Abstract:
A Roots pump comprises a plurality of multi-toothed rotary pumps, each forming a pump stage, and connection channels connecting respective adjacent pump stages. The invention provides that the connection channels are arranged in partitioning walls separating the adjacent pump stages.

Description:
BACKGROUND 
     1. Field of the Disclosure 
     The disclosure relates to a Roots pump. 
     2. Discussion of the Background Art 
     Roots pumps typically comprise two-toothed rotary pumps arranged in a pump chamber. The two rotary pistons are driven in opposite directions so that the individual chambers formed draw gas through a main inlet and expel the gas through a main outlet. Here, the main inlet and the main outlet both extend in a radial direction and are arranged opposite each other. Further, multi-toothed rotary pistons are known, in particular such pistons with three or four teeth. In this case, too, the gas is pumped substantially radially from a radially arranged main inlet to a radially arranged main outlet. 
     Further, for producing low pressures, multi-stage Roots pumps are known. Such Roots pumps comprise one pair of rotary pistons per stage. Here, the gas to be pumped is conveyed from one outlet of a pump stage to the inlet of an adjacent pump stage. This is effected through connection channels. As described, for instance, in U.S. 2010/0158728, these connection channels may be arranged in the housing of the Roots pump, wherein the connection channels surround or are arranged radially outside the pump chambers in which the rotary pumps are arranged. This is necessary in order to convey gas from an outlet of a pump stage situated, for instance, in the lower part of the Roots pump to a pump inlet of the adjacent pump stage, which inlet is situated, for instance, in the opposite, upper part of a Roots pump. Such Roots pumps are disadvantageous in that the design of the channels in the housing is technically complex. Further, the housing volume must be large in order to accommodate the connection channels. This does not only result in large outer dimensions of the Roots pump, but in particular entails high costs. Besides the complex manufacturing process, the high costs are due also to the large quantity of metal used. 
     It is an object of the disclosure to provide a Roots pump of technically simple construction, wherein, further, the necessary structural space and the costs are preferably reduced. 
     SUMMARY 
     The Roots pump of the present disclosure comprises a plurality of multi-toothed rotary piston pairs, each forming a pump stage. Per pump stage, two rotary pistons with more than two teeth are provided, it being preferred that the rotary pistons have at least four, in particular at least six teeth. The two rotary pistons of a pump stage rotate in opposite senses to convey the gas. Preferably, one of the two rotary pumps of each rotary pump pair is arranged on a common shaft, so that the Roots pump comprises two shafts extending in parallel, wherein each shaft carries one of the rotary pistons in each stage. The two shafts can be connected through gears so that only one of the shafts has to be driven. 
     Adjacent pump stages are connected via connection channels. Here, adjacent pump stages may be connected via one or a plurality of connection channels. According to the disclosure, the connection channels are arranged in partitioning walls that separate adjacent pump stages from each other. The partitioning walls are thus provided between the piston chambers of adjacent pump stages. By arranging the connection channels in the partitioning walls, as provided by the disclosure, the outer dimensions of the present Roots pump can be reduced drastically as compared to prior art. This has the advantage that, due to the lower material input, a cost reduction can be achieved. Further, the connection channels provided in the partitioning walls can be manufactured more economically, since it is possible to form the connection channels as straight, in particularly circular cylindrical channels or bores. According to the disclosure, a technically difficult manufacture of curved connection channels situated radially outside the piston chambers is thus not required. The Roots pump, which according to the disclosure is of a very compact structure, has the further advantage that a reduction in weight and a reduction in the number of parts can be achieved. Since they can be designed as dry-running pumps without oil lubrication, Roots pumps further have the advantage that the maintenance requirements are reduced. 
     It is another advantage of the present arrangement of the connection channels in partitioning walls that less pressure loss is experienced due to the short length of the connection channels. 
     Preferably, at least a part of the connection channels is connected with the piston chambers, in which the rotary piston pairs are arranged, such that a channel inlet opening and/or a channel outlet opening is swept over by a side wall of a rotary piston in operation. The channel inlet opening and/or the channel outlet opening of at least one connection channel is thus not arranged radially with respect to a piston chamber, but axially. The opening is not swept over by a radial end face, but by a side wall of a rotary piston. 
     In order to allow for a structure of the present Roots pump that is as compact and as economic as possible, all connection channels are preferably arranged in partitioning walls separating the pump stages from each other. Only one main inlet and/or a main outlet are not arranged in partitioning walls. The main inlet and/or the main outlet may be arranged axially or radially. Preferably the main inlet is arranged radially opposite the main outlet. If, for example, gas is drawn through a main inlet arranged at the top of the pump, the gas is thus expelled, in a preferred embodiment, at the radially opposite bottom of the pump. Of course, the main inlet is radially offset from the main outlet, since the individual pumps are arranged in axial succession, starting from the main inlet to the main outlet. 
     It is possible, in particular with rotary pistons having three or more teeth, to provide connection channels extending axially in the partitioning walls. This can be realized by the fact that a chamber situated between two teeth does not expel the gas only after a rotation of the rotary pistons by about 180°, but already at a smaller angle of rotation. In such a preferred embodiment of the present Roots pump, the gas does not have to be conveyed between two stages by conveying it from a main inlet-side chamber to a main outlet-side chamber. For example, with three-toothed rotary pistons, gas is drawn through a main inlet at a top of the pump. The gas is conveyed from the first to the second stage through a connection channel arranged centrally at a rotation angle of the rotary pistons of about 90°. This connection channel may extend axially so that the gas enters a central of the adjacent rotary pistons. In this pump stage, the gas is then conveyed further towards the outlet side, from which region it flows into an inlet-side chamber of the adjacent pump stage through a channel arranged in particular obliquely or diagonally in the partitioning wall. In particular with rotary pistons having more than three teeth, multiple axial channels can extend between adjacent pump stages. Providing the axial channels has the particular advantage forming the channels is technically simple. These may be axial, in particular circularly cylindrical bores. 
     In order to also allow for a technically simple design of the connection channels extending obliquely or diagonally in the partition walls, partitioning walls, in which such connection channels are arranged, are preferably thicker in the axial direction than partitioning walls in which axial connection channels are provided. Thereby, it is possible to also design the oblique connection channels in a straight manner without bends. 
     In the interest of keeping the power consumption of the pump as low as possible, the connection channels have as large a cross section as possible. In order to increase the cross section it is also possible to provide a plurality of mutually parallel channels. In particular with the channels extending obliquely in the partitioning walls, it should further be observed to make them as short as possible. 
     For an increase in compression, the rotary pistons preferably have different widths in the axial direction, with the width of the rotary pistons decreasing in particular in a step-like manner in the direction of pumping. Thereby, the volume of the individual chambers formed between the teeth of the rotary pistons is reduced. 
     In a preferred embodiment, the two meshing rotary pistons have the same diameter and the same shape. However, it is also possible to provide rotary pistons with different diameters and different numbers of teeth, where the rotary pistons will then rotate at different speeds. Likewise, meshing rotary pistons may also have different tooth shapes. 
     Due to the present design of the Roots pump, it is possible to achieve, in particular, a uniformization of the stress peaks over the rotation of the rotor and, thereby, to also achieve a uniformization of the compression heat. 
     The following is a detailed description of the disclosure with reference to preferred embodiments and to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the Figures: 
         FIG. 1  is a schematic illustration of a three-toothed pressure piston pair of a first pump stage, 
         FIG. 2  is a schematic illustration of a three-toothed pressure piston pair of a second, adjacent pump stage, 
         FIG. 3  is a schematic illustration of a six-toothed rotary piston pair of a first stage, 
         FIG. 4  is a schematic illustration of a six-toothed rotary piston pair of a second stage, 
         FIG. 5  is a schematic illustration of a six-toothed rotary piston pair of a third stage, 
         FIG. 6  is a schematic sectional view of a six-stage Roots pump comprising six-toothed rotary pistons as schematically illustrated in  FIGS. 3-5 , and 
         FIG. 7  is a schematic top plan view on an alternative embodiment of a rotary piston pair. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The three-toothed rotary pistons  10  schematically illustrated in  FIGS. 1 and 2  are arranged in a first pump stage ( FIG. 1 ) in a pump chamber  12 . The two rotary pistons  10  are each rotatably supported on a non-illustrated shaft and are driven in opposite senses in the direction of the arrows  14  and  16 , respectively. Gas is supplied to a chamber  20  via a main inlet  18 . By rotation of the left rotary piston in  FIG. 1 , the gas is enclosed in the chamber  20  that is closed by the curved portion  22  of an outer wall. When the left rotary piston in  FIG. 1  is rotated further in the direction of the arrow  14 , the chamber  20  is opened in a position corresponding to the chamber identified by the reference numeral  24 . The chamber  24  encloses the entire lower portion of the two rotary pistons so that the portions  24 ,  26 ,  28  have the same pressure level. Thereby, the gas initially in chamber  20  is expelled through an axial connection channel  30 , i.e. a channel extending in parallel with the rotary shafts of the rotary pistons. 
     Likewise, the right rotary piston in  FIG. 1  encloses gas in a chamber  32 , which is moved downward in the direction of the arrow  16  in  FIG. 1  by rotation of the rotary piston  10 , and is then expelled through the also axially extending connection channel  34  illustrated in dotted lines. 
     In the next pump stage ( FIG. 2 ), which is arranged axially downstream with respect to the first pump stage ( FIG. 1 ), gas enters a chamber  36  through the connection channel  30 , the chamber being at the same pressure level as the portions  38 ,  40 . By rotating the left rotary piston in  FIG. 2 , a chamber, closed in itself, is formed in combination with the curved wall  42  so that the gas enclosed therein is supplied towards a main outlet  44 . The same principle of conveyance is implemented by the right rotary piston in  FIG. 2 , where gas enters the chamber  40  through the connection channel  34  as soon as the right piston  10  is rotated on in the direction of the arrow. The gas then enclosed in a chamber  46  is also conveyed towards the main outlet  44 . 
     In order to form a third stage, the gas again has to be conveyed from the outlet  44 , which is the main outlet in  FIG. 2 , upwards towards a main inlet. According to the disclosure, this is effected by means of channels extending diagonally or obliquely in a partitioning wall, which channels are not illustrated in this embodiment. 
       FIGS. 3-5  illustrate six-toothed rotary piston pairs  48 ,  49  together with the connection channels relevant to a first stage ( FIG. 3 ), a second stage ( FIG. 4 ) and a third stage ( FIG. 5 ). In a Roots pump with six stages ( FIG. 6 ), for example, the illustration in  FIG. 3  corresponds to a first stage  50 , the illustration in  FIG. 4  corresponds to a second stage  52 , and the illustration in  FIG. 5  corresponds to a third stage  54 . The fourth stage  56  essentially corresponds to the first stage ( FIG. 3 ), where the inlet, however, does not occur radially but through an obliquely or diagonally extending connection channel  77 . The fifth stage  58  corresponds to the second stage or  FIG. 4 , and the sixth stage  60  corresponds to the third stage  54  or the stage illustrated in  FIG. 5 , with the outlet occurring in the radial direction through a main outlet  62 . The individual rotary pistons  48 , whose width decreases in the axial direction or the pumping direction  64 , are supported on a common shaft  66 . Likewise, the rotary pistons  49  are supported on a common shaft  68 . The two shafts  66 ,  68  are rotatably supported in an upper housing half  70  or a lower housing half  72  and can be connected via non-illustrated gears so that only one of the two shafts  66 ,  68  has to be driven by a motor. 
     Partitioning walls  74 ,  76 ,  78 ,  80 ,  82  are provided between adjacent pump stages. In the embodiment illustrated, at least one connection channel  84 ,  86 ,  88 ,  90 ,  77  is arranged. In addition, it is also possible to provide connection channels that are at least partly arranged in an outer portion, as known from prior art. In the embodiment illustrated, gas is drawn through the main inlet  51 . Instead of a radially arranged main inlet  51 , the same may also be formed radially as an inlet  53  ( FIG. 3 ). Of course, it is also possible to provide an oblique inlet or even a combination of different inlets, where the inlet only has to provide a means for the inflow of gas into the chamber  55  ( FIG. 3 ). 
     Thereafter, the gas is conveyed from the first pump stage  50  into the second pump stage  52  through a connection channel  84  extending axially, i.e. in parallel with the shafts  66 ,  68 . The connection channel  84  is arranged in the partitioning wall  74 . Here, according to the principle described for  FIGS. 1 and 2 , the gas is conveyed into a chamber  59  connected with the connection channels  84  via the intermediate chamber. 
     The gas is then conveyed further ( FIG. 4 ) and flows from the second pump stage  52  into the third pump stage  54  through a connection channel  86  also extending axially. The connection channel  86  is arranged in the partitioning wall  76 . 
     When the gas is conveyed further ( FIG. 5 ), it is necessary to convey the gas from the main outlet side towards the main inlet side. For this purpose, a diagonal or oblique channel  77  is provided in the partitioning wall  78  which is thicker in the axial direction than the other partitioning walls  74 ,  76 ,  80 ,  82 . 
     The gas is conveyed from the fourth pump stage  56  into the fifth pump stage  58  through a channel  88  extending axially in the partitioning wall  80 . The conveyance into the next pump stage  60  again occurs through an axial channel  90  provided in the partitioning wall  82 . Since, in the embodiment illustrated, the sixth pump stage  60  is the last pump stage, the same s connected with the substantially radial main outlet  62 . 
     Since, as is obvious in particular from  FIGS. 3-5 , only a part of the chambers is used to convey gas, the chambers, in which the rotary pistons are arranged, require a surface finishing with small tolerance levels only in the region of the active chambers, i.e. the chambers relevant to conveying. Thereby, manufacturing costs can be reduced further. 
     Instead of identically designed rotary pistons, it is also possible to provide rotary pistons with different diameters and in particular different numbers of teeth. Moreover, a combination of rotary pistons having different tooth shapes is possible. An example is illustrated in  FIG. 7  in top plan view. Here, a left rotary piston  92  has teeth that cooperate with five differently shaped teeth of a right rotary piston  94 .