WO2013165840A1

WO2013165840A1 - A low stress turbocharger turbine wheel having a threaded through bore mount

Info

Publication number: WO2013165840A1
Application number: PCT/US2013/038387
Authority: WO
Inventors: Brock Fraser; Paul Diemer
Original assignee: Borgwarner Inc.
Priority date: 2012-05-02
Filing date: 2013-04-26
Publication date: 2013-11-07

Abstract

A turbocharger low stress turbine wheel (1) is provided for an internal combustion engine. The turbine wheel (1) is mounted to the turbine shaft (3) via a through bore threaded mount (10, 13, 19, 27).

Description

A LOW STRESS TURBOCHARGER TURBINE WHEEL HAVING A THREADED

THROUGH BORE MOUNT

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and all the benefits of U.S. Provisional Application No. 61/641,457, filed on May 2, 2012, and entitled "A Low Stress Turbocharger Turbine Wheel Having A Threaded Through Bore Mount."

BACKGROUND OF THE INVENTION

1. Field of the Invention This invention relates to a turbocharger for an internal combustion engine. More particularly, this invention relates to turbocharger having a low stress turbine wheel which may be mounted to a turbine shaft via a through bore threaded mount.

2. Description of Related Art

A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting an engine's horsepower without significantly increasing engine weight. Thus, turbochargers permit the use of smaller engines that develop the same amount of horsepower as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which contributes to the highly desirable goal of a cleaner environment.

Turbochargers typically include a turbine housing connected to the engine's exhaust manifold, a compressor housing connected to the engine's intake manifold, and a center bearing housing coupling the turbine and compressor housings together. A turbine wheel in the turbine housing is rotatably driven by an inflow of exhaust gas supplied from the exhaust manifold. A shaft rotatably supported in the center bearing housing connects the turbine wheel to a compressor impeller in the compressor housing so that rotation of the turbine wheel causes rotation of the compressor impeller. The shaft connecting the turbine wheel and the compressor impeller defines an axis of rotation. As the compressor impeller rotates, it increases the air mass flow rate, airflow density and air pressure delivered to the engine's cylinders via the engine's intake manifold. The turbine wheel operates in a high temperature environment and the turbine wheel may reach temperatures as high as 1922° F (1050° C). In addition, the turbine wheel of a turbocharger rotates very fast. The rotation speed of a turbine wheel is size dependent and smaller turbine wheels can rotate faster than larger wheels. A turbocharger turbine wheel used in conjunction with an internal combustion engine may reach circumferential tip speeds of 530 meters per second. The rapid rotation of the turbine wheel creates large centrifugal forces or centrifugal stress on the wheel. The combination of the high temperature operating environment and the high centrifugal stress limit the materials which can be used in turbocharger turbine wheels. One group of materials which has been used for turbocharger turbine wheels are the nickel-based super alloys. The nickel-based super alloys generally contain nickel, chromium and iron, although, in certain alloys other metals may be included. The Inconel^® alloys are examples of nickel-based super alloys. The nickel-based super alloys have a density between 8 and 9 grams per cubic centimeter depending upon the alloy. Thus, nickel-based super alloy turbine wheels may be heavy and this weight creates high centrifugal stress when the wheel is rotated at high speed. The weight of the turbine wheel also means that when there is a demand for a pressure boost by the turbocharger, the turbine can take some time to reach full speed. This can result in a lag between the initial demand for a turbocharger boost and the actual delivery of the pressure boost. Recently, γ (gamma) titanium aluminide has been introduced as a turbine wheel material, γ (Gamma) titanium aluminide has a density of 4.0 grams per cubic centimeter and thus a γ (gamma) titanium aluminide turbine wheel weighs half as much as an nickel-based super alloy turbine wheel of the same size. The lower density of the gamma titanium aluminide allows the turbine wheel to have a lower weight, and thus, less stress. The lower weight also allows the turbine to come up to speed faster so that there is less lag in pressure boost. Recently Borg Warner introduced EFR turbochargers to the market. These turbochargers have a γ (gamma) titanium aluminide turbine wheel.

Turbocharger turbine wheels have had solid cores because a solid core helps the turbine wheel withstand the severe centrifugal stress created by the rapid rotation of the turbine wheel. Even with the advantage of lower weight provided by constructing the turbine wheel from γ (gamma) titanium aluminide, the centrifugal forces remain high and γ (gamma) titanium aluminide turbine wheels have solid cores. The solid core turbines are either brazed or welded to the turbine shaft. U.S. Patent publication 2007/0119908 relates to a titanium-aluminide turbine wheel which is joined to the end of a shaft by utilizing a titanium surface on the end of the shaft to be joined to the wheel, and electron-beam welding the wheel onto the titanium surface on the shaft. U.S. Patent 6,007,301 relates to a turbine rotor consisting of a wheel made of a TiAl alloy of good heat resistance and a rotor shaft made of a structural steel or a martensitic heat resistant steel with good bonding strength. The TiAl turbine wheel made by precision casting is butted to the shaft with insertion of a brazing filler in the butted interfaces and heated by high frequency induction heating in atmosphere of an inert gas or a reducing gas to a temperature higher than the liquidus temperature of the brazing metal but not exceeding 100° C above the liquidus temperature.

U.S. Patent 5,006,054 and U.S. Patent 4,891,184 disclose low density, high temperature and aluminum-rich intermetallic alloys displaying excellent elevated temperature properties, including oxidation resistance. Based on the aluminum titanium system, specifically modifications of A13 Ti compositions, useful alloys are derived from changes in crystal structure and properties effected by selected-site substitution alloying with manganese and/or chromium, and, where used, vanadium, or equivalent site-substituting alloying elements.

The high temperature processing required for brazing or welding can cause problems. The differential heating of the turbine wheel and shaft can induce stresses in the wheel and shaft. These stresses could cause failure if they are not relieved by heat treatment. In addition, the brazing or welding process may deposit slightly more metal at one part of the wheel than the other. This can create an imbalance which must be corrected in order to allow the turbine wheel to rotate at high speeds.

There are three basic types of turbine wheels. The radial turbine wheel has the fluid flowing around the edge of the turbine wheel. An example of such a wheel is a water wheel. An axial turbine has the fluid flowing through the turbine blades. A windmill is an example of an axial turbine. The mixed flow turbine wheel combines the designs of both the axial flow and radial flow turbines. Mixed flow and axial flow turbine wheels tend to have lower stress than radial flow turbines. SUMMARY OF THE INVENTION

It has been discovered that a turbine wheel having a low stress geometry, may be mounted on a shaft using an axial through bore threaded mounting. Wheels of low stress geometry which in addition are designed to run at lower speed are particularly suited for through bore mounting. Wheels of low stress geometry may be either axial flow or mixed flow wheels. A mixed flow wheel geometry is preferred. Low stress wheels may be made of heat resistant metals such as nickel-based super alloys. However, turbine wheels made from a high strength, low density, heat resistant material are easier to design and are thus preferred.

To make a low stress turbine wheel suitable for through bore mounting, two factors must be addressed. The first factor is the design of the turbine wheel itself. For a turbine wheel of a given size, axial and mixed flow turbine wheels tend to place less of the wheel mass at the edge of the wheel. Accordingly, axial and mixed flow turbine wheels have less centrifugal stress than radial turbine wheels. The second factor is the density of the material from which the turbine wheel is made. A turbine wheel made from a strong low density material will weigh less, and thus have lower centrifugal stress.

Turbine wheels suitable for through bore mounting can be made by carefully designing a wheel with low stress geometry. An axial flow wheel, or a mixed-flow wheel with very high axial content, having a very small-diameter hubline, would have a low level of centrifugal stress. Such wheels can be made from a denser material such as a nickel-based superalloy and be through bore mounted. In addition, a turbine wheel designed for lower rotation speed has less centrifugal stress and can be made from metals such as nickel-based superalloys and be through bore mounted. However, greater freedom in design is possible with a turbine wheel of low stress geometry which is made from a high strength low density material.

Examples of low density heat resistant materials suitable for constructing the turbine wheels of the present invention include the intermetallic alloys of aluminum, titanium, and other metals such as those described in U.S. Patent 5,006,054 and U.S. Patent 4,891,184. γ (Gamma) titanium aluminide is a preferred material for the turbine wheel. The through bore threaded mounting employs a female threaded component which is either female threads on the inside of the turbine wheel or the threads may be on the end of the turbine shaft with the wheel secured by a nut. The turbine wheel may be mounted to the shaft in a cold operation thus avoiding all the problems caused by brazing or welding the shaft.

BRIEF DESCRIPTION OF THE DRAWINGS Advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Figure 1 shows a low stress turbine wheel having an axial bore hole through the center of the hub and a threaded shaft extending through the hole;

Figure 2 shows a low stress turbine wheel having an axial bore hole through the center of the hub resting on a collar on the threaded shaft;

Figure 3 shows a low stress turbine wheel through bore mounted on a threaded shaft having a nut retaining the turbine wheel to the shaft; Figure 4 shows a cut away view of a low stress turbine wheel having an axial bore hole in the hub and a cross section of a shaft suitable for mounting the turbine wheel;

Figure 5A shows a cut away view of a low stress turbine wheel, a bore hole in the hub, and female threads in the bore hole which allow the threads of the shaft to be screwed into the threads of the turbine wheel thereby securing the turbine wheel to the shaft; Figure 5B shows a shaft suitable for mounting the turbine wheel illustrated in Figure

5A;

Figure 6A shows a cutaway view of a low stress turbine wheel having a conical bore hole;

Figure 6B shows a shaft suitable for mounting the turbine wheel illustrated in Figure 6A wherein the turbine shaft has a conical taper suitable for attachment to a turbine wheel having threads at the end of the wheel hub; and

Figure 7 shows a cutaway view of a turbine wheel having a conical bore hole and a cross-sectional view of a turbine shaft, having a conical taper, suitable for attachment to a low stress turbine wheel. DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 shows a low stress turbine wheel being mounted on a threaded shaft. The turbine wheel (1) has an axial bore hole, not shown in this view, in the hub (2) through which a shaft (3) extends. The turbine wheel (1) has a slip fit on shaft (3), that is, when the shaft (3) is placed in the bore hole in the hub (2), the turbine wheel (1) moves down the shaft (3) with relatively little force until the turbine wheel (1) rests on collar (4) on shaft (3). The collar (4) is an integral part of the shaft (3). The turbine wheel (1) has blades (5) shaped to allow it to achieve a low stress geometry. The shaft (3) is threaded on both ends (6) and (7). In use, the turbine rotates clockwise, as viewed facing the turbine. The threads (6) are left hand threads. The threads (7) are right hand threads. There is a gap (8) between the turbine wheel and the collar (4) on the threaded shaft (3). The turbine wheel (1) is being moved down the threaded shaft (3) but has not yet come to rest of collar (4).

Figure 2 shows a low stress geometry turbine wheel mounted on a threaded shaft. The turbine wheel (1) has an axial bore hole, not shown in this view, in the hub (2) through which a shaft (3) extends. The turbine wheel (1) has a slip fit on shaft (3), that is, when the shaft (3) is placed in the bore hole in the hub (2), the turbine wheel (1) moves down the shaft (3) with relatively little force until the turbine wheel (1) rests on collar (4) on shaft (3). The collar (4) is an integral part of the shaft (3). The turbine wheel (1) has blades (5) shaped it to allow to achieve a low stress geometry. The shaft (3) is threaded on both ends (6) and (7). The threads (6) are left hand threads. The threads (7) are right hand threads. In use, the turbine rotates clockwise, as viewed facing the turbine. The turbine wheel (1) rests on collar (4).

Figure 3 shows a low stress turbine wheel mounted on a threaded shaft secured by a nut. The turbine wheel (1) has an axial bore hole, not shown in this view, in the hub (2) through which a shaft (3) extends. The turbine wheel (1) has a slip fit on shaft (3), that is, when the shaft (3) is placed in the bore hole in the hub (2), the turbine wheel (1) moves down the shaft (3) with relatively little force until the turbine wheel (1) rests on collar (4) on shaft (3). The collar (4) is an integral part of the shaft (3). The turbine wheel (1) has blades (5) shaped to achieve a low stress geometry. The shaft (3) is threaded on both ends (6) and (7). The threads (6) are left hand threads. The threads (7) are right hand threads. A nut (9) has been turned onto the threaded end (6) of the turbine shaft.

Figure 4 shows a cut away view of a low stress turbine wheel having a bore hole in the hub and a cross section of a shaft suitable for mounting the turbine wheel. The turbine wheel (1) has an axial bore hole (10) in the hub (2) through which a shaft (3) may extend. The turbine wheel (1) has blades (5) shaped to achieve a low stress geometry. The turbine wheel (1) has a slip fit on shaft (3), that is, when the shaft (3) is placed in the bore hole in the hub (2), the turbine wheel (1) moves down the shaft (3) with relatively little force until the turbine wheel (1) rests on collar (4) on shaft (3). The shaft (3) has a collar (4) which is an integral part of the shaft (3). The shaft (3) is threaded on both ends (6) and (7). The threads (6) are left hand threads. The threads (7) are right hand threads.

Figure 5A shows a cut away view of a low stress turbine wheel having a bore hole in the hub, and threads in the bore hole which allow the threads of the shaft to be screwed into the threads of the turbine wheel thereby securing the turbine wheel to the shaft. Figure 5B illustrates a shaft suitable for mounting the turbine wheel illustrated in Figure 5 A. The turbine wheel (11) has an axial bore hole (13) in the hub (12) and threads (14) at the end of hub (12) to which shaft (3) can attach. The turbine wheel (11) has a slip fit on shaft (3), that is, when the shaft (3) is placed in the bore hole (13) in the hub (12), the turbine wheel (11) moves down the shaft (3) with relatively little force until the threaded portions (6) and (14) engage. The turbine wheel (11) has blades (15) shaped to achieve a low stress geometry. The collar (4) is an integral part of the shaft (3). The shaft (3) is threaded on both ends (6) and (7). In use, the turbine rotates clockwise, as viewed facing the turbine. The threads (6) are left hand threads. The threads (7) are right hand threads.

Figure 6A shows a cutaway view of a low stress turbine wheel having a conical bore hole and a cross-sectional view of a turbine shaft, having a conical taper, suitable for attachment to a low stress turbine wheel and suitable for securing to the turbine wheel using threads in the hub of the turbine wheel. Figure 6B illustrates a shaft suitable for mounting the turbine wheel illustrated in Figure 6A The turbine wheel (16) has an axial bore hole (19) in the hub (20) through which the shaft (3) extends. The bore hole (19) has a conical section (20). In use, the turbine wheel (16) rotates clockwise, as viewed facing the turbine. The turbine wheel (16), when placed on the shaft, moves down the shaft easily until the threaded sections, (22) and (6) meet. The turbine wheel (16) has blades (18) shaped to achieve a low stress geometry. The shaft (3) is threaded on both ends (6) and (7). The shaft has a conical section (21). The threads (6) are left hand threads. The threads (7) are right hand threads. When the threads (22) in the turbine wheel (16) are turned onto the threaded section (6) of shaft (3) conical tapered sections (21) and (22) are forced together securing the turbine wheel (16) to the shaft (3).

Figure 7 shows a cutaway view of a low stress turbine wheel having a conical bore hole, and a cross-sectional view of a turbine shaft having a conical taper. The turbine wheel (23) has an axial bore hole (27) in the hub (24). The bore hole (27) has a conical section (29). The shaft has a conical section (28). The turbine wheel (23) has blades (25) shaped to achieve a low stress geometry. When a nut (26) is turned onto the threaded section (6) of shaft (3) conical tapered sections (28) and (29) are forced together securing the turbine wheel (23) to the shaft (3). The turbine wheel of the present invention has a bore hole through the center of the hub of the wheel along an axis defined by the axis of the turbocharger shaft connecting the turbine wheel to the compressor portion of the turbocharger. In the axial through bore mounting of the present invention the turbocharger shaft is threaded and the turbine wheel is retained on the shaft by a female threaded component which is either female threads in the hub of the turbine wheel or by a nut which is screwed down onto the threads. This mounting of the turbine wheel has many advantages. The turbine wheel may be attached to the shaft in a low temperature process which avoids the high temperatures required for brazing or welding. In particular, the low temperature attachment process avoids differential heating of the turbine wheel and shaft which can induce stresses in the wheel and shaft. These stresses could cause failure if they are not relieved by heat treatment. Thus, the low temperature attachment process removes the extra step of heat treatment to relieve stress. In addition, the brazing or welding process may deposit slightly more metal at one part of the wheel than the other. This can create an imbalance which must be corrected in order to allow the turbine wheel to rotate at high speeds. The through bore mounting also lowers capital equipment costs because it does not require welding or heat treatment equipment.

In order to achieve through bore mounting of the turbine wheel to the turbine shaft the turbine wheel has to have a low stress design. Radial flow turbine wheels tend to have higher centrifugal stress than axial or mixed flow design turbine wheels. For a turbine wheel of a given size, axial and mixed flow turbine wheels tend to place less of the wheel mass at the edge of the wheel. Accordingly, axial and mixed flow turbine wheels have less centrifugal stress than radial turbine wheels. With careful design it is possible to make a turbine wheel of low stress design suitable for through bore mounting from a denser material such as nickel based superalloy. For example, a carefully designed axial, or mixed flow turbine wheel prepared from a nickel-based superalloy, such as an Inconel^® alloy, may be through bore mounted. Such mounting is of a turbine wheel made from a nickel -based superalloy, such as an Inconel^® alloy is particularly feasible when the wheel is also designed to run at lower speeds. However, greater freedom in design is possible with a turbine wheel of low stress geometry which is made from a high strength low density material. A turbine wheel made from a strong low density material will weigh less, and thus have lower centrifugal stress. Accordingly, it is easier to design a low stress turbine if a high strength low density material is used to make it.

Examples of the low density heat resistant materials having sufficient strength to be used as turbine wheels of the present invention include the intermetallic alloys of aluminum, titanium, and other metals such as those described in U.S. Patent 5,006,054 and U.S. Patent 4,891 , 184. γ (Gamma) titanium aluminide is a preferred material for the turbine wheel.

The through bore threaded mounting employs a female threaded component which is either female threads on the inside of the turbine wheel or the threads on the end of the turbine shaft. The turbine wheel is secured to the shaft either by a nut turned onto the threads or by turning the threads in the wheel hub onto the threads of the shaft. In one embodiment, the turbine wheel (1) has a slip fit on the threaded shaft (3). A collar (4) on the shaft prevents the turbine wheel from moving. The turbine wheel (1) is slipped down the threaded shaft (3) until it rests on the collar (4). A nut (9) is placed on the end of the threaded shaft (6) and is turned to place pressure on turbine wheel (1) thereby securing it to the threaded shaft (3). This embodiment is illustrated in Figures 1-4. When facing the turbine wheel, the wheel, in use, rotates clockwise. The threaded end (6) of threaded rod (3) has left handed threads. Accordingly, the rotation of the turbine wheel does not cause the nut to loosen during use. The low density heat resistant materials having sufficient strength to be used as turbine wheels, including intermetallic aluminum alloys, such those disclosed in U.S. Patent 5,006,054, U.S. Patent 4,891,184, and γ (gamma) titanium aluminide, tend to be brittle. Accordingly, care must be taken to control the pressure on the turbine wheel (1) exerted by the nut (9). The pressure must be sufficient to secure the turbine wheel (1) to the shaft but not so large as to cause the turbine wheel (1) to crack. One method of achieving this result, during manufacture, is to tighten the nut to a set torque. In another embodiment, the nut (9) is replaced by female left handed threads (14) in the hub (12) of wheel (11). This is illustrated in Figures 5A and 5B. In this embodiment, the turbine wheel (11) is slipped down the threaded shaft (3) until the female threads (14) engage the threaded end (6) of threaded shaft (3). At that point, the turbine wheel (11) is turned until the turbine wheel (11) is pressed firmly into collar (4). In use, the turbine wheel (11) rotates clockwise, and the left handed female threads (14) in the turbine wheel (11) and on the shaft assure that rotation of the turbine wheel (11) will not dislodge the wheel from the shaft (3).

In another embodiment the turbine wheel (1) does not have a slip fit on the shaft (3). Instead, the axial bore hole (10) in the turbine wheel (1) is slightly smaller than the shaft (3) and must be expanded by heating before it is put on the shaft (3). This particular embodiment is represented by Figures 1-4. The only difference between the slip fit embodiment and the embodiment in which the turbine wheel (1) must be heated is that in the heated embodiment the axial bore hole (10) in the hub is made slightly smaller relative to the shaft (3) than in the slip fit embodiment. The turbine wheel is further secured to the shaft by a nut (9). The left hand threads of the shaft and of the nut securing the turbine wheel to the shaft will assure that the turbine wheel will not loosen from the shaft in use. The difference in size between the axial bore hole (10) and the shaft (3) is not large and thus the temperature to which the wheel must be heated is much lower than the temperature required to braze or weld a steel shaft to a turbine wheel having a solid hub. Accordingly, the mild heating required to expand the axial bore hole (10) does not cause the stresses which brazing or welding would cause.

In another embodiment, a low stress turbine wheel having a conical axial bore hole and threads in the hub is placed on a turbine shaft, having a conical taper, suitable for attachment to a low stress geometry turbine wheel. The turbine wheel (16) has an axial bore hole (19) in the hub (17) through which the shaft (3) extends and blades (18) are shaped to achieve a low stress geometry. The bore hole (19) has a conical section (20) and a threaded section (22). This is illustrated in Figure 6A. There is a shaft suitable for mounting the turbine wheel illustrated in Figure 6A. The shaft has a conical section (21). The shaft (3) is threaded on both ends (6) and (7). In use, the turbine rotates clockwise, as viewed facing the turbine. The threads (6) are left hand threads. The threads (7) are right hand threads. The turbine wheel (6), when placed on the shaft, moves down the shaft easily until the threaded sections, (20) and (6), meet. When the threads (22) in the turbine wheel (16) are turned onto the threaded section (6) of shaft (3), conical tapered sections (20) and (21) are forced together securing the turbine wheel (16) to the shaft (3). This is illustrated in Figure 6B. Because the turbine wheel is secured to the shaft by left hand threads it will not loosen from the shaft in use.

In another embodiment, a low stress turbine wheel, having a conical bore hole, is placed upon a turbine shaft, having a conical taper. The turbine wheel (23) has an axial bore hole (27) in the hub (24). The bore hole (27) has a conical section (29). The turbine wheel (23) has blades (25) shaped to provide low stress geometry. The shaft has a conical section (28). The turbine wheel (23), when placed on the shaft, moves down the shaft easily until the conical sections, (28) and (29), meet. When a nut (26) is turned onto the threaded section (6) of shaft (3) conical tapered sections (28) and (29) are forced together securing the turbine wheel (23) to the shaft (3). Because the turbine wheel is secured to the shaft by left hand threads it will not loosen from the shaft in use. When the turbine wheel is made from a nickel-based superalloy there is no preference for the type of female threaded component. It may be either threads cut in the hub of the turbine wheel or a nut on the turbine shaft to retain the turbine wheel. However, when the turbine wheel is made from a low density, high strength, heat resistant material, the embodiments in which a nut is used to secure the turbine wheel to the shaft are preferred. The low density heat resistant materials having sufficient strength to be used as turbine wheels tend to be hard materials in which it is difficult to cut threads. Intermetallic alloys of titanium, aluminum and other metals are much more difficult to machine than steels, γ (Gamma) titanium aluminide is particularly difficult to machine. Accordingly, when the turbine wheel is made from γ (gamma) titanium aluminide, it is preferred to cut an axial bore hole and use a nut to retain the turbine wheel on the shaft.

When a turbine wheel is through bore mounted on a turbine shaft, the coefficient of thermal expansion of the steel turbine shaft becomes important. The low density heat resistant materials, having sufficient strength to be usable as turbine wheels in the present invention, have rather low coefficients of thermal expansion. For example, γ (gamma) titanium aluminide has a coefficient of linear thermal expansion of 12.2 (μητ/ηι per Kelvin degree). If the coefficient of thermal expansion of the turbine wheel is much less than that of the shaft, tension will be created as the turbine is used in its working environment where temperatures may reach as high as 1922° F (1050° C). If the coefficient of thermal expansion of the shaft is much less than that of the turbine wheel, the turbine wheel could become loose during use as the temperature increases. An exact match of coefficients is not required. High temperature steels having a coefficient of thermal expansion within approximately 2 μιη/m per Kelvin degree of the coefficient of thermal expansion of the turbine wheel material may be used easily. The closeness of fit of the shaft to the turbine wheel axial bore hole can influence the type of steel chosen. For example, if the steel has a lower coefficient of thermal expansion than the turbine wheel, the turbine wheel can be heated before being placed on the shaft. This would allow the shaft turbine wheel fit to tightly at lower temperatures and more normally at higher temperatures. Similarly, if the steel used in the shaft has a higher coefficient of thermal expansion than that of the turbine wheel, a slightly looser slip fit could be used. This would allow the shaft to expand at working temperature without becoming too tight.

Steels can vary markedly in their coefficient of thermal expansion. This allows the shaft to match a wide range of coefficients of thermal expansion of turbine wheel materials. For example, austentitic steels can have coefficients as high as 17 μηι/m per Kelvin degree while martensitic, heat resistant, steels often have lower coefficients of linear thermal expansion of about 12 μητ/ηι per Kelvin degree. 416 Stainless steel is a useful martensitic steel having a coefficient of thermal expansion of 11.6 μητ/ιη per Kelvin degree. It is readily machinable, and retains a Rockwell C hardness of 29 - 36 even at the elevated operating temperatures of a turbocharger. While the invention has been shown and described with respect to the particular embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the present invention as defined in the following claims.

Claims

What is Claimed:

1. A turbocharger turbine assembly comprising a low stress turbine wheel (1), and a threaded turbocharger shaft (3), wherein the low stress turbine wheel (1) comprises an axial bore hole (10, 13, 19, 27); wherein the threaded turbocharger shaft (3) passes through the axial bore hole (10, 13, 19, 27), and wherein the low stress turbine wheel (1) is retained on the turbocharger shaft (3) by a female threaded component (9, 14, 22, 26).

2. A turbocharger turbine assembly according to claim 1 in which the low stress turbine wheel (1) comprises a low density heat resistant material.

3. A turbocharger turbine assembly according to claim 2 in which the female threaded component (14, 22) comprises female threads within the hub of the low stress turbine wheel (1).

4. A turbocharger turbine assembly according to claim 2 in which the female threaded component comprises a nut (9, 26).

5. A turbocharger turbine assembly according to claim 2 in which the low stress turbine wheel (1) comprises γ titanium aluminide.

6. A turbocharger turbine assembly according to claim 3 in which the low stress turbine wheel comprises γ titanium aluminide.

7. A turbocharger turbine assembly according to claim 4 in which the low stress turbine wheel (1) comprises γ titanium aluminide.

8. A turbocharger turbine assembly according to claim 5 wherein the turbocharger shaft (3) further comprises a collar (4) upon which the low stress turbine wheel (1) can rest, and wherein the low stress turbine wheel (1) can slip over the turbocharger shaft (3) with little application of force.

9. A turbocharger turbine assembly according to claim 6 wherein the turbocharger shaft (3) further comprises a collar (4) upon which the low stress turbine wheel (1) can rest, and wherein the low stress turbine wheel (1) can slip over the turbocharger shaft (3) with little application of force.

10. A turbocharger turbine assembly according to claim 7 wherein the turbocharger shaft (3) further comprises a collar (4) upon which the low stress turbine wheel (1) can rest, and wherein the low stress turbine wheel (1) can slip over the turbocharger shaft (3) with little application of force.

11. A turbocharger turbine assembly according to claim 7 wherein the turbocharger shaft (3) further comprises a collar (4) upon which the low stress turbine wheel (1) can rest, and wherein the low stress turbine wheel (1) must be heated to slip over the turbocharger shaft (3).

12. A turbocharger turbine assembly according to claim 6 wherein the turbocharger shaft (3) further comprises a cone (21, 28) upon which the low stress turbine wheel (1) hub can rest.

13. A turbocharger turbine assembly according to claim 7 wherein the turbocharger shaft (3) further comprises a cone (21, 28) upon which the low stress turbine wheel (1) can rest.

14. A turbocharger turbine assembly according to claim 1 in which the low stress turbine wheel (1) comprises a nickel -based superalloy.

15. A turbocharger turbine assembly according to claim 14 in which the female threaded component (14, 22) comprises female threads within the hub of the low stress turbine wheel (1).