BACKGROUND OF THE INVENTION 
     The present invention relates to a cooled screw vacuum pump comprising two rotating systems, consisting each of a screw rotor and a shaft with a floating device supporting the rotors, having, on each shaft, two mutually spaced bearings and an empty space arranged in each rotor, open on the bearing side, wherein is located an element cooling the rotor internally. 
     In an already proposed screw vacuum pump of this type, the bearing of the floating support on the rotor side is located within a central hollow space, open toward the bearing side, inside the rotor. Cooling is effected with the aid of a lubricating oil, which is first passed, inside a central channel in the shaft, to the bearing on the side of the rotor. In known fashion, the transported oil volume is larger than is needed for lubrication of the bearing in order to be able to carry away the maximum amount of heat possible. 
     With respect to the screw vacuum pump, the oil volume which, according to the state of the art, can be passed through the empty space, is limited since it is not only the bearing but also the bearing support that must be accommodated in said empty space. Therefore, there is the risk of inadequate cooling on the pressure-side region of the screw vacuum pump since it is precisely in this region that the generated heat is greatest due to the executed compression work. 
     Because of the existing empty space inside the rotor, the wall thickness of the rotor is also limited in the bearing region of the empty space. As a result, it is only possible at very high temperature gradients, to carry off the heat developing in the pressure-side region of the screw threads via the suction side region of the rotor, the shaft and the cooling oil. High temperature or inadequate cooling of the pressure-side region of a screw vacuum pump results in uneven rotor expansions and thus in local clearance consumption between the rotors and between each of the rotors and the housing. Run-up of rotors may, in fact, be prevented by relatively large clearances. 
     Relatively large clearances, however, result in deterioration of the pump operating properties. Furthermore, with respect to the prior known screw vacuum pump, there exists the danger of overheating the bearing located in the empty space, all the more so since said bearing can only be lubricated with relatively warm oil. Finally, the prior known screw vacuum pump can only be operated with vertically arranged shafts. 
     The present invention is based on the object of equipping a screw vacuum pump of the initially mentioned kind with improved cooling means. 
     According to the invention, this object is solved by making use of the fact that the bearing on the rotor side of the support is located outside the empty space in the rotor. The invention facilitates effective cooling of the rotor from the inside without being impeded by the bearing and bearing support, so that the unwelcome clearance consumption will no longer occur in this critical region. 
     Each rotor appropriately consists of two segments with different thread profiles, whereby the thread depth of the pressure-side segment is smaller than the thread depth of the suction side segment. A lesser thread depth in the pressure-side segment provides more space for accommodation of the empty space needed for the internal cooling. 
     If, in addition, the rotor and housing are stepped in such manner that the pressure-side rotor segment has a smaller diameter than the suction-side rotor segment, then this measure creates more space in the housing for the accommodation of jacket cooling. 
     According to another characteristic of the invention, it is appropriate to additionally provide in the wall of the pump housing, i.e at least at rotor level, channels perfused by a cooling agent. 
     A cooling agent of this type permits, specifically in combination with the interior cooling of the rotor according to the invention, uniform tempering of the entire pump. Consequently, the pump is able to adopt variable temperatures with variable loads, without resulting in gap reductions. It is appropriate to also include in such tempering the bearings, the bearing supports and the driving motor, in order to prevent problems due to variable temperature expansions. Lastly, a jacket cooling of the proposed type has the benefit of having the effect of excellent sound deadening. 
    
    
     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 section through a screw vacuum pump with cooling according to the invention; and 
     FIG. 2 is a partial section according to FIG. 1 with an additional design for cooling according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a section through an exemplary embodiment of a screw vacuum pump  1  according to the invention is depicted, i.e. at the level of that of the two rotating system which is equipped with a driving motor  2 . Synchronization of the two rotating systems is effected with the aid of toothed wheels  3 . 
     The rotating systems, which are arranged in housing  4 , each comprise a rotor  5  and a shaft  6 . Each rotor  5  is overhung, in other words, unilaterally supported. The shaft  6  supports itself in a housing  4  via bearings  7  and  8  and also bearing supports  11  and  12 . Frontally, housing lids  13  and  14  are provided, with lid  13  on the rotor side being equipped with an inlet stub  15 . Bearing support  12  is a component of the gear-side lid  14 . 
     The rotor  5  consists of two positively joined rotor segments  17  and  18  having different profiles  19  and  20 . The suction-side rotor segment  17  has a large volume profile  19  in order to achieve high volume flows in a helical compression chamber. The pressure-side segment  18  of rotor  5  has both a reduced profile volume as well as a lesser diameter. This reduces the cross section of the helical compression chambers or pumping chambers  49 . Internal compression is obtained, and the work done on compression is reduced. 
     The inner wall of housing  4  is adapted to the rotor gradation (Gradation  21 ). A dotted line  22  indicates that the housing may be designed divisible at the level of gradation  21 . As a result, it is possible to replace the suction-side rotor segment  17  and the suction side element  4 ′ of housing  4  with rotor segments having different profiles, lengths and/or diameters as well as having housing segments  4 ′ adapted to same, in order to be able to adapt the pump to different applications. 
     The outlet of pump  1  which is adjacent to the pressure-side end of the thread turns is identified by the numeral  24 . It is laterally conducted outward. A housing bore  25  also issues into the outlet, joining the compression chamber with the outlet at the level at which its cross-section decreases-either by gradation or by change in the thread profile. In the housing bore  25 , there is a non-return valve  26  which opens with excessive pressure in the compression chamber and short-circuits the suction-side thread turns of the rotor segment  17  with the outlet  24 . In order to seal the helical compression chambers from the support, shaft gaskets  27  are provided which are located between bearing  7  and the rotor segment  18 . 
     The cooling system in the depicted exemplary embodiment comprises a rotor with interior cooling arrangement and a housing jacket to facilitate cooling. 
     For realization of the rotor interior cooling, the rotor  5  is equipped with a hollow space  31 , open toward its bearing-side. Said hollow space may extend through almost the entire rotor  5 . 
     With respect to rotor  5 , consisting of two segments  17  and  18 , the delivery or pressure-side segment  18  is appropriately designed hollow. The suction-side segment  17  closes the suction-side end of the hollow space  31 . The shaft  6 , which is appropriately designed in one single piece with rotor  5  or with the pressure-side segment  18  of rotor  5 , is likewise hollow (hollow space  32 ). In the hollow spaces  31 ,  32  there is a central cooling pipe  33 , which is conducted, on the side of the bearing, out of the shaft  6  and ends, on the side of the rotor, shortly before the suction-side end of hollow space  31 . The cooling pipe  33  and the annular space formed by the cooling pipe  33  and the hollow shaft  6  are available for the supply or removal of a coolant. 
     In the represented exemplary embodiment of the present invention, the bearingside opening  34  of the cooling pipe  33  is in communication via line  35  with the outlet of a cooling agent pump  36 . In addition, in the region of housing lid  14  there is a coolant sump  37  in a coolant chamber  50 . Coolant sump  37  is connected via line system  38  with the inlet of cooling agent pump  36 . The sump  37  and the line system  38  are designed in such manner that the represented pump  1  can be operated in any position ranging from vertical to horizontal. Cooling agent levels which occur with horizontal and with vertical position of the pump  1  are indicated. Depending upon whether the cooling agent pump  36  is located outside (as depicted) or inside (for example on the second, not visible shaft of pump  1  at the level of the driving motor  2 ) of housing  4 , the opening  34  of the cooling pipe  33  is located either outside or inside of housing  4 . 
     For operation of the internal cooling of rotor  5 , the cooling agent is transported by the cooling agent pump  36  from the cooling agent sump  37  via the cooling pipe&#39;s inner surface or first channel  47  into the empty space  31  in rotor  5 . From there, it flows back into sump  37  via the annular space or second channel  48  between cooling pipe  33  and shaft  6 . The hollow space  31  is located at the level of the pressure-side region of the thread turns of pump  1 , so that this region in particular is cooled effectively. The cooling agent flowing back outside of the cooling pipe  33  along the second channel  48  tempers, among others, the hollow shaft  6 , the bearings  7  and  8 , the driving motor  2  (on the armature side), and the toothed wheels  3 , so that the thermal expansion problems are reduced. 
     It is advisable for the cross section of the second channel  48  between the cooling pipe  33  and the shaft  6  to decrease at the pressure end; this can be done, for example, by providing the cooling pipe  33  with a larger outside diameter in this area, As a result, a constructed pass-through opening or narrowed region  39  is formed. This constriction ensures that the spaces holding the coolant are completely filled. 
     It is advisable to select a material with poor thermal conductivity (such as plastic/special steel, etc.) for the cooling pipe  33 . As a result, the rotor  5  will be cooled more effectively, and the components of the pump  1  near the shaft will be tempered more uniformly. 
     The housing cooling system shown comprises cavities or a first and a second set of channels  41 ,  42 , respectively, in the housing  4 . The first set of cooling channels provided in the area of the rotor  5  are designated  41 ; the second set of cooling channels in the area of the motor  2  are designated  42 . 
     One of the jobs of the cooling channels  41  in the area of the rotor  5  is to carry away the heat which develops especially on the pressure side of the rotor  5 . Another job of the channels is to temper the housing  4  as uniformly as possible in the entire area of the rotor. Finally, the channels are designed to give up the absorbed heat to the outside. The channels  41  through which the coolant flows therefore extend along the entire length of the rotor  5 . The housing lid  13  serves to seal off the channels  41  on the suction side. The housing  4  is also cooled effectively on the pressure side. 
     Cooling channels  42 , located at the level of the driving motor  2 , have the mentioned objects as well. They produce tempering of the driving motors (on the side of the coils) as well as tempering of the bearing support  11 . Finally, they increase, to a significant extent, the thermal discharge via the exterior surfaces of pump  1 . The pump is appropriately equipped with fins  44 , at least at the level of the cooling channels  41  and  42 . 
     Feeding the cooling channels  41 ,  42  with cooling agent is likewise done with the aid of the cooling agent pump  36 , namely via lines  45  and  46 , if they are to be perfused parallel. Depending upon the thermal requirements, there also exists the possibility of subsequently providing same with cooling agent. One of the lines  45  or  46  could then be eliminated. The cooling agent gets from hollow spaces  41 ,  42  back into the sump  37  via bores which are not represented in detail. 
     With vertical arrangement of shaft  6 , the cooling agent located in the sump cools the bearing support  12 , protruding into the sump  37 . With horizontal arrangement, it is appropriate to let the returning cooling agent flow back over the internal side of lid  14 , in order to cool both the bearing seat  12  as well as improve thermal discharge toward the outside. 
     In the depicted exemplary embodiment of the present invention according to FIG. 1, housing  4  and rotor  5  are—as already mentioned—designed partable at the level of line  22 . Consequently, there exists the possibility of replacing the suction-side segments of rotor  5  (segment  17 ) and housing  4  (segment  4 ′). Pump  1  can be adapted to various applications by installing rotor segments  17  with different profiles  19 , different length, different pitch and/or different diameter, combined in each case with an adapted housing segment. Various large profiles can be selected on the suction side in order to obtain high suction capacities, various long profiles on the suction side in order to obtain low end pressures and/or various volume gradations in order to obtain, for example, higher fluid compatibility with lower gradation or with higher gradation, high suction capacity with relatively small power consumption. Finally, there exists the possibility of providing, at the level of a reduction in the diameter of rotor  5 , a circumferential groove in order to achieve, in certain applications, a release of pressure in this region. 
     A cooling agent flowing through the screw vacuum pump  1  may be water, oil (mineral oil, PTFE-oil or similar) or another liquid. The utilization of oil is appropriate in order to also lubricate the bearings  7  and  8  and the toothed wheels  3 . Separate supply of cooling agent and lubricating agent, as well as corresponding gaskets, can thereby be eliminated. The only need being a controlled supply of oil to the bearings  7  and  8 . 
     The described solutions permit beneficial selection of raw material. For example, the rotors  5  and the housing  4  may consist of relatively inexpensive aluminum materials. The proposed cooling and, most importantly, the uniform cooling of pump  1  have the effect that, even with variable operating temperatures and relatively small gaps, play does not consume local clearance which will result in rotor to rotor contact and/or rotor to housing contact. Further gap reduction is possible if materials are employed for the internal, thermally more stressed components of pump  1  (rotors, bearings, bearing supports, toothed wheels) which have a lower thermal expansion coefficient than the material for housing  4 , which is less thermally stressed. 
     A moderate equilization of the expansion of all components of pump  1  is obtained as a result thereof. An exemplary selection of such material is steel, for example nickel chromium (CrNi) steel, for the interior components and aluminum for the housing. Bronze, brass or nickel silver (China or German silver) may also serve as materials for the interior components. 
     In an exemplary embodiment of the present invention according to FIG. 2, the interior cooling of rotor  5  comprises a cooling bushing  51 , which supports itself, on the bearing side on housing  4  and which projects into hollow space  31 . The cooling bushing  51  surrounds the shaft  6 , which is no longer designed hollow. It traverses the hollow space ( 31 ) and carries rotor  5  in the region of its suction-sided end. For supplying the cooling bushing  51  with cooling agent, one or several cooling channels  52  are provided, which are supplied by the cooling agent pump  36  in a manner not shown in more detail. 
     In order that the cooling bushing  51  will absorb as much heat as possible from rotor  5 , a gap  53  between cooling bushing  51  and rotor  5  is selected as small as possible. In this region, the bushing  51  is equipped with threading  54 , which has a pumping effect directed in the direction of the compression chamber. Dirt particles present there are held back. 
     A gap  55  between bushing  51  and shaft  6  is also relatively small in order to produce, with the aid of threading  56 , a pumping effect on the interior side of bushing  51 . Said pumping effect acts in the direction of gasket  27 /bearing  7  and keeps oil particles out of the compression chamber.
A cooled screw vacuum pump has a housing ( 4) two rotating systems ( 5, 6) consisting each of a screw rotor ( 5) and a shaft ( 6), a floating device supporting the rotors having, on each shaft, two mutually spaced bearings ( 7, 8) and an empty space ( 31) arranged in each rotor ( 5) open on the bearing side, wherein is respectively located an element cooling the rotor internally. In order to improve cooling it is suggested that the bearing ( 7) of the support located on the rotor side, is placed outside the rotor ( 5) empty space ( 31), such that in said empty space ( 31) there is more room available for obtaining efficient cooling.