Gas bearing for concentric piston arrangement

Abstract

The invention relates to a self-sustaining gas bearing for a concentric piston arrangement. The required gas pressure is generated by the compression pressure of the pistons and temporarily stored by check valves in an annular high-pressure reservoir within a hollow cylindrical piston. The high-pressure reservoir is formed by offset axial bores that are introduced into the hollow cylindrical piston from both sides and connect at its center. The gas flow is restricted by radial and helical grooves on the outer surface of restrictor inserts. These inserts are pressed into the axial bores and, via the radial and helical grooves, establish a connection between the high-pressure reservoir and the bearing gaps. The invention enables the use of a thin-walled, hollow cylindrical piston that can be manufactured monolithically without joining elements.The gas bearing can be designed in such a way that, in addition to guiding the hollow cylindrical piston itself, it guides an additional, concentrically arranged piston in the central bore of the hollow cylindrical piston.

Description

Application area

[0001] This invention relates generally to self-supplying gas bearings, such as those used in free-piston Stirling engines or linear compressors, and specifically to self-supplying gas bearings for a free-piston Stirling cryocooler in an alpha arrangement with concentrically arranged pistons for cooling the rotor coils of high-power-density superconducting electric motors. State of the art

[0002] Superconducting electric motors are considered a key technology for the electrification of heavy transportation vehicles such as ships, trains, trucks, and aircraft. A major challenge lies in cooling the superconducting rotor coils. Unlike the stator coils, these cannot be cooled with liquid hydrogen because no rotary coupling is currently available that functions reliably at high speeds and cryogenic temperatures. Previous rotor cooling systems used stationary thermosiphons with a cryogenic refrigerant, sealed by ferrofluid gaskets. To enable higher rotational speeds, rotationally symmetrical Stirling or pulse-tube cryocoolers have been mounted directly on the rotor to cool the rotor coils by heat conduction during rotation. However, the suitability of pulse-tube cryocoolers for aerospace applications is questionable, as their efficiency depends on their orientation during operation.In contrast, Stirling cryocoolers offer higher efficiency than pulse tube cryocoolers due to their orientation independence, especially at low temperatures. However, to date, only commercially available Stirling cryocoolers have been used for cooling superconducting rotor coils, and these were never designed to rotate at high speeds on a shaft. Therefore, they cannot meet the requirements regarding weight, cooling capacity, and reliability. To meet the stringent reliability requirements, particularly in aerospace applications, it is advantageous to guide the pistons of the Stirling engine through a non-contact gas bearing. The design of existing gas bearings for Stirling engines has been disclosed in US 7,600,464 B2, US 6,293,184 B1, and US 6,694,730 B2. US 6,293,184 B1 describes a gas-bearing working piston of a free-piston Stirling engine in a beta configuration.The working piston consists of an outer sleeve into which a hollow cylindrical core is press-fitted. According to US 7,600,464 B2, the core may also be multi-part. The core includes large radial grooves that serve as a high-pressure reservoir, small radial grooves that act as a throttle, and axial grooves connecting the radial grooves. A check valve in the working piston pressurizes the high-pressure reservoir with pressure from the working chamber during each piston stroke. The outer diameter of the working piston slides within a cylinder, and the inner diameter guides a displacer piston rod. Radial bores in the outer sleeve and the core, via the small radial grooves, connect the high-pressure reservoir to the bearing journals.Due to the throttling effect of the radial grooves, the pressure in the bearing gaps immediately behind the restrictors becomes a function of the gap height, thus exerting a self-centering force on the working piston. A disadvantage of this type of gas bearing is that the radial bores must be at least as long as the outer sleeve or core is thick, because the throttling occurs at the interface between these two parts. The dead volume in the radial bores increases the gas bearing's tendency towards self-excited vibrations. Furthermore, the working piston in this type of gas bearing is relatively massive and is therefore preferably made of aluminum to reduce weight. However, this necessitates that the displacer piston be manufactured from multiple materials.The piston rod of the displacer piston, guided by the gas bearing, would also need to be made of aluminum to prevent jamming due to differential thermal expansion, while its dome would preferably be made of stainless steel to reduce parasitic axial heat conduction. This is particularly problematic when the displacer piston consists of a single cylinder that serves as both the piston rod and the dome. Furthermore, both the outer sleeve and the core of the working piston must be manufactured with high precision to meet the tight tolerances required for the gas bearing. This results in high manufacturing costs. Another variant of a gas bearing for a linear compressor is described in US 7,247,007 B2, in which throttling grooves are located in the outer surface of a cylinder liner that is pressed into a guide cylinder.The throttled working gas then enters the bearing gap through radial bores in the cylinder liner. However, since a linear compressor only requires a single piston to be guided, this design cannot be readily transferred to a Stirling engine with two concentrically arranged pistons. Object of the invention

[0003] Therefore, there is a need for an improved gas bearing for free-piston Stirling engines that is suitable for guiding concentrically arranged pistons, does not tend to self-excited vibrations, and whose outer piston has a cost-effectively manufactured lightweight structure made of stainless steel. This problem is solved by a gas bearing with the features of claim 1. Advantageous embodiments and further developments are the subject of the dependent claims. Example description

[0004] The invention is described and explained below with reference to the illustrations. These illustrations depict: Fig. Figure 1 is a sectional view of a rotating free-piston Stirling cryocooler, the piston of which is guided by a gas bearing according to the present invention. Fig. 2 is an enlarged section from Fig. 1 and shows the function of the gas bearing using a throttle insert mounted in the outer piston. Fig. Figure 3 is a perspective front view (from above) of the outer piston. Fig. Figure 4 is a sectional view of the outer piston in Fig. 3 along line 4 - 4. Fig. Figure 5 is a sectional view of the outer piston in Fig. 4 along line 5 - 5. Fig. Figure 6 is a sectional view of the outer piston in Fig. 4 along line 6 - 6. Fig. Figure 7 is a sectional view of the outer piston in Fig. 4 along line 7 - 7. Fig. Figure 8 is a perspective front view (from below) of the throttle insert. Fig. 9 is a sectional view of the throttle insert in Fig. 8 along line 9 - 9. Fig. Figure 10 is a perspective front view (from below) of an alternative embodiment of the throttle insert. Fig. 11 is a sectional view of the throttle insert in Fig. 10 through line 11 - 11. Fig. Figure 12 is a perspective front view (from below) of a distance insert. Fig. 13 is a sectional view of the distance insert in Fig. 12 along line 13 - 13. Fig. Figure 14 is an exploded view of the outer piston assembly.

[0005] Fig. Figure 1 shows a sectional view of a rotating free-piston Stirling cryocooler in Alpha configuration 200, which utilizes a gas bearing according to the invention. The Alpha configuration is characterized in that a piston is arranged in both the compression chamber 242 and the expansion chamber 244, and both act simultaneously as working and displacer pistons. This means that both pistons contribute to the periodic compression and expansion of the working gas. In the compression chamber 242, an outer piston 302 of an outer piston assembly 300 is arranged, which has the form of a hollow cylinder and whose outer surface 304 (see Figure 1) Fig. 3 - 7) is guided in a housing of the outer piston 202. In the expansion chamber 244, an inner piston 402 of an inner piston assembly 400 is arranged, which is guided in the inner surface 306 of the outer piston 302. The compression chamber 242 is connected to the expansion chamber 244 by an ambient heat exchanger 222, a regenerator 226, and a cold heat exchanger 218. According to the known Stirling principle, the heat exchangers 218 and 222 ensure heat absorption and heat release to the environment, respectively. The regenerator 226 increases the efficiency of the process by increasing the temperature difference between the compression chamber 242 and the expansion chamber 244 through the intermediate storage of heat. Furthermore, the inner piston 402 moves concentrically to a cylinder liner 206, which spatially separates the compression chamber 242 from the expansion chamber 244 and directs the gas flow between these chambers through the heat exchangers 222 and 218 and the regenerator 226.A leak at the sealing gap between cylinder liner 206 and inner piston 402 does not cause a pressure loss in the Stirling process, but only a short-circuit flow between the hot and cold sides, or between compression chamber 242 and expansion chamber 244. Since a sufficiently small sealing gap also acts as a regenerator, the sealing gap between cylinder liner 206 and inner piston 402 can be chosen to be larger than the sealing gap between outer piston 302 and inner piston 402, which reduces manufacturing costs and simultaneously prevents the inner piston 402 from seizing. The two pistons 302 and 402 are each connected to separate linear electric machines 500A and 500B, preferably using moving magnet motors or generators, which are already known in the prior art.The linear electric machines 500A and 500B can each generate a magnetic flux in the outer stator 510 and the inner stator 502, both made of a ferromagnetic material, by means of a coil 504 energized with an alternating current. In the air gap between the outer stator 510 and the inner stator 502, a ring magnet 523 is attached to each piston 302 and 402, respectively. This ring magnet interacts with the magnetic flux via the Lorentz force, thus generating the oscillating piston movement. To prevent the movement of the inner piston 402 from dividing the buffer chamber 246 into two separate gas chambers with different spring stiffnesses, axial through-bores 412 are provided in the underside of the inner piston 402.Furthermore, a local constriction 405, which can be designed, for example, as a throttle valve and is located on the underside of the inner piston 402, prevents unintended pressure equalization between the cavity 408 and the buffer chamber 246 from occurring quickly enough during the oscillating piston movement. The outer piston 302 and inner piston 402 are guided without contact by means of a self-sustaining gas bearing, which utilizes the compression pressure of the pistons and will be described in detail later. However, the inner piston assembly 400 can only be guided in the central region of the inner piston 402, while a large portion of the moving mass is concentrated in the rear region on the side facing the buffer chamber 246. Therefore, the inner piston 402 has a counterweight 406 on the side facing the expansion chamber 244.The counterweight 406 serves to shift the center of mass of the inner piston assembly to the center of the area guided by the gas bearing when the two pistons 302 and 402 are in the neutral position. This prevents the inner piston 402 from tilting due to leverage forces, which could negatively affect the load-bearing capacity of the gas bearing. The rotating part of the Stirling engine 200 is supported within the stationary part by means of rolling bearings 232A and 232B. Furthermore, an airflow between the rotating and stationary parts of the Stirling engine 200 is generated by a flow tube 208 with radial inlet openings 248 and a fan 228. This allows the waste heat from a heat sink 224, which is thermally connected to the ambient heat exchanger 222 via heat pipes 223, as well as the waste heat from the coil 504 in the linear electrical machines 500A and 500B, to be dissipated convectively.In the area of ​​the rolling bearings 232A and 232B, the airflow is guided through axial ventilation openings 252A and 252B in the housing of the outer piston 202. The heat sink 224 is attached to the rotating part of the Stirling engine 200 by threaded bolts 239 and nuts 237. A seal 236 seals the working gas, preferably by means of a metal C-ring or an O-ring, against the environment.

[0006] The following section will first describe the structure of the outer piston 302, which is located in Fig. Figures 3-7 are shown in different views. The outer piston 302 is preferably made of the same material as the housing of the outer piston 202, such as stainless steel, so that the bearing gap of the gas bearing 332 (see Figure 3) is as follows: Fig. 2) is not affected by different coefficients of thermal expansion. Furthermore, the outer piston 302 has an annular high-pressure reservoir 326, which is formed by axial blind bores 314 and 316. The axial blind bores 314 are inserted from the side facing the compression chamber 242, and the axial blind bores 316 from the side facing the buffer chamber 246. The axial blind bores 314 and 316 are both located on a pitch circle with preferably the same diameter, and their number is preferably identical. They are also arranged rotationally offset from each other, which allows them to connect inside the outer piston 302 to form the annular high-pressure reservoir 326. First, a spacer insert 340 is inserted into each of the axial blind bores 314 and 316, which is Fig. 12 and Fig. Figure 13 shows that the spacer inserts 340 are preferably made of a low-density plastic, such as polyethylene, and primarily serve to generate a defined gas volume in the high-pressure reservoir 326, independent of the axial blind bores 314 and 316. To keep the weight of the outer piston 302 low, it is advantageous to make the axial blind bores 314 and 316 as large as possible, which would also allow the annular high-pressure reservoir 326 to be larger than required for the gas bearing. Beyond a certain volume of the annular high-pressure reservoir 326, the load-bearing capacity of the gas bearing is no longer improved. However, if the volume of the annular high-pressure reservoir 326 becomes too large, it takes longer for the supporting gas film to build up after the machine starts, which can lead to increased wear.To adjust the volume of the annular high-pressure reservoir 326 to a defined value, the size of the central bore 342 and the size and number of the radial grooves 344A and 344B can be varied.

[0007] The central bore 342 and the radial grooves 344A and 344B ensure that a continuous flow channel is maintained within the annular high-pressure reservoir 326. The outer diameter of the spacer inserts 340 is formed by the outer cylindrical surfaces 346A, 346B, and 346C and is preferably designed such that, together with the diameter of the axial blind bores 314 and 316, a clearance fit is achieved. Furthermore, the spacer inserts 340 serve as depth stops for the subsequently inserted throttle inserts 350A and 350B. The spacer inserts 340 in the axial blind bores 314 collide in the center of the annular high-pressure reservoir 326 with the spacer inserts 340 in the axial blind bores 316 and thus limit the depth to which the throttle inserts 350A and 350B can be inserted from both sides into the axial blind bores 314 and 316.At the same time, the spacer inserts 340 facilitate the disassembly of the throttle inserts 350A and 350B. By striking a mounted throttle insert 350A or 350B with a punch, the two opposing throttle inserts are driven out (since each spacer insert 340 rests against two spacer inserts 340 on the opposite side due to the rotational offset of the axial blind holes 314 and 316). The throttle inserts 350A and 350B, which are in . Fig. The spacer inserts 340 are press-fitted into the axial blind bores 314 and 316, thus sealing the annular high-pressure reservoir 326 to the compression chamber 242 and the buffer chamber 246. The throttle inserts 350A and 350B are preferably made of aluminum. This prevents a significant increase in the weight of the outer piston assembly 300 and ensures that the forming forces during press-fitting have a greater impact on the aluminum of the throttle inserts 350A and 350B than on the stainless steel of the outer piston 302, which must be manufactured with high precision for gas bearing. The throttle inserts 350A have a check valve 352 with a flow direction indicated by arrow 354 (see Figure 8-11). Fig. 2) in the direction of the annular high-pressure reservoir 326. The throttle inserts 350B do not have a check valve, but are otherwise identical in construction to the throttle inserts 350A. The throttle inserts 350A can be installed either only on the side of the outer piston 302 facing the compression chamber 242, or only on the side facing the buffer chamber 246, or on both sides. In a preferred embodiment, the throttle inserts 350A are installed on both sides of the outer piston 302 in every second axial blind bore 314 and 316. The throttle inserts 350B are installed in the remaining axial blind bores 314 and 316. The check valves 352 are preferably designed as ball check valves with an outer diameter of 5.5 mm.Many small check valves are preferable to a few large check valves to prevent inertial forces from negatively affecting the firing behavior due to the oscillating piston movement. The function of the check valves 352 is to ensure an internal pressure supply for the gas storage by means of the compression pressure from the combined movement of the outer piston 302 and the inner piston 402.

[0008] The operating principle of gas storage is most easily explained in Fig. 2. This process is to be understood and will be described below. As soon as the pressure in the compression chamber 242 or in the buffer chamber 246 is greater than in the annular high-pressure reservoir 326 due to the oscillating piston movement, the check valves open and working gas flows under high pressure into the annular high-pressure reservoir 326 in the flow direction indicated by arrow 354. From there, the working gas passes through radial through-bores 356A and 356B into radial throttle grooves 362A and 362B, which are formed in the outer surfaces 358A and 358B of the throttle inserts 350A and 350B. From the radial throttle grooves 362A and 362B, the working gas then passes into helical throttle grooves 366A and 366B. From the helical throttle slots 366A and 366B, the working gas enters further radial throttle slots 364A and 364B.The radial throttling grooves 364A and 364B are located, in the assembled state, at the same level as radial through-holes 312 on the side of the outer piston 302 facing the compression chamber 242, or at the same level as radial through-holes 308 on the side of the outer piston 302 facing the buffer chamber 246. The division into two radial and one helical throttling groove is primarily for manufacturing purposes; however, other paths establishing a throttling gas connection between the radial through-holes 356A and 356B and the radial through-holes 308 or 312 would function equally well. The throttled working gas passes through the radial through-bores 308 and 312 into the bearing gap 332 between the outer piston 302 and the housing of the outer piston 202, and into the bearing gap 328 between the outer piston 302 and the inner piston 402.The length and cross-section of the throttle grooves 362A, 362B, 364A, 364B, 366A, and 366B are preferably designed such that the pressure drop during flow through the throttles is the same as during the outflow of the working gas from the bearing gaps 328 and 332. Because working gas continuously flows out of the annular high-pressure reservoir 326, the maximum pressure there never reaches the same level as in the compression chamber 242. Nevertheless, a load-bearing gas film can be maintained continuously, since the working gas flows alternately through the bearing gaps 328 and 332 into the chamber where the pressure is currently lower, as indicated by arrows 334 and 336.For example, if the combined movement of the outer piston 302 and the inner piston 402 causes compression in the compression chamber 242, then the opposite buffer chamber 246 is automatically expanded, causing the working gas of the gas storage to preferentially flow into the buffer chamber 246 and thereby create a supporting gas film. Conversely, if the combined movement of the outer piston 302 and the inner piston 402 causes expansion in the compression chamber 242, then the opposite buffer chamber 246 is automatically compressed, causing the working gas of the gas storage to preferentially flow into the compression chamber 242 and thereby create a supporting gas film.

[0009] Fig. Figure 14 shows an exploded view of the outer piston assembly 300 to illustrate the assembly of the previously described throttle inserts 350A and 350B, the spacer inserts 340, and the ring magnet 523. Reference symbol list 200 Stirling cryocoolers 202 Housing of the outer piston 204 Inner piston housing 206 Cylinder liner 208 Flow tube 212 stationary mounting flange 214 rotating mounting flange 216 Throw flange 218 cold heat exchanger 222 Ambient heat exchangers 223 Heatpipe 224 Heat sink 226 Regenerator 228 Fan 232A Rolling bearing 232B Rolling bearing 236 Seal 237 Mother 239 threaded bolts 242 Compression chamber 244 Expansion area 246 Buffer space 248 radial ventilation opening 252A axial ventilation opening 252B axial ventilation opening 300 outer piston assembly 302 outer piston 304 outer surface 306 inner surface area 307 inner surface area 308 radial through hole 312 radial through hole 314 axial blind hole drilling 316 axial blind hole drilling 326 High-pressure reservoir 328 Bearing gap 332 Bearing gap 334 Flow direction 336 Flow direction 340 Distance deployment 342 central bore 344A radial groove 344B radial groove 346A outer surface 346B outer surface 346C outer surface 350A throttle insert with check valve 352 Check valve 354 Flow direction of the check valve 356A radial through hole 358A outer surface 362A radial throttle groove 364A radial throttle groove 366A helical throttle groove 350B Throttle insert without check valve 356B radial through hole 358B outer surface 362B radial throttle groove 364B radial throttle groove 366B helical throttle groove 400 inner piston assembly 402 inner piston 405 local bottleneck 406 Counterweight 408 Cavity 412 axial ventilation opening 500A Linear electric machine of the outer piston 500B Linear electric machine of the inner piston 502 inner stator 504 coil 510 outer stator 523 Ring magnet QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Zitierte Patentliteratur

[0000] US 7 600 464 B2

[0002] US 6 293 184 B1

[0002] US 6 694 730 B2

[0002] US 7 247 007 B2

[0002]

Claims

[1] Gas bearing for a concentric piston arrangement having an outer piston (302) which is slidably mounted in a cylinder with its outer piston surface (304) by means of a gas bearing, wherein the cylinder is formed by a housing of the outer piston (202), wherein the outer piston (302) with its inner piston surface (306) slidably guides an inner piston (402) by means of a gas bearing, wherein the gas bearing is based on the throttling of the working gas by means of small grooves in the outer surface of a cylinder, characterized by, that the outer piston (302) is formed by a monolithic hollow cylinder without a joining connection and has a high-pressure reservoir (326) which is formed by axial blind bores (314 and 316) on rotationally offset bolt circles in the two axial outer surfaces of the outer piston (302), wherein the outer piston (302) has throttle inserts (350A and 350B) with which the axial blind bores (314 and 316) of the outer piston (302) are gas-tightly closed from both sides, wherein the throttle inserts (350A and 350B) have throttle grooves (362A, 362B, 364A, 364B, 366A and 366B) on their outer cylindrical surfaces (358A and 358B) which are connected by means of radial through bores (356A, 356B, 308 and 312) establish a connection between the high-pressure reservoir (326) and the bearing gaps of the gas storage (328 and 332). [2] Gas bearing for concentric piston arrangement according to claim 1, characterized by, that at least one of the throttle inserts (350A and 350B) has a check valve (352) that allows flow only in the direction of the high-pressure reservoir (326). [3] Gas bearing for concentric piston arrangement according to claim 1, characterized by , that the outer piston (302) has throttle inserts (350A and 350B) which are made of a softer material than the outer piston (302). [4] Gas bearing for concentric piston arrangement according to claim 1, characterized by , that the outer piston (302) has spacer inserts (340) which are inserted slidably from both sides into the axial blind bores (314 and 316) of the outer piston (302) and whose axial outer surfaces meet in the middle of the high-pressure reservoir (326). [5] Gas bearing for concentric piston arrangement according to claim 1, characterized by, that the outer piston (302) has spacer inserts (340) which serve as depth limiters when inserting the throttle inserts (350A and 350B) and whose length is designed such that the throttle inserts (350A and 350B) are exactly flush with the axial outer surfaces of the outer piston (302) in their lowest position. [6] Gas bearing for concentric piston arrangement according to claim 1, characterized by , that the outer piston (302) has spacer inserts (340) which, by means of central bores (342) and radial grooves (344A and 344B), maintain a continuous flow channel within the high-pressure reservoir (326) and simultaneously generate a defined gas volume in the high-pressure reservoir (326) that is independent of the axial blind bores (314 and 316).

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