所谓一次约束就是连接或构成某一事物能够正常运作的最简形式。譬如：电机被螺栓固定在底座上，窗被安在窗框上，钻孔时钻头在钢件中转动等，都是一次约束的表现形式。所谓二次约束，就是在一次约束的基础上再加上一道约束，或叫二次防护。其特点是：二次约束虽然一开始就参与? The so-called primary constraint is the simplest form of connecting or composing a thing that can function normally. For example: the motor is bolted to the base, the window is placed in the window frame, drill when the drill bit in the steel rotation, etc., are a form of constraint. The so-called secondary constraint, is a constraint on the basis of a constraint, or called secondary protection. Its characteristics are: Although the two constraints involved in the beginning?