2100 The Crescent Oy/Be/bec (2022006397)

Birmingham Business Park Q22007WQ00

Birmingham, West Midlands B37 7YE

Rotor assembly for an electric machine

The application refers to a rotor assembly for an electric machine comprising a rotor and a shaft with a thrust portion forming an abutment for the rotor, wherein the rotor is preloaded against the thrust portion.

Such rotor assemblies are known with laminated sheets forming the rotor arranged upon the shaft. Various elements are used to provide a reliable connection for transmitting torque between the lamination stacks and the shaft, such as axial tie rods or positive locking means.

US 2017/1 17766 A1 relates to a method for mounting laminated sheets onto the shaft of a rotor for an electrical machine, whereby the laminated sheets are slid onto the shaft and braced between two thrust portions connected with the shaft in a rotationresistant manner. Axially preloading the rotor assembly, however, implies increased structural stress and cost impact.

It is an objective to reduce the axial preload force without reducing the transmissible torque of the rotor assembly.

The objective is achieved by a rotor assembly of claim 1 . Further embodiments are subject to the dependent claims.

The rotor assembly for an electric machine comprises a rotor and a shaft with a thrust portion forming an abutment for axially supporting the rotor, wherein the rotor is axially preloaded against the thrust portion. A friction increasing means is arranged between a contact surface of the thrust portion abutting against the rotor and a contact surface of the rotor.

Increasing the friction between the contact surface of the thrust portion abutting against the rotor and the contact surface of the rotor advantageously allows to reduce the axial preload force without reducing the transmissible torque. Depending on the intended amount of the reduction in the axial preload force, the transmissible torque may even be increased.

Further, during operation of an electric machine, it is excited to vibrate. If the frequencies of the vibrations and a natural frequency of the electric machine are identical or close to each other, resonance occurs, by which components of the electric machine can be strongly deflected. Such deflections cause acoustic noises, which are disturbing and can impair the service life of the electrical machine. The resonance area can be shifted to an acceptable frequency range by the axial preload of the rotor assembly. However, high preloads normally lead to structural stress and cost impact. Hence, the rotor assembly with increased friction between the contact surface of the thrust portion abutting against the rotor and the contact surface of the rotor has the further advantage, to provide a beneficial compromise between noise, vibration, harshness (NVH) reduction, torque transmission and structural stress.

The rotor and the shaft are rotatably mounted for rotation about a longitudinal axis, wherein axial refers to a direction parallel to the longitudinal axis. Electrical machines may have a stator provided with permanent magnets or stator coils and the rotor may be provided with permanent magnets or rotor coils. The stator may be located in and fixed to a housing. The housing may further comprise bearings for supporting the rotor assembly to be rotatable about the longitudinal axis. The thrust portion may be an integral part of the shaft or connected to the shaft.

The friction increasing means may be provided in the form of a surface treatment of the contact surface of the thrust portion abutting against the rotor and/or the contact surface of the rotor. The friction increasing means may as well be regarded as a friction increasing element.

According to an embodiment, the friction increasing means or element is a friction shim, which has a coating on on both sides. The friction shim may be disc-shaped, round, with a central hole, for being supported on the shaft, arranged between the thrust portion and the rotor, so that the one side of the friction shim faces the contact surface of the thrust portion abutting against the rotor, and the other side of the friction shim faces the respective contact surface of the rotor. An outer diameter of the friction shim may be adapted to an outer diameter of the thrust shoulder.

According to an embodiment, the coating comprises particles embedded in a matrix. The particles may be diamond particles having a mean particle size between 10 and 35 micrometre. The matrix may be an electroless nickel-phosphorus plating having a thickness between 5 and 25 micrometre. The friction shim may be made of a quenched and tempered high-carbon steel having a thickness between 0.12 and 0.20 millimetre. The friction shim may have a surface roughness (Rz) in a range of 4 and 17 micrometre, where Rz is the average value of the absolute values of the heights of five highest- profile peaks and the depths of five deepest alleys within an evaluation length along the surface of the friction shim.

According to a further embodiment, the shaft has a preloading washer for exerting an axial preload force onto the rotor at an axial distal end located opposed to the thrust portion.

According to a further embodiment, a shim dry static friction coefficient (p_th) between the contact surface of the thrust portion and the contact surface of the rotor is at least twice as high as a washer dry static friction coefficient (p_pr) between a contact surface of the preload washer and an opposite contact surface of the rotor, with the friction increasing means being applied between the contact surface of the thrust portion and the contact surface of the rotor, and not between the contact surface of the preload washer and the opposite contact surface of the rotor. In particular, the shim dry static friction coefficient (p_th) is at least three times as high as the washer dry static friction coefficient (p_pr).

The axial preload force (Fp) multiplied with the shim dry static friction coefficient (p_th), a mean radius (Rm_th) of the contact surface of the thrust portion abutting against the rotor plus the axial preload force (Fp) multiplied with the washer dry static friction coefficient (p_pr), a mean radius (Rm_pr) of the contact surface of the preloading washer exceeds a maximum torque (Tmax) of the electric machine: Fp ■ p_th ■ Rm_th + Fp ■ p_pr ■ Rm_pr > Tmax.

Fulfilling the above condition prevents slipping of the rotor relative to the shaft. Assuming that the mean radius (Rm_th) of the contact surface of the thrust portion abutting against the rotor equals mean radius (Rm_pr) of the contact surface of the preloading washer abutting against the opposite end of the rotor (Rm_th = Rm_pr), the shim dry static friction coefficient (p_th) three times as high as the washer dry static friction coefficient (p_th = 3 ■ p_pr), for example, allows the axial preload force (Fd) to be reduced by 75% while still resisting the same torque. The axial preload force (Fp) may be in a range of 20 kN to 150 kN.

The rotor may comprise an end cap at its axial distal end facing the friction increasing means and/or at its axial distal end facing the preloading washer. The rotor may comprise stacks of laminated metal sheets.

According to a further embodiment, the friction increasing means or element is connected to the shaft in a form-locking manner. The shaft may have at least one axially extending groove fitting together with a tongue at the friction shim to circumferentially secure the friction shim on the shaft.

An exemplary embodiment and further advantages of the rotor assembly will be illustrated as follows with reference to the accompanying drawings, wherein

Figure 1 shows an exemplary embodiment of the rotor assembly in a perspective exploded view;

Figure 2 the embodiment of Figure 1 in a further perspective view, partly assembled;

Figure 3 a detail of the embodiment of Figure 1 in an assembled state in a schematic illustration.

An exemplary embodiment is described with respect to Figures 1 and 2, showing rotor assembly in a perspective exploded view and in a further perspective view, partly assembled. Figures 1 and 2 are described together. The rotor assembly for an electric machine comprises a rotor 2 and a shaft 1 with a thrust portion 3 forming an abutment for axially supporting the rotor 2. The rotor 2 and the shaft 1 are rotatably mounted for rotation about a longitudinal axis L. The rotor 2 is axially preloaded against the thrust portion 3 by means of a preloading washer 5 for exerting an axial preload force onto the rotor 2 at an end located opposed to the thrust portion 3. The rotor 2 comprises an end cap 10 at each axial distal end. The rotor 2 further comprises stacks of laminated metal sheets, stacked between the end caps 10.

As a friction increasing means, a friction shim 4 is arranged between a contact surface of the thrust portion 3 abutting against the rotor 2 and a contact surface of the rotor 2, which is in this embodiment formed on the surface of the respective end cap 10. The friction shim 4 is connected to the shaft 2 in a form-locking manner. The shaft 1 has two axially extending grooves 6 fitting together with respective tongues 7 at the friction shim 4 to circumferentially secure the friction shim 4 on the shaft 1 .

In Figure 3, a detail of the embodiment of Figure 1 is illustrated. Depicted is a schematic illustration of the thrust portion 3 and the rotor 2, or respectively its end cap 10, in an assembled state with the friction shim 4 arranged between the contact surface of the thrust portion 3 abutting against the rotor 2 and a contact surface of the rotor 2. The person skilled in the art acknowledges that the rotor 2 and the thrust portion 3 are the abutting parts of the rotor assembly, not the friction shim 4 between the abutting parts, as the thickness of the friction shim is insignificantly small, for example in the range of 0.1 to 0.2 millimetre. The illustration of Figure 4 is thus not true to scale, as the thickness of the thrust portion 3 in axial direction is, for example, at least 25 times the thickness of the friction shim 4.

The friction shim 4 has a coating on each side, the coating comprising particles 9 embedded in a matrix 8. The particles 9 may be diamond particles having a mean particle size between 10 and 35 micrometre. The matrix 8 may be an electroless nickel-phos- phorus plating having a thickness between 5 and 25 micrometre. The shim 4 may be made of a quenched and tempered high-carbon steel having a thickness between 0.12 and 0.20 millimetre including the coatings. The friction shim 4 has a surface roughness Rz in a range of 4 and 17 micrometre. The resulting shim dry static friction coefficient (p_th) between the contact surface of the thrust portion 3 and the contact surface of the rotor 2 may be at least 0.5, for example higher than 0.7. The shim dry static friction coefficient (p_th) between the contact surface of the thrust portion 3 and the contact surface of the rotor (2) may be, for example, three times as high as a washer dry static friction coefficient (p_pr) between a contact surface of the preload washer 3 and an opposite contact surface of the rotor 2, with the friction increasing means being applied between the contact surface of the thrust portion 3 and the contact surface of the rotor 2, and not between the contact surface of the preload washer 3 and the opposite contact surface of the rotor 2.

To prevent slipping of the rotor relative to the shaft, the axial preload force (Fp) multiplied with the shim dry static friction coefficient (p_th), a mean radius (Rm_th) of the contact surface of the thrust portion 3 abutting against the rotor 2 plus the axial preload force (Fp) multiplied with the washer dry static friction coefficient (p_pr), a mean radius (Rm_pr) of the contact surface of the preloading washer exceeds a maximum torque (Tmax) of the electric machine:

Fp ■ p_th ■ Rm_th + Fp ■ p_pr ■ Rm_pr > Tmax.

Assuming that the mean radius (Rm_th) of the contact surface of the thrust portion 3 abutting against the rotor 2 equals mean radius (Rm_pr) of the contact surface of the preloading washer abutting against the opposite end of the rotor 2 (Rm_th = Rm_pr), the shim dry static friction coefficient (p_th) three times as high as the washer dry static friction coefficient (p_th = 3 ■ p_pr), for example, allows the axial preload force (Fd) to be reduced by 75% while still resisting the same torque.

Reference Numerals

1 Shaft

2 Rotor

3 Thrust portion

4 Friction shim

5 Preload washer

6 Groove

7 Tongue

8 Matrix

9 Particle

10 End cap

L Longitudinal axis.