pic: Telescoping Arm Warning

[daltore]

loading photo ...

[Tom Line]

Thanks for the pic. Your post brings up a good point. Not only do you have to be wary of mechanical advantage. Putting a knot in a line ('rope' for you non-sailors) will lower the breaking strength by nearly 40%! The 'correct' way once you know your required line lengths would be to use a thimble (not the sewing kind) and have the line spliced, or you can mearly account for the reduction in strength due to the knot and use larger line.

[gblake]

Originally Posted by Tom Line

Thanks for the pic. Your post brings up a good point. Not only do you have to be wary of mechanical advantage. Putting a knot in a line ('rope' for you non-sailors) will lower the breaking strength by nearly 40%! The 'correct' way once you know your required line lengths would be to use a thimble (not the sewing kind) and have the line spliced, or you can mearly account for the reduction in strength due to the knot and use larger line.

This is an asterisk on Tom's good advice:

Anything (including a thimble) that puts a curve into the line lowers the force it can survive without breaking. The fibers on the inside of the curve go slack a little bit and contribute less to the strength of the total line; and consequently the fibers on the outside (that are now carrying more load) break under less load than a straight line could survive. Each individual fiber is as strong as as ever, but fewer of them are sharing the total load.

Using a thimble's "gentle" curve reduces the breaking point less than an ordinary knot with a severe (small radius) initial curve; but I wouldn't want anyone to think that using a thimble allowed you to avoid the strength reduction entirely. Even with a thimble you have to account for some loss of load-carrying ability.

Blake

[Ahnxlazyman]

your stick figures weren't wearing safety glasses. unsafe!

but I hope you guys solved the problem though, and I hope no one got hurt

[IndySam]

This is my biggest fear when it comes to suspending from another robot. Will you be willing to trust all your hard work to another teams calculations and design?

[vivek16]

It seems to me that the easiest solution would be to triple or quadruple up your line. Effectively gives you a 4000 pound breaking limit instead of 1000.

We're using parachute cord (which has a breaking point of 500 pounds) and are quadrupling it. It's a lot easier to tie than spectra.

[Sh1ine]

Check out AmSteel Line. It is designed to replace stainless steel rigging on sailboats. Size for Size AmSteel is stronger then stainless, at 1/7th the weight. Plus, it is very flexible.

[daltore]

Yeah, I'm actually going to West Marine Supply tomorrow to pick up some 3/16" AmSteel Blue line. Instead of the 1/8" harpoon tether cord we've been using with a breaking strength of 1050 pounds (made of Spectra A, the weaker of the UHMWPE forumlations), the new stuff will have a breaking strength of 5000 pounds and is made of Dyneema (next step up, right before Spectra B).

Thanks for all of your suggestions, and the concerns for our safety. Richard and I were fine, as we were expecting a hitch pin to break anyway, so we were holding on in a way that allowed us to both catch the robot and land on our feet.

Unfortunately, we don't have the luxury of using a thimble in our setup, as it would be too large, and our tensioning system uses eye loops that are welded shut, so the standard overhand knot using the loop won't work. Do you have any suggestions for the best kind of knot to use in this situation?

[KC1AJT]

Originally Posted by daltore

Yeah, I'm actually going to West Marine Supply tomorrow to pick up some 3/16" AmSteel Blue line. Instead of the 1/8" harpoon tether cord we've been using with a breaking strength of 1050 pounds (made of Spectra A, the weaker of the UHMWPE forumlations), the new stuff will have a breaking strength of 5000 pounds and is made of Dyneema (next step up, right before Spectra B).

Thanks for all of your suggestions, and the concerns for our safety. Richard and I were fine, as we were expecting a hitch pin to break anyway, so we were holding on in a way that allowed us to both catch the robot and land on our feet.

Unfortunately, we don't have the luxury of using a thimble in our setup, as it would be too large, and our tensioning system uses eye loops that are welded shut, so the standard overhand knot using the loop won't work. Do you have any suggestions for the best kind of knot to use in this situation?

i would recommend 2 half hitches http://meritbadge.org/wiki/images/5/...cout-Knots.pdf

[gblake]

Originally Posted by daltore

...
Unfortunately, we don't have the luxury of using a thimble in our setup, as it would be too large, and our tensioning system uses eye loops that are welded shut, so the standard overhand knot using the loop won't work. Do you have any suggestions for the best kind of knot to use in this situation?

A Google search using "thimble" "knot" "load" "line" turned up this link to a Knot blog, along with a few other leads. The blog entry is a bit tedious; but it has some useful info in it and it has some references to knot-tying reference material (near the bottom).
http://allaboutknots.blogspot.com/20...hitch-for.html

Blake

[Rion Atkinson]

Originally Posted by IndySam

This is my biggest fear when it comes to suspending from another robot. Will you be willing to trust all your hard work to another teams calculations and design?

If that team was FRC 111 (Simbotics) or FRC 148 (Robowranglers) I would do it in a heart beat.

[Vikesrock]

Originally Posted by Formerly Famous

If that team was FRC 111 (Simbotics) or FRC 148 (Robowranglers) I would do it in a heart beat.

Note:

FRC 111 is Wildstang

FRC 1114 is Simbotics

[EricH]

And if it was 111, 1114, or 148, I'd do it in a heartbeat... (There are a few other teams, too.)

[ltdboarder101]

this is why you do not use your arm to pull up the robot.
you use something else to pull u up

[JamesCH95]

If your second telescoping stage was moving twice as fast as the first then it has a 2x mechanical advantage and sees only 1/2 the force of the first stage, not twice the force. Otherwise you'd magically be getting free work, which we all know is impossible.

Edit: We're using 1/4" vectran rope from McMaster, which has a working load of 1500lbs. Make sure whatever rope or cable you're using has a WORKING LOAD in the range you want that that you're never exceeding any bending allowance.