Patent Publication Number: US-2021180613-A1

Title: Pump

Description:
TECHNICAL FIELD 
     The present disclosure is directed to a pump, in particular to a single stage or multistage centrifugal pump with a power of up to 300 kW. 
     BACKGROUND 
     Single stage or multistage centrifugal pumps usually comprise at least one impeller fixed to a rotor shaft driven by a motor. The rotor shaft is immersed in the fluid to be pumped and/or extends into a pump housing. The rotor shaft is usually centred by a radial bearing within the pump housing. In particular for large shafts of big pumps, it is a challenge to quickly and reliably mount the bearing to the rotor shaft. 
     It is known to mount the bearing to the rotor shaft by means of a flange connection. Such known solutions consume significant time and material for mounting the radial bearing to the rotor shaft. 
     SUMMARY 
     In contrast to such solutions, embodiments of the present disclosure provide a pump for which the radial bearing can be quickly and reliably mounted to the rotor shaft with less material. 
     In accordance with the present disclosure, a pump is provided comprising
         a rotor shaft extending along a rotor axis,   a bearing body circumferentially encompassing the rotor shaft and comprising a radially outer bearing surface, and   a locking ring circumferentially encompassing the rotor shaft and limiting an axial movement of the bearing body relative to the rotor shaft, wherein the locking ring comprises at least two radially inwardly protruding teeth, wherein the locking ring is radially expandable from a locking state to a mounting state against an elastic restoring force of the locking ring, wherein the locking ring is, in the mounting state, positionable at a desired axial position on the rotor shaft, wherein the teeth are configured to press, in the locking state, against a radial outer surface of the rotor shaft by the elastic restoring force of the locking ring.       

     The locking ring thus serves as a “snap ring” without needing a prefabricated groove in the shaft to snap into. The teeth concentrate the normal forces exerted by the elastic restoring force of the locking ring between the locking ring and the radial outer surface of the rotor shaft on a small arear so that there is a high pressure between teeth and the radial outer surface of the rotor shaft. This high pressure provides for sufficient frictional force to axially fix the locking ring to the shaft. The locking ring thus facilitates a quick and reliable mounting of the radial bearing to the rotor shaft without a flange connection. Over time and use, and particularly in an abrasive environment of the fluid to be pumped, the teeth of the locking ring, when in the locking state, may even impress or cut into the surface of the rotor shaft and thereby, at the desired axial position, may create grooves for the teeth to engage with. Such created grooves provide for a positive-fit in addition to the frictional force for securing the locking ring axially to the rotor shaft. 
     Optionally, the pump may further comprise a first axial stop body and a second axial stop body, wherein the first axial stop body is the locking ring. The bearing body is thus fully axially locked between the two axial stop surfaces. Optionally, the pump may further comprise an impeller nut encompassing the rotor shaft for fixing an impeller to the rotor shaft, wherein the impeller nut is the second axial stop body. 
     Optionally, the second axial stop body may define N 1  engagement location(s) for preventing a rotational movement of the bearing body relative to the rotor shaft, wherein the bearing body comprises a first axial end facing the locking ring and a second axial end facing away from the locking ring, wherein the second axial end comprises N 1  engagement location(s) positive-locking with the engagement location(s) of the second axial stop body. These positive-locking engagement location(s) thus prevent a rotational movement of the bearing body relative to the rotor shaft. Optionally, the second axial stop body and the second axial end of the bearing body may comprise N≥2 engagement locations, wherein the engagement locations are arranged in an N-fold symmetry with respect to the rotor axis. “N-fold symmetry” shall mean herein “N evenly distributed locations along the circumference”, e.g. N=2 locations would have an angular distance of 180° to each other, i.e. at diametral opposite sides, whereas N=3 locations would have an angular distance of 120° to each other. The N-fold symmetric arrangement of engagement locations provides for N different angular orientation options how the bearing body can be mount the rotor shaft. This may be beneficial to find the best positive fit among the N options for being less prone to manufacturing tolerances. 
     Optionally, a first one of the engagement locations may be configured fora positive-locking fit having a lower tolerance in tangential and/or axial direction than a second one of the engagement location(s). This means that the second engagement location has a has larger clearance or “wiggle room” between the bearing body and the second axial stop body in tangential and/or axial direction. The first one of the engagement locations may be considered as the designated “best” positively fitting engagement location. The other engagement location(s) may be considered as auxiliary engagement locations for backing the prevention of rotational movement of the bearing body relative to the rotor shaft in case the first engagement location wears out. 
     Optionally, at least said first engagement location at the second axial stop body and/or the second axial end of the bearing body may comprise a convex axial contact surface for providing the only axial contact between the bearing body and the second axial stop body. The convex axial contact surface thereby provides a well-defined point of axial contact for the bearing body. 
     Optionally, the locking ring may define a circumferential gap between a first circumferential end portion of the locking ring and a second circumferential end portion of the locking ring, wherein the gap is smaller in the locking state than in the mounting state. The locking ring with the gap provides for a certain resilient flexibility to be widened into the mounting state. 
     Optionally, the locking ring may comprise a security hook extending from the first circumferential end portion and overlapping the circumferential gap, wherein the security hook is configured to hook into the second circumferential end portion of the locking ring for preventing, in the mounting state, a further radial expansion of the locking ring. The security hook may thus prevent an over-expansion of the locking ring into a plastic deformation. A plastic deformation should be avoided, because it would reduce the elastic restoring force of the locking ring for pressing the teeth against the shaft surface. A tool may be used to widen the locking ring into the mounting state. Alternatively, or in addition to the security hook, the tool may comprise means for preventing an over-expansion of the locking ring. 
     Optionally, the locking ring may comprise a stress portion between a first circumferential end portion of the locking ring and a second circumferential end portion of the locking ring, wherein the annulus area of the locking ring reduces from the stress portion towards the first circumferential end portion and towards the second circumferential end portion. In other words, the radial thickness of the locking ring may reduce towards the gap between the circumferential end portions. The radially thicker stress portion may provide structural stability and the radially thinner circumferential end portions may provide structural elasticity both facilitating resilient elastic deformation between the mounting state and the locking state. 
     Optionally, the locking ring may define an envelope of maximal radial expansion, wherein the maximal radial expansion is equal to or smaller than the radius of the radially outer bearing surface. Thereby, the rotorshaft can be mounted to the pump as a pre-assembled unit with the bearing body being fixed to the rotor shaft by means of the locking ring. 
     Optionally, each of the teeth of the locking ring may form an inward blade. Such blades may cut grooves into the rotor shaft surface, wherein the grooves extend essentially perpendicular to the rotor axis. Such blades may be sharpened inwardly to increase the frictional contact and/or to facilitate the cutting of grooves. 
     Optionally, the teeth may be located at M≥2 locking ring segments with a central angle α of 30°≤α≤90°, wherein the locking ring segments are preferably arranged symmetrically with respect to a symmetry plane spanned by the rotor axis and a direct virtual connecting line between the centre of the circumferential gap and the centre of the stress portion, and wherein the circumferential gap is preferably located centrally in one of the locking ring segments. There may thus be M≥2 tooth-free segments, each located between two neighbouring locking ring segments of said M 2  locking ring segments having teeth. Thereby, the teeth may be arranged at those locations of the locking ring, where the elastic restoring force provides the highest normal force on the rotor shaft surface to press the teeth against the rotor shaft surface. 
     Optionally, the locking ring may comprise a first one of the teeth at a first circumferential end portion of the locking ring and a second one of the teeth at a second circumferential end portion of the locking ring, wherein the first tooth and the second tooth preferably extend over an arc length with a central angle β of less than 10°. The optimal length of the teeth may be a compromise between their ability to exert a force on the rotor shaft and their structural stability to secure the bearing body axially. 
     The shorter they are the better, i.e. with higher pressure, they may press against the rotor shaft. The longer they are the more locking ring material actually participates in the axial locking function. A range of arc length teeth between 5° and 10° was found to be a good compromise. 
     Optionally, the locking ring may comprise a third one of the teeth and a fourth one of the teeth at a stress portion between the first circumferential end portion of the locking ring and the second circumferential end portion of the locking ring, wherein the third tooth and the fourth tooth each preferably extend over an arc length with a central angle γ of less than 60°. The third and fourth tooth may have the same arc length as the first and second tooth, e.g. 5°-10°. However, the normal forces provided at the radially thicker stress portion may be higher so that the third tooth and the fourth tooth are preferably longer to involve more material in the axial locking function while pressing against the rotor shaft surface with a similar pressure as the first tooth and the second tooth. It is desired that the pressure with which the teeth press against the rotor shaft is about the same for all teeth even if the teeth differ in size and/or length and/or shape. 
     Optionally, the pump may be a single stage or multistage centrifugal pump for pumping a fluid, wherein the fluid to be pumped serves as a lubricant on the radially outer bearing surface of the bearing body. 
     Optionally, the pump may be a single stage or multistage centrifugal pump for pumping a fluid, wherein the locking ring is configured to be bathed in the fluid to be pumped so that the fluid provides an abrasive environment facilitating, in the locking state, an impressing of the teeth of the locking ring into the radial outer surface of the rotor shaft. 
     The pump according to any of the preceding claims, wherein the teeth may be harder than the surface of the rotor shaft. This facilitates the impressing of grooves into the surface of the rotor shaft by the teeth of the locking ring. For instance, the teeth and/or the whole locking ring with the teeth being an integral part thereof may be made of stainless steel, e.g. EN 1.4410 (Alloy 2507; X 2 CrNiMoN 25-7-4), which is highly resistant to crevice corrosion, erosion corrosion and corrosion fatigue under tension. The rotor shaft and/or the rotor shaft surface may be made of another type of stainless steel, e.g. EN 1.4462 (Alloy 2205; X 2 CrNiMoN 22-5-3), which is also highly resistant to erosion corrosion, but slightly softer than the teeth of the locking ring. The hardness made be determined in terms of Vickers hardness number (HV) as set forth in DIN EN ISO 6507. The teeth may thus have a hardness of HV 290, whereas the rotor shaft surface may have a hardness of HV 270. 
     SUMMARY OF THE DRAWINGS 
     Embodiments of the present disclosure will now be described by way of example with reference to the following figures of which: 
       FIG. 1  shows a perspective view on a pump according to an example of an embodiment of the present disclosure; 
       FIG. 2  shows a perspective view on a section of a rotor shaft with assembled impeller, impeller nut, bearing body and locking ring as parts of a pump according to an example of an embodiment of the present disclosure; 
       FIG. 3  shows a top view on a section of a rotor shaft with assembled impeller, impeller nut, bearing body and locking ring as parts of a pump according to an example of an embodiment of the present disclosure; 
       FIG. 4  shows a side view on a section of a rotor shaft with assembled impeller, impeller nut, bearing body and locking ring as parts of a pump according to an example of an embodiment of the present disclosure; 
       FIG. 5  shows a longitudinal cut view on a section of a rotor shaft with assembled impeller, impeller nut, bearing body and locking ring as parts of a pump according to an example of an embodiment of the present disclosure; 
       FIG. 6  shows a top view on a first embodiment of the locking ring as part of a pump according to an example of an embodiment of the present disclosure; 
       FIG. 7  shows a top view on a second embodiment of the locking ring as part of a pump according to an example of an embodiment of the present disclosure; 
       FIGS. 8 a,b    show detailed top views illustrating how the teeth of a locking ring may impress into the rotor shaft surface according to an example of an embodiment of the present disclosure; 
       FIG. 9  shows a side view on a section of a rotor shaft with assembled, impeller nut, bearing body and locking ring as parts of a pump according to an example of an embodiment of the present disclosure; 
       FIG. 10  shows a longitudinal cut view on the plane A-A as indicated in  FIG. 9 ; 
       FIG. 11  shows another side view on a section of a rotor shaft with assembled, impeller nut, bearing body and locking ring as parts of a pump according to an example of an embodiment of the present disclosure; 
       FIG. 12  shows a longitudinal cut view on the plane B-B as indicated in  FIG. 11 ; 
       FIG. 13  shows a cross-sectional cut view on the plane C-C as indicated in  FIG. 12 ; 
       FIG. 14  shows a perspective view on a section of a rotor shaft with assembled impeller, impeller nut, and bearing body as parts of a pump according to an example of an embodiment of the present disclosure with a detail zoom on an engagement location; and 
       FIG. 15  shows a perspective view on a bearing body a part of a pump according to an example of an embodiment of the present disclosure. 
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a big multistage centrifugal pump  1  with a 75 kW electric motor in an upright standing vertical configuration. The motor is located in a motor housing  3  being mounted on a motor stool  5 , sometimes referred to as lantern, located between the motor housing  3  and a pump housing  7 . A rotor shaft (not visible in  FIG. 1 ) extends along a vertical rotor axis R from the motor through the motor stool  5  into the pump housing  7 , where a stack of impellers (not visible in  FIG. 1 ) is mounted to the rotor shaft. When the pump  1  is connected with its inlet  9  and outlet  11  to a pipe system, the impellers are immersed into the fluid to be pumped. The motor drives the rotor shaft with the impellers to convey fluid from the inlet  9  to the outlet  11 . The rotor shaft is radially held in position within the pump  1  by one or more radial bearings circumferentially encompassing the rotor shaft in order to allow for low-friction rotation of the rotor shaft. The fluid to be pumped, e.g. water, may serve here as a lubricant on a radially outer bearing surface of the radial bearing(s). The radial bearing(s) and/or the radially outer bearing surface may comprise ceramic material for low-friction. 
       FIG. 2  gives a better view on a section of a rotor shaft  13  with an impeller  15  mounted to it by way of an impeller nut  17 . The impeller nut  17  comprises an inner thread screwed on an outer thread at the impeller  15 . The impeller nut  17  comprises a portion of hexagonal cross-sectional shape for a spanner or wrench to engage with for screwing. By tightening the impeller nut  17 , the impeller  15  and the impeller nut  17  are axially and rotationally fully fixed to the rotor shaft  13 . 
     A bearing body  19  circumferentially encompasses the rotor shaft  13  axially above the impeller nut  17 . Thereby, the bearing body  19  rests on the impeller nut  17 . The impeller nut  17  thus forms an axial stop body  21  for an axial bottom end  23  of the bearing body  19 . In order to fully axially fix the bearing body  19 , a locking ring  25  circumferentially encompasses the rotor shaft  13  axially above bearing body  19 . The locking ring  25  is an axial stop body  27  for an axial top end  29  of the bearing body  19 . The bearing body  19  is thus axially fixed between the two axial stop bodies  21 ,  27 . 
     The locking ring  25  comprises at least two radially inwardly protruding teeth (see  FIGS. 6, 7 and 8   a,b ), which are configured to press against a radial outer surface  31  of the rotor shaft  13  by the elastic restoring force of the locking ring  25 . Thereby, the locking ring  25  is fixed by a frictional force and preferably by a positive-fit with impressed grooves against axial displacement when it is in a locking state as shown in  FIG. 2 . However, it should be noted that the rotor shaft  13  does not comprise a pre-fabricated circumferential groove for the locking ring  25  to engage with. The teeth of the locking ring  25  press against the radial outersurface  31  of the rotor shaft  13  and may thereby create grooves to positively engage with over time and use. The locking ring  25  is radially expandable from the shown locking state to a widened mounting state against an elastic restoring force of the locking ring  25 . The teeth of the locking ring  25  are, in the widened mounting state, disengaged from the rotor shaft  13  so that the locking ring  25  in then positionable at a desired axial position on the rotor shaft  13 . This allows an easy mounting and dismounting of the locking ring  25  and the bearing body  19 . 
     The bearing body  19  is not rotationally fixed by the locking ring  25  or the rotor shaft  13 , but by means of at least one engagement location  33  at the axial bottom end  23  of the bearing body  19 . The engagement location  33  is here a female recess in the axial bottom end  23  of the bearing body  19  being engaged in a positive form-fit with a correspondingly formed engagement location  35  in form of a male axial protrusion at the axial stop body  21 , i.e. the impeller nut  17 . Alternatively, the engagement location  33  may be a male axial protrusion in the axial bottom end  23  of the bearing body  19  being engaged in a positive form-fit with a correspondingly formed engagement location  35  in form of a female recess at the axial stop body  21 , i.e. the impeller nut  17 . The engagement locations  33 ,  35  prevent a rotational movement of the bearing body  19  relative to the rotor shaft  13 . 
     The top view of  FIG. 3  illustrates nicely that the radially outer contour of the locking ring  25  does not radially extend the radius of a radial outer bearing surface  32 . In other words, the locking ring  25  defines an envelope of maximal radial expansion, wherein the maximal radial expansion is equal to or smaller than the radius of the radially outer bearing surface  32 . 
     Thereby, the locking ring  25  does not impede or complicate mounting or dismounting of the rotor shaft  13  together with the impeller  15  and the bearing body  19  as a pre-assembled unit into or out of the pump  1 . 
       FIGS. 4 and 5  show the impeller nut  17  fixes the impeller  15  to the rotor shaft  13  by frictional force. By tightening the impeller nut  17  onto an upper threaded portion  39  of the impeller  15 , an inner wedge element  37  is pushed downward and squeezed between the upper threaded portion of the impeller  15  and the rotor shaft  13 . The radial normal forces between rotor shaft  13 , wedge element  37 , upper threaded portion of impeller  15  and the impeller nut  17  are then high enough to provide a secure connection by frictional force. It should be noted that the axial positioning of the impeller  15  is adjustable to the desired position. In combination with the locking ring  25  that does not require any specific feature at the rotor shaft  13  to be fixed thereto, the assembly is more “resilient” or less prone to manufacturing tolerances. The axial position of the impeller  15  and the bearing body  19  can be chosen as seem best fit relative to other parts of the pump  1 . 
       FIG. 6  shows a first embodiment of the locking ring  25  in more detail. The locking ring  25  is not circumferentially closed, but comprises two circumferential end portions  41 ,  43  defining a gap  45  therebetween. The locking ring  25  is shown in a relaxed state of minimal radial expansion with a minimal width of the gap  45 . The locking ring  25  can be widened, e.g. by a tool, into a mounting state in which the locking ring  25  may be put onto the rotor shaft  13 . The locking ring  25  is not flexible enough to open the gap  45  wider than the diameter of the rotor shaft  13  so that the locking ring could be “clicked” sideways onto the rotor shaft  13 . However, the widened locking ring  25  in the mounting state can be slipped over the rotor shaft  13  from one axial end into the desired axial position to axially fix the bearing body  19 . The maximal expansion in the mounting state is limited by structural constraints not to reach a point of plastic deformation. It has shown that the gap  45  may approximately be doubled in width to stay within a region of essentially elastic deformation in which the elastic restoring force of the locking ring is able to essentially fully restore the relaxed state of minimal radial expansion as shown in  FIG. 6 . In order to prevent a wider expansion into a plastic deformation, a security hook  47  extends from the first circumferential end portion  41  and overlaps the circumferential gap  45 . The security hook  47  hooks into the second circumferential end portion  43  of the locking ring  25  for preventing, in the mounting state, a further radial expansion of the locking ring  25 . The length of the security hook  47  thereby defines the maximal width of the gap  45 . The locking state of the locking ring  25  is somewhere between the relaxed state as shown in  FIG. 6  and a widened mounting state allowing to axially move the locking ring  25  along the rotor shaft  13 . When slipped over the rotor shaft  13 , the locking ring  25  cannot fully relax as shown in  FIG. 6 , but will always be under tension of its elastic restoring force. 
     The locking ring  25  comprises four radially inwardly protruding teeth  49 ,  51 ,  53 ,  55 , wherein a first tooth  49  is located at the first circumferential end portion  41  and a second tooth  51  is located at the second circumferential end portion  43 . The first tooth  49  and the second tooth  51  have essentially the same size and shape. They extend in form of inward blades over an arc length with a central angle β of less than 20°, here 13° . A third tooth  53  and a fourth tooth  55  are longer and extend in form of inward blades over an arc length with a central angle β of less than 60°, here 48°. The third tooth  53  and a fourth tooth  55  are located at a circumferential position such that their angular distance to each other is about the same as their angular distance to the first tooth  39  and the second tooth  51 , respectively. Thus, the teeth  49 ,  51 ,  53 ,  55  are located at three locking ring segments  57   a,b,c  each with a central angle α of 60° arranged in a three-fold symmetry with respect to the rotor axis R. The 60°-segments between the locking ring segments  57   a,b,c  comprise no teeth. The gap  45  is located centrally in one  57   a  of the locking ring segments  57   a,b,c.  The position and length of the teeth  49 ,  51 ,  53 ,  55  are thus optimised for pressing effectively against the rotor shaft surface  31 . 
     The radial width of the locking ring  25  reduces towards the gap  45 . The locking ring  25  thus comprises a radially thicker stress portion  59  between the radially thinner circumferential end portions  41 ,  43 . In other words, the annulus area of the locking ring  25  reduces from the stress portion  59  towards the circumferential end portions  41 ,  43 . The stress portion  59  provides for the structural stability and a large fraction the elastic restoring force of the locking ring  25 . In addition, the radially thinner circumferential end portions  41 ,  43  allow for the security hook  47  to be placed within an envelope of maximal radial expansion that is equal to or smaller than the radius of the radially outer bearing surface  32 . 
       FIG. 7  shows a second embodiment of the locking ring  25  that differs in two separate and independent aspects from the first embodiment shown in  FIG. 6 . The first aspect is that the security hook  47  and the second circumferential end portion  43  comprise correspondingly inclined surfaces  61 ,  63  that are configured to engage with each other in the widened mounting portion so that the security hook  47  safely engages with the second circumferential end portion  43 . The safety hook  47  cannot slip off by a radially outward deformation, because the mutually engaging inclined surfaces  61 ,  63  prevent radial separation. 
     The second aspect in which the embodiment shown in  FIG. 7  differs from the embodiment shown in  FIG. 6  is that the third tooth  53  and fourth tooth  55  are actually formed by pair of teeth  53   a,b  and  55   a,b , wherein each of the teeth  53   a,    53   b,    55   a,    55   b  has a similar arc length as the first tooth  49  and the second tooth  51 . Various arrangements of teeth may be possible here. Alternatively or in addition, there could be a plurality of teeth distributed at the radially inner side of the locking ring  25 , wherein it is advantageous to have M≥2 segments without any teeth between M≥2 segments that comprise teeth. 
       FIGS. 8 a  and 8 b    illustrate how a tooth  55   b  presses against the rotor shaft surface  31 .  FIG. 8 a    shows the locking ring  25  in a locking state preventing an axial displacement of the locking ring  25  along the rotor shaft  13  by frictional force between the tooth  55   b  and the rotor shaft surface  31 . There is no pre-fabricated groove in the rotor shaft surface  31  for the locking ring  25  to engage with.  FIG. 8 b    shows the the locking ring  25  in a mounting state after it has been used over some time in the locking state during which it has impressed the tooth  55   b  into the rotor shaft surface  31 . The rotor shaft surface  31  was thereby plasticly deformed so that a groove  65  remains in the rotor shaft surface  31  even after the locking ring  25  is expanded for demounting into the mounting state as shown in  FIG. 8 b   . In the not shown locking state, the groove  65  and the tooth  55   b  are engaged with each other in a positive form-fit. 
       FIGS. 9 and 10  show that there is small axial gap  67  between the axial bottom end  23  of the bearing body  19  and the axial top face of the impeller nut  17 . There is very low tolerance fit radially between the bearing body  19  and the rotor shaft  13 , but the small axial gap  67  allows for higher tolerances in terms of axial alignment. For instance, the axial top face of the impeller nut  17  may not be precisely perpendicular to the rotor axis R. 
       FIGS. 11, 12 and 13  shows that there are two pairs of engagement locations  33   a,b ,  35   a,b  located at diametrical opposite sides of the axial bottom end  23  of the bearing body  19  and the axial top face of the impeller nut  17 . A first one  33   a , 35   a  of these pairs of engagement locations  33   a,b ,  35   a,b  has a tighter positive-locking fit, i.e. it has a lower tolerance in both tangential and axial direction. In this case, the axial female recess  33   a  in the bearing body  19  is axially shorter and tangentially narrower than the other axial female recess  33   b  in the bearing body  19 , wherein the male axial protrusions  35   a,    35   b  at the axial top face of the impeller nut  17  are of essentially identical shape and size. Thereby, the first pair of engagement locations  33   a,    35   a  actually provides a defined axial point of contact between the bearing body  19  and the impeller nut  17 . The second pair of engagement locations  33   b,    35   b  with a relatively looser axial and tangential fit only functions as a back-up when the first pair of engagement locations  33   a,    35   a  wears out. During assembly, there are actually two options for mounting the bearing body  19 . The bearing body  19  can be rotated by 180° around the rotor axis R so that axial female recess  33   a  engages with the other male protrusion  35   b.  This is useful if manufacturing tolerances result in the male axial protrusions  35   a,    35   b  at the axial top face of the impeller nut  17  not being of exactly identical shape and size. If the axial female recess  33   a  does not fit on the axial male protrusion  35   a,  the other male protrusion  35   a  may be tried for fit. The assembly is thus less prone to manufacturing tolerances. It is possible to have N≥1 pair(s) of engagement locations arranged in an N-fold symmetry so that there are N options to find a best fit. 
       FIG. 14  shows the shape of the second back-up pair of engagement locations  33   b,    35   b  with a relatively loose axial and tangential fit. There is no contact between the female recess  33   b  and the male protrusion  35   b  as long as the tight fit of the first pair of engagement locations  33   a,    35   a  on the other side (not visible in  FIG. 14 ) wears out. The female recess  33   b  has a rounded M-shape, wherein lateral faces  69   a,b  are slightly inclined so that the recess  33   b  narrows in the direction away from the impeller nut  17 . A ceiling of the recess  33   b  forms a convex axial contact surface  71 . The male protrusion  35   b  has essentially a cuboidal shape. 
     As shown in  FIG. 15 , where the bearing body  25  is illustrated alone, the first recess  33   a  has the same shape as second recess  35   b  as described above for  FIG. 14 , but smaller both in height and width. The narrowing lateral faces  69   a,b  and the convex axial contact surface  71  of the first recess  33   a  allow fora tight fit for rotational fixing and a well-defined single point of axial contact between the bearing body  19  and the impeller nut  17 . The corresponding first male protrusion  35   a  has, analogous to the second male protrusion  35   b  shown in  FIG. 14 , essentially a cuboidal shape providing a top face abutting against the convex axial contact surface  71  of the first recess  33   a  of the bearing body  19 . 
     Where, in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. 
     Reference should be made to the claims for determining the true scope of the present disclosure, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the disclosure that are described as optional, preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. 
     The above embodiments are to be understood as illustrative examples of the disclosure. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. While at least one exemplary embodiment has been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art and may be changed without departing from the scope of the subject matter described herein, and this application is intended to cover any adaptations or variations of the specific embodiments discussed herein. 
     In addition, “comprising” does not exclude other elements or steps, and “a” or “one” does not exclude a plural number. Furthermore, characteristics or steps which have been described with reference to one of the above exemplary embodiments may also be used in combination with other characteristics or steps of other exemplary embodiments described above. Method steps may be applied in any order or in parallel or may constitute a part or a more detailed version of another method step. It should be understood that there should be embodied within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of the contribution to the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the disclosure, which should be determined from the appended claims and their legal equivalents. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  pump 
           3  motor housing 
           5  motor stool 
           7  pump housing 
           9  inlet 
           11  outlet 
           13  rotor shaft 
           15  impeller 
           17  impeller nut 
           19  bearing body 
           21  axial stop body 
           23  axial bottom end of the bearing body 
           25  locking ring 
           27  axial stop body 
           29  axial top end of the bearing body 
           31  rotor shaft surface 
           32  radially outer bearing surface 
           33   a,b  engagement location in form of a recess 
           35   a,b  engagement location in form of a protrusion 
           37  inner wedge element 
           39  threaded portion of the impeller 
           41  first circumferential end portion of the locking ring 
           43  second circumferential end portion of the locking ring 
           45  circumferential gap in the locking ring 
           47  security hook 
           49  first tooth 
           51  second tooth 
           53  third tooth 
           55  fourth tooth 
           57   a,b,c  locking ring segments with teeth 
           59  stress portion 
           61  inclined surface of security hook 
           63  inclined surface of second circumferential end portion 
           65  impressed groove in the rotor shaft surface 
           67  axial gap between the bearing body and impeller nut 
           69   a,b  lateral faces of engagement location in form of a recess 
           71  convex axial contact surface 
         R rotor axis 
         D direct virtual connecting line between the centre of the circumferential gap and the centre of the stress portion 
         α central angle of locking ring segments with teeth 
         β arc length of first and second tooth 
         γ arc length of third and fourth tooth