Originally Posted by
Tobin
I don't think this is correct. There are a lot of posts in various threads to look through, so forgive me for being lazy and not quoting the exact one, but a while ago I did see an explanation like this:
The clutch mechanism is such that even a relatively small opposing force on the trim wheel (such a pilot grabbing the wheel without the handle) will engage the clutch. However, absent any resistance of the wheel, the electric trim motor can drive the jackscrew itself with very high torque and against a high resistance; far more than a human could.
This implies that the mechanism distinguishes between resistance from the wheel versus resistance from the jackscrew, yet still maintains direct correspondence between the wheel and jackscrew motion.
It's not immediately obvious to me how such a mechanism would be designed, but I don't doubt that it is possible to design one given sufficient smarts. Can someone confirm that this is how it actually works on a 737?
I puzzled over this strange two way game of rock/paper/scissors some weeks ago, but I've not been able to find anything conclusive as to how it might actually work. However it's possible to imagine a fairly simple mechanical system that would allow it. For example there could be two discs, one attached to the jackscrew and one to the trim wheels, face to face and with several rollers between them and with a spring pressing them together. In one of the discs there would be grooves that were circular segments that varied in depth, so that they got deeper and then shallower and the rollers would normally sit at the deepest point, but any rotation between the discs would make the rollers run up the slope and push the discs apart, but they would soon hit the end of the groove so they couldn't roll any more and then the two discs would be forced to turn together. The moving apart of the discs would push apart a set of clutch plates/dogs disengaging the electric motor drive. I'm sure there are other ways it could be done.