Re: Bungy and Surgical tubing
 

Bruce,

If you show up at a competition hoping that you will pass inspection, then you are approaching this wrong.

If you have finalized your design, then you should have a good number for the kinetic energy that will be absorbed by the shocks (0.5*M*v*v) where M is the mass of your kicker, the piston of the cylinder and any linkages.

Alternate computation: If the KE is derived solely from the air pressure on the piston without an external spring, conservation of energy states that the KE will be the pneumatic force on the piston head times the stroke of the piston and you don't have to calculate mass...

Now, if you come to the competition with a spec sheet saying that the cylinder can safely absorb that KE, you will pass inspection.

If you come to the competition with an email from the piston manufacturer saying that your design is safe, you will pass inspection (at least on that point).

If you can not prove a safe design, who knows?

<R94> gives you the power to control your destiny.

Right now, the decision as to whether to add a hard stop is yours. Without supporting engineering documentation, the decision at the competition may be out of your hands.

Mike

Re: Bungy and Surgical tubing
 

I'm more interested in safety than requiring hard stops. That said, assisting the accelleration of an large inertial mass faster than the cylinder is normally capable (surgical tube assistance, etc) is an indication that you may be overstressing the end cap (beyond manufacturing specs) and need a hard stop. There is a reason that cylinders have pressure ratings and specific port sizes.

BTW, just because you do not 'hear' the clunk of the cylinder bottoming out doesn't mean that you are not over pressurizing the back side of the cylinder. This is not so much for you Bruceb but for others that may not understand that the outgoing Cv is just as important as the incoming Cv.

Now, I'm not trying to 'screw up' anyones design. I'm only interested in safety here. The forces we're talking about here could throw a soccer ball (or end cap) into the stands, and therefore need to be made as safe as possible. Lets not let 'go fever' create an unsafe condition.

Most cylinder manufacturers build in a x1.5 safety factor. FIRST adds another x2 safety factor. Do I expect to see a problem? no. That said, we all need to be cognizant of the dangers our machines present, and we need to minimize those dangers.

as per your 8" cylinder above.
Cost to team: Free (max 3)


8" Cylinder cost: $87.60($64.80[base]+$22.80[length adder])+shipping+pivot bracket. Oh, BTW, this is the cost you should have on your BOM ... Not what they charged you.

Just because of the costs involved, I doubt very many teams will have cushions. My team has a hard stop in our design, not because we need one, but because we felt it was warranted.

Re: Bungy and Surgical tubing
 

Daniel,

If you don't mind me asking, where did that "2X" come from? I don't see it in the rules...

Mike

Re: Bungy and Surgical tubing
 

FIRST requires all pneumatics to run at a working pressure of 60PSI and requires all devices (including cylinders) on the pneumatic lines to have 120PSI working pressure capability.

Re: Bungy and Surgical tubing
 

I understand the safety concernes but our design has an additional safety feature that will keep us all safe. We are using the piston in a pulling operation so I don't think there is any way for the piston to come appart as it is hitting a hard stop per say, that is cushioned by the internal air cushion. The back of the piston(the receiving end of the force) is bolted to the rest of the structure so the forces are transmitted to the cap and directly to the chassis. Does that make sense?
Bruce

Re: Bungy and Surgical tubing
 

Yes it does

Re: Bungy and Surgical tubing
 

Daniel,

OK. I was just making sure that yet another requirement hadn't sneaked by me...

Mike

Re: Bungy and Surgical tubing
 

Not to be argumentative here, but this seems to put an undue burden on the inspector. How does the inspector know "that KE" was correctly calculated by the team.

Even if the team shows their calculations, the inspector has no way to verify the mass, angular momentum, spring force, pneumatic damping, etc inputs that went into the calculation.

Just my 2cents


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Re: Bungy and Surgical tubing
 

While I understand the desire to make sure that every robot is completely safe and the power that inspectors hold based on <R94>, some of the talk in this thread confuses me a bit.

How is a hard stop any different then the cylinder cap with regards to proving that it is a safe method of stopping the mechanism? Will teams be required to provide material info and design calculations regarding the structure of their hard stops to prove that they will not fail and go flying off the robot?

Re: Bungy and Surgical tubing
 

very true

Re: Bungy and Surgical tubing
 

Are you saying we can't use our slide rules? :eek:

Seriously... We, the inspectors, act subjectively. You, as the designers, act empirically.

We have on, average, less than an hour to inspect each team per inspector. I usually try and send out inspectors two at a time. That's 1/2 hour per team.

Can teams make mistakes and the inspectors miss it? You bet ya!

We have a checklist and we have experienced eyes and ears (I've been mentoring FIRST for 16 years now).

We will focus on anything that looks or sounds wrong. Stored energy devices will always get a second look...

Can things be hidden from the inspectors? You bet ya!

It's usually the students who talk to the inspectors. Any mentor who would knowingly allow his or her students to cheat doesn't "get it" and usually has a short half life in FIRST.

Lastly... You will be surrounded by dozens of teams with hundreds of mentors (many with more experience than me) and thousands of students. These folks have been eating and breathing these rules for 6 weeks now.

Our job is not to "catch" you. Our job is to get you ON to the playing field in a safe and equitable manner.

I will always assume that you have done your job as a mentor until proved otherwise. However, if I think that you have not done YOUR job, I can and will do mine...

Regards,

Mike

Re: Bungy and Surgical tubing
 

Those posted were with a quick vent exhaust valve , approx Cv= 1.6. This corresponds to about a 28mm area valve. Coast Pneumatics said their quick vent valves were 25mm.

Re: Bungy and Surgical tubing
 

That teams would cheat or try to deceive the inspectors never even crossed my mind Mike.

Maybe that's just my innocence showing. I thought FIRST was different.



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Re: Bungy and Surgical tubing
 

It's a matter of failure modes and probability.

With an external stop, it would require two failures for the piston to explode. Not likely.

The external stop can be inspected; the endcap not so much.

If an external stop is in the design, one hopefully can assume the endcap has not been subjected to multiple merciless beatings and has full integrity and is not on the verge of failure.

Finally, with an external stop it is very easy and inexpensive to incorporate impact-absorbing material. Even a small piece of hard rubber can reduce the impact force by an order of magnitude.


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