Abstract:
A reversible reciprocating piston compressor includes a crankcase defining at least one cylinder therein and a crankshaft rotatably supported by the crankcase. The crankshaft includes a drive portion and a crankpin eccentrically positioned relative to an axis of rotation of the crankshaft. A piston is reciprocable within the cylinder and a connecting rod assembly is provided between the crankpin and the piston to reciprocally drive the piston in response to forward or reverse rotation of the crankshaft. A cam assembly is operably connected to the crankpin and is engageable with the drive member to effectuate a first stroke length in a first direction of rotation of the crankshaft and a second stroke in a second direction of rotation of the crankshaft. The cam assembly includes a cam, a driven portion and a counterweight. The cam is interposed between the connecting rod assembly and the crankpin and the driven portion is attached to the cam and is in a contacting relationship with the drive portion through at least one contact interface. The contact interface is oriented at a non-zero angle to a radial reference originating from a centerline axis of the crankpin. The counterweight is attached to the cam and has a center of mass located radially adjacent to or through the contact interface. The drive portion is engageable and disengageable with the driven portion through sliding movement of the drive portion relative to the driven portion along the contact interface.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention pertains to reversible reciprocating piston machines, and particularly to reversible hermetic reciprocating piston compressors. More specifically, the present invention relates to compressors including an eccentric cam operably engaged with a crankpin and connecting rod to provide a first piston stroke in a first direction of crankshaft rotation and a second piston stroke in a second direction of crankshaft rotation.  
           [0002]    Reciprocating piston compressors, such as the compressor disclosed in U.S. Pat. No. 5,281,110, which is assigned to the present assignee, the disclosure of which is incorporated herein by reference, are generally of fixed displacement and powered by an electric motor which rotates in a single direction. Also known in the art are reversible hermetic reciprocating piston compressors in which a piston has a first stroke length when driven by a crankshaft rotating in a first, forward direction, and a second stroke length when driven by the crankshaft rotating in a second, reverse direction. Two separate stroke lengths are achieved through use of an eccentric cam which rotates relative to the crankshaft between stops thereon corresponding to first and second angular cam positions which, in turn, correspond to the first and second stroke lengths. These reversible compressors provide the advantage of having one displacement when the crankshaft is rotated in the forward direction, and another displacement when the crankshaft is rotated in the reverse direction. Typical variable stroke, reversible drive compressors, however, do not provide means for positively maintaining engagement between the cam stop and the crankshaft corresponding to the greater stroke length during rotation of the crankshaft without a latching mechanism which holds the cam and crankshaft in engagement during rotation in one of these two directions. If the cam and crankshaft are not continually maintained in engagement during crankshaft rotation, the reexpansion of gas in the cylinder after the piston reaches top-dead-center (TDC) may force the piston away from its TDC position at such a speed that the cam may rotate relative to the crankshaft, separating the cam and crankshaft stops. The separation of these stops result in their subsequently slamming together as the rotating crankshaft catches up to the cam, causing considerable component stresses, adversely affecting durability, and producing undesirable noise.  
           [0003]    To prevent this undesirable loss of contact between the crank and the cam a reversible reciprocating compressor was adapted with a centrifugally activated latching mechanism which coupled the crank with the cam when the crank was rotating in the forward direction. The disclosure of a reversible reciprocating compressor employing a latching mechanism is provided in U.S. Pat. Nos. 5,951,261 to Paczuski and 6,190,137 to Robbins et al., both of which are assigned to the assignee of the present application, the disclosures of which are expressly incorporated herein by reference. Although effective in maintaining contact between the cam and crankshaft, implementing the latching mechanism requires multiple parts and additional machining at a significant additional cost.  
           [0004]    U.S. Pat. No. 6,132,177 to Loprete et al. discloses a reversible reciprocating compressor having a flyweight incorporated into the cam assembly exerting a centrifugal force which is transmitted to the crankshaft from the cam assembly to prevent separation of the cam and crankshaft. The flyweight is located opposite the engagement between the cam assembly and the crankshaft. As the rotational speed of the crankshaft increases, the flyweight imparts a force influencing the cam assembly and crankshaft into engagement. However, since the centrifugal force is effective after the crankshaft has gained significant rotation, the flyweight has significantly less effect at low crankshaft speeds, i.e., at start-up. As a result, undesirable noise and damage due to impact may occur during insignificant crankshaft speeds.  
           [0005]    What is needed is a reversible compressor assembly which is simple in construction and is adapted to avoid undesirable impact between the cam and crankshaft at any crankshaft speed and in either direction. Further, a reversible compressor which significantly reduces wear or other damage of the contacting surfaces defined by the crankshaft and the cam assembly, is desirable.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention overcomes the disadvantages of prior reversible compressor assemblies by providing a reversible, variable stroke compressor assembly including a crankshaft having a drive portion coacting with a driven portion of a cam assembly through lubricated sliding engagement and disengagement between the contacting surfaces to reduce impact, noise and damage.  
           [0007]    The present invention provides a reversible reciprocating piston compressor including a crankcase defining at least one cylinder therein, and a crankshaft which rotates in opposite, forward and reverse directions and is rotatably supported by the crankcase. The crankshaft includes a drive portion and a crankpin eccentrically positioned relative to an axis of rotation of the crankshaft. A piston is reciprocable within the cylinder and a connecting rod assembly is provided between the crankpin and the piston to reciprocally drive the piston in response to forward or reverse rotation of the crankshaft. A cam assembly is operably connected to the crankpin and is engageable with the drive member to effectuate a first stroke length in a first direction of rotation of the crankshaft, and a second stroke in a second direction of rotation of the crankshaft. The cam assembly includes a cam, a driven portion and a counterweight. The cam is interposed between the connecting rod assembly and the crankpin. The driven portion is attached to the cam and is in a contacting relationship with the crankshaft drive portion through at least one contact interface. The contact interface is oriented at a non-zero angle to a radial reference originating from a centerline axis of the crankpin. The counterweight is attached to the cam and has a center of mass located radially adjacent to or through the contact interface. The drive portion is engageable and disengageable with the driven portion through sliding movement of the drive portion relative to the driven portion along the contact interface.  
           [0008]    The present invention further provides a reversible reciprocating piston compressor including a counterweight attached to the cam and being structured and arranged to provide an inertial force directed through a center of mass of the cam. The center of mass of the cam is located radially adjacent or through the contact interface. The driven portion and the drive portion resist separation under the influence of the inertial force.  
           [0009]    The present invention further provides a reversible reciprocating piston compressor having a counterweight attached to the cam being structured and arranged to provide a centrifugal force on the cam assembly when the crankshaft has attained a running speed. The centrifugal force urges a reduction in a force of contact exerted by the drive portion on the driven portion to thereby retain a film of lubricating oil between the drive and driven portions.  
           [0010]    The present invention further provides a reversible reciprocating piston compressor including a crankcase defining at least one cylinder therein, and a crankshaft which rotates in opposite, forward and reverse directions and is rotatably supported by the crankcase. The crankshaft includes a drive portion and a crankpin eccentrically positioned relative to an axis of rotation of the crankshaft. A piston is reciprocable within the cylinder and a connecting rod assembly is provided between the crankpin and the piston to reciprocally drive the piston in response to forward or reverse rotation of the crankshaft. A cam assembly is operably connected to the crankpin and is engageable with the drive member to effectuate a first stroke length in a first direction of rotation of the crankshaft, and a second stroke in a second direction of rotation of the crankshaft. The cam assembly includes a cam and a driven portion. The cam is interposed between the connecting rod assembly and the crankpin. The driven portion is attached to the cam and is in a contacting relationship with the crankshaft drive portion through at least one contact interface. The contact interface is oriented at a non-zero angle to a radial reference originating from a centerline axis of the crankpin. The compressor includes structure for slidingly engaging and disengaging the drive portion with the driven portion through sliding movement of the drive portion relative to the driven portion along the contact interface.  
           [0011]    The present invention further provides a method for compressing gas with a reciprocating piston compression device, including receiving a gas to be compressed into a cylinder of the compression device; rotating a crankshaft drive member in a first rotational direction; engaging a first surface of a camshaft driven member with a first surface of the crankshaft drive member through sliding movement between the drive and driven members; driving the cam in the first rotational direction; moving a piston operably connected to the cam assembly a first stroke distance; compressing the gas within the cylinder of the compression device; rotating the crankshaft drive member in a second rotational direction such that a second surface of a crankshaft drive member is rotated in a second rotational direction opposite the first rotational direction; engaging a second surface of the camshaft driven member with the second surface of the crankshaft drive member through sliding movement between the drive and driven members; driving the cam in the second rotational direction; and moving the piston operably connected to the cam assembly a second stroke distance. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    The above mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0013]    [0013]FIG. 1 is a sectional side view showing a first embodiment of a compressor according to the present invention;  
         [0014]    [0014]FIG. 2 is a fragmentary side view of the crankshaft of the compressor of FIG. 1;  
         [0015]    [0015]FIG. 3 is an end view of the crankshaft viewed along line  3 - 3  of FIG. 2;  
         [0016]    [0016]FIG. 4 is a fragmentary side view of the crankshaft with a cam assembled thereto;  
         [0017]    [0017]FIG. 5 is a perspective view of the cam assembly of the compressor of FIG. 1;  
         [0018]    [0018]FIG. 6A is an end view of the cam assembly of FIG. 1, illustrating the drive flange of the crankshaft in ghosted lines and the crankshaft in section;  
         [0019]    [0019]FIG. 6B is an enlarged view of the area encircled in FIG. 6A, depicting the components of force exerted by the drive flange on the cam assembly;  
         [0020]    [0020]FIG. 7 is an end view of the cam assembly of FIG. 5 in the direction of arrow  7 , showing the cam counterweight;  
         [0021]    [0021]FIG. 8 is a perspective view of the crankshaft and cam assembly of FIG. 1, illustrating the crankshaft in section, engaged with its corresponding piston and connecting rod assembly;  
         [0022]    [0022]FIG. 9 is an end view of the crankshaft and cam assembly of FIG. 8, showing the cam assembly driven in a forward direction of rotation of the crankshaft;  
         [0023]    [0023]FIG. 10 is an end view of the crankshaft and eccentric cam assembly of FIG. 8, showing the cam assembly driven in a reverse direction of rotation of the crankshaft;  
         [0024]    [0024]FIG. 11 is an end view of a second embodiment of a cam assembly of a compressor according to the present invention, showing the crankshaft drive flange in ghosted lines and the crankshaft in section;  
         [0025]    [0025]FIG. 12 is a perspective view of the crankshaft and cam assembly of FIG. 11, showing the crankshaft in section, shown engaged with its piston and connecting rod assembly;  
         [0026]    [0026]FIG. 13 is an end view of the crankshaft and eccentric cam assembly of FIG. 12, showing the cam assembly driven in a forward direction of rotation of the crankshaft; and  
         [0027]    [0027]FIG. 14 is an end view of the crankshaft and eccentric cam assembly of FIG. 12, showing the cam assembly driven in a reverse direction of rotation of the crankshaft.  
     
    
       [0028]    Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of the present invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the present invention. The exemplifications set out herein illustrate embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    Referring to FIG. 1 compressor assembly  20 , which may be utilized in a refrigeration or air conditioning system (not shown), includes hermetically sealed housing  22  having top portion  24  and bottom portion  26  welded or brazed together. Mounting bracket  28  is attached to bottom housing portion  26  to position compressor  20  in an upright or vertical position. Although compressor assembly  20  is shown having a vertical orientation, the scope of the present invention encompasses compressors having a horizontal orientation as well.  
         [0030]    Reversible electric motor assembly  30  is located within housing  22  and includes cylindrical rotor  32  extending through the center of annular stator  34 . Crankshaft  36  is attached to rotor  32  by means of an interference fit, for example. Stator  34  is supported in housing  22  by means of its attachment to crankcase  38 , as is customary. Stator  34  includes windings  40  comprised of two individual portions separately and selectively energized for forward and reverse rotation of rotor  32  activated by a switch (not shown) mounted external to the compressor. A terminal cluster (not shown) is provided in housing  22  for connecting the windings to a switched source of electrical power.  
         [0031]    Crankcase  38  has central bearing portion  42  which radially supports upper journal portion  44  of crankshaft  36 . Shock mounts  46 , attached to crankcase  38  and lower housing portion  26 , support electric motor assembly  30  and compressor mechanism  48  within housing  22 . Outboard bearing  50 , attached to crankcase  38  by bolts  52 , radially supports crankshaft lower journal portion  54 . Additionally, bolts  52  attach thrust bearing plate  56  to outboard bearing  50 , and thrust bearing plate  56  axially supports end surface  58  of crankshaft  36 .  
         [0032]    Lower housing portion  26  forms sump  60 , containing liquid lubricant, such as oil, therein, to lubricate compressor mechanism  48 . Pistons  62  and  64  respectively reciprocate within cylinders  66  and  68  of equal diameter formed in crankcase  38 . Refrigerant gas is drawn into cylinders  66  and  68  at suction pressure and is expelled therefrom in a compressed state at discharge pressure through respective, valved suction and discharge ports in valve plate  70 . In a well known manner, refrigerant gas is drawn through the suction ports of plate  70  and into the cylinders through the suction valves from suction chamber  72  of head  74 . Head  74  is attached to crankcase  38  by means of bolts (not shown) which extend through valve plate  70 . Suction chamber  72  is fluidly connected to the interior chamber  76  of compressor assembly  20 , which receives low pressure refrigerant gas from the system. Compressed refrigerant gas is forced from the cylinders through the discharge ports of plate  70  and into discharge chamber  78  of head  74 . The discharge gas then exits through a tube (not shown) which extends through the housing wall and provides compressed refrigerant to the system.  
         [0033]    Referring to FIG. 2, crankshaft  36  includes outboard crankpin  80  and inboard crankpin  82 . Outboard and inboard crankpins  80 ,  82  each include respective centerline axes  84 ,  86 . Crankshaft  36  includes axis of rotation  88  which is offset relative to centerline axis  84  of crankpin  80  by distance “a” and offset relative to centerline axis  86  of crankpin  82  by distance “e”. Centerline axes  84 ,  86  and axis  88  lie in a plane, with axis  84  located 180° about centerline axis  88  from centerline axis  86 . Centerline axes  84 ,  88  are offset by distance e, the eccentricity of inboard crankpin  82 , which corresponds to one half the stroke distance of piston  64  in cylinder  68 . Pistons  62 ,  64  are reciprocatively driven by respective crankpins  80 ,  82  through connecting rod assemblies  90 ,  92  (FIG. 1). Connecting rod assemblies  90 ,  92 , comprising connecting rods  94 ,  96  and rod straps  98 ,  100 , are pivotally attached to pistons  62 ,  64  through wrist pins  102 ,  104  (FIG. 1).  
         [0034]    Crankshaft  36  includes drive flange  106  situated adjacent to outboard crankpin  80  and has first and second drive surfaces  108  and  110 , respectively (FIG. 3). Drive flange  106  extends substantially perpendicularly to axis  88  and coacts with annular cam  112  provided between connecting rod assembly  90  and crankpin  80  to rotate cam  112  either in the forward or reverse direction (FIG. 8). Referring to FIG. 5, located on lateral surface  114  of cam  112  is raised driven portion  116  which includes first and second driven surfaces  118 ,  120  alternatively driven by respective first and second driving surfaces  108 ,  110  (FIG. 3) of drive flange  106  as hereinafter described. As best seen in FIG. 8, outer periphery  122  of cam  112  is rotatably engaged with annular bearing surface  124  of connecting rod assembly  90 . Crankpin  80  of crankshaft  36  extends through eccentrically positioned hole  126  in cam  112  and periphery  122  of cam  112  orbits about crankpin  80  to provide varying piston strokes corresponding to forward rotation (arrow  128 ) and reverse rotation (arrow  130 ) of crankshaft  36  (FIG. 4).  
         [0035]    Referring to FIGS.  5 - 7 , there is shown a first embodiment of a cam assembly according to the present invention. Cam assembly  112  includes first and second members  132 ,  134  which join along parting line  136  to form cam assembly  112 . First and second members  132 ,  134  may be heat treated and nitrided sintered, powder metal, for example, and are assembled about outboard crankpin  80  as shown in FIGS. 6A and 8. First member  132  and second member  134  of cam  112  are a matched pair and are joined by screws  138 ,  140  and  142  (FIG. 7). Second member  134  includes through holes  144 ,  146  and  148  which include counterbores  150 ,  152  and  154  to recess heads  156 ,  158 ,  160  of respective screws  138 ,  140 ,  142  such that the screw heads do not outwardly project from outer margins of cam  112 . Notably, screw heads  156  and  158  are completely recessed below bearing surface  162  of cam  1112  such that cam  112  may rotate freely within inner surface  124  of connecting rod assembly  90 . First member  132  of cam  112  includes corresponding threaded holes  164 ,  166  and  168  which align with through holes  146 ,  148 ,  150 . As an alternative to bolting first and second members, it is contemplated that first and second members  132 ,  134  may be retained in place without using fasteners. Specifically, the cam may be radially retained by inner cylindrical surface  124  (FIG. 8) of connecting rod assembly  90  and axially retained by adjacent, abutting axial surfaces  170 ,  172  of crankshaft  36  (FIG. 2). As a further alternative, it is envisioned that the cam may comprise a single piece having the same overall shape and features as interfitted portions  132 ,  134 , illustrated in FIG. 5. The single piece eccentric cam may be assembled with the crankshaft by either moving it axially along a single piece crankshaft and onto its corresponding crankpin, or by providing a segmented crankshaft which is accordingly assembled subsequent to placement of the cam upon the crankpin.  
         [0036]    Referring to FIG. 7, first and second members  132 ,  134  of cam  112  define cylindrical outer surface  162  having central axis  174  which is parallel to and offset relative to central axis  176  of interior cylindrical surface  178  of eccentric hole  126  in cam  112 . When cam  112  is assembled to crankshaft  36 , axis  176  is substantially coincident with central axis  84  of outboard crankpin  80  (FIG. 2). As best seen in FIG. 6B, annular clearance  180 , located between outer surface  182  of crankpin  80  and inner surface  178  of cam  112 , is provided to allow crankpin  80  to freely rotate relative to cam  112 . Axes  174  and  176  are offset by distance “b” which, in the exemplary embodiment of compressor assembly  20 , is equivalent to distance “a”, illustrated in FIG. 2.  
         [0037]    Driven portion  116  of cam  112  is positioned along a first edge portion  184  (FIG. 6A) of offset hole  126 . Driven surfaces  118 ,  120  are alternatively engaged by drive surface  108  (FIG. 9) of drive flange  106 , in the forward direction  128 , and drive surface  110  (FIG. 10), in the reverse direction  130 . Referring to FIG. 6B, when rotated in forward direction  128 , drive and driven surfaces  108 ,  118  form contact interface  186  in a plane parallel with axis  84 . Contact interface  186  continuously changes orientation relative to axis of rotation  88  of crankshaft, however, those having ordinary skill will understand that contact interface  186  forms a fixed angle θ relative to a radially extended reference  188  originating from centerline axis  84  of crankpin  80  and extending through centerpoint  191  of contact interface  186 . Since cam  112  and drive flange  106 , concomitantly rotate about centerline  84  of crankpin  80 , angle θ is fixed as long as drive and driven portions  108 ,  118  are engaged. Similarly, drive and driven surfaces  110 ,  120  (in the reverse direction) form planar interface  190  positioned at fixed angle α relative to radially extended reference  193  originating from centerline axis  84  of crankpin  80  (FIG. 10). Referring to FIGS. 5 and 7, lateral inboard face  192  of cam  112  includes counterweight  194  attached thereto or integrally formed therewith, and which extends in an axial direction opposite that of which raised driven portion  116  extends from cam outboard face  114 . Counterweight  194  projects radially from edge portion  196  (FIG. 7) of through hole  126  of cam  112  and prevents impact between drive flange  106  and driven portion  116  as described hereinafter.  
         [0038]    Referring to FIG. 6A, it may be seen that counterweight  194  is located radially adjacent raised driven portion  116 , and consequently, radially adjacent contact interfaces  186 ,  190  (FIG. 10). By locating center of mass  198  of cam assembly  112  proximate to contact interfaces  186 ,  190  (FIG. 10), an inertial force provided by the counterweight opposes the separation of drive flange  106  and driven portion  116  during low torque operation, and reexpansion. Low torque operation of compressor  20  generally occurs during the suction stroke of the piston, and as a result, an insignificant amount of force is transmitted between driven portion  116  of cam  112  and flange  106  of crankshaft  36 . Prior art reversible reciprocating compressor assemblies, not employing a latching mechanism, are susceptible to separation of the crankshaft and cam corresponding to low torque operation of the compressor, resulting in undesirable impact and noise. In sharp contrast, the inventive compressor assembly  20  includes drive and driven surfaces  108 ,  118  which gradually and slidably coact to prevent separation and the ensuing slamming impact between the cam and the crankshaft.  
         [0039]    During engagement of drive and driven surfaces  108 ,  118 , central axis  176  of cam  112  tends to shift off center, or become misaligned, relative to centerline axis  84  of crankpin  80 . Consequently, annular clearance  180  deforms from its uniformly annular shape and drive and driven surfaces  108 ,  118  begin to slide relative to one another. This sliding engagement results in a damped or shock absorbing phenomena during engagement. Similarly, when drive and driven surfaces  108 ,  118  disengage, sliding movement occurs prior to separation as clearance  180  is being restored. Thus, a significant degree of dampening is also associated with drive and driven surfaces  108 ,  118  as they disengage.  
         [0040]    Referring to FIG. 6B, it may be seen that angle θ of interface  186  relative to radial reference line  188  enhances the aforesaid sliding engagement between drive and driven surfaces  108 ,  118  by providing a component of force F 1  in the direction of sliding motion along contact interface  186 . Drive surface  108  of drive flange  106  contacts driven surface  118  of cam  112  exerting a tangentially directed force          relative to centerline  84  of crankpin  80 . The maximum torque transferred from crankshaft  36  to cam  112  is tangentially directed relative to centerline  84  of crankpin  80 , hence, force          is tangentially directed or perpendicular relative to radial reference line  188  having a first end located at centerline  84  and a second end extended through centerpoint  191  of interface  186 . Cam  112  is urged to move by drive flange  106  when the inertial force, provided by counterweight  194 , and inherent frictional forces are overcome by force          exerted by drive flange  106  of the crankshaft  36 . Due to the position of angle θ at contact interface  186 , a component F 1  of force          is directed along interface  186  as illustrated. In contrast, an angle θ of 0° would direct a negligible force F 1  along interface  186 , resulting in insignificant sliding engagement between cam  112  and crankshaft  36 , hence, angle θ is preferably a non-zero value. The force F 1  urges movement of cam  112  along the direction of interface  186  and as a result sliding engagement between drive and driven surfaces  108 ,  118  ensues. An angle θ between 5° and 60° produces a sufficient force F 1 , directed along interface  186 , to promote sliding engagement between drive and driven surfaces  108 ,  118  to prevent direct sudden abutment of these surfaces.  
         [0041]    The sudden and significant impact of the cam and crank as they engage presented by prior art compressors is avoided by compressor assembly  20  since energy is dissipated during engagement, over a period of time, through sliding engagement between drive and driven surfaces  108 ,  118 . Referring to FIG. 10, in the reverse direction of crankshaft rotation  130 , drive and driven surfaces  110 ,  120  comprise interface  190  at angle α formed relative to radially extended reference  193  originating from centerline axis  84  of crankpin  80  and extending through centerpoint  195  of contact interface  190 . Similar to angle θ of interface  186 , angle α, which may be between 5° and 60°, produces a force directed along interface  190  which facilitates sliding engagement rather than direct, abutting impact.  
         [0042]    Referring to FIG. 2, inboard crankpin  82  includes a pair of radially positioned oil passages (only passage  200  shown) extending from opposite locations on surface  202  into crankpin  82  and communicate with longitudinally extending oil passage  204  (FIG. 8) within crankshaft  36 . In a well known manner, oil from sump  60  (FIG. 1) is pumped through longitudinal passage  204  and provided to the sliding interface between surface  202  and the surrounding interior bearing surface (not shown) of connecting rod assembly  92 . In a similar manner, outboard crankpin  80  includes a pair of radial positioned oil passages (only passage  206  shown) extending from opposite locations on surface  182  and into crankpin  80 . The radial passages are fluidly connected with the above-mentioned longitudinal oil passage  204  in the crankshaft  36 .  
         [0043]    Referring to FIG. 5, cam  112  includes oil passages  208  and  210  which respectively extend through first and second members  132 ,  134  of cam  112  to allow oil to communicate between bearing surface  124  and crankpin surface  182  through cam  112 . In each of the forward and reverse rotational directions, passages  208  and  210  are both respectively aligned with the respective oil passages provided radially through crankpin  80 , thereby providing a supply of oil to the interface between surface  162  of cam  112  and the interfacing surface  124  of surrounding connecting rod assembly  90 . A portion of the oil which flows from radial passages in crankpin  80  is also supplied to the interface between lateral face  114  of cam  112  and lateral surface  170  of camshaft  36  (FIG. 1).  
         [0044]    Referring to FIG. 6A, an oil film is captured between drive surface  108  of drive flange  106  and driven surface  118  of raised member  116  as the drive member engages the driven member. Consequently, as the oil film is squeezed from interface  186  a dampening effect is produced and as a result wear on the engaging surfaces is significantly reduced. The squeezing of oil from interface  186  coincides with gradual energy dissipation, as a shock absorbing effect, as engagement and disengagement ensues.  
         [0045]    Referring to FIG. 6B, the force          exerted by drive flange  106  of crankshaft  36  on cam  112  includes a component of force F 2  directed normal or perpendicular to interface  186 . The thickness of the oil film between drive surface  108  and driven surface  118  depends on the magnitude of force F 2 . For example, a large force F 2  tends to squeeze a significant amount of oil from interface  186 . The force F 2  may be varied by varying the angle θ. For instance, if θ was selected to be substantially zero, coinciding with a value of          substantially equal to F 2 , a significant amount of oil would be squeezed from interface  186  corresponding to a high degree of dampening. However, since a significant amount of oil is expelled from between drive surface  108  and driven surface  118 , only an insignificant amount of oil would remain therebetween for lubrication. Hence, a non-zero angle θ between 5° and 60° is preferred.  
         [0046]    A centrifugal force F CF  develops as cam  112  begins to rotate and is outwardly and radially directed relative to the centerline  84  of crankpin  80 . The centrifugal force F CF  acts to radially displace the cam  112 , albeit slightly, relative to the crankshaft. As a result, a sliding action between drive surface  108  and driven surface  118  develops, having a dampening or shock absorbing effect located at interface  186 . Moreover, sliding caused by centrifugal force F CF  prevents separation and corresponding impact during low torque operation or reexpansion, for example, of the compressor since cam  112  is urged into contact with drive surface  108  of crankshaft  36  by centrifugal force F CF . Furthermore, counterweight  194  is positioned about the cam to increase the oil film thickness between the drive surface  108  and driven surface  118  to accordingly facilitate lubricated sliding at interface  186 . The centrifugal force FCF acting on cam  112  reduces the component of force F 2  perpendicular to interface  186  and consequently less oil is squeezed from interface  186 .  
         [0047]    Again referring to FIG. 6B, it may be seen that dampening between cam  112  and crankshaft  36 , is provided when the oil film, located in clearance  180  between inner surface  178  of cam  112  and outer surface  182  of crankpin  80 , is displaced. Upon engagement of drive surface  108  of drive flange  106  and driven surface  118  of cam  112 , clearance  180  is decreased at location  212  proximate interface  186 . By decreasing clearance  180  at location  212 , a gradual dampening effect occurs as drive and driven surfaces  108 ,  118  engage and oil is squeezed from the clearance. It will be understood by those having ordinary skill in the art that the contact interface angle θ directing force F 1  along interface  186 , resulting in oil being squeezed from clearance  180  and from between drive and driven surfaces  108 ,  118 , produces a significant dampening effect as drive flange  106  engages driven portion  116 .  
         [0048]    Referring to FIG. 9, in operation, drive and driven surfaces  108  and  118  are in abutment as cam  112  is driven in the forward direction of rotation  128  and piston  62  has a stroke of twice the eccentricity ( 2   e ) and the stroke is equivalent to the distance between crankshaft axis of rotation  88  (FIG. 2) and central axis  174  of cam  112 . During forward rotation in the direction of arrow  128 , axes  84  and  174  are equally eccentric (each having eccentricity e) relative to the crankshaft axis of rotation  88  and pistons  62  and  64  have a common stroke distance (i.e., 2×e) and common displacement. Forward rotation of crankshaft  36  causes compressor assembly  20  to have its maximum displacement.  
         [0049]    In contrast, with reference to FIG. 10 during reverse rotation of crankshaft  36 , eccentric cam assembly  112  is driven in a reverse direction of rotation, as illustrated by arrow  130 , compressor assembly  20  achieves only a portion (as shown, one half) its maximum displacement and piston  62  has zero stroke. Those having ordinary skill in the art will appreciate that, between the two cylinders, different stroke lengths or cylinder bore sizes may also be employed, and it is envisioned that the above described arrangement may be modified to produce a reduced displacement which is greater than or less than one half of the maximum displacement. Further, the present invention may be adapted to single cylinder compressors which have a first displacement when rotated in the forward direction, and a second, different displacement when rotated in reverse direction.  
         [0050]    Referring to FIGS.  11 - 14 , a second embodiment of a compressor assembly including a modified cam according to the present invention is depicted. Certain elements include primed reference numerals which indicate that the corresponding element previously described within the first embodiment has been modified. The second embodiment of a compressor assembly includes cam  112 ′ and differs from cam  112  of the first embodiment by having contact interface  186 ′ positioned at angle θ′ relative to a radially extended reference  188 ′ originating from centerline axis of crankpin  80 . In the exemplary embodiment, during forward rotation of the compressor assembly, angle θ′ is between 5° and 60° and during reverse rotation (FIG. 14) of the compressor assembly, angle α′ is between 5° and 60°, for example. In operation, which is depicted in FIG. 13 (forward rotation) and FIG. 14 (reverse rotation), the second embodiment compressor assembly, and corresponding modified cam assembly  112 ′, operates substantially identical to the first embodiment compressor assembly previously described. Those having ordinary skill in the art will understand that by altering angle θ′ of interface  186 ′, components of force F 1 ′, F 2 ′ may be predetermined to cause sliding engagement and disengagement, and control the oil film thickness, between the drive and driven surfaces.  
         [0051]    While this invention has been described as having exemplary designs, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.