Patent Publication Number: US-7213683-B2

Title: Piston lubricating system for a reciprocating compressor with a linear motor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
   This application is a U.S. national phase application under 35 U.S.C. §371 of International Application No. PCT/BR02/000040 filed Mar. 12, 2002, and claims the benefit of Brazilian Patent Application No. PI0101017-4 filed Mar. 13, 2001 which is incorporated by reference herein in its entirety. 
   FIELD OF THE INVENTION 
   The present invention refers, in general, to a piston lubrication system for a reciprocating compressor of the type driven by a linear motor and used in small refrigeration appliances, such as refrigerators, freezers, water fountains, etc. and, more particularly, to the lubrication system of a piston with a vertical shaft for the type of reciprocating compressor mentioned above. 
   BACKGROUND OF THE INVENTION 
   In a compressor of the type driven by a linear motor, the gas compression and gas suction operations are performed by the axial movement of a piston inside a cylinder, which is closed by a cylinder head and mounted inside a hermetic shell, in the cylinder head being positioned a discharge valve and a suction valve, which control the admission and discharge of gas in the cylinder. The piston is driven by an actuating means that carries magnetic components such as an electrical coil, which is exposed to a magnetic field generated by a magnetic element, such as a magnet, and operatively associated with a linear motor affixed to the shell of the compressor. 
   These constructions generally present two helical springs under constant compression and which are operatively mounted against the actuating means, each seated against an adjacent surface of the latter. The piston, the movable parts of the actuating means, the magnetic components, and the helical springs form together the resonant assembly of the compressor, which is driven by the linear motor and has the function to develop a reciprocating linear movement, making the movement of the piston inside the cylinder exert a compression action on the gas admitted by the suction valve, until a point is reached in which said gas is discharged to the high pressure side, through the discharge valve. 
   The reciprocating movement of the piston requires a lubrication system to minimize frictions and wears, in order to assure an adequate performance to the compressor. 
   In constructions known from the art, the supply of lubricant oil to the cylinder and piston is made through a lubrication system using the principle of centrifugal force and mechanical drag, with channels contained in the cylinder, such as presented in document PI0004286-2, of the same applicant, or in documents WO97/01032 and WO97/01033. These channels are of complex construction, due to the difficult access to the lubricated region and require careful sealing, since they pass through some components. 
   In one of the known solutions, in order to supply oil to the piston/cylinder assembly, it is necessary to make the compressor suction gas flow, which generates a small pressure differential in relation to the oil sump, to draw said oil through a capillary tube, mixing it with the gas drawn by the compressor, said mixture being admitted to the inside of the cylinder by the suction valve, so that the oil lubricates the contact regions between the piston and the cylinder. As a function of the low gas flow drawn by the compressor in certain situations, this construction presents low efficiency regarding lubrication. 
   In another known construction (WO97/01033), the compression and suction forces of the piston are used to displace the lubricant oil from the sump, through a capillary tube, to an upper reservoir formed around the cylinder, said upper reservoir being connected to the inside of the cylinder by a plurality of orifices formed in the wall of the latter and which serve for admitting oil into the piston-cylinder gap, when the piston is performing the suction movement, and for discharging said oil when the piston is performing the reverse movement. The oil is discharged into a number of channels formed in the valve plate of the compressor, further supplying the suction flow, causing said oil to re-enter the cylinder. 
   Other known solution (WO97/01032) uses a resonant mass that moves reciprocally inside a cavity formed in the external side of the cylinder, said resonant mass drawing oil from the sump while moving in one direction, said oil passing through a tube and through a check valve, which allows only the admission of oil into said cavity, the latter being connected to the inside of the cylinder by a plurality of orifices formed in the wall thereof. The oil in said cavity is expelled when the resonant mass moves in the other direction and passes through a check valve, which allows only the discharge of oil from said cavity. Although being functional, this solution is difficult to manufacture and its construction presents many components. 
   OBJECTS OF THE INVENTION 
   It is an object of the present invention to provide a piston lubrication system for a reciprocating compressor, of simple construction, which results in a continuous and adequate lubrication of the piston and cylinder, allowing said lubrication to be obtained without the provision of channels for conducting oil through the cylinder. 
   It is another object of the present invention to provide a piston lubrication system for a reciprocating compressor, which, besides the characteristics mentioned above, allows lubrication to be achieved with minimum pressurization in the region between the external sidewall of the piston and the internal sidewall of the cylinder. 
   This and other objects are attained through a piston lubrication system for a reciprocating compressor with a linear motor, of the type comprising: a hermetic shell, defining therewithin an oil sump and affixing a cylinder, inside which is lodged a reciprocating piston presenting a top region and a body region defining a radial gap with the cylinder; and a pump means supplying lubricant oil from the oil sump to the radial gap, said system comprising: at least one oil supply duct having a lower end coupled to the pump means, and an upper end carried by the piston and opened to the radial gap; and at least one oil return duct carried by the piston and having an end opened to the radial gap, and an opposite end opened to the oil sump. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described below, with reference to the attached drawings, in which: 
       FIG. 1  is a schematic longitudinal diametrical sectional view of a hermetic compressor of the reciprocating type driven by a linear motor, in which the lubrication of the piston is made according to the present invention; 
       FIG. 2  is a schematic lateral view of the piston of the present invention, constructed according to a way of carrying out the invention; 
       FIG. 3  is a schematic diametrical cross-sectional view of the piston illustrated in  FIG. 2 , according to line III—III in said figure; 
       FIG. 4  is a schematic lateral view of the piston of the present invention, constructed according to another way of carrying out the invention; and 
       FIG. 5  is a schematic diametrical cross-sectional view of the piston illustrated in  FIG. 4 , according to line IV—IV in said figure. 
   

   DESCRIPTION OF THE ILLUSTRATED EMBODIMENT 
   The present invention describes a piston lubrication system for a reciprocating compressor with a linear motor (of the type applied to a refrigeration system), presenting a hermetic shell  1 , defining in the interior thereof an oil sump  2 , wherefrom the lubricant oil is pumped through a pump means  3 , for example, a pump of the type described in Patent Application PI0004286-2, from the same applicant, to the inside of a cylinder  4 , which is affixed inside the shell  1  and lodges a reciprocating piston  10 . The illustrated piston  10  is of the tubular type, and presents a top region  11  and a body region  12  defining a radial gap  5  with the cylinder  4 , said pump means  3  supplying lubricant oil from the oil sump  2  to said radial gap  5 . 
   Cylinder  4  is closed by a cylinder head  20 , where are positioned a suction valve  21  and a discharge valve  22 , which control the admission and discharge of gas in the cylinder  4 . Between the top region  11  of the piston  10  and the cylinder head is defined a compression chamber  23 . 
   In the illustrated embodiment, the region of the radial gap  5  is defined by a diametrical reduction, which is provided along part of the axial extension of the piston  10  inferiorly to the top region  10  thereof. In the reciprocating hermetic compressor with a linear motor, the reciprocating movement of the piston  10  is made through an actuator  6 , which sustains a magnetic component  7  driven by the linear motor. The piston  10  is connected to a spring means  8 , which is formed for example, by a pair of helical springs under constant compression against the actuator  6 , the piston  10  forming together with the magnetic component, the resonant assembly of the compressor. The non-resonant assembly of the compressor comprises the cylinder  4 , a suction-discharge system of the compressor, and its linear motor. 
   According to the present invention, the oil pumped by the pump means  3  reaches the radial gap  5  through at least one oil supply duct  30 , to be described ahead, which has a lower end  31 , coupled to the pump means  3 , and an upper end  32 , which is carried by the piston  10  and opened to the radial gap  5 , said oil returning to the oil sump  2 , by at least one oil return duct  40 , to be described ahead, which is carried by the piston  10  and has an inlet end  41  opened to the radial gap  5 , and an outlet end  42  opened to the oil sump  2 . 
   In a constructive option to be described below, at least one of the parts of the oil supply duct  30  and the oil return duct  30  presents part of its extension internal to the piston  10 . 
   According to the present invention, the oil supply duct  30  comprises at least one supply axial portion  33  and at least one supply radial portion  34 , which are defined inside the piston  10 , each said supply axial portion  33  being in fluid communication with the oil sump  2  and with at least one supply radial portion  34 , each supply radial portion  34  being opened to the lateral surface of the piston  10  in the region of the radial gap  5 . 
   In the construction illustrated in  FIG. 1 , the supply axial portion  33  of the oil supply duct  30  is defined inside a tubular piston rod  13 , which is coupled by a lower end to the pump means  3  and has an upper end opened to a plurality of supply radial portions  34 , for example in the form of radial channels, which are substantially rectilinear and orthogonal to the axis of piston  10 , and produced in the body region  12  of the piston  10 , inferiorly to the top region  11  thereof. 
   According to a constructive form of the present invention, the oil return duct  40  is defined for example by a plurality of radial bores, generally rectilinear and orthogonal to the axis of piston  10  and which are produced in the sidewall of said piston  10 , inferiorly to the supply radial portions  34  thereof. 
   In the constructive option illustrated in  FIG. 1 , the lubricant oil reaching the inlet end  41  of the oil return duct  40  arrives to the oil sump  2 , flowing down along the internal lateral surface of said piston  10 , and the oil may be conducted by one or more internal grooves (not illustrated). 
   In another constructive option of the present invention, the lubricant oil reaches the oil sump  2  through an oil return duct  40  defined by a groove, which is produced in the external lateral surface of the piston  10 , or by an axial channel produced along part of the axial extension of said piston  10  (the latter being solid), opened to the oil sump  2 . 
   In the construction illustrated in  FIGS. 2–5 , the piston lubrication system also presents another oil return duct  40 , which conducts the oil existing in the radial gap  5  to the oil sump  2  by the external lateral surface of the piston  10 . In this construction, the piston  10  presents a bearing ring  16 , which is defined by a diametrical enlargement in a portion of its extension, inferior to that presenting a diametrical reduction and which defines the radial gap  5 , said bearing ring  16  presenting at least one oil return duct  40 , for example in the form of an axial or helically developed groove produced along the extension of said bearing ring  16 . 
   In the construction of the piston  10  of the present invention, the latter has, according to the illustrations, two functional regions: the piston top region  11 , which operates with the oscillation of the piston  10 , and the bearing ring  16 , which works as a sealing means, for better sealing the compression chamber  23 , as a bearing means, and as a guiding means, absorbing the radial efforts on the piston  10  and generating minimum viscous friction. 
   The piston top region  11  presents narrow gaps, which substantially isolate the region of the radial gap  5  from the compression chamber  23 . 
   The bearing ring  16  is provided in a certain position of the axial extension of the piston  10 , which position is calculated in order to minimize the effect of moments during operation of the compressor. 
   With the construction of the present invention, the lubricant oil, which is pumped to the radial gap  5 , is contained in this region and returns to the oil sump  2  through the oil return duct. Part of this oil returns to said oil sump  2  through each oil return duct  40  produced through the bearing ring  16 . Besides actuating in the oil return, the oil return duct  40  provided on the external lateral surface of the piston  10  also provides a pressure relief.