1. Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208 e-mail:
2. State Key Laboratory of Mechanical Transmission, Chongqing University, Chongqing 400030, China e-mail:
3. School of Mechanical Engineering, Northwestern Polytechnical University, Xi'an, Shaanxi 710072 China
4. Department of Civil and Environmental Engineering and Mechanical Engineering, Northwestern University, Evanston, IL 60208
5. Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208;