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.