Patent Description:
The bush parts are typically interference fitted into the structural metallic bush housing. This achieves several purposes - (i) the bushes are rigidly fixed and retained in the structural housing, (ii) the bushes are prevented from rotating, in reacting the rotation of the rotor part(s), and (iii) the bush material is good in compression, but may crack in tension and so interference fitting the bush into a bush holder ensures that the bush stays compressed.

The disadvantages of this interference fitted bush system construction include (a) a wide range of metallurgies and/or interference fits have to be employed to cater for process liquid compatibility and differences in application temperature ranges, respectively, for example, variations in material coefficient of thermal expansion between the bushes and bush holder have to be accounted for in the design of the interference fits, and (b) interference fitting these parts together requires specialist equipment and is extremely difficult to facilitate in the field, should a breakdown occur.

Thus, it would be an advantage to provide an improved system for retaining a bush in a bush holder, preferably addressing one or more of the above problems.

<CIT> discloses an electrical submersible pump assembly includes a centrifugal pump having impellers and diffusers. A thrust runner is coupled to a motor shaft for rotation along with the impellers.

<CIT> discloses an oil-free bushing having a fixing seat, a bushing and two annular retaining rings. A similar bushing system is also known from <CIT>.

According to the present invention, there is provided a bushing system for use in a magnetic drive or canned motor pump, the system comprising a bush holder having an axial recess for receiving a bush, a bush for insertion into the axial recess, the bush and bush holder each having: one or more cooperating features which abut against each other when the bush is fitted into the recess; characterised in that the bush and bush holder each have a recess for receiving at least part of a retention member; wherein, when the cooperating features abut, the retention member recesses on the bush holder and the bush are axially offset.

The cooperating features preferably includes one or more shoulders.

The retention member recesses are preferably axially misaligned. The retention member recesses may overlap in the axial direction.

One or both retention member recesses are preferably annular.

The retention member recess in the bush holder may be axially further than the retention member recess in the bush from the cooperating features.

The system above may further comprise a retention member. The retention member may be compliant.

The retention member and the axially offset recesses may combine to provide a biasing force which urges the bush and bush holder towards each other.

The bush and bush holder preferably each have one or more axially extending mating surfaces which are in contact when the bush is inserted in the bush holder.

The mating surfaces are preferably complementary to limit relative rotation of the bush and bush holder.

The mating surfaces are preferably configured such that they prevent relative rotation of the bush and bush holder.

A portion of each mating surface preferably matches the opposite surface exactly.

One or more of the retention member recesses may be annular.

One or more of the retention member recesses may be partially circumferential.

One or more of the retention member recesses may be arcuate sections.

Multiple retention member recesses may be provided on one or both of the bush and bush holder.

The retention member may be toroidal in shape.

The retention member may have a circular, square, parallelogram or hexagonal cross-section.

<FIG> shows the three individual components that form the bush retention system <NUM>. The three components are a bush <NUM>, a bush holder <NUM> and a retention member <NUM>. The left-hand half of <FIG> shows the components <NUM>,<NUM> and <NUM> as an axial section, and the right-hand half of <FIG> shows an outer view of the solid component.

Bush <NUM> has a main body section <NUM> which is generally cylindrical in form with, at the upper end in <FIG>, a wider flange section <NUM>. The flange need not be at an end, but it is generally preferable that it is at an end. The flange and main body section define a shoulder <NUM> therebetween. The shoulder <NUM> includes a chamfered section <NUM>, in this case a curve which may have a relatively small radius of curvature when compared to the diameter of the main body section. The chamfer may take an alternative form including one or more straight sections. The chamfer may be omitted and the flange <NUM> and main body section <NUM> could meet a hard angle, such as a right angle.

The main body section <NUM> of bush <NUM> defines an axially extending passageway <NUM>. The passageway will typically, as in this example, extend all the way through the bush such that it takes the form of an open passageway. The passageway is typically of a constant cross section, both in shape and size. The recess/passageway <NUM> is for receiving in use a shaft which is intended to rotate with the bush <NUM>. The passageway <NUM> is generally circular in cross section and is defined by an inner surface <NUM> of a wall <NUM> which forms the main body section <NUM>. The wall <NUM> has an outer surface <NUM>. One or more (three in this case at equally spaced radial positions) axially extending recesses or indents <NUM> are formed in the inner surface <NUM> and these are always open at each end to permit liquid to flow along the inner surface <NUM>.

The main body section <NUM> includes a retention member recess <NUM> formed in the outer surface <NUM>. The recess may have a curved cross-section, which may be a complete semi-circle or may only be arcuate. The cross section is preferably constant along the entire length of the recess. Alternative cross sections are possible and may be dependent upon the size and/or shape of retention member <NUM> being used. In this Figure, the recess <NUM> extends all the way around the main body section <NUM>. In this example, the main body section <NUM> is substantially circular in cross section, and thus the recess <NUM> is circular. The recess may not be a single recess extending all the way round the main body section, but instead may be two or more partially circumferential recesses. These may be evenly spaced around the main body section.

Bush holder <NUM> is typically a structural component, usually formed from a metallic material (typically stainless steel <NUM>). The bush holder <NUM> is formed by a wall <NUM> having inner <NUM> and outer <NUM> surfaces. An axially extending passage <NUM> passes axially from one end to the other (normally, at the other end, a matching formation is provided for receiving another bush <NUM>). The outer surface <NUM> of the wall <NUM> is stepped in several locations to aid the location of the bush holder within other components. The upper (in the figures) end <NUM> of the bush holder <NUM> has a larger diameter than other portions of the holder <NUM>, thereby defining a bush holder neck <NUM>.

The upper end <NUM> of the passageway <NUM> in bush holder <NUM> is intended to receive the lower end of bush <NUM>. The inner surface of the wall <NUM> is formed with a pair of shoulders/seats <NUM>, <NUM>. A bush holder shoulder <NUM> is formed toward the upper end of the inner surface of wall <NUM>. The shoulder <NUM> may be identical in size and/or shape to the shoulder <NUM> of the bush <NUM> such that, when the bush is inserted into the bush holder, shoulder <NUM> rests on shoulder <NUM>. Alternatively, the shoulder <NUM> may take a different shape and/or size, for example a <NUM> degree angled surface relative to the curve of shoulder <NUM>, so as to act as a lead-in so the retention member <NUM> can more easily slide into the bore of the bush holder. Seat <NUM> in the bush holder is preferably positioned, as can be seen in <FIG>, such that when the bush has been inserted into the bush holder, there is a small gap between seat <NUM> and the lower most surface of the bush <NUM>, so that the axial bearing load is reacted by shoulder <NUM>.

A bush holder retention recess <NUM> is formed axially between the shoulder <NUM> and seat <NUM>. The recess may have a curved cross-section, which may be a complete semi-circle or may only be arcuate. The cross section is preferably constant along the entire length of the recess. Alternative cross sections are possible and may be dependent upon the size and/or shape of retention member <NUM> being used. In this Figure, the recess <NUM> extends all the way around the inner surface <NUM>. In this example, the inner surface <NUM> is substantially circular in cross section, and thus the recess <NUM> is circular. The recess may not be a single recess extending all the way round the main body section, but instead may be two or more partially circumferential recesses. These may be evenly spaced around the main body section. The partial recesses of the bush holder are preferably located at the same circumferential positions as any partial recesses in the bush <NUM>.

Retention member <NUM> typically takes the form of an o-ring as these are commonly available and easy install, especially as there is no specific orientation required. The retention member may have a different cross section. The shape of the cross section of the retention member may correspond to the recesses, i.e. an o-ring would typically have semi-circular/arcuate cross section recesses, a square cross section retention member would typically have square/rectangular recesses. The retention member <NUM> is preferably formed from a compliant material. When the retention member extends around the full circumference of the bush main body <NUM>, it may additionally act as a seal, e. g to prevent liquid traveling across the axial or radial bearing surfaces and lubricates the axial and radial 'bearings' respectively from short-circuit the bearing system by travelling along the clearance gap between the bore of the bush-holder and the outside diameter of the bush. The compliant material may be an elastomer (e.g. FKM, FFKM, EPDM, etc) or a thermosetting plastic (e.g. PTFE, nylon, etc) depending on process liquid compatibility. The range of retention materials which can be employed not only caters for a wide range of process liquid chemical compatibilities, but also enables the 'spring stiffness' of the journal bearing system to be optimised by the choice of the most appropriate material.

If the retention member is for fitting within a recess that is only partially circumferential, then a circlip or other appropriate device could be used in place of an o-ring.

<FIG> shows the bush <NUM>, bush holder <NUM> and bush retention member <NUM> in a constructed arrangement in which the shoulder <NUM> and shoulder <NUM> abut. It can be seen that the bush retention recess <NUM> and the bush holder retention recess <NUM> are at different axial positions, i.e. they are axially misaligned or offset. In particular, bush retention recess <NUM> is located axially closer than the bush holder retention recess <NUM> to the flange <NUM>. This axially misalignment means that the retention member <NUM> causes a biasing force to be applied between the bush <NUM> and the bush holder <NUM>, such that the two components are urged axially towards each other into face to face contact.

The bush <NUM>, in the example, extends out of the upper end <NUM> of the bush holder <NUM>, so as to prevent the rotor part of the bearing reacting with the stainless steel bush-holder, as opposed to the silicon carbide bush, causing wearing, particulate and damage. The lowermost end of the bush <NUM> is axially spaced from the seat <NUM> in the bush holder <NUM> thereby defining an annular gap <NUM>.

The retention member also serves to centre the bush <NUM> in relation to the bush holder <NUM> and provides stiffness and a radial compressive load to the bush. The use of the retention member with the axial offset allows the bush and bush holder faces to be presented in the optimum position with no distortion (such as dishing or coning) of the faces due to the interference fitting of the parts. This results in improved operation, that would be expected to give longer life as a consequence.

The fitting diameter of the bush <NUM> is preferably a clearance fit to the bush holder bore <NUM>. This eliminates the need for an interference fitting operation and the specialist (heating) equipment that this involves. It can also mean that the retrofit operation becomes a simple push fit of one component into another, an operation that is more conducive to field servicing of magnetic drive pumps. Finally, it also reduces the number of permutations of bush to bush holder fits that are needed to cover a wide range of duty temperatures, thereby allowing one bush holder size to fit all applications, simplifying manufacture, stocking and supply of spare parts.

In general, the present design of bush and bush holder is much simplified taking complexity out of the parts required and the sub- assembly build process. This lends itself to field retrofit and distributor building or servicing of equipment, avoiding the need to return an item to factory for repair, thereby maximising the time the item in which the bush and bush holder are located can remain operational.

Whilst the figures show a single retention member recess in each of the bush and bush holder, it may be possible to use multiple pairs of recesses, and provide multiple retention members, one for each pair of recesses. Two or more retention members may be used, each with a respective pair of retention member recesses in the bush and bush holder. However, any increase in the number of retention members and recesses also increases the assembly friction, which makes assembly and disassembly more difficult. As such, it is better to keep the number of retention members as low as possible, and most preferably a single retention member is used.

<FIG> illustrates a further feature of the bush and bush holder fitting. The engagement between the bush <NUM> and the bush holder <NUM> features an anti-rotation system <NUM> whereby a flat edge <NUM> on the outside diameter of the bush <NUM> (typically on the flange <NUM>), fits to a corresponding flat feature <NUM> milled into the end of the inner surface <NUM> of the bush holder <NUM>. This is shown end-on in <FIG>. Anti-rotation features often require a separate part, e.g. a pin, wire, circlip, etc., which can shake loose and again requires different metallurgies for the sake of chemical compatibility with the process liquid. The integration of the flat feature into the bearing system, provides an anti-rotation method that is simple, effective and further reduces the 'parts-count'.

Claim 1:
A bushing system for use in a magnetic drive or canned motor pump, the system comprising:
a bush holder (<NUM>) having an axial recess for receiving a bush,
a bush (<NUM>) for insertion into the axial recess, the bush and bush holder each having:
one or more cooperating features (<NUM>, <NUM>) which abut against each other when the bush is fitted into the axial recess; characterised in that
the bush and bush holder each have
a recess (<NUM>, <NUM>) for receiving at least part of a retention member (<NUM>);
wherein, when the cooperating features abut, the retention member recesses (<NUM>, <NUM>) on the bush holder and the bush are axially offset.