在皮带头部安装倒闸的目的是为了防止皮带因自重的原因倒转。
参考答案：对
It is nice to do something for someone else out of the blue and not to expect anything ______.
A．in relief
B．in return
C．in turn
D．in need
密封圈的内径与电缆外径差不应超过多少？
