On 12/23/2012 9:20 AM, JohnDeRosa wrote:
We had an overhead bi-fold hangar door cable snap yesterday (one of
five). Snip...

The cable had rusted through inside the guide tube on the wind up
spool. We had one hell of a time getting the old rusty cable out of
that old rusty tube. We are wondering when the other cables will
suffer the same fate.

Everything else being equal, and painting with a broad brush here, the other
cables are likely "in similar condition" as the one that has already failed,
though under 25% higher load now. Maybe in summer you'd feel warmer and
fuzzier. :-)


Questions;

- What type of cable (3/16") should be used? Aviation? Stainless?
Galvanized?
More "common sense engineering" follows...
- Given the service life rendered by the existing cables, anything of similar
material (you used the word "rust", so we're talking steel?) and cross section
should be in the same life ballpark. I'm sure you're aware aluminum has lower
tensile strength than steel for a given cross sectional area. Point being, you
won't go too far wrong "replacing with like." Assuming the same cross
sectional area, the number of strands affects only the minimum recommended
bend radius of whatever corners the cable must go around. More strands, more
bendy.
- In your use, my take is corrosion is your long-term enemy (Duh!). IOW, if
what's rusted rusted because it was a non-CRES steel (cheaper than CRES
steel), then pocketbook reality will need to be balanced against cable-life
realities (as in...it's rusted once, what are you going to change to preclude
rusting in the future? Personally, when I have to do "major/PITB structural
repairs" I like to do what I can to ensure the next repair need will occur
after I croak.)
- galvanized cable (if available) achieves its corrosion resistance from a
surface coating, meaning that once the coating is violated, corrosion has a
direct entry path. Presuming the interior metal is not CRES steel (else, why
coat it?), I'd do a kentucky windage guess the life of galvanized cable would
be closer to that of non-CRES bare steel cable, than of a CRES steel cable.

- What kind of swedged stop sleeves should be used? Aluminum?
Copper? Steel?
You won't go far wrong using a swage of similar (as possible) materials, both
for strength-matching and galvanic reasons. I'd reckon strength information is
readily available on the web for the number crunchers in your Club. In the
absence of number crunching, certainly don't go smaller than what's already
there, dry chuckle...

- How to prevent rusting inside the tube? We greased the cable end in
hopes that this will help.
Short of obtaining a tube of CRES material, regular usage will be your best
preventer of tube corrosion (if I'm understanding the question correctly).
Activation abrasion, if you will...


Thanks, John


Rotsa ruck!!!

Bob W.