A spring is the elastic component that can make the transformation between potential and kinetic energy by storing the energy through deformation. Because of this, springs are utilized in various applications, such as shock mitigation, mechanical shape recovery, etc., So the kinds of spring are differentiated into various forms such as coil spring, leaf spring, etc. However, even though various kinds of springs already exist, it is getting harder to meet requirements when springs are utilized in narrow and complex design spaces. For this reason, researchers tried to develop custom springs that can achieve custom requirements. Through the development of custom springs, the performance of mechanical systems has improved, and new mechanical systems have been developed, such as series elastic actuators, variable stiffness actuators, and compliant mechanisms. However, the customising of springs only concentrates on the stiffness modelling or function of load-displacement, not on maximizing energy storage. It is challenging to maximize the energy storage under the narrow and complex design space without interference. The case of the compact variable gravity compensator, called CVGC, which was developed by our group, was facing the same difficulties. Therefore, to maximize energy storage, we propose a hybrid spring system, called CoiLeaf spring that maximizes space utilization. By dividing the design space based on the characteristics of each spring, we set the design constraints of each spring. In the case of the coil spring, we have optimized using the mathematical model and selected commercially available products which was adjacent to the optimized result. The leaf spring especially was optimized by linking finite element analysis software and numerical software through Python scripting. The energy storage of the CoiLeaf spring was 11.38 J. Compared with general systems utilized in the design space of CVGC, the CoiLeaf spring system achieved a 5.47 times higher energy storage.

Funding Information
* The Industrial Strategic Technology Development Program (No. 20007058, Development of safe and comfortable human augmentation hybrid robot suit) funded by the Ministry of Trade, Industry, & Energy (MOTIE, Korea)