GB2500167A

GB2500167A - Impeller to shaft connector

Info

Publication number: GB2500167A
Application number: GB1200405.7A
Authority: GB
Inventors: Ian Pinkney; Osarobo Famous Okhuahesogie; Paul Eifion Roach; Neil Ryan Thomas; Ian Patrick Clare Brown; Peter Kay; Stephen Wilson; David Leslie Smith; Robert Neil George; Paul Leslie Jacklin; Geoff Kinpoy Ngao; Kevin John Musson; Matthew Elijah Moore; Jamie Clare; Thomas Jarlath Murray; Stuart Michael Potter; Christopher John Monaghan; Alan Martin Taylor; Francis Joseph Geoffrey Heyes; Trevor Knighton
Original assignee: Napier Turbochargers Ltd
Current assignee: Napier Turbochargers Ltd
Priority date: 2012-01-10
Filing date: 2012-01-10
Publication date: 2013-09-18
Anticipated expiration: 2032-01-10
Also published as: GB2500167B; GB201200405D0

Abstract

A connector, for connecting an impeller 1 to a shaft 2, has first second subcomponents. The first subcomponent has a first sleeve portion 3, and the second subcomponent has a second sleeve portion 14 which is sandwiched between the first sleeve portion and the hub extension. The second sleeve portion is frictionally connected on one side with the first sleeve portion and on the opposing side with the hub extension. The first subcomponent further has a threaded portion which screws onto a corresponding threaded portion of the shaft. The second subcomponent is formed of a material having a coefficient of thermal expansion which is intermediate the coefficients of thermal expansion of the materials of the impeller and the first subcomponent. The impeller may be made of an aluminium alloy, the first subcomponent may be made of a high tensile or medium carbon steel, and the second subcomponent may be made of stainless steel, bronze or brass.

Description

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CONNECTOR

Field of the Invention

The present invention relates to a connector for connecting an impeller to a shaft, and in particular, but not exclusively, for connecting an impeller of a turbocharger to a turbocharger 5 shaft.

Background of the Invention

Turbocharger impellers are typically made of aluminium alloys to provide low rotational inertia with reasonable strength at a commercially-acceptable cost. Attachment of the impeller to the steel turbocharger shaft is achieved in various ways. For example, because 10 of the relative weakness of aluminium and the small diameter of the shaft, one option is to provide the impeller with a steel insert containing a screw-threaded socket which can be threaded on to the shaft. This arrangement can take a higher torque than a connection in which the shaft is directly threaded into the aluminium body (the torque is proportional to the power transmitted across the joint, and so the impeller can be used at a higher pressure ratio 15 than one in which there is a direct threaded connection).

Typically, such an insert is fitted into the impeller by shrink fitting; the aluminium body of the impeller is heated to expand the bore which is to receive the steel insert, while the insert is cooled, for example using liquid nitrogen, before being inserted into the bore. The resultant t.

interference connection is restricted by the temperature to which the aluminium can be 20 heated before its material properties are affected, and by the temperature to which the steel can be cooled.

While the arrangement described can perform satisfactorily, a problem can arise during cycling of the turbocharger from rest to full load. As the turbocharger starts to spin, the joint is affected by centrifugal forces, whereby the aluminium grows outwards away from the steel 25 insert. This reduces the interference force between the insert and the impeller, and due to design constraints it has been found that this reduction tends to be greater at one end of the insert than at the other. Consequently, the insert is gripped more firmly at one of its ends than at the other. The turbocharger then starts to heat up, and because of the different thermal coefficients of expansion of the aluminium alloy and the steel, the aluminium grows 30 axially more than the steel, causing the two metals to slide over each other, except at the

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location where the impeller still grips the insert firmly. On shutdown, the centrifugal stresses are removed, but the thermal stresses remain for some minutes as the turbocharger cools. In this process, the point of grip of the impeller on the insert changes from one end to the other, and as the turbocharger cools, the insert "walks" along the impeller.

5 In certain very cyclic conditions (for example fast ferry applications in high ambient temperatures), it has been observed that the insert can move so far along the impeller that turbocharger failure can occur. Although the effect can be mitigated to some degree by increasing the original interference between the components, for the reasons mentioned above this solution is limited, and it is therefore desirable to achieve a design which ensures 10 that the point of grip remains at the same location during the operating cycles, rather than shifting from one end of the insert to the other.

Accordingly, EP1394387 proposes an outer steel constraining ring which reinforces the frictional contact between aluminium impeller and the insert. Since the ring does not expand as much as the impeller body as the turbocharger heats up, the point of grip between the 15 impeller and the insert remains within the axial extent of the ring during the whole operating cycle of the turbocharger, thereby preventing the tendency of the impeller to "walk" along the insert. As a consequence, the operating life of the turbocharger can be considerably extended in comparison with the conventional turbocharger without the constraining ring.

However, the assembly of such a joint is relatively complex. First the insert and impeller 20 bore are manufactured to tight tolerances. Then typically the insert is cooled and the impeller heated, and the insert is placed within the impeller bore at a hub extension of the impeller. As the insert warms up and the impeller cools, a shrink fit joint is formed, but because of the non-axisymmetrical shape of the impeller, some distortion occurs within the impeller. Generally, the outer surface of the impeller hub extension must therefore be 25 reground to be axisymmetric so that it will be suitable for the outer joint with the constraining ring. A further ring may then be shrunk onto a flange portion of the insert to prevent the constraining ring from coming off the impeller.

Summary of the Invention

It would be desirable to provide a connection between an impeller and a shaft which is 30 simpler to install, but one that can transmit high torques and can prevent or reduce any tendency of the impeller to "walk".

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Accordingly, in a first aspect the present invention provides a connector for connecting an impeller to a shaft, in particular for connecting an impeller of a turbocharger to a turbocharger shaft, the connector having a first subcomponent and a second subcomponent, wherein:

the first subcomponent has a first sleeve portion which is coaxial with a shaft-side hub 5 extension of the impeller, and the second subcomponent has a second sleeve portion which is sandwiched between the first sleeve portion and the hub extension, such that the second sleeve portion is frictionally connected on one side with the first sleeve portion and on the opposing side with a surface of the hub extension;

the first subcomponent further has a threaded portion carrying a thread which screws 10 onto a corresponding threaded portion of the shaft, such that the connector provides a rotationally fixed connection between the impeller and the shaft; and the impeller and the first subcomponent are formed of respective materials having different coefficients of thermal expansion, and the second subcomponent is formed of a material having a coefficient of thermal expansion which is intermediate the coefficients of 15 thermal expansion of the impeller and the first subcomponent.

The two subcomponents can be fixed together to form the connector before the connector is fitted to the impeller. As both sleeve portions are typically axisymmetric, the fixing together of the two subcomponents does not usually result in non-axisymmetrical distortion of the connector. Advantageously, a regrinding operation of the connector can thereby be avoided. 20 In addition, regrinding of the hub extension can also be avoided after fitting of the connector, as it is usually unnecessary to fit a constraining ring of the type described in EP1394387 to the hub extension. That is, by forming the second subcomponent from a material having an intermediate coefficient of thermal expansion, the differential thermal forces which encourage the impeller to "walk" along the insert can be spread across two interfaces reducing any 25 tendency of the impeller to "walk".

A second aspect of the invention provides an impeller having a shaft-side hub extension and fitted with a connector according to the first aspect, the second sleeve portion of the second subcomponent being frictionally connected on one side with the first sleeve portion of the first subcomponent and on the opposing side with a surface of the hub extension.

30 A third aspect of the invention provides the impeller fitted with a connector of the second aspect, which impeller is connected to a shaft having a corresponding threaded section, the

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thread of the threaded portion of the first subcomponent screwing onto the corresponding threaded portion of the shaft.

A fourth aspect of the invention provides a turbocharger having the connected impeller and shaft of the third aspect.

5 Optional features of the invention will now be set out. These are applicable singly or in any combination with any aspect of the invention.

The first and second sleeve portions of the connector may be approximately cylindrically shaped. The surface of the shaft-side hub extension of the impeller which frictionally connects with the second sleeve portion may be correspondingly approximately cylindrical.

10 The frictional connection between the second sleeve portion and the hub extension can be achieved by e.g. press fitting or shrink fitting. Likewise, the frictional connection between the second sleeve portion and the first sleeve portion can be achieved by e.g. press fitting or shrink fitting.

The first subcomponent (which provides the first sleeve portion and the threaded portion) can 15 be formed as a unitary body.

To provide the rotationally fixed connection, the threads can be positive-locking, e.g. tapered. However, another option is for the connector to have an abutment surface which engages a corresponding abutment surface of the shaft when the thread portions are screwed together, thereby tightening the threads to provide the rotationally fixed connection.

20 The second sleeve portion may be frictionally connected with a radially inner surface of the hub extension, e.g. at a central recess of the hub extension. The first sleeve portion is then radially inwards of the second sleeve portion. Typically, the impeller does not have a through-hole extending from one side to another of the impeller. Generally, the central recess is thus a blind hole (i.e. with an end surface), the hole opening to the shaft-side end 25 face of the hub extension. Advantageously, when the hub extension has a central recess, the threaded portion of the connector can be within the recess. In this way, an axially compact arrangement can be achieved.

Alternatively, however, the second sleeve portion may be frictionally connected with a radially outer surface of the hub extension. Then, if the coefficients of thermal expansion of

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the first and second subcomponents are less than that of the impeller, during operation, as the impeller assembly heats up, the hub extension grows to a greater degree than the second sleeve portion, and the second sleeve portion grows to a greater degree than the first sleeve portion, whereby the joints between the hub extension and the sleeve portions 5 tighten, reducing any tendency of the impeller to "walk" and increasing the torque capacities of the joints. Alternatively, because of this tightening of the joints, the degrees of interference required on fitting the sleeve portions together and/or fitting the connector to the impeller can be reduced, while maintaining the "walk" resistances and torque capacities of the joints. In addition, as such a connector is not typically required to frictionally connect to a radially inner 10 surface of the hub extension by shrink fitting, heating of the impeller on fitting can be avoided, whereby accelerated ageing of the impeller during assembly can be prevented.

The connector may be configured to contact the impeller only on the radially outer surface of the hub extension. Alternatively, the hub extension may have an end face, and the connector may have an abutment portion which bears against the end face. In this case, the 15 connector may be configured to contact the impeller only on the radially outer surface of the hub extension and the end face of the hub extension. Preferably the end face is at the radially outer and impeller side of the hub extension so that "walking" of the impeller can be readily monitored by measuring any gap that opens up between the end face and the abutment portion.

20 Preferably, the frictional connections between the sleeve portions and the surface of the hub extension transmit, in use, substantially all of the torque between the shaft and the impeller.

Even when the second sleeve portion is frictionally connected with a radially outer surface of the hub extension (i.e. so that the first sleeve portion is radially outwards of the second sleeve portion), the hub extension may have a central recess (e.g. a blind hole), and a part of 25 the first subcomponent may be inserted into the recess. However, a clearance may be provided between the inserted part of the first subcomponent and the side surface of the recess. Thus the first subcomponent may not contact the side surface of the recess, i.e. if the first subcomponent contacts a surface of the recess at all, it may only contact an end surface of the recess. The part of the first subcomponent inserted into the recess may 30 include the threaded portion of the first subcomponent. In this way, the threaded portion of the connector may still be within the recess such that an axially compact arrangement can be achieved.

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The thread of the first subcomponent can face radially outwardly. For example, the threaded portion of the first subcomponent can be formed as a substantially cylindrical boss coaxial with the shaft, and the threads can be formed on a radially outer surface of the boss.

Alternatively, the thread of the first subcomponent can face radially inwardly. For example, 5 the threaded portion can be within a central recess of the hub extension or adjacent a shaft side end face of the hub extension.

The first subcomponent may be formed of a material having a greater strength than the material of the impeller. The first subcomponent may be formed of a material having a lower coefficient of thermal expansion than the material of the impeller. For example, the first 10 subcomponent can be formed of steel, and preferably high tensile steel, and the impeller can be formed of aluminium alloy. The second component can be formed, for example, of stainless steel, bronze or brass.

The second sleeve portion can extend over at least 50%, and preferably at least 80%, of the axial length of the hub extension. More preferably the second sleeve portion can extend 15 over substantially the entire axial length of the hub extension.

Frictional contact between the second sleeve portion and the hub extension can extend over at least 25% of the axial length of the overlap region between the second sleeve portion and the hub extension, preferably over at least 30% or 50% of the axial length of the overlap region, and more preferably over the entire axial length of the overlap region. Frictional 20 contact between the first sleeve portion and the second sleeve portion can extend over at least 25% of the axial length of the further overlap region between the first sleeve portion and the second sleeve portion, preferably over at least 30% or 50% of the axial length of the further overlap region, and more preferably over the entire axial length of the further overlap region.

25 The connector and/or the impeller may have one or more centring portions having respective engagement surfaces which engage with one or more corresponding centring portions of the shaft, the threaded portion of the first subcomponent and the centring portions of the connector and/or the impeller being distributed along the impeller axis. The thread surface of the connector and the engagement surfaces of the connector and/or the impeller can face 30 radially inwardly, and the respective diameters of the thread and the engagement surfaces can then decrease towards the impeller. Alternatively, the thread and engagement surfaces

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of the connector can face radially outwardly, and the respective diameters of the thread and the engagement surfaces can then increase towards the impeller.

Generally the impeller has a casing, and the connector and/or the hub extension can then form a seal with a section of the casing. For example, the seal can include a sealing ring, 5 which may be carried by the casing section and which may be received by a corresponding circumferential recess formed on an outer surface of the connector and/or the hub extension. The sealing ring may have one or more annular grooves on its radially inner face, and the recess may have corresponding circumferential ribs which are received in the grooves. Another option is for the seal to include a labyrinth seal, with formations on facing surfaces of 10 the casing section and the connector and/or the hub extension forming the labyrinth.

The connector may be formed with or may carry a circumferential oil thrower formation at its radially outer surface.

Further optional features of the invention are set out below.

Brief Description of the Drawings

15 Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a sectional elevation through a turbocharger impeller joined to a shaft by a connector in accordance with an embodiment of the invention;

Figure 2 is a close-up schematic view of a seal between a section of a casing of the impeller 20 of Figure 1 and a hub extension of the impeller;

Figure 3 shows schematically a sectional elevation of a further embodiment of the connector;

Figure 4 shows schematically a sectional elevation of a further embodiment of the connector;

Figure 5 is a sectional elevation of a further embodiment of the connector;

Figure 6 is a close-up schematic view of a seal between a section of a casing of an impeller 25 and a sleeve portion of a further embodiment of the connector; and

Figure 7 shows schematically a sectional elevation of a further embodiment of the connector.

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Detailed Description and Further Optional Features of the Invention

Referring first to Figure 1, an aluminium alloy impeller 1 is fitted on to a steel turbocharger shaft 2 by means of a connector having a steel first subcomponent and a second subcomponent formed of a material having a coefficient of thermal expansion which is 5 intermediate the coefficients of thermal expansion of the impeller and the first subcomponent. The alloy of which the impeller is made (known in the U.S.A. by the designation "2618A") has a relatively high strength for use up to a temperature of about 200°C, having a composition of aluminium with about 2.5wt.% copper and smaller amounts of magnesium, iron and nickel. The first subcomponent may be made of a high tensile steel such as EN26, whose 10 composition includes about 2.5wt.% nickel, and is located at a hub extension H of the body of impeller 1. Alternatively the first subcomponent may be made of a medium carbon steel such as EN8. The second subcomponent may be made of stainless steel, bronze or brass.

The first subcomponent is of cup-like shape and has a first sleeve portion 3 forming the wall of the cup, a threaded portion 12 with a threaded bore 11 forming the base of the cup, and a 15 flange portion 8 around the mouth of the cup. The second subcomponent is a cylindrical second sleeve portion 14. To form the connector, the second sleeve portion 14 is shrink fitted onto the first sleeve portion 3 such that the two sleeve portions 3, 14 are coaxial and frictionally connected, with the second sleeve portion 14 covering the outer surface of the first sleeve portion 3.

20 The shaft 2 is formed at its end with a first shoulder 4 surrounding a cylindrical centring portion 5, and a screw-threaded portion 7 of further reduced diameter extending from the end of the centring portion. The connector is inserted into a central recess formed in the hub extension H, with the outer surface of the second sleeve portion 14 frictionally connected to the radially inner surface of the hub extension H. The flange portion 8 of the first 25 subcomponent engages against a shaft-side end face 9 of the hub extension H to determine the relative axial positions of the sleeve portions 3, 14 and the hub extension H. The flange portion 8 is engaged on its other side by the shoulder 4 on the shaft 2. The centring portion 5 of the shaft is received in a corresponding centring portion 10 of the first subcomponent in a close, but not tight, fit. The threaded bore 11 engages on the screw-threaded portion 7 of 30 the shaft. The threaded portion 12 has a small clearance from the end of the recess.

The connector is fitted on to the hub extension H by cooling the connector to cause the sleeve portions 3, 14 to shrink and by heating the impeller to cause the hub extension H to

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expand, and then inserting the connector into the central recess of the hub extension H until the flange portion 8 contacts the end face 9 of the hub extension H. On returning from their thermal excursions, the connector and hub extension H frictionally grip across the outer surface of the second sleeve portion 14 and the radially inner surface of the hub extension H.

5 The second sleeve portion 14 extends over and thereby frictionally contacts most of the axial length of the hub extension H, although in other embodiments the second sleeve portion 14 can be extend over only a portion of the axial length, and/or frictional contact can extend between the second sleeve portion 14 and the hub extension H over only a portion of the overlap region between the second sleeve portion 14 and the hub extension H.

10 The outer diameter of the flange portion 8 is provided with an oil capture/thrower ring R, which in this embodiment of the invention is machined into the flange portion 8. Another option, however, is to form the ring R as a separate component.

As shown better in Figure 2, a section 15 of the impeller casing and the outer surface of the hub extension H are in close proximity to help provide a rotating oil and pressure seal 15 between the impeller 1 and the casing. To improve the seal, the hub extension H has a recess 13 on its outer surface which is bounded at one end by the flange portion 8 of the first component of the connector and which receives a sealing ring 16 carried by the casing section 15. To reduce wear between the sealing ring 16 and the hub extension H, the casing section 15 has a small abutment surface 20 on the shaft side (right hand in Figure 1) of the 20 seal ring 16 and against which the sealing ring 16 rests. To provide enhanced sealing, the sealing ring 16 has annular grooves 18 on its radially inner face, and the recess has corresponding circumferential ribs 17 which are received in the grooves, as described in EP A 1130220. Alternatively, however, the sealing ring can be a plain ring (i.e. without grooves) received in a plain recess (i.e. without ribs). The sealing ring 16 co-operates with the casing 25 section 15 and serves to retain lubricating oil to the shaft side of the assembly and compressed air to the impeller side of the assembly (left hand in Figure 1). The compressed air is contained between the body of the impeller 1, the hub extension H with its sealing ring 16, and the impeller casing, within which the impeller assembly is mounted for rotation on overhung bearings (not shown).

30 After the connector is fitted on to the hub extension H, the screw-threaded portion 7 of the shaft 2 is screwed onto the threaded portion 12 of the connector, the respective centring portions 5, 10 ensuring the shaft aligns with the axis of the impeller. The threads are

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screwed until opposing surfaces of the flange portion 8 and shoulder 4 come into abutment, which causes the threads to tighten and provides a rotationally fixed connection between the impeller 1 and the shaft 2.

Advantageously, by forming the second subcomponent from a material having an 5 intermediate coefficient of thermal expansion, the differential thermal forces acting across each frictional connection can be reduced, whereby the tendency for the impeller to "walk" can also be reduced . In addition, by containing the threaded connection between the connector and the shaft 2 in the central recess of the hub extension H, an axially compact arrangement is achieved. The frictional connections between the sleeve portions 3, 14 and 10 the radially inner surface of the hub extension H transmit, in use, substantially all of the torque between the shaft 2 and the impeller 1. In addition, regrinding operations can be avoided during fitting of the connector.

If there is any tendency for the impeller 1 to "walk", advantageously this can be monitored by measuring the size of the gap that would open up between the flange portion 8 and the end 15 face 9. For this reason, it is preferred that the flange portion 8 and the end face 9 determine the relative axial positions of the sleeve portions 3, 14 and the hub extension H. Alternative pairs of facing features that could be configured to abut each and thereby determine the relative axial positions (such as the threaded portion 12 and the end of the recess) are less amenable to inspection.

20 Figure 3 shows schematically a sectional elevation of a further embodiment of the connector. In this embodiment there is no central recess in the hub extension H of the impeller 1 and the connector grips onto the radially outer surface of the hub extension H. The first subcomponent of the connector again has a first sleeve portion 3, but the flange portion 8 of the first subcomponent extends radially inwardly from the first sleeve portion 3. The 25 threaded portion 12 of the connector is then at the radially inner end of the flange portion 8 of the first subcomponent. The centring portion 10 is formed on the radially inner side and at the shaft-side end of the first sleeve portion 3. The recess 13 for receiving the sealing ring and the oil capture/thrower ring R is formed on the outer surface of the first sleeve portion 3. The second subcomponent again is a cylindrical second sleeve portion 14. To form the 30 connector, the second sleeve portion 14 is shrink fitted onto the first sleeve portion 3 such that the two sleeve portions 3, 14 are coaxial and frictionally connected, with the second sleeve portion 14 covering the inner surface of the first sleeve portion 3.

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The impeller assembly is built up as follows. The connector is warmed, and the sleeve portions 3, 14 are slid on to a cylindrical outer surface of the hub extension H until an abutment portion 21 at the end of the sleeve portion 3 contacts an end face 22 of the hub extension H. When the connector cools, a frictional connection is thus formed between the 5 second sleeve portion 14 and the hub extension H.

As the material of the second sleeve portion 14 has a lower coefficient of thermal expansion than the aluminium alloy of the impeller, the sleeve portion 14 does not expand as much with rising temperature as the hub extension H. Likewise, as the steel of the first sleeve portion 3 has a lower coefficient of thermal expansion than the material of the second sleeve portion 10 14, the first sleeve portion 3 does not expand as much with rising temperature as the second sleeve portion 14. These differences in their respective coefficients of expansion ensure that during operation, as the impeller assembly heats up, the joints between the hub extension and the sleeve portions tighten, reducing any tendency for relative movement between impeller and connector under the influence of centrifugal and thermal stresses, and 15 increasing the torque capacity of the joint. Thus the degree of interference required on build up can be reduced while maintaining the "walk" resistances and torque capacities of the joints. Further, heating of the impeller 1 on build up can be avoided, whereby material properties of the impeller 1 may not be degraded by accelerated ageing during assembly. However, as the hub extension H does not have a central recess, an axially longer 20 arrangement results.

Figure 4 shows schematically a sectional elevation of a further embodiment of the connector. In this embodiment, which is otherwise similar to the embodiment of Figure 3, the threaded portion 12' of the first subcomponent of the connector is formed as a cylindrical boss which fits into a cylindrical recess formed in the shaft 2. The threaded portion 12' carries its thread 25 on a radially outer surface of the boss, while the corresponding threaded portion 7' of the shaft 2 is formed around the cylindrical recess and is threaded on a surface which faces radially inwards.

Figure 5 is a sectional elevation of a further embodiment of the connector. In this embodiment, which is otherwise similar to the embodiment of Figure 1, the second 30 subcomponent also has a flange portion 23 which is sandwiched between the end face 9 of the hub extension H and the flange portion 8 of the first subcomponent. The flange portion

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23 may be used to form secondary features, such as the recess 13 for receiving the sealing ring.

Figure 6 is a close-up schematic view of a seal between a section of a casing of an impeller and the flange portions 8, 23 of a further embodiment of the connector. In this case, instead 5 of a seal formed by a sealing ring, the hub extension H and flange portions 8, 23 on one side and the casing section 15 on the other side have engaging surfaces 19 carrying respective sets of machined grooves which interlock to form a labyrinth seal.

Figure 7 shows schematically a sectional elevation of a further embodiment of the connector. This embodiment is similar to the embodiment of Figure 3 except that the shaft 2 has two 10 centring portions 5a, 5b, and the connector has two corresponding centring portions 10a, 10b. Further the hub extension H has a central recess, and an insertion part of the first component, which forms the centring portion 10b and the threaded portion 12, is inserted therein, although with a clearance C from the wall of the recess. The threaded portions 7, 12 of the shaft 2 and the connector are located axially between the engaging pairs of centring 15 portions such that, on each of the shaft and the connector, the respective diameters of the threaded portions and the centring portions decrease towards the impeller. A further difference relative to the previous embodiments is that the threads are tapered, so that merely screwing the threaded portions 7, 12 together results in a rotationally fixed connection between the impeller 1 and the shaft 2.

20 While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. For example, in embodiments such as that of Figure 7, instead of the connector having a centring portion 10a, the impeller may have a centring portion at the base of the recess that engages with the centring portion 5b of the 25 shaft. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

All references referred to above are hereby incorporated by reference.

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Claims

1. A connector for connecting an impeller (1) to a shaft (2), the connector having a first subcomponent and a second subcomponent, wherein:

the first subcomponent has a first sleeve portion (3) which is coaxial with a shaft-side 5 hub extension (H) of the impeller, and the second subcomponent has a second sleeve portion (14) which is sandwiched between the first sleeve portion and the hub extension,

such that the second sleeve portion is frictionally connected on one side with the first sleeve portion and on the opposing side with a surface of the hub extension;

the first subcomponent further has a threaded portion (12, 12') carrying a thread 10 which screws onto a corresponding threaded portion (7, 7') of the shaft, such that the connector provides a rotationally fixed connection between the impeller and the shaft; and the impeller and the first subcomponent are formed of respective materials having different coefficients of thermal expansion, and the second subcomponent is formed of a material having a coefficient of thermal expansion which is intermediate the coefficients of 15 thermal expansion of the impeller and the first subcomponent.

2. A connector according to claim 1, wherein the impeller is formed of a material having a higher coefficient of thermal expansion than the first subcomponent.

3. A connector according to claim 1 or 2, wherein the first subcomponent is formed of a material having a greater strength than the material of the impeller.

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4. A connector according to any one of the previous claims, wherein the frictional connections between the first sleeve portion and the second sleeve portion, and between the second sleeve portion and the surface of the hub extension transmit, in use, substantially all of the torque between the shaft and the impeller.

5. A connector according to any one of the previous claims, wherein the hub extension 25 has a central recess, and the threaded portion of the first subcomponent is within the recess.

6. A connector according to any one of the previous claims, wherein the second sleeve portion extends over at least 50% of the axial length of the hub extension.

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7. A connector according to any one of the previous claims, wherein the first subcomponent and/or the impeller has one or more centring portions (10; 10a, 10b) having respective engagement surfaces which engage with one or more corresponding centring

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portions (5; 5a, 5b) of the shaft, the threaded portion of the first subcomponent and the centring portions of the connector and/or the impeller being distributed along the impeller axis.

8. A connector according to any one of the previous claims, wherein the impeller has a 5 casing and the connector and/or the hub extension forms a seal with a section (15) of the casing.

9. A connector according to any one of the previous claims, wherein the connector is formed with or carries a circumferential oil thrower formation (R) at its radially outer surface.

10. An impeller having a shaft-side hub extension and fitted with a connector according to 10 any one of the previous claims, the second sleeve portion of the second subcomponent being frictionally connected on one side with the first sleeve portion of the first subcomponent and on the opposing side with a surface of the hub extension.

11. The impeller fitted with a connector of claim 10, which impeller is connected to a shaft having a corresponding threaded section, the thread of the threaded portion of the first

15 subcomponent screwing onto the corresponding threaded portion of the shaft.

12. A turbocharger having the connected impeller and shaft of 11.