1．OUTLINE

This technology employs Mg (magnesium) alloy (AZ61 (Mg with 6% Al and 1% Zinc) and AZ91), which is an advanced　ultralightweight metallic material, for athletic wheelchairs (tennis wheelchairs and badminton wheelchairs), achieving a weight reduction of 30% or more compared to conventional wheelchairs made of Al (aluminum) alloy. The wheelchair is said to be difficult to implement in society in this field. The application of Mg alloy to wheelchairs, which has been said to be difficult to implement in society in this field, was achieved by (1) detailed structural analysis of materials, members, and structures, (2) evaluation of the wheelchair’s drivability and operability using various measurement technologies such as AI technology, and (3) strength and rigidity evaluation using destructive testing with large heavy machinery. The wheelchair has achieved a strength of more than 1 ton under vertical load (strength that caused cracks in the frame), and has completed a competition wheelchair with high mobility, operability, and human friendliness (Fig. 1).

The products of this technology have contributed to the winning of three gold medals and one bronze medal in the women’s single and doubles wheelchair badminton, and bronze medals in the women’s doubles and quad doubles wheelchair tennis at the 2021 Tokyo Paralympic Games.

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Fig. 1 Mg alloy tennis wheelchair (Rio Games model)

2.TECHNOLOGY CONTENT

The Mg alloy used in this technology is the lightest metal material among practical metals, with a specific gravity of 1.74 g/cm³ compared to 2.70 g/cm³ for general Al alloys and 4.51 g/cm³ for Ti (titanium) alloys, but it is slightly inferior in terms of rigidity. Therefore, the selection of material thickness and other factors greatly affects the strength, rigidity, and even operability of the wheelchair. The technology has solved this problem by conducting sufficient structural analysis at each stage of materials, components, and wheelchair frame, and by developing and introducing AI measurement technology in destructive tests and actual running tests.

Figure 2 shows the results of structural analysis when a vertical load of 3 G was applied to the wheelchair in the downward vertical direction. Structural analysis was performed on Mg alloy, Al alloy (including Al-Sc (scandium) alloy), and CFRP. As a result, the stress concentration areas on the frame were identified, the Mg alloy used for the wheelchair was selected (AZ61 and AZ91 wheelchairs with a hybrid structure of CFRP), the thickness of the Mg alloy frame material was determined, and the superiority of the Mg alloy in reducing weight was assured. Next, the wheelchair was subjected to a vertical load fracture test using a Large-scale material testing machine (Figure 3), and the load at which the first crack or rupture on the frame occurred with a clapping sound was used as an index of rigidity and durability. The developed wheelchair was guaranteed to withstand a load of more than 1 ton. The fractured parts were analyzed by electron microscopy to confirm the importance of welding technology as well as material selection and adjustment in obtaining wheelchair durability, and the information was shared with wheelchair designers and fabricators.

Fig. 2 Structural analysis (vertical 3G)

Fig. 3 Fracture testing using Large-scale material testing machine

Table 1 Structural analysis results when frame materials are replaced with Al alloy and Mg alloy

Table 1 shows a comparison of structural analysis when the wheelchair frame materials used as the basis for this technology were replaced with Al alloy and Mg alloy. The deformation and vibration modes for both materials were similar, with the Al alloy having a higher Young’s modulus and therefore lower deformation. However, no significant difference in maximum stress was observed. Since the material strengths are almost the same, it is confirmed that the weight reduction effect of the Mg alloy is significant, as mentioned above (The table also includes the results for the case where a scandium (Sc) alloy is used for reference. (For reference, the table also includes the results of replacing Mg with Sc (scandium) alloys, where Sc alloys are actually Al-Sc alloys, and are listed as Sc alloys to distinguish them from conventional Al alloys in the table．

Akira SHIONOYA(Nagaoka University of Technology)
Yukio MIYASHITA(Nagaoka University of Technology)
Shigeharu KAMADO(Nagaoka University of Technology)
Taiki NAKATA(Nagaoka University of Technology)
Ryuichi IIHOSHI(OX ENGINEERING CO.,LTD.)