Help with a Science Fair Project - Breaking strength of rope

[MrToot]

Greetings all. My 14 year old son is doing an 8th grade science fair project on the impact of various knots on the breaking strength of a rope. He is a Boy Scout and enjoys hanging in his Blackbird by dynaglide whoopies. See him pictured on a recent trip here. Going to progressively lighter and thinner whoopies is what caught his interest in addition to doing some rock climbing this past summer.

He has a nice project outlined and is in search of a reasonable mechanism to actually measure the breaking strength of different types of rope when various different knots are tied in them. Of course a solution needs to be relatively affordable to build or buy and needs to be able to be accomplished by 14 year old.

Thoughts, ideas and pointers greatly appreciated.

[Muskrat]

I would shoot Bruce Smith at On Rope an email. He lives for this sort of thing.

Thats about all the help I can provide. . . Good luck. Sounds like a great project.

[MrToot]

I would shoot Bruce Smith at On Rope an email. He lives for this sort of thing.

Thats about all the help I can provide. . . Good luck. Sounds like a great project.

Thank you. Could not find a link to email to or an email address at the link you provided. Any further help on those appreciated. Again, thank you

[Muskrat]

Thank you. Could not find a link to email to or an email address at the link you provided. Any further help on those appreciated. Again, thank you

On the link I sent you, click on the "Click here for more info" link. That will bring up a new page where you will see a link for "Contact". Should be good to go from there.

[Rug]

Simple solution that any science fair judge will love:

1 VERY strong base (a steel box frame would work perfect, but is heavy and perhaps out of his league, or you could try a frame made from steel tubing (cast-iron used in high-pressure lines like natural gas and air-compressers.)
2 (or more pulleys as needed)
1 Torque wrench
1 ratchet-style winch

attach the end to be 'tested' to the base. (this would normally be the end that is picking something up)
run the rope through the pulleys (using math and the multipliers to determine the forces applied at the anchor point).
use the torque wrench on the ratcheting winch (keeping a close eye on the guage) and measure at what point the rope breaks (using the multipliers from the pullys to estimate the effective load on the rope at time of breakage.

repeat multiple times to get averages and limit the margine of error.

Test each knot, graph it out, send me a pic of the winning ribbon!

[Nicodumas]

+1 to this, as it has math involved and will have a great "look n feel" on the floor. Judges seem to love looking at number crunched by hand.

Simple solution that any science fair judge will love:

1 VERY strong base (a steel box frame would work perfect, but is heavy and perhaps out of his league, or you could try a frame made from steel tubing (cast-iron used in high-pressure lines like natural gas and air-compressers.)
2 (or more pulleys as needed)
1 Torque wrench
1 ratchet-style winch

attach the end to be 'tested' to the base. (this would normally be the end that is picking something up)
run the rope through the pulleys (using math and the multipliers to determine the forces applied at the anchor point).
use the torque wrench on the ratcheting winch (keeping a close eye on the guage) and measure at what point the rope breaks (using the multipliers from the pullys to estimate the effective load on the rope at time of breakage.

repeat multiple times to get averages and limit the margine of error.

Test each knot, graph it out, send me a pic of the winning ribbon!

[zukiguy]

If I were going to try something like this I might start out with a frame something like an engine hoist. The idea would be something freestanding or better yet if you have a place to connect it to a structural beam.

The idea would be to have a lever, maybe around 6' or 7' long for portability. This would be attached to a pivot point maybe a foot off the ground and extend out horizontally. A foot from this pivot point you need a big eyebolt to attach one end of the rope under test. The other end of the rope will go straight up a few feet to a fixed point (the same vertical beam as supports the lever).

At the end of the lever you might put some kind of platform where you can stack weights or maybe a bucket. The idea is to use force multiplication so you can exert a lot of force but still be able to weigh it.

So, you've got your rope under test and a couple of 5-gal buckets of sand, steel weights, or whatever to preload the rope. Then you can slowly pour sand into yet another bucket to slowly add more weight until the rope fails.

When you get done you can weigh all the stuff on a kitchen scale and use the lever ratios to figure out the actual tension (approximately) you've exerted on the rope. You won't get really exact numbers but this should be close enough to tell what difference the knots make and the relative strengths of various ropes.

I'd think you'd end up with a frame that looks something like a trebuchet but with the weights and projectile ends reversed.

Sorry for the rambling...hopefully this makes some sense. Just like any source of kinetic energy though....SAFETY FIRST. If something lets go it's going to be pretty catastrophic so take the proper precautions.

[MrToot]

Thanks to all for your ideas. Just wanted to share the final approach used by my son. See photo:

http://www.hammockforums.net/gallery...hp?i=16247&c=3

He is still doing testing and analysis. Will pot final results when he finishes.

Thanks again.

[flannery]

Maybe use a hydrolic jack and a lift scale (see links below for examples). You could attach the scale to the bottom of the jack or maybe mount the jack on a thick piece of steel and attach the scale to an eyebolt attached to the steel plate. Attache the top of the rope to the top of the jack and start pumping it up. I'm sure there are details that would have to be worked out but it would be pretty exact in showing at which weight the rope broke...

scale -> http://www.northerntool.com/shop/too...duct_7185_7185
Jack -> http://www.northerntool.com/shop/too...5429_200345429

[MrToot]

These are some great ideas. Thank you and please keep them coming.