Patent Document

BACKGROUND OF THE INVENTION 
     This invention relates to centrifugal gas compressors and, more particularly, to an impeller and shaft assembly used in a high-speed gas compressor in a refrigeration plant or other chiller. 
     Centrifugal gas compressors have one or more impellers rotated in a cavity for compressing a gas, such as refrigerant vapor. The one or more impellers are mounted on a pinion shaft that is turned by a motor. In centrifugal gas compressors, it is important that the impellers and pinion shaft mounting arrangements are simple and efficient to manufacture, install and operate. In particular, overly complex attachment arrangements involving the machining of complementary grooves and threads in male and female parts pose a greater burden on highly skilled machinists, a resource that is both finite and costly. More particularly, such arrangements are more likely to be damaged during transport, installation and normal running of the compressor. 
     U.S. Pat. No. 4,257,744 describes an impeller and shaft assembly that includes a cap screw, a Belleville washer or spring, a deformable socket machined into the rear of an impeller, a drive shaft with a frusto-conical shaped extremity, and a steel washer. The impeller has an axial bore extending through its center and a counterbored recess at its front. The frusto-conical shaped extremity includes axially extended grooves that are circumferentially spaced and alternate with intervening lands. A high torque applied to the cap screw results in plastic deformation of the lining of the socket in the rear of the impeller. 
     The manufacture of the frusto-conical shaped extremity is complex and adds to the cost of the impeller and shaft assembly. In addition, the counterbored recess is sized to accommodate the cap screw. As a result, the protective steel washer and single spring are both sized to correspond to the cross-section area of the counterbored recess and screw cap. Thus, the torque results in a clamping force being directly transmitted from the cap screw, without dissipation, through the single Belleville washer and steel washer. This arrangement may damage the single Belleville washer and cause stress fractures in the front face of the impeller immediately around the counterbored recess, necessitating the costly replacement of the entire impeller. Thus, there is a need for a simple impeller and shaft assembly that minimizes the risk of damage to the front face of the impeller necessitating the costly replacement of the entire impeller. 
     Maintenance personnel may use an ordinary wrench when a torque wrench is more appropriate. Dramatic over or under-torquing of pinion shafts in centrifugal impeller configurations leads to increased maintenance and downtime costs. An impeller assembly that is less vulnerable to such problems is needed. 
     Additionally, cap screws increase the diameter of the impeller eye. The impeller eye is the terminal area on the cap screw end which is located radially inward of the impeller contour. 
     Other factors are the effect of thermal expansion of the aluminum impeller versus the steel drive shaft, and the fretting between the parts. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, an object of this invention is to provide a simpler and improved impeller and shaft assembly. 
     Another object is to provide an impeller and shaft assembly that employs an arrangement that more effectively dissipates the clamping load. 
     Yet another object is to avoid stress fractures in the front face of the impeller leading to replacement of the entire impeller. 
     A further object is to provide an impeller assembly that is less prone to damage resulting from failure to use a torque wrench. 
     It is an object, feature and advantage of the present invention to expand the impeller contour into the area of the impeller eye. It is a further object of the invention to provide a contour to the fastener or washer located in that eye area. It is still a further object, advantage and feature of the invention that the contour added in the area of the impeller eye should be continuous with the contour of the impeller itself. 
     It is an object, feature and advantage of the present invention to provide a collapsible washer that counteracts the effects of thermal expansion between the aluminum impeller versus a steel drive shaft. 
     It is a further object, feature and advantage of the present invention to reduce fretting between the components of a high speed impeller, shaft and fasteners. 
     At least one of these objects is addressed, in whole or in part, by the present invention. The invention is a rotatable impeller assembly for a refrigerant compressor. The assembly includes an impeller, a protective washer, a contoured spacer body, and at least one spring. (In this specification, an element introduced with an article “a,” “an,” or “the,” such as “a spring” or “the bore,” should be read to include one or more of the element.) 
     The impeller has an axial bore through it, a front face intersecting with the axial bore, and a rear face that is adapted to fit the driving end of a rotatable shaft. The protective washer is seated against the front face of the impeller. The rear face of the protective washer is seated against the front face of the impeller. The protective washer has an aperture registered with the axial bore. The contoured spacer body has a front face, a rear face, a recessed spring bearing surface in its rear face, a spring spacing abutment positioned to seat against the protective washer, and a central bore. At least one spring is seated between the protective washer and the spring bearing surface to provide a spacer assembly. The protective washer is used to keep the at least one spring from damaging the impeller. 
     A fastener (such as a bolt), including a headed front end and a rear end, is positioned through the axial and central bores. The rear end of the fastener is connected to the rotatable shaft. The headed front end of the fastener is seated against the front face of the contoured spacer body to provide a clamping load. The front face of the contoured spacer body may further comprise a recess sized to accommodate the headed front end of the fastener. 
     An advantage of this invention is that the cross-section area of the headed front end of the fastener does not govern the cross-section area of the protective washer and the at least one spring. Instead, the protective washer and the spring are sized to correspond to the much larger cross-section area of the rear face of the contoured spacer body, which itself closely matches the cross-section area of the front face of the impeller. Hence, the clamping load, after bolt tightening, is dissipated over a relatively large area of the front face of the impeller. 
     This arrangement has two immediate and very advantageous consequences. First, the front face of the impeller is less likely to suffer stress fractures. Second, even in the event that the clamping load causes stress fractures in the region immediately around the headed front end, such damage will only require the replacement of the contoured spacer body rather than the replacement of the impeller. 
     Another advantage of this invention is that the use of a collapsible washer counteracts the effects of the thermal expansion caused by the difference in materials between an aluminum impeller and a steel drive shaft. The use of the collapsible washer also reduces fretting between the parts since the washer absorbs some of the tension generated in axial directions. 
     Yet another advantage of the present invention is that the impeller contour is extended closer to the axis of impeller rotation. This is accomplished by modifying the contoured spacer body to extend the impeller contour over the fastener and washer area. 
     A further advantage of the present arrangement is that a maintenance engineer not using a torque wrench is far less likely to damage the impeller shaft assembly by applying too great a clamping load at the headed front end of the fastener. This is because in one aspect of the invention the contoured spacer body includes a spring spacing abutment positioned to seat against the protective washer. Once the spring spacing abutment comes into contact with the front face of the protective washer, the maintenance engineer will notice that it is suddenly harder to tighten the headed front end. This is a signal to stop tightening and hence avoid grossly over-torquing the impeller shaft assembly. 
     Yet another advantage is that the impeller shaft assembly is more tolerant to rough treatment. For example, a maintenance engineer who is in the habit of using a hammer or other rough treatment to loosen the fastener is more likely to damage the contoured spacer body rather than the front face of the impeller. Replacing a damaged contoured spacer body is preferable to replacing a damaged impeller. 
     Alternatively the contoured spacer body and the headed front end of the fastener can be combined to convert the headed front end into a contoured front end. In this aspect of the invention the contoured front end of the fastener would include at least some of the elements of the contoured spacer body and the headed front end. The contoured front end includes a front face, a rear face, a recessed spring bearing surface in its rear face, and a spring spacing abutment positioned to seat against the protective washer. However, the contoured front end does not require a central bore. The at least one spring is seated between the protective washer and the spring bearing surface of the contoured front end to provide a spacer assembly. 
     The above mentioned and other features of this invention and the manner of obtaining them will become more apparent, and the invention itself will be best understood, by reference to the following drawings taken in conjunction with the accompanying description of preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a block diagram of a chiller showing the major components and the flow of the refrigerant through the chiller. 
     FIG. 2 is a side elevation, cut away to show some of the interior features, of a refrigeration compressor. The refrigerant inlet and outlet are also shown. 
     FIG. 3 is a longitudinal section of the impeller and shaft assembly comprising a contoured spacer body according to one aspect of the invention. 
     FIG. 4 is an enlarged section, in isolation, of the contoured spacer body as illustrated in FIG.  3 . 
     FIG. 5 is an enlarged section, in isolation, of the contoured spacer body and Belleville springs employed in an alternative embodiment of the invention. 
     FIG. 6 is an enlarged section in isolation, of the contoured spacer body and Belleville springs employed in another alternative embodiment of the present invention. 
     FIG. 7 is an enlarged section, in isolation, of the contoured spacer body and pair of Belleville springs employed in another preferred embodiment of the invention. 
     FIG. 8 is an enlarged section, in isolation, of the contoured spacer body employed in yet another preferred embodiment of the invention. 
     FIG. 9 is a longitudinal section of the impeller and shaft assembly comprising a contoured front end. 
     FIG. 10 is a longitudinal section of the impeller and shaft assembly comprising a contoured front end according to another preferred embodiment of the invention. 
     FIG. 11 is a longitudinal section of the impeller and shaft assembly comprising a contoured front end according to yet another preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the invention will be described in connection with one or more embodiments, it will be understood that the invention is not limited to those embodiments. On the contrary, the invention includes all alternatives, modifications, and equivalents as may be included within the spirit and scope of the appended claims. 
     FIG. 1 schematically shows a mechanical chiller  10  including a compressor  12 , a heat exchanger such as a condenser  14 , an expansion device such as an expansion valve  16 , and a heat exchanger such as an evaporator  18 . These components are connected to form a refrigerant circuit by refrigerant conduits  20 ,  22 ,  24  and  26 . Refrigerant gas enters the compressor  12  from the conduit  20  and is compressed in the compressor  12 , thus raising its temperature. The compressed gas from the compressor  12  enters the condenser  14  via the conduit  22 . In the condenser  14 , the hot, compressed gas is condensed into liquid form and contacted with a heat sink, such as ambient air, ground water, or another cooler medium, to remove heat from the condensing refrigerant. The condensed refrigerant passes through the conduit  24  and through an expansion valve  16 . The expansion valve  16  allows a limited quantity of refrigerant to enter the evaporator  18 , while maintaining the pressure difference between the condenser  14  (at higher pressure) and the evaporator  18  (at lower pressure). The refrigerant entering the evaporator  18  evaporates after contacting a heat load, such as the refrigerator interior or ventilation air that is to be cooled, thus absorbing heat from the heat load. The refrigerant vapor leaves the evaporator  18  via the conduit  20 , returning to the compressor  12  to repeat the cycle. 
     Now refer to FIGS. 2 and 3, and specifically to the interior of a centrifugal compressor  12 . The compressor  12  includes an impeller assembly including impellers  40 ,  50  mounted on a rotatable shaft  64 . The compressor  12  has a gas inlet  30 , a gas outlet  32 , and internal passages  34  directing refrigerant gas from the inlet  30 , into and through the first stage impeller  40 , the second stage impeller  50 , and to the outlet  32 . The rear end  264  of a fastener  62  such as a bolt (or other device allowing radial rotation while providing axial clamping force) is connected to the rotatable shaft  64  to removably attach the impeller  40  to the rotatable shaft  64 . Although the preferred embodiment of this invention is shown as a gear drive centrifugal compressor, the impeller assembly is generally applicable to all centrifugal compressors as well as to other compressors having an impeller  40  mounted on a terminal end  66  of a rotatable shaft such as rotatable shaft  64 . Exemplary centrifugal compressors are sold under the registered trademark CenTraVac by The Trane Company, a Division of American Standard Inc. having a principal place of business in La Crosse, Wis. Exemplary centrifugal compressors are shown in commonly assigned U.S. Pat. No. 3,805,547 to Eber and U.S. Pat. No. 3,853,433 to Roberts et al., both of which are incorporated by reference herein. 
     Referring to FIGS.2 and 3, a first stage impeller and shaft assembly  90  including the first stage impeller  40  depicting an aspect of this invention is disclosed. The impeller  40  has an axial bore  100  through it, a front face  102  intersecting with the axial bore  100 , and a rear face  104  that is adapted to fit the driving end  66  of the rotatable shaft  64 . FIG. 3 does not show the details of the connection between the impeller  40  and the shaft  64 , which can be conventional. For two examples, either a conventional splined joint or the three-lobed connection described in co-pending U.S. Ser. No. 09/204,867, filed by the present assignee on Dec. 3, 1998 can be used. 
     The front face  102  of the impeller  40  is truncated at an end  105  and optionally has a recess  110  to accommodate a contoured spacer body  200 , a protective washer  120  and an expansor such as a spacer assembly  150 . For purposes of this application, a contoured spacer body is a device having an external surface which is aerodynamically contoured and having an internal portion acting as a spacer. The spacer assembly  150  provides a known resistance when compressed. 
     The protective washer  120 , preferably a hardened steel washer, has a front face  122  and a rear face  124  . The rear face  124  is seated against the front face  102  (the recess  110  if present) of the impeller  40 . The protective washer  120  has an aperture  126  registered with the axial bore  100 . 
     Referring to FIGS. 3 and 4, the contoured spacer body  200  includes a front surface  202  and a rear surface  204 . The contoured spacer body  200  is symmetrical about an axis  206 , and the front surface  202  includes a contoured surface  210  at an angle or a curve relative to the axis  206 . The rear surface  204  includes a spring spacing abutment  220  including a washer contact surface  222  at the end of the abutment  220 . The spring spacing abutment  220  is axially dimensioned relative to the axis  206  so that the spacer assembly  150  deflects at a desired amount. The contoured spacer body  200  includes a center portion  224  having a rear recess  226  arranged in the rear surface  204  about the spring spacing abutment  220 . A central bore  230  runs through the center portion  224  symmetrical about the axis  206 . The washer contact surface  222  engages the protective washer  120 . The recess  226  provides a spring bearing surface  234  for engagement with the spacer assembly  150 . The front surface  202  of the contoured spacer body  200  preferably includes a recess  235  and a forward facing shoulder  236  in the recess  235 . At least one tension providing device such as a spring  232 , which in the illustrated embodiment is a Belleville spring (though another type of spring, or a lock washer, or a compressible gasket or washer can be used instead), is seated between the protective washer  120  and the spring bearing surface  234  to provide the spacer assembly  150 . 
     The fastener  62 , including a headed front end  260 , a front face  262  and a rear end  264 , is positioned through the axial bore  100 , the aperture  126 , and the central bore  230 . The rear end  264  of the fastener  62  is connected to the rotatable shaft  64  (here, the rear end  264  is threaded into a cavity  270  in the shaft  64 ), and the headed front end  260  is seated against the front surface  202  of the contoured spacer body  200 , preferably in the recess  234  and against the shoulder  236 , to provide a clamping load. 
     After torquing the fastener  62 , the spacer assembly  150  collapses to about 75% of its maximum deflection. The abutment  220  of the contoured spacer body  200  is seated against the protective washer  120  and is spaced by the depth of the spring spacing abutment  220  to control the deflection of the springs  232  in the spacer assembly  150 . At 75% maximum deflection, the clamp load will exceed the axial thrust load imposed upon the impeller  40 . 
     FIG. 4 is an enlarged isolated side elevational view, in section, of the contoured spacer body  200  including the spring spacing abutment  220  as positioned to seat against the protective washer  120  (as shown in FIG.  3 ). In this embodiment, the surface  222  comes into contact with the front face  122  of the protective washer  120 . At least one spring  232  is sized to fit in the recessed pocket  226  formed between the contoured spacer body  200  and the protective washer  120 . The protective washer  120  is used to keep the at least one spring  232  from damaging the impeller  40 . A skilled mechanic would slack off slightly to avoid over-torquing the impeller shaft assembly in response to the surface  222  seating hard against the protective washer  120 . 
     The front surface  202  of the contoured spacer body  200  can desirably be continuous from the front face  102  of the impeller  40  to the central bore  230 . The front surface  202  of the contoured washer  200  optionally has a recess  235  to accommodate the headed front end  260  of the fastener  62 . The recess  235  in the front surface  202  of the contoured spacer body  200  can be sized to ensure that the front face  262  of the headed front end  260  is seated flush across the central bore  230  in order to make a substantially continuous surface (shown in FIG.  3 ). A substantially continuous surface across the front surface  202  of the contoured spacer body  200  provides improved refrigerant flow during normal operation. 
     In one aspect of this embodiment (as depicted in FIG. 3) the truncated end  105  in the front face  102  of the impeller  40  is sized to accommodate the protective washer  120 , the spacer assembly  150  and the contoured spacer body  200 . In this embodiment of the invention, the rear face  124  of the protective washer  120  seats against the recess  110  in the front face  102  of the impeller  40 . 
     In an alternative embodiment shown in FIG. 5, the body  224  of the contoured spacer body  200  has an aerodynamic portion  270  extending slightly around the spring spacing abutment  220  but not contacting either the impeller  40  or the protective washer  120 . In this manner, the front face  102  of the impeller  40  need only provide a recess  110  sized to accommodate the protective washer  120 . One advantage of this embodiment is that the front face  102  of the impeller  40  around such a recess would be less vulnerable to stress fractures. 
     In another embodiment shown in FIG. 6, the contoured spacer body  200  has an aerodynamic portion  272  which extends around the spring  232  and the protective washer  120  to make contact with the front face  102  of the impeller  40 . 
     In still another embodiment, the spring spacing abutment  220  is spaced radially outwardly so that the surface  222  seats against an outer edge  280  of the protective washer  120  (FIG.  7 ). 
     In yet another embodiment, the rear surface  204  of the contoured spacer body  200  provides two shoulder surfaces  274  and  276  (FIG. 8) including an outer shoulder  274  spaced radially outwardly and an inner shoulder  276  spaced radially inwardly. In this embodiment each shoulder,  274  and  276 , seats against the washer  120  to provide a pocket  277  to accommodate the at least one spring  232 . 
     Referring to FIG. 9, the contoured spacer body  200  (not shown in FIG. 9) and the headed front end  260  (not shown in FIG. 9) of the fastener  62  are combined to convert the headed front end  260  into a domed front end  300  of the fastener  62 . In this aspect of the invention, the domed front end  300  has a front face  302 , a rear face  304 , a recessed spring bearing surface  306  in its rear face  304 , and a spring spacing abutment  308  positioned to seat against the protective washer  120 . In this arrangement, the spacer assembly  150  is seated between the protective washer  120  and the spring bearing surface  306 . 
     As in FIG. 3, the front face  102  of the impeller  40  may comprise a recess  110  in order to accommodate the protective washer  120 . The rear face  304  of the domed front end  300  (including the surface  306 ) can be sized to correspond to the cross section area of the truncated end  105  of the impeller  40  (or to the forward facing area of the recess  110 ). In this arrangement the rear face  124  (and by default, the front face  122 ) of the protective washer is sized to correspond to the cross-section area of the truncated end  105  of the impeller  40  (or the forward facing area of the recess  110 ). Thus, the clamping force is transmitted from the domed front end  300  and through the relatively large surface area of the protective washer  120 . Hence, large torquing may be applied without causing stress fractures in the front face  102  of the impeller  40  or the rear face  304  of the domed front end  300 . 
     The fastener&#39;s ability to carry more torque results in higher energy yield. In addition, the front face  302  of the domed front end  300  provides a continuous aerodynamic surface  309  across the front face  102  of the impeller  40 . Compressors fitted with a contoured front end will result in higher speeds and higher work rates and a concomitant decrease in compressor size. 
     The front face  302  of the domed front end  300  may be designed with indents or holes  320  to allow a suitable tool bit to attach to the aerodynamic surface  309 . This tool bit in turn attaches to a suitable torque wrench. Alternatively, the tool bit might form part of a torquing tool. This would ensure that appropriate tools are used in the installation and removal of the impeller and shaft assembly thus decreasing the likelihood of damage to the impeller and shaft assembly. 
     FIG. 10 schematically shows a different aspect of the arrangement disclosed in FIG.  9 . In this aspect of the invention, the rear face  304  of the domed front end  300  makes contact with the front face  102  of the impeller  40  at a shoulder area  312  of the domed front end  300 . The recess  110  in the front face  102  of the impeller  40  is less pronounced compared to that disclosed in FIG.  9 . 
     In another aspect of the invention, the front face  102  of the impeller  40  has a truncated end  314  which lacks the recess  110  and is essentially flat as shown in FIG.  11 . In this embodiment of the invention, the protective washer  120  is sized to correspond more closed to the cross section area of the truncated end  314  of the impeller  40 . The protective washer  120  preferably includes a contoured, radially outward end  318  having an aerodynamic contour matching that of the domed front end  300  and the front face  102 . The domed front end  300  has an additional shoulder  322 . The comparatively large cross section area of the rear face  304  in contact with the protective washer  120  ensures maximum dissipation of the clamping load. 
     While the invention is described above in connection with preferred or illustrative embodiments and examples, they are not intended to be exhaustive or limiting of the invention. Rather, the invention is intended to cover all alternatives, modifications and equivalents included within its spirit and scope of the invention, as defined by the appended claims.

Technology Category: 2