion space is minimum. During operation of the cryogenic refrigerator 10, a temperature gradient occurs in which the temperature decreases from top to bottom, so the top side can also be referred to as the high-temperature side and the bottom side as the low-temperature side.

[0015] The cold head cylinder 16 includes a first cylinder 16a and a second cylinder 16b. The first cylinder 16a and the second cylinder 16b are, for example, cylindrical members, and the second cylinder 16b has a smaller diameter than the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are arranged coaxially, and the lower end of the first cylinder 16a is rigidly connected to the upper end of the second cylinder 16b.

[0016] The displacer assembly 18 includes a first displacer 18a and a second displacer 18b that are connected to each other and move together. The first displacer 18a and the second displacer 18b are, for example, cylindrical members, and the second displacer 18b has a smaller diameter than the first displacer 18a. The first displacer 18a and the second displacer 18b are arranged coaxially.

[0017] 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 is reciprocatingly movable in the axial direction along the first cylinder 16a, and the second displacer 18b is reciprocatingly movable in the axial direction along the second cylinder 16b.

[0018] 2, the first displacer 18a accommodates a first regenerator 26. The first regenerator 26 is formed by filling a cylindrical main body of the first displacer 18a with a first regenerator material, such as a wire mesh made of copper or other suitable material. The upper and lower covers of the first displacer 18a may be provided as separate members from the main body of the first displacer 18a, and the upper and lower covers of the first displacer 18a may be fixed to the main body by suitable means such as fastening or welding, thereby accommodating the first regenerator material in the first displacer 18a.

[0019] Similarly, the second displacer 18b accommodates a second regenerator 28. The second regenerator 28 is made of a non-magnetic regenerator material such as bismuth, HoCu, etc., housed in a cylindrical body of the second displacer 18b. ２ The second displacer 18b is formed by filling it with a magnetic regenerator material such as a refrigerant or other suitable second regenerator material. The second regenerator material may be formed in a granular form. The upper and lower lids of the second displacer 18b may be provided as separate members from the main body of the second displacer 18b, and the upper and lower lids of the second displacer 18b may be fixed to the main body by suitable means such as fastening or welding, thereby containing the second regenerator material in the second displacer 18b.

[0020] 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. The expander 14 includes a first cooling stage 33 and a second cooling stage 35 for heat exchange with the desired object or medium to be cooled by the cryogenic refrigerator 10. The upper chamber 30 is formed between the upper cover of the first displacer 18a and the top of the first cylinder 16a. The first expansion chamber 32 is formed between the lower cover of the first displacer 18a and the first cooling stage 33. The second expansion chamber 34 is formed between the lower cover of the second displacer 18b and the second cooling stage 35. The first cooling stage 33 is fixed to the lower part of the first cylinder 16a to surround the first expansion chamber 32, and the second cooling stage 35 is fixed to the lower part of the second cylinder 16b to surround the second expansion chamber 34.

[0021] The first regenerator 26 is connected to the upper chamber 30 through a working gas passage 36a formed in the upper lid of the first displacer 18a, and is connected to the first expansion chamber 32 through a working gas passage 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 passage 36c formed from the lower lid of the first displacer 18a to the upper lid of the second displacer 18b. The second regenerator 28 is also connected to the second expansion chamber 34 through a working gas passage 36d formed in the lower lid of the second displacer 18b.

[0022] A first seal 38a and a second seal 38b may be provided so that the flow of working gas between the first expansion chamber 32, the second expansion chamber 34, and the upper chamber 30 is directed to the first regenerator 26 and the second regenerator 28, rather than through the clearance between the cold head cylinder 16 and the displacer 18. The first seal 38a may be attached to the top cover of the first displacer 18a so as to be positioned between the first displacer 18a and the first cylinder 16a. The second seal 38b may be attached to the top cover of the second displacer 18b so as to be positioned between the second displacer 18b and the second cylinder 16b.

[0023] 3, the cold head housing 20 is provided with an intake port 20a, which is an inlet for the working gas of the cryogenic refrigerator 10 to the expander 14, and an exhaust port 20b, which is an outlet for the working gas from the expander 14. The intake port 20a is connected to the high-pressure side of the compressor 12, and the exhaust port 20b is connected to the low-pressure side of the compressor 12. The working gas is supplied from the intake port 20a to the expander 14 and discharged from the exhaust port 20b to the compressor 12.

[0024] The cold head drive unit includes a cold head motor 40, a rotary valve 42, and a motion conversion mechanism 43. These components of the drive unit are housed 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.

[0025] The cold head motor 40 is provided in the expander 14 as a drive source for the displacer 18 and the rotary valve 42. The cold head motor 40 may be an appropriate electromagnetic motor configured to rotate a motor shaft 40a at a constant rotational speed, or the rotational speed of the motor shaft 40a may be variably controlled.

[0026] The rotary valve 42 is configured to alternately connect the high-pressure side and low-pressure side of the compressor 12 to the cold head cylinder 16 (i.e., the upper chamber 30, the first expansion chamber 32, and the second expansion chamber 34) and periodically switch between intake and exhaust of the cold head cylinder 16.

[0027] 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 so as to rotate while sliding relative to the valve stator 42b. The valve stator 42b is fixed to the cold head housing 20. An elastic body such as a spring may be interposed between the valve stator 42b and the cold head housing 20 to press the valve stator 42b toward the valve rotor 42a in the direction of the rotation axis of the valve rotor 42a.

[0028] The cold head housing 20 is formed with an internal housing passage 24 that connects the rotary valve 42 to the upper chamber 30. Internal valve passages are formed in the valve rotor 42a and valve stator 42b of the rotary valve 42 so as to alternately connect the internal housing passage 24 to the intake port 20a of the cold head housing 20 and the low-pressure gas chamber 22. Various known forms of the internal valve passage can be used, and will not be described in detail here.

[0029] The motion conversion mechanism 43 is configured to connect the cold head motor 40 to the rotary valve 42 and the displacer 18 so as to transmit the rotation of the motor rotary shaft 40a to the rotary valve 42 and convert it into linear reciprocating motion of the displacer 18. An example of the motion conversion mechanism 43 will be described later. One rotation of the motor rotary shaft 40a causes one reciprocating motion of the displacer 18 via the motion conversion mechanism 43, thereby periodically changing the volume of the expansion space for the working gas. At the same time, 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 in the expansion space for the working gas.

[0030] FIG. 4 is a schematic exploded perspective view of the main components of the cold head drive unit according to an embodiment. FIG. 4 shows a motor rotating shaft 40a and a motion conversion mechanism 43. In this embodiment, the motion conversion mechanism 43 includes a Scotch yoke mechanism. Therefore, as shown in FIGS. 3 and 4, the motion conversion mechanism 43 includes a crank 44 having 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.

[0031] The crank 44 is fixed to the motor shaft 40a and is thereby connected to the cold head motor 40. The crank 44 is housed in the cold head housing 20 together with the cold head motor 40. The crank 44 has a crank pin 44a that extends toward the Scotch yoke mechanism on the side opposite the motor shaft 40a. The crank pin 44a extends parallel to the motor shaft 40a at a position eccentric from the motor shaft 40a.

[0032] The Scotch yoke plate 45a is a rectangular plate-like member having a horizontally elongated window 47. The horizontally elongated window 47 extends in the axial direction of the cold head and in a direction perpendicular to the motor rotary shaft 40a. A crankpin bearing 46 is rotatably disposed in this horizontally elongated window 47. The crankpin bearing 46 may be, for example, a roller bearing. An engagement hole 46a that engages with the crankpin 44a is formed in the center of the crankpin bearing 46, and the crankpin 44a passes through the engagement hole 46a.

[0033] On the opposite side of the scotch yoke plate 45a from the crank 44, the valve rotor 42a of the rotary valve 42 is arranged with its central axis aligned with the motor rotary shaft 40a, and the tip of the crank pin 44a, which passes through the engagement hole 46a, is fixed to the valve rotor 42a.

[0034] The upper rod 45b extends upward from the center of the upper frame of the Scotch yoke plate 45a, and the lower rod 45c extends downward from the center of the lower frame of the Scotch yoke plate 45a, and these rods are arranged coaxially. The Scotch yoke plate 45a and the upper rod 45b are housed in the low-pressure gas chamber 22, and the lower rod 45c extends through the cold head housing 20 into the cold head cylinder 16. The tip of the lower rod 45c is connected to the displacer 18 inside the cold head cylinder 16.

[0035] 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 recess in its upper portion to receive the upper rod 45b. The first sliding bearing 48a is disposed in this recess and supports the upper rod 45b axially slidably. 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 supports the lower rod 45c axially slidably. The second sliding bearing 48b is provided with a seal, such as a slipper seal or clearance seal, and is configured to be airtight, thereby isolating the low-pressure gas chamber 22 from the upper chamber 30. There is no direct gas communication between the low-pressure gas chamber 22 and the upper chamber 30.

[0036] 3 , the crank 44 is rotatably supported relative to the cold head housing 20 by a crank support bearing 50. The crank support bearing 50 may include an inner ring fixed to the outer periphery of the crank 44, an outer ring fixed to the cold head housing 20, and rolling elements disposed between the inner ring and the outer ring, and may be an appropriate bearing such as a deep groove ball bearing or an angular contact ball bearing. The crank support bearing 50 allows the crank 44 to rotate and can support a load in the cold head axial direction (i.e., the direction in which the Scotch yoke shaft 45 extends) that is applied to the crank 44 from the displacer 18 due to the intake and exhaust of the working gas.

[0037] The crank support bearing 50, together with the crank 44, is attached to the cold head housing 20 so as to divide the low-pressure gas chamber 22 in the cold head housing 20 into a first chamber 22a and a second chamber 22b. The cold head motor 40 is housed in the first chamber 22a, and the Scotch yoke shaft 45 and the rotary valve 42 are housed in the second chamber 22b.

[0038] As an example of a method for attaching 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-like shape surrounding the crank 44 with a gap between it and the crank 44, and is fixed to the cold head housing 20. The bearing retaining member 52 is disposed adjacent to the crank support bearing 50 on the cold head motor 40 side of the crank support bearing 50. A bearing retaining portion 53 is formed in the cold head housing 20 on the Scotch yoke shaft 45 side of the crank support bearing 50. The crank support bearing 50 is positioned with respect to the direction of the motor rotating shaft 40a by being sandwiched between the bearing retaining member 52 and the bearing retaining portion 53. A buffer material 54, such as an O-ring, may be sandwiched between the bearing retaining member 52 and the crank support bearing 50.

[0039] Like the crank 44, the valve rotor 42a of the rotary valve 42 is also rotatably supported relative to the cold head housing 20 by a valve support bearing 56. The valve support bearing 56 may be disposed symmetrically to 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 approximately equidistant from the Scotch yoke shaft 45. In this manner, the axial load acting on the displacer 18 and the Scotch yoke shaft 45 due to the intake and exhaust of the working gas can be evenly supported by the crank support bearing 50 and the valve support bearing 56.

[0040] In this embodiment, the crank 44 also 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, i.e., the first chamber 22a and the second chamber 22b, are connected to the exhaust port 20b.

[0041] The pressure equalizing passage 58 may be at least one through hole formed in the crank 44 and may extend in the direction of the motor rotary shaft 40a. As shown in Fig. 4, the crank 44 may have a plurality of through holes as the pressure equalizing passage 58, and these through holes may be formed around the shaft hole 44b of the crank 44 into which the motor rotary shaft 40a is fitted.

[0042] The diameter of the pressure equalizing passage 58 may be equal to or smaller than the diameter of the shaft hole 44b, for example, 90 to 100% of the diameter of the shaft hole 44b. By making the hole diameter of the pressure equalizing passage 58 the same as or slightly smaller than the shaft hole 44b, the pressure equalizing passage 58 can be machined simultaneously in the process of machining the shaft hole 44b in the crank 44. This allows the crank 44 to be manufactured efficiently.

[0043] The pressure equalizing passage 58 is not limited to a through hole. For example, the pressure equalizing passage 58 may have any shape that connects the first chamber 22 a and the second chamber 22 b across the crank support bearing 50, such as a groove or notch formed in the outer periphery of the crank 44.

[0044] In the above-described configuration of the cryocooler 10, when the cold head motor 40 is driven, the motor shaft 40a is rotated. The rotation is transmitted to the rotary valve 42 and the motion conversion mechanism 43. The rotation of the motor shaft 40a causes the crank pin bearing 46 engaged with the crank pin 44a to rotate in a circular motion. At this time, the crank pin bearing 46 reciprocates through 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 axial reciprocation of the displacer 18 and rotates the rotary valve 42 in synchronization with this.

[0045] In this manner, synchronized volume fluctuations and pressure fluctuations are generated in the expansion space, forming a refrigeration cycle for the cryogenic refrigerator 10, thereby enabling the cryogenic refrigerator 10 to provide the 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 the range of about 10 K to about 100 K, or in the range of about 20 K to about 40 K. The second cooling temperature may be, for example, about 20 K or less, or about 10 K or less, or about 1 K to about 4 K.

[0046] During operation of the cryogenic refrigerator 10, a cyclic load acts on the displacer 18 in response to the intake and exhaust of the working gas. When the displacer 18 moves upward, the working gas is drawn into the expansion space (32, 34) via the first regenerator 26 and the second regenerator 28, and the displacer 18 is subjected to a downward load from the working gas. When the displacer 18 moves downward, the working gas is exhausted from the expansion space, and the displacer is subjected to an upward load from the working gas. This load, which is in the opposite direction to the movement of the displacer 18, acts as a load on the cold head motor 40. In large cryogenic refrigerators with high refrigeration capacity, the load on the motor due to the intake and exhaust of the working gas tends to be particularly large. Excessive load can cause the motor to malfunction or malfunction.

[0047] Supporting the crank 44 with the crank support bearing 50 helps to reduce this load on the cold head motor 40. Because the crank 44 is located closer to the Scotch yoke shaft 45 than the motor rotary shaft 40a, supporting the crank 44 with a bearing can more effectively support the axial load from the Scotch yoke shaft 45 than if the motor rotary shaft 40a were supported.

[0048] However, existing cryogenic refrigerators use a solid crank. When the crank is supported by a bearing, the crank and the bearing divide the space inside the cold head housing in two. The working gas pressure differs between the space on the motor side and the space on the scotch yoke side, with the crank and the bearing as the boundary, creating a pressure difference between the two spaces. The load acting on the bearing due to the pressure difference could adversely affect the operation and reliability of the bearing.

[0049] According to this embodiment, the pressure equalizing passage 58 is provided in the crank 44, which prevents a pressure difference from occurring on either side of the crank support bearing 50, i.e., between the first chamber 22a and the second chamber 22b. This reduces the load on the crank support bearing 50, thereby improving the long-term reliability of the cryogenic refrigerator 10.

[0050] The present invention has been described above based on examples. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiments, and that various design changes and modifications are possible, and that such modifications are also within the scope of the present invention. Various features described in relation to one embodiment can also be applied to other embodiments. A new embodiment created by combining embodiments will have the combined effects of the respective combined embodiments.

[0051] Fig. 5 is a schematic diagram of a cryogenic refrigerator 10 according to another embodiment. Similar to Fig. 3, Fig. 5 shows the internal structure of the cold head drive unit. As shown in Fig. 5, the pressure equalization passage 58 may be formed in the cold head housing 20. This also makes it possible to avoid a pressure difference between both sides of the crank support bearing 50, i.e., between the first chamber 22a and the second chamber 22b.

[0052] Although the above embodiment has been described with reference to a two-stage GM refrigerator, the present invention is not limited thereto, and the pressure equalization passage 58 according to the embodiment can be applied to a single-stage or multi-stage GM refrigerator, or to other cryogenic refrigerators in which the crank 44 is supported on the cold head housing 20 by the crank support bearing 50.

[0053] The present invention has been described above based on examples. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiments, and that various design changes and modifications are possible, and that such modifications are also within the scope of the present invention.

[0054] The present invention can be used in the field of cryogenic refrigerators.

[0055] 10 cryogenic refrigerator, 12 compressor, 14 expander, 20 cold head housing, 20a intake port, 20b exhaust port, 22 low-pressure gas chamber, 22a first chamber, 22b second chamber, 40 cold head motor, 44 crank, 50 crank support bearing, 58 pressure equalizing passage.

Claims

1. A cryogenic refrigerator comprising: a cold head motor; a crank connected 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 so as to divide the housing together with the crank into two chambers; and a pressure equalizing passage connecting the two chambers.

2. The cryogenic refrigerator according to claim 1, wherein the pressure equalizing passage is formed in the crank.

3. The cryogenic refrigerator according to claim 2, wherein the pressure equalizing passage includes at least one through hole extending in the direction of the rotation shaft of the cold head motor.

4. The cryogenic refrigerator according to claim 3, characterized in that the crank has a shaft hole into which the rotating shaft of the cold head motor is fitted, and the diameter of the through hole is equal to or smaller than the diameter of the shaft hole.

5. The cryogenic refrigerator according to claim 1, wherein the pressure equalizing passage is formed in the housing.

6. The cryogenic refrigerator according to claim 1, further comprising a bearing retaining member fixed to said housing and disposed adjacent to said bearing on the cold head motor side with respect to said bearing.

7. The cryogenic refrigerator according to claim 6, wherein the bearing is positioned in the direction of the rotation axis of the cold head motor between the bearing holding member and the bearing holding portion of the housing.

8. The cryogenic refrigerator according to claim 6, further comprising a buffer material sandwiched between said bearing and said bearing retaining member.

9. A cryogenic refrigerator as claimed in any one of claims 1 to 8, characterized in that the housing has an exhaust port for discharging the working gas of the cryogenic refrigerator, and the two chambers are connected to the exhaust port.

10. A cryogenic refrigerator as described in any one of claims 1 to 8, further comprising: a motion conversion mechanism including a Scotch yoke shaft; a rotary valve connected to the cold head motor via the motion conversion mechanism; and a valve support bearing that rotatably supports the rotary valve relative to the housing, wherein the bearing that supports the crank is disposed symmetrically to the valve support bearing with respect to the Scotch yoke shaft.

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