Hollow shaft arrangement

A hollow shaft arrangement may include a hollow shaft, such as a rotor shaft of an electric motor, for example, through which a fluid can pass for cooling purposes. Surface-enlarging cooling structures for transferring thermal energy from the hollow shaft to the fluid may be arranged in an inner space of the hollow shaft. The surface-enlarging cooling structures may be connected to the hollow shaft, but may be part of a cooling body that is formed separately from the hollow shaft. Further, the cooling body may include a sleeve-like main body from which the surface-enlarging cooling structures project radially inwardly, and the sleeve-like main body may include an axial gap that permits a circumference of the main body to be adjustable. In some cases an outer diameter of the cooling body is larger than an inner diameter of the bearing.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Entry of International Patent Application Serial Number PCT/EP2016/062316, filed Jun. 1, 2016, which claims priority to German Patent Application No. DE 10 2015 108 817.7, filed Jun. 3, 2015, the entire contents of both of which are incorporated herein by reference.

FIELD

The present disclosure generally relates to hollow shaft arrangements and methods for assembling hollow shaft arrangements.

BACKGROUND

DE 10 2008 043 367 A1 discloses a hybrid drive device for a motor vehicle, having an internal combustion engine and an electric motor, in each case for driving the motor vehicle. A rotor of the electric motor is arranged on a hollow shaft in the drivetrain. Means for conveying a cooling fluid convey said cooling fluid through the inner space of the hollow shaft in order to cool the rotor and the hollow shaft.

Cooling fins are arranged in the inner space of the hollow shaft in order to transfer the heat to the cooling fluid in the best way possible. The cooling fins are formed integrally on the inner wall of the hollow shaft in a fairly complex manner. Also, the inner diameter of the hollow shaft, and thus the diameter of the region which is able to be used for the cooling, is quite small in size, this resulting in a limited cooling performance.

Thus a need exists for a hollow shaft arrangement that offers a simple construction, very good cooling behavior and ease of assembly.

DETAILED DESCRIPTION

According to the invention, a hollow shaft arrangement, in particular for the drivetrain of a hybrid motor vehicle, comprising a hollow shaft, in particular a rotor shaft of an electric motor, through which shaft a fluid is able to pass for the purpose of cooling, is provided. Surface-enlarging cooling structures, in particular cooling fins, for transferring thermal energy from the hollow shaft to the fluid are arranged in an inner space of the hollow shaft. The cooling structures are connected to the hollow shaft. The hollow shaft arrangement is characterized in that the cooling structures are part of a cooling body which is formed separately from the hollow shaft.

The cooling body is thus produced separately from the hollow shaft. In this case, materials which have high thermal conductivity but are not subject to high requirements regarding strength are suitable for the production of the cooling body. In particular, aluminum-based alloys have proven to be advantageous. The production of the hollow shaft itself is simplified since no complex cooling structures are to be formed in the interior of the relatively long hollow shaft.

The cooling body is advantageously held in the inner space in a force-fitting manner. In particular, the cooling body is clamped in radially in the inner space. Advantageously, no further radial fastening is provided, and the cooling body is thus held in the inner space only by way of the force-fitting connection acting in the radial direction. For this purpose, a radially compressible cooling body is used in particular, which, in an unstressed state, has an outer diameter which is larger than the inner diameter of the hollow shaft into which the cooling body is inserted. For the assembly, the cooling body is radially compressible, that is to say the outer diameter of the cooling body can be reduced by radial, mechanical loading and in the process in particular prestressed, such that the outer diameter is smaller than the inner diameter of the hollow shaft. The cooling body is then inserted into the hollow shaft; subsequently, the radial loading is removed, whereby the cooling body re-expands, in particular in this case springs back elastically, and is supported radially against the inner wall of the hollow shaft in a force-fitting manner.

An outer diameter of the cooling body is advantageously larger than an inner diameter of a bearing, in particular of two bearings, for mounting the hollow shaft. Consequently, good cooling capacities are possible due to a large cooling body and large contact surfaces between the cooling body and the hollow shaft; at the same time, radially small and thus low-cost and light bearings for mounting the hollow shaft are used, said bearings also being characterized by relatively low friction losses during operation.

An outer circumferential surface of the cooling body is advantageously formed so as to complement an inner circumferential surface of the hollow shaft, in particular the outer circumferential surface and the inner circumferential surface are formed cylindrically with a mutually corresponding outer diameter and inner diameter, respectively. “The mutually corresponding outer and inner diameters” means in particular the diameters in the assembled state of the arrangement. As a result of the complementary configuration, a large contact surface is provided between the hollow shaft and the cooling body, this promoting a favorable heat transfer capacity. Moreover, a hollow shaft with a cylindrical inner surface is easily producible.

The cooling body advantageously comprises a sleeve-like main body from which the cooling structures project radially inwardly. The sleeve-like main body provides in particular the cylindrical surface which serves for abutment with the hollow shaft. In particular, with corresponding dimensioning, the sleeve-like form is conducive to the elasticity required for the radial compressibility.

The sleeve-like main body advantageously acquires the radial compressibility in that the main body has a circumferential region which is able to be reduced in the circumferential direction. Said circumferential region is formed in particular by an axial gap. Said circumferential region allows the outer circumference and thus the outer diameter of the sleeve-like main body to be reducible without plastic deformation. Furthermore, said circumferential region makes it possible for there to be at all times an optimal abutment between the cooling body and the hollow shaft, both at low temperatures and at high temperatures, despite the different thermal expansion coefficients of the materials of the hollow shaft and the cooling body. Said reducible circumferential region, in particular the axial gap, advantageously extends over the entire axial length of the main body.

In an advantageous refinement, two ends of the sleeve-like main body, which are separated from one another by the above-described circumferential region, are connected to one another via a radially inwardly projecting expansion fold. Even in the case of a relatively small reduction in the circumference of the sleeve-like main body, said expansion fold is subjected to relatively large deformation, as a result of which high prestressing can be achieved. The parameters of the deformability, and thus the dimension of the prestressing, can be set by dimensioning the expansion fold.

The hollow shaft is advantageously of multi-part form and comprises, prior to the final assembly, at least one, in particular sleeve-like, receptacle part with an axial opening for inserting the cooling body and comprises at least one closure part for closing off the axial opening. The receptacle part and the closure part are connected fixedly to one another, for example by way of an interference fit, during the assembly. Prior to this, however, the cooling body is inserted into the inner space of the hollow shaft through the opening. Subsequently, the hollow shaft is closed off by the closure part. The closure part is formed in particular as a flange, which is, for example, formed in one piece with a joinable shaft section.

The invention further relates to a method for assembling a hollow shaft arrangement of the above-described type, comprising the following method steps: reducing, in particular reducing in an elastic manner, the radial dimension of the cooling body by way of radially inwardly acting force loading; inserting the cooling body, which is thereby reduced in the radial direction, through an axial opening into the inner space; removing the radial loading, with the result that the cooling body increases radially, in particular in an elastic manner, and undergoes force-fitting connection to the hollow shaft. The cooling body in the installed state advantageously has an outer diameter which is reduced by at least 0.5% in comparison with the cooling body in the non-installed state, which results in the radial prestressing which is necessary for the force-fitting connection between the hollow shaft and the cooling body over a large temperature range. Reference is made to the advantages specified in relation to the device and to further configuration possibilities. Due to the elastic shape-changing behavior, in particular full contact between the hollow shaft and the cooling body, and thus high heat transfer capacity, is ensured over different operating temperatures.

FIG. 1shows a hollow shaft arrangement1according to the invention, which may be used for example in a drivetrain of a hybrid vehicle. The hollow shaft arrangement1comprises a rotor shaft2which is in the form of a hollow shaft. A rotor of an electric motor is fastened on an outer circumference of the rotor shaft2. The lamination pack4of the rotor can be seen. On one axial end of the rotor shaft2, said shaft comprises a first shaft section16, to which an internal combustion engine is able to be connected. A fan wheel19, via which a cooling fluid is conveyed into the inner space3of the hollow shaft, is arranged between the internal combustion engine and the rotor shaft2. On the other axial end of the rotor shaft2, said shaft comprises a second shaft section17with an internal toothing20, to which a shift gearbox (not illustrated), which is connected to a drive axle, is able to be connected. The rotor shaft2is mounted relative to a vehicle body or a vehicle frame via two rolling bearings13. Situated on the two shaft sections16,17are in each case bearing seats14for said rolling bearings13.

The rotor shaft2is of multi-part form and comprises a separate receptacle part5in an axially central region. The receptacle part5is of sleeve-like form and has both a cylindrical outer surface and a cylindrical inner surface12. Said sleeve-like receptacle part5defines a radially internal inner space3in which there is arranged a cooling body8which is presented in even more detail on the basis ofFIGS. 2 and 3. At both axial ends, the sleeve-like receptacle part5comprises in each case openings15which are closed off by a first closure part6or a second closure part7. Joined integrally to said first and second closure parts6,7are the first and the second shaft sections16,17, respectively.

The rotor shaft2is thus formed from the first shaft section16, the first closure part6, the receptacle part5, the second closure part7and the second shaft section17. The two closure parts6,7are each connected fixedly to the receptacle part5, for example by way of an interference fit or a weld seam. It can be seen that, in the assembled state, an outer diameter D8of the cooling body8and an inner diameter d12of the cylindrical inner surface12are significantly larger than the inner diameters d13of the two rolling bearings13. It is thus possible, in the case of the present rotor shaft2, to provide small rolling bearings, but nevertheless to use a cooling body8having a relatively large diameter. Until now, these two properties have been mutually exclusive.

FIGS. 2aand 2billustrate the cooling body8in detail. The cooling body8comprises a sleeve-like main body18from which a multiplicity of cooling fins9project radially inwardly. An outer circumferential surface10of the sleeve-like main body18is of almost cylindrical form. The sleeve-like main body18has an axial gap11extending over the entire axial length, whereby the main body18is in principle compressible in a radially elastic manner. Radial compression allows the cooling body8to be radially prestressed. It is possible for the main body18to expand from this compressed state automatically, as soon as loading which produces the compression is not present. Formed as a result of the axial gap11are two exposed ends21of the main body18as seen in the circumferential direction U.

The radial compressibility is utilized during the assembly of the arrangement. The cooling body8comprises, in the non-assembled state, an outer diameter D8which is initially larger than the inner diameter d12of the inner surface12of the receptacle part5. For the assembly, the cooling body8is radially compressed by way of radial loading and is thus prestressed. In this case, the outer diameter D8of the cooling body8is reduced in an elastic manner such that said diameter is smaller than the inner diameter d12of the inner surface12. In this state, the cooling body8is inserted through one of the axial openings15into the inner space3. The radial loading is then removed, and the cooling body springs back in an elastic manner. In this case, the cooling body18expands radially and strives to return to its starting state. Since the inner diameter d12of the receptacle part5is smaller than the outer diameter D8of the cooling body8in the unstressed state, the cooling body8then presses against the inner surface12of the receptacle part5radially from the inside and thus undergoes force-fitting fastening to the receptacle part5. Further radial fastening is no longer necessary. As can be seen fromFIG. 1, it is possible, however, for axial, form-fitting positioning of the cooling body8inside the inner space3to be performed by way of the two closure parts.

Cooling bodies8whose outer diameters D8in the non-installed state are at least 0.5%, and in particular at most 2.5%, larger than the expected radial installation space, which is defined by the inner diameter d12, are particularly suitable. Although the compressibility of the cooling body could be increased by enlarging the axial gap in the circumferential direction, this leads to the outer surface of the cooling body being reduced, which lowers the heat transfer capacity of the cooling body due to there being less contact with the hollow shaft. It is therefore preferable to limit the axial gap and thus the compressibility to what is absolutely necessary.

In a particularly suitable material pairing, the receptacle part5is a steel part and the cooling body8is an aluminum part.

In the case of an exemplary outer diameter D8of approx. 80 mm, the axial gap11has a gap width B between 0.5 mm and 10 mm, advantageously approximately 1.2 mm in the installed state and at 20° C.

In the configuration according to the invention, the dimensions of the cooling body8are independent of the bearing inner diameter d13of the rolling bearings13. The bearing inner diameter d13may thus be chosen to be quite small, and at the same time the cooling body8may, independently thereof, be designed according to the required heat transfer capacity. Owing to the large cylindrical contact surface between the inner surface12of the receptacle part5and the cooling body8, there is a large contact surface for the heat transfer, wherein the full-surface contact between the cylindrical inner surface12of the receptacle part5and the cylindrical outer surface10of the cooling body8is ensured by the elastic prestressing over a large temperature range.

FIG. 3shows a refinement of the cooling body as perFIG. 2. The two ends21′ of the sleeve-like main body18′, which are separated from one another by the axial gap11′, are connected to one another via a radially inwardly projecting expansion fold22. The expansion fold is of U-shaped form in the present case. The design of the expansion fold has considerable influence on the elastic parameters of the sleeve-like main body. The elasticity of the expansion fold and thus the elasticity of the main body can be set by way of the wall thickness, the size and the shape of the expansion fold22.

The invention is not restricted, with regard to its embodiment, to the exemplary embodiment specified above. Rather, numerous variants which make use of the presented solution even in fundamentally different embodiments are conceivable. All the features and/or advantages which emerge from the claims, the description or the drawings, including structural details or spatial arrangements, may be essential to the invention both individually and in a wide variety of different combinations.

LIST OF REFERENCE SIGNS

1Hollow shaft arrangement2Hollow shaft/Rotor shaft3Inner space4Lamination pack of a rotor of an electric motor5Receptacle part6First closure part7Second closure part8Cooling body9Cooling fins10Cylindrical outer surface of the cooling body11Axial gap12Cylindrical inner surface of the receptacle part13Rolling bearing14Bearing seat15Opening in the central part16First shaft section17Second shaft section18Sleeve-like main body19Fan wheel20Internal toothing21End of the sleeve-like main body22Expansion foldD8Outer diameter of the cooling bodyd12Inner diameter of the cylindrical inner surface of the receptacle partd13Inner diameter of the rolling bearingB Gap widthU Circumferential direction