Out stretching surgical tubing.

[Deke]

Originally Posted by menns

The curious thing we discovered about stretching surgical tubing is that the restoring force decrease by about 10% for about 40 seconds to a minute after it's stretched. We get the most catapulting force if we stretch it and release it immediately. We suspect this was throwing off our two ball autonomous.

I'm guessing this a thermal effect. Rubber produces heat when stretched. It then cools to room temperature but rubber expands when it cools so less restoring force.

Something doesn't quite add up here, are you sure it was the surgical tubing causing variation? If there is a power loss from the tubing, it should be a steady decline and not self restored. Maybe over time the tubing slides or rotates where it is mounted and changes its tension, just a thought.

[shewejff]

Originally Posted by menns

The curious thing we discovered about stretching surgical tubing is that the restoring force decrease by about 10% for about 40 seconds to a minute after it's stretched. We get the most catapulting force if we stretch it and release it immediately. We suspect this was throwing off our two ball autonomous.

I'm guessing this a thermal effect. Rubber produces heat when stretched. It then cools to room temperature but rubber expands when it cools so less restoring force.

This is what's called viscoelastic creep. It's common for elastomeric materials. That reduction in force will start to happen fairly rapidly when the elastomer is placed under load, but the final force will eventually level out (sometimes after hours under load). I used to perform analyses of EPDM rubber, and our EPDM rubbers would eventually level out at about 70-75% of the original load.

If you remove the load, the elastomeric material is none the worse for wear, and will load again exactly how it did the first time. For example, if I pull the surgical tubing to 100 lb and hold it at that distance for a long time, the force will eventually decay to 75 lb (for example). If I release the force and pull it back to that same distance, it will take the original 100 lb of force to do so, not the 75 lb that it was just at for that distance.

I haven't performed any tests on the surgical tubing to look at its viscoelastic creep behavior, but this might be a good experiment for a team to do.

[RoboChair]

1678 Greases our spear tubing with lithium I believe, it was rubbing together so bad it was tearing its self apart. Now with grease it has gone through 2 regionals of heavy use with no ill signs, but it seems to absorb the grease and isn't causing any problems.

[Ether]

Originally Posted by RoboChair

it seems to absorb the grease

This earlier post in this thread may be of interest:

http://www.chiefdelphi.com/forums/sh....php?p=1372063

[Deke]

Originally Posted by shewejff

This is what's called viscoelastic creep. It's common for elastomeric materials. That reduction in force will start to happen fairly rapidly when the elastomer is placed under load, but the final force will eventually level out (sometimes after hours under load). I used to perform analyses of EPDM rubber, and our EPDM rubbers would eventually level out at about 70-75% of the original load.

If you remove the load, the elastomeric material is none the worse for wear, and will load again exactly how it did the first time. For example, if I pull the surgical tubing to 100 lb and hold it at that distance for a long time, the force will eventually decay to 75 lb (for example). If I release the force and pull it back to that same distance, it will take the original 100 lb of force to do so, not the 75 lb that it was just at for that distance.

I haven't performed any tests on the surgical tubing to look at its viscoelastic creep behavior, but this might be a good experiment for a team to do.

That is very useful information, I stand corrected.

The prototyping we did with surgical tubing was very inconsistent, so we went to steel springs.

Is there a range you can design the tension to eliminate the viscoelastic creep?

[Deke]

http://en.wikipedia.org/wiki/File:El...oelastic_2.JPG

Looks like you cannot design out of it; it's inherent with the material. So the best would be to reduce its variation from the viscoelasticity as best as possible.