What keeps the trim from slipping once you have it where you want it? A brake?
Originally Posted by
Winemaker
The threaded rod in the trim assembly is rotating, effectively, in a nut moving the nut up or down the rod. Rotating the rod moves the nut; pushing on the nut doesn't rotate the rod. No brake required.
Originally Posted by
CONSO
Pushing on the ' nut ' will not move the stabilizer over a few thousands of an inch up or down- there is little to NO slack absent extreme wear >
NOTE added - technically, Boeing uses a BALLscrew on the 767/747/and SOME 737 - the difference being the " nut " contains a path for recirculating ball bearings -
Unless the screw turns, nothing moves unless a major part breaks. Note the same concept has been used in automobile steering for decades - although newer models use electric motors to assist turning ( power steering )
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?