FIELD OF THE INVENTION 
       [0001]    The present invention relates to linear compressors, and in particular linear compressors of the type suitable for use in a vapour compression refrigeration system. 
       BACKGROUND TO THE INVENTION 
       [0002]    Linear compressors of a type for use in a vapour compression refrigeration system are the subject of many documents in the prior art. One such document is our co-pending PCT patent application PCT/NZ2004/000108. That specification describes a variety of developments relating to such compressors, many of which have particular application to the linear compressors. The present invention relates to further improvements to compressor embodiments such as are described in that patent application which provides a general exemplification of a compressor to which the present invention may be applied. However the present may also be applied beyond the scope of the particular embodiments of a linear compressor disclosed in that application. Persons skilled in the art will appreciate the general application of the ideas herein to other embodiments of linear compressors such as are found in the prior art. 
       SUMMARY OF THE INVENTION 
       [0003]    It is an object of the present invention to provide improvements relating to linear compressors or to at least provide the industry with a useful choice. 
         [0004]    In a first aspect the invention may broadly be said to consist in a linear compressor comprising: 
         [0005]    a cylinder part including a cylinder bore, 
         [0006]    a piston disposed in said bore and slidable therein, 
         [0007]    a main spring connecting directly or indirectly said cylinder part to said piston, 
         [0008]    a connecting member connecting between said main spring and said piston, 
         [0009]    a stator having an air gap, said connecting member passing through said air gap, and 
         [0010]    one or more substantially flat blocks of permanent magnet material secured to said connecting member with the large faces of said blocks facing the stator, said permanent magnet material magnetised to define at least one armature pole; and wherein said main spring comprises a combination of a plurality of individual spring elements acting in parallel. 
         [0011]    In a further aspect the invention may broadly be said to consist in a linear compressor comprising: 
         [0012]    a cylinder part including a cylinder bore, 
         [0013]    a piston disposed in said bore and slidable therein, 
         [0014]    a main spring connecting directly or indirectly said cylinder part to said piston, 
         [0015]    a connecting member connecting between said main spring and said piston, 
         [0016]    a stator having an air gap, said connecting member passing through said air gap, and 
         [0017]    at least one armature pole located along said connecting member, 
         [0018]    wherein said connecting member is supported only by said piston at one end and by said main spring away from said one end, and said main spring comprises a combination of individual spring elements acting in parallel. 
         [0019]    In a still further aspect the invention may broadly be said to consist in a linear compressor comprising: 
         [0020]    a cylinder part including a cylinder bore, 
         [0021]    a piston disposed in said bore and slidable therein, 
         [0022]    a main spring connecting directly or indirectly said cylinder part to said piston, and 
         [0023]    a connecting member connecting between said main spring and said piston, with a compliant element in said connecting member transmitting side and axial loads but allowing rotation about axes transverse to the axis of reciprocation of said piston in said bore; 
         [0024]    wherein said main spring comprises a combination of a plurality of individual spring elements acting in parallel. 
         [0025]    In relation to the invention as set forth in any of the above paragraphs said main spring may for example comprise a combination of coil springs, a combination of coil springs and planar springs or a combination of planar springs. Coil springs may be formed from suitable high fatigue wire or springs machined from thin walled cylinder stock. Preferably the combination includes at least one planar spring element contributing higher lateral stiffness. Most preferably the combination includes at least one planar spring and at least one coil spring. 
         [0026]    In a still further aspect the invention may broadly be said to consist in a linear compressor comprising: 
         [0027]    a cylinder part including a cylinder bore, 
         [0028]    a piston disposed in said bore and slidable therein, 
         [0029]    a main spring connecting directly or indirectly said cylinder part to said piston, 
         [0030]    a connecting member connecting between said main spring and said piston and connecting said main spring to said piston, 
         [0031]    a stator having an air gap, said connecting member passing through said air gap, 
         [0032]    one or more substantially flat blocks of permanent magnet material secured to said connecting member with the large faces of said blocks facing the stator, said permanent magnet material magnetised to define at least one armature pole, and 
         [0033]    a lateral support acting between said cylinder part and said connecting member, at a location intermediate said permanent magnet material and said piston, said lateral support allowing axial movement of said connecting rod, but transferring side loads to said cylinder part. 
         [0034]    In relation to the invention as set forth in the above paragraph said main spring may comprise a single spring element or a combination of a plurality of spring elements acting in parallel. Preferably the main spring also provides lateral support acting between said cylinder part and said connecting member, at a location such that said armature pole or poles are between said main spring location and said lateral support located so that the armature of said motor is supported at one end by said main spring and at the other end by said lateral support. 
         [0035]    According to a further aspect of the invention, said lateral support comprises one or more planar springs, for example cut from sheet material or formed from spring wire bent into a spring line within a plane. Alternatively said radial support may comprise one or more sliding bearings acting on the connecting member. 
         [0036]    According to a further aspect of the invention, in the region of the connecting member between the lateral support and the piston the connecting member is laterally flexible or includes one (or preferably two) flexible portion, so as to effectively transmit axial forces but to have lateral and angular compliance of the piston relative to the axis and line of reciprocation of the connecting member. 
         [0037]    The cylinder part may include provision for aerostatic gas bearings receiving compressed gases and supplying these through a plurality of spaced bearing ports spaced along and around the cylinder bore to support the piston in operation. However the armature radially (or laterally) supported at both ends and compliancy in the connecting member between the lateral support and the piston the inventors expect that the benefits of the gas bearings and reduced friction may be exceeded by the consumption of compressed gas in the gas bearings. 
         [0038]    To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]      FIG. 1  is a plan elevation in cross-section of a linear compressor according to a first embodiment. The first embodiment has a main spring comprising a combination of a flat spring and a coil spring. The flat motor armature is radially supported at one end by the main spring and at the other end by the piston.  FIG. 1  is a cross-section taken through line DD of  FIG. 2 . 
           [0040]      FIG. 2  is a side elevation in cross-section of the embodiment of  FIG. 1 , taken through line CC of  FIG. 1 . 
           [0041]      FIG. 3  is a plan elevation in cross-section of a linear compressor according to a second embodiment. The second embodiment has a main spring comprising a stack of flat springs. The flat motor armature is radially supported at one end by the main spring and at the other end by another flat spring. There is a compliant connection to the piston.  FIG. 3  is a cross-section taken through line EE of  FIG. 4 . 
           [0042]      FIG. 4  is a side elevation in cross-section of the embodiment of  FIG. 2 , taken through line BB of  FIG. 3 . 
           [0043]      FIG. 5  is a plan elevation in cross-section of a linear compressor according to a third embodiment. The third embodiment has a main spring comprising a combination of a flat spring and a coil spring. The flat motor armature is radially supported at one end by the main spring and at the other end in a sliding bearing. There is a compliant connection to the piston. 
           [0044]      FIG. 5  is a cross-section taken through line FF of  FIG. 6 . 
           [0045]      FIG. 6  is a side elevation in cross-section of the embodiment of  FIG. 5 , taken through line AA of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0046]    Referring to  FIGS. 1 to 6  the compressor for a vapour compression refrigeration system includes a linear compressor  1  supported inside a housing  2 . Typically the housing  2  is hermetically sealed and includes a gases inlet port  3  and a compressed gases outlet port  4 . Uncompressed gases flow within the interior of the housing surrounding the compressor  1 . These uncompressed gases are drawn into the compressor during intake stroke, compressed between the piston crown  14  and valve plate  5  on the compression stroke and expelled through discharge valve  6  into a compressed gases manifold  7 . Compressed gases exit the manifold  7  to the outlet port  4  in the shell through a flexible tube  8 . To reduce the stiffness effect of discharge tube  8 , the tube is preferably arranged as a loop or spiral transverse to the reciprocating axis of the compressor. Intake to the compression space may be through the piston (with an aperture and valve in the crown) or through the head, divided to include suction and discharge manifolds and valves. The illustrated compressors have suction through the head, with suction manifold  13  and suction valve  29 . 
         [0047]    The illustrated linear compressor  1  has, broadly speaking, a cylinder part and a piston part connected by a main spring. The cylinder part includes cylinder housing  10 , cylinder head  11 , valve plate  5  and a cylinder  12 . The cylinder part also includes stator parts  15  for a linear electric motor. An end portion  18  of the cylinder part, distal from the head  11 , mounts the main spring relative to the cylinder part. In the embodiment illustrated in  FIGS. 1 and 2  and the embodiment illustrated in  FIGS. 5 and 6 , the main spring is formed as a combination of coil spring  19  and flat spring  20 . In the embodiment illustrated in  FIGS. 3 and 4  the main spring comprises a stack of a plurality of planar springs  16 . 
         [0048]    The piston part includes a hollow piston  22  with sidewall  24  and crown  14 . A rod  26  connects between the crown  14  and a supporting body  30  for linear motor armature  17 . The linear motor armature  17  comprises a body of permanent magnet material (such as ferrite or neodymium) magnetised to provide one or more poles directed transverse to the axis of reciprocation of the piston within the cylinder liner. An end portion  32  of armature support  30 , distal from the piston  22 , is connected with the main spring. 
         [0049]    In the embodiment of  FIGS. 1 and 2  the rod  26  has a flexible portion  28 , located at approximately the centre of the hollow piston  22 . In the embodiment of  FIGS. 3 and 4  and the embodiment of  FIGS. 5 and 6  the rod  21  is narrow over its whole length. 
         [0050]    The linear compressor  1  is mounted within the shell  2  on a plurality of suspension springs to isolate it from the shell. In use the large outer body of the linear compressor, the cylinder part, will oscillate along the axis of reciprocation of the piston part within the cylinder part. In the preferred compressor the piston part is purposely kept very light compared to the cylinder part so that the oscillation of the cylinder part is small compared with the relative reciprocation between the piston part and cylinder part. In the illustrated form the linear compressor is mounted on a set of four suspension springs  31  generally positioned around the periphery. Alternate suspension spring arrangements are illustrated in PCT/NZ2004/000108. The ends of each suspension spring fit over elastomeric snubbers connected with the linear compressor  1  at one end of each spring and connected with the compressor shell  2  at the other end of each spring. 
         [0051]    Referring to the compressor embodiment of  FIGS. 1 and 2 , this illustrates a variation of a compressor of a type disclosed in our earlier patent application, PCT/NZ2000/000201. In that application we disclosed a compressor including a linear motor with a substantially flat permanent magnet armature operating in an air gap of a stator carried by the cylinder part. The flat armature was positioned part way along a connecting member extending from the piston, to one side of the stator, to the main spring, on the other side of the stator. The connecting member, and therefore the side forces exerted by the linear electric motor, were laterally supported at one end by the piston within the cylinder and at the other end by the lateral stiffness of the main spring. 
         [0052]    In that earlier PCT application we disclosed a main spring of substantially singular construction involving a double helical loop of heavy gauge high fatigue strength steel wire. This main spring provides sufficient lateral stiffness and appropriate axial stiffness in a single essentially unitary element, but is not a stock item and is complex to manufacture. 
         [0053]    In one aspect the present invention is a variation of main spring involving a plurality of separate spring elements working in combination. For example in the embodiment of  FIGS. 1 and 2  and the embodiment of  FIGS. 5 and 6  the main spring comprises a combination of a coil spring  19  and a planar spring  20 . The planar spring  20  provides the lateral stiffness, while the coil spring  19  may add any desired additional axial stiffness. The planar spring  20  may be of any conventional form, for example cut from a spring steel sheet, or may be of a form such as illustrated in our earlier patent application, PCT/NZ2000/000202. 
         [0054]    Another embodiment is disclosed with reference to  FIGS. 3 and 4  in which the main spring comprises the combined stack of four planar springs  16  all operating together. In this case each of the planar springs offers both lateral stiffness and axial stiffness. Planar springs are generally very stiff laterally compared with their axial stiffness and an embodiment as illustrated in  FIGS. 3 and 4  will probably exhibit unnecessarily high lateral stiffness to obtain a suitable axial stiffness, although it would be appreciated that the desired axial stiffness will depend on the desired running speed for the compressor. 
         [0055]    The embodiments of  FIGS. 3 and 4  and  FIGS. 5 and 6  illustrate a further aspect of the present invention. In the compressor embodiment of  FIGS. 1 and 2  and in the aforementioned patent application PCT/NZ2000/000201, the piston rod, carrying the armature  17 , is supported against lateral loading by the main spring at one end and through the piston at the other end. This is desirable for its compactness and simplicity however it does result in increased side loading of the piston within the cylinder bore. This extra side loading can be managed and examples of how to manage it are given in our patent applications, including in relation to the embodiment of  FIGS. 1 and 2  herein. 
         [0056]    However the embodiments of  FIGS. 3 and 4  and  5  and  6  herein provide an alternative approach to dealing with the lateral forces resulting from the flat permanent magnet linear motor, where the motor is located on the member connecting between the main spring and the piston. 
         [0057]    According to this aspect of the invention a radial or lateral support is provided to act between the cylinder part  1  and the connecting member at a location between the armature magnets and the piston. The support transmits the side loads from the connecting member directly to the cylinder part  10 . 
         [0058]    In the embodiment of  FIGS. 3 and 4  the radial support comprises a planar spring  40  connected at its outer edge  41  to said cylinder part  10  and at its hub  43  to an end  45  of the armature supporting body  30 . The planar spring  40  offers substantial lateral stiffness and the armature supporting body  30  is substantially rigid. Accordingly the lateral loads from the flat permanent magnetic linear electric motor, which can be substantial, are supported at one end by flat spring  40  and at the other by the main spring, which includes further planar springs  16 . The planar spring  40  may be mounted within an annular ring portion  42  of cylinder part  10 . 
         [0059]    In an alternative embodiment illustrated in  FIGS. 5 and 6  the lateral support is provided by an axial sliding bearing. The end portion  50  of armature support member  30  is formed to provide a substantially cylinder shaft of constant diameter. This shaft portion passes through a sliding bearing  52  forming part of the cylinder part  10 . The sliding bearing  52  may for example comprise a bush of a suitable low friction hardwearing material. The bush may for example be a spherical bush of PTFE plastic material (or similar) retained within a suitable internally spherical housing. This arrangement will also allow for certain misalignment of the armature support member  30  relative to the cylinder part  10 . 
         [0060]    It is preferred in either case to retain reasonable gas flow in the vicinity of the armature. Accordingly an open frame construction, such as illustrated in  FIGS. 4 and 5 , is used to support the lateral support (e.g. planar spring or sliding bearing) relative to the cylinder part  10 . Alternatively a plurality of windows or apertures, such as openings  56  in  FIGS. 5 and 6  may be provided which communicate both with the region of the cylinder part housing the linear electric motor and with the region of the cylinder part housing the cylinder and piston. This gases flow capability into the inside of the cylinder part  10  is also useful to reduce any gas pressure effects on the back face of the piston  22  and to provide gas flow paths to the back face of piston  22  in embodiments where suction gases flow is provided through the crown of the piston rather than through the compressor head. 
         [0061]    In the embodiments of  FIGS. 3 to 6  where the armature supporting member  30  is fully supported against lateral loading, a preferred connection between the armature supporting member  30  and the piston  22  has considerable lateral compliancy while retaining axial stiffness. A suitable linkage would include a narrow metal rod embedded at one end in the end of the armature supporting member  30  and at the other end in the piston crown  14 . The thin rod  21  should have sufficient compliancy to allow the orientation of piston  22  to adapt to any misalignment between the armature support member  30  and the cylinder  12 , and sufficient axial stiffness that it will not buckle as the linear motor and springs drive the piston toward the cylinder head during the compression stroke of the compressor in operation. 
         [0062]    While a compressor according to these embodiments, where the flat permanent magnetic armature is fully supported, may still provide for aerostatic gas bearings to operate between the cylinder  12  and piston  22  it is expected that the side loads from the piston  22  to the cylinder  12  will be very low. With modern hardware and coatings the arrangement may operate effectively and with sufficient longevity without either oil lubrication or aerostatic bearings.
A linear compressor ( 1) includes a cylinder ( 21) with a piston ( 22) connected through a main spring ( 19) and a planar spring ( 20). One or more flat blocks of permanent magnet material ( 17) with large faces of the blocks facing a stator ( 15) and defining armature poles are secured to a connecting rod ( 30). A lateral support ( 52) acts between the cylind ( 21) and the connecting rod ( 30) at a location midway between the permanent magnet material and the piston ( 22) allowing axial movement of the connecting rod ( 30) but transferring side loads to the cylinder ( 21).