Field of the Invention

The present invention relates to a connection system for connecting an impeller to a shaft, and in particular, but not exclusively, for connecting an impeller of a turbocharger to a turbocharger 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 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 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 interference connection is restricted by the temperature to which the aluminium can be 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 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 axially more than the steel, causing the two metals to slide over each other, except at the 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.

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 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 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 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 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 can transmit high torques but has little or no susceptibility to the "walking". Accordingly, in a first aspect the present invention provides a connection system for connecting an impeller to a shaft, the impeller having a shaft-side hub extension with a central hole, wherein the connection system has:

a connector body which is inserted into the hole, the connector body having a threaded portion carrying a thread which screws onto a corresponding threaded portion of the shaft such that the connection system provides a rotationally fixed connection between the impeller and the shaft;

a plurality of circumferentially distributed elongate threaded members (such as screws or bolts) which join the connector body to the hub extension such that the connector body is axially fixed relative to the impeller; and

a plurality of circumferentially distributed torque transfer members which extend between the connector body and the hub extension to transmit a majority of the torque between the shaft and the impeller.

As the torque transfer members transmit a majority of the torque between the connector body and the impeller, the strength of any interference connection between the connector body and the hub extension can be reduced, or indeed such a connection can be avoided entirely, for example with the connector body having merely a location or transition fit in the central hole. With little or no interference fit, the "walking" mechanism can then be eliminated.

A second aspect of the invention provides an impeller having a shaft-side hub extension with a central hole and fitted with a connection system according to the first aspect, the connector body being joined to the hub extension by the elongate threaded members, and the torque transfer members extending between the connector body and the hub extension.

A third aspect of the invention provides the impeller fitted with a connection system of the second aspect, which impeller is connected to a shaft having a corresponding threaded section, the thread of the threaded portion of the connector body 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.

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 central hole may be a blind hole (i.e. with an end surface). Thus, the impeller may not have a through-hole extending from one side to another of the impeller. The threaded portion of the connector body is typically within the central hole. In this way, an axialiy compact arrangement can be achieved.

Preferably, the torque transfer members transmit substantially all of the torque between the shaft and the impeller. The connector body can be of cup-like shape and can have a flange portion around the mouth of the cup which, when the connector body is inserted into the central hole, engages a shaft side end face of the hub extension. Conveniently, the elongate threaded members can then join the connector body to the hub extension at the flange portion.

The connector body may be formed of a material having a greater strength than the material of the impeller. For example, the shaft can be formed of steel (e.g. a high strength steel), the impeller can be formed of aluminium alloy, and the connector body can also be formed of steel (e.g. a high strength steel).

One option is for the elongate threaded members also to be the torque transfer members. That is they have a dual function. However, as such an arrangement can result in high shear loading on the threads of the threaded members, another option is for the torque transfer members to be formed as separate members from the threaded members. For example, the torque transfer members can conveniently be formed as sleeves which surround the threaded members. More particularly, each threaded member may have a first portion which makes a threaded engagement to axialiy fix the connector body relative to the impeller, and a second portion which is surrounded by the respective sleeve.

The elongate threaded members may screw into the hub extension to join the connector body to the hub extension. In this case, the corresponding threads of the hub extension may be protected by helicoil formations. Another option, however, is to fit (e.g. by press or shrink fitting) threaded inserts into respective circumferentially distributed cavities provided in the hub extension, and then to screw the threaded members into the inserts. Conveniently, the inserts can be formed of a stronger material (e.g. steel) than the material of the impeller.

In general there may be from six to twelve elongate threaded members. Likewise, there may be from six to twelve torque transfer members.

The torque transfer members can be arranged to provide a reaction force which has an axial component urging the impeller towards the connector body when the torque is transmitted between the connector body and the impeller. Typically, the reaction force increases as the transmitted torque increases. In this way, the torque transfer members can supplement the axial fixing provided by the elongate threaded members at operational conditions when the loads on the impeller are high. For example, the length direction of each threaded member can have an axial component and a tangential component. When the threaded members are the torque transfer members, or when the torque transfer members are formed as sleeves which surround the elongate threaded members, the length direction of each torque transfer member then has an axial component and a tangential component. Optionally, the length direction of each threaded member can also have a radial component, i.e. when the threaded members are the torque transfer members, or when the torque transfer members are formed as sleeves which surround the elongate threaded members, the length direction of each torque transfer member then has an axial component, a tangential component and a radial component. In this way, the threads of the threaded members can be more optimally positioned. The angle between the length direction of each threaded member and the axial direction may be more than 15°, and preferably may be more than 30°. However, the angle between the length direction and the axial direction may be less than 50°, and preferably may be less than 45°.

The connector body may be arranged such that, when inserted into the central hole, a radially inward facing surface of the hub extension opposes a radially outward facing surface of the connector body. The torque transfer members can then project from the radially outward facing surface of the connector body to locate in respective recesses formed in the radially inward facing surface of the hub extension. Such torque transfer members may be integrally formed with the connector body. Alternatively, however, the torque transfer members may be separate components from the connector body and may be located in respective recesses formed in the radially outwardly facing surface of the connector body.

Preferably, the torque transfer members are elongate and extend in the axial direction of the impeller, the recesses being correspondingly elongated and extended. However, the torque transfer members (and the recesses) may also extend in the circumferential direction in order to provide a reaction force which has an axial component urging the impeller towards the insert when the torque is transmitted between the connector body and the impeller. For example, the torque transfer members may take the form of helical splines on the outwardly facing surface of the connector body which insert into corresponding helical recesses formed in the inwardly facing surface of the hub extension. The connector body 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 connector body and the centring portions of the connector body and/or the impeller being distributed along the impeller axis. The thread surface of the connector body and the engagement surfaces of the connector body and/or the impeller can face radially inwardly, and the respective diameters on the shaft of the thread and the engagement surfaces can then decrease towards the impeller.

Generally the impeller has a casing, and the connector body 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, 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 body 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 the casing section and the connector body and/or the hub extension forming the labyrinth.

The connector body 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

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 connection system 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 of Figure 1 and a hub extension of the impeller;

Figure 3 is a close-up schematic view of a seal between a section of a casing of an impeller and a sleeve portion of a further embodiment of the connection system; Figure 4 shows schematically a sectional elevation of a further embodiment of the connection system;

Figure 5 shows schematically a sectional elevation of a further embodiment of the connection system; Figure 6 shows schematically a sectional elevation of a further embodiment of the connection system;

Figure 7 shows schematically a sectional elevation of a further embodiment of the connection system;

Figure 8 shows schematically a sectional elevation of a further embodiment of the connection system;

Figure 9 shows schematically (a) a side view and (b) a developed view of a further embodiment of the connection system;

Figure 10 shows schematically a side view of a further embodiment of the connection system; and Figure 11 shows schematically (a) a sectional elevation and (b) a cross-section view on plane A-A of a further embodiment of the connection system.

Referring first to Figure 1 , an aluminium alloy impeller 1 is fitted on to a steel turbocharger shaft 2 by means of a connection system having a connector body 3 located at a hub extension H on the shaft side of the body of the impeller 1. 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 material of the connector body 3 can be medium carbon steel such as EN8. The connector body 3 is of cup-like shape and has a threaded portion 12 with a threaded bore 11 forming the base of the cup, and a flange portion 8 around the mouth of the cup.

The shaft 2 is formed at its end with a 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 body 3 is inserted into a blind central hole formed in the hub extension H, with a radially outer surface 14 of the connector body 3 making a location fit with the radially inner surface of the hub extension H. The flange portion 8 of the connector body 3 engages against a shaft-side end face 9 of the hub extension H to determine the relative axial positions of the connector body 3 and the hub extension H. The centring portion 5 of the shaft is received in a corresponding centring portion 10 of the connector body 3 in a close, but not tight, fit. The threaded bore 11 engages on the screw-threaded portion 7 of the shaft. The threaded portion 12 has a small clearance from the end of the recess.

The hub extension H has between six and twelve, circumferentially distributed, axially extending tapped holes 21 formed in the shaft-side end face 9. The holes 21 can be provided with helicoil formations to protect their respective threads. The flange portion 8 of the connector body 3 has an equal number of through holes 22 which align with the tapped holes 21 when the connector body 3 is inserted into the hub extension H, and which each receives a capscrew 23 which threads into the corresponding tapped hole 21 to urge the flange portion 8 engages the end face 9.

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 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 connector body 3 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 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 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).

After the connector body 3 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 body 3, the respective centring portions 5, 10 ensuring the shaft 2 aligns with the axis of the impeller 1. The threads are screwed until the shoulder 4 of the shaft 2 engages with flange portion 8, which causes the threads to tighten and provides a rotationally fixed connection between the impeller 1 and the shaft 2.

Capscrews 23 axially fix the connector body 3 relative to the impeller 1 , and, as the outer surface 14 of the connector body 3 only makes a location fit with the hub extension H, they also transmit most, if not all, of the torque between the shaft 2 and the impeller 1. Thus by avoiding transferring torque over an interference fit, the problem of "walking" by the impeller can be avoided.

By containing the threaded connection between the connector body 3 and the shaft 2 in the central hole of the hub extension H, an axially compact arrangement is achieved. Further, as there is no need to fit a constraining ring of the type described in EP1394387 to the hub extension H, reg rinding operations can be avoided during fitting of the connector body 3.

Any unintended loosening of the capscrews 23 can be monitored by measuring the size of the gap that would open up between the flange portion 8 and the end face 9. Figure 3 is a close-up schematic view of a seal between a section of a casing of an impeller and the flange portion 8 of a further embodiment of the connector body 3. In this case, instead of a seal formed by a sealing ring, the hub extension H and flange portion 8 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 4 shows schematically a sectional elevation of a further embodiment of the connection system. This embodiment is similar to the embodiment of Figure 1 except that the shaft 2 has two centring portions 5a, 5b, and the connector body 3 has two corresponding centring portions 10a, 10b. The threaded portions 7, 12 of the shaft 2 and the connector body 3 are located axially between the engaging pairs of centring portions such that, on each of the shaft 2 and the connector body 3, the respective diameters of the threaded portions and the centring portions decrease towards the impeller. Figure 5 shows schematically a sectional elevation of a further embodiment of the connection system. This embodiment is similar to the embodiment of Figure 1 except that instead of tapping the impeller 1 to form each hole 21 , a cylinder 24 is press or shrink fitted into a hole in the impeller, and then the cylinder 24 is drilled and tapped to form the hole 21. The cylinder 24 is formed of steel or some other relatively strong material. In this way, although the capscrews 23 still transmit the torque between the shaft 2 and the impeller 1 , the risk of damage to the threads of the holes 21 is reduced.

An option in the embodiment of Figure 5 is to modify the cylinders 24 so that they extend into the flange portion 8. This allows torque to be transmitted from the connector body 3 to the impeller via the cylinders 24, and bypassing the threads of the capscrews 23. Figure 6 shows schematically a sectional elevation of a further embodiment of the connection system, in which the function of axial fixation of the connector body 3 is further separated from that of torque transfer. In this embodiment, threaded end portions of the capscrews 23 still screw into the tapped holes 21 to axially fix the connector body 3. However, a separate sleeve 25 (formed of steel or some other relatively strong material) surrounds a shank portion of each capscrew 23. The sleeve 25 contacts the sides of the holes 21 and 22, but is spaced at its inner surface from the shank portion of the capscrew 23. Thus the sleeve 25 transmits torque between the connector body 3 to the impeller, and avoids directing torque across the threads of the hole 21 and the capscrew 23. Figure 7 shows schematically a sectional elevation of a further embodiment of the connection system. In this embodiment, the hub extension H has between six and twelve,

circumferentially distributed, radially extending through holes 21. The connector body 3 has an equal number of radially extending through holes 22 which align with the holes 21 when the connector body 3 is inserted into the hub extension H, and which each receives a bolt 23. The head of each bolt 23 can be recessed in the connector body 3, while the nut of each bolt 23 can be recessed in the outer surface of the hub extension H. In this arrangement, the bolts 23 both axially fix the connector body 3 and transmit the torque.

Figure 8 shows schematically a sectional elevation of a further embodiment of the connection system. This embodiment is similar to the embodiment of Figure 6 except that the hub extension H has an outwardly projecting flange formation 26 through which the hole 21 in the hub extension traverses. A ring 27 is positioned behind the flange formation 26 and is tapped to receive the threads of the capscrews 23. The ring 27 can be split so that it can be assembled onto the impeller 1. Alternatively, the ring 27 can be formed in two or more pieces which can fit into separate pockets in the flange formation 26.

Figure 9(a) shows schematically a side view of a further embodiment of the connection system. This embodiment is similar to the embodiment of Figure 6 except that the direction of extension of the capscrews 23 has both the axial and tangential components. This is better shown in the developed view of one of the capscrews 23 of Figure 9(b). By providing a substantial tangential component, the sleeves 25 which transmit the torque provide a reaction force which has an axial component urging the impeller 1 towards the connector body 3 when the torque is transmitted between the connector body 3 and the impeller 1. The reaction force increases as the transmitted torque increases. Advantageously, the torque transfer members can thus supplement the axial fixing provided by the capscrews 23 at operational conditions when high loads are imposed on the impeller 1.

Figure 10 shows schematically a side view of a further embodiment of the connection system. This embodiment is a variation of the embodiment of Figure 9 in that the direction of extension of the capscrews 23 has axial, tangential and radial components. Since the capscrews 23 are straight, rather than helical, without the radial component the threaded ends would be at higher radial locations in the impeller than the sleeves 25. Thus, by introducing the radial component, the threaded ends can be located at positions which can better accept the screw stresses. Indeed, as shown in Figure 10, the radial component can be such that the holes 21 which take the capscrews 23 enter the connector body 3 at their threaded ends. This has an additional advantage that the threads on the screw can engage in the (generally stronger) connector body 3 rather than in the impeller 1.

The same principle of extending the screws or bolts 23 in axial, tangential and optionally radial directions can also be applied to the previous embodiments.

Figure 11 (a) shows schematically a sectional elevation of a further embodiment of the connection system, and Figure 11(b) shows a cross-section view on plane A-A. This embodiment is similar to the embodiment of Figure 1 except that the torque transfer between the connector body 3 and the impeller 1 is primarily via a plurality of pins which extend in the axial direction of the impeller 1 along the interface between the outer surface 14 of the connector body 3 and the radially inner surface of the hub extension H. Each pin 26 is located in facing recesses formed in these surfaces. However, an alternative would be to integrate the pins with the connector body 3 by forming them as splines projecting from the outer surface 14.

The other embodiments discussed above can also be modified by including pins, splines or other torque transfer members, such as drive wedges, at the interface between the outer surface 14 of the connector body 3 and the radially inner surface of the hub extension H.

Also, rather than extending only in the axial direction, the pins or splines can follow a helical paths on the outer surface 14 of the connector body 3, such that they extend also in the circumferential direction. The helical pins or splines can then provide a reaction force which has an axial component urging the impeller 1 towards the connector body 3 when the torque is transmitted between the connector body 3 and the impeller 1.

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 4, instead of the connector body 3 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 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.