pic: Telescoping Arm Warning

Tonight we finally got our telescoping arm working and mounted it on the robot. Lifted itself up just fine! To test the weight capacity, Richard and I hung very carefully on either side of the robot, expecting this one complaining hitchpin to snap. It didn’t. The 1000-pound breaking strength Spectra we were using in the pulley system did.

Let this be a warning to everyone using telescoping arms that you MUST consider mechanical advantage. Together, the three of us weighed about 350 pounds. Since this is a two-stage arm, that was the force experienced at the hook on the last (inner) stage. However, the inner stage moves twice as fast as the outer stage, causing the outer stage to experience twice that force, so we were pulling on a single knot with 700 pounds of force. That’s about half the weight of your average compact car.

If you’re using a telescoping arm, good luck to you, and do the math!

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.


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

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?

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.

Check out AmSteelLine. 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.

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/5/5d/Six-Boy-Scout-Knots.pdf

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).


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


FRC 111 is Wildstang

FRC 1114 is Simbotics

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

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

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.

Calling the stages “first” and “second” implies a reference point. You’re labeling things from the point of view of the driving motor, in which case I’m not sure the term “mechanical advantage” is being applied properly.

Aaron described it from the other perspective, with the slower-moving stage experiencing twice the force applied to the faster-moving stage. They were testing it by pulling down on the end, applying force to what you labeled the “second” stage.

You’re right, I had it mixed up. Apologies! That’s what I get for reading posts at 730am :eek:

The problem is that the rope’s BREAKING load and WORKING load are different criterion. The breaking load can be approached once, whereas the working load can be reached safely many many times.

You see… I verified that on TBA… I knew the difference… I forgot to add the 4… Sorry about that.

Either way. 111, 148, or 1114, I would still do it in a heartbeat. :smiley:

…The 1000-pound breaking strength Spectra we were using in the pulley system did…

While Spectra is extremely strong for it’s diameter, stronger than steel of the same diameter, it is also VERY slippery! If you intend to use it as your primary lifting line, make sure you understand all of it’s properties and how to deal with them.

Here is a hintfrom the kiting community: Sleeve the ends of the line. Here is another source of info on sleeving, this has picture to help guide you.

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.

The problem with UHMWPE is that it’s used for so many different purposes and it’s properties change so easily that they never rate it for working load, only breaking strength. The actually working properties are pretty interesting, the molecular bonds are fairly weak (Van der Waals forces hold it all together), but the molecules are very long, so the surface area between the molecules is enough to provide a lot of strength. That force changes pretty quickly when there’s any kind of bend in the line, temperature difference, if it’s wet, if it’s a Saturday… Vectran is quite a bit more stable, but lower strength per cross-sectional area, which is the reason we went with UHMWPE (not to mention price, it’s a little older and proven material, and used more commonly in commercial applications like boating and rock climbing).

Thanks for all of the suggestions, this is very helpful!