Originally Posted by
MechEngr
When the Alaska jackscrew failed there were two nuts, one for "backup,"
No, the DC-9, MD-80, MD-90, and B-717 all have a single nut but that single nut has two threads. ( A two start thread.). Jack screws with two independent nuts are typically arranged so the backup carries no load until the primary fails. In the two thread single nut design both nut threads wear together and they wear quickly if they are not lubricated.