When designing a "machine", estimating its life is an important study point not only for ball screws, but also for all mechanical elements.
Life is "the time from birth to death"; however, for manufactured products, it is " the period of time (or distance or number of times) the product can be used without problems".
Then, what is the life of a ball screw?

Then, what happens to a ball screw that reached its life?
The screw shaft breaks, the nut comes off, and the circulating balls fall off ... Nothing like that will happen.
In the screw groove of a ball screw that reached its life, a small round depression (of approximately 0.2 mm in diameter) called pitting appears with a reliability of 10%. This is called rated life of 90% (L-10Life).

When ten ball screws with the same characteristics are assumed, pitting appears in one. Naturally, pitting will start to gradually appear in the remaining nine as well. However, the presence of pitting does not mean that the ball screws will stop working; they can be used without problems in general applications such as conveyance.０

The pitting that appeared in the screw groove of a ball screw grows to become a crack, and then a flaking where the screw groove is peeled off. This is similar to how damages grow on roads transited by cars. When a road starts wearing out, small gaps appear in the wheel track; and the pavement starts to peel off like flakes from those gaps. This is often seen in roads with heavy truck traffic, where the peeled off portions expand gradually as a result of the weight of trucks that is repeatedly applied to the damaged portions (weak points) of the road through the tires.

Neither ball screws nor roads become unusable just because they have pitting or gaps. However, if asked whether "they can be used without problems", the answer is tricky.
Accurate positioning cannot be expected from the ball screw, and the road cannot be used as a racing track or runway.
This is the reason why the ball screw life is the "period during which the ball screw can be used without problems", and is defined as the rated life of 90%.