This article shows how the S-Life FKM software from PART Engineering can be used to easily design and optimize the shape, material, weight and load capacity of an electric mobility scooter chassis in accordance with the FKM guideline. With obese users in mind, on the one hand the scooter was to be modified so that its maximum load capacity could be increased from 150 kg to 200 kg. On the other hand, the aim was to reduce the overall weight of the vehicle from 87 kg to a maximum of 65 kg in conjunction with other optimizations so that it can be transported on a standard car trailer hitch rack.
Load case determination and analysis of the current chassis
The first step was to determine which accelerations and forces are to be assumed in the event of a curb crossing at a maximum speed of 15 km/h and a ride over rough cobblestones. The latter load case served primarily as the basis for the fatigue strength analysis, whereby a payload of up to 200 kg was considered.
Based on these load assumptions, the current frame shown in Figure 2 was then analyzed regarding the static stresses and safety factors to be encountered. The current version of the scooter has already been in use for several years so that its operational stability was considered proven. Thus, the new frame was initially designed in such a way that the resulting safety factors at least corresponded to the pre-determined ones.
Entwicklung eines neuen Rahmenkonzeptes
Die Ausgestaltung einer optimierten Rahmenvariante basierte auf den Ergebnissen einer software-gestützten Formoptimierungsanalyse und resultierte letztlich in einem Rahmen bestehend aus Aluminium-Hohlprofilen der Abmessungen 50 x 25 x 1,5 mm, welcher in kritischen Bereichen durch Einlegeprofile verstärkt wurde und entweder als ausschließliche Schweißkonstruktion oder überwiegende Biegekonstruktion umgesetzt werden kann (Bild 3). Bei beiden Varianten konnte beobachtet werden, dass die maximalen Spannungen in dem in Bild 3 markierten Bereich liegen. Nachdem der statische Festigkeitsnachweis erbracht war, wurde daher insbesondere diese Region hinsichtlich der Betriebsfestigkeit untersucht.
Praktischer Einsatz von S-Life FKM im Kontext der Rahmenoptimierung
Entsprechend der Handhabung von S-Life FKM konnte u.a. auch der in Bild 3 gezeigte Rahmen untersucht werden (Bild 4). Konkret wurde untersucht, ob ein positiver Betriebsfestigkeitsnachweis resultiert, wenn das Hinsetzen und Aufstehen einer 200 kg schweren Person simuliert wird. Hierfür war dementsprechend nur ein Lastfall zu berücksichtigen, welcher mit einem Spannungsverhältnis von R = 0 in die Berechnungen einging.
Development of a new frame concept
The design of an optimized frame variant was based on the results of a software-supported shape optimization analysis and ultimately resulted in a frame consisting of hollow aluminum profiles measuring 50 x 25 x 1.5 mm, which was reinforced in critical areas by insert profiles. It can be implemented either as an exclusively welded construction or as a predominantly bent construction (Fig. 3). For both variants, it was observed that the maximum stresses are in the range marked in Fig. 3. After the static strength verification had been provided, this region was therefore investigated regarding the fatigue strength.
Practical use of S-Life FKM in the context of frame optimization
In accordance with the handling of S-Life FKM, the frame shown in Figure 3 was also examined (Figure 4). Specifically, it was investigated whether a fatigue strength assessment result is uncritical if a person, weighing 200 kg, sits down and stands up multiple times. For this purpose, only one load case had to be considered, which was included in the calculations with a stress ratio of R = 0.