I have learned in my life that anything made of rubber -
be it a hose, belt or o-ring - should be replaced after 10
years. This is true on your airplane, motorcycle or car.
If you can replace the rubber, and stop the metal from
grinding / cracking / rusting, it can last a very long time
indeed. And work very well.
Eventually the metal crystallizes and becomes brittle,
but by that time, all the people that designed and built
it are long dead, so it's difficult to complain, unless you
have the resources of Mackenzie King. Time to anneal.
Reminds me: how can you spot a warbird pilot? He's the guy
in the flight suit that's always on his cellphone to maintenance
after every flight :^)
Those aircraft were intended to last six months, a year tops.
At the end of that time, they were either reduced to rubble
by their combat usage, or they were obsoleted by updated
designs. Often both. And here we are, 80 years on, and they're
still flying. Amazing. All the guys that designed and built them
wouldn't believe it. Long dead, all of them. Hell, the guy that
designed the Spitfire died before WWII.
https://en.wikipedia.org/wiki/R._J._Mit ... e_Spitfire
No one gives a shit, but the pretty wings of the Spitfire were
designed by a Canadian:
https://en.wikipedia.org/wiki/Beverley_Shenstone
-- EDIT --
Very few people have any interest in chemistry, and pretty well
zero people give a shit about metallurgy, despite how incredibly
important they both are to your continued survival.
Some light reading:
https://www.kitplanes.com/coming-into-the-cold/
It’s important to note cryo is not hardening, nitriding, tempering or anything else. It’s a rationalization of the material’s crystalline structure, and as such, the treatment is effective completely through the material. It is not a surface treatment and can’t be machined off like a heat treatment or typical hardening processes. In part, it can be considered a speeding up of the aging process metals go through at vastly slower rates at room temperature.
What really got Ly-Con principal Ken Tunnell hot on cold treatment was Lycoming 540 crankshafts. Ly-Con has long supplied engines—almost all of them lightweight, parallel-valve 540 Lycomings—to major airshow aerobatic performers (Jim LeRoy, Sean Tucker, Skip Stewart), and between 2003 and 2010, about 75% of the Red Bull air racers. This group of Type A hard chargers could figuratively wreck an iron ball, and like a frat house property manager, Ly-Con has certainly learned much from these guys.
One such lesson was the best-prepared Lycoming 540 crankshaft, if consistently subjected to tumbling aerobatics, will break the pendulum counterweight mounting ears at between 400 and 500 hours. The failure will occur only during cruise flight at low engine speed, usually around 2200 rpm and never during aerobatics. “For a while I thought I had a propeller problem,” says Tunnell, “but then I broke them with Hartzell, McCauley, and composite props, so that wasn’t it.”
Thanks to so much exposure to hardcore aerobatics, Ly-Con broke 27 crankshafts in precisely the same way. Then they tried cryogenics and haven’t broken one since. Same engine builds, mainly the same pilots, same routines, and no failures in over eight years. That does make you say, “Hmmm…”
At first having parts cryogenically treated elsewhere, and then buying his own equipment, Tunnell has experimented with chilling seemingly anything, including spark plug wires, ignition coils, golf balls, car parts, industrial items such as 1-inch bolts for oil wells and even a tuning fork. The latter was a test, as Tunnell thinks the rationalization of a metal’s structure could have measurable affects on its harmonic properties. After treatment the fork rang only half as long, yet test labs he sent it to said it didn’t change the vibration frequency, “not one iota.”
Tunnell admits cryogenics is still not understood—at least by laypeople—and ultimately sums up by saying, “It’s a black art…and there’s no cookbook for what works, exactly.”
But he also notes, “No one really knows precisely why cryo works, but it does.” And to that end, he cites a litany of informal experience with items made better by cryo.
Those golf balls, for example, drive 25 yards farther; Ly-Con’s carbide cutters last twice as long; auto racers using Ly-Con’s cryo services report clutches that stay crisp much longer, better ring seal, less driveline gear wear, and on and on; farmers say plows and discs last longer; and those Grade 8 1-inch bolts used in industrial applications used to snap off right at the head, but a batch of junk, at best Grade 3, overseas bolts was cryoed and proved at least as durable as the untreated Grade 8 fasteners.
That last example introduces the well-regarded theory that the lower the quality of the metal being cryoed the better the results. More supporting evidence for that comes from a paper by Debra Lynn Smith of Marquette University studying silicon steel valve springs; she found the better the spring material the less effect cryo had on it, but it did help in all cases.
More Ly-Con anecdotal experience is valve springs cryoed as individual pieces and left at their free height work great, but if the springs are compressed during cryo, they won’t return to their free height.
If a Lycoming flywheel and ring gear are cryoed as an assembly, the ring gear becomes loose on the flywheel. But if the flywheel and ring gear are separated first and cryoed separately then the ring gear still must be heated to be refit to the flywheel and remains tight in service.
Following the same logic, if Ly-Con wants a set of low-tension piston rings, they cryo the rings in an undersized cylinder. The rings seem to relax, offering reduced-drag cylinder sealing to racers wanting the last bit of power, or efficiency specialists looking for the best fuel economy.
Any lessons from the above? Anyone? Bueller? Bueller?
