Nut out edge is hinged with another elastic rod.Another ends of the two elastic rods are respectively hinged with the wrist to form isosceles triangle.

• 螺母外缘上也铰接有一弹性杆，两根等长的弹性杆的另一端分别铰接在集中质量块的同一销轴上，构成一底边长可变的等腰三角形；