Assembly for a turbine

Abstract

An assembly for a turbomachine comprises a bush 74 and a cap 86. The bush defines a bore for receiving at least part of a shaft 39 and which defines a bore axis A. An end of the bush comprises a reces

Description

The present invention relates to an assembly for a turbomachine, a valve assembly, a wastegate assembly, and an associated method of aligning and retaining a seal in a recess of a bush of a turbomachine. Turbomachines are machines that transfer energy between a rotor and a fluid. For example, a turbomachine may transfer energy from a fluid to a rotor or may transfer energy from a rotor to a fluid. Two examples of turbomachines are a power turbine, which uses the rotational energy of the rotor to do useful work, for example, generating electrical power; and a turbocharger, which uses the rotational energy of the rotor to compress a fluid. Turbochargers are well known devices for supplying air to an inlet of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to an engine inlet manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings. The turbine of a conventional turbocharger comprises: a turbine chamber within which the turbine wheel is mounted; an annular inlet defined between facing radial walls arranged around the turbine chamber; an inlet volute arranged around the annular inlet; and an outlet passageway extending from the turbine chamber. The passageways and chamber communicate such that pressurised exhaust gas admitted to the inlet volute flows through the inlet to the outlet passageway via the turbine and rotates the turbine wheel. It is also known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet so as to deflect gas flowing through the inlet. That is, gas flowing through the annular inlet flows through inlet passages (defined between adjacent vanes) which induce swirl in the gas flow, turning the flow direction towards the direction of rotation of the turbine wheel. Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that characteristics of the inlet (such as the inlet’s size) can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suit varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the inlet using a variable geometry mechanism. Turbochargers provided with a variable geometry turbine are referred to as variable geometry turbochargers. Nozzle vane arrangements in variable geometry turbochargers can take different forms. Two known types of variable geometry turbine are swing vane turbochargers and sliding nozzle turbochargers. Generally, in swing vane turbochargers the inlet size (or flow size) of a turbocharger turbine is controlled by an array of movable vanes in the turbine inlet. Each vane can pivot about an axis extending across the inlet parallel to the turbocharger shaft and aligned with a point approximately half way along the vane length. A vane actuating mechanism is provided which is linked to each of the vanes and is displaceable in a manner which causes each of the vanes to move in unison, such a movement enabling the cross sectional area available for the incoming gas and the angle of approach of the gas to the turbine wheel to be controlled. Generally, in sliding nozzle turbochargers the vanes are fixed to an axially movable wall that slides across the inlet. The axially movable wall moves towards a facing shroud plate in order to close down the inlet and in so doing the vanes pass through apertures in the shroud plate. Alternatively, the nozzle ring is fixed to a wall of the turbine and a shroud plate is moved over the vanes to vary the size of the inlet. The compressor of a conventional turbocharger comprises a compressor housing defining compressor chamber within which the compressor wheel is mounted such that it may rotate about an axis. The compressor has a substantially axial inlet passageway defined by the compressor housing and a substantially annular outlet passageway defined by the compressor housing between facing radially extending walls arranged around the compressor chamber. A volute is arranged around the outlet passageway and an outlet is in flow communication with the volute. The passageways and compressor chamber communicate such that gas (for example, air) at a relatively low pressure is admitted to the inlet and is pumped, via the compressor chamber, outlet passageway and volute, to the outlet by rotation of the compressor wheel. The gas at the outlet is generally at a greater pressure (also referred to as boost pressure) than the relatively low pressure of the gas which is admitted to the inlet. The gas at the outlet may then be pumped downstream of the compressor outlet by the action of the compressor wheel. It is known to provide a turbocharger turbine with a valve controlled bypass port referred to as a wastegate, to enable control of the turbocharger boost pressure and / or shaft speed. A wastegate valve (typically a flap type valve) is controlled to open the wastegate port (bypass port) when the boost pressure of the fluid in the compressor outlet increases towards a pre-determined level, thus allowing at least some of the exhaust gas to bypass the turbine wheel. Typically, the wastegate port opens into a wastegate passage which diverts the bypass gas flow to the turbine outlet or vents it to atmosphere. The wastegate valve may be actuated by a variety of means, including electric actuators, but is more typically actuated by a pneumatic actuator operated by boost pressure delivered by the compressor wheel. The wastegate valve actuator is typically connected to the wastegate valve by a linkage, part of which passes through an actuator conduit in the turbine housing. Where the linkage passes through the actuator conduit it is possible that fluid from the turbine outlet may leak into the actuator conduit and then to atmosphere. In some cases, after the turbine has been assembled, the turbine is installed in an engine (as part of a turbocharger), and the engine undergoes a pass-off test in order to evaluate aspects of the performance of the engine to ensure that the engine meets specified performance standards. One portion of the pass-off test is a pressure leak down test. This test is used to check the engine for leaks which may have an adverse effect on the operating performance of the engine. If the engine fails the leak test (e.g. an amount of leakage is unacceptably high) it means that: i) there is a leakage path somewhere within the engine (not including the turbine); or ii) there is no leak within the engine itself but the turbine is leaking. It is a challenge for an operator to then confirm which of the above options applies without costly and time-consuming manual intervention, such as manually sealing part of the turbine and / or the complete removal of the turbine from the engine. It is also comparatively challenging to quantify a turbine leakage so that the turbine leak ‘contribution’ can be factored into an overall engine leak amount, with a view to confirming that the engine leak amount is below an acceptable threshold. As well as the above ‘combined’ leakage tests, it may also be desirable to test the turbine in isolation of the engine (again with a view to determining a leakage amount attributable to the turbine). For most turbines, a leakage path exists from the turbine outlet, through an actuator conduit and to the atmosphere. This leakage path may result in the engine ‘artificially’ failing the pressure leak down test, despite the turbine leakage path being a normal, and acceptable, leakage path. For turbines tested in isolation, turbines which fail a portion of the pass-off test (e.g. the pressure leak down test) may be recalled so that they can be inspected and / or reassembled before being retested and, if passed, released for dispatch. It will be appreciated that inspection, reassembly and / or retesting of a turbine which fails the pressure leak down test due to a leakage path via the actuator conduit (generally, a non-significant fault) may be a waste of time, effort and money. This is because a turbine with a leakage path via the actuator conduit will, in general, still exhibit acceptable performance and hence such a turbine could have been released for dispatch without the need for inspection, reassembly and / or retest. In view of the above, it is desirable to provide a sealing solution which seals a turbine leakage path, which may otherwise exist through the actuator conduit, to improve the efficiency and reliability of engine, and turbine-in-isolation, leak testing. Existing sealing solutions have been found to be problematic insofar as they may be difficult and / or costly to install, and lack robustness. There exists a need to overcome the disadvantages associated with existing assemblies, and associated methods, whether mentioned in this document or otherwise. According to a first aspect of the invention, there is provided an assembly for a turbomachine, the assembly comprising: a bush which defines a bore for receiving at least part of a shaft, the bore defining a bore axis, and wherein an end of the bush comprises a recess for a seal; and a cap at least partly received in the recess for retaining the seal. The turbomachine may be a turbocharger. The term “bush” encompasses a cylindrical tube or sleeve, and may form part of a bearing. The bush may be provided in an actuator conduit and may be provided to reduce friction between the actuator conduit and a shaft that is received in the bore of the bush. “Receiving at least part of a shaft in the bore of the bush” encompasses receiving an axial length of the shaft in the bore of the bush. The bore may be an annular bore. The bush may be an annular bush. The seal that may be received in the recess may be a mechanical seal. The term “recess” encompasses a depression or groove or counter bore in the bush. The term “end of the bush” encompasses an axially outermost end of the bush. That is to say that an end face of the bush comprises a recess, the recess being sized to receive a seal. The bush may be formed from any suitable material, but the material may be a metallic material, for example steel or steel alloy. The recess may be machined into an end face of the bush. The recess, may be referred to a sealing recess, a leakage recess, or a leakage path recess. That is to say, the provision of the recess is to allow for one or more features (e.g. a seal, a cap) to be at least partly received in the recess to reduce leakage through the bore and / or through an actuator conduit. The term “cap” encompasses a closing member and / or alignment member. The cap may be for example a top hat shim. At least part of the cap being received in the recess has a number of advantages. A first advantage of providing the cap at least partly in the recess is that in use it serves a purpose of leakage reduction of fluid (for example exhaust gas) passing from an upstream location through the bore of the bush and / or through an actuator conduit, where it is not intended to pass. Another advantage of the cap being partly received in the recess is that when a seal is provided in the recess, the cap promotes retention of the seal in the recess, even with relative movement (rotational and / or axial) of the shaft. A further advantage of the cap being received at least partly in the recess is that it may serve the purpose of aligning a seal in the recess during an assembly process. That is to say that the cap may also function as a seal insertion tool. Providing a recess in the end of the bush is particularly beneficial as it allows for a seal to be easily inserted during a manufacturing process. Furthermore, the assembly time is reduced compared to sealing arrangements wherein a seal is provided in a middle portion of a bush. In addition, line of sight access is improved when locating a seal in the end of a bush, in comparison to know methods of locating a seal in a groove along a length of a bush, which is inserted blindly. The assembly may further comprise a shaft which defines a shaft axis, and wherein at least part of the shaft may be rotatably received in the bore. At least part of the shaft being received in the bore encompasses receiving an axial length of the shaft in the bore of the bush. The shaft may be a wastegate shaft for use in a wastegate assembly. The shaft axis may be coincident with the bore axis. The shaft may be a cylindrical shaft. The shaft may have a shape that is complementary to the shape of the bore. For example, if the bore of the bush is annular the shaft may also be an annular shaft. The shaft having a complementary shape to the bore helps to reduce leakage pathways through the bore. The shaft being rotatably received in the bore encompasses the shaft being able to rotate about the shaft axis. The shaft may be able to move in an axial direction. The movement of the shaft in the axial direction may be restricted. Restriction of movement of the shaft in the axial direction may be achieved by the cap, and / or the shaft may be fixedly attached to other members that cannot move in an axial direction. By way of example, where the shaft is a wastegate shaft, one end of the shaft may be fixed to a wastegate lever arm, and another end of the shaft may be fixed to a wastegate linkage lever (or actuator lever), the linkage lever may be further attached to an actuator rod. The recess may be adjacent the bore. That is to say that the recess is disposed next to the bore. In particular, the recess may be axially adjacent the bore. Put another way, the recess and the bore may together form a continuous passage in which at least part of a shaft is received. That is to say that that recess and the bore may be arranged such that at least part of a shaft can pass through the recess and the bore. The recess may extend around the bore axis. The recess may be an annular recess. The recess extending around the bore axis allows for a shaft the pass through the bore and the recess. The recess may have a variable cross-sectional profile. That is to say that a cross section profile of the recess taken in a plane that is parallel, with, and extends through the bore axis, is not constant. In other words, a perpendicular distance measured from the bore axis to an inner surface of the recess, would not be constant along a length of the recess relative to the bore axis. This is particularly beneficial as it may allow for a section of the recess to have a greater tolerance such that a seal can be easily received in the recess, and then another section to have a smaller tolerance, such that the seal is accurately located. The recess may comprise a seal retaining portion and a seal guiding portion. The term “seal retaining portion” encompasses a portion of the recess, which is configured for a seal to be received and retained in. In use, the seal retaining portion may be at least in part defined by a shaft that extends through the recess. Further, an end of the seal receiving portion may also be partly defined by a wall of the bore that is an interface between the recess and the bore. The seal retaining portion may be sized such that it forms an interference fit with the seal in use. The walls of the sealing retaining portion may cause the seal received in it to be compressed. The term “seal guiding portion” encompasses a portion of the recess that is configured to guide a seal into the seal retaining portion. The seal guiding portion may comprise a tapered or constricting profile. The seal guiding portion may be configured to promote movement of the seal into the seal retaining portion. The seal guiding portion may have a greater diameter than the seal retaining portion, such that the alignment of the seal in the recess initially does not need to be as precise as when in the seal retaining portion. The cap may be at least partly received in the seal guiding portion of the recess. The cap may at least partly bound the seal retaining portion of the recess. That is to say the seal retaining portion, in use, may be at least partly defined by the cap. As already described above, the recess, and hence the seal retaining portion may be at least partly defined by a wall of the bush, and the shaft. The cap bounding at least part of the seal retaining portion promotes retention of the seal in the seal retaining portion. That is to say, the cap may prevent the seal from moving out of the seal retaining portion. The cap may comprise a guiding surface. That is to say that the cap comprises a surface which may be used to guide a seal into the recess. In particular, where the recess comprises a seal guiding portion guiding surface of the cap may have a complementary profile to the seal guiding portion. The cap guiding surface may promote movement of the seal into a sealing retaining portion of the recess. The guiding surface may have a converging profile. The term “converging profile” encompasses the surface being tapered. Only a portion of the guiding surface may be converging. In some embodiments, the entirety of the guiding surface may be converging. The at least part of the cap that is received in the recess may comprise the guiding surface. The portion of the cap that is received closest to an end of the recess that may define a sealing retaining portion may be narrower than a portion of the cap that is further away from the seal retaining portion. By the guiding surface being converging, it, in use, helps guide a seal into the seal retaining portion. That is to say the converging profile of the guiding surface may act as a location or alignment feature. Such that the cap can initially help locate a seal into the recess and then cause movement of the seal into the recess in an axial direction. The cap may comprise a projection, the projection may comprise the guiding surface; and at least part of the projection guiding surface may be complementary to an inner surface of the seal guiding portion of the recess. The term “projection” encompasses an extension or protrusion of the cap. The projection may be the at least part of the cap that is received in the recess. The projection comprising the guiding surface which is complementary to the guiding surface of the seal guiding portion of the recess, encompasses the projection having a complementary profile. This is beneficial in that the cap may function as a location tool or insertion tool to accurately guide the seal into the recess. The cap may be a top hat shim, wherein a “top hat section” of the shim is the protrusion that is received in the recess. At least part of the protrusion may comprise a converging profile. The protrusion may be at least partly received in the seal guiding portion of the recess. An end portion of the protrusion may be partly received in the seal retaining portion of the recess. Part of the cap may comprise a flange, wherein the flange is provided at a base of the protrusion and extends in a direction generally orthogonal to the direction of the protrusion. The flange may be sized such that it cannot be received in the recess. The presence of the flange may reduce leakage through the bore of the bush and / or through an actuator conduit. In addition, the flange may prevent the protrusion from being inserted too far into the recess. The flange may also provide means for a user or a tool to hold / fix to when inserting the protrusion into the recess. The cap may comprise a spigot, and the spigot may be at least partly received in the recess. The term “spigot” encompasses an elongate peg or plug. The spigot may act as a location feature for inserting a seal into the recess. At least part of the spigot may comprise a converging profile. All of the spigot may comprise a converging profile. The spigot may be at least partly received in the seal guiding portion of the recess. An end portion of the spigot may be partly received in the seal retaining portion of the recess. The spigot may form part of a top hat shim. The assembly may further comprise a seal disposed in the recess. The seal may be a mechanical seal. For example, the seal may be an O-ring seal. The seal may be formed from an elastomeric material. The seal being formed from an elastomeric material allows for it to deform and undergo compression as it is inserted into the recess. Accordingly, when received in the recess, the seal may form an interference fit with a surface of the bush that defines the recess. The seal may extend around the shaft. The recess may form a groove for the seal. Such that when the seal is received in the groove, a seal is provided between the seal and the bush, and the seal and the shaft, and unwanted leakage flow through the bore is prevented or reduced. The seal may be formed from a silicon based material. The seal may be received in the seal retaining portion of the recess. The seal may be guided into the recess with the cap. In particular, the seal may be guided into the recess by a guiding surface of the cap. The seal may be a gasket. The seal may be a degradable seal. The term “degradable seal” encompasses a seal that when subjected to an environment, for example, an environment with a temperature above a predetermined value, the seal degrades. When the seal degrades it does not and cannot form a seal between the bush and / or the shaft. In particular, the seal may degrade to a powder. Providing a degradable seal in the recess is particularly useful for end of line leakage testing or a pressure leak down test, which is a test for identifying unintended or undesirable leaks through a turbomachine. A small leakage flow path through the bore of the bush is generally acceptable during operation of the turbomachine and does generally not cause operational problems, however, it is desirable for this leakage flow path to be reduced or minimised during end of line leakage testing. Once the end of line leakage testing has been complete the seal can be “burned off” by subjecting the seal to a known pre-determined temperature such that the seal degrades. The seal may be compressed between the bush and the shaft. That is to say that the seal may be compressed radially relative to the bore axis. The seal being compressed promotes the formation of a good seal between the seal and the bush, and the seal and the shaft, such that leakage flow paths through the bore are reduced. According to a second aspect of the invention there is provided a valve assembly, comprising the assembly of the first aspect of the invention. The valve assembly further comprises a valve head, wherein the valve head is coupled to the shaft. The term “valve head” may be interchangeably referred to as a valve member. The valve member may be used to substantially prevent a fluid from passing through an opening. The shaft may be configured to rotate so as to move the valve head between an open position in which fluid is permitted to pass through the opening, and a closed position in which the valve head substantially prevents fluid from passing through an opening. The valve head may be directly or indirectly coupled to the shaft. That is to say the valve head may be fixed to the shaft. In some embodiments the valve head may be coupled to the shaft via an intermediate connector such as a lever arm. The valve head or lever arm may be in some instances welded to the shaft. The valve assembly may form part of an exhaust throttle valve, a throttle valve, a rotary turbine control valve, a wastegate valve, a variable geometry turbocharger actuator. According to a third aspect of the invention there is provided a wastegate assembly for a turbocharger, comprising the assembly of the first aspect of the invention, wherein the shaft is a wastegate shaft, the assembly further comprises: a wastegate lever arm for supporting a valve head, wherein the wastegate lever arm is coupled to the wastegate shaft. The term “wastegate lever arm” encompasses an intermediate member for coupling the wastegate shaft to the valve head. The lever arm may be fixed to the wastegate shaft such that the lever arm and the wastegate shaft rotate about the shaft axis together. The wastegate lever arm may be fixedly attached to the wastegate shaft. In particular, the wastegate lever arm may be welded to the wastegate shaft. The wastegate lever arm may also be fixedly attached to the valve head. The wastegate lever arm may be welded to the valve head. In some embodiments the wastegate lever arm may be integrally formed with the valve head. The cap may abut the wastegate lever arm. That is to say that an end of the cap which is opposite the to end which is partly received in the recess abuts the wastegate lever arm. The cap abutting against the wastegate lever arm is beneficial as it reduces undesired leakage flow through the bore of the bush and / or through an actuator conduit during use. The cap may comprise a flange or a shim which extends radially outwards of the bush which defines the recess, so as to further prevent leakage flow through the bore of the bush. The wastegate lever arm may comprise the cap. The cap may be integrally formed with the wastegate lever arm. The cap being integrally formed with the wastegate lever arm may reduce assembly time. According to a fourth aspect of the invention, there is provided a kit of parts for a turbomachine, the kit of parts comprising: a bush which defines a bore for receiving at least part of a shaft, the bore defining a bore axis, and wherein an end of the bush comprises a recess for a seal; and a cap, wherein the cap is configured to be at least partly received in the recess for retaining the seal. According to a fifth aspect of the invention, there is provided a method of aligning and retaining a seal in a recess of a bush, the method comprising: placing the seal around a shaft, the shaft defining a shaft axis; urging at least one of the shaft and the bush towards the other, such that the shaft is at least partly received in a bore of the bush, to insert the seal into the recess of the bush using a cap; and retaining the seal in the recess, using the cap, by at least part of the cap being received in the recess. The seal may be aligned in a recess of a bush of a turbomachine, and the turbomachine may be a turbocharger. The term “bush” encompasses a cylindrical tube or sleeve, and may form part of a bearing. The bush may be provided in an actuator conduit and may be provided to reduce friction between the actuator conduit and the shaft that is received in the bore. The term “placing the seal around a shaft” encompasses providing a seal, which may be a mechanical seal, such that it extends around a circumference of the shaft. The seal may be an O-ring seal. Urging at least one of the shaft and the bush towards each other encompasses moving the shaft in an axial direction towards and / or through the bore of the bush. Urging at least one of the shaft and the bush towards each other encompasses fitting the bush around the shaft, such that the bush moves radially inwards towards the shaft axis. The term “cap” encompasses a closing member and / or alignment member. The cap may be for example a top hat shim. Inserting the seal into the recess using the cap reduces assembly time in comparison to trying to inset the seal without the cap. The cap promotes alignment of the seal in the recess, and that the seal is generally coincident about the shaft, i.e. the seal is not at an angle in the recess. Once the seal is received in the recess, the cap also being at least partly received in the recess mitigates against the seal from being urged out of the recess, for example through movement, including rotation, of the shaft. The cap therefore promotes retention of the seal in the recess The method may further comprise inserting the seal into the recess using at least one of a guiding surface of the cap and a seal guiding portion of the recess. The term “seal guiding portion” encompasses a portion of the recess that is configured to guide a seal into a seal retaining portion. The seal guiding portion may comprise a tapered or constricting profile. The seal guiding portion is configured to promote movement of the seal into the seal retaining portion. The seal guiding portion may have a greater diameter than the seal retaining portion, such that the alignment of the seal in the recess initially does not need to be as precise as when in the seal retaining portion. The term “guiding surface of the cap” encompasses a surface which may be used to guide a seal into the recess. The guiding surface of the cap may have a complementary profile to the seal guiding portion. The cap guiding surface may promote movement of the seal into a sealing retaining portion of the recess. Urging at least one of the shaft and the bush towards the other may comprise inserting the shaft into the bore of the bush. Urging at least one of the shaft and the bush towards the other may comprise urging the bush over the shaft. The cap may be placed over the shaft before the seal is placed around the shaft. Inserting the seal into the recess may comprise compressing the seal between at least two of the bush, the shaft and the cap. The seal may be formed from an elastomeric material. The seal being formed from an elastomeric material allows for it to deform and undergo compression as it is inserted into the recess. Accordingly, when received in the recess, the seal may form an interference fit with a surface of the bush that defines the recess. The recess may form a groove for the seal. Such that when the seal is received in the groove, a seal is provided between the seal and the bush and the seal and the shaft, and unwanted leakage flow through the bore is mitigated. When the seal is compressed between the shaft and / or the bush which defines the recess, it is compressed in a radial direction with respect to the shaft axis. When the seal is compressed at least by the cap, it may be compressed in a radial and / or axial direction with respect to the shaft axis. The seal being compressed between at least two of the bush, the shaft and the cap promotes the formation of a good seal between the seal and the bush, and the seal and the shaft, such that leakage flow paths through the bore are reduced. The method may further comprise rotating the shaft in the bore. It will be appreciated that features of the assembly according to the first aspect of the invention may be combined with the second, third and fourth aspects of the invention; and that the method of the fifth aspect of the invention may be applied to the assemblies and kit of parts of the first to fourth aspects of the invention. A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a schematic cross-section through a portion of a known turbocharger; Figure 2 shows a schematic perspective view of a portion of a turbocharger including a turbine according to the present invention; Figure 3 shows a schematic end-on perspective view of a portion of the turbocharger shown in Figure 2; Figure 4 shows a perspective cross-section side view of a wastegate assembly comprising an assembly according to an embodiment the present invention; Figure 5 shows a cross-section side view of a magnified region of interest of the assembly shown in Figure 4; and Figure 6 shows a further cross-section side view of the region of interest of the assembly shown in Figure 4 and Figure 5. Figure 1 shows a schematic cross-section through a known turbocharger. The turbocharger comprises a turbine 1 joined to a compressor 2 via a central bearing housing 3. The turbine 1 comprises a turbine wheel 4 for rotation within a turbine housing 5. Similarly, the compressor 2 comprises a compressor wheel 6 which can rotate within a compressor housing 7. The compressor housing 7 defines a compressor chamber within which the compressor wheel 6 can rotate. The turbine wheel 4 and compressor wheel 6 are mounted on opposite ends of a common turbocharger shaft 8 which extends through the central bearing housing 3. The turbine housing 5 has an exhaust gas inlet volute 9 located annularly around the turbine wheel 4 and an axial exhaust gas outlet 10. The compressor housing 7 has an axial air intake passage 11 and a volute 12 arranged annularly around the compressor chamber. The volute 12 is in gas flow communication with a compressor outlet 25. The turbocharger shaft 8 rotates on journal bearings 13 and 14 housed towards the turbine end and compressor end respectively of the bearing housing 3. The compressor end bearing 14 further includes a thrust bearing 15 which interacts with an oil seal assembly including an oil slinger 16. Oil is supplied to the bearing housing from the oil system of the internal combustion engine via oil inlet 17 and is fed to the bearing assemblies by oil passageways 18. The oil fed to the bearing assemblies may be used to both lubricate the bearing assemblies and to remove heat from the bearing assemblies. In use, the turbine wheel 4 is rotated by the passage of exhaust gas from the exhaust gas inlet 9 to the exhaust gas outlet 10. Exhaust gas is provided to exhaust gas inlet 9 from an exhaust manifold (also referred to as an outlet manifold) of the engine (not shown) to which the turbocharger is attached. The turbine wheel 4 in turn rotates the compressor wheel 6 which thereby draws intake air through the compressor inlet 11 and delivers boost air to an inlet manifold of the engine via the volute 12 and then the outlet 25. The exhaust gas inlet 9 is defined by a portion of the turbine housing 5 which includes a turbocharger mounting flange 27 at the end of the exhaust gas inlet 9 remote from the turbine wheel 4. Figure 2 shows a schematic perspective view of a turbocharger 30, in accordance with the present invention. Figure 3 shows a schematic end-on perspective view of a portion of the turbocharger shown in Figure 2. The turbocharger 30 includes all of the features of the turbocharger described above in relation to Figure 1. The same numbering is used within Figures 2 to 5 for features of the turbocharger 30 shown in Figures 2 to 5 which are equivalent to features shown in the turbocharger of Figure 1. In addition to the features of the turbine shown in Figure 1, the turbine according to an embodiment of the present invention shown in Figures 2 to 5 further includes a wastegate assembly. Turning to Figures 2 to 5, the turbine 32 of the turbocharger 30 includes a turbine housing 5 which defines a turbine inlet 9 upstream of the turbine wheel 4, and a turbine outlet 10 downstream of the turbine wheel 4. Although not visible in Figure 2, the wastegate arrangement includes a wastegate passage which extends between the turbine inlet 9 and the turbine outlet 10. The wastegate arrangement also includes a wastegate valve comprising a movable valve member 36 and a valve seat. The valve seat is a surface of the turbine housing 5 which is configured to be contactable with a surface of the valve member 36 in order to produce a substantially gas-tight seal between the valve seat and the valve member 36. The wastegate valve (and hence valve member 36) has an open state in which gas may pass between the turbine inlet 9 and turbine outlet 10 via the wastegate passage. The wastegate valve (and hence valve member 36) also has a closed state in which the wastegate valve member 36 contacts the valve seat and thereby in which gas is substantially prevented from passing between the turbine inlet 9 and turbine outlet 10 via the wastegate passage. Turning to Figures 4 and 5, Figure 4 shows a perspective cross section side view of a wastegate assembly comprising an assembly according to an embodiment of the present invention. Figure 5 shows a magnified cross section side view of the area 43 indicated in Figure 4. Figure 6 also shows a cross section side view of the area 43 indicated in Figure 4. Figure 6 differs only to the area shown in Figure 5, in that a larger area can be seen, showing the entire cross section of the annular seal 44. The valve member 36, which in this embodiment is of a flap type, is mounted to a shaft 39 having a shaft axis A, the shaft axis A may be interchangeably referred to as a longitudinal axis A, the shaft axis A is generally coincident with an axis of the bore 71. The shaft 39 passes through an actuator conduit 40 of the turbine housing 5. The shaft 39 is movable so as to move the wastegate valve between the open and closed states and, in particular, so as to move the valve member 36 between corresponding open and closed states. The shaft 39 may be moved in any appropriate manner so as to move the valve member 36 of the wastegate valve between the open and closed states. In the embodiment of turbine shown within Figures 2 to 5 this is achieved as follows. The shaft 39 is a generally cylindrical shaft which extends along the shaft axis A. The shaft 39 is connected at a first end to the valve member 36 via a wastegate lever arm 41. In the illustrated embodiment the valve member 36 is coupled to the wastegate lever arm 41 by a rivet 35. However, other attachment means may otherwise be used. The shaft 39 is connected at a second end to a linkage lever 46. The linkage lever 46 is connected to an actuator 56 (only visible in Figure 2). An actuator bracket 47 supports the actuator 56. The linkage lever 46 may be actuated by an actuator 56. The actuator 56 may be a pneumatic actuator; however, any appropriate actuator may be used. The mounting and operation of an actuator (and any associated linkage) in order to move a valve member of a wastegate valve is well-known, and hence further discussion of this is omitted within this description. However, it is worth noting that movement of the linkage lever causes the shaft 39, to pivot (e.g. rotate) about axis A. The pivoting movement of the shaft 39 about the axis A results in the lever arm 41 and hence valve member 36 also pivoting about axis A. Hence the valve member 36 can pivot about axis A between the open state (corresponding to the open state of the wastegate valve) in which the valve member 36 is spaced from the valve seat 38, and the closed state (corresponding to the closed state of the wastegate valve) in which the valve member 36 contacts the valve seat 38. The shaft 39 is located within the actuator conduit 40 as follows. A bush 74 is received by the actuator conduit 40 of the turbine housing 5. The bush 74 is generally annular and the shaft 39 passes through the central opening, which may be referred to as a bore 71 of the bush 74. As described above, the shaft 39 is rotatably received in the bore 71, such that it is rotatable about the axis A, and remains rotatable in the bore 71 of the bush 74. The bush 74 may be formed from any suitable material, but the material may be a metallic material, for example steel or steel alloy. When the bush 74 is used in turbomachines, in particular turbochargers the bush is made from a material which is able to withstand high operating temperatures, for example temperatures of about 100 degrees C to about 1200 degrees C, about 300 degrees C to about 1100 degrees C, and / or about 500 degrees C to about 1000 degrees C. In some embodiments a leakage path may form from the turbine outlet 10 to atmosphere via the actuator conduit 40 such that gas from the turbine outlet 10 may pass through the actuator conduit 40 to the atmosphere. In embodiments in which the shaft 39 is located within the actuator conduit 40 by a bush 74 the leakage path via the actuator conduit may include a leakage path between the shaft 39 and the bush 74 -e.g. between the outer diameter of the shaft 39 and the inner diameter of the bore 71 of the bush 74. In some cases, after the turbine has been assembled, the turbine, or more generally an engine on which the turbine has been installed, may undergo a pass-off test (an initial performance test) in order to evaluate aspects of the performance of the turbine or engine to ensure that they meet specified performance standards. One portion of the pass-off test may be a pressure leak down test, or an end of line leakage test (e.g. a leakage test performed at an end of an assembly line, once assembly is complete). This test is used to check the turbine, or engine, for leaks which may have an adverse effect on the operating performance. If, in a particular turbine or engine, a leakage path exists from the turbine outlet 10, through the actuator conduit 40 and thence to atmosphere, then this may result in the turbine, or engine more generally, failing the end of line leakage test. However, in such a case, although the leakage path via the actuator conduit 40 has caused the turbine to fail the end of line leakage test, the presence of such a leakage path via the actuator conduit 40 is not, generally, a significant fault which will result in unacceptable operating performance of the turbine. The embodiment of the invention shown in Figures 2 to 5, includes a sealing arrangement. An annular seal 44, which in this instance is an O-ring, but may be any suitable type of seal, for example a gasket, is disposed around the shaft 39. The annular seal 44 is made from silicon, but in other embodiments may be made from any suitable material. In particular, the seal may be made from an elastomeric material. The bush 74 comprises a recess 76 which the seal 44 is received in. The recess 76 is provided in an end of the bush 74 that is closest to the wastegate lever arm 41. As already described above, the bore 71 is defined by the bush 74. Likewise, the recess 76 is also defined by the bush 74. In particular, the recess 76 may be readily machined in to the bush 74, using standard tooling and fixtures. The recess 76 extends around the axis A, and hence extends around the bore axis. In the present embodiment the recess 76 is a countersunk bore that is machined in to the end of the bush 74. Accordingly, the recess 76 comprises a first portion 78 which is the larger bore, and a second portion 80 which is the smaller bore. In other embodiments, the recess 76 may simply be a depression, groove, or counter bore, or any other suitable shape. As will be described in further detail, at least one purpose of the recess 76 to retain the seal 44. The first portion 78 of the recess is a seal guiding portion 78, and the second portion 80 is a seal retaining portion 80. As can be best seen in Figure 5 and Figure 6, the seal 44 is located in the seal retaining portion 80. The seal 44, in the seal retaining portion 80 is compressed in a radial direction relative to the axis A. The seal 44 is compressed by a radially inner surface 82 of the bush 74 that defines the seal retaining portion 80 and a radially outermost surface 84 of the shaft 39. The seal 44 is therefore retained in the seal retaining portion 80 of the recess 76. Compression of the seal 44 in the radial direction promotes a robust seal between the seal 44 and the bush 74, and between the seal 44 and the shaft 39. The embodiment further comprises a cap 86. The cap 86 is partly received in the recess 76. The cap 86 comprises a protruding portion 88, which extends in a generally axial direction relative to the axis A. The cap 86 further comprises a flanged portion 90. The flanged portion 90 extends in a generally radial direction relative to the axis A. The flanged portion 90 is not received in the recess 76. Instead, the flanged portion 90 acts as a stop to prevent the cap from entering too far into the recess 76. As such, a first face 91 of the flanged portion 92 abuts an end face 93 of the bush 74. The cap 86 may be referred to as a top hat shim, wherein the flanged portion 90 is a shim and the protruding portion 88 is a “top hat”. The cap 86 also abuts against the wastegate lever arm 41 (seen in Figure 4). In particular, a second face 94 of the cap 86 abuts against the wastegate lever arm 41. The cap 86 abutting against the wastegate lever arm 41 and being at least partly received in the recess 76, during use, mitigates against leakage flow through the bore 71 and / or the actuator conduit 40. The cap 86, further promotes location and retention of the seal 44 in the seal retaining portion 80. However, the cap 86 provides a number of additional benefits in relation to the seal 44. As described above, the seal 44 is disposed around the shaft 39. When assembling the wastegate assembly, at least one of the shaft 39 and the bush 74 are urged towards each other such that the shaft 39 is at least partly received in a bore 71 of the bush 74. In some embodiments, urging at least one of the shaft 39 and the bush 74 towards the other comprises providing the bush 74 in the actuator conduit 40 and then inserting the shaft 39 into the bore 71 of the bush 74. In other embodiments, urging at least one of the shaft 39 and the bush 74 towards the other comprises providing the shaft 39 in the actuator conduit 40 and then urging the bush 74 over the shaft 39. Once the shaft 39 is received in the bore 71 of the bush 74, the cap 86 is used to urge or insert the seal 44 into the recess 76 of the bush 74. During an assembly process, a user who is inserting the seal 44 into the recess 76 is able to see where the cap 46 is being guided into, due to the recess being provided at an end of the bush 74. In some embodiments, the cap 46 may be inserted into the recess 76 at the same time the shaft 39 is inserted into the bore 71 of the bush 74. The protruding portion 88 of the cap 86 is used to push the seal 44 into the seal retaining portion 80 of the recess. The protruding portion 88 of the cap 86 abuts against the seal 44 to prevent the seal 44 from moving out of the recess 76. When the protruding portion 88 abuts against the seal 44, it may cause further compression of the seal 44 in an axial and / or radial direction relative to the axis A. The protruding portion 88 at least partly bounds the seal retaining portion 80 of the recess 76. In other embodiments the protruding portion 88 may be sized such that it is received in the seal guiding portion 78 of the recess 76 but not in the seal retaining portion 80 of the recess 76. As described above, in some embodiments, the wastegate linkage lever 46 is coupled to the shaft 39. In this instance, the shaft 39 may be first urged into the bore 71 of the bush 74 before the seal 44 is placed around the shaft 39. The shaft 39 may then be moved in an axial direction so as to promote movement of the seal 44 into the recess 76. Further, in some embodiments, the cap 86 may be integral with the wastegate lever arm 41. In this instance the seal 44 may be placed around the shaft 39 prior to the shaft 39 and the bush 74 being urged towards one another. The recess 76 is a countersunk recess, such that the seal guiding portion 78 comprises a tapered converging surface 96. The recess 76 therefore has a variable cross sectional profile. The tapered surface 96 is a guiding surface 96. The tapered surface 96 converges towards the seal retaining portion 80. By the surface 96 being tapered aids in locating the seal 44 in the recess 76. This is because the diameter of the tapered surface 96 at an end of the recess 76 furthest from the seal retaining portion 80 is larger than the outermost diameter of the seal 44. Therefore, there is a larger tolerance for providing the seal 44 in the recess 76 in the seal guiding portion 78. The converging tapered surface then promotes movement of the seal 44 towards the seal retaining portion 80. In other embodiments, the recess 76 may be any suitable shape, such that it is able to retain seal and receive at least part of a cap 86. The protruding portion 88 of the cap 86 also promotes guidance of the seal 44 through the seal guiding portion 78 and into the seal retaining portion 80. The protruding portion 88 further comprises a guiding surface 98. The guiding surface 98 of the cap 86 also has a tapered profile. The tapered profile of the guiding surface 98 of the cap 86 is complementary to the tapered profile of the guiding surface 96 of the seal guiding portion 78. The protruding portion 88 of the cap 86 having a complementary profile the guiding surface 96 of the seal guiding portion 78 further aids in providing the seal 44 in the seal retaining portion 80. It will be appreciated that the cap 86 having a complementary profile is not essential, but that the cap 86 should be sized such that it can be at least partly received in the recess 76. It will be appreciated that in other embodiments the cap 86 may simply comprise a spigot that is received in the recess 76. And that any other suitable cap may be used and / or provided. In some embodiments the cap 86 may be integrally formed with the wastegate lever arm 41. When the seal 44 is subjected to a predetermined temperature, for example a temperature in which the turbine normally operates, the seal 44 degrades. The seal 44 may degrade to a powder, such that it cannot form a seal with the bush 74 and with the shaft 39. The seal 44 may combust, melt and / or vaporise, when subjected to a temperature that causes degradation. For example, the initial turbine test may be performed at relatively low temperatures, such as room temperature. The relatively low temperatures of the first environment in which the initial turbine performance test is conducted may be in the range of about 0 degrees C to about 50 degrees C, for example in the range of about 10 degrees C to about 40 degrees C and / or in the range of about 10 degrees C to about 30 degrees C. The second environment in which the turbine normally operates will generally have a temperature which is relatively high compared to the temperature of the first environment. This is because the turbine will generally receive hot exhaust gas from an engine to which the turbine is attached. Examples of the range of temperatures within which the temperature of the second environment may fall include: about 100°C to about 1200 degrees C, about 300 degrees C to about 1100 degrees C, and / or about 500 degrees C to about 1000 degrees C. An example of a material which may be used to form the seal such that the seal degrades when subjected to a predetermined temperature is silicon or a silicon based material. Whilst, in the embodiment of the invention described above, it is a difference in temperature between the first and second environment which causes the sealing arrangement to degrade in the second environment, in other embodiments the difference between the first environment and second environment which causes the sealing arrangement (e.g. sealing ring of the sealing arrangement) to degrade in the second environment may be any appropriate environmental difference. For example, in some embodiments it is envisaged that, if the turbine is configured to be supplied with exhaust gas from an engine, the exhaust gas may include at least one substance which is either not present in air, or is present in air in relatively limited amounts. In this situation, air the first environment includes air, whereas the second environment includes exhaust gas. Exposure of the sealing arrangement to the at least one substance within the exhaust gas during the normal operation of the turbine may lead to chemical decomposition of the sealing arrangement (e.g. the sealing ring of the sealing arrangement). In other words, in the preceding embodiment, the first environment is an environment which includes substantially only air and the second environment includes the exhaust gases produced by the engine to which the turbine is attached. The lack, or relatively low concentration of at least one substance within the air means that the sealing arrangement substantially does not degrade. However, the sealing arrangement is formed of a particular material which, when in the presence of or subjected to a relatively high concentration of said at least one substance within the exhaust gas, the sealing arrangement is caused to chemically degrade by said at least one substance. Following an end of line leakage test the seal 44 will still be present. If a subsequent ‘hot’ test is carried out, the hot test may degrade the seal 44. If a hot test isn’t carried out, the seal 44 will degrade in normal use. Nevertheless, the cap 86 will still be partly received in the recess 76. During normal operation of the turbine, the cap 86 mitigates against leakage flow paths through the bore 71 and / or the actuator conduit 40. It is to be appreciated that numerous modifications to the above-described embodiments may be made without departing from the scope of the invention as defined in the appended claims. The assembly and sealing arrangement has been described in the context of incorporation in a wastegate assembly. However, it will be appreciated that the invention may be used in a wide range of different applications. For example, the assembly may be used in various valve assemblies (e.g. which comprise a valve head coupled to a shaft) for turbomachines (e.g. turbochargers, power turbines). In particular, the assembly can be used in a variety of automotive applications, which in use are subject to high temperatures and where end of line leakage tests may be required. The valve assembly may form part of an exhaust throttle valve, a throttle 5 valve, a rotary turbine control valve, a wastegate valve, or a variable geometry turbocharger actuator.

Claims

1. An assembly for a turbomachine, the assembly comprising:a bush which defines a bore for receiving at least part of a shaft, the bore defining a bore axis, and wherein an end of the bush comprises a recess for a seal; anda cap at least partly received in the recess for retaining the seal.

2. An assembly according to claim 1, further comprising a shaft which defines a shaft axis, and wherein at least part of the shaft is rotatably received in the bore.

3. An assembly according to claim 1 or claim 2, wherein the recess is adjacent the bore.

4. An assembly according to any preceding claim, wherein the recess extends around the bore axis.

5. An assembly according to any preceding claim, wherein the recess has a variable cross-sectional profile.

6. An assembly according to any preceding claim, wherein the recess comprises a seal retaining portion and a seal guiding portion.

7. An assembly according to claim 6, wherein the cap at least partly bounds the seal retaining portion of the recess.

8. An assembly according to any preceding claim, wherein the cap comprises a guiding surface.

9. An assembly according to claim 8, wherein the guiding surface has a converging profile.

10. An assembly according to claim 8 or claim 9, when dependent, directly or indirectly, on claim 6, wherein the cap comprises a projection, the projection comprises the guiding surface; andwherein at least part of the projection guiding surface is complementary to an inner surface of the seal guiding portion of the recess.

11. An assembly according to any preceding claim, wherein the cap comprises a spigot, and the spigot is at least partly received in the recess.

12. An assembly according to any preceding claim, further comprising a seal disposed in the recess.

13. An assembly according to claim 12, wherein the seal is a gasket.

14. An assembly according to claim 12 or claim 13, wherein the seal is adegradable seal.

15. An assembly according to any of claims 12 to 14, when dependent on at least claim 2, wherein the seal is compressed between the bush and the shaft.

16. A valve assembly, comprising the assembly of any preceding claim, when dependent on at least claim 2, the valve assembly further comprising a valve head, wherein the valve head is coupled to the shaft.

17. A valve assembly according to claim 16, wherein the valve assembly forms part of an exhaust throttle valve, a throttle valve, a rotary turbine control valve, a wastegate valve, a variable geometry turbocharger actuator.

18. A wastegate assembly for a turbocharger, comprising the assembly of any claims 2 to 15, when dependent at least on claim 2, wherein the shaft is a wastegate shaft, the assembly further comprises:a wastegate lever arm for supporting a valve head, wherein the wastegate lever arm is coupled to the wastegate shaft.

19. A wastegate assembly according to claim 18, wherein the cap abuts the wastegate lever arm.

20. A wastegate assembly according to claim 18, wherein the wastegate lever arm comprises the cap.

21. A kit of parts for a turbomachine, the kit of parts comprising:a bush which defines a bore for receiving at least part of a shaft, the bore defining a bore axis, and wherein an end of the bush comprises a recess for a seal; anda cap, wherein the cap is configured to be at least partly received in the recess for retaining the seal.

22. A method of aligning and retaining a seal in a recess of a bush, the method comprising:placing the seal around a shaft, the shaft defining a shaft axis;urging at least one of the shaft and the bush towards the other, such that the shaft is at least partly received in a bore of the bush, to insert the seal into the recess of the bush using a cap; andretaining the seal in the recess, using the cap, by at least part of the cap being received in the recess.

23. A method according to claim 22, further comprising inserting the seal into the recess using at least one of a guiding surface of the cap and a seal guiding portion of the recess.

24. A method according to claims 22 or 23, wherein urging at least one of the shaft and the bush towards the other comprises inserting the shaft into the bore of the bush.

25. A method according to claims 22 or 23, wherein urging at least one of the shaft and the bush towards the other comprises urging the bush over the shaft.

26. The method according to any one of claims 22 to 25, wherein the cap is placed over the shaft before the seal is placed around the shaft.

27. The method according to any one of claims 22 to 26, wherein inserting the seal into the recess comprises compressing the seal between at least two of the bush, the shaft and the cap.

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