The screw jack has strong load-bearing capacity and can be mechanically braked. The internal threads can ensure that the heavy objects supported on the lifting sleeve and top cover remain stationary and static, providing convenience for operators while also protecting surrounding people and things. Therefore, the performance is reliable and the safety factor is relatively high.

However, with the prolonged use of the screw jack and the influence of multiple overloading uses, the internal threads of the jack will gradually wear out, thereby reducing its maintenance function and causing the heavy object to slide down to varying degrees during the lifting process.

Therefore, when we find that the screw jack cannot maintain a stationary state after lifting a heavy object to a certain height and begins to slowly descend, we need to consider whether the heavy object has fallen due to severe thread wear. At this time, the threads should be scrapped and replaced with threads of the same specification and model.

It is explicitly stated in the "Regulations on the Safe Use of Jacks" that when the thread wear of a screw jack reaches 20% of the size, it must be strictly prohibited from use.