Abstract:
An automotive air conditioning assembly has a fan that is molded by a technique that inevitably leaves the lower hub and upper rim radially staggered relative to one another. Therefore, a substantial length of the edges of the blades&#39; bases are unsupported by the incomplete hub, and the air forced radially outwardly between the blades has no fan structure to confine it at that point. The invention provides a fan housing having a wall portion specially shaped so as to provide the air confinement function that the missing section of the fan hub cannot.

Description:
TECHNICAL FIELD 
     This invention relates to air conditioning and ventilation systems in general, and specifically to centrifugal fan assembly therefor. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 5,588,803 describes some of the basic structural and manufacturing issues involved in producing molded plastic centrifugal fans for automotive air conditioning systems. The ultimate in molding simplicity is a one piece design, which can be made only by designing the fan with a shape that is amenable to the so called axial draw or by pass molding technique. In order to be moldable by that technique, the part, be it a fan or anything else, must have a certain structural relationship relative to its central axis, such as the central axis of a bearing cage or the central axis of a fan. All “upper” and “lower” surfaces of the part must be divisible in such a way that they have no radial overlap with one another. If so designed, all part surfaces may be divided up so that some can be molded by one die, and the rest by the other die, and the pair of dies (or molds) can be pushed together and pulled apart freely along the same central axis. This represents the absolute minimum both in terms of the number of molds used (two) to produce the part, and the number of pieces (one) in the part produced. 
     A dilemma is faced in designing a centrifugal fan with such a “no radial overlap” design, especially for so called rearwardly inclined fan blade designs, which are wide in the radial direction. Both the lower blade bases and the upper blade tips need adequate structural support. The blade bases may be easily integrally molded to the central area of the fan, which has a thick center hub. However, to provide complete support to the upper tips of the blades, an upper ring is needed, which is axially spaced from the hub, and inevitably overlaps with it. The issue then becomes the best way to physically attach this non integral blade tip supporting ring. This may be done by separate fasteners, heat staking, or, as in the above referenced patent, by a twist lock technique. 
     This is not to say that it&#39;s impossible to by pass mold a plastic centrifugal fan, even one with radially wide blades. A design capable of being molded that way is relatively simple, and an example of such a design is disclosed in U.S. Pat. No. 5,352,089. The design involves basically splitting off the radially outermost section of the hub at an imaginary cylindrical line and moving it up to support the tips of the fan blades. Then, the two molds can part along that imaginary cylinder, which is arrayed around the central axis. Inevitably, the entire width of the base and tips of the blades cannot both be structurally supported, however. Only the radially inner portions of the base of the fan blades are supported, by the hub, and the radially outer portions are unsupported by the hub. Likewise, only the radially outer portions of the tips of the blades are supported, by the upper rim, and the radially inner portions are unsupported. Sufficient structural stiffness can be achieved simply by making the hub, rim and blades thick enough, of course. 
     However, in a two piece fan design, the hub at the blade bases, and the radially overlapped ring at the blade tips, provide more than just blade stiffness. The air that is pulled axially in and then driven radially outwardly between the blades is also confined between the axially opposed lower hub and upper ring. The upper ring generally slopes axially downwardly relative to the lower hub (to maintain a constant volume as the radius increases), and both the hub and ring generally slope axially downwardly relative to the air capturing, torroidal volute that surrounds the fan. With a by pass molded, one piece fan design, both the hub and upper ring are “incomplete,” and cannot alone do an efficient job of confining the radially outwardly moving air stream. For example, in the design disclosed in U.S. Pat. No. 5,352,089, the unsupported outer portions of the blade bases are simply left wide open, decreasing the effectiveness of the fan assembly as a whole. 
     SUMMARY OF THE INVENTION 
     An automotive air conditioning fan assembly according to the present invention is characterised by the features specified in claim  1 . 
     In the preferred embodiment disclosed, a centrifugal fan with the same basic “split lower hub and upper ring” design described above is incorporated within a housing that uniquely cooperates therewith to compensate for the fact that the axial space between the blades in not totally bounded or confined by the fan itself. The housing volute is configured with a circumferentially continuous inner wall which, in effect, takes the place of the inevitably missing outer section of the hub. The volute wall has a cylindrical inner coaming that surrounds and is closely radially opposed to the terminal edge of the fan hub. Air driven outwardly by the fan blades, therefor, does not have a large leak path available through the fan hub-volute wall clearance. From its inner coaming, the volute wall slopes radially out and axially down, generally matching and closely paralleling the contour of the hub. In the embodiment disclosed, the edges of the unsupported outer portions of the fan blade bases also closely follow the contour of the volute wall, with a close clearance. Therefore, air moving radially outwardly between the blades is axially well confined between the volute wall and the axially opposed upper rim of the fan. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the invention will appear from the following written description, and from the drawings, in which: 
     FIG. 1 is a perspective view of a centrifugal fan incorporated in the fan assembly of the invention; 
     FIG. 2 is a cross section through the fan of FIG. 1; 
     FIG. 3 is a cross section of the whole fan assembly; 
     FIG. 4 is an enlargement of the directed portion of FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIGS. 1 and 2, a molded plastic centrifugal fan, indicated generally at  10 , is generally defined about a central axis A, and also spins about the same axis in operation. The structural foundation of fan  10  is a central hub  12 , which is basically an annular disk that slopes radially outwardly and, in the particular embodiment disclosed, axially downwardly from, a center bore  14  that lies on axis A. Bore  14  is the attachment point the motor shaft that spins the fan  10 , and the bottom point of bore  14 , indicated at X, is the point relative to which the fan  10  would bend or vibrate if unbalanced. Hub  12  is as thick and as structurally stiff as it practically can be, within cost and weight constraints, but it does not, and cannot, extend radially all the way out to the radial outermost edge of fan  10 . Instead, it ends at a cylindrical outermost edge, in this case, a lower cylindrical flange  16 , disposed about axis A. As indicated by the double headed arrow in FIG. 2, the lower flange  16  is diagonally opposed to the point X, that is, it is spaced both radially outwardly from and axially below the point X. Lower flange  16  is also located just radially inboard of an imaginary cylinder C, which is also coaxial to central axis A. A series of circumferentially spaced, radially disposed blades  18  have the radially inner portion of their bases integrally molded with, and supported by, the central hub  12 . That support ends, however, at the imaginary cylinder C, where hub  12  ends. Radially outboard of cylinder C, the lower edges  20  of the bases of blades  18  are open and unsupported. Those unsupported lower edges  20  continue to slope radially outwardly and axially downwardly from flange  16 , for a significantly greater distance than the supported inner portion of the bases of blades  18 , continuing on with the basic contour and shape of the hub  12 . Axially above the hub  12 , a generally annular rim  22  slopes radially outwardly and axially downwardly from an inner lip  24  to a circular terminal edge in the form of an upstanding cylindrical flange  26 . Inner lip  24  lies just radially outboard of the cylinder C, while flange  26  is contiguous to the outer edges of the tips of the blades  18 . Upper flange  26 , like lower flange  16 , is diagonally opposed to, but axially above, the point X. The flanges  16  and  26  help to stiffen the fan  10 , but also provide conveniently located structures on which to place balance weights, or from which to shave material, or both, so as to dynamically balance fan  10  relative to its attachment point X. They also provide other functions, described below. 
     Referring next to FIGS. 3 and 4, a fan housing, indicated generally at  28 , encases a motor  30  with central shaft  32 , which is attached through bore  14  to fan  10 . The outer reaches of housing  28  comprise a generally torroidal volute  34  that surrounds the fan  10 , and which acts as a trough to catch and gather the pressurized air forced radially outwardly by fan  10 . The volute  34  increases in width and volume at its outer perimeter, moving around its circumference, and also moves axially down, so as to move the pressurized air radially outwardly and axially down to a non illustrated outlet. The inner perimeter of volute  34  comprises a lower wall portion  36  that has a substantially constant size and shape. Generally, as best seen in FIG. 4, wall portion  36  slopes radially out and axially downwardly in a contour that generally matches and continues the contour of the fan hub  12 , beyond the flange  16  where hub  12  ends. Specifically, wall portion  36  runs below and parallels the open, unsupported lower edges  20  of the bases of the blades  18 , with a slight, substantially constant clearance therefrom, indicated at G1, of 2-8 millimeters. Wall portion  36  has a generally circular inner edge in the form of an integral, cylindrical coaming  38 , which is radially opposed to and spaced from fan lower flange  16  by a clearance G2 of similar size. The upper wall of housing  28  includes an annular, upstanding trough  40  that surrounds the upper fan flange  26  with a clearance G3 comparable in size range to G1 and G2. 
     Referring again to FIG. 3, the operation of fan  10  within housing  28  is illustrated. As fan  10  is spun by motor  30  about its central axis, air is pulled axailly in from above, and through the open, unsupported inner edges of the tips of blades  18 . This unsupported tip length, standing out from lip  24 , is not particularly long, and a much greater proportion of the blade tip is supported by rim  22  than is unsupported, so blade tip stiffness is not an issue. Air pulled axially in is then forced radially outwardly between the blades  18 , axially confined below by the upper contoured surface of the fan hub  12 , and above by the inner surface of the fan rim  22 . However, since the hub  12  and rim  22  cannot radially overlap one another, they are never axially opposed, and cannot concurrently axially confine the moving air stream physically between them. Instead, as the air moves radially outwardly (as shown by the arrows) it moves past the radial gap G2, with little pressure loss, because of the controlled size of G2. Thereafter, the air stream smoothly follows the contour of the housing wall portion  36 , because of the fact that it continues on with the basic contour of the upper surface of hub  12  (sloping axially down and radially out), and because of the fact that it is so closely spaced relative to the open, unsupported lower edges  20  of the fan blades  18 . Above the hub  12 , the air stream smoothly follows the contour of the upper blade rim  22 , flowing past the upper flange  26  with minimal pressure loss, due to the tightly controlled radial gap G3. Upper blade rim  22  is axially opposed to the housing wall portion  36 , and slopes down even more steeply, thereby maintaining a relatively constant total volume as the confined area expands with the growing radius. Thus, before as it is expelled from between the fan blades  18 , the air stream is forced radially out and axially downardly into the volute  34  under pressure. The closely contoured housing wall portion  36 , with its particular shape and closely controlled gap G1, makes up for and replaces the “missing” portion of the hub  12 , cooperating with the fan rim  22 . An operation comparable to a two piece fan is achieved, that is, a fan in which the hub can and does run radially out all the way along the entire base of the blades. This performance is achieved by a molded, one piece fan, however, which is inherently less costly to manufacture and handle. 
     Variations in the disclosed embodiment could be made. For example, the outer edge of hub  12  could be abrupt and sharp, instead of the cylindrical flange  16  shown, just as the inner edge of wall portion  36  could be sharp, rather than the cylindrical coaming  38  disclosed. However, the flange  16 , being concentric to the cylinder C, can be created without mold pull interference, and provides both extra fan stiffness, as well as extra axial length to the gap G2, which aids in non contact sealing. The coaming  38 , as well, can be easily molded and provides extra axial length to the gap G2. The same considerations apply to the upper flange  26  and the way it fits within trough  40 . That is, rim  22  could also end, instead, an abrupt edge, but upper flange  26  provides the same benefits as the lower flange  16 , and the two flanges  16  and  26 , as noted above, together provide improved fan balancing potential. The upper surface of hub  12  and the wall portion  36  could be sloped axially downwardly to a lesser degree, even nearly flat, in a case where the volute itself did not recede in the axial direction, so long as they still essentially matched each other in shape and contour.