Abstract:
The inventions relates to a feed screw for eccentric screw pumps which, to avoid bridge formation of the medium and optimisation of the drive power, has a screw, which in the axial region, has perforations and webs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority of German patent application No. 10 2006 036 243.8 filed on Aug. 3, 2006, the content of which is incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     The invention relates to an eccentric screw pump with a feed screw which supplies the suction region of the screw rotor mainly with medium to highly viscous media.  
       BACKGROUND OF THE INVENTION  
       [0003]     DE 101 60 335 A1 shows an eccentric screw pump in this regard where a feed screw is arranged in the pump housing before the pump rotor. The screw is connected with the screw core via its entire inner contour.  
         [0004]     DE 101 18 071 A1 shows an eccentric screw pump about the coupling rod of which a hollow screw is arranged. This hollow screw is connected with a disc on the drive side. The other end in the suction-side region of the screw rotor has no connection to a joint or the coupling rod.  
         [0005]     With a mixing device according to DE 1 277 819 dry substances are mixed with liquid and subsequently delivered by a pump. To this end, a mixing screw is seated in the region of a storage vessel, which mixing screw is connected on the one side with a motor and, on the other side with a screw pump. Liquid enters the screw region between the storage vessel and the pump. The feed screw consists of a helical band which is only fastened to the mixer shaft at one end by means of four braces.  
         [0006]     A mixing and feed device is also shown in DE 43 18 177. The dry substances enter the region of a mixing screw via a hopper, while a liquid feed line also leads into the region of said mixing screw. Following the mixing operation, the mixture is transported onwards by a screw pump. The mixing screw itself consists of a region with a solid screw and a region with paddle and web-shaped mixing elements.  
         [0007]     Each pump is designed for a determined delivery rate. To this end, adequate medium must always be available for the pump region on the suction side. The feed screws, which are arranged upstream of the actual screw or eccentric screw pump, can therefore deliver a multiple volume of the pump capacity. Because of this, a back-up effect develops in the so-called stuffing space in the suction region which is associated with danger of bridge formation in the hopper above the screw. Because of this stuffing effect, substantially higher drive power than necessary must be made available.  
         [0008]     The object of the invention consists in adapting the stability of the feed screw to the required output while keeping the drive power constantly low even with different media.  
       SUMMARY OF THE INVENTION  
       [0009]     This object is solved through the characteristics of claim  1 . Further developments of the feed screw according to the invention are indicated from the characteristics of the sub-claims.  
         [0010]     The design of a corresponding feed screw between the pump rotor and the drive has shown that the devices known from the prior art solve only part problems of the object according to the invention.  
         [0011]     The design according to the invention is obviously dependent on which products with which viscosities, and, if applicable, present solid materials have to be pumped.  
         [0012]     According to the invention the normal embodiment concerns a feed screw having at least two perforations, wherein the webs formed between these perforations are connected with their screw root with the coupling shaft. Depending on which viscosity the product has it may be practical to increase the number of perforations to at least four in order to facilitate the return flow of the medium and ensure more homogenous mixing-through, through which bridge formation of the medium is already counteracted on the suction-side end of the pump.  
         [0013]     Through the perforations according to the invention, low-loss drive power compared with delivery devices from the prior art, designed to the condition of the medium can be installed.  
         [0014]     In order to obtain the advantage according to the invention also in the regions of the couplings (joints) the screw also extends beyond this region for the purpose of which the pipe employed as coupling rod has strip-shaped pipe segments which are connected with the screw. If the number of the pipe segments corresponds to the number of the perforations, a corresponding number of webs for their fastening is available.  
         [0015]     Since for the return flow of the medium not only the number of perforations but also their area place a substantial role, it can be provided according to a version according to the invention to dimension the perforations between 30% and 70% of the height of the screw start. With low-viscosity substances the height of the perforations can be selected in the range from 20% to 60% of the height of a screw pitch.  
         [0016]     As is shown in an exemplary embodiment the width of the perforations will correspond to the width of the webs because of the homogenous return feed and even loading of the screw. With high-viscosity media the return flow possibility must certainly be improved wherein the width of the perforations is greater than the width of the webs. With low viscosity substances the danger of bridge formation is relatively low so that here the width of the webs can be greater than that of the perforations.  
         [0017]     To improve the return flow and thus reduce the stagnation effect the flow along the coupling shaft can be improved in that the webs have an inclination and thus produce a flow direction which is opposite to the course of the screw.  
         [0018]     Depending on which design of the screw is required, the webs can be offset to one another by 30° to 120° per screw start. In order for the feed screw to be stabilised about its entire length through the coupling shaft the length of the pipe segments is adapted to the course of the feed screw.  
         [0019]     Easier affixing of the feed screw to the coupling shaft is obtained in that the feed screw consists of several parts which enable better handling during the mostly employed welding operation.  
         [0020]     The invention is exemplarily described in the following by means of exemplary embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  lateral view of the feed screw;  
         [0022]      FIG. 2 a  three-dimensional representation of the feed screw;  
         [0023]      FIG. 3 a  cross section of the feed screw;  
         [0024]      FIG. 4  feed screw with joint part on both sides;  
         [0025]      FIG. 5  lateral view of the feed screw according to  FIG. 4 ;  
         [0026]      FIG. 6  cross section of a feed screw;  
         [0027]      FIG. 7  screw cross section with 6 perforations;  
         [0028]      FIG. 8  screw cross section with 4 perforations; and  
         [0029]      FIG. 9  screw cross section with various perforation distances from the coupling rod.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]      FIG. 1  shows a design possibility of a feed screw  10  with a screw pipe  12 . A screw  14  is welded on to the circumferential surface of the screw pipe  12 . On both ends  16 ,  18  of the screw pipe  12 , pipe segments  20  for fastening the screw  14  are provided. Each of the pipe segments  20  ends at the point at which the screw  14  ends in axial direction. In the region of the pipe elements  20 , couplings  22  for the joints not shown are provided at both ends. The screw  14  is manufactured from flat band-shaped material.  
         [0031]     The design of the screw  14  can be more clearly seen in  FIG. 2 . From the perspective representation it becomes evident that the screw  14  is provided with perforations  24  and webs  26 . Each screw root  34  of the webs  26  is connected with the pipe segment  20  or the coupling shaft  32  for example through a welding operation. While the medium in axial direction flows from the rotor region back to the pump inlet through the perforations of the feed screw closely along the screw pipe, the screw  14  with its end faces  28  transports the medium in the direction towards the pump rotor. The pump rotor, through a joint which is not completely shown, and which joint is fastened to the coupling  22 , is in positive contact with the feed screw.  
         [0032]      FIG. 3  shows the embodiment and arrangement of the perforations  24  and webs  26  for a feed screw for highly viscous media. Here, the large free areas of the perforations  24  enable very good return flow possibilities for the medium in order to adapt the stagnation pressure in the stuffing space to the pump output. Here, dehydration of the medium and increased tendency toward bridge formation is prevented and undesirably high drive power avoided.  
         [0033]     The perforations  24  according to this exemplary embodiment are wider than the webs  26 . The middle of the perforations in each case is located on the midperpendicular and is thus offset by 90° relative to one another which produces 4 perforations  24  and 4 webs  26  per screw pitch. The height of the perforations corresponds to approximately 50% of the screw height.  
         [0034]     A feed screw  10  is also shown in  FIG. 4  and  FIG. 5 . Here, a coupling shaft  32  is provided as drive component. A screw  14  is welded on to the coupling shaft  32  in the region between the joint components  30  which, as with all other exemplary embodiments, consists of individual screw segments. This embodiment of the screw  14  is employed for instance with low-viscosity media. By way of the large face areas  28  compared with the areas of the perforations  24 , more medium enters the rotor region while the smaller dimensioned perforations nevertheless prevent an increased need for drive power.  
         [0035]     The arrangement of the perforations  24  and their size of the feed screw  10  shown in  FIGS. 4 and 5  is shown in  FIG. 6 . Per screw pitch, three perforations each offset by 120° are provided in the screw  14 .  
         [0036]     Additional exemplary embodiments for the screw design for one winding each are shown in  FIGS. 7, 8 ,  9 .  
         [0037]      FIG. 7  represents an even distribution of 6 perforations  24  and webs  26 . The height HD of the perforations is 50% compared with the height HS of the screw  14 . The width BD of the perforations corresponds to the width BS of the webs.  
         [0038]     In  FIG. 8  the screw  14  has four perforations  24  and four webs  26  while the width BD of the perforations is greater than the width BS of the webs. The height HD of the perforations  24  is 50% of the height HS of the screw  14 .  
         [0039]     The interrupted lines of  FIG. 9  shows different size relationships with regard to the height HD of the perforations to the height HD of the screw  14 , while three perforations  24  are shown with a division by 120.