Out stretching surgical tubing.

[Chief Hedgehog]

We found that we would have the same issues after working with our Twinbot 'Betsy'. So we ordered 7/16" speargun tubing (from Amazon). That coupled with our lead screw we used for tensioning, we found that we could dial in as we needed. We used a donated pressure scale (much like a fish or deer scale) - it was from an old prison. The story is that it was used to check the pressure from the boiler unit. No matter, it DOES look like something from a prison.

In the photo, you can barely see the leadscrew - it is just to the lower-left of the speargun tubing.

[JohnFogarty]

Ive found that speargun tubing doesnt stretch out as quickly/easily as regualr surgical tubing. Definitely not in the time of a regional espcially if tension isn't always left fully applied to it throughout the course of the event. I would suggest looking to see if you can adapt some for your team's use.

[themccannman]

Greasing your surgical tubing greatly increases it's life since the tubing tends to crack over time from frictional rubbing with other surgical tubing or with the bars it's mounted to. Elastic materials lose their stretchiness based on how far they get stretches relative to their starting length.

I'm not a professional so this may be wrong, but with most elastic materials it seems that stretching the material a longer distance has an exponential affect on it's life; e.g. stretching the same tubing twice the distance for a half the number of repetitions seems to wear it out faster than stretching it half the distance for twice as many repetitions.

Thicker walled surgical tubing tends to live longer before it loses it's stretchiness due to the fact you can get the same force while stretching it a shorter distance. We also found that using more wraps of tubing while reducing the overall travel also increases the life of each piece as each one is being stretched less but you're getting a larger net force between all of the pieces.

Aside from that, just spend a lot of time checking your shot, if it's losing power re-wrap the tubing or replace it.

[Josh Fox]

When 27 ran our surgical tubing catapult in '08 we made "cartridges" using carabiners (rated for climbing and with a proper safety factor) that would provide 50 pounds of force when stretched the desired amount, and would test them after events to determine if they needed replacing or not.

I don't recall for sure, but I believe after every event we just replaced the tubing.

[Chief Hedgehog]

What type of grease did you use?


QUOTE=themccannman;1372023]Greasing your surgical tubing greatly increases it's life since the tubing tends to crack over time from frictional rubbing with other surgical tubing or with the bars it's mounted to. Elastic materials lose their stretchiness based on how far they get stretches relative to their starting length.

I'm not a professional so this may be wrong, but with most elastic materials it seems that stretching the material a longer distance has an exponential affect on it's life; e.g. stretching the same tubing twice the distance for a half the number of repetitions seems to wear it out faster than stretching it half the distance for twice as many repetitions.

Thicker walled surgical tubing tends to live longer before it loses it's stretchiness due to the fact you can get the same force while stretching it a shorter distance. We also found that using more wraps of tubing while reducing the overall travel also increases the life of each piece as each one is being stretched less but you're getting a larger net force between all of the pieces.

Aside from that, just spend a lot of time checking your shot, if it's losing power re-wrap the tubing or replace it.[/quote]

[themccannman]

Originally Posted by Chief Hedgehog

What type of grease did you use?

Lithium grease works well, so does high pressure grease. I'd have to ask T^2 what specific brand we're using. It's important to get the grease in places where it's either contacting other tubing, or where it's being bent around something such as a cross-bar that it's wrapped around.

[nuclearnerd]

Originally Posted by themccannman

Lithium grease works well, so does high pressure grease. I'd have to ask T^2 what specific brand we're using. It's important to get the grease in places where it's either contacting other tubing, or where it's being bent around something such as a cross-bar that it's wrapped around.

Be careful. Most oil-based lubricants are not compatible with latex. I can't say how long it would be until you have problems, but if you want to be safe, pick up a vial of silicone o-ring grease instead: http://www.mcmaster.com/#9463k33/=rh3f7n

[Jon Stratis]

We use a fish scale to periodically measure the force on our catapult at a specific pull distance. This tells us if the tubing is losing its pull, and lets us either adjust it or replace it as needed, ensuring we can get consistent results each time we modify it.

[Peter Matteson]

Use the Bobcat solution:
Buy these: http://www.mcmaster.com/#extension-springs/=rh55nd
Replace every event for consitant performance.

Trusted source of spring force on Bobcat designs since 2008.

[jijiglobe]

Our team had a similar problem but we came up with a 3 point solution to it that made us not have to change our surgical tubing for both of our regionals(besides the ruined stuff from build season). The system is as follow:

1) Have a system where you can take off surgical tubing without cutting the zipties or whatever holds it. We accomplished this by using an eye bolt with a hole to hook surgical tubing into. Then only put surgical tubing onto robot while in queue.

2) Never stretch surgical tubing more than half of its initial length.

3) Find out how much force it takes to move your catapult using a spring scale(use the same one every time in case it's off) You have to measure from a specific point in the motion of the part every time. Then check that you match that number before each match.

It's a little tedious but we went through 2 days of competition at the Buckeye and 3 days at New York without having to change and without having noticeable drops in accuracy.

[themccannman]

Originally Posted by nuclearnerd

Be careful. Most oil-based lubricants are not compatible with latex. I can't say how long it would be until you have problems, but if you want to be safe, pick up a vial of silicone o-ring grease instead: http://www.mcmaster.com/#9463k33/=rh3f7n

This is definitely true. However, the majority of the friction our tubing (with our design) is encountering is not abrasive friction but more so caused by being stretched around a curved surface. I'm not sure how much oil based lubricants will affect latex in lower friction environments but definitely keep this mind as large amounts of latex tubing can be hazardous if it snaps.

We also use a fishscale to check the power of our shot, it's a good consistent way to track changes in your tubing.

[Deke]

If you can tension in the elastic range of the material, you drastically increase the life. I'm not sure i have seen a material or mechanical specification stating the elastic range of surgical tubing, but some testing can go a long ways. One way would be reducing pre tension and adding more tubes, or the tubes may have to be longer depending on the stroke required.

We used steel springs but went through a few iterations to find springs with the correct force and range required in the elastic range. We ruined a number of springs by over stretching into the plastic range of the material.

Our last spring combo made it through two competitions and numerous test shots without any loss off power.

[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.

[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.