The importance of environmentally-friendly energy production has been growing globally, and studies on energy storage technologies are underway, to supply produced energy to consumers. Flywheel Energy Storage System (FESS) is physical energy storage technology, that stores generated electric energy into kinetic energy in the rotor. To design the FESS with a high-strength steel rotor, that is inexpensive, recyclable and easy to manufacture, mechanical and electrical components such as a rotor, bearings, etc. are required. Among these, safety of rotor and bearings is critical, because the rotor with high rotating speed may cause axis failure or fracture of the rotating body. Proper size of a rotor for required energy storage and radial, axial forces generated by the spinning rotor was calculated, considering gyroscopic forces acting on the rotating body. Based on the calculation, adequately sustainable angular ball bearings were selected. As a result, by conducting structural, modal and critical speed analysis, safety verification is presented pursuant to the American Petroleum Institute (API) publication 684.

Critical speed analysis was conducted for a active steering bogie prototype, developed to improve the curving performance of railway vehicles. The critical speed for the design concept was about 169.2 ㎞/h. To validate the analysis result, we performed a critical speed test for the prototype bogie using a roller-rig tester. The test results showed that the critical speed for the prototype bogie was about 165 ㎞/h. From the analysis and test results, The critical speed for the prototype bogie was determined to be 165 ㎞/h. Considering the maximum operating speed of the test vehicle is 100 ㎞/h, the prototype bogie is considered stable.