Cylinders may be single-acting or double-acting. In the case of single-acting cylinders, motion is accomplished by the pressure when the piston as moved back to its original position by a spring or some external force, or gravity. In the case or spring return, the output force available hydraulic pressure is modified by the resistance of the spring. 液压缸起重器和柱塞英文文献和中文翻译(4):http://www.751com.cn/fanyi/lunwen_47968.html