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以前用切线支距法设置平曲线时,一般都惯用这样一个算式,即y=x~2/2R(y、x、R表支距、横距、半径)。读者根据自己在测算中的体会,认为这仅是一个近似的而且是有条件的算式,也就是说只能基本上临时应用于半径较大、横距较近即弦切角较小之处,若半径较小而横距较远即弦切角愈大之处,则其计算结果就往往小于应有的y值,愈远愈悬殊。我们从下面附图可知:因为切线x与弦c夹着一个弦切角,而c、y、x三者系一直角三角形,很显然斜边c要大于直角边x,愈远则其悬殊愈大,也就是说:y=c~2/2R而≠x~2/2R但事实上c要按所对弧长a引用
In the past, when setting the flat curve by the tangent offset method, an equation is usually used, that is, y = x ~ 2 / 2R (y, x, R table distance, horizontal distance, radius). Readers according to their own experience in the calculation, that this is only an approximate and conditional formula, which means that can only be basically temporarily applied to a larger radius, the shorter the distance that is less stringers, If the radius is smaller and the horizontal distance is longer, that is, the bigger the chord cut angle is, the calculation result is often less than the value of y that should be. We can see from the following figure: Because tangential x and string c sandwiched by a chord cut angle, and c, y, x are three right-angled triangle, it is clear that the bevel c is greater than the right-angle edge x, the farther the more its disparity In other words: y = c ~ 2 / 2R and ≠ x ~ 2 / 2R but in fact c according to the arc length a reference