Two combined four-bar mechanisms have two functions: lift and collapse. In the current design, high effort was found for the collapse function. Axiomatic Design was used to analyze and optimize the current design. The customer domain was mapped into the functional domain by specifying customer needs in terms of functional requirements (FRs) and constraints (Cs). Design parameters (DPs) were identified in the physical domain for each functional requirement. Design matrices were then defined to characterize the product design. The two combined four-bar mechanisms have two functional requirements at the highest level: lift and collapse. The corresponding DPs are: lift four-bar linkage and collapse four-bar linkage. Through zigzagging to decompose to the next level, the design was found to be coupled. At this level, a torsion spring was selected as the DP for minimizing the lift effort. However, this DP was also found to affect three next-level FRs for the collapse: the front leg rotation, the rear leg rotation and the float link rolling. This coupling greatly increased the collapse operation effort because the spring was useful for the lift function but was a hindrance for the collapse function. The design was decoupled by adding two locks, which are called floating link lock and front leg lock, and an extra plate which is called locking plate. The new modified design can improve customer satisfaction because when the rear lock and the floating link lock are released, the torsion spring will assist the lift, and when front lock and front leg lock are released, the torsion spring will not resist the collapse.
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