其二为一种修整的线接触，展成的等螺距齿面与理论轨迹曲面沿某一空间曲线近似相切。

The new tooth surface can be prolonged by the normal profile of the internal toroidal surface, or replaced by an equal pitch helical surface.

youdao

That limit will be some arrow we can call the velocity at the time and it will always be tangent to the curve.

那个极限也就是一个矢量,我们称之为瞬时速度,并且它总是和轨迹相切的

耶鲁公开课 - 基础物理课程节选