Abstract:
A vacuum pump of the rotary vane type, comprises a casing ( 50 ) having a cylindrical inner wall surface ( 52 ), a first ( 54; 54′ ) and a second ( 56; 56′ ) end wall at opposite sides of said casing defining a fluid cavity therein, fluid inlet ( 60 ) and outlet ( 62 ) ports in open communication with said fluid cavity, and a rotor ( 64; 64′ ) extending between said end walls carried by a drive shaft ( 70 ) for rotation about an axis eccentric to said casing inner wall surface, said rotor being provided with a plurality of longitudinally extending radial slots ( 66 ) about the periphery thereof. Further, there are provided a plurality of vanes ( 68 ), each being radially slidably carried within a respective of said slots. The invention comprises that at least one of said end walls and said rotor comprise, at oppositely facing surfaces, an annular recess ( 84, 86; 84′, 84″, 86′, 86″ ) and an annular rib ( 88, 90; 88′, 88″, 90′, 90″ ), respectively, said rib and recess being interengaging so as to define a radial clearance ( 92, 96; 92′, 96′ ) and an axial seal ( 94, 98; 94′, 98′ ), respectively, between said at least one of said end walls and said rotor, and that the rotor/drive shaft combination is axially biased.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to vacuum pumps, and more specifically to the kind of device in which a plurality of vanes are fitted to slide substantially radially in a respective slot of a rotor eccentrically mounted within a casing.  
         DESCRIPTION OF RELATED ART AND BACKGROUND OF THE INVENTION  
         [0002]    A previously known vacuum pump of such kind is illustrated in FIGS. 1 a - e.  The pump includes a cylindrical-shaped casing or housing  10  which has an inner cylindrical wall surface  12  and is closed at its opposite ends by end walls  14 ,  16  such as by means of machine screws  18  or the like. As shown, the pump includes circumferentially spaced fluid input  20  and output  22  ports intercommunicating the interior cavity. Output  22  is preferably held at atmospheric pressure, while input  20  is held at a vacuum of about 50 kPa during operation.  
           [0003]    The rotor  24  of the pump is provided with a number of elongated vane slots  26  cut therein from the circumference thereof; and wherein a plurality of vanes  28  are mounted in freely slidable relation within these slots. A pump drive shaft  30 , provided with an axle spindle  32  for coupling, is keyed to the rotor  24  and is rotatably mounted in the end walls  14 ,  16  as by means of bearings  32 ,  34 . The rotor  24  is eccentrically mounted relative to the cylindrical inner wall  12  of the casing  10 . Accordingly, for efficient operation of a pump of this type, as the rotor turns within the casing it is required for the outboard edges of the vanes  28  to be in pressure-sealing contact with the inner surface  12  of the casing  10  while sliding in slots  26  back and forth; and that pressure losses around the longitudinal ends of vanes  28  and rotor  24  permitting escape of fluid to the exhaust, must also be prevented.  
           [0004]    To such end, the pump comprises radial seals  35 ,  36  between the rotor  24  and the end walls  14 ,  16 , respectively, and also between the vanes  28  and the end walls  14 ,  16 . The rotor is not axially locked, but is freely movable between the end walls, in order not to exhibit unacceptable losses caused by e.g. axial slackness of the ball bearings and manufacturing tolerances of the pump components. Due to such freely movable mounting, however, the pump is very sensitive to axial forces and in unfortunate situations such forces may lead to seizing of the pump. Additionally, such radial seals need large amounts of evenly distributed lubrication in order to work satisfactorily and very precise clearances  38 ,  40  of the seals  35  and  36 , respectively, have to be provided and maintained irrespective of variations in the temperature of the pump. This may be hard to fulfill due to different length expansions of casing  10  and rotor  24 .  
           [0005]    The latter problem has been addressed in the art. For instance, U.S. Pat. No. 2,312,655 issued to LAUCK discloses a rotary impeller type of vacuum pump, which provides for a precise clearance between the walls and the adjacent impeller assembly irrespective of the materials of the housing and of the impeller assembly. The pump includes the main housing of a light weight material, the impeller assembly of a heavier material, and an intermediate housing assembly, being composed of a thin sleeve member of a material having substantially the same characteristic temperature expansion as the heavier material of the impeller assembly, an axially adjustable end plate, and a plurality of coil springs. The thin sleeve member is arranged between the main housing and the impeller assembly and has a length slightly greater than the overall coaxial dimension of the impeller assembly by an amount exactly equal to the desired total clearance to be provided. The end plate is arranged to engage at the periphery thereof with the end of the sleeve member and urging the same into such engagement by means of the plurality of coil springs. In such manner the initially provided clearance is maintained irrespective of the differential temperature expansion between the housing and the impeller assembly.  
           [0006]    U.S. Pat. No. 2,098,652 issued to BUCKBE discloses a similar type of vacuum pump provided with annular members arranged in spaces provided between the rotor-vane combination and the casing heads of the pump. These annular members are maintained pressed against the end surfaces of the rotor-vane combination by means of directing a suitable pressure fluid against the annular members, preferably between annular recesses of the annular members and the casing heads, such that they are forced to rotate with the rotating rotor-vane combination. The longitudinal dimensions are set such that there will always be a clearance between the rotating parts and the casing heads. Further, the annular members and the casing heads are provided with a number of interengaging annular ribs as a further means of preventing internal leakage.  
           [0007]    However, such vacuum pumps comprise additional parts, which make them more complicated and costly to fabricate. Further, the former pump needs provision of a plurality of coil springs, and it does not provide for maintenance of the radial clearance if there are spatial temperature gradients, such as if the impeller was to be more heated than the sleeve member. The latter pump needs the provision of a pressure fluid and seals to prevent such pressurized fluid from leaking into the low pressure pump chamber. Additionally, there are extensive frictional movements between the vanes and the annular members, as these members are pressed against the vanes, while the vanes are sliding substantially radially within their respective slots continuously.  
           [0008]    Further, U.S. Pat. No. 4,397,620 issued to INAGAKI et al. discloses a rotary compressor including disc-shaped members having a diameter slightly smaller than that of a rotor each disposed on opposite ends of the rotor and supported on the same rotary shaft as the rotor for rotation, and two disc-shaped recesses each formed on one of inner opposite end surfaces of a housing for receiving therein one of the rotary disc-shaped members. A small gap is formed between the inner end surfaces of the housing and the end surfaces of the rotor, and small gaps are formed between surfaces of the rotary disc-shaped members and surfaces of the disc-shaped recesses.  
           [0009]    However, such pump is not suitable to be used with a coupling, which generates axial forces since the pump then may seize. Further, the pump may be noisy and the bearings used may be exposed to stress, and thus have a short lifetime. Also, it is doubtful if the pump may withstand its own weight, and maintain the radial gaps if mounted on a support which is not horizontal.  
         SUMMARY OF THE INVENTION  
         [0010]    It is an object of the present invention to provide a vacuum pump of the rotary vane type, which is in lack of the problems discussed above in connection with vacuum pumps of the prior art.  
           [0011]    It is yet a further object of the invention to provide such a vacuum pump that is efficient, simple, reliable, of low cost, and easy to manufacture.  
           [0012]    It is still a further object of the invention to provide such a vacuum pump that allows for axial biasing of the rotor.  
           [0013]    These objects among others are, according to the present invention, attained by vacuum pumps as claimed in the appended claims.  
           [0014]    By providing the rotor and the end walls at oppositely facing surfaces, by annular recesses and annular ribs, respectively, wherein the ribs and the recesses are interengaging so as to define radial clearances and axial seals, respectively, between the end walls and the rotor, a pump is obtained, which provides for a clearance between the rotor and end walls irrespective of the materials thereof or any temperature gradients, while the pump is simple and reliable and has very few movable parts. Very same end walls may be used in a large variety of pumps having different pump capacities.  
           [0015]    The rotor and the end walls may be provided with a plurality of annular recesses and ribs, respectively, such that axial labyrinth seals between the end walls the said rotor are obtained. In such manner any leakages occurring, are further reduced.  
           [0016]    By axially biasing the rotor/drive shaft combination of the vacuum pump, preferably by means of axial stops provided in the end walls and a loaded spring, e.g. a cup spring, mounted between the rotor and the axial stops, a vacuum pump, which is insensitive to axial forces is obtained. In such instance, a plurality of different transmission systems or gearboxes may be used with the vacuum pump. Further, an axially biased pump is easier to manufacture, and the pump may be mounted upon a support, which is not horizontal.  
           [0017]    Bearings, such as ball bearings, in which the rotor/drive shaft combination may be mounted at the end walls would have a longer lifetime, be less noisy and cause less vibrations, when being axially biased. Further, the radial and axial plays of the bearings would not affect the sealing properties of the inventive vacuum pump.  
           [0018]    Further, by providing the end walls with a respective inner annular rib for axially guiding the vanes when sliding substantially radially within the slots of the rotor, it is prevented that vanes may move sideways and get stuck at the inner corners of the end walls. Additionally, each of the inner annular ribs may be provided with a respective through hole for lubrication of the vanes.  
           [0019]    By providing a rotor wherein the longitudinally extending radial slots are at least partly, or completely, radially sealed at the longitudinal ends thereof, the internal leakage is even further reduced. Hereby, the casing and the end wall located at the motor side, may be an integrated single part.  
           [0020]    Further characteristics of the invention and advantages thereof will be evident from the following detailed description of embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The present invention will become more fully understood from the detailed description of embodiments of the present invention given hereinbelow and the accompanying FIGS.  1 - 3 , which are given by way of illustration only, and thus are not limitative of the present invention.  
         [0022]    [0022]FIG. 1 a  is a front elevation view of a vacuum pump of the rotary vane type according to prior art.  
         [0023]    [0023]FIG. 1 b  is a sectional view along the line A-A of FIG. 1 a.    
         [0024]    [0024]FIG. 1 c  is a radial cross sectional view of the vacuum pump of FIG. 1 a.    
         [0025]    [0025]FIG. 1 d  displays, in a perspective view, a rotor as being comprised in the vacuum pump of FIG. 1 a.    
         [0026]    [0026]FIG. 1 e  displays, in a perspective view, a casing end wall as being comprised in the vacuum pump of FIG. 1 a.    
         [0027]    [0027]FIG. 2 a  is a front elevation view of a vacuum pump of the rotary vane type when its front-end wall is demounted according to a first embodiment of the present invention.  
         [0028]    [0028]FIG. 2 b  is a sectional view along the line B-B of FIG. 2 a.    
         [0029]    [0029]FIG. 2 c  is a radial cross sectional view of the vacuum pump embodiment of FIG. 2 a.    
         [0030]    [0030]FIG. 2 d  displays, in a perspective view, an inventive rotor as being comprised in the vacuum pump embodiment of FIG. 2 a.    
         [0031]    [0031]FIG. 2 e  displays, in a perspective view, an inventive casing end wall as being comprised in the vacuum pump embodiment of FIG. 2 a.    
         [0032]    [0032]FIG. 3 a  is a front elevation view of a vacuum pump of the rotary vane type when its front-end wall is demounted according to a second embodiment of the present invention.  
         [0033]    [0033]FIG. 3 b  is a sectional view along the line C-C of FIG. 3 a,  in which also fragmentary enlarged scale views of encircled portions are shown.  
         [0034]    [0034]FIG. 3 c  is a radial cross sectional view of the vacuum pump embodiment of FIG. 3 a.    
         [0035]    [0035]FIG. 3 d  displays, in a perspective view, an inventive rotor as being comprised in the vacuum pump embodiment of FIG. 3 a.    
         [0036]    [0036]FIG. 3 e  displays, in a perspective view, an inventive casing end wall, and also a fragmentary enlarged scale view of an encircled portion thereof, as being comprised in the vacuum pump embodiment of FIG. 3 a.   
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0037]    In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular techniques and applications in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and apparatuses are omitted so as not to obscure the description of the present invention with unnecessary details.  
         [0038]    The vacuum pump of the present invention is primarily intended to be used with equipment such as an automatic milking machine and other equipment present at a dairy farm. Nevertheless, the pump may be suitable for use in other fields, and as far as the present invention concerns there is no limitation whatsoever as to where the pump may find applications.  
         [0039]    With reference to FIGS. 2 a - e  a first exemplary embodiment of the vacuum pump of the present invention will be described.  
         [0040]    The pump includes a cylindrical-shaped casing or casing  50 , which has an inner cylindrical wall surface  52  and is closed at its opposite ends by end walls  54 ,  56  such as by means of machine screws  58  or the like, being received in holes  59  of end wall  54  and similar holes in the longitudinal end of casing  50 . Similarly, end wall  56  is mounted to the opposite end of casing  50 . As shown, end wall  56  is integrated in a larger detail  57  referred to as a motor axle casing to be mounted to a motor casing housing a motor for driving the pump. Further, casing  50  includes circumferentially spaced apart fluid inlet  60  and outlet  62  ports intercommunicating the interior cavity of the pump.  
         [0041]    The rotor  64  of the machine is provided with a number of elongated vane slots  66  cut therein on the radius thereof, and within these slots are mounted in freely slidable relation therein a plurality of vanes  68 . The pump drive shaft  70  is press-fitted into the rotor  64  (or otherwise keyed thereto) and is rotatably mounted in the end walls  54 ,  56  as by means of bearings  72 ,  74 . In an alternative version the rotor and the pump drive shaft are fabricated as a single unit. The bearings are preferably slide fitted to the end walls  54 ,  56 , and interference fitted to the rotor/drive shaft combination  64 ,  70 .  
         [0042]    The rotor  64  is concentrically mounted and positioned with respect to the axis of the drive shaft  70  as shown in FIG. 2d, but the shaft  70  is eccentrically mounted relative to the cylindrical inner wall  52  of the casing  50 . Accordingly, it will be understood that for efficient operation of a machine of this type, as the rotor turns within the casing it is required for the outboard edges of the vanes  68  to be at all times in pressure-sealing contact with the inner surface  52  of the casing  50  while reciprocatively sliding in the slots  66 ; and that pressure losses around the ends of the vanes permitting escape of fluid to the exhaust, has also to be prevented.  
         [0043]    To attain the aforesaid objectives, end walls  54 ,  56  are provided with annular recesses  84 ,  86  and the rotor  64  is provided with annular ribs  88 ,  90  at its respective end faces. Recess  84  and rib  88  are interengaging so as to define a radial clearance  92  and an axial seal  94 , respectively, between end wall  54  and rotor  64 . Similarly, recess  86  and rib  90  are interengaging so as to define a radial clearance  96  and an axial seal  98 , respectively, between end wall  56  and rotor  64 . It shall be appreciated in this respect that a radial clearance signifies a play between the rotor and the end walls, said play extending in the radial direction. Correspondingly, an axial seal signifies a thin slit or a gap between the rotor and the end walls, said thin slit or gap extending in the axial direction and operating as a seal between said parts.  
         [0044]    The rotor/drive shaft combination  64 ,  70  (joined in fixed relation or fabricated as a single piece) is axially biased by means of axial stops  110 ,  102 , respectively, provided in the end walls  54 ,  56  and a loaded spring, preferably a cup spring  104 , mounted between rotor  64 , or more precisely one of the bearings  74 , and the axial stop  102  of end wall  56 . In such manner the thermal expansion of rotor  64  is balanced by means of spring  104  in the direction of end wall  56  (i.e. on the motor side). Such axial biasing is very advantageous since it allows for the use of a coupling (not illustrated), which generates axial forces. Preferably then, the drive shaft  70  is provided with an axle spindle, to which the coupling is mounted, and via which the motor can drive the rotor/drive shaft combination  64 ,  70 . Further, the use of axial biasing of the rotor/drive shaft combination  64 ,  70  provides for a more silent-running pump with a longer lifetime.  
         [0045]    End walls  54 ,  56  comprise a respective inner annular rib or ring  106 ,  108  for axially guiding the vanes  68  when sliding substantially radially within said slots. This guiding rib guides the vanes from their innermost position (e.g. at startup) towards their outermost position without allowing them to move sideways and thus to possibly get stuck in the end walls  54 ,  56 . Annular ribs or rings  106 ,  108  may further be provided with a respective through hole (not illustrated) for lubrication of the vanes.  
         [0046]    The longitudinally extending radial slots  66  are in this embodiment preferably extending along the complete longitudinal extension of said rotor. The vanes  68  extend along the entire casing  50  and in this respect, an essentially radial sealing between vanes  68  and end walls  54 ,  56  is provided as in the prior art device of FIG. 1. However, vanes  68  are preferably made of a plastic or other low friction material, such that very small clearances between vanes  68  and end walls  54 ,  56  can be employed. The need of lubrication of the vanes may in such instances be dispensed with. Further, the material of vanes  68  is preferably chosen such that the thermal expansion of vanes  68  and of casing  50 , respectively, are comparable. Further, vanes  68  are easily exchangeable simply by demounting end wall  54 , drawing the vanes axially out of their respective slots, inserting new vanes, and finally remounting end wall  54 .  
         [0047]    Further notably, slots  66  are arranged not entirely radially, but parallelly translated therefrom, to be oriented in a radial-tangential direction. Such design is intended to be included in the expression “substantially radially” as used within the present patent application. Accordingly, vanes  68  are sliding in a substantially radial direction.  
         [0048]    Advantages of this particular embodiment of the invention comprise:  
         [0049]    An axial sealing is not working as a sliding bearing, which indicates that no lubrication is needed between rotor and end walls.  
         [0050]    The location for lubrication of the vanes may be freely selected. Hence, the material of the vanes as well as the type of lubrication may be more freely selected. Possibly, the pump may be driven entirely without lubrication.  
         [0051]    The critical thermal expansion is now related to the diameter of the rotor and not to the length thereof. Thus, there are possibilities to manufacture pumps of longer lengths. Further, very same end walls may be used for both short and long vacuum pumps. Different material combinations for the casing, rotor, and end walls may be used with the risk of seizing reduced to a minimum.  
         [0052]    The axial biasing of the rotor/drive shaft combination enables the use of a coupling, which generates axial forces.  
         [0053]    The manufacturing will be easier due to less stringent tolerances.  
         [0054]    The pump may be located on a surface, which is inclined with respect to the horizontal plane.  
         [0055]    The axial biasing of the rotor/drive shaft combination will result in longer lifetimes of the ball bearings.  
         [0056]    Further, the bearings will cause less noise and less vibrations. The kind of bearings is more freely chosable and any radial and/or axial play of the bearings does not affect the sealing between the rotor and the end walls.  
         [0057]    In FIGS. 3 a - e  a second exemplary embodiment of the present invention is shown. This second embodiment is similar to said second embodiment and all identical parts and features of the two embodiments are given identical reference numerals in the Figures. However, the second embodiment is differing from the first embodiment as regards the following.  
         [0058]    End walls  54 ′ and  56 ′ are provided with respective first and second annular recesses  84 ′,  84 ″ and  86 ′,  86 ″, and rotor  64 ′ is provided with respective first and second annular ribs  88 ′,  88 ″ and  90 ′,  90 ″ at each of its longitudinal end faces. Thus, annular recesses  84 ′,  84 ″ and  86 ′,  86 ″ and ribs  88 ′,  88 ″ and  90 ′,  90 ″ are interengaging so as to define radial clearances  92 ′,  96 ′ and a plurality of axial seals  94 ′  98 ′, respectively, between end walls  54 ′,  56 ′ and rotor  64 ′. Thus, axial labyrinth seals are provided, which may further reduce the internal leakages of the pump.  
         [0059]    End wall  56 ′ is as in previous embodiment integrated in a motor axle casing  57 ′.  
         [0060]    Annular ribs or rings  106 ′,  108 ′ as defined between respective annular recesses  84 ′,  84 ″ and  86 ′,  86 ″ are adapted to guide the vanes  68  axially when sliding substantially radially within the slots. Annular ribs or rings  106 ′,  108 ′ are further provided with a respective through hole (only through hole  110  in rib  106 ′ is illustrated, FIG. 3 e ) for lubrication of the vanes. Preferably, vanes  68 , fluid inlet port  60 , and through hole  110  for lubrication, are arranged circumferentially such that there are, at all times during operation, at least one of the vanes  68  located between fluid inlet port  60  and the through hole  110  for lubrication. Thus, as through hole  110  never will be in open communication with inlet port  60  the internal leakages are further reduced.  
         [0061]    Furthermore, the longitudinally extending radial slots  66  are at least partly, but preferably completely, radially sealed  112  at the longitudinal ends thereof, e.g. by means of sealing rings  114 ,  116  attached to the body of rotor  64 ′ by means of screws  118  or other fastening means. Such sealing rings may extend along the entire radial extension of slots  66  as illustrated, or they may extend only partly along the radial extension of slots  66 . Alternatively, the rotor  64 ′ is made as a single piece with integrated radial seals.  
         [0062]    Particular advantages of this latter embodiment comprise:  
         [0063]    The internal leakage is further reduced.  
         [0064]    A larger play between end walls and vanes may thus be acceptable, which facilitates the choice of vane material.  
         [0065]    A larger “smallest distance” between the eccentrically arranged rotor  64 ′ and the inner surface  52  of casing  50  may be acceptable. This would make it possible to manufacture end wall/motor axle casing  56 ′,  57 ′ and casing  50  integrated in a single piece.  
         [0066]    Simpler manufacturing and logistics if tolerances are higher, fewer pieces are to be manufactured.  
         [0067]    Simpler mounting if fewer pieces (integrated casing/end wall) are to be mounted.  
         [0068]    No need of uniquely fastening end walls to casing by pins; the end walls are thus exchangeable.  
         [0069]    Simple and even lubrication of the vanes, if at all necessary, through holes  110  provided in annular end wall ribs  106 ′,  108 ′.  
         [0070]    It will be obvious that the invention may be varied in a plurality of ways. Such variations are not to be regarded as a departure from the scope of the invention. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims.