Originally Posted by
A Squared
Not a 737 systems expert, but I'm not sure the answer is quite so simple as is being made out here. In fact, jackscrew driven mechanisms *can* move if the actuating nut is loaded and there's not enough resistance to the jackscrew turning. The landing gear on the airplane I fly is actuated by a jackscrew and recirculating ball nut. There is a braking device on the jackscrew to keep the landing gear retracted. without it, the weight of the landing gear on the ball-nut would be enough to turn the screw and the gear would descend. In fact that is the emergency gear extension procedure, to unlock the screw and the gear extends by it's own weight. On the other end of the cycle, there is a friction washer assembly whcih keels it from turning when extended and the aircraft weight is on the wheels. Also, Cessna 180's have a trimmable stabilizer actuated by a jackscrew and on the early models, the air loads at high airspeed would cause the trim system to move. I happen to know this because I own one. So there are two examples of aircraft systems in whcih "Pushing on the nut" does indeed "rotate the Rod" So what does keep the jackscrew on the 737 from turning under air loads? Is there a braking device? Does the gear train of the drive motor provide enough resistance?
Whether or not a 'jackscrew" can be back driven depends on the thread pitch and friction ( braking ) factors. . For a simple example- a course ( large) pitch ( few threads per inch ) is often used for jacking up buildings and heavy objects and are NOT likely to be back driven ..
BALL screws can be back driven, depending on design parameters. SDo just how much ' braking' is needed to prevent rotation is a design factor. Thus we have the problems/questions that arise when a simplified explanation in a blog is used instead of a multifactor detailed design analysis. Does BA use a brake ? probably- what friction factors are used for braking torque and backup operation really are are unknown by this SLF, and no doubt over 90 percent of the posters. One can assume that horiz stabs are NOT designed for ' free fall' like a landing gear !
Note in my prior comments above that I did carefully say " UNLESS THE SCREW TURNS " which I admit was an over simplification
No attempt at a good deed goes unpunished !
https://www.linearmotiontips.com/how...ll-back-drive/ may provide some clues