There appears some flexibility at the top of tower with high ratio of height to diameter (H/D), caused by wind load.

高径比（H/D）较大的塔在风载荷的作用下，在塔顶处会引起一定的静挠度。

youdao

However, the diameter of the filling groove (34) is smaller than the diameter (d) of the ball, thereby the ball (25) does not fall off.

然而，填充槽(34)比滚珠直径(D)略小，使得滚珠(25)不会脱落。

youdao

Pitch diameter (d) is the diameter of the pitch circle.

节圆直径(D)：节圆直径是节圆所在圆的直径。

youdao