How to understand the principle of patent equivalence that basically achieves the same function and basically achieves the same effect by basically the same means?

The example you gave is not appropriate. If the metal bar connection needs protection, then it is infringement to connect with copper bar.

If it is clearly stated in the claim that products are connected by iron bars and actual products are connected by copper bars, there is no infringement, which involves the inclusion of upper and lower positions.

A more appropriate example may be that in the claim, the chain is used to connect the transmission, while the belt is actually used to connect the transmission. This may be judged as "basically achieving the same function and achieving the same effect by basically the same means".