Maybe a dumb question, but wouldn’t you have expected it to break at the target max load? I would have thought it being stronger than required would mean it was heavier than necessary?
One day, I got called into the stress group, where they told me they were unhappy with my parts. I asked why, they said there just barely strong enough for the ultimate load, only about 1% over. They said they'd feel more comfortable with 10% extra strength.
I said this was no coincidence, I used math to size the part to the load, rather than guessing at a size and checking to see if it was strong enough. The requirement was the ultimate load, which is 150% of the maximum load that could ever be expected.
I asserted that I designed to the design requirement, and an extra 10% would make it overweight. Brat that I was, I said if you guys were unhappy with the design requirement, increase it. They grudgingly signed off on it.
The reason it didn't break on the test stand is because all parts vary in size due to manufacturing tolerances. It was sized to pass the ultimate load test under the most adverse size allowed under the tolerances. Odds are, it'll be a little stronger.
For comparison, spacecraft have (I think) an ultimate load of 110% of the max load. The margins are awfully thin, but they have little choice. If you really want a math heavy engineering job, design spacecraft.
BTW, the stress group signed off on the jackscrew size. If it had failed the ultimate load test, both them and I would have had a black eye. As it turned out, the test guys were embarrassed by their bent rig :-) and fortunately that wasn't really a problem.
Did they want the extra 10% buffer for any particular reason?
Reason I ask, I always agreed with your take on not over-designing, philosophically. I had a couple of professors who made a big deal about being conservative/ round-up on calculating design elements, but once you tally up the different loading cases, you shouldn't ever bump the total required strength (again) beyond whatever the required multiplier is.
However, when faced with real life projects, I learned fairly early to ignore that advise, mostly because of the risk of having to redesign due to a changed design requirement. Of course I've also never worked for a company as structured as one like Boeing, and I've also encountered LRFD 99% of the time over ASD.
It's one of those life lessons that's never really sat right with me, but I still consider to be the rational choice.
They said they just were more comfortable with more margin. The stress group was an independent organization to avoid conflicts of interest with the design group. Our job was to design, their job was to verify. If parts broke, they got the black eye. If the airplane was overweight, design got the black eye. We both had to agree, and it was a system that worked well.
Most design engineers didn't work out the stresses themselves, they just hoped to pass the stress group. It was pass/fail for them. In my not so humble opinion, they were making overweight parts. I was interested in using math to sculpt perfect parts :-)
For example, the inside diameter of the jackscrew was specified to the ten-thousandth of an inch, something like 2.1834..2.2096. So I'd get the "why not make it a nice round 2.19..2.20?" I'd reply because if it came in at 2.205, it would get rejected, even though it was perfectly usable. And I got my way, because the jackscrew forging was a very, very expensive part and rejecting a usable part did not make anyone happy.
(Rejected expensive parts often got bounced back to engineering to find a way to salvage them. I just was doing the math in advance so they wouldn't have to.)
If it's a rabbit hole you're interested in, then check out the history of geodesy / geodetic surveying & the design of mechanical / optical surveying equipment- especially theodolites & auto-levels (and even chronometers and astronomic observatories, if you want to go 4-dimensional).
It was the "industry" that had a large hand in innovating this technology, which led to the smaller versions used in manufacturing (made even more interesting due to the fact it was all funded to better artillery, and ultimately ballistic missle targeting, sans-GPS.)
I'd be happy to steer you in the right direction, if interested. It's a personal favorite.
Going beyond that, to tens of millionths or sub-micron, is where things get nutty with special climate controlled rooms. Digital indicators can easily read with such precision, but controlling for factors like radiated body heat become important.
Reference measurement is usually done with gauge blocks and pins. The development and popularization of these sets is basically how mechanical parts became standardized.
If you're interested in that sort of thing - bootstrapping precision and the like - check out "The Foundations of Mechanical Accuracy".
https://www.youtube.com/watch?v=dzIsR4Mg158
"ball screw" and "lead screw" are your search terms. For aerospace it's gonna also be center drilled to save weight as well.
It's forged slightly oversized, then machined down to spec.
Forgings are about 3x stronger than a casting, and are of a more consistent quality, which is why when I increased the HP for my Dodge, I spent the extra bucks to get forged spinning parts (usually they are cast).
For example, if you're testing the integrity of a pressure vessel, the standard is to test it to 110% the rated maximum allowable working pressure if the medium is a compressible fluid (like air) or 150% if it's a 'non-compressible' fluid (like water, although everything is compressible to a certain extent). This is because the compression of a fluid stores a lot more internal energy if suddenly released. (Don't quote me exactly on these numbers because it's been a looooong time since I've had my nose in ASME standards)