Automotive auxiliary unit with an electric motor

The invention is directed to an automotive auxiliary unit, preferably an automotive vacuum pump for generating vacuum.

Such auxiliary units in automobiles can be, for instance, vane cell vacuum pumps for supplying other devices, such as brake boosters, with an absolute pressure of about lOOmbar. Conventional automotive auxiliary units are driven mechanically by an internal combustion engine of a motor vehicle. In order to be independent of the rotational speed of the internal combustion engine, electric auxiliary units as, for example, vacuum pumps driven electrically by an electric motor become more and more common. The electric motor is provided with a rotor shaft and a separate motor rotor fixed to the shaft.

DE 10 2005 046 285 A1 and DE 10 2016 216 476 A1 disclose two different arrangements for mounting the motor rotor positively with the rotor shaft. In DE 10 2005 046 285 A1 the rotor shaft is provided with a gear shaped cross section, whereas in DE 10 2016 216 476 A1 the rotor shaft has a polygonal cross section. In either case, the motor rotor is provided with a corresponding inner profile in order to be fixed co-rotatably to the shaft to thereby transfer relatively high torques. Furthermore, in DE 2017 011 969 A1 the motor rotor is fixed to the rotor shaft using a casting process. Moreover, DE 10 2017 115 229 A1 discloses a fixation arrangement including a bushing for mounting the motor rotor at the rotor shaft. However, prior art arrangements require a considerable assembly effort or additional components. It is an object of the invention to provide an automotive auxiliary unit with a reliable and cost-effective fixation arrangement for mounting the motor rotor at the rotor shaft so that high torque transmission is possible.

This object is achieved with an automotive auxiliary unit with the features of claim 1.

The automotive auxiliary unit according to the invention is provided with a motor stator. The motor stator is housed in a motor housing that separates the auxiliary unit from the environment. The motor stator is arranged coaxially to a motor rotor rotation axis and can be provided with a predetermined number of permanent magnets or with an electromagnetic arrangement. Furthermore, the motor stator is separated by an air gap from a motor rotor that is, preferably, arranged radially inside the motor stator.

The automotive auxiliary unit according to the invention is also provided with a rotatable metallic rotor shaft. The rotor shaft is provided with material bulgings at the cylindric shaft surface, hereby creating a local shaft surface roughness. The shaft surface area that is provided with bulgings defines a rotor fixation section, the uneven surface serving as a mechanical connecting means between the shaft surface and the motor rotor.

The motor rotor is co-rotatably fixed to the rotor shaft by pressing at the rotor fixation section. The term "pressing" means that the sheet metal pack is provided with an undersize with respect to the rotor shaft. As a result of the pressing, a microscopic form-fitting connection between the rotor shaft and the motor rotor is realized. This connection ensures a high-torque transmission capability of the automotive auxiliary unit, as it is required, for example, in automotive displacement pumps.

According to the invention, a laser textured shaft surface is created in order to improve and increase the friction properties between the motor rotor and the rotor shaft so as to create a strong and solid fixation between the two said rotor components. For this purpose, a laser beam that can create shaft surface temperatures of more than 1600°C is used to generate material bulgings on the shaft surface. The high surface temperatures result in local melting of the shaft surface material. A portion of the molten shaft surface material re-solidifies after removing the laser beam so that lateral bulges and micro formations are generated. These effects change the shaft surface topology and generate a plurality of local groove-like or indentation-like recesses and re-solidified material accumulations at the shaft surface. The specific shaft surface structure depends on the nature of the used laser type and working method. The laser can, for example, generate single pulse laser radiation or high-power continuous wave laser radiation. While the invention provides an automotive auxiliary unit with a reliable and cost-effective fixation arrangement for mounting the motor rotor at the rotor shaft so that high torque transmission is possible, the bulging generation procedure is speeded up due to the elimination of setup times. There are no longer tool wear parts, thereby reducing costs. The quality of the generated bulgings is better as the progressing wear of tool parts is avoided. Finally, quality control is automatable.

In a preferred embodiment of the invention, a minimum height of the material bulgings of 15pm is realized in order to ensure sufficient torque transfer capability between the motor rotor and the rotor shaft. This is needed for transmitting relatively high torques which is typical for displacement pumps.

Preferably, the motor rotor is defined by a sheet metal pack that is a stack of punched ferromagnetic metal sheets. The individual metals sheets are axially joined together to a single rotor body. The metal sheets, for example, can be provided with pin-like surface elevations and corresponding indentations on the back side of each metal sheet, the pin like surface elevations of one metal sheet being pressed into the indentations of the adjacent metal sheet, hereby creating a mechanical connection between the sheets. Other joining techniques are laser welding or adhesive techniques in order to join the metal sheets. The resulting sheet metal pack is defined by a plurality of radial recesses that extend axially through the stack, to thereby define a plurality of magnetic poles.

In a particularly preferred design of the invention, the shaft surface is completely hardened so that the shaft surface is protected against abrasive wear. The more flexible shaft center remains elastic so that high shear stresses caused by torsional torque can be transmitted which is necessary for displacement pump applications.

Preferably, the rotor shaft is provided with at least three continuous material bulgings at the shaft surface so as to realize a good fixation between the motor rotor and the rotor shaft. A continuous laser beam is guided along the shaft surface, hereby melting material and displacing it laterally to generate continuous bulgings. The resulting re-solidified material trace is similar to a mountain chain, is coherent and radially protrudes from the shaft surface. The material bulgings are preferably designed as lines. However, the material bulgings do not necessarily define a strictly straight line, but could define a curved line or a helix. An alternative embodiment of the invention is defined by ring-like material bulgings. The shaft surface is provided with a plurality of circular material throw-ups arranged along a line, the number of lines preferably being at least three. In contrast to continuous material bulgings, this type of surface topology with individual and separate material throw-ups can be generated by means of pulsed laser radiation, generating recesses in the center of the ring-like bulges. The structural properties of the shaft surface, such as the bulging height, diameter and hardness, depend on the laser type, the laser power, the radiation duration and the process gas.

Preferably, the continuous and longitudinal material bulgings are orientated approximately axially. Regardless of the shape of the bulges, continuous or ring-like, the axial orientation of the bulgings can be simply realized by pivoting the laser device to generate a line at the shaft surface or by axially moving the shaft while the laser device is stationary and focused to the shaft surface.

The automotive auxiliary unit is preferably provided with at least one electromagnetic coil wire. The coil wire is wound through the radial grooves of the sheet metal pack to define at least two electromagnetic coils.

According to a preferred embodiment of the invention, the motor stator comprises at least one permanent magnet. The total torque performance of the auxiliary unit correlates with the number of the permanent magnets within the electric motor.

Further advantages will become evident by the following detailed description of embodiments of the present invention in combination with the enclosed drawings, wherein figure 1 shows a schematic arrangement of the automotive auxiliary unit with an electric motor, figure 2 shows a cross section of the electric motor of figure 1, figure 3a shows a section of the cross section of the rotor shaft of the electric motor of figure 2, figure 3b shows a section of an alternative embodiment of the rotor shaft of the electric motor in top view, and figure 4 shows a side view of a part of the rotor shaft of figure 2.

As figure 1 shows, the described automotive auxiliary unit 10 according to the invention comprises an electric motor 16 that drives a pump rotor 14. The pump rotor 14, preferably being a part of a displacement pump, such as a vane cell pump, pumps air in order to generate vacuum. The mechanical connection between the electric motor 16 and the pump rotor 14 is realized by a rotor shaft 12. The electric motor 16 comprises a motor stator 18, a motor rotor 11 and the rotor shaft 12.

The motor stator 18 is arranged in a motor housing not shown in the figures and comprises ten permanent magnetic poles 29. As the stationary part of the electric motor 16, namely the motor stator 18, is static and is fixed to the motor housing, the electric motor 16 of the present invention is designed and operated as a so-called internal rotor motor.

The motor rotor 11 comprises a sheet metal pack 22 consisting of numerous identical metal sheets 22'. Each ferromagnetic metal sheet 22' is produced by punching defining a plurality of radial recesses 25. The metal sheets 22' are then piled and joined axially to define the laminated sheet metal pack 22. The radial recesses 25 of the metal sheets 22' are arranged during the piling and joining procedure such that the entirety of radial recesses 25 define axial notches 23 throughout the sheet metal pack 22, thereby defining rotor pole heads.

The motor rotor 11 is provided with at least one coil wire 24 defining eight electromagnetic rotor coils 24' that can be electrically energized so as to generate electromagnetic fields. These electromagnetic fields interact with the permanent magnets 28 of the motor stator 18 and thereby create and maintain rotation of the motor rotor 11. The coil wire 24 is part of the motor rotor 11.

The motor rotor 11 is rotatably fixed to the rotor shaft 12 by pressing at a rotor fixation section 34. The rotor fixation section 34 describes a section on the shaft surface 32 that defines the axial position of the sheet metal pack 22. In order to realize sufficient pressing, a height 30 of material bulgings 26 or 26' with respect to the shaft surface 32 of 15-30pm is realized. The figures 3a and 3b show two different embodiments of the present invention. Figure 3a shows a continuous axial material bulging line 26" which has been generated using a laser beam. The laser beam causes high temperatures at the hardened shaft surface 32 to thereby melting the shaft surface material and displacing the melted shaft material laterally to thereby produce substantially linear grooves 20 and the adjacent continuous axial material bulgings 26.

An alternative embodiment of the invention is shown in Figure 3b. In contrast to continuous material bulgings 26, the material bulgings 26' are ring-like and the ring-like bulgings 26' are arranged along an axial line 27 at the shaft surface 32. The indentations 20' surrounded by the ring-like bulgings are substantially circular.

Reference List

10 automotive auxiliary unit

11 motor rotor

12 rotor shaft

14 pump rotor

16 electric motor

18 motor stator

20 linear grooves

20' indentations

22 sheet metal pack

22' metal sheets

23 notches

24 coil wire

24' coil

25 radial recesses

26 continuous material bulgings

26' ring-like material bulgings

26" continuous material bulging lines 27 lines of ring-like material bulgings

28 permanent magnet

29 magnetic poles

30 minimum height of material bulgings

32 shaft surface

34 rotor fixation section