Transfer of Torque by Elastic Element

The possible use of an elastic element as a coupling to reduce the loads and thus the deformations transferring on the rotor can fulfill requirements of integration into the in-wheel motor. However, the determination of arrangement, type and position of the elastic element becomes a major factor, because this part of design depends on a great variety of factors.

Elastic element in the sense of the machine part represents a coupling. Couplings are machine elements which main function it is to transmit torque and rotation by frictional or positive connection from one rotating part to another. As an additional task, couplings can compensate axial, radial and/or angular misalignment. A flexible but torsionally rigid compensating coupling (see Figure 3.10, (b), 3.2.2) is required between rotor housing and wheel. It is also possible to compensate torsional shocks and vibrations with coupling. If the connection between two elements is to be disconnected during operation, switchable couplings are used. Non-switchable couplings are used if it is not necessary or possible to make the coupling releasable/switchable. The next classification criterium is flexibility of couplings, while the flexible couplings are distinguished between torsionally stiff and torsionally elastic ones. Flexible couplings or compensating couplings are designed to be moveable and torsionally flexible in order to compensate misalignments, vibrations and torque shocks. In order to ensure that the selected coupling fulfils all the functions required, coupling must be selected according to various criteria in addition to some basic factors such as mounting, manufacturing, wear, service life and susceptibility to failure.

In order to use a coupling in an in-wheel motor, it must fulfil more requirements in addition to the main requirements mentioned above. For example, the installation space is very important, because the space between the rotor and the wheel is very limited. Another important requirement of the application on vehicles is driving comfort, which has its origin in unsprung weights. Therefore, it must be ensured, that the unsprung weight, which includes the weight of the coupling, is reduced to its minimum value. In principle, there are several standard variants of such couplings, which can find an application on developing the in-wheel motor.

Standard solution in the area of flexible couplings can be represented for example by metal bellow couplings or lashing couplings.

33 Figure 3.18, (a) defines a typical metal bellow coupling. An approach of metal bellow coupling enables to compensate horizontal and angular movements [155]. However, in-wheel motors are also loaded with vertical forces by the transmission of torque and by deformations of the wheel. The metal bellow coupling could break by the application of loads of this kind, what results in the complete functional failure. This fact makes the use of the metal bellow couplings impossible and they are not further considered for the in-wheel application.

As a transmission element by lashing coupling, shown in Figure 3.18, (b), configure integrated lashes, what allows to transmit a force from one coupling part to another. In the core of the lashes there is an elastic package, which function it is to compensate for radial, axial and angular misalignments. An advantage of this coupling type is the option to vary the flexibility of the lashes to become a different characteristic of compensation. At first glance, it might seem logical that the lashing coupling as a coupling system for the application in in-wheel motor requires an additional space in axial direction, but that does not meet the requirements of minimal installation space. A further difficulty relates to the adaptability of the in-wheel motor through the use of such a coupling, requiring the integration of lashing accessories for the connection to the rotor and wheel.

a b

Figure 3.18 – Standard variants of couplings: a - Metal bellow coupling [115], b - Lashing coupling [166]

The standard methods of torque transmission and decoupling of load influence are implemented in many units of the modern vehicle. Further development is focused on the solution of the principal problem with the help of the integration of the elastic element directly into the rotor housing and using available interfaces of the multiple wheel. That allows to solve the problem at a significant level and at the same time to satisfy the main requirements of compensation of loads and side requirements of weight, certain flexibility by material selection and mountability in in-wheel motor.

The task above can be solved by special designed tooth coupling. The main requirement on the tooth is to demonstrate an elasticity, preferably bending elasticity, with value which prevents tilting and thus an unwanted transmission of force without reducing the torsional stiffness for transmission of the driving torque. And the tooth must also be connected to each other in such a way that a form-fit connection is created that blocks one degree of freedom, while the other degrees of freedom are not restricted in their movement and a movable, detachable connection is created. Developed solutions in the area of transmission elements and their contact areas with separate additional part for the rotor (a) and for the wheel (b) are presented in Figure 3.19.

34

a b

Figure 3.19 – Variants of contact areas for developed coupling structure: a - With separate additional part for the rotor, b - With separate additional part for the wheel

The approach with a separate additional part for the wheel (Figure 3.19, (b)) allows to compensate the rotor body for loads coming from the wheel by the degrees of freedom in the directions shown in Figure 3.20, with 𝐹_𝐴 as an axial and 𝐹𝑅 as a radial load represented. An additional part of this solution is mounted directly to the three-piece wheel with screws that realize the assembly of the wheel. A disadvantage of this solution is obtained by the connection of the second part of the structure on the rotor, which brings an additional junction to the whole system.

Figure 3.20 – Loads compensation by the elastic element with separate additional part for the wheel [91]

The compensation possibilities of the second variant of solution with a separate additional part for the rotor is shown in Figure 3.21. The elastic coupling element presents a ring with cut-outs, further called slots, and the second part of the transferring structure are the screws of the three-piece wheel with turned ends. The cylindrical screw ends engage in these slots and have the function to connect outer and inner ring. These screws are movable in radial and axial direction in the slots, thus no deformations from the wheel are transmitted to the rotor of the in-wheel motor. Additionally, the screws have a connection on the side surface of the slots in the tangential direction and are able to forward the torque to the wheel. The main requirement for this solution is expected from the material of coupling element, which relates to a high torsional strength caused by the functional use of the in-wheel motor.

35 Figure 3.21 – Load compensation by the elastic element with separate additional part for the rotor [91]

Both solutions, with a separate additional part for the wheel or for the rotor, comply with the main requirements for the elastic coupling element. As the next step of the analysis it was decided to implement an additional requirement as manufacturing complexity in order to develop more of the variants. At first sight, both solutions are thin-walled rings with big diameter/length relation and additional slots in longitudinal direction by the first variant and keys in transversal direction by the second. In the first production step, both coupling elements can be produced by turning. The solution with a separate additional part for the wheel has rectangular slots with sharp edges. The required slots have an important design feature - two flat surfaces are perpendicular to each other. The use of conventional production methods in this case involves rounding off the shape of the teeth and the slots. However, the necessary slot geometry can be achieved with a special tool such as the angle milling head. An angle milling head allows to produce a slot that cannot be reached with standard axis kinematics and tools. In addition, the rotor must be equipped with teeth that can be engaged as an elastic coupling element.

The elastic element with a separate additional part for the rotor is in technological advantage by the usage of a standard end milling process. The favorite design also requires an application of standard parts – screws of the three-part wheel, which must be adapted by conventional turning.

As an additional requirement, a contact between two parts of the coupling can be analyzed. As already mentioned in 3.3.1, a flexible coupling element is also cyclically loaded by the drive. Due to the specific design of the flexible coupling element, a contact surface is more advantageous for possible wear and occurring contact stress, since a contact in this case is two lines (contact between a cylindrical body lying on the plane surface).

Based on the additional requirements for the elastic coupling element, the solution with an additional part for the rotor has the highest value and is therefore selected as the preferred solution for further detailed design and simulation.