After the linear rail is used for a period of time, why does it rust?

On the one hand, there may be no maintenance. On the other hand, the product you purchase is not stainless steel.

Why are the linear rails slightly bent?

This is mainly due to shipping and packaging issues.

Why is carriages moving hard´╝č

There is no place to install or do not take measures to prevent rust.

Of course, it is also possible that the inside of the steel ball is not adjusted.