Help with a Science Fair Project - Breaking strength of rope

**MrToot** · 10-12-2011, 13:35

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** · 10-12-2011, 13:53

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.

**Rug** · 10-12-2011, 14:29

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!

**MrToot** · 10-12-2011, 14:59

Originally Posted by 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.

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

**flannery** · 10-12-2011, 15:02

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** · 10-12-2011, 15:26

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

**Muskrat** · 10-12-2011, 15:43

Originally Posted by MrToot

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.

**MAD777** · 10-12-2011, 19:12

How about using smaller and smaller ropes on a hammock with dear ole Dad in the hammock. When Dad hits the dirt, he can declare his project a success!!

Sorry, I couldn't resist - I'm a Dad too and that's the picture that came to my mind when I read your post

**nothermark** · 10-12-2011, 19:53

Originally Posted by 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.

If he stays away from things like amsteel.... I would pick a small diameter line that breaks at around 50 lbs. That way you could use a fishing scale as a load cell. Make a simple frame with a fixed eye bolt in one end for the cell and a threaded hook in the other end. Make string samples with various knots in them. First problem is to find a way to attach the sample. Start with loop knot in each end, hook over the scale hook and the threaded hood. Tighten the nut on the hook watching the scale until the string breaks.
Change knots and repeat.
FWIW I think my electronic fish scale records the max weight. something like braided fishing line might be the best sample material.

**Syb** · 10-12-2011, 21:21

Send opie a message. He originally had the whoopieslings.com business but sold it. I believe he did some pretty wild testing. Hope that helps and best of luck on the science project.