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.
Take a look here for how the 737 - and virtually all other models of boeing and probably airbus jackscrews work to move the horizontal stabilizer . braking the turning of the jackscrew can stop movement.
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 -
see
https://www.google.com/search?q=767+ball+screw+details&client=firefox-b-1-ab&tbm=isch&source=iu&ictx=1&fir=8R_i7jhyn421DM%253A%252C5Mq 2-dsu2Z5RgM%252C_&usg=AI4_-kS87Ir4HB0qg4Sz928mdwMREx_8eg&sa=X&ved=2ahUKEwiX9J2ihufeAhWG HXwKHXRqDigQ9QEwAXoECAQQBg#imgrc=8R_i7jhyn421DM:
http://www.beaver-online.com/documen...Stabilizer.pdf
https://www.aerospaceonline.com/doc/...actuators-0001
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 )
And FWIW - in my earlier days at Boeing ( 1980's ) as an engineer in tooling/manufacturing area, I had to rig up a simple way to check the jackscrew install and horiz stab movement of the first 767 - long before the installation of the electrical ' drivers '. The spline drive connection for the electric motor would not fit any standard nut driver- so I made a polyurethane mold of the spline in a larger socket, and then used a simple low power ( air motor ) low speed nutrunner to turn the jackscrew and move the stabilizer over the full travel and adjustments.
Given that - I'm guessing that the cockpit trim wheel probably drives by cable a ' pulley ' attached to the jackscrew - which of course requires multiple turns to get a relatively small movement- and vice versa.