Balancing procedure for a turbocharger

Abstract

A method for balancing a rotating assembly (50) of a turbocharger (1), the rotating assembly comprising a shaft (6), a turbine wheel (4) connected to one end of the shaft (6), a compressor wheel (5) connected to the other end of the shaft (6), and a bearing (20) supporting the shaft (6) within a bore (7) in a bearing housing (8) of the turbocharger (1), the method comprising the following steps: Performing a low speed balancing test of the turbocharger rotating assembly (50), comprising measuring low speed vibrations while the turbocharger rotating assembly (50) is positioned within the bore (7); based on the results of the low speed balance test, performing a first rebalancing of the rotating assembly (50) of the turbocharger by removing material from the turbine wheel (4) of the shaft / turbine wheel assembly (40) while the shaft / turbine wheel assembly (40) is installed in the bearing (20) and the bearing (20) is installed within the bore (7); Carrying out a balance test of the rotating assembly (50) of the turbocharger at high speed, comprising measuring the vibrations at high speed while the rotating assembly (50) of the turbocharger is positioned within the bore (7); and based on the results of the high speed balance test, performing a second rebalancing of the turbocharger rotating assembly (50) by removing material from one of the compressor wheel (5) and the compressor wheel nut (21) while the turbocharger rotating assembly (50) is installed within the bearing housing (8).

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 61 / 955,896, filed on March 20, 2014, entitled "Balancing Method for a Turbocharger," which is incorporated herein by reference. BACKGROUND 1. FIELD OF THE INVENTION

[0002] The present invention relates to a method for balancing a shaft / wheel assembly of a turbocharger. 2. Description of the state of the art

[0003] Turbochargers are installed on an engine to deliver air to the engine intake at a higher density than would be possible with a normal intake configuration. This allows more fuel to be burned, thereby increasing an engine's horsepower without increasing engine weight.

[0004] Generally, turbochargers utilize the exhaust flow from the engine exhaust manifold, which enters the turbine housing at a turbine inlet, to drive a turbine wheel located within the turbine housing. The turbine wheel is fixed to one end of a shaft, which drives a compressor wheel mounted on the other end of the shaft. Thus, the turbine wheel provides rotating power to drive the compressor wheel and, in turn, the turbocharger's compressor. This compressed air is then delivered to the engine intake as described above.

[0005] The compressor stage of the turbocharger comprises the compressor wheel and its associated compressor housing. Filtered air is drawn axially into a compressor air inlet, which defines a passage extending axially to the compressor wheel. The rotation of the compressor wheel forces the pressurized airflow radially outward away from the compressor wheel into the compressor volute, where it is further pressurized and flows to the engine.

[0006] The efficiency of turbochargers has been increased by the use of rolling element bearings (REBs) to support the rotating assembly comprising the turbine wheel, compressor wheel, and the shaft connecting them. For example, the REB system results in an improvement in the transient response of the turbocharger due to the reduction in power losses, particularly at low turbocharger speeds, compared to some typical turbocharger bearing systems, such as sleeve-type hydrodynamic bearing systems. REB systems can also support much larger axial shock loads than typical turbocharger bearing systems, making the axial component more robust. However, REBs have a limited fatigue life that is sensitive to the loads they transmit. The bearing loads depend on the degree of imbalance of the wheels, so the fatigue life of the REB is sensitive to the imbalance of the turbine and compressor wheels.Therefore, there is a need to keep the imbalance levels of the turbine and compressor wheels as low as possible during turbocharger operation.

[0007] DE 10 2009 021 725 A1 discloses a balancing device for balancing a turbocharger, wherein the balancing device has at least one laser device for removing material by means of a laser in order to at least reduce or substantially eliminate an imbalance of the turbocharger. DE 11 2013 002 879 T5 discloses a turbine wheel for a turbocharger. The turbine wheel has a hub and extends in an axial direction between a nose and a rear wall. The hub defines an axis of rotation, and the rear wall has a circumferential edge. At least one arc is arranged in the circumferential edge of the rear wall for balancing the turbine wheel. The at least one arc is arranged along the circumferential edge such that the circumferential edge is not symmetrical in the circumferential direction about the axis of rotation. SUMMARY

[0008] The invention relates to a method according to claim 1. In some aspects, a method for balancing a rotating assembly of a turbocharger may comprise performing a first balance of a shaft / turbine assembly to provide a first-time balanced shaft / turbine assembly; inserting the first-time balanced shaft / turbine assembly into a bearing, the bearing being located in a bore of a bearing housing of the turbocharger; securing a compressor wheel to the balanced shaft / turbine assembly to form the rotating assembly of the turbocharger; performing a low-speed balance test of the rotating assembly of the turbocharger, comprising measuring low-speed vibrations while the rotating assembly of the turbocharger is positioned within the bore;Comparing the measured low-speed vibrations with a first predetermined acceptable vibration level; based on the results of comparing the measured low-speed vibrations with a first predetermined acceptable vibration level, determining whether rebalancing of the turbocharger rotating assembly is necessary; if the measured low-speed vibrations exceed the predetermined level, performing rebalancing of the turbocharger rotating assembly by removing material from a turbine wheel of the shaft / turbine wheel assembly while the shaft / turbine wheel assembly is installed in the bearing and the bearing is installed within the bore; performing a high-speed balance test of the turbocharger rotating assembly, comprising measuring the high-speed vibrations while the turbocharger rotating assembly is positioned within the bore;Comparing the measured high-speed vibrations with a second predetermined acceptable vibration level; based on the results of comparing the measured high-speed vibrations with a second predetermined acceptable vibration level, determining whether rebalancing of the turbocharger rotating assembly is necessary; and if the measured high-speed vibrations exceed the second predetermined level, performing rebalancing of the turbocharger rotating assembly by removing material from the compressor wheel while the turbocharger rotating assembly is installed in the bearing housing.

[0009] The method may also comprise one or more of the following steps and / or features: The step of removing material from the turbine wheel comprises removing material from a peripheral edge of a back wall of the turbine wheel. The step of removing material from a peripheral edge of a back wall of the turbine wheel comprises removing material between a pair of adjacent turbine blades such that the peripheral edge is not symmetrical in the circumferential direction about a rotational axis of the turbine wheel. The step of removing material from a peripheral edge of a back wall of the turbine wheel comprises the step of machining at least one hollow cut that is elongated in the circumferential direction of the turbine wheel. The step of removing material from a peripheral edge of a back wall of the turbine wheel comprises the step of machining at least one hollow cut that is substantially semicircular.The step of removing material from the turbine wheel includes advancing a cutting tool toward a rear wall of the turbine wheel, approaching from a nose side of the turbine wheel. Checking the balance of the unbalanced shaft / turbine wheel assembly includes: . Causing rotation of the shaft; and measuring vibrations of at least one of the shaft and the turbine wheel. Installing a shaft / turbine assembly in a bearing within a bearing housing of a turbocharger comprises inserting a free end of the shaft of the unbalanced shaft / turbine assembly into the bore on a turbine side of the bearing housing until the free end protrudes outwardly from the bore on a compressor side of the bearing housing; and mounting the bearing to the shaft of the unbalanced shaft / turbine assembly by inserting the bearing into the bore on the compressor side of the bearing housing while performing the step of inserting the free end of the shaft until the bearing abuts against the portion of the bearing housing and such that the bearing is disposed between the shaft and the bore. The bearing is a rolling element bearing insert.

[0010] In some aspects, a method is disclosed for balancing a rotating assembly of a turbocharger, the rotating assembly comprising a shaft, a turbine wheel connected to one end of the shaft, a compressor wheel connected to the other end of the shaft, and a bearing assembly supporting the shaft within a bore of the turbocharger, the method comprising the steps of: performing a low speed balance test of the rotating assembly of the turbocharger, comprising measuring vibrations at low speed while the rotating assembly of the turbocharger is positioned within the bore;and based on the results of the low speed balance test, performing a first rebalance of the turbocharger rotating assembly by removing material from a turbine wheel of the shaft / turbine wheel assembly while the shaft / turbine wheel assembly is installed in the bearing and the bearing is installed within the bore;

[0011] The method further comprises performing a high-speed balance test of the rotating assembly of the turbocharger, comprising measuring the high-speed vibrations while the rotating assembly of the turbocharger is positioned within the bore; and based on the results of the high-speed balance test, performing a second rebalance of the rotating assembly of the turbocharger by removing material from the compressor wheel of the shaft / turbine wheel assembly while the rotating assembly of the turbocharger is installed within the bearing housing. The method may also comprise one or more of the following steps and / or features: The step of removing material from the turbine wheel comprises removing material from a peripheral edge of a backwall of the turbine wheel.The step of removing material from a peripheral edge of a back wall of the turbine wheel comprises removing material between a pair of adjacent turbine blades such that the peripheral edge is not symmetrical in the circumferential direction about a rotational axis of the turbine wheel. The step of removing material from the turbine wheel comprises advancing a cutting tool toward a back wall of the turbine wheel, approaching from a nose side of the turbine wheel.

[0012] A method for balancing a rotating assembly of an exhaust gas turbocharger is also provided. The rotating assembly includes a shaft / turbine wheel assembly installed within a bearing insert in the bearing housing of the turbocharger, and the compressor wheel connected to the shaft of the assembly. The method reduces the imbalance on the turbine wheel after assembly with the ball bearing insert and the compressor wheel, but before balancing the core at high speed. The method includes checking and correcting the balance of the rotating assembly while the rotating assembly is installed within the bearing housing.

[0013] This method allows balancing without additional installation and removal steps and reduces the likelihood of contamination of the ball bearing insert with abrasive debris, which can occur in some conventional methods where balancing of the assembly is performed outside the bearing housing. Advantageously, the method does not require a special fixture to balance the assembly, and furthermore, it does not require a design change to the central bearing housing by expanding the seal ring diameter on the turbine side. By avoiding an expanded seal ring diameter in the central bearing housing, increased oil leaks from the bearing housing can be avoided. Furthermore, increased ingress of exhaust gas into the bearing housing can be avoided by an enlarged turbine-side seal ring, thereby preventing the ingress of "blow-by exhaust" into the engine crankcase.

[0014] In some aspects, a turbine wheel for a turbocharger includes a hub extending in an axial direction between a nose and a backwall. The backwall includes a circumferential rim, and the hub defines an axis of rotation extending in the axial direction. A plurality of turbine blades are coupled to the hub, and the turbine blades are arranged in a circumferential direction substantially equally spaced about the axis of rotation. At least one scoop cut is formed in the circumferential rim of the backwall to balance the turbine wheel by approaching a cutting tool from the nose side of the turbine wheel to the backwall while disposing the shaft / turbine wheel assembly within the bearing insert within the bore of the center bearing housing. In some embodiments, the scoop cut is elongated in the circumferential direction such that material is removed from the backwall over a predetermined angle.In other embodiments, the scoop cut is substantially semicircular, so that material is removed from the back wall with a single feed of a cutting tool. The scoop cut is positioned along the peripheral edge such that the peripheral edge is not symmetrical in the circumferential direction about the rotational axis. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Advantages of the present invention will become apparent when the same is understood by reference to the following detailed description taken in conjunction with the accompanying drawings in which: Fig. 1 is a schematic view of an engine with an exhaust gas turbocharger; Fig. 2 is a side sectional view of a turbocharger bearing housing with a rotating assembly of the turbocharger disposed in an axial bore thereof; Fig. 3 is a side sectional view of a shaft / turbine assembly; Fig. 4 is a side sectional view of the bearing insert; Fig. 5A and Fig. 5B shows a flow chart showing the steps of the method for balancing the rotating assembly of the turbocharger, wherein block “A” represents the connection between the portion of the flow chart in Fig. 5A with the section of the flowchart in Fig. 5B indicates; Fig. 6 is a side sectional view of the bearing housing during the step of assembling the shaft / turbine assembly with the bearing insert within the bore of the bearing housing; Fig. 7 is a side view of the turbine wheel illustrating the step of advancing a cutting tool toward the back wall from a nose end of the turbine wheel, with the arrow indicating the direction of advance. Fig. 8 is a front view of the turbine wheel and illustrates a hollow cut formed in the peripheral edge between a pair of adjacent blades. Fig. 9 is a rear view of the turbine wheel of Fig. 8 and illustrates the hollow cut formed in the peripheral edge. Fig. Figure 10 is a front view of the turbine wheel illustrating an alternative hollow cut formed in the peripheral edge between a pair of adjacent blades. Fig. 11 is a rear view of the turbine wheel of Fig. 10 and illustrates the alternative hollow cut formed in the peripheral edge. Fig. 12 is a rear view of the turbine wheel of Fig. 10 and illustrates another alternative hollow cut formed in the peripheral edge. Detailed description

[0016] With reference to Fig. 1 and Fig. 2, an exhaust gas turbocharger 1 comprises a turbine section 2, the compressor section 3, and a central bearing housing 8, which is arranged between the compressor section 3 and the turbine section 2 and connects them to one another. The turbine section 2 comprises a turbine housing 11 defining an exhaust inlet 13, an exhaust outlet 10, and a turbine volute 9 arranged in the fluid path between the exhaust inlet 13 and the exhaust outlet 10. A turbine wheel 4 is arranged in the turbine housing 11 between the turbine volute 9 and the exhaust outlet 10.

[0017] The compressor section 3 comprises a compressor housing 12 defining the air inlet 16, an air outlet 18, and a compressor scroll 14. A compressor wheel 5 is arranged in the compressor housing 12 between the air inlet 16 and the compressor scroll 14. The compressor wheel is connected to a shaft 6. The shaft 6 connects the turbine wheel 4 to the compressor wheel 5. The shaft 6 is supported within an axial bore 7 in the bearing housing 8 via a rolling bearing insert 20, as explained further below.

[0018] During use, the turbine wheel 4 in the turbine housing is rotatably driven by an incoming exhaust gas flow from the exhaust manifold 15a of an engine 15. Since the drive shaft 6 connects the turbine wheel 4 to the compressor wheel 5 in the compressor housing 12, the rotation of the turbine wheel 4 causes the compressor wheel 5 to rotate. As the compressor wheel 5 rotates, it increases the rate of air mass flow, air density, and air pressure delivered to the engine cylinders by an outgoing air flow from a compressor air outlet 18 connected to the exhaust manifold 15b of the engine.

[0019] With reference to Fig. 3, the turbine wheel 4 includes a hub 30 extending in an axial direction between a nose 31 on a front side of the turbine wheel 4 and a weld boss 32 on a rear side of the turbine wheel 4. The hub 30 defines a hub line 33 extending in the axial direction from a point generally adjacent the nose 31 and then diverging outwardly in a radial direction toward a circumferential edge 35 of a back wall 34. The circumferential edge 35 of the back wall 34 coincides with an inlet tip 39 of a plurality of turbine blades 36, thereby defining a "full back" turbine wheel 4. The turbine blades 36 are arranged at substantially equal intervals in a circumferential direction around a rotational axis 38 of the turbine wheel. A proximal end 6a of the shaft 6 is fixed to the welding boss 32 of the turbine wheel 4 to form a shaft / turbine wheel assembly 40.The type of connection between the turbine wheel 4 and the shaft 6 is determined, at least in part, by the material used to form the turbine wheel 4. For example, a turbine wheel 4 formed from a nickel-based superalloy (i.e., an Inconel™) may be friction welded to the proximal shaft end 6a, whereas a turbine wheel 4 formed from titanium aluminide may not be welded but is instead secured to the proximal shaft end 6a by brazing.

[0020] Manufacturing the shaft / turbine assembly 40 may, for example, include the following steps: A turbine wheel casting may be held in a chuck to drill a central hole into the boss 31 on a front side of the turbine wheel casting. The shaft 6 is then welded to the weld boss 32 on a rear side of the turbine wheel casting. After heat treating the weld, the shaft / turbine assembly is machined, which includes finishing a plurality of the turbine blades 32. The distal shaft end 6b is then threaded, resulting in a shaft / turbine assembly 40.

[0021] With reference to Fig. 2 and Fig. 4, the shaft / turbine assembly 40 is supported within the bore 7 via the rolling bearing insert 20. The bearing insert 20 includes an inner race 24, two bearing sets 23, 23', and an outer race 22. The bearing insert 20 is prevented from rotating within the bore 7 relative to the bearing housing 8 by an anti-rotation ring 28 interposed between the bearing insert 20 and the bearing housing 8, as described in co-pending US patent applications US 2012 / 0 045 326 A1, the contents of which are incorporated herein by reference. The US 2013 / 0 202 432 A1 anti-rotation ring 28 includes at least one anti-rotation feature for engagement with the bearing housing 8 and at least one anti-rotation feature for engagement with the bearing insert 20 to prevent rotation of the outer race 22 relative to the housing 8.

[0022] When the shaft / turbine wheel assembly 40 is installed within the bearing insert 20 in the bearing housing bore 7 and connected to the compressor wheel 5, it provides the rotating assembly 50 of the turbocharger 1.

[0023] In some embodiments, the turbine wheel 4 is cast from a nickel-based superalloy containing over seventy percent (70%) nickel by weight. As a result, the turbine wheel 4 represents approximately five percent (5%) of the weight of the entire turbocharger 1. Because the relatively heavy turbine wheel 4 is subject to rotational speeds typically ranging from 80,000 rpm to 300,000 rpm, balancing the shaft / turbine wheel assembly 40 is critical to both the performance and life cycle of the bearing insert 20.

[0024] With reference to Fig. 5A and Fig. 5B, a method for balancing the rotating assembly of the turbocharger 50 is described that reduces the degree of imbalance of the compressor and turbine wheels 4, 5 compared to some conventional approaches, thereby increasing the service life of the bearing insert 20. The method for balancing the rotating assembly 50 comprises the following steps: A first balancing of the shaft / turbine wheel assembly 40 is performed in isolation from the other turbocharger components to eliminate any imbalance of the cast turbine wheel 4 or the assembly 40 as a whole (step 101). During the first balancing step, the shaft 6 is held in a fixture, and the assembly 40 is rotated. For example, an air stream may be directed at the turbine wheel 4 to achieve rotation of the assembly 40. The vibration of the shaft 6 is measured to determine the degree of imbalance and identify corrections. The imbalance is corrected by selectively removing material from the back plate 34 and / or nose 31 of the turbine wheel 4 (step 102).

[0025] With reference to Fig. 6, the shaft / turbine assembly 40, once sufficiently initially balanced, is installed in the bearing housing 8 (step 103). As used herein, the term "initially balanced shaft / turbine assembly" refers to the shaft / turbine assembly 40 in its assembled and initially balanced state prior to installation in the turbocharger 1. To achieve installation in the bore 7, the free end 6a of the shaft 6 is inserted into the turbine-side opening of the bore 7 until the free end 6a protrudes outwardly from the bore 7 on a compressor side of the bearing housing 8 and the back wall 34 of the turbine wheel 4 is adjacent to an axial end surface of the bearing housing 8 with only minimal clearance.Furthermore, the rolling bearing insert 20 is mounted on the shaft 6 by inserting the bearing insert 20 into the bore 7 on the compressor side of the bearing housing 8, while inserting the free shaft end 6a from the opposite end of the bore 7 until the bearing insert 20 abuts against a shoulder on the shaft of the shaft / turbine wheel assembly 40, and such that the bearing insert 20 is arranged between the shaft 6 and the bore 7 (. Fig. 6). The insert 20 is held by the ring 28 (in Fig. 2), which is arranged between the insert 20 and the bore 7, is prevented from rotating relative to the bore 7.

[0026] Next, the compressor wheel 5 is assembled with the initially balanced shaft / turbine wheel assembly 40, which remains supported within the bore 7 by the bearing insert 20. The compressor wheel 5 is secured to the free shaft end 6b by the nut 21 (e.g., a clamp nut), as shown in Fig. 2 (step 104). The resulting rotating assembly of the turbocharger 50, which includes the compressor wheel 5, the bearing insert 20, and the shaft / turbine wheel assembly 40, is placed in the bore 7.

[0027] However, when the nut 21 is tightened, the shaft 6 is displaced and, as the components are clamped together, is slightly bent, resulting in increased unbalance at the compressor wheel end of the rotor assembly 50. At this point, in some conventional balancing methods, unbalance corrections are made only at the compressor wheel end, for example, by removing material from the compressor wheel 5. However, geometric variations in the inner race 24 of the ball bearing insert 20 can offset the mass of the shaft 6 and the turbine wheel 4 from the rotational axis of the bearing insert 20, resulting in an unbalance much greater than the unbalance limit of the shaft / turbine wheel assembly 40 prior to installation in the bearing housing 8. In some cases, the resulting unbalance cannot be adequately corrected by unbalance corrections at the compressor side of the rotor assembly 50.

[0028] In addition, clearances may exist within the bearing and between the bearing insert 20 and the bore 7, which may prevent balancing of the rotating elements of the turbocharger 1.

[0029] For example, with reference to Fig. 2 and Fig. 4 an internal radial clearance Δr internal between each set of bearings 23, 23' and the outer surface of the inner race 24 (e.g. at r IR,O ) and the inner surface of the outer race 22 (e.g. at r OR,i ). These gaps allow the inner race 24 to move slightly off-axis and tilt with respect to the outer race 22. In Fig. 4, the broken axial line represents the axis of the outer race 22 (z OR ), the dotted axial line represents the axis of rotation of the inner race 24 (z IR ) and the angle θ internalis formed between these two axes, and it may vary over time. Furthermore, the inner race 24 may shift relative to the outer race 22, as indicated by a thick double-headed arrow, and the axial distance Δz internal may vary over time (e.g., measured as a difference between an axial center of the outer race 22 and an axial center of the inner race 24). Since the radius of the inner surface of the outer race 22 (r OR,i ) increases towards the ends of the bearing insert 230, the displacement of the inner race 24 with respect to the outer race 22 can reduce the internal clearances (Δr internal ), as well as changes in the angle of inclination (θ internal ).

[0030] Furthermore, the outer race 22 can move in the bore 7 of the bearing housing 8 because it floats on a film of lubricant. Such movement can include a displacement from the axis and / or a tilt, wherein the tilt has an angle θ damper between the axis of the outer race 22 (z OR ) and the axis of the bore of the bearing housing 8 (z B ). Parameters such as θ damper , Δz damper , Δr damper , which may vary with respect to time (where, for example, Δz damper a difference between an axial center of the outer race 22 and an axial center of a housing bore, and where Δr dampera difference between an outer diameter of the outer race 22 and an inner diameter of a housing bore). The air drive of a shaft / turbine assembly 40, in a conventional balancing process, typically results in movement of the inner race 24 with respect to the outer race 22 and / or the outer race 22 with respect to the bore of the bearing housing 8. Thus, when a bearing insert (e.g., bearing insert 20) is positioned in a housing, several angles (e.g., θ damper , θ internal ), margins (Δr damper , Δr internal ) and axial displacements (Δz damper , Δz internal ) exist that can prevent balancing.

[0031] Once the initially balanced shaft / turbine assembly 40 is installed in the bore 7, with the bearing insert 20 disposed between the shaft 6 and the bore 7, and the compressor wheel 5 connected to the free shaft end 6b, a low-speed balance check of the rotating assembly 50 is performed (step 105). The low-speed balance check involves causing the shaft 6 to rotate at a low speed within the bore 7 while supported by the bearing insert 20. This is achieved, for example, by directing an airflow toward the turbine wheel 4 or by connecting the assembly 40 to an output shaft of a high-speed engine via a belt. As used herein, the term "low speed" refers to speeds equal to or lower than the first bending mode of the rotating assembly 50.This depends on the size and geometry of the rotating assembly 50. For example, in some cases, a low speed may refer to speeds less than or equal to 5000 rpm, while in other cases, a low speed may refer to speeds less than or equal to 20,000 rpm.

[0032] Checking the balance further includes measuring the speed and vibration of the rotating assembly 50 while rotating the shaft 6. The vibration measurements may be made using an accelerometer and are taken in one or more planes along the rotational axis R of the turbocharger 1. For example, measurements may be taken in a first plane transverse to the rotational axis R, including the peripheral edge 35, and in a second plane transverse to the rotational axis R, including the nose 31 of the turbine wheel 31. A conventional speed sensor is used to obtain measurements of the speed of rotation of the shaft 6 and / or the turbine wheel 4.

[0033] To eliminate the above-described anti-balance angles, clearances, and axial displacements associated with the bearing insert 20, the shaft / turbine assembly 40 and the bearing insert 20 are preloaded within the bore 7 during the balance check using a combination of techniques. These techniques may include, but are not limited to, one or more of the following: using the anti-rotation ring 28 alone, or in conjunction with a dampening ring (not shown), to axially position the bearing insert 20, dampen its axial movement, and absorb axial shocks; applying a radial preload to the bearing insert 20 via a device inserted through the oil drain 26 of the bearing housing; and applying a bending moment to the shaft via a drive belt connected to the nose 31 of the turbine wheel.

[0034] The measured vibrations are compared with a first predetermined acceptable level of unbalance (step 106). This value depends on the type and size of the turbocharger 1, as well as the type and size of the turbine wheel 5 and the shaft 6, and was determined by testing. Based on the results of the comparison, it is determined whether an unbalance correction is necessary (step 107).

[0035] If, with reference to Fig. 7, the measured vibrations exceed the first predetermined level, the unbalance correction is performed by removing material from the turbine wheel 4 while the shaft / turbine wheel assembly 40 is installed within the bearing housing 8 (step 108). In particular, material is removed from a peripheral edge 35 of the rear wall 34 of the turbine wheel. It should be understood that the material is removed from a portion of the peripheral edge 35, such as between a pair of adjacent turbine blades 36a, 36b, such that the peripheral edge 35 is not symmetrical in the circumferential direction about the rotational axis 38 of the turbine wheel 4. Removing material from the peripheral edge 35 is advantageous because the peripheral edge 35 is accessible from the nose end of the turbine wheel and is a portion of the turbine wheel 4 that is not in the flow path of air through the turbine wheel 4 and / or only minimally affects it when in operation in the turbocharger 1.The removal of material is achieved by advancing a cutting tool 60 toward the turbine wheel back wall 34 by approaching the back wall 34 from a nose side of the turbine wheel 4.

[0036] With reference to Fig. 8 and Fig. 9, in some embodiments, the step of removing material from a peripheral edge 35 of the turbine wheel back wall 34 includes using the cutting tool 60 to machine at least one hollow cut 42 into the peripheral edge 35 that is generally semicircular in shape. This may be accomplished by using a rotating cutting tool with a circular cross-sectional shape and performing a single advance of the cutting tool 60 through the peripheral edge 35 at a position between a pair of adjacent turbine wheel blades 36a, 36b.

[0037] With reference to Fig. 10 and Fig. 11, in some embodiments, the step of removing material from a peripheral edge 35 of the turbine wheel back wall 34 comprises using the cutting tool 60 to machine at least one hollow cut 44 into the peripheral edge 35 that is slightly elongated in the circumferential direction of the turbine wheel 4. This may be accomplished by using a rotating cutting tool with a circular cross-sectional shape and performing multiple overlapping advances of the cutting tool 60 through the peripheral edge 35 at a position between a pair of adjacent turbine wheel blades 36a, 36b. Alternatively, this may be accomplished by performing a single advance of the cutting tool 60 through the peripheral edge 35 at a position between a pair of adjacent turbine wheel blades 36a, 36b and then moving the advanced cutting tool 60 along the circumferential direction.

[0038] The scoop cuts 42, 44 are shallow, so their radial dimension is small relative to their circumferential dimension. For example, the depth of each scoop cut 42, 44 (e.g., the maximum dimension of the scalloped edge 42, 44 in the radial direction) is in a range of approximately 1 mm to 2 mm. The circumferential length of the scoop cut 42, 44 depends on the spacing between adjacent turbine blades 36, which in turn depends on the wheel diameter and the number of blades 36.

[0039] Although Fig. While Figures 7-11 illustrate the turbine wheel 4 with a single scoop cut 42, 44, it is contemplated that multiple scoop cuts may be employed, with a single scoop cut being provided between each pair of adjacent blades. In other embodiments, multiple scoop cuts may be provided between each adjacent blade pair, as long as sufficient spacing remains between the scoop cuts and the blade fillet to avoid increased blade loading.

[0040] With reference to Fig. 12, in some embodiments, the step of removing material from a peripheral edge 35 of the turbine wheel back wall 34 includes using the cutting tool 60 applied from the nose side of the turbine wheel 4 to machine at least one scoop cut 46 into the peripheral edge 35 that is substantially elongated in the circumferential direction of the turbine wheel 4 so as to begin and end between adjacent blades 36a, 36b and extend across at least one blade 36. This may be accomplished by using a rotating cutting tool with a circular cross-sectional shape and making multiple overlapping passes of the cutting tool 60 through the peripheral edge 35 along an arc A that extends across one or more blades 36.Alternatively, this can be accomplished by performing a single advance of the cutting tool 60 through the peripheral edge 35 at a position between a pair of adjacent turbine wheel blades 36a, 36b and then moving the advanced cutting tool 60 along an arc extending across one or more blades 36. In particular, the start and end of the scoop cut 46 should be spaced apart from the nearest blade 36. For example, depending on the size and configuration of the turbine wheel 4 and the cutting blade 36, the start and end of the scoop cut 46 should be spaced apart by at least 2 mm from the nearest blade 36. Furthermore, the scoop cut 46 does not have any sharp curves and does not overlap with other scoop cuts 46.The circumferential length and depth d of the hollow cut 46 are calculated based on the required amount of unbalance correction, and the hollow cut 46 may be formed as a single elongated hollow cut (as shown in FIG. Fig. 12) or divided into two or more shorter excavation cuts 46. The maximum length of arc A or the sum of several shorter cuts corresponds to an angle of 180 degrees. Furthermore, the depth d of the excavation cut 46 is uniform along arc A.

[0041] Once material has been removed from the peripheral edge 35 of the turbine wheel back wall 34, the balance test of the rotating assembly 50 is repeated at low speed. If necessary, additional material is removed from the peripheral edge 35 of the turbine wheel, and the test is repeated until sufficient balance (e.g., a vibration level less than the first predetermined level) of the rotating assembly 50 is achieved.

[0042] Once sufficient low-speed balancing of the rotating assembly 50 is achieved, a high-speed balancing test of the turbocharger rotating assembly 50 is performed (step 109). The high-speed balancing test involves causing the shaft 6 to rotate at high speed within the bore 7 while supported by the bearing insert 20. This is achieved, for example, by directing an airflow toward the turbine wheel 4 or by connecting the turbocharger rotating assembly 50 to an output shaft of a high-speed engine via a belt. As used herein, the term "high speed" refers to speeds of 60,000 rpm to 150,000 rpm or more.

[0043] The high-speed balance test further includes measuring the speed and vibration of the rotating assembly 50 while rotating the shaft 6. As before, the vibration measurements may be performed using an accelerometer and are taken in one or more planes along the rotational axis R of the turbocharger 1. A conventional speed sensor is used to obtain measurements of the speed of rotation of the shaft 6 and / or the compressor wheel 5.

[0044] The measured vibrations are compared with a second predetermined acceptable level of unbalance (step 110). This value depends on the type and size of the turbocharger 1, as well as the type and size of the compressor wheel 5, and was determined by testing. Based on the results of the comparison, it is determined whether an unbalance correction is necessary (step 111).

[0045] If the measured vibrations exceed the second predetermined level, the unbalance correction is performed by removing material from the compressor wheel 5 while the rotating assembly of the turbocharger 50 is installed within the bearing housing 8 (step 112). In particular, material is removed from the nut 21 or a hub line of the compressor wheel 5.

[0046] If material has been removed from the nut 21 or a hub line of the compressor wheel 5, the balance check is repeated. If necessary, additional material is removed, and the check is repeated until sufficient balance (e.g., a vibration level lower than the second predetermined level) of the turbocharger's rotating assembly is achieved.

[0047] The method described herein, including steps 101-112, is advantageous compared to some conventional balancing methods that involve performing a low-speed balancing of the shaft / turbine assembly 40 outside the bearing housing 8, followed by inserting a bearing insert 20 from the compressor side of the bearing housing 8 while inserting the shaft of the shaft / turbine assembly 40 from the turbine side of the housing. In this conventional approach, once the shaft / turbine assembly is installed in the bore 7, it is difficult to eliminate any effects of the bearing insert 20 on the balancing of the assembly from the turbine side.Other balancing methods have been proposed, which include the use of a special clamp to support a unit comprising the shaft / turbine assembly 40 together with the bearing insert 20, wherein the unit is balanced outside the turbocharger 1, and then the unit is inserted through an enlarged diameter seal ring opening at the turbine end of the bearing housing 8. Although this conventional method includes balancing the shaft / turbine assembly 40 together with the bearing insert 20, it requires the special clamp to hold the unit during balancing and increases the risk of damage to the bearing insert 20, for example, by allowing abrasive debris to enter the bearing housing when the unit is removed from the fixture and installed in the bearing housing 8.Furthermore, it requires that the diameter of the turbine-side bearing housing seal be larger than the outer diameter of the bearing insert so that the unit can be installed through the turbine side of the bearing housing 8, compared to a conventional housing in which the diameter of the turbine-side seal is smaller than the outer diameter of the bearing insert. In some cases, the increased diameter of the turbine-side seal can lead to increased oil leakage from the bearing housing 8 and increased "blowby," with exhaust gases flowing through the enlarged opening, entering the bearing housing, and flowing through the engine crankcase via the lubrication lines. Advantageously, in the method described in steps 101-112, the rotating assembly 50, including the shaft / turbine assembly 40, is balanced while installed within the bearing housing 8 and supported by the bearing insert 20.The procedure described in steps 101-112 allows balancing without additional installation and removal steps, reduces the likelihood of contamination of the bearing insert 20, and allows the use of the optimized diameter of the turbine-side seal.

[0048] Although the compressor wheel 5 illustrated herein includes a bore that receives the shaft 6 and is secured to the shaft using the nut 21, the turbocharger 1 is not limited to this configuration. For example, in some embodiments, a compressor wheel without a bore may be used.

[0049] A selected illustrative embodiment of the invention has been described above in relatively great detail. It should be understood that only structures considered necessary to clarify the present invention have been described herein. Other conventional structures, as well as supporting and additional components of the system, are believed to be known and understood by those skilled in the art. Moreover, while a working example of the present invention has been described above, the present invention is not limited to the working example described above; rather, various modified constructions may be implemented without departing from the present invention as set forth in the claims.

Claims

[1] A method for balancing a rotating assembly (50) of a turbocharger (1), the rotating assembly comprising a shaft (6), a turbine wheel (4) connected to one end of the shaft (6), a compressor wheel (5) connected to the other end of the shaft (6), and a bearing (20) supporting the shaft (6) within a bore (7) in a bearing housing (8) of the turbocharger (1), the method comprising the following steps: Performing a low speed balancing test of the turbocharger rotating assembly (50), comprising measuring low speed vibrations while the turbocharger rotating assembly (50) is positioned within the bore (7); based on the results of the low speed balance test, performing a first rebalancing of the rotating assembly (50) of the turbocharger by removing material from the turbine wheel (4) of the shaft / turbine wheel assembly (40) while the shaft / turbine wheel assembly (40) is installed in the bearing (20) and the bearing (20) is installed within the bore (7); Carrying out a balance test of the rotating assembly (50) of the turbocharger at high speed, comprising measuring the vibrations at high speed while the rotating assembly (50) of the turbocharger is positioned within the bore (7); and based on the results of the high speed balance test, performing a second rebalancing of the turbocharger rotating assembly (50) by removing material from one of the compressor wheel (5) and the compressor wheel nut (21) while the turbocharger rotating assembly (50) is installed within the bearing housing (8). [2] The method of claim 1, further comprising the following steps: performing a first balancing of the shaft / turbine assembly (40) to provide a first-time balanced shaft / turbine assembly (40); Inserting the initially balanced shaft / turbine wheel assembly (40) into the bearing (20); Securing the compressor wheel (5) to the balanced shaft / turbine wheel assembly (40) to form the rotating assembly (50) of the turbocharger (1); wherein the results of the low speed balance test are based on a comparison of the measured low speed vibrations with a first predetermined acceptable vibration level; and wherein the results of the high speed balance test are based on a comparison of the measured high speed vibrations with a second predetermined acceptable vibration level; [3] A method according to claim 1 or claim 2, wherein the step of removing material from the turbine wheel (4) comprises removing material from a peripheral edge (35) of a rear wall (34) of the turbine wheel (4). [4] A method according to claim 3, wherein the step of removing material from a peripheral edge (35) of a rear wall (34) of the turbine wheel (4) comprises: Removing material between a pair of adjacent turbine blades (36a, 36b) such that the peripheral edge (35) is not symmetrical in the circumferential direction about a rotational axis (38) of the turbine wheel (4). [5] A method according to claim 3, wherein the step of removing material from a peripheral edge (35) of a rear wall (34) of the turbine wheel (4) comprises: the step of machining at least one hollow cut (44) which is elongated in the circumferential direction of the turbine wheel (4). [6] A method according to claim 3, wherein the step of removing material from a peripheral edge (35) of a rear wall (34) of the turbine wheel (4) comprises: the step of machining at least one hollow cut (42) which is substantially semicircular. [7] A method according to claim 3, wherein the step of removing material from a peripheral edge (35) of a rear wall (34) of the turbine wheel (4) comprises: the step of machining at least one hollow cut (46) which extends longitudinally in the circumferential direction of the turbine wheel (4), between a first pair of adjacent blades (36a, 36b), between a second pair of adjacent blades (36a, 36b), and extends over at least one blade (36). [8] A method according to claim 1 or claim 2, wherein the step of removing the material from the turbine wheel (4) comprises advancing a cutting tool (60) towards a rear wall (34) of the turbine wheel (4) approaching from a nose side of the turbine wheel (4). [9] The method of claim 1 or claim 2, wherein checking the balance of the unbalanced shaft / turbine assembly (40) comprises: causing the shaft (6) to rotate; and Measuring the vibrations of at least the shaft (6) and / or the turbine wheel (4). [10] The method of claim 2, wherein inserting a shaft / turbine assembly (40) into a bearing (20) within a bearing housing (8) of a turbocharger (1) comprises: Inserting a free end (6b) of the shaft (6) of the unbalanced shaft / turbine wheel assembly (40) into the bore (7) on a turbine side of the bearing housing (8) until the free end (6b) projects outwardly from the bore (7) on a compressor side of the bearing housing (8); and Mounting the bearing (20) on the shaft (6) of the unbalanced shaft / turbine assembly (40) by inserting the bearing (20) into the bore (7) on the compressor side of the bearing housing (8) while performing the step of inserting the free end (6b) of the shaft (6) until the bearing (20) abuts against a portion of the bearing housing (8) and such that the bearing (20) is disposed between the shaft (6) and the bore (7). [11] Method according to one of the preceding claims, wherein the bearing (20) is a rolling bearing insert.

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