Seal assembly

Abstract

A seal assembly for a cryo-cooler displacer has a first ring 12 and at least one separate second ring 14, an axial surface of the first ring 12 in direct contact with an axial surface of the second ri

Description

The present invention relates to a seal assembly for a displacer of a cryo-cooler. Further, the present invention relates to a method of manufacturing of such a seal assembly. Further, the present invention relates to a displacer for a cryo-cooler comprising such a seal assembly and a cryo-cooler comprising such a displacer. Cryo-cooler are designed to reach cryogenic temperatures below 120K. Common cryocoolers, such as Gifford-McMahon (GM-coolers) comprise a displacer having a displacer body in a housing that is movable within the housing from a first position to a second position in order to provide compression and expansion of a coolant such as helium. A seal assembly must be connected to the displacer body in order to provide a seal between an inner surface of the housing and the displacer body. Due to the low temperatures, seal elements are contracted and still must provide a reliable sealing between the displacer body and the inner surface of the housing. In common cryo-coolers it is known to provide a sealing ring, wherein the sealing ring has a Z-shaped cut from a first axial surface to the opposite axial surface of the sealing ring to compensate thermal contraction of the sealing ring. The Z-cut comprises a first substantially vertical section starting from the first axial surface of the sealing ring, a substantial parallel section providing two parallel contact surfaces and a second axial section towards the second axial surface of the sealing ring. Therein, the first axial section and the second axial section are offset to each other. By the parallel section a sealing effect is provided even if the sealing ring is contracted at low temperatures since the two parallel contact surfaces can slide alone each other but maintain contact and consequently also maintain the seal. However, such a Z-cut is difficult to manufacture and thus, increasing the costs of the sealing ring. In addition, if the opposing contact surfaces of the parallel section are not exactly parallel to each other, the contact surfaces are deflected or buckled and pushed apart which results in a break of the seal and seal failure. Consequently, testing of the parallel surface during manufacturing becomes mandatory which further increases the costs of the seal ring. In addition, a larger percentage of production rejects occur due to failures of the parallel surfaces in the Z-cut. If the failure is only noted when mounted in the cryo-cooler itself, replacement of the seal ring is necessary which leads to interruption of operation and increased service costs. It is an object of the present invention to provide an improved seal assembly for a displacer of a cryo-cooler that is easy to manufacture and provides a reliable seal. The problem is solved by a seal assembly according to claim 1, a method for manufacturing of a seal assembly according to claim 12, a displacer according to claim 13 and a cryo-cooler according to claim 14. The seal assembly according to the present invention for a displacer of a cryo-cooler comprises a first ring element and at least one separate second ring element. Each ring element comprises a first axial surface and an opposite second axial surface, a central opening to receive a displacer body of the displacer, wherein the central opening is preferably defined by a radial inner surface of the respective ring element. Further, each ring element comprises a radial outer surface to provide a seal surface in contact with a housing and in particular, an inner surface of a housing of the displacer. By contact between the radial outer surface in the inner surface of the housing, a seal is created. Therein, the second axial surface of the first ring element is in direct contact with the first axial surface of the second ring element. In other words, the first ring element and the at least one separate second ring element are stacked along an axial direction. Here and in the following the axial direction is defined by either the central opening of the first ring element and the at least one second ring element or is defined by the displacer body and coincides with the moving direction of the displacer body. Contrary to the prior art providing only a single sealing ring, the present invention provides two separate elements, i.e. the first ring element and the at least one separate second ring element acting together. Thereby, the manufacturing of the first ring element and the second ring element is simplified. No need of a complex shaped sealing ring is present anymore. Instead, contact between the first ring element and the second ring element is along the respective axial surfaces, i.e. the second axial surface of the first ring element and the first axial surface of the second ring element, providing a reliable seal. In addition, the external surfaces providing the contact between the first ring element and the second ring element, i.e. the second axial surface of the first ring element and the first axial surface of the second ring element, can be manufactured with high accuracy. If necessary also testing of planarity of these surfaces is simplified compared to the internal contact surfaces of the Z-cut in the prior art. Preferably, the first ring element and the at least one second ring element have the same inner diameter, i.e. defining the same size of the central opening. Or in other words, their radial inner surfaces have the same diameter. Preferably, the outer diameter of the first ring element is equal to the outer diameter of the at least one second ring element. In particular, the radial outer surfaces of the first ring element and the at least one second ring element flushes with each other to provide both a contact with an inner surface of the housing. Hence, the seal effect is combinedly provided by the first ring element and the second ring element. Preferably, the first ring element and the at least one second ring element are identical. Thus, manufacturing of the seal assembly is further simplified, and two identical ring elements can be used as first ring element and at least one second ring element combined together to form the seal assembly. Preferably, the first axial surface is parallel to the second axial surface. This applies to the first ring element, as well as the second ring element. Preferably, the second axial surface of the first ring element is parallel to the first axial surface of the second ring element. Thus, by the parallel surfaces in contact with each other, a sealing effect between the first ring element and the second ring element is provided. Hence, although separating the seal assembly into two separate parts, i.e. the first ring element and the at least one separate second ring element, compared to the prior art, due to the parallel surfaces in contact with each other, a sufficient sealing effect is provided. No gas or coolant can flow in-between the first ring element and the at least one second ring element. Preferably, the first ring element and / or the second ring element comprises a gap. In particular, the gap is closed at room temperature or smaller than 1 mm and preferably smaller than 0.254 mm. Thus, by the gap thermal contraction of the first ring element and the second ring element can be compensated. Under lower temperatures and in particular cryo-tem-peratures, the gap opens and increases. Thus, thermal stress in the respective ring elements can be avoided. Therein, the gap is a full gap connecting the first axial surface and the second axial surface of the first ring element and / or second ring element. In particular, the gap extends substantially perpendicular to the first axial surface and / or second axial surface. Hence, the gap can be easily manufactured by a cut and does not have any complex shape as in the prior art. The need of a complex shaped Z-cut is avoided and also the necessity of an internal parallel contact surface, i.e. the parallel section of the Z-shape, to be perfectly parallel can be prevented. Preferably, the gap of the first ring element and the gap of the second ring element are offset to each other. In particular, the first ring element and the at least one second ring element are arranged relative to each other, such that the gap of the first ring element is offset to the gap of the second ring element. In particular, the offset is larger than 2cm and preferably larger than 6cm along the perimeter of the first ring element and / or second ring element. In other terms, the offset is larger than 10° and preferably larger than 30°. Due to the offset, a sufficient sealing effect is provided and the coolant in the cryo-cooler cannot flow via the first gap and the second gap across the seal assembly. Due to the parallel axial surfaces of the first ring element and the second ring element in contact with each other and the offset between the gaps of the first ring element and the second ring element, no direct connection for any coolant is provided and the sealing effect is maintained. At the same time, the seal assembly is still easy to manufacture and reliable. Preferably, the first ring element and / or the second ring element are made from fiber reinforced plastic and in particular fiber reinforced Teflon® or the like. Preferably, an expander spring is in contact with an inner surface of the first ring element and / or an inner surface of the at least one second ring element. By the expander spring, a radial force is applied to the first ring element and / or the second ring element to provide a sufficient contact pressure between the radial outer surfaces of the first ring element and / or the second ring element with the inner surface of the housing. In particular, the expander spring is made from a metal such as stainless steel. In particular, a common expander spring is implemented combinedly providing a radial force to the first ring element and the second ring element. Hence, only a single expander spring needs to be implemented to the seal assembly reducing the number of parts of the seal assembly and further reducing the costs of manufacture. Preferably, the expander spring comprises an orientation feature engaging with a corresponding orientation feature of the first ring element and / or the second ring element. In particular, the expander spring comprises a first orientation feature at a first axial side engaging a corresponding orientation feature of the first ring element and a second orientation feature at an opposite second axial side of the expander spring coming into engagement with the corresponding orientation feature of the second ring element. Thereby, angular orientation between the first ring element and the second ring element is fixed by the expander spring. Hence, even under constant movement of the seal assembly, the gap of the first ring element cannot align with the gap of the second ring element due to the respective orientation features. Preferably, the seal assembly comprises more than one second ring element arranged in a stacked manner in the axial direction. Therein, the more than one second ring elements may all be identical or at least two of the more than one second ring elements are different. In particular, each of the more than one second ring elements comprise a gap, wherein the gaps of the directly adjacent ring elements are preferably offset to each other. In particular, one second ring element is in direct contact with a previous second ring element, wherein direct contact is provided between a first axial surface of the one ring element and the second axial surface of the previous second ring element. In an aspect of the present invention, a method of manufacturing a seal assembly for a displace of a cryo-cooler is provided including the steps: Providing a first ring element and at least one second ring element; Applying a cut to each of the first ring element and the second ring element; and Arranging the first ring element and the second ring element in direct contact with each other, wherein the cut of the first ring element is offset to the cut of the second ring element. Preferably, the cut is made by a razor blade cut, a laser cut or the like. In particular, the cut has a simple shape being substantially perpendicular to the first axial surface and the second axial surface of the respective ring element. Hence, various methods of creating the cut are feasible and can be implemented in the present invention. In particular, the seal assembly manufactured by the present method is further built along the features as described in connection with the seal assembly before. In an aspect of the present invention, a displacer for a cryo-cooler is provided comprising a housing and a displacer body movably disposed in the housing. Further, the displacer comprises a seal assembly as described before, wherein the radial outer surfaces of the first ring element and the at least one second ring element are in contact with an inner surface of the housing to provide a seal between the displacer body and the housing. In particular, the first ring element and / or the second ring element are in contact with and preferably direct contact with the displacer body. Preferably, the first axial surface of the first ring element is in direct contact with the displacer body to provide a seal between the displacer body and the housing. In an aspect of the present invention, a cryo-cooler is provided comprising a displacer as described before. Hence, by the present invention a seal assembly for a displacer of a cryo-cooler is provided which is reliable, less prone to manufacturing defects compared to a ring seal of the prior art and consequently, cost effective. The present invention is described in more detail with reference to the accompanying figures. The figures show: Fig. 1 a seal assembly according to the present invention Figs. 2a - 2c detailed views of the seal assembly of Fig. 1 and Fig. 3 a detailed view of the cryo-cooler according to the present invention. In the following it is referred to Fig. 1 showing a seal assembly 10 according to the present invention comprising a first ring element 12 and a second ring element 14. The first ring element 12 comprises a gap 16, wherein the second ring element 14 also comprises a gap 18. Therein, the gap 16 of the first ring element 12 is offset from the gap 18 of the second ring element 14. The offset is at least 6cm along the perimeter of the seal assembly or at least 30°. In the following it is referred to the Figs. 2a - 2c showing details of the seal assembly 10 of Fig. 1. In Fig. 2a, the first ring element 12 is shown. The first ring element 12 comprises a first axial surface 20 and an opposite axial surface 26. Therein, the first axial surface 20 and the second axial surface 26 are directed in the axial direction and are parallel to each other. Further, the first ring element 12 comprises a radial inner surface 22 defining a central opening 28 configured to receive the displacer body of the cryo-cooler. Opposite to the radial inner surface 22, the first ring element 12 comprises a radial outer surface 24 which comes into contact with an inner surface 33 of a housing 32 (see Fig. 3) of the cryo-cooler in order to provide a seal. Therein, the first ring element 12 comprises a gap 16 as shown in Fig. 2c. Therein, the gap is closed at room temperature and in particular, smaller than 0.254mm. In particular, at room temperature the gap has a size between 0.025mm und 0.254mm. During manufacturing of the seal assembly, the first ring element 12 is provided and subsequently the gap 16 is inserted for example by a razor blade cutting, laser cutting or the like. Therein, the gap 16 extends from the first axial surface 20 completely through the first ring element 12 to the second axial surface 26. The extension of the gap 16 is substantially perpendicular to the axial surfaces 20, 26 and thus, is easy to manufacture and does not require any complex shape. Preferably, the outer diameter of the first ring element 12 is between 10mm and 300mm and preferably between 20mm and 200mm. Preferably, the axial thickness of the first ring element 12 is between 1mm and 3mm and preferably between 1mm and 2mm. Preferably, the radial width of the first ring element 12 is between 1mm and 4mm and preferably between 1mm and 3mm. Although not shown, the second ring element 14 may be substantially identical to the first ring element 12 as described before. Therein, the second axial surface 26 of the first ring element 12 is in direct contact with the first axial surface of the second ring element 14. The first axial surface of the second ring element 14 is parallel to the second axial surface 26 of the first ring element 12 such that a full surface contact between the first ring element 12 and the at least one second ring element 14 is provided. However, the first ring element 12 and the second ring element 14 may differ for example by the material, the axial thickness or the like. In addition, there might be more than two ring elements stacked in the axial direction. Therein each ring element may be identical or at least two ring elements may differ. In the following it is referred to Fig. 3 showing a detailed view of a cryo-cooler comprising a housing 32 defining an inner surface 33 and a displacer body 34 movably disposed in the housing 32. Therein, the displacer body 34 is moved by a stem 30. The displacer body 34 may be a displacer body of a first stage or a second stage of a cryo-cooler. The first axial surface 20 of the first ring element 12 is in direct contact with an axial surface of the displacer body 34. The radial outer surfaces 24 of the first ring element 12 and the second ring element 14 is in direct contact with the inner surface 33 of the housing 32. Therein, contact pressure is applied by an expander spring 36 which is disposed radial inside the first ring element 12 and the second ring element 14. Opposite to the displacer body 34, an L-seal element 38 is provided which is in contact with the second axial surface of the second ring element 14. Axial pressure is applied to the seal assembly 10 by an axial spring 42 applying pressure via pressure distribution element 40. Alternatively, no axial spring is implemented and the seal assembly is implemented in a shuttle configuration eliminating the need for the axial spring. In particular, the expander spring 36 comprises an orientation feature engaging with a corresponding orientation features 44A, 44B of the first ring element 12 and / or the second ring element 14. In particular, the expander spring 36 comprises a first orientation feature at a first axial side engaging a corresponding orientation feature 44A of the first ring element 12 and a second orientation feature at an opposite second axial side of the expander spring 26 coming into engagement with the corresponding orientation feature 44B of the second ring element 14. Thereby, angular orientation between the first ring element 12 and the second ring element 14 is fixed by the expander spring 36. Hence, even under constant movement of the seal assembly 10, the gap 16 of the first ring element 12 cannot align with the gap 18 of the second ring element 14 due to the respective orientation features 44A, 44B. The position of the orientation features 44A, 44B of the first ring element 12 and / or the second ring element 14 lean be located at the same angular distance to the respective gaps 16, 18 such that the first ring element 12 and the second ring element 14 are substantially identical and can be manufactured in the same way. Hence, the one or more orientation features of the expander spring 36 may be offset to each other and thereby define the offset between the gaps 16, 18 of the first ring element 12 and the second ring element 14. Alternatively, the orientation feature 44A of the first ring element 12 and the orientation feature 44B of the second ring element 14 are at the same position and in different angular distance to the gaps 16, 18 being offset to each other. Upon decrease of the temperature applied to the seal assembly 10, the gaps 16, 18 open due to thermal contraction. However, due to the offset between the gaps 16, 18 of the first ring element 12 and the second ring element 14, no break of seal occurs and a reliable and easy to manufacture seal assembly 10 is provided. signs: assembly first ring element second ring element gap gap first axial surface orientation feature orientation feature radial inner surface radial outer surface second axial surface central opening stem housing inner surface displacer body expander spring L-seal pressure distribution element axial spring

Claims

1. Seal assembly for a displacer of a cryo-cooler, comprising:a first ring element andat least one separate second ring element,wherein each ring element comprise a first axial surface and an opposite second axial surface, a central opening to receive a displacer body of the displacer, and a radial outer surface to provide a seal surface in contact with a housing of the displacer, andwherein the second axial surface of the first ring element is in direct contact with the first axial surface of the second ring element.

2. Seal assembly according to claim 1, wherein the first ring element and the at least one second ring element are identical.

3. Seal assembly according to claim 1 or 2, wherein the first axial surface is parallel to the second axial surface.

4. Seal assembly according to any of claims 1 to 3, wherein the second axial surface of the first ring element is parallel to the first axial surface of the second ring element.

5. Seal assembly according to any of claims 1 to 4, wherein the first ring element and / or the second ring element comprise a gap.

6. Seal assembly according to claim 5, wherein the gap is closed at room temperature or smaller than 0.1 mm.

7. Seal assembly according to claim 5 or 6, wherein the gap of the first ring element and the gap of the second ring element are offset to each other.

8. Seal assembly according to claim 7, wherein the offset is larger than 2cm and preferably larger than 6cm and / or larger than 10° and preferably larger than 30°.

9. Seal assembly according to any of claims 1 to 8, wherein the first ring element and / orthe second ring element are made from fiber reinforced plastic and in particular fiber reinforced Teflon®.

10. Seal assembly according to any of claims 1 to 9, wherein an expander spring is in contact with an inner surface of the first ring element and / or an inner surface of the at least one second ring element.

11. Seal assembly according to any of claims 1 to 10, comprising more than one second ring element arranged in a stacked manner.

12. Method of manufacturing a seal assembly for a displacer of a cryo-cooler including:Providing a first ring element and at least one second ring element;Applying a cut to each of the first ring element and the second ring element; andArranging the first ring element and the second ring element in direct contact with each other, wherein the cut of the first ring element is offset to the cut of the second ring element.

13. Displacer for a cryo-cooler, comprising:a housing,a displacer body movably disposed in the housing, anda seal assembly according to any of claims 1 to 11, wherein the radial outer surfaces of the first ring element and the at least one second ring element are in contact with an inner surface of the housing to provide a seal.

14. Cryo-cooler comprising, a displacer according to claim 13.

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