Abstract:
A gas restrictor particularly usrful for applicatio in gas bearing as used, for example, in free-piston Striling cycle machinery. Porous strip material together with a backing plate and orifice (bleed hole) is used to provide the restriction to the flow of gas into an annular gap between a piston and a cylinder.

Description:
This application claims the benefit of provisional 60/345,472, filed on Oct. 19, 2001. 

   BACKGROUND OF THE INVENTION 
   1. Field Of The Invention 
   This invention relates generally to gas bearings as used, for example, in free-piston Stirling machines, and relates more particularly to a restrictor apparatus used with gas bearings. 
   2. Description of the Related Art 
   In many different machines, pistons reciprocate in a cylinder formed in a housing. Due to accurate machining, a thin annular gap is formed between the cylinder wall and the piston wall. In Stirling cycle machines, for example, the housing encloses a work space bounded by one end of the piston and a back space bounded by the opposite end of the piston. The term “piston” can refer generically to any piston-like body, including the displacer in a Stirling cycle machine. A working gas, such as helium, fills the workspace, back space and other regions of the machine within the housing. 
   Because of the close proximity of the piston and cylinder walls during operation, the annular gap formed between the walls must be lubricated to prevent rapid wear. The most effective lubrication has been found to be a thin layer of the working gas forming a gas bearing. Such gas bearings are described in U.S. Pat. Nos. 4,412,418, 4,802,332 and 4,888,950, all to Beale, which are incorporated by reference. 
   In order to lubricate the moving piston, gas must be directed into the gap at three or more points around the peripheral surface of the piston after being routed from the workspace or back space. Transporting the gas into the annular gap often requires a network of small passages. The passages that route the working gas directly into the annular gap are often extremely small to restrict the flow of gas. Restricting the flow of gas is necessary to maintain a constant gas pressure, but very small passages and other structures that restrict the flow of gas into a clearance gap, commonly referred to as “restrictors”, are especially susceptible to blockage. 
   Conventionally, gas bearing restrictors have been provided by a number of means: capillary tubes, screws and close fitting parts with accurate passages used to direct the gas into the clearance gap. All of these previous techniques and structures suffer from cost or sensitivity to blockage by small particles in the working gas. A desirable restrictor would have low cost, temperature and creep stability and little or no susceptibility to blockage. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention is a porous body, preferably a porous plastic strip covering the upstream side of an orifice leading into the clearance gap. In a preferred embodiment, the plastic strip is supported by a backing ring that biases outwardly to keep the strip in place. The diameter of the orifice, the porosity of the plastic strip and the width and the degree of compression of the backing ring control the degree of restriction imposed to the gas flow. The downstream side of the orifice is directly adjacent to its associated gas bearing cavity in the clearance gap. 
   The space upstream of the gas bearing restrictor is the charge volume. In a preferred embodiment, the charge volume is pressurized to the maximum pressure in the Stirling cycle by the use of a small reed check valve. The charge volume bleeds through the restrictive porous strip and via the orifice to the gas bearing clearance gap and then through the clearance gap. By arranging the restrictors to have a similar restriction to the resistance of the clearance gap, it is possible to obtain close to maximum gas bearing stiffness. 
   When the component with the gas bearing, the piston in one embodiment, moves eccentrically, one or more cavities are moved closer to the cylinder wall. This restricts the flow of gas locally through the clearance gap even more, which tends to increase the local pressure in the clearance gap. The cavity or cavities on the opposite side of the piston become less restricted and therefore bleed down and lose pressure. This causes a net force on the piston tending to move the piston away from the cylinder wall, and maintains a gas film for lubrication. 
   The use of the plastic porous strip as a restrictor has the advantage of being multi-pathed and therefore far less likely to become blocked with loose particles. There are distributed orifices for the gas to flow through, and if one becomes blocked, it will have little effect on the gas bearing. A further advantage is that the gas bearing restrictor becomes its own filter which prevents small particles from entering the close fitting clearance seals typically found on machines that employ gas bearings. 
   The porous material is easily fitted into a piston sleeve by providing a backing ring or spring that squeezes the porous material against the orifices of the gas bearing. Installation could be done by an automatic machine and appears to be advantageous for high volume production. 
   Costs in material and labor appear to be extremely low compared to conventional techniques. Reliability is expected to be far better due to lower likelihood of blockage and prevention of particles coming into the clearance gap. This gas bearing restrictor appears particularly favorable for implementation in Stirling cycle machinery. 
   The proposed invention appears to offer all these advantages and has the additional advantage of providing a filter for particles that might damage or wear the close fitting bearing surfaces. Restriction performance has been found to be comparable to precision restrictors consisting of 60-micron glass capillaries of 5-mm length. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a side view in section illustrating a preferred embodiment of the present invention. 
       FIG. 2  is an end view in section through the line  2 — 2  of FIG.  1 . 
       FIG. 3  is an end view in section similar to that of  FIG. 2 , and illustrating the piston at a position away from radial dead center. 
       FIG. 4  is a view in perspective illustrating a preferred piston. 
       FIG. 5  is a schematic view in section illustrating an alternative porous sheet and its attachment to the piston. 
       FIG. 6  is a schematic side view in section illustrating an alternative embodiment of the present invention. 
       FIG. 7  is an exploded view illustrating a piston assembly with gas bearings and restrictors according to this invention. The porous restrictor material is in the form of a ring and the backing is facilitated by a compression ring that forces the porous ring against the orifices. Two sets of gas bearings are shown (four gas bearings for each set). Also shown in this view is the reed check valve for pressurizing the charge cavity. 
   

   In describing the preferred embodiment of the invention which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific term so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word connected or term similar thereto are often used. They are not limited to direct connection, but include connection through other elements where such connection is recognized as being equivalent by those skilled in the art. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The preferred embodiment of the present invention is shown in  FIG. 1 , in which the free piston Stirling cycle apparatus  10  includes a cylindrical housing  12  having an internal sidewall surface  14 , which is a circular cylinder. The piston  16 , also shown in  FIG. 4 , has an outer cylindrical surface  18  on the sidewall  20  that is disposed in close proximity to the internal cylindrical surface  14  of the housing sidewall  12 . 
   There is an annular gap  22  formed between the piston  16  and the housing  12  in which the working fluid, such as helium gas, flows. The size of the annular gap is, exaggerated in the drawings. The diametrical difference of the outer surface of the piston  16  and the housing inner surface  14  is between about 15 and 35 microns in a contemplated embodiment. Thus, the annular gap is half of that difference when the piston  16  is radial dead center (eccentricity of zero), which is 7.5 to 17.5 microns. The gas flows through the annular gap  22 , thereby providing a fluid bearing as is known conventionally. 
   The gas supplied to the annular gap  22  comes out of the charge cavity  40 , which is the chamber within the piston  16  that is sealed off with the piston cap  42 . Gas is supplied to the charge cavity  40 , in the preferred embodiment, from an alternating pressure source through a reed valve consisting of the reed  44 , the orifice  46  and the holding screw  48 . The purpose of the reed valve is to prevent gas from leaving the charge cavity  40  other than into the gap  22 , and to allow gas into the charge cavity  40  only when the gas pressure in, for example, the compression space  41 , is higher than that in the charge cavity  40 . The ideal maximum pressure in the charge cavity  40  is the peak pressure subjected to the piston cap  42 . This pressure variation is usually generated by the motion of the piston  16  as is known in the Stirling cycle machinery art. A check valve filter (not shown) can be added to protect the ability of the reed valve to seat properly by keeping debris from contaminating the sealing components thereof. 
   Orifices  30 ,  31 ,  32  and  33  are formed in the sidewall  20  near one end of the piston  16  at four equally spaced intervals around the piston  16 . Another set of four similar orifices is formed close to the opposite end of the piston  16  as shown. These orifices convey gas in the charge cavity  40  into the annular gap  22 . The orifices formed on the piston  16  preferably do not restrict the flow of gas therethrough, and are approximately 1.0 millimeter in diameter in the preferred embodiment. Of course, more or fewer than four orifices can be formed near each end of the piston  16 , and the sizes, relative positions, shapes and angles of orientation can be varied according to principles understood by those having ordinary skill in the gas bearing technology. 
   In a preferred embodiment, ports  34 ,  35 ,  36  and  37  are formed on the outer cylindrical surface  18  of the piston  16  at the ends of the orifices  30 - 33 , as is conventional. Similar ports are formed at the ends of the orifices near the opposite end of the piston  16 , so that there are two sets of four gas bearings near each end of the piston  16 . 
   A fluid-permeable, porous body, preferably the gas-permeable, porous plastic strip  50 , is mounted against the inner surface  19  of the piston  16 . A similar strip  60  is similarly mounted near the opposite of the piston  16 . The strip is described as porous, which means that it contains many extremely small passages extending entirely through the strip. These passages function as capillary passages that restrict or meter the flow of fluid therethrough. The strip has a thickness significantly smaller than its width and its length. One material contemplated for use as the strips  50  and  60  is sold under the name POREX T3 Bacteria Sheet #7744 having a pore size in the range of 7 to 150 microns with void volumes of 35-50%. The product has a thickness of 0.025 inches and is made of polyethylene, although it is contemplated that polypropylene could work. 
   Mounting means, preferably backing springs  52  and  62 , bias outwardly against the strips  50  and  60 , respectively, to force the outer surfaces of the strips against the inner surface  19 . The flow of gas is illustrated by the arrows in  FIG. 1  extending along a fluid flow path extending from the charge cavity  40 , through the strips  50  and  60 , the orifices  30 - 33 , the ports  34 - 37  and into the annular gap  22 . 
   Referring to  FIG. 2 , the strip  50  is mounted, with the aid of the backing spring  52 , against the piston sidewall  20 . The strip  50  and compression spring  52  are arranged to seat on the radially inwardly facing surface  19  of the piston  16 , and are positioned upstream of the orifices  30 - 33 . The term “upstream” has its usual meaning and therefore a first object in a gas stream will be contacted by gas molecules before a second object if the first object is upstream of the second. Tabs  54  allow convenient compression of the spring  50  to aid in rapid assembly and disassembly. 
   The present invention operates in the following manner with reference to  FIGS. 1 ,  2 ,  3  and  4 . When the piston  16  begins to veer away from radial dead center, one of its sides approaches the cylinder housing wall  14 . This is illustrated, again in exaggerated relative dimensions, in  FIG. 3  in which the piston&#39;s surface  18  comes closer to the wall  14  near the orifice  30  and the port  34 . The gas bearing passages tend to close off on the side where the piston  16  is closest to the cylinder wall  14 , and the passages tend to open on the side where the piston  16  is farthest from the cylinder wall  14 , which is at the opposite side. The orifice  30  and port  34  are unable to bleed off as much gas due to the restriction caused by the piston and cylinder coming closer together, and therefore tend to increase the local pressure in the annular gap  22  from the gas bleeding in through the restrictive porous strip  50 . The orifice  32  and port  36  on the opposite side are not as closed off so they tend to bleed down and lose pressure. The pressure difference causes a net force that opposes the piston  16  motion towards the cylinder wall  14 , thus avoiding contact between the piston  16  and cylinder wall  14  and tending to push the piston back to radial dead center. 
   For optimum stiffness (defined as righting force per unit radial displacement) of the gas bearings, the designed restriction of the combination of each orifice  30 - 33  and the strip  50  is approximately the same as the leakage restriction caused by the annular clearance gap  22 . The same relationship exists at the strip  60  and its associated orifices. Because the orifices  30 - 33  are essentially free flowing in the preferred embodiment, the restriction of the combination is made up essentially entirely of the restriction to flow of the fluid through the strip  50 . Of course, a different compromise could be established between a more restrictive orifice than in the preferred embodiment, and a less restrictive porous strip than in the preferred embodiment. 
   The dimensions of the ports  34 - 37  are chosen to maintain stability to the radial motion of the piston  16  within its cylinder  12 . This means that when the piston  16  is displaced radially, its righting motion is such that no radial oscillations are induced that would allow the piston  16  to eventually collide with the cylinder  12 . 
   An alternative embodiment is shown in  FIG. 5 , in which a porous body, such as the sheet  80 , is bonded directly to the upstream, inner surface  82  of the piston sidewall  81 , thereby covering the orifices  84 ,  86  and others not shown. This can be accomplished by the use of adhesive, for example. The porous sheet  80  is then backed with a non-porous film or sheet  88 , such as aluminum tape, for example. Compression of the porous sheet is minimal since sealing is provided by the adhesive film. Fluid may only flow through the orifices by first passing through the porous sheet  80 , and fluid can only enter the porous sheet  80  from the edges of the porous sheet  80  that are not sealed by the non-porous sheet  88 . 
   In this embodiment, the restriction to the flow of fluid is a function of the orifices&#39; diameters and the porosity and width of the porous sheet. As in the preferred embodiment, the restriction is designed to have a similar restriction to the resistance to the fluid flow in the annular gap  90  between the piston  81  and the cylinder housing  83  after the gas-bearing orifice. Such a design provides maximum gas bearing stiffness. 
   Another alternative embodiment of the present invention is shown in  FIG. 6 , in which a piston  100  is slidably mounted within the cylindrical housing sidewall  102 . Rather than the orifices being formed in the piston sidewall as described above, in the embodiment of  FIG. 6  the orifices are formed in the cylindrical housing sidewall  102 . Orifices  104 ,  106 ,  108  and  110  extend entirely through the sidewall  102  from the charge cavity  140 . The charge cavity  140  extends around the periphery of the sidewall  102  and is in fluid communication with the annular gap  122  between the piston  100  and sidewall  102 . 
   The charge cavity  140  is charged by gas entering through the passageway  110  past the reed  112 , which is held in place by the screw  114 . Fluid flows from the charge cavity  140  through the porous body, such as the porous strip  120  through the orifices  104 ,  106 ,  108  and  110  and into the annular clearance gap  122 . The backing spring  124  produces a radially inwardly directed bias to hold the strip  120  in place over the orifices  104 - 110 . The fluid, such as helium gas, in the charge cavity  140  therefore must flow through the edges of the strip  120  as shown by the arrows in  FIG. 6  to reach the orifices  104 - 110 . The illustration of  FIG. 6  shows that it is possible to vary the positioning of the orifices, the porous strips, the charge cavity and other structures and yet stay within the bounds of the instant invention. 
   While certain preferred embodiments of the present invention have been disclosed in detail, it is to be understood that various modifications may be adopted without departing from the spirit of the invention or scope of the following claims.