He developed a safety break that kept the elevator from freefalling of the rope broke.

他开发了一种安全装置，防止电梯因绳索断裂而自由下落。

He developed a safety brake that kept the elevator from freefalling if the rope broke.

他开发了一种安全制动器， 可以防止电梯在绳索断裂时自由下落。