Bungee Cord Mechanical Behavior

Hello CaptnPea

How much energy are you intending to store?

For how long are you intending to store that energy?

Kind Regards....

I think the bungee cord manufacturers produce such a wide variety of product that there is no standard at all. You would have to do complete experimentation on your own.

Here's is some basic information on shock cord. The vendor might be able to point you in the right direction for furthers specs.

I recall reading a very interesting article a few years back. It delved into the same type of issue you are questioning here. I believe the conclusion was that it was too inconsistent to rate or scale properly.

Any natural substances had too many variables. Any synthetic substances had extreme variations due to temperature, age, forces applied and manufacturing process variability.

Not much of an answer, I know. But I guess it suggests the question:

How specific do you need to be with your specifications?

Also, if you intend to hold an elastomer in a "stretched" condition for any significant time, I suggest you question it's memory characteristics. As I understand it, a bungee springs back due to the materials structural memory. Being held in an extended state for too long will probably affect this.

Just my thoughts…..

Guy

Once, bungee cord was the most common form of spring for aircraft landing gear, and it is STILL used there. Note that manufacturers have a date coding system that goes by year and quarter on a five-year repeat cycle, which gives a rough idea of lifespan for the stuff. Here is one source: http://aircraftspruce.com/catalog/lgpages/shock_rings.php; they may be able to provide specifications or put you into contact with the manufacturer. Or enter landing gear bungee cord into Google and go searching - there is a list of manufacturers down the right column of results.

Thanks for the help...

The situation is indoor use, and I have seen it being used as a spring in other products so that sort of sparked the idea. I am concerned about relaxation over time. The duty cycle is really low, and it is not a saftey item.

Just noticed your tag line. I've done a bit of sailing, and designed this boat:

Its flap (at the rear of the vertical wing) is bungee-centered. When the flap is centered, the tension on the bungees is near zero, and they are relatively large diameter, and quite stiff in the relaxed condition. During sailing, they are fully extended all the time (the flap is engaged at all times other than in the middle of a tack -- or if there is very little wind). Under that condition (and being mounted out of the sun) they seem to hold up well -- and could easily be replaced or re-tensioned as required.

Can't see the boat pic, Ken. And I'd like to, since it sounds like something I've wanted to try for an automatic yoke adjuster on a kite. Can you provide me a link to the pic? Or repost it, so we can see the bungie in active service?

Micah

Hi Micah,

This is not a great photo for the purpose, but it may give you the idea. The red circle is a reinforced area on the flap in which a hinge pin (red) is mounted. (There are also hinge pins inside the flap, at 2' intervals -- the flap is 10' long) The green thing is the bracket on the wing proper from which the base of the flap is pivoted. The stops are the barely visible yellow lines, and they set the flap deflection (20^o, I think). The blue lines are bungees. The bungees were terminated in about a foot of 1/8 inch Kevlar cord, which was secured to cleats, sailor style, so I could adjust the tension on the bungees if need be.

The bungees are like a stacked coil spring in that some minimum amount of tension must be overcome to extend them at all. So, in ordinary operation (with the wing installed in the boat, and generally pointed skyward) the flap would remain centered, until the sailor pulled on the rope attached to the back of the flap. That would cause a small amount of lift to be developed (the lift being oriented to the side, rather than upward as in an airplane wing). The wing, being pivoted forward of its center of lift would then generate a moment, tending to resist the sailor's pull, and extending the bungee on one side. This would cause more lift to be generated, causing more moment, etc... and quite quickly the bungee would be fully extended, and the sailor would be scrambling around trying to keep the boat flat and avoid being thrown off the back.

With the wing horizontal (as shown here on my workbench, or when on the boat's trailer) the flap was pinned to prevent motion or its sagging under it's own weight.