GB2544033A

GB2544033A - Mounting a component to a shaft

Info

Publication number: GB2544033A
Application number: GB1515021.2A
Authority: GB
Inventors: Jiang Kyle
Original assignee: Birmingham High Performance Turbomachinery Ltd
Current assignee: Birmingham High Performance Turbomachinery Ltd
Priority date: 2015-08-24
Filing date: 2015-08-24
Publication date: 2017-05-10
Also published as: GB201515021D0; CN106438023A; CN106438023B

Abstract

The component has an axial bore (20 fig. 5) with at least two longitudinal keyways 48. The shaft has a corresponding number of longitudinal keys 40, each key being received in a respective keyway to form a key/keyway combination 56', 56". The keyways are wider than the keys and each key is circumferentially offset relative to its respective keyway. At least one key/keyway combination (e.g. 56) is offset such that the leading side surfaces of the key and keyway abut one other and a clearance 58 is provided between their trailing side surfaces. At least one other key/keyway combination (e.g. 56") is offset such that the trailing side surfaces of the key and thekeyway abut one another and a clearance 60 is provided between their leading side surfaces. Further clearances 62 may be provided between the radially outer surfaces of the keys and the keyways. The component may be a turbine wheel for a turbocharger and may allow for thermal expansion of the components during use.

Description

Mounting a Component to a Shaft

Technical Field of the Invention

The present invention relates to improvements in or relating to arrangements for mounting components to a shaft, and in particular, in relation to mounting of a turbine wheel to a shaft, especially for use in turbochargers and the like.

Background to the Invention

Turbochargers are known which intake hot exhaust gases from an internal combustion engine and convert the energy contained within the exhaust gas to kinetic (rotational) energy by a turbine wheel mounted both rotationally and axially fast to a shaft. The rotational motion of the turbine wheel is transferred along the shaft to rotate a compressor wheel, which draws air into the turbocharger and compresses the air for delivery through an inlet manifold to the cylinders of the engine. The use of a turbocharger can considerably improve the overall efficiency of a combustion engine. Important considerations in the design of a turbocharger include reducing energy losses in order to maximise efficiency and ensuring a fast response to engine exhaust so as to reduce turbo lag.

Turbine assemblies comprising a turbine wheel mounted rotationally fast to a shaft are also used in other applications where the energy in a gas stream is converted into kinetic (rotational) energy by the turbine wheel mounted and transferred via the shaft to another component of the system

It is known to use ceramic turbine wheels, especially in applications where the temperature experienced by the turbine wheel is high, for example, when used within a turbocharger. Ceramic turbine wheels are generally used as an alternative to metallic turbine wheels, which are typically made from Inconel, due to the high temperature strength and relatively low density of ceramic materials. This allows use of the turbine wheel at elevated operating temperatures while the mass moment of inertia is reduced in comparison to an Inconel turbine wheel. Ceramic turbine wheels are also lighter, have a higher strength, and are harder and more corrosion resistant than their Inconel equivalents. Use of a ceramic turbine wheel allows turbochargers to be designed which have the following advantages over turbocharger having an equivalent Inconel turbine wheel: • more rapid response, reducing turbo lag; • ability to work at a lower engine speed, which helps cutting emissions of vehicles in urban area where cars do not move fast but produce high emissions; • ability to run faster, which allows a turbocharger to be reduced in size for a given flow rate. • longer lasting.

Whilst there are many advantages to using a ceramic turbine wheel, it is often preferable to use a metallic shaft in turbine assemblies as metallic shafts may tolerate a larger load to other materials and are easier to manufacture. Problems arise however in mounting a ceramic turbine wheel to a metallic shaft, due in part to the much larger coefficient of thermal expansion of the metallic materials used in the shaft when compared to the ceramic materials used in the construction of turbine wheels. A common method of mounting a component rotationally fast to a shaft comprises the use of a longitudinal key of the shaft which engages in a corresponding groove or keyway in the component. A portion of the shaft is located within a bore in the component with a close sliding fit. The outer circumferential surface of the portion of the shaft has a longitudinally (axially) extending key which engages in a corresponding longitudinally (axially) extending keyway in the component. Opposing side surfaces of the key and keyway engage to transmit torque. For example, if the component is rotated, a trailing side surface of the keyway abuts the trailing side surface of the key to transmit torque from component to the shaft. The key is a close sliding fit in the keyway to limit rotational free play between the component and the shaft. The key may be a spline integrally formed with the shaft or it may be a separate component which locates in aligned keyway groves in the shaft and the wheel. This arrangement is known for mounting a pulley wheel to a shaft for example.

The known types of keyed connections are not generally used for mounting a turbine wheel to a shaft in high speed turbo-machinery as it would case a high imbalance and vibration. To overcome this problem, it is known to shrink fit a conventional metallic turbine wheel rotationally fast to a metallic shaft. However, this arrangement cannot be adopted for mounting a ceramic turbine wheel directly to a metallic shaft as the shaft will expand at a faster rate than the turbine wheel when the assembly heats up. If the turbine wheel is shrunk fit about the shaft, the greater thermal expansion of the shaft will place the turbine wheel under stress and may lead to structural failure of turbine wheel.

In view of the above difficulties, various alternative approaches have been used to attach a ceramic turbine to a metallic shaft. In one known arrangement, the ceramic turbine wheel is produced with an integral shaft portion that is attached to the end of a metallic shaft using a joint designed to accommodate the differing rates of thermal expansion. Such an arrangement is disclosed in US 4, 704, 074. Whilst these arrangements work, they are relatively complex and limit design options for use in complex machinery such as turbochargers.

There is a need therefore for an arrangement for mounting components rotationally fast to a shaft which overcomes or at least mitigates the problems of the prior art. There is a particular need for an arrangement for mounting a turbine wheel to a shaft which overcomes or at least mitigates the problems of the prior art. More particularly, there is a need for an improved arrangement for mounting a ceramic turbine wheel to metallic shaft which overcomes or at least mitigates the problems of the prior art. There is also a need for such arrangements suitable for use in a turbocharger and for a turbocharger incorporating such an arrangement.

Summary of the Invention

According to a first aspect of the present invention, there is provided an assembly comprising a component mounted rotationally fast to a shaft for co-rotation about a longitudinal axis of the shaft, the component including a surface defining an axial bore at least partially therethrough, and comprising at least two longitudinal keyways in the surface defining the bore, each keyway having a leading side surface and a trailing side surface; the shaft having a portion received within said bore, the portion of the shaft having a corresponding number of longitudinal keys about its outer surface, each key having a leading side surface and a trailing side surface and being received in a respective keyway to form a key/keyway combination; wherein, at least at room temperature, the keyways are wider than the keys and each key is circumferentially offset relative to its respective keyway, with at least one key/keyway combination having a first offset in which the leading side surfaces of the key and keyway abut one other and a clearance is provided between their trailing side surfaces, and at least one other key/keyway combination having a second offset in which the trailing side surfaces of the key and the keyway abut one another and a clearance is provided between their leading side surfaces.

In an assembly as defined, the asymmetric arrangement of the contact surfaces ensures that the component is held rotationally fast to the shaft to within acceptable tolerances even at room temperature, whilst the clearances are selected to accommodate thermal expansion of the shaft as the assembly heats up to operating temperature.

The component may be a turbine wheel.

An even number of key/keyway combinations may be provided. In which case, the combinations may be arranged symmetrically about the longitudinal axis of the shaft and they may be evenly distributed about the circumference of the shaft. The combinations may be arranged in diametrically opposed pairs, in which case, the combinations in each diametrically opposed pair may have the same first or second offset. Thus in an embodiment having four key/keyway combinations, the combinations may be arranged in two diametrically opposed pairs, each key/keyway combination in a first one of the pairs having said first offset and each key/keyway combination in a second one of the pairs having said second offset.

Each key has a radially outer end surface interconnecting its side surfaces and each keyway has a radially outer bottom surface interconnecting its side surfaces and which opposes the radially outer end surface of its respective key, and a clearance may be provided between the radially outer end surface of each key and the radially outer bottom surface of each keyway, at least at room temperature. A clearance may be provided between the outer surface of the shaft and the surface defining the bore, at least at room temperature.

In an embodiment, the only points of contact provided between the component and the shaft at room temperature are between the leading side surfaces of the key and keyway in the at least one key/keyway combination having said first offset and the trailing side surfaces of the key and key way in at least one key/keyway combination having said second offset, clearances being provided elsewhere between the shaft and keys and the component to accommodate thermal expansion.

The keys may extend over the full length of the keyways or they may only extend over part of the length of the keyways.

Each key may comprise a key spline integrally formed with the shaft.

The component may be formed of a ceramic material and the shaft may be formed of a metallic material. However, the invention is not limited in this sense and is intended to cover all combinations of components and shafts in which the material of the shaft has a higher coefficient of thermal expansion than material of the component. For example, the component and/or the shaft may be formed of a ceramic material, or may be formed from a plastics material or may be formed from a metallic material. In some embodiments the component and/or the shaft may be formed of a composite material.

Each key may have a central longitudinal axis and each keyway may have a central longitudinal axis, wherein in the at least one key/keyway combination having said first offset, the central longitudinal axis of the key is offset to the leading side of the central longitudinal axis of the keyway and in the at least one key/keyway combination having said second offset, the central longitudinal axis of the key is offset to the trailing side of the central longitudinal axis of the keyway

According to a second aspect of the present invention there is provided a turbocharger comprising an assembly in accordance with the first aspect of the present invention, wherein the component is a turbine wheel mounted to one and of the shaft and the turbocharger further comprising a compressor wheel connected to a distal end of the shaft to the turbine wheel.

Detailed Description of the Invention

In order that the invention may be more clearly understood embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a cross sectional view of an embodiment of a turbocharger in accordance with an aspect of the present invention.

Figure 2 is an exploded perspective view of a turbine wheel and shaft assembly forming part of the turbocharger shown in Figure 1.

Figure 3 is a view from the front of the turbine wheel and shaft assembly of Figure 2 showing the turbine wheel mounted to shaft and with a central hub section of the turbine wheel and the shaft shown in cross section.

Figure 4 is an enlarged view of detail “D” in Figure 3.

Figure 5 is a view similar to that of Figure 4 but omitting the shaft.

Figure 6 is a view similar to that of Figure 4 but omitting the turbine wheel.

Figure 1 illustrates an embodiment of a turbocharger 1 in accordance with an aspect of the invention. The turbocharger 1 has a housing 2 which is made up of a turbine housing section 4, a central housing section 6, compressor housing section 8 and a back cover 9 for the compressor wheel housing. Mounted within the housing 2 are the working parts of the turbocharger 1 including a turbine wheel 10, a connecting shaft 12 and a compressor wheel 14.

The turbine housing section 4 is mounted to one side of central housing section 6 and surrounds the turbine wheel 10. The turbine wheel 10 includes a series of turbine blades 16 disposed about a central hub 18. A blind bore 20 extends axially part way through a central hub 18 from a rear wall 22. The turbine wheel 10 is mounted rotationally fast to a first end of the shaft 12 via the bore 20 for co-rotation about a longitudinal axis X of the shaft 12. In the present embodiment the turbine wheel and shaft rotate in the direction indicated by arrow A in Figs. 2 and 4 during normal operation of the turbocharger. In this specification, references to “leading” and “trailing” in relation to features the turbine wheel 10, shaft 12, and any other components mounted rotationally fast therewith, should be understood as referring to the relative disposition of the feature when rotating in the direction of arrow A unless otherwise specified.

The turbine housing section 4 defines an exhaust gas inlet 26 for directing exhaust gases from an associated combustion engine (not shown) onto the blades 16 of the turbine wheel 10 and an exhaust gas outlet 28. It is envisaged that in use, the exhaust gas outlet will be suitably connected to an exhaust system (not shown) in the usual manner.

The compressor wheel 14 is mounted rotatably fast to a second end of the shaft 12. The compressor wheel is housed between the back plate 9, which is mounted to the opposite side of the central housing portion 6 from the turbine, and the compressor housing section 8 which is mounted to the back cover 9. The compressor housing section 8 defines an air inlet 30 and a compressed air outlet 32. Rotation of the compressor wheel 14 as it is driven by the turbine wheel 10 draws in ambient air through the inlet 30, which air is compressed and expelled through the outlet 32. In use, the outlet 32 is connected to an inlet manifold (not shown) of the associated combustion engine (not shown) for directing the compressed air in to the cylinders of the associated combustion engine in the usual manner.

The shaft 12 passes through the central housing section 6 and the back cover 9 and is rotatably supported and axially located in the housing by a bearing system in a known manner.

Figures 2 to 6 illustrate an arrangement for mounting the turbine wheel 10 to the shaft 12 in accordance with a further aspect of the invention.

The turbine wheel 10 is manufactured from a ceramic material as a single integral component. Any suitable ceramic material can be used but in an embodiment, the turbine wheel 10 is manufactured from Si3N4, its alloy. The turbine shaft 12 is manufactured from a metallic material such as steel and other alloys, for example Inconel. A first end of the turbine shaft 12 has a large diameter sealing boss 36 and a spigot portion 38 which projects axially from the boss 36 to be received in the bore 20. The boss 36 locates in an aperture in an end wall of the central housing portion 6 at the turbine end and suitable sealing means are provided between the boss 36 and the aperture The spigot portion 38 and the bore 20 are dimensioned so that the turbine wheel 10 can be mounted on the spigot portion 38 with a rear wall 22 of the turbine wheel 10 in contact with an outer face of the boss 36.

The turbine wheel 10 is held rotationally fast with the shaft 12 by means of keyed driving connection. Four longitudinally extending key splines 40 are spaced about the circumference of the spigot portion 38. Each key spline 40 extends along at least part of the length of the spigot portion from its free end towards the sealing boss 36. With reference to the direction of rotation A, each key spline has a leading side surface 42, a trailing side surface 44 and a radially outer end surface 46 which extends between the side surfaces. A corresponding number of keyways or grooves 48 are provided in the turbine wheel disposed about the surface 20A of the wheel that defines the bore 20. The keyways 48 open at the rear wall 22 of the turbine wheel and extend longitudinally in an axial direction of the bore towards the closed end of the bore 20. With reference to the direction of rotation, each keyway 48 has leading side surface 50, a trailing side surface 52 and a radially outer bottom surface 54 which extends between the side surfaces 50, 52.

The key splines 40 and keyways 48 are arranged and configured so that each key spline 40 is a sliding fit in a respective keyway 48 to form a key spline/keyway combination 56. The key splines 40 may extend along the whole of the length of the keyways 48 or they may extend only along part of the length of the keyways 48. In this later case, the key splines 40 will extend only along part of the length of the spigot portion 38 from its free end towards the boss 36.

At least when the assembly is at room temperature, the keyways 48 are wider than the key splines 40. Each key spline/keyway combination 56 is configured so that the key spline 40 is circumferentially offset relative to its respective keyway 48 so that the side surfaces of the key spline and keyway contact one another on one side only, with a clearance between the side surfaces of the key spline 40 and the keyway 48 on the other side. In accordance with the invention, at least one of the key spline/keyway combination 56’ has a first offset and at least one other key spline/keyway combination 56” has a second, opposite offset. In key spline/keyway combinations 56’ having the first offset, the key spline 40 is offset towards a leading side of the keyway 48 so that the leading side surfaces 42, 50 of the key and keyway abut one another and a clearance 58 is provided between the trailing side surfaces 44, 52 of the key spline 40 and keyway 48. In key spline/keyway combinations 56” having the second offset, the key spline 40 is offset towards a trailing side of the keyway 48 so that the trailing side surfaces 44, 52 of the key and keyway abut one another and a clearance 60 is provided between the leading side surfaces 42, 50 of the key and keyway. In the embodiment as shown, the four key spline/keyway combinations 56 are arranged in diametrically opposed pairs in which the combinations in a first opposed pair have the same offset, whilst the combinations in the other pair have an opposite offset. Accordingly, a first of the opposed pairs the key spline/keyway combinations 56’, 56’ each have the first offset and in a second of the pair the key spline/keyway combinations 56”, 56” each have the second offset.

In the present embodiment, in addition to the clearances 58, 60 provided between the sides surfaces of each key spline 40 and respective keyway 48 on one side, at least at room temperature, there is a clearance 62 between the radially outer end surface 46 of each key spline 40 and the opposed radially outer bottom surface 54 of its respective keyway 48 and a clearance 64 between the outer circumferential surface 66 of the spigot portion 38 between the key splines 40 and surface 20 A defining the bore 20. It will be noted that in this arrangement, the only points of contact between the shaft 12 and the turbine wheel 10 are the leading side surfaces 42, 50 of the key splines and keyways in the combinations 56’ having the first offset and the trailing side surfaces 44, 52 of the key splines 40 and keyways 48 in those combinations 56” having the second offset. Clearances 58, 60, 62, 64 are provided between all the other opposed surfaces of the shaft 12, including its key splines 40, and the turbine wheel 10 to allow for thermal expansion of the shaft 12.

The asymmetrical arrangement of the key spline/keyway combinations 56’ 56” holds the turbine wheel 10 rotationally fast to the shaft 12 whilst the clearance 64 between the shaft 12 and the surface 20 A of the bore 20 and the clearances 58, 60, 62 between those surfaces of the key splines 40 and their respective keyways 48 that are not in contact are selected to allow for thermal expansion of the shaft 12, including the key splines 40, as the assembly heats up to its normal operating temperature range without placing the turbine wheel 10 under an unacceptable stress which could lead to damage of the turbine wheel.

Whilst reference is made herein to the clearances at room temperature, it will be appreciated that keyed mounting arrangement can be configured as required for the full temperature range over which the turbocharger is expected to operate. For example, if the turbocharger 1 is likely to be subject to temperatures below freezing when cold, then the mounting arrangement will be configured to provide a suitable fit at the lowest expected temperatures with sufficient clearances to accommodate the greater thermal expansion of the shaft 12 as it warms up to normal operating temperature range.

The turbine wheel 10 is a relatively close sliding fit on the key splines 40, which in many applications will be sufficient to hold the turbine wheel axially in position on the shaft. In an alternative embodiment, the bore 20 can extend fully through the turbine wheel 20 and the spigot portion 38 provided with a threaded section at its free end to which a nut can be fastened to secure the turbine wheel axially on the shaft.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

For example, the keyed connection arrangement between the turbine wheel 10 and the shaft 12 as described is not limited to use in mounting a ceramic turbine wheel to a metallic shaft but could be used to mount any turbine wheel to a shaft, or indeed any component having a bore to a shaft, particularly where the material of the shaft has a greater coefficient of thermal expansion than the component. Accordingly, whilst the invention has particular application in mounting a turbine wheel to a shaft for use in a turbocharger, it can be adopted for use in other applications.

It has been found that the use of four key spline/keyway combinations 56 as shown in the embodiment described works particularly well in high speed applications such as turbo-machinery. However, provided there are at least two key spline /keyway combinations, one having the first offset and one the second offset, the number of key spline/keyway combinations 56 can be selected as required. For example, it is believed that an arrangement using as few as two key spline/keyway combinations may be suitable in low speed applications and more than four key spline/keyway combinations can be adopted where required. Arranging the key spline/keyway combinations 56 symmetrically helps to reduce out of balance forces and for this an even number of key spline/keyway combinations may be advantageous. Nevertheless, in some applications an odd number of key spline/keyway combinations may be suitable.

Claims

1. An assembly comprising a component mounted rotationally fast to a shaft for co-rotation about a longitudinal axis of the shaft, the component including a surface defining a central, axial bore at least partially therethrough, and comprising at least two longitudinal keyways in the surface defining the bore, each keyway having a leading side surface and a trailing side surface; the shaft having a portion received within said bore, the portion of the shaft having a corresponding number of longitudinal keys about its outer circumferential surface, each key having a leading side surface and a trailing side surface and being received in a respective keyway to form a key/keyway combination; wherein, at least at room temperature, the keyways are wider than the keys and each key is circumferentially offset relative to its respective keyway, with at least one key/keyway combination having a first offset in which the leading side surfaces of the key and keyway abut one other and a clearance is provided between their trailing side surfaces, and at least one other key/keyway combination having a second offset in which the trailing side surfaces of the key and the keyway abut one another and a clearance is provided between their leading side surfaces.

2. An assembly as claimed in claim 1, where the component is a turbine wheel.

3. An assembly as claimed in claim 1 or claim 2, wherein an even number of key/keyway combinations are provided.

4. An assembly as claimed in claim 3, wherein the key/keyway combinations are arranged symmetrically about the circumference of the shaft.

5. An assembly as claimed in claim 3 or claim 4, wherein the key/keyway combinations are arranged in diametrically opposed pairs.

6. An assembly as claimed in claim 5, wherein both key/keyway combinations in each diametrically opposed pair have the same first or second offset.

7. An assembly as claimed in claim 1, wherein there are four key/keyway combinations arranged in two diametrically opposed pairs, each of key/keyway combination in a first one of the pairs having said first offset and each key/keyway combination in a second one of the pairs having said second offset.

8. An assembly as claimed in any one of the preceding claims, wherein each key has a radially outer end surface interconnecting its side surfaces and each keyway has a radially outer bottom surface interconnecting its side surfaces and which opposes the radially outer end surface of its respective key, a clearance being provided between the radially outer end surface of each key and the radially outer bottom surface of each keyway, at least at room temperature.

9. An assembly as claimed in any one of the preceding claims, wherein a clearance is provided between the outer circumferential surface of the shaft and the surface defining the bore, at least at room temperature.

10. An assembly as claimed in any one of the preceding claims, wherein the only points of contact provided between the component and the shaft at room temperature are between the leading side surfaces of the key and keyway in the at least one key/keyway combination having said first offset and the trailing side surfaces of the key and keyway in at least one key/keyway combination having said second offset, clearances being provided elsewhere between the shaft and keys and the component.

11. An assembly as claimed in any one of the preceding claims, wherein the shaft has a larger coefficient of thermal expansion than the component, the clearances being configured to accommodate thermal expansion of the shaft and keys during use.

12. An assembly as claimed in any one of the preceding claims, wherein each key extends only over part of the length of its respective keyway.

13. An assembly as claimed in any one of the preceding claims, wherein the keys are key splines integrally formed with the shaft.

14. An assembly as claimed in any one of the preceding claims, wherein the component is formed of a ceramic material and the shaft is formed of a metallic material.

15. An assembly as claimed in any one of the preceding claims, wherein each key has have a central longitudinal axis and each keyway has a central longitudinal axis, wherein in the at least one key/keyway combination having said first offset, the central longitudinal axis of the key is offset to the leading side of the central longitudinal axis of the keyway and in the at least one key/keyway combination having said second offset, the central longitudinal axis of the key is offset to the trailing side of the central longitudinal axis of the keyway.

16. An assembly comprising a component mounted rotationally fast to a shaft for co-rotation about a longitudinal axis of the shaft as hereinbefore described, with reference to and as illustrated in the accompanying drawing.

17. A turbocharger comprising an assembly as claimed in any one of the preceding claims, wherein the component is a turbine wheel mounted to one and of the shaft and the turbocharger further comprising a compressor wheel connected to a distal end of the shaft to the turbine wheel.

18. A turbocharger as claimed in claim 17, wherein the shaft has a larger coefficient of thermal expansion than the turbine wheel, the clearances being configured to accommodate thermal expansion of the shaft during use.

19. A turbocharger as claimed in claim 16 or claim 17, wherein the turbine wheel is made of a ceramic material and the shaft is made of a metallic material.

20. An assembly comprising a component mounted rotationally fast to a shaft for co-rotation about a longitudinal axis of the shaft as herein before described, with reference to and as illustrated in the accompanying drawing.