rubber band properties

http://design.caltech.edu/Archive/2001/handouts/elasticband32.html
this shows the properties of #32 rubber bands, and the graph shows how the energy in is less than energy out. It seems like you shouldn’t stretch the rubber bands to their maximum length, and the maximum you should stretch is ~1ft before diminishing returns, and at lower stretches, the bands are more efficient.

For the rubber bands we usually use in Vex, the length is 3 inches, and what I found was that they stretch to around a maximum of 4 times their original length (12 inches) before they start breaking, which agrees with what you said.

The energy outputted is less than the energy inputted because elastic bands release heat when they’re stretched.

Each time you stretch and release a rubber band, it becomes fractionally longer because you break some of the polymer chains that make up its structure. If it is stretched beyond its range of elasticity (past about 80% of the length it ruptures at) it will rapidly deteriorate, lose elasticity and end up behaving like a piece of spaghetti. :slight_smile:

I think rubber bands are such an awesome way to store energy.

Could rubber band energy loss to the release of heat be reduced by insulating them with latex tubing?

No. The energy is converted to heat and once it’s converted it’s not going back.

That would be a bit like putting on a winter jacket while running a race so that the heat your body produces while running could be used as energy to make you run faster. That wouldn’t work, it would just make you warmer.

You can try to equate this heat loss to friction in gears. While there is some energy lost due to friction (which in turn becomes heat) it’s pretty negligible and can be ignored for most stuff.

Are you using the same Vex flat bearing as the rest of us?

Okay so not negligible haha but still pretty small (at least compared to, say, no bearing)

The point is that friction in gears is a bit like energy loss in rubber bands.

The “No” answer, provided above, while largely correct from a practical point of view is… well, let’s say “incomplete” from a theoretical point of view.

The rubber bands do convert some mechanical energy into thermal energy as they stretch and relax. This is common for all elastic materials and is known as “hysterisis”. Some materials are more efficient in this regard than others, but in short the laws of thermodynamics make it exceptionally difficult to store energy 100% efficiently.

So, anyway, we have this rubber band giving off heat. This heat has to go somewhere. This requires a thermal gradient between the rubber band and its surroundings. The more insulation you put around the rubber bands, the higher that thermal gradient has to be in order for the heat to get out of the rubber band.

So if you put insulation around the rubber band, the heat energy will start to build up in the rubber band, making the temperature of the rubber band increase.

So the question is, does a hot (well, maybe slightly warm) rubber band return more energy than a cold rubber band? I couldn’t find anything specifically related to rubber bands, but I did find an interesting paper studying the coefficient of restitution of a rubber ball.

In their research the rubber ball did return energy more efficiently up to about room temperature (300 degrees Kelvin). I suspect the results might be similar for a rubber band.

So… keeping in mind that the practical answer is “No, insulating the rubber band will not make your robot arm work any better.” I’ll suggest that the more complete answer from a theoretical point of view is “Yes.” Particularly if you are playing in a cold room, have well insulated rubber bands, and use them a LOT.

How can an answer be both “Yes” and “No”? Well, that’s the difference between the science answer and the engineering answer. It is important to know both of them.

Good question!

Jason

P.S. The same sort of thing goes for wearing a jacket when you’re running… up to a certain point the additonal warmth will make your muscles work more efficiently so you will be able to run faster and farther with a jacket on… if it is cold out! That’s one of the reasons that speed skaters wear insulated suits!

It also gets super cold XD
That was quite the analysis, to say the least.

Thanks, but now that I look at it again, it also was incomplete.

I forgot to mention that if the insulation stretches and relaxes along with the rubber bands, that may be an additional source of hysterisis-related energy loss.

In reality, however the gains from running your motors at a cooler temperature would likely outweigh any additional gains from more efficient energy return from the elastic bands in any case.

Of course the increase in motor efficiency due to cooler temperatures would likely be offset by reduced battery output as chemical reactions tend to proceed more slowly at lower temperatures.

And I said that just to add three more layers of “ridiculously irrelevant” to my original analysis. I mean, they are all factors worth considering… chilling motors and breakers while warming batteries to an optimal temperature before a match could actually provide a small but likely measurable increase in robot performance…

Jason

I also found some properties of latex tubing, and these should be more accurate than the rubber band properties (source: http://www.chiefdelphi.com/forums/showthread.php?t=66497).
The force from latex tubing is (cross sectional area)*(force constant).
The vex latex tubing has a csa of 0.00920388472 and the force constants are (105, 154, 202, 331, 460, 1100, 2800, and 4400) corresponding to 100-900% elongation. The maximum elongation before permanent deformation is 300% (or 4 times, or 12" for #32 rubber bands). These force constants may also apply to rubber bands, which are also latex, but rubber bands may be a different durometer hardness or have different properties.
When heated a rubber band will contract, when cooled, it will expand. When a rubber band is stretched, it will release heat, when it is released it will absorb heat. If the rubber band were perfectly insulated, the heat that it gave off when stretched would be constantly forcing it to contract, resulting in much less loss of energy than an uninsulated rubber band… at least that’s my theory.