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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!
Discussion
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07-02-2010 17:21
Tom Line
Re: pic: Telescoping Arm Warning
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.
07-02-2010 17:50
gblake
Re: pic: Telescoping Arm Warning
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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.
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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
07-02-2010 18:42
Ahnxlazyman
Re: pic: Telescoping Arm Warning
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
07-02-2010 19:10
IndySam
Re: pic: Telescoping Arm Warning
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?
07-02-2010 19:39
vivek16
Re: pic: Telescoping Arm Warning
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.
07-02-2010 20:59
Sh1ine
Re: pic: Telescoping Arm Warning
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.
07-02-2010 21:30
daltore
Re: pic: Telescoping Arm Warning
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?
07-02-2010 21:44
KC1AJT
Re: pic: Telescoping Arm Warning
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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? |
07-02-2010 22:49
gblake
Re: pic: Telescoping Arm Warning
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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? |
http://allaboutknots.blogspot.com/20...hitch-for.html
Blake
07-02-2010 22:58
Rion Atkinson
Re: pic: Telescoping Arm Warning
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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?
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08-02-2010 02:59
Vikesrock
Re: pic: Telescoping Arm Warning
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Originally Posted by Formerly Famous
If that team was FRC 111 (Simbotics) or FRC 148 (Robowranglers) I would do it in a heart beat.
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FRC 111 is Wildstang
FRC 1114 is Simbotics
08-02-2010 03:14
EricH
Re: pic: Telescoping Arm Warning
And if it was 111, 1114, or 148, I'd do it in a heartbeat... (There are a few other teams, too.)
08-02-2010 07:48
ltdboarder101
Re: pic: Telescoping Arm Warning
this is why you do not use your arm to pull up the robot.
you use something else to pull u up
08-02-2010 07:54
JamesCH95
Re: pic: Telescoping Arm Warning
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.
08-02-2010 08:16
Alan Anderson
Re: pic: Telescoping Arm Warning
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Originally Posted by 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.
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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.
08-02-2010 08:34
JamesCH95
Re: pic: Telescoping Arm Warning
You're right, I had it mixed up. Apologies! That's what I get for reading posts at 730am 
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.
08-02-2010 09:53
Rion Atkinson
Re: pic: Telescoping Arm Warning
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Originally Posted by Vikesrock
Note:
FRC 111 is Wildstang FRC 1114 is Simbotics |
Either way. 111, 148, or 1114, I would still do it in a heartbeat.
08-02-2010 10:08
billbo911
Re: pic: Telescoping Arm Warning
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Originally Posted by daltore
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| ...The 1000-pound breaking strength Spectra we were using in the pulley system did... |
Here is a hint from the kiting community: Sleeve the ends of the line. Here is another source of info on sleeving, this has picture to help guide you.
10-02-2010 21:16
daltore
Re: pic: Telescoping Arm Warning
| 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. |
Thanks for all of the suggestions, this is very helpful!
10-02-2010 21:44
travis
Re: pic: Telescoping Arm Warning
Some people will tell you that there is no rope on a sailboat, but this only holds true for an ill-prepared one. Even fancy pants spinneret-ed high tech fiber used for holding stuff together on an america's cup boat is called rope when it is on the spool, as "rope" is the term for the raw material. A line is a usefully knotted/spliced/whatever amount of rope. So next time "captain" Bill castigates you with the old chestnut, tell him to wait while you go get some, cause how else are you getting home if the halyard snaps?
13-02-2010 02:13
daltore
Re: pic: Telescoping Arm Warning
Update: The 3840 lb. Dyneema works! To weight test the system, I stood on the arm and the motor lifted me right up, no problem! Did the same for the robot, and then Richard and I hung on it. The first time we hung, something dropped down, but the Dyneema was fine, it was just under enough tension that it forced a spacer on the axle (a Delrin shroud that extended the 1/2" steel axle to 1") away from the sprocket and popped down to the next level. We fixed that problem and got all 4 feet off the ground for several seconds. It was quite impressive! We're adding in a tensioning system for the one knot we had stationary because it undid a little bit and detensioned the rope, but overall, I'm very happy with this material!
I highly recommend this stuff, just a few suggestions if you use it in the future:
1) It's a little hard to work with, but as was said before, 2 half hitches or a taut-line hitch work well when tying it, and always use a much bigger rating than you think you will need. Example: 500 pound load, didn't hold until we went to almost a 2-ton line!
2) Spectra/Dyneema (brand names of Ultra High Molecular Weight Polyethylene, UHMWPE) is very slick, and does not like any kind of friction-based system. It will not work like a belt, it has to be tied down (the knots only work because there's so much pressure between the strands of material). To drive it off of our shaft, we actually had to drill a hole through the shaft, and then tie it down with a constrictor knot to keep it from moving in the hole.
3) If there is ANY kind of rubbing or abrasion, it will VASTLY weaken the rope. Just because the breaking strength is high doesn't mean it's hard to cut. The rating is just for a tension load, not shearing. We broke our first rope with 50 pounds of weight on it alone because we sheared it when it jumped a pulley.
Thanks for all the help, and good luck in competition!
16-02-2010 15:18
JamesCH95
Re: pic: Telescoping Arm Warning
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Originally Posted by daltore
1) It's a little hard to work with, but as was said before, 2 half hitches or a taut-line hitch work well when tying it, and always use a much bigger rating than you think you will need. Example: 500 pound load, didn't hold until we went to almost a 2-ton line!
2) |
Can't stress that enough!Also, one physical test is worth a thousand models and calculations.
16-02-2010 20:57
DonRotolo
Re: pic: Telescoping Arm Warning
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Originally Posted by JamesCH95
Can't stress that enough!
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Anyway, we're going with 5/32" steel cable, as I insisted upon a 10:1 factor of safety for our single bot (we don't expect to be lifting anyone else), and 1/8" was just at the limit.
Simply put: If that cable breaks, we're toast. Lesson to be learned: Don't let that cable break.
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