Originally Posted by
DaveReidUK
Correct me if I'm wrong, but the coarser the thread pitch (all other things being equal), the more likely a jackscrew is to be back driveable (the spiral ratchet screwdriver principle). Is that what you're saying?
I can't see air loads on the stab being capable of overcoming the combined braking friction in the jackscrew, gearbox and actuator unless we're talking about a previously undisclosed failure in addition to the issues already discussed.
Mr Reid, you are correct.
There is a point where a coarse pitch, usually on an Acme threaded bar, will become "non-reversible" as a function of reduction in pitch and concomitant increase of friction between the nut and the leadscrew. However, with recirculating ball systems where the leadscrew has a rounded threadform conforming to the radius of the balls, the frictional element substantially is reduced which, of course, is the purpose of having the ball race. Nevertheless, even with this system, there will be a point at which the mechanism becomes non-reversible. Hardly a definitive observation but looking at the video above, it would seem that the Boeing arrangement possibly might be sufficiently low-geared to prevent a reversed actuation.
Also, it is possible that in order to achieve optimal screwjack speed, the motor drive would be via a worm/wheel arrangement, thus making the system truly non-reversible.