Spindle static stiffness unloading is more than that of loading. The elastic deformation of spindle system resumes stagnantly when unloading. So the loading curve and unloading curve are not closed.

主轴静刚度在卸载时要大于加载时，卸载时主轴系统弹性恢复迟滞，并且加载曲线与卸载曲线不封闭，卸载时主轴变形恢复不到起点，进而影响加工精度。

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They believed that, unlike free-living species, parasites did not evolve, but rather sponged, stagnantly, on their hosts.

FORBES: Those Resourceful Bugs