Hard Reserve Pull

[hackish 8]

  Quote

If you felt the resistance for the seal thread you must hate it when there is a pea under your mattress.
As for an aerospace engineer just what is it you are going to ask him, how mush force does it take to break a double wrap of 4+ lb seal thread?

Something that may contribute about 1/3 of the pull force you may feel. I don't know.

What will I ask the engineer? How to best determine the static versus kinetic friction, if they are usually significantly different or not. I'll go to the aerospace lab because it tends to pique their interest when you ask questions for something that flies. I also know they have instruments that are much better than my "Made in China" fish scale.

-Michael

[Scrumpot 1]

C'mon guys, puh-leeze. Enough of these silly games. If the pull force being exerted already EXCEEDS the breaking / tensile strength rating of the thread, when it gets to its fully taught point (which it clearly does/will) - the thread will not STOP it, and it will break!

It will not ADD, even anywhere near pound-for-pound, ADDITIONAL pull force being required, unless it was part of the initial structure (static friction as it is being referred to), that was tieing down / holding the pin in place, in the first place. And in that case if it was - the seal/thread was installed wrong.

Watching this thread now is truly getting painful. You guys keep playing with each other and your words over this. Have fun. I'm out.

Blues,
-Grant

coitus non circum - Moab Stone

[hackish 8]

  Quote

C'mon guys, puh-leeze. Enough of these silly games. If the pull force being exerted already EXCEEDS the breaking / tensile strength rating of the thread, when it gets to its fully taught point (which it clearly does/will) - the thread will not STOP it, and it will break!

You are wrong.

Did some quick testing last night. Took a grommet in a vise, ratcheted a piece of cypress cord down down on a pece of 3/32 polished stainless wire until it was at 20 pounds of pull force. Believe it or not this is a lot of force!

Then I measured the static and kinetic friction. 2 pounds different. Once it was moving I reduced the force until it stopped. Stopped at just under 18.

I didn't have a sealing thread around but applying 20 pounds of pull force would have applied at maximum 2 pounds on the sealing thread whether it had been tight or not.

-Michael

[Scrumpot 1]

I said with the exception of an extremely slow pull. It is clear that in your test however, that is precisely what you are instead doing!

The reg states very clearly how and to what spec's to test, yet you insist on adding your OWN (non-applicable and inappropriate) variables. It is clear as well that you already absolutely and abundantly already know everything, and so much better than everybody and anybody else. This dialogue as such then, is also clearly with you, not worth continuing. It's already gotten way too far afield of the OP's original post and intent anyway - and I certainly don't think that either of us with this, are any longer adding anything substantive to that here with this any longer.

All my best wishes to you Michael, in any case for your practical (and not just theoretical) both jumping and rigging career.

Blue Skies,
-Grant

coitus non circum - Moab Stone

[hackish 8]

Trust me this wasn't an extremely slow pull and that's why I used a 42" long "pin". The only real difference in speed is the amount of inertia your hand has. Although kinetic friction does change with V it doesn't change a lot in this situation. The dept of aerospace may be willing to let me use some of their equipment to prove the real life situation.

The guy I spoke to said that it's nearly impossible to calculate the kinetic friction so you just have to measure it. He concurred with my evaluation that you should end up with on average the kinetic force required plus the ultimate strength of the sealing thread * the number of strands.

-Michael

[erdnarob 1]

Some physics here; when you pull your reserve handle, you have generally 2-3 inches of slack at the end of the cable (the cable ending ball is 2-3 inches lower than the top of the handle where the cable goes thru). Then at the pull, for 2-3 inches there is virtually no resistance and when the cable ending ball hits the top of the handle, your arm has already acquired some momentum and therefore the cable is subject to a sudden accelerating force (this is for D shape metal handle). Because of the momentum one shouldn't really feel a big resistance from the pin. Plus as you said adrenaline makes it nice and easy. About the kinetic coefficient of friction being less than the static one, OK, but I don't think that kind of consideration makes a big difference again because of the slack on the cable and the arm momentum. I like to pack my reserve tight (about 20 lbs) for safety reason. I have had 5 cut away but never I felt a significative resistance. But before the beginning of the jumping season, I always practice myself on a suspended harness. Maybe "practice makes perfect" is the way!!!

Learn from others mistakes, you will never live long enough to make them all.

[riggerrob 643]

I am not quite sure why you guys are fussing about a tiny piece of thread.

Half the time, the thread frays or breaks half way through the repack cycle .... ergo .... the thread contributes nothing to the pull force.

That is why I quit installing sealing thread on school parachutes ... I got tired of people panicing and running about worrying about a tiny piece of broken seal thread!

[hackish 8]

We lack anything else to fuss about. I'm purely curious about the tiny piece of thread. We know with the existing specs it works so further research is unlikely to contribute anything to safety but my objective is to determine the actual amount of pull force required.

-Michael