Practical Implementation of Functionally Graded Lattice Structures in a Bicycle Crank Arm

Functionally graded lattice structures (FGLS) were studied thoroughly for the past years, mostly focusing on specific synthetic tests in the context of additive manufacturing while rarely being actually applied outside of this specific domain. This paper examines a way to practically implement them in a commonly used appliance and study their potential for its improvement. A bicycle crank arm was chosen for this purpose, and a solid aluminium reference model (Shimano FC-R450-453) is used as a performance baseline. The novel design is composed of a hollow body containing a beam-based, non-stochastic, functionally graded lattice structure and is planned to be manufactured on a Markforged Metal X system using 17-4 PH stainless steel. It aims to increase the total stiffness under EN ISO 4210-8 norm loading conditions compared to the reference model while limiting mass and stress values to acceptable degrees. Two crank arm variants, containing a face-centred cubic (FCC) and re-entrant auxetic lattice respectively, are optimised by locally altering their beam radii in eight separate regions. The displacement at the load application point is minimised using Altair OptiStruct and HyperStudy. The reference crank, weighing 213g, exhibits a deflection magnitude of 7.1mm in the most demanding load case while the newly designed and optimised versions only showed displacements of 2.52mm (FCC lattice, 340g) and 2.58mm (re-entrant lattice, 339g) respectively. In addition, the stress distribution was significantly enhanced compared to the reference model, as the latter would not pass the fatigue tests required by the norm. This demonstrates that FGLS, in combination with high-strength materials and additive manufacturing, can increase the performance of many parts, although in this case, with a trade-off in terms of its mass. In future projects, it might be considerably reduced by utilising alternative lattice types lattices or other materials while preserving the benefits of FGLS.