Abstract:
A screw vacuum pump ( 1 ) has shafts ( 7, 8 ) and rotors ( 3, 4 ) secured to the shafts. Each rotor has a central hollow chamber ( 31 ) which receives the shaft or contains in built-in components rotating along with the rotor to define a relatively thin annular slot segment ( 32 ) for cross-flow circulation of coolant. The annular slot segment is disposed between the built-in components and an inner wall of the rotor&#39;s hollow chamber and between a suction side and a delivery side of the rotor. Separate feed and discharge lines ( 41, 55, 63, 77, 78, 81, 89, 99, 101, 103 ) circulate the coolant through the annular slot segment in such a way that the coolant remains cavitation-free and bubble-free.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a screw vacuum pump comprising two shafts and two rotors secured to the shafts, whereby each rotor has a central hollow chamber provided with devices for guiding a coolant flow. 
     Screw vacuum pumps of this kind are known from the German patent applications 197 45 616, 197 48 385 and 198 00 825. Since these are operated in a dry manner (without coolant or lubricant in the pump chamber), there exists the problem of dissipating the heat produced during operation caused chiefly by compressing the pumped gases. 
     In the instance of the screw vacuum pump known from DE-A-197 45 616 with a cantilevered rotor, both rotor and shaft are equipped with a pocket hole which is open towards the bearing side. A central coolant pipe affixed to the casing extends into the pocket hole, said coolant pipe being guided out of the bore on the side of the bearing and opening out on the rotor side just in front of the end of the pocket hole. With the aid of a coolant pump, the coolant is pumped through the central pipe into the bore. It flows back via the annular chamber between the stationary coolant pipe and the rotating inner wall of the pocket hole. The diameter of the bore in both the rotor and the shaft is relatively small so that the surfaces over which the coolant flows are also small. Moreover, shearing forces occur between the pipe fixed to the housing and the rotating wall of the hollow chamber, producing in the coolant unwanted friction and thus an increase in the temperature. The effectiveness of the desired cooling facility for the rotors is limited for these reasons. 
     In DE-A-197 48 385 two cooling methods are disclosed. In the instance of a first solution each of the rotors is equipped with a hollow chamber open on one side, into which the coolant is injected. Owing to the rotation, a film forms on the inside of the rotor said film flowing back to the opening of the hollow chamber. In the instance of a film cooling arrangement of this kind there exists the danger of the film breaking down so that the desired cooling effect is interrupted. In addition, it is proposed to equip the hollow chamber in the rotor with conical sections so as to be able to increase the dwell time of the coolant in the hollow chamber of the rotor and thus also influence the amount of heat dissipated. In the instance of such conical sections which support pumping of the flow, however, there all the more exists the danger of the coolant flow breaking down. Finally film cooling arrangements have the general disadvantage, that a stationary flow profile forms, in which that part of the film which is close to the wall flows much more slowly compared to the section of the film further away from the wall. The area close to the wall thus practically forms an isolating layer hampering heat dissipation. For the purpose of removing this disadvantage it has already been proposed to provide obstructions in the flow so that the stationary flow profile is interrupted by turbulence. Thus an exchange of heat can be attained between the film sections close to the wall and those further away from the wall. However, installing obstructions is involved and in all slows down the velocity of the coolant flow. 
     DE-A-198 20 523 discloses a cooling system similar to the one detailed above. Cooled oil is injected into a hollow section of the shaft. Due to centrifugal forces the oil is displaced outwards to the inner wall of the hollow chamber extending conically in the direction of the discharge side. 
     In a further embodiment disclosed in DE-A-197 48 385, the coolant flows through an annular slot between rotor shaft and a bearing base extending into the rotor&#39;s hollow chamber. As to cooling of the rotor itself, a coolant flow of this kind has very little effect. 
     DE 198 00 825 discloses a screw pump with entirely hollow rotors. Coolant/lubricant is continuously supplied to, respectively discharged from the hollow chambers. As to the bearings which are arranged on the side where the coolant is discharged, a kinematic reversal has been implemented, i.e., they have a stationary inner ring and a rotating outer ring. The hollow chambers may be designed to be cone-shaped (widening in the direction of the flow) or may have an inner pumping thread. In the instance of such a screw pump there forms a coolant film which requires high rotational or circumferential velocities. The already mentioned danger of the coolant film breaking down exists. Monitoring facilities for an even spread of the coolant quantity to both rotors is, for this reason, absolutely recommended. Also, the necessity detailed of producing turbulence in the film exists, in order to attain an effective cooling effect. Finally equipping a rotor with a conical hollow chamber has several disadvantages: the conical hollow chamber is difficult to manufacture. In the instance of a cantilevered rotor on the delivery side and feeding in the coolant on the suction side of the rotor, the mass of the rotor in the area distant from the bearing is large. Correspondingly, when employing a cantilevered rotor on the delivery side the design needs to be involved. Finally, the fact that the coolant needs to be discharged relatively far out on the delivery side (at a great radial distance), limits the design options available. 
     DE-A-198 00 825 discloses a further embodiment, in which the rotors are each cantilevered on a shaft stub which extends into a hollow chamber in the rotor, said chamber being only open on the bearing side. The disadvantages detailed above also apply to this embodiment. 
     In the instance of all cooling systems detailed, there exists in addition the disadvantage that cooling is not performed in a counterflow. The coolant is in each instance supplied to the suction side of the rotor and not to the delivery side which is exposed to a significantly greater extent to the heat produced within the pump. 
     It is the task of the present invention to equip a pump of the aforementioned kind with an effective cooling arrangement permitting the pump to be manufactured in a simple, compact and cost-effective manner. 
     SUMMARY OF THE INVENTION 
     This task is solved through the characterising features of the patent claims. 
     It has been found that the cooling arrangement in accordance with the present invention in which the coolant flows through a relatively narrow, preferably cylindrical slot in the rotating system at sufficient velocity, has an unexpectedly good cooling effect, particularly since the annular slot can be arranged far to the outside, i.e. in the immediate vicinity of the root circle of the rotor&#39;s profile. Since the coolant is not injected, any hollow cavities which might interrupt the cooling effect are not present. Finally there exists the advantage that the direction for the coolant flow may be selected freely so that there is no obstacle as to cooling by way of a counterflow. This results in an equalisation of the temperature spread so that on the delivery side and the suction side narrow rotor/casing slots can be maintained. 
     As to the selection for the thickness of the annular slot it is relatively narrow, 0.2 to 5 mm for example, preferably 0.5 to 2 mm, whereby the thickness of the slot also depends on the coolant employed, the oil commonly used in vacuum pumps, for example. Apparently it is important that the distance between the two boundary films close to the walls be relatively small so that they will mutually influence each other in a turbulent manner. A laminar flow not influenced by the boundary layers keeping these separated and impairing the transfer of heat is apparently not present or is of negligible thickness. 
     In order to effectively cool the rotors, the velocity of the coolant (again depending on the type of coolant employed) must be sufficiently high. Flow velocities in the order of 0.1 to 1 m/s, preferably 0.3 to 0.7 m/s have been found to be expedient in the instance of cooling oil. With the known oil supply pumps, be they centrifugal, gear or similar pumps, the required pressure differences can be generated. 
     Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. 
     FIG. 1 is a sectional view through a screw vacuum pump with cantilevered rotors; 
     FIG. 2 is a sectional view through a screw vacuum pump with rotors having bearings at both sides; 
     FIGS. 3,  4 , and  5  are sectional views through each of two rotors of a screw vacuum pump in which the coolant is supplied to and discharged from the cooling slot by a central hollow chamber in the rotor shaft; 
     FIG. 6 is a sectional view of a rotor with a means for displacing the cooling slot to the outside; 
     FIGS. 7 a ,  7   b  and  8  illustrate an alternate embodiment in which a component is disposed in a hollow chamber of the rotor independent of shaft limits on the cooling slot; 
     FIG. 9 is an alternate embodiment with a two section rotor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The screw vacuum pump  1  depicted in FIG. 1 comprises pump chamber casing  2  with the rotors  3  and  4 . Inlet  5  and outlet  6  of the pump  1  are schematically marked by arrows. The rotors  3  and  4  are affixed on to the shafts  7  and  8  respectively, said shafts being each supported by a cantilevered manner by two bearings  11 ,  12  and  13 ,  14  respectively. One bearing pair  11 ,  13  is located in a bearing plate  15  which separates the pump chamber being free of lubricant from a gear chamber  16 . Located in casing  17  of the gear chamber  16  are the synchronising toothed wheels  18 ,  19  affixed to the shafts  7  and  8 , as well as a pair of toothed wheels  21 ,  22  serving the purpose of driving the pump  1 , where one toothed wheel is coupled to the shaft of the drive motor  23  arranged vertically besides the pump  1 . Moreover, the gear chamber has the function of an oil sump  20 . The second pair of bearings  12 ,  14 , of the shafts  7 ,  8  is located in bores  24 ,  25  said bores penetrating the bottom of the gear chamber housing  17 . The shafts  7 ,  8  in turn penetrate through bores  24 ,  25  and end in an oil containing chamber  26  being formed by casing  17  and a thereto affixed trough  27 . 
     From FIG. 1 it is apparent that the rotors  3  and  4  each have a hollow chamber  31  in which the shaft  8  extends and in which the cooling slot  32  is located. Since only rotor  4  is depicted by way of a partial section, the present embodiment is explained only with reference to this rotor  4 . In the embodiment according to FIG. 1, the annular slot section  32  is located directly between shaft  8  (resp.  7 ) and rotor  4  (resp.  3 ). To this end the cylindrical inner wall of the rotor containing the hollow chamber  31  is equipped in its middle area with a section  33  turned off on a lathe, the depth of which corresponds to the thickness of the cooling slot  32 . On the suction side and the delivery side, the shaft  8  rests flush against the inner wall of the hollow chamber  31 . In addition, the shaft  8  is equipped in these areas with grooves  35  and  36  for sealing rings  37  and  38  which ensure a leak tight separation of the cooling slot  31  from the pump chamber. 
     The cooling slot  32  is supplied with the coolant through the shaft  8 . It is equipped with a first bore  41  extending from the bottom end of the shaft  8  to the end of the cooling slot  32  on the delivery side. Via a cross bore  42  the bore  41  is linked to the cooling slot  32 . The coolant is supplied to the cooling slot  32  through bores  41  and  42 . The coolant flows through the cooling slot  32  from the delivery side to the suction side of the rotor  4 . Since most of the heat which needs to be dissipated is generated on the delivery side of the rotor  4 , the rotor  4  is cooled in a counterflow. 
     The coolant is evacuated through the second bore  43  in the shaft  8 . Said bore extends from the suction side of the cooling slot  32  up to the level of the gear chamber  16 . The cross bores  41 ,  45  provide in each instance the link between bore  43  with the cooling slot  32  respectively the gear chamber  16 . 
     Reliable cooling of the rotors  3 ,  4  is attained when the coolant is capable of flowing through the relatively narrow cooling slots  32  relatively quickly and undisturbed (free of cavitation and contamination). For this reason it is expedient to ensure, besides cooling and filtering of the coolant, a sufficient pumping force. In the design example in accordance with FIG. 1, therefore, the gear chamber  16 , resp. the oil sump  20  is linked to the chamber  26  through a line  51  in which there is located besides a cooler  52  and a filter  53 , an oil pump  54  which may be designed by way of a gear pump, for example. The oil pump  54  ensures that the coolant enters at the necessary pressure and free of cavitation from chamber  26  into the bore  41 . 
     Moreover, there exists the possibility of arranging oil pumps (centrifugal pumps, gear pumps) in the area of the bottom ends of the shafts  7 ,  8 . However, these need to be so designed that they are capable of meeting the requirements as to the desired pumping properties. 
     Depicted in FIG. 2 is an embodiment in which the shafts  7 ,  8  of the rotors  3 ,  4 , are supported by bearings on both sides, specifically at bearing plate  15  (bearing  11 ,  13 ) and in the pump chamber housing  2  (bearing  12 ,  14 ). The lower ends of the shafts  7 ,  8  end in gear chamber  16 . 
     Owing to the fact that the shafts  7 ,  8  are supported by bearings at both sides, there exists the possibility of supplying the coolant on the suction side in a simple manner. To this end the shafts are equipped on the suction side with a preferably cylindrical pocket hole  55  which extends up to the end of the cooling slot  32  at the suction side. 
     Via a cross bore  56  the bore  55  is linked to the cooling slot  32 . On the delivery side the shafts  7 ,  8  are equipped with a pocket hole  57  which extends up to the end of the slot  32  on the delivery side and which is linked to said slot via the cross bore  58 . 
     For the purpose of supplying the coolant to the pocket holes  55 , these are linked via line  51 , said line being connected to oil sump  20  whereby the line incorporates oil pump  54 , filter  53  and cooler  52 . In the instance detailed, the coolant flows through the cooling slot  32  from the suction side towards the delivery side. 
     In the design example according to FIG. 3 in which only the rotor  4  and the shaft  8  are depicted, further means of supplying to, respectively evacuating the coolant from the cooling slot are detailed. The shaft  8  is equipped with a central pocket hole being open on the delivery side and extending over the end of the cooling slot  32  at the suction side. Said pocket hole forms a hollow chamber  61  in which a guide component  62  for the coolant is located. The guide component  62  extends from the bottom end of the shaft  8  up to and past the end of the cooling slot  32  on the delivery side. The coolant is supplied via the longitudinal bore  63  in the guide component  62 , said bore being linked via truly aligned cross bores  64  through the component  62  and the shaft  8  to the end of the cooling slot  32  on the delivery side. At the level of the cooling slot  32  on the suction side, the shaft  8  is equipped with one or several cross bores  66  which open out into the chamber formed by the pocket hole  61  and the face side of the guide component  62 . Said chamber is linked via the longitudinal bore  68  and the truly aligned cross bores  69  (in the guide component  62  and in the shaft  8 ) to the gear chamber  16  (not depicted in FIG.  3 ). 
     Depicted in FIG. 4 is an embodiment in which the guide component  62  comprises three sections  71 ,  72 ,  73  which divide the hollow chamber  61  in the shaft  8  in to three partial chambers  74 ,  75 ,  76  which are each located at the level of the cross bores  69 ,  64  and  66  respectively. Through suitable bores in the sections  71  to  73  as well as line sections  77  and  78  linking said bores, separate supply and evacuation of the coolant may be implemented. The guide component may be fitted easily, since bores which need to be truly aligned are not present. Cooling in a counterflow can be implemented in a simple manner. 
     In the embodiment in accordance with FIG. 5, the coolant is supplied in contrast to the embodiments in accordance with FIGS. 3 and 4, through a central bore  81  in the guide component  62 . The coolant passes into the hollow chamber  61  formed by the pocket hole as well as the guide component  62  and through the cross bore  66  into the cooling slot  32 . The evacuation bores  64  are linked to lateral longitudinal grooves or an annular chamber  82  turned off on a lathe said annular chamber being located in the guide component  62 . The longitudinal grooves or the annular chamber  82  extend up to the level of the gear chamber  16  where they are linked to the cross bores  69 . 
     The embodiment in accordance with FIG. 6 differs from the embodiments detailed above in that a bore is provided fully penetrating the shaft  8  and the rotor  4 . For the formation of the hollow chamber  31 , a cover  85  is provided on the suction side, this cover being linked via a bolt  86  with the guide component  62 . The guide component  62  is firmly inserted from the suction side. Together with bolt  86  and the cover  85  it serves the purposes of axially affixing the rotor  4 . The shaft  8  is equipped with an outer sleeve  87  which together with the rotor  4  forms the cooling slot  32 . This slot extends substantially only at the level of the delivery side of the rotor  4 . Radially displacing the cooling slot  32  towards the outside improves the cooling effect. The coolant is only supplied through a relatively short section  88  turned off on a lathe said section being located in guide component  62 . Before it enters into the section  88  turned off on a lathe, it flows through bores  89 ,  90  in the bearing plate  15  as well as the chamber  92  on the bearing side of an axial face seal  93  where it ensures the formation of the necessary barrier pressure. The coolant is returned through the central bore  81  in the guide component  62 , resp. in the shaft  8 . 
     In the embodiment of FIGS. 7 a  and  7   b , the shaft  8  does not extend into the rotor&#39;s hollow chamber  31 . Said shaft is linked to the rotor  4  at the level of the delivery side. The guide component  62  in the rotor&#39;s hollow chamber  31  has a section  94  with an increased diameter which together with the inner wall of the rotor  4  forms the cooling slot  32 . A second section  95  having, compared to the section  94  a smaller diameter, penetrates the bore  61  in the shaft  8 . 
     For thermal reasons of permitting on the one hand the supply of the coolant from the open side of the bore  61  through a central bore  81  in the guide component  62  and on the other hand to permit cooling of the rotor  4  in a counterflow, it is required that the guide component  62  provides a crossing for the coolant flows. This is implemented through cross bores and outer groove sections in the guide component  62  which are designed as detailed in the following (see FIGS. 7 a ,  7   b  and  8 ): Coolant supplied centrally through the pocket hole  81  enters through a cross bore  98  into two groove sections  99  facing each other and then the coolant enters into the hollow chamber  31  (delivery side). Thereafter the coolant flows through the cooling slot  32  and enters through cross bores  66  into a line section  101  located centrally in the guide component. Said line section extends to a second cross bore  102  placed on the suction side with respect to the first cross bore  98 . The two cross bores  98  and  102  are arranged approximately perpendicular to each other. The cross bore  102  opens out into groove sections  103  facing each other, which are offset by about  90  degrees with respect to groove sections  99 . Thus it is possible to guide the returning coolant through these groove sections  103  to the cross bores  69  in the area of the gear chamber  16 . 
     In the design example in accordance with FIG. 9, the rotor  4  comprises two sections  4 ′ and  4 ″ having differently designed threads as well as each with a hollow chamber  31 ′ and  31 ″ respectively. The shaft  8  extends into the hollow chamber  31 ″ of the rotor section on the delivery side  4 ″ and thus forms the cooling slot  32 ″. The guide component  62  is similarly designed as in the embodiment in accordance with FIGS. 7,  8 . It has a section  94  with an increased diamter which is located in hollow chamber  31 ′ of the rotor section  4 ′ and which forms together with the inside wall of this rotor section  4 ′ the cooling slot  32 ′. A further section  95  of the guide component  62  having a smaller diameter penetrates the central bore  61  in shaft  8 . The guide component  62  is equipped with a central bore  81  extending to the suction side of the rotor  4 . 
     For simplicity and better overview, an embodiment is presented in which the coolant is supplied through the central bore  81  and where the coolant flows through lateral bores  64 ′ in section  94  on the suction side into the cooling slot  32 ′. Through a section  66 ′,  105  turned off on a lathe (or also through longitudinal grooves) as well as cross bores  64 ′ the end of the cooling slot  32 ′ on the delivery side is linked to the end of the cooling slot  32 ″ on the suction side so that the coolant passes sequentially through the two cooling slots  32 ′,  32 ″. Through a further section  106  turned off on a lathe, the evacuation opening  66 ″ on the delivery side of the cooling slot  32 ″ is linked to the evacuation opening  69  at the level fo the gear chamber  16 . Also in the instance of this solution there exists the possibility of also employing the guide component  62  as a tie rod, specifically for affixing the rotor section  4 ′. 
     Of course there also exists the possibility in the instance of the embodiment in accordance with FIG. 9 of designing the supply and evacuation lines for the coolant in such a manner that the cooling slots  32 ′,  32 ″ are supplied separately and/or in a counterflow. 
     The embodiments of FIGS. 7 to  9  are of particular advantage when the rotors  3 ,  4  are cantilevered, since then there exists the possibility of manufacturing the guide component  62  of light materials like plastic, for example. Thus the mass of the rotors far from the bearing can be kept small. The usage of plastic or similar materials also offers the general advantage that there are located between the in flowing and the outflowing coolant materials and do not conduct heat very well. 
     The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.