Abstract:
A vacuum pump ( 1 ) comprising a pump chamber casing ( 5 ) accommodating two co-operating rotors ( 2, 3 ) which are respectively arranged on a shaft ( 8, 9 ). A bearing/gear chamber ( 6 ) is disposed adjacent to the pump chamber casing ( 5 ) in which the rotor shafts ( 8, 9 ) are cantilevered and provided with a synchronisation gear ( 17 ). A drive motor ( 25 ) whose drive shaft ( 28 ) extending parallel to the rotor shafts ( 8, 9 ) and is provided with a drive gear ( 35 ). A gear stage ( 37 ) is disposed between the drive shaft ( 28 ) and one of the rotor shafts ( 8, 9 ). In order to provide a machine of this type which can be embodied in a compact form, the drive gear ( 35 ) of the drive shaft ( 28 ) engages directly with a driven gear ( 36 ) on one of the rotor shafts ( 8, 9 ), forming the gear stage ( 37 ).

Description:
In U.S. Pat. No. 5,197,861, the drive motor is accommodated in a casing at the side next to the pump. In order to be able to operate the rotors at a higher speed compared to that of the motor, a gear is provided. The driving toothed gear of the motor shaft is coupled via a further toothed gear to a toothed gear arranged on one of the rotor shafts. A solution of this kind requires much space. Moreover, four shafts are present which each need to be equipped with bearings. 
   Vacuum pumps of this kind belong to the class of two-shaft vacuum pumps. Typical examples of two-shaft vacuum pumps are Roots pumps, claws pumps and screw pumps. The two rotors of such pumps are located in a pump chamber and effect pumping of the gases from an inlet to an outlet. The cantilevered bearing offers, in the instance of axially pumping machines, the benefit that on the suction side (high-vacuum side) shaft seals are not necessary. 
   In two-shaft machines with synchronised shafts, direct driving of one of the two shafts is common (c.f. DE 198 20 523 A1, for example). If in machines of this type common AC drive motors are employed, there result rotor speeds of 3000 rpm. (at 50 Hz) and 3600 rpm. (at 60 Hz) respectively. Pumps being operated at such speeds have a low power density, require narrow slots and/or many stages and are for this reason relatively large, heavy and costly. Increasing the speed would be possible with the aid of a frequency converter; however, frequency converters for large drive power ratings are expensive. 
   It is the task of the present invention to design a vacuum pump of the kind affected here in a more simple and more compact manner. 
   SUMMARY OF THE INVENTION 
   A primary advantage of the present invention is, that the means which are required for a transmission to the higher speed—doubling of rotor speed, for example—are much simpler compared to the state-of-the-art. 
   Another advantage is that commonly employed motor technology can be retained. 
   Other advantages reside in extremely slim and compact designs and in addition, simplified cooling of the electric motor. 
   Still further advantages will be apparent to those of ordinary skill in the art upon reading and understanding the following detailed description. 

   
     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. 
     Further advantages and details of the present invention shall be explained with reference to examples of embodiments depicted schematically in drawing  FIGS. 1  to  10 . Depicted in 
     drawing  FIGS. 1  to  3  are examples of embodiments according to the present invention, in which the motor rotor runs on a separate motor shaft arranged beside the rotor shafts and 
     drawing  FIGS. 4  to  10  are examples of embodiments in which the motor rotor and one of the rotor shafts have a joint axis of rotation. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the drawing figures, the two-shaft vacuum pump is designated as  1 , its rotors as  2 ,  3 , its pump chamber as  4  and its pump chamber casing as  5 . Adjoined to pump chamber casing  5  is the bearing/gear chamber  6 , the casing of which is designated as  7 . The rotor shafts  8 ,  9  extend into the bearing/gear chamber  6 . The axes of rotation of the rotors and the shafts are designated as  11  and  12 . The shafts are supported by bearings on the side of the pump chamber and on the side of their ends (bearings  13  to  16 ) so that the rotors  2 ,  3  are supported in a cantilevered manner. The rotor shafts  2 ,  3  are coupled via a synchronising gear  17  being formed by two engaging toothed gears  18 ,  19 . Gaskets  21 ,  22  are provided for the purpose of sealing off the pump chamber  4  against the bearing chamber  6 . 
   In all examples of the embodiments depicted, the drive motor  25  is located in the bearing/gear chamber  6 . The stator  26  encompasses the armature  27  being affixed on to the motor shaft  28 . The motor shaft  28  extends in each instance in parallel to the rotor shafts  8 ,  9  and is supported by bearings in the area of its ends (bearings  31 ,  32 ) in the bearing/gear chamber  6 . Its axis of rotation is designated as  29 . 
   There also exists the possibility of arranging a standard motor outside of casing  7  and to link said motor to a shaft extending within the bearing/gear chamber  6  in parallel to the rotor shafts  8 , 9  with the motor shaft carrying the driving toothed gear  35 . This and any other toothed gears can be advantageously constructed from plastic. A solution of this kind is outlined through the dashed line  30  in drawing FIG.  1 . 
   As an example, a screw vacuum pump  1  is depicted in drawing FIG.  1 . Plane  23  (drawing  FIGS. 2 ,  3  and  4 ) formed by the axes of rotation  11 ,  12 , is perpendicular to the plane of the drawing figure, so that only one rotating unit is visible. During their operation, the rotors  2 ,  3  pump gases from inlet  33  to an outlet which is not depicted. 
   In the screw vacuum pump in accordance with drawing  FIG. 1 , the motor shaft  28  is adjoined at the side of the plane formed by axes of rotation  11 ,  12 . The motor shaft carries a driving toothed gear (driving gear  35 ) which directly engages with a toothed gear (driven gear  36 ). Driving gear  35  and driven gear  36  form a gear stage  37 . The driven gear  36  is affixed on to one of the rotor shafts  8 ,  9 . Synchronous driving of the second rotor shaft is in each instance effected through the toothed gears  18 ,  19  of the synchronising gear  17 . 
   Drawing  FIGS. 2  to  4  outline coupling options of the kind detailed. In the solution according to drawing  FIG. 2 , the driving gear  35  engages with one ( 18 ) of the two synchronising toothed gears  18 ,  19 . The toothed gear  18  acts at the same time as the driven gear  36 . The transmission ratio is determined by the ratio between the diameters of the toothed gears  35  and  18 . 
   The embodiment in accordance with drawing  FIG. 3  substantially corresponds to the solution depicted in drawing FIG.  1 . Located under the synchronising toothed gear  18  on the shaft  8 , there is a further, toothed gear  36  preferably smaller in diameter, which engages with the driving toothed gear  35 . The same also applies to the solution in accordance with drawing FIG.  4 . The difference compared to drawing  FIG. 3  is that the axes of rotation  11 ,  12  and  29  are located in one plane. 
   From drawing  FIGS. 2  to  4  it is apparent that on the one hand the usable space between the rotor shafts  8 ,  9  may be utilised in part for the motor stator  26  (drawing  FIGS. 2 ,  3 ) so that compact solutions will result. On the other hand the angle between the respective axes of rotation may be selected almost free of any restrictions. 
   In the embodiments in accordance with drawing  FIGS. 5  to  10 , the motor shaft  28  is designed to be hollow, so that there then exists the possibility of letting one of the rotor shafts penetrate the hollow shaft  28  in such a manner that the axes of rotation  29  and  11  respectively  12  are identical. In embodiments of this kind the usable space between the rotor shafts  8 ,  9  may be utilised even better. In all, there results from this an optimally compact and slim shape. 
   Some of the design options for such embodiments are depicted in drawing  FIGS. 5  to  10 . In the solutions in accordance with the drawing  FIGS. 5 and 6 , the hollow shaft  28  carries in each instance the driving gear  35  which engages with the driven gear  36  on the rotor shaft located besides the hollow shaft  28 . The synchronisation gear  17  offset which respect to this is employed for synchronised driving the rotor shaft  8  penetrating the hollow shaft  28 . Also in the drawing  FIGS. 7 and 8  a driving gear  35  and a driven gear  36  form the gear stage  37 . Outlined in drawing  FIG. 7  is, that the gear stage is designed by way of chain or belt stages. The solution in accordance with drawing  FIG. 8  is equipped with a planet gear. 
   The bearing arrangement for motor shaft  28  may be effected independently of the bearings  13  to  16  for rotor shafts  8 ,  9  by means of carriers affixed to the casing (drawing  FIG. 8 , upper bearing  31 ). An especially compact solution is attained when the motor shaft  28  is supported by at least one (drawing  FIG. 8 , bearing  32 ) preferably both bearings  31 ,  32  (drawing  FIGS. 5 and 7 ) on the rotor shaft  8  penetrating the hollow shaft  28 . Moreover, the rotor shaft  8  penetrating the hollow shaft  28  may be supported within the hollow shaft (bearings  15  in drawing FIG.  6 ). Finally there is depicted in drawing  FIG. 7  as a special feature that the transmission ratio of the synchronising stage may differ from 1:1. The toothed gears  18  and  19  have differing diameters, outlining a transmission ratio of 2:1. Required for this is that the rotors  2 ,  3  be designed correspondingly. 
   Drawing  FIGS. 8  to  10  depict the way in which the hollow motor shaft  28  is coupled to the rotor shaft  8  centrally penetrating the hollow shaft, said coupling being effected by a planet gear  41 , forming the gear stage  37 . The planet gear comprises in a manner which is basically known the outer ring gear  42 , for example two planet gears  43 ,  44  as well as the sun gear  45  affixed to the rotor shaft  8  with axis of rotation  29 . Schematic diagram  9  depicts the solution presented in drawing  FIG. 8  with a fixed ring gear  42 . The plant gears  43 ,  44  which are joined via cranks  46 ,  47  to the motor shaft  28 , form the driving gears  35 ,  35 ′. Only one planet gear  44  would suffice as the driving gear  35  (drawing FIG.  9 ). The sun gear  45  forms the driven gear  36 . 
   In the solution in accordance with drawing  FIG. 10  the ring gear  42  forms the driving gear  35 . For the planet gear  44  a fixed carrier is provided. The sun gear  45  again forms the driven gear  36 . Although in the instance of this solution the driving gear  35  and the driven gear  36  no not engage directly, the aims of the invention—compact, simple—can be attained. 
   It has already been proposed to equip at least one of the rotor shafts  8 ,  9  with a central bore and to utilise said bore(s) for conveying a lubricating and cooling agent (preferably oil). Where the drive motor  25  is arranged within the bearing/gear chamber, said motor may also be cooled with the oil. An oil pump for pumping the oil may be arranged on one of the shafts  8 ,  9  or  28 . If the motor shaft  28  should be located besides the rotor shafts  8 ,  9  then a particularly expedient solution is such that the oil pump—preferably designed as an eccentric pump—be arranged on the motor shaft  28 , specifically in the area of its upper end. This embodiment is depicted in drawing FIG.  1 . The oil pump is designated as  51 . In addition, one of the shafts  8 ,  9  or  28  may be run out, on its side adjacent to the pump chamber, from the bearing/gear chamber  6  in a sealed manner and carry a ventilating wheel  52 . Expediently, to this end also the motor shaft  28  is utilised in accordance with drawing FIG.  1 . 
   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.