Abstract:
A substantially planar suspension spring for supporting a linear compressor housed within a hermetic shell. A hub portion ( 36 ) connects to the body of the compressor assembly while a spiral arm portion ( 38 ) curves around the hub portion ( 36 ) at least one full turn before attaching to the wall of the compressor housing. Provides lateral stability to the reciprocating compressor assembly while maintaining axial flexibility. 46

Description:
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 compressors, many of which have particular application to linear compressors. The present invention relates to further improvements to compressor embodiments such as are described in that patent application, which provides a general description of an example compressor to which the present invention may be applied. However the present invention may also be applied beyond the scope of the particular embodiments of 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. 
         [0003]    The present invention relates generally to suspension springs for suspending the compressor assembly within the hermetic shell. 
       SUMMARY OF THE INVENTION 
       [0004]    It is an object of the present invention to provide a suspension spring with improved characteristics with particular application to linear compressors and/or to provide refrigeration compressors incorporating such springs, or to at least provide the industry with a useful choice. 
         [0005]    In a first aspect the invention may broadly be said to consist in a suspension spring for use in supporting a linear compressor within a hermetic shell, said spring comprising: 
         [0006]    a body of substantially planar form having a hub portion for connection with the compressor, a spiral arm extending from said hub portion in the plane of said hub portion, said spiral arm curving around said hub portion in a first direction, said arm extending greater than one complete pass around said hub portion and terminating in an attachment portion for fixing to said compressor housing. 
         [0007]    According to a further aspect of the invention, said arm is singular and is the only said connection between said hub portion and said attachment portion. 
         [0008]    According to a further aspect of the invention, said hub portion includes means for connection to said compressor assembly. Said means may for example be one or more apertures by which said compressor may be bolted or otherwise fixed to said hub portion, or could alternatively be a clip pressed from the plane of the sheet material constituting the suspension spring. 
         [0009]    According to a further aspect of the invention, said arm leaves said hub portion in a direction substantially tangential with the perimeter of said hub portion and follows the path of a gradually expanding spiral to said attachment portion. 
         [0010]    According to a further aspect of the invention, said attachment portion comprises a tab extending substantially radially with respect to said spiral. 
         [0011]    According to a further aspect of the invention, said tab includes means for attachment to said housing. The means for attachment may comprise, for example, an aperture, slot or other deformity for assisting fixing to the housing by, for example, rivet, screw, adhesive or weld. 
         [0012]    According to a further aspect of the invention, said spring is cut from a thin flat plate of high carbon steel, pre-hardened and tempered. 
         [0013]    According to a further aspect of the invention, said spiral arm has a width, taken in a substantially radial direction with respect to said spiral, increasing from a minimum where the arm leaves said hub portion to a maximum adjacent said attachment portion. 
         [0014]    According to a further aspect of the invention, said tab includes a bend across it, such that a portion of said tab distal from said hub portion of said spring lies generally in a plane that is at a substantial angle to the plane of the hub portion and spiral arm of said spring. 
         [0015]    According to a further aspect of the invention, said spring arm includes a tuning mass at a location along said arm intermediate between said hub portion and said attachment portion. 
         [0016]    According to a further aspect of the invention, said tuning mass comprises a short length of said arm substantially wider than the adjacent parts of said arm. 
         [0017]    In a further aspect the present invention may broadly be said to consist in a refrigeration system compressor comprising a hermetic housing, a linear compressor within said hermetic housing, said compressor including at least two relatively reciprocating parts, with one part typically being much greater mass than the other part, the relative reciprocation of the centre of mass of each part occurring along an axis of reciprocation, and 
         [0018]    at least a pair of suspension springs substantially as set forth in one or more of the above paragraphs, the hub portion of each said suspension spring being connected with said compressor part of greater mass, such that the centre of said spiral at least substantially coincides with said axis of reciprocation, and the attachment portion of each said spring being fixed to one part of said hermetic housing. 
         [0019]    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  
         [0020]      FIG. 1  is a side elevation in cross-section of a refrigeration compressor including a linear compressor suspended in a housing. The compressor is suspended in the housing at each end by a suspension spring according to a preferred embodiment of the present invention. 
           [0021]      FIG. 2  is a perspective view of a suspension spring according to a first embodiment of the present invention. 
           [0022]      FIG. 3  is a plan elevation of a blank for forming the spring of  FIG. 2 . 
           [0023]      FIG. 4  is a side elevation of the blank of  FIG. 3 . 
           [0024]      FIG. 5  is an elevation of a suspension spring including a tuning mass intermediate along the spring arm. 
       
    
    
     DETAILED DESCRIPTION  
       [0025]    Referring to  FIG. 1 , 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. The 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. 
         [0026]    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 chassis  10 , cylinder head  11 , valve plate  5  and a cylinder liner  12 . It 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 illustrated embodiment the main spring is a combination of coil spring  19  and flat spring  20 . 
         [0027]    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 rod  26  has a flexible portion  28  approximately at the centre of the hollow piston  22 . 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  19 ,  20 . 
         [0028]    This briefly describes a linear compressor of a type for which the suspension spring of the present invention is useful. However it will be appreciated that the usefulness of the suspension spring of the present invention is not restricted to linear compressors of the type and configuration illustrated. It is generally applicable where operation of the linear compressor results in the relative reciprocation of the centre of mass of the piston carrying part and the centre of mass of the cylinder part along the linear axis. 
         [0029]    The suspension spring of the present invention is most usefully applied to support the heavier of the relatively moving assemblies, typically the cylinder part assembly. In the preferred manner, such as illustrated in  FIG. 1 , a suspension spring is provided at each extreme end of the compressor. This is so that a centre of the suspension spring can be aligned with the axis of relative reciprocation of the centres of mass of the two main assemblies. 
         [0030]    Referring to  FIGS. 2 and 4  the suspension spring of the preferred embodiment of the present invention has a hub portion  36  and a spring arm  38  extending from the hub portion  36 . The spring arm  38  terminates in an attachment portion  40 . The hub portion  36 , spring arm  38  and attachment portion  40  are preferably integrally formed. The whole component may, for example, formed from a flat sheet material of suitable elastic property. An example of a suitable material is 0.8 mm thick sheet of T302 spring steel. 
         [0031]    The precise shape or form of hub portion  36  is not critical although a generally circular or volute shape is preferred to provide a suitably large flat area to clamp the hub portion  36  to the compressor end. 
         [0032]    The spring arm  38  spirals around the hub portion  36 , preferably through greater than a complete turn, staying in the same plane as the planar hub portion  36 . As best seen in  FIGS. 3 and 5  the spiral arm  38  is preferably tapered from one end adjacent the attachment portion  40  to the other end adjacent the hub portion  36 . The spiral arm  38  merges tangentially into the hub portion  36  at end  42 . 
         [0033]    The hub portion  36  is for attachment to an end of the compressor. The hub portion  36  may include suitable feature to facilitate attachment. In the illustrated embodiments the hub portion  36  includes one or more apertures  45  which may be used to screw the spring to the compressor. To prevent gradual rotation of the compressor about its axis the two outer holes may be used at one end (as at end  50  in  FIG. 1 ) and the central hole used at the other end (as at end  52  in  FIG. 1 ). A flexible, for example rubber, grommet may be provided as desired. Other forms of connections such as clip or adhesive fixing are also possible. 
         [0034]    The attachment portion  40  is for mounting the spring to the housing. Typically the spring will be mounted to the lower internal surface of the housing. For that application the attachment portion  40  may, as illustrated, include an extended tab bent to a suitable angle such that with the bent tab flush against the housing the main planar portion of the suspension spring extends away from the housing at an angle to be perpendicular to the axis of reciprocation of the compressor. So, for example, the angle at which the tab  46  is bent through at bend  44  will depend on the slope of the part of the surface of the housing to which the tab  44  is to be fixed. The attachment portion  40  (or tab  44 ) may be attached to the housing in any convenient fashion so that the planar portion of the support spring is cantilevered from the housing. For this the tab  44  may include suitable features to facilitate attachment. For example, for attachment to the housing by a fastener, or to provide keying for attachment by an adhesive, the tab  44  may include an aperture  46 . Alternatively the tab may include one or more protrusions or dimples to facilitate spot welding or projection welding to the housing. 
         [0035]    When suspension of the compressor in the housing is by a conventional coil spring there is the disadvantage that when the coil springs are made soft to minimise vibration along the axis of the compressor they allow too much movement at right angles to this axis. This can compromise robustness during transport and handling of the compressors or the appliance in which they are fitted. Conventional coil springs can also be noisy as in use they tend to slide over the snubbers that locate them at each end. 
         [0036]    The spiral flat spring of the present invention, when carefully designed, is very soft in the axis of reciprocation and stiff in directions transverse to this axis. Accordingly it does not compromise between isolation and robustness. 
         [0037]    One possible disadvantage is the many modes of resonance a spring of this type can have. Such a spring, when designed to be very soft in the direction of axial movement, will have low fundamental frequencies, (e.g. a frequency below 50 hz for all six rigid body modes) and will also have a large number of higher mode resonant frequencies where the spring vibrates within itself. Our linear compressor is also based around a resonance spring system. In preferred embodiments that we use the compressor runs at a varying natural frequency due to the variable stiffness of the compressed gas associated with the current running conditions. The compressor resonant system allows the compressor to move almost sinusoidally but there are higher order harmonics due mainly to the non linearity of the compressed gas stiffness. These higher harmonics can excite resonance in the suspension spring. Accordingly, it is important that the spring design is such that internal resonances of the suspension spring do not coincide with the running frequency or low order harmonics of the compressor. 
         [0038]    If this interference cannot be avoided it is possible to add a mass at an appropriate point on the spring so that the resonant frequency of one internal resonance mode (which would otherwise be excited by the compressor operation) is reduced. The mass can be an additional quantity of spring material, or an added mass such as a piece of polymer which is dense and has high internal damping. Additional spring material may be included for example by providing a short wide portion  58  along the spiral arm  38  at a location between the end joining into the hub portion  36  and the end joining to the attachment portion  40 . The mass is located at a point expected to exhibit maximum amplitude in the problematic resonant mode.