Originally Posted by
DaveReidUK
I can't recall ever having seen an O-ring used as a seal between moving parts. Maybe I've led a sheltered life.
No problem, let me open your eyes.
It's not "moving" in the sense it is supposed to move (freely).
But, when one side of the connection is (for example) connected to the sink and the other side of the connection is connected to the toilet side wall, it looks "fixed" at first sight. However, the sink and the side wall are probably not connected rigidly, more just like normal kitchen cabinets, it's connected, but apply some force and the connection moves. Even, when it's only a little bit.
So, when the whole airplane becomes "dynamic", rattling, shaking, bending, etc (even when just a little bit), that O-ring connecting starts moving, just a little bit. Though that is enough to let an O-ring fail, over time.
I hope, it's clear, what is meant with "moving".