Abstract:
A lower shell for a sealed compressor includes a shape with a plurality of intricate bends. In particular, u-shaped sections are stamped into a plainer sheet of material to form the lower shell. This stamping process can cause brittleness in the lower shell. Thus, after the stamping process, the lower shell is subjected to a heat-treating process to reduce the brittleness. After the heat-treating process a center shell is welded to the lower shell.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a process for strengthening the lower shell of a sealed compressor housing. 
     Refrigerant compressors typically include a compressor pump driven by a motor within a sealed housing. In such compressors, the refrigerant is allowed to flow over the motor driving the compressor pump to cool the motor during operation. Therefore the sealed housing must provide a fluid tight seal. Most housings are constructed from upper and lower shells attached to corresponding ends of a center cylindrical shell. The motor and compressor are secured within the center shell. 
     One common type of compressor used for commercial and residential refrigeration applications is a scroll compressor. A scroll compressor operates by trapping refrigerant within compression chambers formed between interfitting scrolls. Each scroll has a generally spiral wrap extending from a base. Typically one of the scrolls is an orbiting scroll and the other is non-orbiting. The motor drives the orbiting scroll relative to the non-orbiting scroll to progressively reduce the volume of the compression chambers, thereby compressing the refrigerant. 
     In one recently developed lower shell, a guide portion engages an inner part of the center shell to align the two pieces. The guide portion has a u-shaped section formed in the lower shell. The guide portion of the lower section is typically formed from a cold forming process such as stamping. The stamping process introduces stresses into the lower shell that increases the brittleness of the material in sections having the most intricate shapes. The increase in brittleness corresponds to a reduction in material elongation properties caused by cold work hardening of the stamping process. Further, the center shell is attached to the lower shell by a welding process that creates heat-affected zones that further hardens the material. 
     Currently pending patent application Ser. No. 09/816,178 titled “Weld Strengthening Component for Sealed Compressors” assigned to the applicant includes the addition of a strengthening member to the u-shaped section in order to increase the static pressure level that can be held by the sealed compressor. Such a method improves the static pressure capability of the sealed housing, however, it would be desirable to further increase this capability. Further, the addition of the strengthening member requires additional parts and manufacturing steps that may be undesirable in the cost conscious production environment. 
     Thus it would be desirable to strengthen the lower shell and particularly any region with dramatic and acute shapes such as the above-described u-shaped sections without additional parts. 
     SUMMARY OF THE INVENTION 
     A disclosed embodiment of this invention is a sealed compressor housing with a heat-treated lower shell that increases the capability of the sealed housing to withstand high static pressures. 
     The heat-treated lower shell is attached by a welding operation to a center shell. The lower shell of the subject compressor is heat treated to relieve stresses built up from previous processes. In the preferred embodiment the lower shell includes a u-shaped bend that is formed by a stamping process. The stamping process cold works the part and creates regions or zones of increased hardness. As the hardness of the material is increased the capability of the material to expand is decreased. This capability is related to elongation properties of the material. 
     The cold working process introduces high stress areas particularly in regions with the most dramatic changes in shape such as in the u-shaped guide section of the lower shell. Heat treatment of the lower shell relieves stress in the material and restores the materials elongation properties to a level substantially that of the original material. Heat-treating of the lower shell before the welding process restores the material to substantially the original elongation properties of the material such that stresses introduced by the welding process do not change the elongation properties of the material to the magnitude of the combined process without heat treat. 
     This invention provides a lower shell with improved material properties such that the static pressure capability of the sealed housing is significantly improved without additional parts or expensive additional manufacturing process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
     FIG. 1 is cross sectional view of a compressor; 
     FIG. 2 is a plane view of the lower shell with discrete guide sections; 
     FIG. 3 is a plan view of the lower shell with a continuous guide section; and 
     FIG. 4 is a cross-sectional view of the joint between the lower shell and the center shell. 
     FIG. 5 schematically shows a process for manufacturing and assembling the lower shell of a sealed compressor housing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the Figures, wherein like numerals indicate like or corresponding parts throughout the several views, the subject invention is a sealed compressor assembly generally shown at  10  in FIG.  1 . The sealed compressor  10  includes a compressor  12  and motor  14  disposed within a sealed housing  16 . The compressor  12  is preferably a scroll type compressor. The scroll compressor includes an orbiting  18  and a non-orbiting scroll  20 . The sealed housing  16  includes a center shell  26  with first and second ends  22 ,  24 . The compressor  12  is disposed near the first end  22  of the center shell  26  and the motor  14  is disposed closer to the second end  24  of the center shell  26 . The scrolls  18 ,  20  include generally spiral wraps  21  extending from a base  19 . The spiral wraps  21  interfit to form compression chambers  23 . The motor  14  drives the orbiting scroll  18  relative to the non-orbiting scroll  20  to progressively compress a refrigerant within the compression chambers  23 . It is understood by those skilled in the art that a scroll compressor is only one type of compressor that would benefit from the application of this invention and that other types of sealed compressors fall within the scope of this invention. An upper shell  28  is attached to the first end  22  of the center shell  26  and a lower shell  30  is attached to the second end  24 . 
     Referring to FIG. 2, the lower shell  30  is a generally square shaped plate with legs  34  extending from each corner. The lower shell also includes additional features that provide guiding and support functions for the motor and compressor. Preferably these features are formed from a stamping process as explained below. In the preferred embodiment the compressor  10  is of an upright configuration and the legs  34  provide stability and mounting points. Although the application illustrates an upright compressor  34  it should be understood that other configurations of a sealed compressor are within the contemplation of the invention, such as a horizontally disposed compressor. 
     The lower shell  30  includes guide sections  36  that align with an inner diameter  42  of the center shell  26 . The guide sections  36  extend upward from the lower shell about a central point  38  of the lower shell  30  at a radius such that the inner diameter  42  of the center shell  26  fits to the outside of each of the guide sections  36 . The guide sections  36  are equally spaced at intervals about the central point  38 . 
     Referring to FIG. 3, another embodiment of the lower shell includes a guide section  50  that extends uninterrupted about the central point  38 . A worker knowledgeable in the art would understand that other configurations of the guide sections  36 ,  50  are possible and are within the scope of the subject invention. 
     Referring to FIG. 4, fabrication of the sealed housing  12  includes placing the central shell  26  onto the lower shell  30  with the guide sections  36  abutting the inner diameter  42  of the central shell  26 . The lower shell  30  is attached to the center shell  26  by a weld  52  between the outer diameter  44  of the center shell  26  and the lower shell  30 . The cross-section of the guide section  36  is generally u-shaped having an inner leg  46  and an outer leg  48 . A worker skilled in the art would understand that the specific cross-sectional shapes of the guide sections  36  are application sensitive and many variations are possible within the scope of this invention. Further, for purposes of this application, the term “u-shaped” should be taken as a general descriptive term of a shape having two legs with a center section connecting the two legs. Although the shape is generally shown as being generally symmetrical, in practice, the shape will typically vary from such a symmetric shape. 
     The guide sections  36 ,  50  are preferably formed from a stamping process that cold works the material. As the material is hardened the elongation properties of the material are reduced such that the capability of the material to expand under pressure is impaired. The reduced ability to expand under pressure correlates to a reduction in the amount of static pressure that the sealed housing  12  can withstand. The cold work hardening of the material is the result of stresses introduced during the stamping process. Stresses from cold working hardening of the material are of the greatest concern where the shape of the lower shell  30  magnifies the affects of pressure within the sealed housing  16 , such as at the extreme bends of the u-shaped guide section  36 , 50 . 
     The welding process used to attach the lower shell  30  to the center shell  26  further contributes to weakening the material and thereby reducing the magnitude of static pressure that the sealed housing can withstand. The weld between the center shell and the lower shell creates heat-affected zones within the guide sections  36  and the center shell  26  that further reduces the elongation properties of the material used to fabricate the sealed housing  16 . Heat treatment of the lower shell  30  prior to attachment to the center shell  26  restores the original elongation properties of the material. 
     It is shown in FIG. 5, a method of arriving at the final lower shell  30  includes starting with a generally flat sheet of metal. This sheet has been stamped to have the u-shaped sections. A heat-treating process, as what we describe below next occurs. Once this is complete, the lower shell maybe assembled to the center shell by a welding process as described above. 
     The heat treat process applied to the lower shell relieves the stress built up in the material. The heat treat process includes soaking the lower shell at a temperature between 1000 and 1050 degrees Fahrenheit (F.) for approximately one hour. After the one-hour soak of the lower shell  30  the temperature is ramped down to 500 degrees F. Upon reaching 500 degrees F., the lower shell  30  is air cooled at an ambient temperature. After this heat treat process is complete, the elongation properties of the material are restored to approximately that of the original material before the stamping process. Preferably the material is common grade steel, the specific temperatures and duration of the heat treat process will differ dependent on the type and grade of material used and the amount of cold work hardening introduced into the material from the stamping process. A worker knowledgeable in the art would understand that other temperatures and durations may be used and would fall within the scope of this invention. 
     The end effect of the heat-treat processes it to substantially restore the original elongation properties of the lower shell  30  prior to the stamping process. The construction of the sealed housing requires the weld  52  between the center shell  26  and the lower shell  30 . Heat-treating of the lower shell  30  prior to the welding process restores the material to substantially the original elongation properties such that stresses introduced by the welding process do not change the elongation properties of the material to the magnitude of the combined process without heat treat. Therefore the static pressure that the sealed housing  16  can withstand is significantly improved. 
     The foregoing description is exemplary and not just a material specification. The invention has been described in an illustrative manner, and should be understood that the terminology used is intended to be in the nature of words of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. It is understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.