Originally Posted by
OldnGrounded
Hmmm, indeed. It's really inconceivable that friction between the jackscrew and the ball nut could even come close to the axial forces transmitted from aero loads on the stabilizer.
I'm probably going to regret the following post as it is made to *** possibly *** explain why some posters are so insistant on friction- lubrication -sideloads - issues on the jackscrew
But here goes
The MD80 ( DC-9 ) crash off the coast of california investigation shoed the problem was with a worn thread- pooorly lubricated jackscrewe,
https://www.ntsb.gov/investigations/...ts/AAR0201.pdf
And this excerpt
The interior of the recovered acme nut contained no intact threads, although therewere small ridges where the worn nut thread remnants had previously been attached. Theinner diameter of those ridges was almost identical to the outer diameter of the acmescrew threads, which is consistent with the worn acme nut threads having been sheared offby the acme screw threads during airplane-nose-down trimming movements
Note this refers to an ACME screw thread as shown later in the report on page 145
page 145
Figure 20. A graphical depiction of the stages of acme nut thread wear to the point of fracture.
HOWEVER- as has been shown and discussed for months in the 737 MAX threads, Boeing has used on 737 and following- ( and perhaps on 707-727 ) a recirculating ball "nut" on a jackscrew ( similar to that used on automobiles steering systems since the 1950's at least.
yes it does require lubrication- yes it can fail under extreme conditions- BUT it is unlikey- and VERY very low probability to fail by stripping threads or having extreme friction issues like that of the old fashioned house moving jackscrews designed to eliminate backdriving which is a design issue much more significant in the ballscrew-jackscrew
Hopefully - the above comparison will tamp/slow down the ' friction' side load- lubrication ' problem discussion with a ' new MAX " as being a significant issue in the MAX crashes.
Curious to know how many reports of a lube problem with the Boeing used ball-jackscrew system ?