Compressor wheel

Description

BACKGROUND OF THE INVENTION

This invention relates generally to compressor wheels or impellers as may be used in a turbocharger, supercharger, and the like.

Locomotives equipped with internal combustion engines, e.g., diesel engines, designed to meet stringent emissions regulations may require relatively high air flow rates and high manifold pressures, as may be provided by a turbocharger, to meet air quality targets while maximizing fuel economy and reliability. The high boost ratios, typically 3.8 or higher, may require high tip speeds on a turbocharger compressor wheel, which may lead to high levels of tensile stress in a bore area and shortened wheel life due to low cycle fatigue. It is known that compressor wheels have used a through bore that extends through the center of the wheel along a rotation axis, and where the wheel may be securely mounted onto a shaft with a locking nut.

It is further known that so called boreless compressor wheels are able to rotate at higher speeds than compressor wheels having a through bore since a through bore removes load carrying material and thereby increases the stress level in the remaining material. That is, more wheel material exists at a point of maximum centrifugal load that results in higher load carrying capability.

In one known compressor wheel, a hub section of the compressor wheel that axially corresponds with the radially outermost portion of the wheel experiences the maximum centrifugal load. That is, a plane indicative of typical maximum stress exists in substantial axial alignment with the maximum radial extent of the hub. In this known compressor wheel, a threaded counterbore is provided in a hub extension for receiving the shaft. However, the counterbore must terminate well short of the plane indicative of typical maximum stress to avoid the high level of stress at that location. Since the length of the shaft/threaded interface is generally constant for any given application, the foregoing arrangement (that causes the designer to position such an interface away from the high stress plane) may result in excessive overhang of the compressor wheel. This detrimentally affects rotor dynamics and increases the axial length of the hub extension, thus increasing the overall footprint of the compressor wheel and turbocharger.

BRIEF DESCRIPTION OF THE INVENTION

Aspects of the present invention propose to improve compressor life by using a hub with a counterbore including a base surface treated to impart residual compressive stresses for enhanced endurance to stress-induced fatigue. The surface treatment allows extending the counterbore relatively closer to a plane indicative of typical maximum stress. This design flexibility advantageously allows avoiding or reducing overhang of the compressor wheel, thereby improving rotor dynamics and reducing the axial length of the hub, and the overall foot print of the compressor wheel and turbocharger.

Generally, the present invention fulfills the foregoing needs by providing in one aspect thereof, a method of manufacturing a compressor wheel. The method allows providing a hub extension of the compressor wheel that defines a counterbore. The method further allows imparting residual compressive stresses to a base surface of the counterbore.

In another aspect thereof, the present invention further fulfills the foregoing needs by providing a compressor wheel comprising a hub including a hub extension that defines a counterbore. The counterbore includes a base surface and the base surface of the counterbore is treated to impart residual compressive stresses thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become apparent from the following detailed description of the invention when read with the accompanying drawings in which:

FIG. 1 illustrates a cutaway view of an exemplary turbocharger that may benefit from the teachings of the present invention.

FIG. 2 shows a cross-sectional view of an exemplary compressor wheel embodying aspects of the present invention.

FIG. 3 shows configurational details regarding the compressor wheel of FIG. 2 that allow positioning a mounting counterbore relatively closer to a plane indicative of typical maximum stress of the wheel.

FIG. 4 shows a rotatable shaft assembled into a counterbore embodying aspects of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a cutaway view of an exemplary turbocharger (10) that may benefit from the teachings of the present invention. Turbocharger (10) generally comprises respective compressor and turbine stages (12) and (14) including a compressor wheel (16) and a turbine wheel (18) coupled through a rotatable shaft (20). The turbine wheel (18) is disposed within a turbine housing, which includes an inlet for receiving exhaust gases from an internal combustion engine (not shown). The turbine housing guides the engine exhaust gases for communication with and expansion through the turbine wheel (18) for rotatably driving the turbine wheel. Simultaneously, the turbine wheel rotatably drives the shaft (20) and compressor wheel (16), as may be disposed within a compressor housing. The compressor wheel (16) and housing allow drawing in and compressing ambient air for supply to the intake of the engine.

Referring to FIG. 2, a cross-sectional view of an exemplary embodiment of the compressor wheel (16) is shown. Compressor wheel (16) includes a hub portion (21). The hub portion (21) defines a front face surface (22) for the compressor wheel (16), and supports a plurality of circumferentially spaced apart compressor blades (23) (only two of which are visible in FIG. 2) extending both radially outwardly and axially thereon. The hub portion (21) also includes a radially enlarged disc-like portion (24) which serves to support the compressor blades (23) as well as to define a floor surface (25) for the air flow channels defined between blades (23). The disc-like portion (24) also defines a radially outer circumferential surface (26) for hub (21), as well as an axially disposed back side or back face surface (27).

As will be recognized by those skilled in the pertinent art, in operation of compressor wheel (16), a plane indicative of typical maximum stress (30) typically exists substantially in axial alignment with the maximum radial extent of the hub (21). That is, the plane of maximum stress (30) is typically coincident with surface 26 and reaches a maximum at the point where the rotation axis (34) transects plane (30), approximately at point (31).

To avoid the undesirable stress concentration of a conventional through bore and preserve the strength of solid metal adjacent to point (31), compressor wheel (16) includes a hub extension (38) integrally defined by hub (21) and extending axially away from plane (30). Hub extension (38) defines an axially extending counterbore (36).

The inventors of the present invention have innovatively recognized that one may advantageously improve rotor dynamics as well as reduce the axial length of the compressor wheel when a base surface (40) of counterbore (36) is treated to impart residual compressive stresses to such a base surface. Examples of techniques, such as cold working techniques, that may be used to treat the counterbore base surface may comprise shot peening, laser peening, glass beading, roll burnishing, etc. Cold working provides plastic deformation of a metal (e.g., aluminum) below its annealing temperature to cause permanent strain hardening.

Peening, as understood in the art and as used herein, means to compress a portion of a surface by forming a depression or indentation on the surface. Peening equipment generally is utilized to create a compressively stressed protection layer at the outer surface of a workpiece. The protection layer considerably increases the resistance of the workpiece to fatigue failure. A shot used in shot peening may comprise spherical particles constructed from a hard metal or any other suitable material. With shot peening systems, a stream of shot particles traveling at a high velocity is directed at an outer surface of a workpiece, e.g., the base of the counterbore. Each shot particle that impacts with sufficient force upon the outer surface of the workpiece causes plastic deformation of the surface and a dimple is formed therein. In this manner, a compressively stressed layer is formed at the outer surface of the workpiece to increase fatigue strength of the workpiece.

In laser peening, a laser beam from a laser beam source is used to produce a strong localized compressive force on a surface. Laser peening may be utilized in lieu of shot peening to create a compressively stressed protection layer at the outer surface of a workpiece. This type of treatment also considerably increases the resistance of the workpiece to fatigue failure. Thus, peening is typically a very effective means for producing surface compression residual stress, and therefore, prolonging the useful life of the workpiece.

As shown in FIG. 3, the surface treatment of the base of the counterbore allows extending the base surface (40) of counterbore (36) relatively closer to the plane (30) indicative of typical maximum stress. This is exemplarily illustrated in FIG. 3 by the representation of the counterbore base (40′) relative to the representation of the counterbore base (40). This design flexibility advantageously allows avoiding or reducing overhang of the compressor wheel, thereby improving rotor dynamics and reducing the axial length of the hub extension, and the overall foot print of the compressor wheel and turbocharger. For example, assuming the base (40) of counterbore (36) is configured to extend a distance L towards plane (30), this would allow reducing the axial length of the hub extension (38) by distance L.

In yet another aspect of the invention, as may be appreciated in FIG. 4, the extended base (40) of the counterbore allows deeper penetration of shaft (20) relative to the plane (30) of typical maximum tensile stress, thereby reducing axial overhang of the compressor wheel. In one exemplary embodiment, counterbore (36) includes alignment pilots (42) disposed to facilitate the centering of the shaft (20) received in counterbore (36). That is, alignment pilots (42) are configured to minimize eccentricity of shaft (20) relative to the walls of the counterbore. As will be appreciated by those skilled in the art, the compressor wheel, shaft, and thrust collar (44) may rotate relative to a bearing configured to provide radial support to the rotating structures.

While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.