two level action ; two position action
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双活塞推动，齿条齿轮传动，结构紧凑，装配位置对称，动作迅速，通过简单的活塞颠倒可以改变输出轴转向。
Dual-piston driven. toothed bar and gear driven, structure is compact, position of assembly is symmetrical. Motion is swift. turning of output axis can be changed through simple reversal of piston.
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模块上移
模块下移
不移动