Surgical tubing stretched too far
 

We have a catapult powered with surgical tubing on our bot. The tubing is stretched from about 5 inches untensioned to about 20 inches as the winch pulls down the catapult arm. That's a 300% increase in length.

During the Hub City regional, we suffered from frequent decreases in shooting power and had to re-tension the tubing several times over the three days, and we ended up replacing some of it. I suspect that the tubing is being stretched past its elastic limit. It displays "stretch marks".

We can modify the bot's superstructure to allow for a much longer untensioned length on the tubing, reducing the extension from 300% to about 100%. Does this sound like it will solve the tubing over-stretching problem?

Re: Surgical tubing stretched too far
 

You'd probably need a stronger grade tubing since you'll get less energy stored with less extension (E=1/2kx^2). Unless you're modifying your superstructure considerably.

Re: Surgical tubing stretched too far
 

Had same issues a few years back until we went to spear gun tubing.

Re: Surgical tubing stretched too far
 

Yes, you will either need to switch out to a "stiffer" type of surgical tubing or add more strands of it; or both.

Note that while the equation for potential energy of a spring is E=.5*k*x^2, note that the k value for surgical tubing isn't constant as you stretch the surgical tubing. I would do some quick tests to determine the force required to deform to a certain length.

Re: Surgical tubing stretched too far
 

Spear gun tubing, that sounds interesting. Care to share any details? Like differences, mounting, pricing, sourcing, etc.

Re: Surgical tubing stretched too far
 

Go with spear gun tubing...the stuff is great.

http://www.amazon.com/Speargun-Sling...peargun+tubing

Re: Surgical tubing stretched too far
 

What Keegan said.

Re: Surgical tubing stretched too far
 

This equivalent to McMaster P/N 5234K65. You can search surgical tubing on McMaster and get a variaty of different wall thicknesses and outer diameter.

We have tried a couple of different ones and are also happy with the 1/2 OD and 3/16 Wall.

Re: Surgical tubing stretched too far
 

On team 900 we've used surgical tubing power our catapult and we can shoot ~45 feet. We made sure to build it so that there's only 100% to 150% extension and we've not had to re-tension the surgical tubing.

So yes, it sounds like that probably will solve the over-stretching problem, but you'll probably have to tweak the # of wraps or something until it goes the distance you want.

Re: Surgical tubing stretched too far
 

Can anyone confirm that the McMaster part is equivalent to the amazon link for the spear gun tubing (other than the dimensions) in terms of quality or rubber type?
We ran the McMaster tubing at our first competition and it definitely showed some wear and caused some issues in eliminations, I am wondering if the spear gun tubing will be better.

We have been using 3/8 bungee to power our catapult. We found that it holds it's elasticity very well and we have yet to replace it.

Re: Surgical tubing stretched too far
 

I can't verify that they are the same as we haven't gotten the amazon stuff but they are both latex rubber. So can't be much different. What size did you try from McMaster?

There are many variables to what causes wear, such as attachment points and stretch %.

Re: Surgical tubing stretched too far
 

I bought both the 'Spear it' brand spear tubing, and McMaster Latex Tubing and they appear to be identical, at least for our purposes.

Specifically, we've used both 12mm (1/2") Spear-it tubing, which seems to be identical to 1/2" OD, 3/16" wall latex tubing from McMaster.

Re: Surgical tubing stretched too far
 

Another option, we went with the fitness bands. Find the force and corresponding diameters that work for your application and have fun. Our experience was a band reached its "advertised" force at approximately twice its original length. The brand carried by the chain sporting goods stores around here sells them in increments up to 100 lbs.

Re: Surgical tubing stretched too far
 

We are very happy with the 'KOP' linear force springs from Vulcan

Hundreds of shots, no change.

Re: Surgical tubing stretched too far
 

Be careful leaving bungee tensioned for long periods of time, we used bungees on our climbing arms last year and when tensioned they lose a lot of strength over time. If you can take the tension off when the robot is bagged or stored it will last a lot longer.

Re: Surgical tubing stretched too far
 

The stretching of your rubber tubing is a phenomenon known as hysteresis. As you pull on the tubing, long-chain rubber molecules absorb the energy. Releasing the force returns most but not all the energy. Some stays in the deformed molecules which don't all bounce back to their starting shapes.

What this means is that any rubber tubing will exhibit a loss of power over many stretches, so plan on an easy way to tension or replace the tubing, and try to design your mechanism so that you aren't stretching it so much. Changing the tubing to a better grade may help, but you most likely will still have a significant loss of power until you redesign your mechanism.

Enough for today's physics lesson, let's go play robots!

Dr. Bob
Chairman's Award is not about building the robot. Every team builds a robot.

Re: Surgical tubing stretched too far
 

Thanks for all the useful information. We went ahead and implemented this change.

We were able to attach the tubing to a pair of eyebolts on the top of our practice bot's superstructure, and tested it yesterday. The results were quite gratifying.

The ball has a lower trajectory, implying that the energy transfer is occurring through a greater portion of the arm's stroke. This means that we can tune the shot for greater distance without over-stressing the winch system.

Re: Surgical tubing stretched too far
 

We use surgical tubing and love it. We scored 2 perfect 75's in auton at Howell last weekend and have a system that gives us a very reliable dynamic ranging shooter.

Get a little creative and you can shoot any distance and trajectory you want. The surgical bands on our bot remain loose until the second we shoot. so set is not an issue. We have a max shot wrap. Large amount of loops with minimum starting length. We can shoot most distances on the fly (pretention hapens in less than 1/2 second). Full load is just over 1 for full field shot.

Re: Surgical tubing stretched too far
 

My team did a fair amount of testing with surgical tubing and our robot has a surgical tubing powered catapult.

A few things we learned from testing (On our practice robot we've fired our catapult at least 500 times and it's still consistent):

1. We had some problems with the KOP tubing and found that it lost its elasticity much faster than what we bought off McMaster.
2. After testing, we opted for more wraps that are only slightly tensioned when the catapult is up and ~150% stretched when the catapult is down. More than that and we started noticing wear and loss of elasticity.

Best of luck!

Re: Surgical tubing stretched too far
 

Another follow-up - we used the lower stretch of about 50% on the bot at Arizona today, and we're shooting quite well. Consistency is way up. We're in fourth place! Thanks again for the help.