The present invention relates, in general, to a pump for pumping liquid, and in particular to a pump for pumping contaminated liquid comprising solid matter such as sewage water which may comprise polymers, hygiene articles, textiles, rags etc.

In accordance with a first aspect, the present

invention relates to a pump comprising a pump chamber and an impeller arranged to rotate in said pump chamber, said impeller being suspended in a lower end of an axially extending drive shaft unit. The lower end of the drive shaft unit is received in a cylinder-shaped recess of the

impeller, wherein the impeller is displaceable back and forth in the axial direction in relation to the drive shaft unit between a lower and an upper position.

In accordance with a second aspect, the present invention relates to an impeller assembly for placement in a pump chamber of a pump for pumping liquid.

Background of the invention and state of the art

In plants such as sewage treatment plants, septic tanks, wells etc., it occurs that solid matter or other pollutants such as socks, sanitary towels, paper etc.

obstruct the pump of the plant, for example a submersible pump that is submerged in the basin of the plant. When the impeller and the impeller seat are positioned at a fixed distance in relation to each other, the solid matter is sometimes too large to pass through the pump. Large pieces of the solid matter may in worst case cause the impeller to wedge, thus seriously damaging the pump. Such an

unintentional shutdown is costly since it entails expensive, tedious and unplanned maintenance work.

A known method of getting rid of the solid matter is to provide centrifugal pumps with means for cutting the solid matter into smaller pieces and subsequently remove said smaller pieces together with the pumped liquid. However, the cutting up of solid matter requires a lot of energy, which is negative, in particular since pumps of this kind operate for long time periods. Another conventional way of getting rid of the solid matter is to use an impeller with a single blade that has a large through-channel capable of allowing solid matter to pass. One disadvantage associated with pump of this type is that the solid matter often gets entangled around the leading edge of the impeller. A third effort of solving the problem of large pieces of solid matter

obstructing the pump is to use an arrangement where the impeller is arranged at a fixed distance from the seat of the impeller, e.g. 30-40 mm. A big disadvantage is

nevertheless that the pump in such a case always has a very limited efficiency.

European patent EP 1,899,609 discloses a pump

comprising a pump housing with a rotatable impeller, said impeller being suspended by a drive shaft, and an impeller seat. The impeller is movable in the axial direction in relation to the impeller seat so that it may allow larger pieces of solid matter to pass through, pieces that

otherwise would block the pump or wedge the impeller. The impeller has a cylinder-shaped recess in which the lower end of the drive shaft unit is received, and the impeller is completely freely displaceable in the axial direction between a lower position and an upper position. According to this solution, the impeller lacks a well-defined rest position/basic position - the lower position is adopted exclusively due to the own weight of the impeller. This renders difficult the adjustment of the axial gap between the impeller and the impeller seat so that said gap ends up within the preferred range 0,2-0,8 mm. The only basic position discussed in above-cited patent document is that the impeller may adopt the upper position by means of a spring bias that overcomes the own weight of the impeller so as to facilitate starting-up of the pump. It is, however, an extremely imprecise method for adjusting the axial gap when the impeller is in the upper position.

Certain sold products based on above-cited European patent have used a helical spring arranged between the end surface of the drive shaft unit and a bottom of the

cylinder-shaped recess of the impeller in order to permit due adjustment of the axial gap. This helical spring is relatively strong so as to obtain the well-defined rest position, and the helical spring entails the disadvantage that when the impeller is to be displaced in axial direction in relation to the drive shaft, the helical spring acts with increasing force in relation to increasing distance of displacement. This renders difficult the passage of the solid matter through the pump.

It should also be mentioned that submersible pumps of the above kind are used to pump liquid from basins that are difficult to maintain and that the pumps often operate for 12 or more hours daily. It is therefore utterly desirable to provide a pump with long longevity.

Object of the Invention

The present invention aims at obviating the above- mentioned disadvantages and failings of the previously known pumps and to provide an improved pump. A primary object of the invention is to provide an improved pump and impeller assembly of the type defined in the introduction, wherein the impeller has a well-defined rest position/lower position that entails that adjustment of the axial gap between the impeller and the suction cover of the pump may be done with great precision.

A further object of the present invention is to provide a pump and an impeller assembly wherein, when the pump is operating, the displacement of the impeller away from the impeller seat, due to an axially applied force, isn't counteracted .

It is also an object of the present invention to provide an improved pump of the type defined in the

introduction, wherein said pump in a reliable manner allows large pieces of solid matter to pass through the pump.

Short Description of the Invention

In accordance with the invention, at least the primary object is achieved by means of the initially descried pump, having the features defined in the independent claims.

Preferred embodiments of the present invention are further defined in the dependent claims.

In accordance with the present invention, a pump of the type defined in the introduction is provided, said pump being characterized in that the pump further comprises a snap-lock coupling arranged at the interface between the drive shaft unit and the cylinder-shaped recess, wherein the snap-lock coupling is adapted to position the impeller in the lower position when an applied force acting to displace the impeller in a direction away from the lower position is less than a threshold value.

Hence, the present invention is based on the

understanding that by using a snap-lock coupling, a well- defined rest position/basic position is obtained. Said rest position/basic position may be used both when the pump is operating and when the pump is being mounted, and while the axial gap is being adjusted.

According to a preferred embodiment of the present invention, the snap-lock coupling is adapted to, in the axial direction, disengage the impeller when the impeller is positioned at a distance from said lower position. This entails that the snap-lock coupling doesn't act on the impeller with any axially directed force, when said impeller is displaced away from the rest position/lower position. According to a preferred embodiment of the present invention, a locking element belonging to the snap-lock coupling is arranged in a recess of the locking element, said recess being arranged in the impeller's cylinder-shaped recess, wherein the locking element of the snap-lock coupling is made up of an annular spring and the recess of the snap-lock coupling is made up of a groove. This entails that the present invention necessitates few parts and that the annular ring also may be used to secure the impeller onto the drive shaft unit.

In a further preferred embodiment, the circumference of the annular spring is variable, when viewed in axial direction. This entails that a weaker annular ring may be used, which renders possible the use of a lower threshold value .

According to a preferred embodiment of the present invention, the drive shaft unit comprises a drive shaft and a sleeve, wherein the sleeve surrounds and is releasably connected to the drive shaft and wherein the sleeve makes up part of the lower end of the drive shaft unit. This entails that the sleeve may be mounted in the cylinder-shaped recess of the impeller while the impeller assembly is being assembled, whereby the impeller assembly may be sold as an up-grading kit for existing pumps with axially displaceable impeller .

Further advantages and inventive features of the invention will be clear from the remaining dependent claims and from the following, detailed description of preferred embodiments .

Short Description of the Drawings

A more complete understanding of the above-mentioned and other features as well as advantages of the present invention will be clear from the following, detailed description of the preferred embodiments with reference to the accompanying drawings, where:

Fig. 1 is a schematic cutaway side view of a hydraulic unit of an inventive pump, the figure showing the impeller in the lower position,

Fig. 2 is a schematic cutaway side view of an inventive impeller assembly according to a preferred

embodiment, wherein the impeller is in the lower position,

Fig. 3 is a schematic cutaway side view of the inventive impeller assembly corresponding to figure 2, wherein the impeller is in a position at a distance from the lower position,

Fig. 4 is a schematic cutaway side view of the drive shaft unit according to a first embodiment,

Fig. 5 is a schematic cutaway side view of the drive shaft unit according to a second embodiment,

Fig. 6 is a schematic cutaway view from above of the drive shaft unit corresponding to figure 4, taken along the line VI -VI, as well as a part of the impeller, Fig. 7 is an enlarged partial view of Fig. 2,

Fig. 8 is an enlarged partial view of Fig. 2, said view showing a detail of the snap-lock coupling according to a first embodiment,

Fig. 9 is a partial view corresponding to Fig. 8, said view showing a detail of the snap-lock coupling according to a second embodiment,

Fig. 10 is an enlarged partial view of Fig. 2, said view showing another detail of the snap-lock coupling according to the first embodiment,

Fig. 11 is a partial view corresponding to Fig. 10, said view showing the detail of the snap-lock coupling according to an alternative embodiment,

Fig. 12 is a schematic perspective view of a locking element according to a first embodiment, Fig. 13 is a schematic cutaway view from above of the drive shaft unit and the impeller in accordance with Fig. 7, taken along the line A-A, and also comprising the locking element according to Fig. 12,

Fig. 14 is a schematic cutaway view from above of the drive shaft unit and the impeller in accordance with Fig. 7, taken along the line B-B, and also comprising the locking element according to Fig. 12,

Fig. 15 is a schematic perspective view of a locking element according to a second embodiment,

Fig. 16 is a schematic cutaway view from above of the drive shaft unit and the impeller in accordance with Fig. 7, taken along the line A-A, and also comprising the locking element according to Fig. 15,

Fig. 17 is a schematic cutaway view from above of the drive shaft unit and the impeller in accordance with Fig. 7, taken along the line B-B, and also comprising the locking element according to Fig. 15,

Fig. 18 is a schematic perspective view from above of an

impeller seat.

Detailed Description of the Preferred Embodiments

Reference is initially made to Fig. 1, where a part of an inventive pump is shown, more specifically its hydraulic unit, generally designated 1. In Fig. 1, the remaining parts of the pump are removed for the sake of clarity. These parts are inter alia a drive unit and a sealing unit positioned between the hydraulic unit and the drive unit. The present invention relates to pumps in general, but in the preferred embodiment the pump is a submersible, centrifugal pump. The present invention will be described in conjunction with such a pump, without in any way being limited thereto.

The hydraulic unit 1 comprises a pump housing or volute 2 that delimits a pump chamber 3, an impeller 4 arranged to rotate in said pump chamber 3, the impeller being suspended in a lower end 5 of an axially extending drive shaft unit, generally designated 6, and a suction cover 7 with a

centrally located inlet opening 8 for incoming liquid flow. The suction cover 7, also known as the impeller seat, is preferably releasably connected to the pump housing 2, e.g. by means of a plurality of bolts, in such a way that the suction cover 7 cannot rotate relative the pump housing 2. The impeller 4 is rotatably driven by the drive shaft unit 6 when the pump is operating. Furthermore, the pump housing 2 comprises an outlet opening 9 for outgoing liquid flow, said outlet opening 9 being radially directed in the shown embodiment .

Central feature of the present invention is that the impeller 4 is displaceable back and forth in the axial direction in relation to the drive shaft unit 6 between a lower position (shown in Fig. 1) and an upper position. When the impeller 4 is displaced from the lower position, the impeller 4 is also displaced in direction away from the suction cover 7 so as to let pass large pieces of solid matter present in the pumped liquid.

Now, the reference is primarily made to Figs. 2 and 3 showing the inventive impeller 4. The impeller 4 shown in Fig. 2 is positioned in the lower position and in Fig. 3 the impeller 4 is positioned at a distance from the lower position. As discussed above, when the impeller 4 is

positioned in the upper position, the impeller 4 may have been displaced even further in relation to the drive shaft unit 6 then the position shown in Fig. 3. The impeller 4 comprises a cylinder-shaped recess 10, the lower end 5 of the drive shaft unit 6 being received in said cylinder- shaped recess 10.

Reference is now also made to Figs. 4-6. In the

embodiment shown in Fig. 4 the drive shaft unit 6 comprises a drive shaft 11 and a sleeve 12, wherein the sleeve 12 surrounds and is releasably connected to the drive shaft 11. Hence, the sleeve 12 makes up part of the lower end 5 of the drive shaft unit 6. The sleeve 12 is connected to the drive shaft 11 in any suitable way, and in the shown embodiment the sleeve 12 is connected to the drive shaft 11 by means of a conventional tool cone 13. The drive shaft 11 is cone- shaped and the tool cone 13 is pressed onto the drive shaft

11 using a bolt 14 that is tightened, whereupon the tool cone 13 is forced radially outwards such that the sleeve 12 is braced or clamped onto the drive shaft 11. The advantage of this embodiment is that the axial position between the sleeve 12 and the drive shaft 11 may be adjusted by

loosening the bolt 14, axially displacing the sleeve 12 and subsequently retightening the bolt 14. In the embodiment shown in Fig. 5 the drive shaft unit 6 is a homogenous detail that makes up the lower end 5 of the drive shaft unit 6.

According to another, not shown embodiment, the sleeve

12 is screwed onto the end of a cylinder-shaped, non- conical, drive shaft 11 and this embodiment entails

adjustment of the axial position between the sleeve 12 and the drive shaft 11 by arranging a desired number of shims between the sleeve 12 and the drive shaft 11. It should be pointed out that that this embodiment functions and is perceived as if the lower end 5 of the drive shaft unit 6 is made up of a homogenous detail when the latter is mounted.

The drive shaft unit 6 and the impeller 4 are jointly rotatable. In the embodiment shown in Fig. 6 the pump comprises a carrier in the shape of a bar or rod 14, positioned at the interface of the lower end 5 of the drive shaft unit 6 and the cylinder-shaped recess 10 of the impeller 4. The rod 14 is positioned in oppositely arranged recesses of the lateral surface of the lower end 5 of the drive shaft unit 6 and in an inner surface of the cylinder- shaped recess 10. According to an alternative embodiment, a plurality of rods 14, or carriers, may be distributed along said interface, preferably equidistantly distributed. The carrier may be fixedly connected to, or be a part of, the lower end 5 of the drive shaft unit 6. In an alternative, not shown embodiment, a spline coupling is arranged at said interface .

The lower end 5 of the drive shaft unit 6 has in a preferred embodiment a lower, thicker part that radially abuts the inner surface 16 of the cylinder-shaped recess 10, and an upper, thinner part that is radially positioned at a distance from the inner surface of the cylinder-shaped recess 10. The lower, thicker part guides the impeller 4 so that it doesn't become tilted relative the rotational axis of the pump.

An annular sealing 15 is arranged in the upper part of the impeller's cylinder-shaped recess 10, said sealing 15 abutting the lower end 5 of the drive shaft unit 6, or alternatively abutting the drive shaft 11 and prevents the pumped liquid and solid matter from entering the cylinder- shaped recess 10 from above.

The impeller 4 is of the open type and comprises a hub 16, an upper cover plate 17 and at least one blade 18, also known as paddle, axially extending from the cover plate 17. When viewed in the axial direction, the blade 18 is

preferably spiral-shaped in direction that is opposite to normal direction of rotation of the impeller 4, i.e.

direction of rotation when the pump is in normal operation. The number of blades 18 and their length may vary

significantly so as to fit different liquids and fields of application. The cylinder-shaped recess 10 is arranged in the hub 16. Said at least one blade 18 is in the shown embodiment also connected to said hub 16 and, in the preferred embodiment, the impeller 4 comprises two blades 18. Furthermore, the impeller 4 comprises a hole 19 in the hub 16, said hole 19 connecting the cylinder-shaped recess 10 and the pump chamber 3. One purpose of said hole 19 is to allow for introduction of a suitable tool in order to connect the sleeve 12 to the drive shaft 11. A plug 20 is shown in Fig. 3, said plug being introduced in the hole 19 with the purpose of preventing the pumped liquid and solid matter to enter the cylinder-shaped recess 10 from below.

Reference is now primarily made to Figs. 7-11. The essential feature of the present invention is that the pump comprises a snap-lock coupling arranged at the interface between the drive shaft unit 6 and the cylinder-shaped recess 10. The snap-lock coupling is adapted to position the impeller 4 in the lower position when an applied force acting to displace the impeller 4 in direction away from the lower position is less than a predetermined threshold value.

The snap-lock coupling is preferably arranged so as to disengage the impeller 4 in the axial direction when the latter is positioned at a distance from the lower position.

In the shown embodiments the snap-lock coupling

comprises a seat 21 arranged at the interface between the lower, thicker part of the lower end 5 of the drive shaft unit 6 and the upper, thinner part of the lower end 5 of the drive shaft unit 6. The seat 21 of the snap-lock coupling is preferably delimited by a boss 22 belonging to the snap-lock coupling. Furthermore, a locking element 23 belonging to the snap-lock coupling is arranged in a recess 24 of the locking element, said recess 24 being arranged in the inner surface of the cylinder-shaped recess 10 of the impeller 4. The locking element 23 is preferably made up of an annular spring and said recess 24 is preferably made up of a

circumferentially extending groove.

In Figs. 7-8 and 10-11, the locking element 23 is made up of an annular spring and the recess 24 of the locking element is preferably made up of a groove. The annular spring 23 is arranged in the groove 24 and projects radially inwards in the cylinder-shaped recess 10 of the impeller 4, whereby the lower, thicker part of the lower end 5 of the drive shaft unit 6 is positioned below the annular spring 23. As shown in Figs. 10 and 11, the impeller 4 is hereby prevented from falling off the drive shaft unit 6.

Furthermore, at least a portion of the annular spring 23 is in engagement with the seat 21 of the snap-lock coupling whereby the impeller 4 is kept in its lower position. In other words, the boss 22 belonging to the snap-lock coupling is positioned above the annular spring 23.

The locking element 23 of Fig. 9 is made up of a tube

25, a spring 26 and a ball 27. The tube 25 is arranged in the recess 24 of the locking element and projects radially inwards in the cylinder-shaped recess 10 of the impeller 4, thereby preventing, as shown in Fig. 9, that the impeller 4 falls off the drive shaft unit 6. In other words, the lower, thicker part of the lower end 5 of the drive shaft unit 6 is positioned below the tube 25. The ball 27 and the spring 26 are arranged in the tube 25, whereupon the ball 27 is spring-biased radially inwards and is in engagement with the seat 21 of the snap-lock coupling whereby the impeller 4 is retained in its lower position, i.e. the boss 22 belonging to the snap-lock coupling is positioned above the ball 27. In an alternative, not shown, embodiment the ball and the tube are replaced by an elongate locking element arranged in the recess of the locking element and projecting radially inwards in the cylinder-shaped recess of the impeller, preventing thereby that the impeller falls off the drive shaft unit, i.e. the lower, thicker part of the lower end 5 of the drive shaft unit 6 is positioned below the elongate locking element. The elongate locking element is, similarly to the ball 27 of fig. 9, spring-biased radially inwards and is in engagement with the seat 21 of the snap-lock coupling whereby the impeller is retained in its lower position. The inner end of the elongate locking element is preferably spherical so as to facilitate interaction with the boss 22 belonging to the snap-lock coupling.

In the embodiment shown in Fig. 10, the recess 24 of the locking element is wedge-shaped and tapered radially outwards. The wedge-shaped recess 24 of the locking element has an upper surface 28 that preferably is horizontal, a lower surface 29 that is inclined relative the upper surface, and a bottom surface 30 that connects said upper surface 28 with said lower surface 29. The annular spring 23 is biased radially outwards why parts thereof abut the upper surface 28 and the lower surface 29 of the recess 23 of the locking element, preferably without being in contact with the bottom surface 30. In this way, the annular spring 23 is pressed against the upper surface 28 whereby a precise positioning of the annular spring 23 relative the impeller 4 is obtained.

In the embodiment shown in Fig. 11, the recess 24 of the locking element has an upper surface 28 that is

horizontal, a lower surface 29 that is parallel with the upper surface 28, and a bottom surface 30 that connects said upper surface 28 with said lower surface 29. The annular spring 23 is biased radially outwards why parts thereof abut the bottom surface 30. Due to the own weight of the impeller 4, the annular spring 23 also abuts the upper surface 28 whereby a precise positioning of the annular spring 23 relative the impeller 4 is obtained.

In an alternative, not shown embodiment, the annular spring 23 abuts the upper surface 28, but is positioned at a distance from the bottom surface 30 why the annular spring 23 may be allowed to expand radially outwards once the boss

22 belonging to the snap-lock coupling is passed by the annular spring 23. The annular spring 23 may in this embodiment be a circular annular spring.

The radius of the annular spring 23 preferably varies along its circumference and the spring has oval, triangular or quadratic basic shape, when viewed in axial direction. This entails that certain sections of the annular spring 23 are in contact with the seat 21 of the lower end 5 of the drive shaft unit 6 and other sections of the annular spring

23 are in contact with the recess 24 of the locking element of the cylinder-shaped recess 10 of the impeller 4.

Increased understanding of the annular spring 23 is obtained when looking at Figs. 12-17 that show two exemplifying embodiments of the annular spring 23. The annular spring 23 shown in Figs. 12-14 has oval basic shape, i.e. the annular spring 23 has smaller radius in two

mutually opposite directions (vertically in Figs. 13 and 14) and larger radius in two other, mutually opposite directions (horizontally in Figs. 13 and 14) . Hence, where the annular spring 23 is in contact with the seat 21 of the drive shaft unit 6 is where the annular spring 23 has the smallest diameter. Also, where the annular spring 23 is in contact with the recess 24 of the locking element of the impeller 4 is where the annular spring 23 has the largest diameter. As it may be seen in Figs. 13 and 14, the annular spring 23 also retains the rod 14. The annular spring 23 shown in Figs. 15-17 has quadrangular basic form, i.e. the annular spring 23 has smaller radius in four directions (diagonally in Figs. 16 and 17) and larger radius in four other

directions (horizontally and vertically in Figs. 16 and 17) . Hence, where the annular spring 23 is in contact with the seat 21 of the drive shaft unit 6 is where the annular spring 23 has the smallest diameter. Also, where the annular spring 23 is in contact with the recess 24 of the locking element of the impeller 4 is where the annular spring 23 has the largest diameter. As it may be seen in Figs. 16 and 17, the annular spring 23 also retains the rod 14.

More correctly, the annular spring 23 should have a smaller radius at least in one direction and larger radius in at least one other direction. Both the smaller and the larger radii, respectively, are oppositely arranged when the annular spring 23 has oval or quadrangular basic shape. In the embodiment with triangular annular spring 23, a smaller radius is arranged opposite a larger radius.

The variable radius entails that the annular spring 23 may be spring-biased both in the seat 21 and in the recess 24 of the locking element whereby an accurate positioning of the impeller 4 in its lower position without axial play is obtained. At the same time, a relatively small axially applied force is required for the boss 22 belonging to the snap-lock coupling to pass the annular spring 23.

If a circle-shaped annular spring 23 is used then said spring cannot be spring-biased in the recess 24 of the locking element since the annular spring 23 must have space to expand radially outwards once the boss 22 belonging to the snap-lock coupling passes by the annular spring 23.

When a large piece of solid matter forces the impeller 4 to leave its lower position, no counteracting force is acting after the snap-lock coupling has disengaged. Once the solid matter has passed, the impeller 4 adopts the lower position due to presence of a higher hydraulic pressure on the upper side of the cover plate of the impeller 4 compared to the lower side of the impeller 4, and in those cases the pump is vertically oriented, as shown in the figures, the own weight of the impeller 4 also acts to bring the impeller 4 back to its lower position. Once the impeller 4 has been displaced from its lower position, the annular spring 23 is accordingly still positioned in the recess 24 of the locking element, and once the impeller 4 returns to its lower position, the annular spring 23 is once more positioned in the seat 21 of the drive shaft unit 6.

Reference is now made to Fig. 18 showing an embodiment of a suction cover 7.

At least one groove or clearance groove 31 is arranged in the upper surface of the suction cover 7 and the

adjoining inlet 8 of the pump chamber 3. The groove 31 extends from the inlet 8 of the suction cover 7 towards its periphery. The groove 31 is preferably spiral-shaped and sweeps outwardly in the rotational direction of the impeller 4, i.e. in direction opposite to that of the rotating blades 18. The number of grooves 31 and their shape and orientation may vary significantly so as to fit different liquids and fields of application. The function of the groove 31 is to guide the solid matter in the pumped liquid outwardly, towards the periphery of the pump housing 2. Some of the solid matter passing through the pump will get stuck underneath the blades 18 of the impeller 4 and reduce the rotational speed of the impeller, sometimes even downright completely stop its movement. The groove 31 contributes in keeping the blades 18 clean by scraping off the solid matter each time the blade 18 passes said groove. If the solid mater is too large to fit into the groove 31, between the impeller 4 and the suction cover 7, the impeller 4 will, by means of the solid matter, be displaced upwards and away from the suction cover 7 allowing thereby the solid matter to pass through the pump.

The shape of the lower edge of the blade 18 corresponds in the axial direction to the shape of the upper surface of the suction cover 7. The axial distance between said lower edge and said upper surface should be less than 1 mm when the impeller 4 is in the lower position. Said distance is preferably less than 0,8 mm and most preferred less than 0,5 mm. Said distance should at the same time be greater than 0,1 mm and preferably greater than 0,2 mm. If the impeller 4 and the suction cover 7 are too close to each other, than a friction force or a brake force acts on the blade 20 of the impeller 4.

In order to ensure that the inlet 8 of the pump doesn't become obstructed, the suction cover 7 is preferably provided with means that guide the solid matter towards the groove 31. The guiding means comprise at least a guide pin 32 extending from the upper surface of the suction cover 7, more particularly from the section of the upper surface that faces the inlet 8. The guide pin 32 generally extends in the radial direction of the suction cover 7 and is positioned below the impeller and has an upper surface 33 that extends from a position adjoining the innermost part of the blade 18 of the impeller 4 towards or to the upper surface of the suction cover 7. More specifically, the innermost part of the upper surface 33 of the guide pin 32 is placed at approximately the same radial distance from the centrum of the impeller 4 as the innermost part of the blade 18 of the impeller 4. The upper surface 33 of the guide pin 32 preferably ends in immediate proximity of the "inlet" of said groove 31. When the impeller 4 is in the lower

position, the axial distance between the upper surface 33 of the guide pin 32 and the leading edge of the blade 18 should be less than 1 mm.

The present invention also relates to an impeller assembly for placement in a pump chamber 3. Such an impeller assembly may be sold as an up-grading kit for a pump with axially displaceable impeller, said pump belonging to the prior art. The impeller assembly comprises an impeller 4 with a cylinder-shaped recess 10 and a sleeve 12. The sleeve 12 is received in said cylinder-shaped recess 10, wherein the sleeve 12 is arranged to be connected to an axially extending drive shaft 11. The impeller 4 is displaceable back and forth in the axial direction in relation to the sleeve 12 between a lower position and an upper position. Furthermore, a snap-lock coupling is arranged at the interface between the sleeve 12 and the cylinder-shaped recess 10, wherein the snap-lock coupling is adapted to position the impeller 4 in the lower position when an applied force acting to displace the impeller 4 in the direction away from the lower position is less than a threshold value. In addition, everything that has been mentioned as regards the snap lock, the sleeve 12 and the impeller 4 is applicable to the impeller assembly as well. In this context, the sleeve 12 also belongs to the impeller assembly and when the impeller assembly is mounted on the drive shaft 11, the sleeve 12 belongs to the lower end 5 of the drive shaft unit 6.

Conceivable Modifications of the Invention

The invention is not limited only to the above- described embodiments nor to the embodiments featured in the drawings. In this context, the drawings only have an

illustrative and exemplifying purpose. This patent

application is intended to cover all adaptations and

variants of the preferred embodiments described above. The present invention is consequently defined by the wording of the attached patent claims and may hence be modified in any conceivable way within the frame established by said claims.

It should also be noted that all information regarding terms such as above, below, upper, lower etc. should be construed with the equipment being oriented according to the figures, having the drawings oriented in such a way that the reference numerals can be read in a correct manner.

Thus, similar terms indicate only mutual relations in the shown embodiments, wherein these embodiments may be changed if the equipment of the present invention is provided with a different construction/design. It should also be noted that although not explicitly stated that the feature (s) belonging to a specific embodiment may be

combined with the feature (s) belonging to another

embodiment, such a combination, if feasible, should be deemed obvious .