combined critical speed的用法和样例：
例句
- This paper is directed against the reality problem that it is very difficult to measure the critical speed of the shaft system in a large scale hydrogenerator unit in operating.
针对大型水轮发电机组轴系临界转速难以实际测定的现实问题，从理论分析的角度出发，抓住水力机组轴系结构的基本特点，提出采用较为简单的模拟结构，试验研究水力机组轴系固有频率及其振型特征。
