Abstract:
A pump ( 1 ) with one or more pistons ( 3; 103; 203; 303; 403 ), which can be rotated, especially for conveying essentially liquid media, the piston ( 3; 103; 203; 303; 403 ), having at least regionally a surface layer ( 17; 117; 217; 317; 417 ) of a polymer, especially of an elastomer, is constructed so that at least one recess ( 19; 119; 219; 319; 419 ), into which a supporting body ( 20; 120; 220; 320; 420 ) can be introduced for expanding the polymer layer ( 17; 117; 217; 317; 417 ), is assigned to the polymer layer.

Description:
BACKGROUND OF THE INVENTION 
     The inventions relates to a pump with one or more pistons which can be rotated, as well as to a method for compensating for abrasion-wear of a pump piston. 
     For rotating piston pumps, especially for conveying liquids laden with solids, there must be a seal between the rotating piston or pistons and the wall of the pump space and, in the case of several pistons, also between the individual pistons. The efficiency achieved varies with the quality of the seal with the wall and the quality of the seal between the pistons. 
     The DE 20 02 518 C3 shows an elastomeric coating for rotating pistons of pumps, which are used to convey solid-laden suspensions. Such a coating can yield elastically, if solid particles penetrate into the gap between the piston and the wall or into the gap between the rotating pistons, which are as close to one another, as possible. By these means, blockage of the rotational movement by solids is prevented. 
     However, the outer elastic layer or the piston is subject to increased wear, particularly by such particles penetrating into the respective gap. As the wear increases, the thickness of the plastic layer becomes less and, with that, the gap between the piston or pistons and the wall becomes greater, as a result of which the efficiency of the pump decreases. However, an exchange of pistons, which then becomes necessary, is very expensive. 
     The EP 0 599 333 B1 therefore shows piston parts, which can be exchanged individually, without having to exchange the whole piston. For this purpose, the blade tips are constructed as sealing strips, which can be slipped on and are fastened in a dovetail guide and can be exchanged when necessary. However, the manufacturing costs of such a design are very high. Moreover, in the case of multi-bladed pistons, all piston tips generally must be exchanged simultaneously, so that the maintenance costs, as a whole, are increased for such an exchange, since the number of parts, which must be exchanged, has grown appreciably in comparison to exchanging a piston. Moreover, the practical construction of such a dovetail guide is very expensive. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to counteract the wear-induced abrasion of an outer polymer layer of a pump piston with the least possible expense. 
     An adjustment of the outer, wear layer of the piston is made possible by the inventive construction of a pump. Due to the introduction, as required, of a supporting body in the designated recess, a thinning of the material of the polymer layer can be taken into account, in that the recess is expanded by introducing the supporting body. By these means, the polymer layer is stretched approximately to its original thickness. Such a recess can extend essentially along a line of contact between the piston and the wall of the pump space and thus counteract a radial thinning of the outer polymer layer by introducing a supporting body. The recess can also be disposed essentially perpendicularly to the axis of rotation, so that the introduction of the supporting body expands the piston regions, which form a seal, transversely to the axial course against the wall of the pump space and are also exposed to wear. 
     If an assortment of different supporting bodies of different dimensions is available, a wear-related material thinning of different extent can be taken into account particularly advantageously. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages and details arise out of the example of the object of the invention which are described in the accompanying drawings. 
     FIG. 1 shows a diagrammatic plan view of a two-bladed rotating piston pump with, in each case, a polymer layer surrounding the pistons and with supporting bodies introduced parallel to the axis, 
     FIG. 2 shows a piston of FIG. 1 in a perspective, partially broken open representation, 
     FIG. 3 shows a twisted piston with recesses, following its maximum radial extent, and introduced supporting bodies in a perspective, partially broken open representation, 
     FIG. 4 shows the diagrammatic course of the polymer layer, in the original state, worn on the outside and expanded by introducing an enlarged supporting body, in a diagrammatic, truncated plan view, in which 
     FIG. 4 a  shows the original state, 
     FIG. 4 b  shows the piston with the polymer layer partly worn, 
     FIG. 4 c  shows the piston with the diameter enlarged by the introduced supporting body and the resulting expanded wear layer, 
     FIG. 5 shows a rotating piston pump with a multi-blade, flexible rotating piston, 
     FIG. 6 shows a piston in a view similar to that of FIG. 1, the outer periphery of the piston and the polymer layer being matched to one another and held to prevent rotation relative to one another, 
     FIG. 7 shows a view, similar to that of FIG. 2, of a piston which has recesses for supporting bodies at edge regions, which are at a distance from one another axially, the supporting bodies, which have been introduced, extending transversely to the axial extent, and 
     FIG. 8 shows a plan view of FIG.  7 . 
    
    
     The pump  1  of the construction shown in FIG. 1 has a pump space  2 , in which two pistons  3 ,  4  of similar construction roll on one another and convey a liquid from an inlet gap  5  to an outlet opening  6 . The pistons  3  and  4  each are constructed as two-blade pistons. However, this is not essential. The pistons run in the direction of the arrows  7 ,  8  about the axes of rotation  9 ,  10  and are held removably on the shafts  13 ,  14  by feather keys  11 ,  12 . 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     During the rotation, the blade tips  3   a ,  3   b  and  4   a ,  4   b  pass closely by the walls  15 ,  16  of the pump space  2  as well as by the respective other piston  3  or  4 . On the outside, the pistons  3 ,  4  are surrounded, at least at the highly stressed blade tips  3   a ,  3   b  and  4   a ,  4   b , by a layer of an elastic polymer, especially by a layer of elastomer. These form a seal between the pistons  3 ,  4  and between the pistons  3 ,  4  and the walls  15 ,  16  of the pump space  2 . In the piston of FIG. 1 (see also FIG.  2 ), a recess is disposed within the layer  17 ,  18  of elastomer in the blade tips  3   a ,  3   b ,  4   a ,  4   b , extends parallel to the axis of rotation  9  or  10  and is provided to accommodate a supporting body  20 . According to the example, the recess  19  is disposed completely within the layer  17  and  18  of elastomer. It is also possible to dispose such a recess  19  between the layer of elastomer and the piston, especially the metallic piston, below this layer. 
     FIG. 3 shows a twisted piston  103 , which is also fixed by means of a feather key  111  on a shaft  113  and can be rotated about an axis of rotation  109 . The recesses  119  in the surface layer  117  are introduced spirally into the blade tips  103   a ,  103   b ,  103   c , which also extend spirally with respect to the shaft  113 . The introduced supporting bodies  120 , which are shown here, may be flexible or produced with an appropriate curvature. In the example, the piston  103  has three blades. Once again, this is not essential. Several pistons  103  can slide on one another in a pump  1 . 
     In a further example (FIG.  5 ), a multi-blade piston (eight blades)  203  of a rotary vane pump is shown. The piston is held on a shaft  213  and rotates in the direction  207  about an axis of rotation  209 . The piston  203  has a surface layer  217 , which consists completely of a polymer and surrounds the piston  203  essentially completely. In the piston tips  203   a  to  203   h , the recesses  219  are molded essentially parallel to the axis and can accommodate supporting bodies  220 . 
     In FIG. 6, a further piston  303  is shown, which corresponds essentially to piston  3  in FIG. 1, but has moldings  321  and  322  at the blade tips  303   a  and  303   b , in order to ensure, by these means, that the polymer layer  317  is held securely, so that it does not lift off and also does not rotate. At the lower end in FIG. 6 (blade tip  303   a ), a recess  319 , which is constructed as a channel parallel to the axis, is shown before a supporting body is introduced. A flattening of the outer polymer layer  317 , which has resulted from wear, can be seen here. At the upper blade tip.  303   b , the situation after the introduction of a supporting body  320  is shown. As a result of this introduction, the recess  319  is expanded and, accordingly, the surrounding elastomeric layer is stretched in the direction of arrow  323 . 
     In the embodiment of FIGS. 7 and 8, a piston  403  includes recesses  419  which extend perpendicularly to the axis of rotation  409 , and which are formed in a polymer surface layer  417  present in a region of the piston edges  403   a ,  403   b . By introducing supporting bodies  420 , the upper and lower boundary planes defining the piston edges  403   c ,  403   d , and which are perpendicular to the axis of rotation  409 , are expanded. As a result, an expansion of the areas, which extend transversely, is also possible instead of, or in addition to, the expansion of the blade tips. 
     In FIG. 4, the course of the inventive method for compensating for wear at the outer polymer layer is shown. In FIG. 4 a , the polymer layer  17  is intact also in the region of the blade tip  3   a  and has its full, original width. A supporting body  20 , such as a steel or plastic stud, the diameter of which corresponds to that of the channel formed by the recess  19 , is introduced into the recess  19 . 
     In the representation of FIG. 4 b , the polymer layer  17  is thinned and a gap is formed between the wall  15  and the outer boundary of the piston  3 . 
     For this reason, the supporting body  20  is removed and a supporting body  20   a , the periphery of which is expanded, is inserted. Due to its larger diameter, the supporting body  20   a  brings about an expansion of the recess  19  and, with that, also of the polymer layer  17  surrounding it, so that this expansion once again closes the gap that had been formed (FIG. 4 c ). 
     On the one hand, an existing supporting body  20  can be exchanged for an expanded supporting body  20   a . On the other, it is also possible that, originally, a supporting body  20  was not vulcanized in the recess  19 . In that case, either an existing borehole is expanded by introducing a supporting body  20   a , which is oversized with respect to the borehole, or a borehole with a diameter of 2 mm to 3 mm, for example, is introduced and an oversized supporting body  20   a  of, for example, 10 mm to 12 mm is pushed into the recess. Because an expulsion of an earlier supporting body  20  of smaller diameter is omitted in this procedure, the polymer layer  17  can be expanded in this way without removing the respective piston  3 , 103 ,  203 ,  303  or  403 . 
     Grubscrews with a self-cutting thread can be used particularly advantageous as supporting bodies  20 ,  20   a . They can also be unscrewed once again from the recesses  19 , even when the piston  3  is inserted, so that an expulsion of a supporting body  20 , which would require removal of the piston  3 , is unnecessary for exchanging the supporting bodies  20 ,  20   a . Supporting bodies with a thread may also consist of different materials and optionally be curved. 
     An assortment of supporting bodies  20 ,  20   a ,  120 ,  220 ,  320 ,  420  of different diameters with or without a thread, can be kept on hand. In order to decide, which supporting body is to be fitted in, the gap between the wall  15  and the elastomeric layer  17  is measured first and the appropriately fitting supporting body  20  or  20   a ,  120 ,  220 ,  320 ,  420  is then selected. 
     The supporting body need not have the circular, cross sectional configuration shown in FIG. 4 c . In adaptation to the thinned region, it may also, for example, have an oval, elliptical or a differently shaped peripheral configuration. It is also possible to provide several recesses  19  next to one another, as a result of which the expanded region as a whole is enlarged. 
     Overall, an elastic deformation of the layer  17  is achieved, which also experiences therewith an expansion towards the outside and accordingly compensates for wear-induced thinning of the material.