Help with a Science Fair Project - Breaking strength of rope

[I Splice]

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.

We did that at a Scout meeting. We used a come-along and two 8 inch steel columns that our meeting place had handy. We could have used Suburbans in the parking lot too.

Bending the rope at a small angle is what causes the weakness, in my understanding. So, you want to avoid tight bends where you attach the rope.

I made a couple of devices out of some small steel cable and some 3 inch pipe nipples and 3 ferrules per device. See the attached picture.

I used good quality paracord. That was a bad choice because paracord keeps a lot of strength when it's tied in knots. The cord often failed between the knots. Next time, I'll use something different

Our plan was to have each patrol tie a knot on one side and see whose knot was the strongest.

If you have some weak enough rope and a couple of 5 gallon buckets, you could suspend the buckets from the rope and add sand until the rope fails.

[Jsaults]

When rope is tested by a certified laboratory the ends are anchored by wrapping them numerous times around polished metal cylinders. This allows the rope to slide as it is tensioned and for the rope or knot to break in between.

You could use black pipe that has been smoothed with progressively finer grades of wet sand paper until it was shiny.

Seems like a great project!

Jim

[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.

[nothermark]

neat setup. one suggestion. If the rope between the pulley's is a sample put a second slack rope in parallel so that when the sample breaks things don't fly around too much. Safety goes a long way. ;-) If that rope is the slack rope "nevermind".