渐伸线又称渐屈线，渐变线。当一根绳正沿着另一曲线绕上或脱下时，它描出一条渐伸线。渐伸线的形状见于鹰嘴、鲨鱼背鳍和棕榈树悬叶尖端。机器齿轮，齿两侧曲线(齿郭曲线）大多采用渐伸线。
在测量既有铁路曲线时，传统的测量方式是偏角法，计算曲率和拨距的方法是渐伸线法。
The traditional method to measure the curves in exiting railway is the method of deflection Angle, while the method to calculate the curvatures and the deflection distance is the involute method.
当点沿曲线移动时，相应的曲率中心的轨迹曲线称为曲线的渐屈线，而曲线称为曲线的渐伸线。
When the point moves along the plane curve , the trace curve of the point is called the evolute of the curve , denoted by, and is called the involute of the curve .
应用推荐