Re: Bungy and Surgical tubing
 

Daniel,

Great. Where did this "recommendation" originate? It does not appear in the spec sheets...

Mike

Re: Bungy and Surgical tubing
 

Hi Mike,

I think it first became a topic of discussion in this forum when a poster mentioned a few days ago in another thread that he had been told this on the phone with a Bimba representative (not sure if it was a salesperson or a technical support person).

I will try to find the post, and provide a link to it.


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Re: Bungy and Surgical tubing
 

I have that... I want the source.

The more I think on this, it makes no good engineering sense. As I said, the cylinder will get to faster speeds than 20 IPS in almost all cases for almost all robots.

If this low number is, in fact, a limit of operation, it should be in the specs. It is not.

I do not want to and will not act on hearsay. A verbal number given by someone to someone to someone can not be a reason for me to rule a team's design unsafe at a competition.

Mike

Re: Bungy and Surgical tubing
 

Hi Daniel,

I think you are correct about the high impact force on the cylinder endcap being the primary safety issue, especially with the large inertial forces arising from the sudden deceleration of the additional mass attached to the shaft.

It is possible that the piston speed recommendation (if indeed it is a valid recommendation - still TBD) was more related to piston seal wear. BUT it could also be a recommended max impact speed with NO inertial mass loading.

In either case, it seems most prudent to provide a robust mechanical stop to prevent the piston from smashing into the cylinder endcap. I wouldn't mind seeing a hard and fast rule about this, to make the inspector's job easier.


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Re: Bungy and Surgical tubing
 

Hi Boomergeek,

We don't know at this point that 20ips is an "engineering rule". The provenance and authority of the 20ips statement is still not known. Because of the continued debate about this, I would imagine that eventually (probably sooner than later) someone will track it down and undoubtedly post what they find.

But just assuming for the moment that it IS authoritative, we still don't know the underlying reason - seal wear? or maybe end-cap impact.

If the concern is about seal wear, my best guess would be that you are correct: it doesn't matter whether the speed is caused pneumatically or by spring load. The same applies if the concern is end-cap impact loading.


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Re: Bungy and Surgical tubing
 

I for one would like to see a bright-line rule that robot designs must have external stops to prevent the pistons from colliding with the endcaps of the cylinders, regardless of whether this 20ips rumor is authentic or authoritative.

That's the safest approach, and gives the inspectors clear guidance.


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Re: Bungy and Surgical tubing
 

Some actuator manufacturers require bumpers if application results in piston speed is greater than 4 ips.
Teams should prove their cylinders alwaysmove slower than 4 ips or stow robust external stops.
Applications of a kicker mechanism, the only realistic approach is to have external stops. The real question is: if you have external stops, is there a maximum piston speed that Bimba will certify conforms to their design parameters and their safety testing? My guess is that the official Bimba answer is going to come back as 20 ips. If it was higher, IMO it's not likely that a Bimba representative would have said 20 ips.

I doubt Bimba will get an actuator engineer to stick his/her neck out and put out a blanket statement that say 20 ips can be exceeded (by a a factor of X ) if the external stops are good. That would be great for FIRST if he/she did because I think substantial rework may need to be done if Bimba makes a definitive statement on piston speed: and much of the rework will potentially make the modified mechanisms less safe, not more safe.

(I say potentially less safe because it will result in more connections and disconnections from the piston and kludged mechanisms for attaching and detaching from pistons in order to keep the designed piston speed low.
More misfires are likely to result. Every misfire is a potential to hurt a careless person (those persons that don't assume that the kicker will misfire EVERY time someone does something lazy or risky).

Re: Bungy and Surgical tubing
 

I would think that the GDC has seen the discussion here and is talking over the issue as we speak/read. They and certainly the inspection team are concerned for the welfare of the participants and the volunteers. Safety is our most important product. (I stole that but I can't remember who to give credit to.) Expect that UL safety people will be consulted at your events if in the opinion of an inspector, the part seems to operating in an unsafe manner.

<R02> ROBOT parts shall not be made from hazardous materials, be unsafe, or cause an unsafe condition.

Re: Bungy and Surgical tubing
 

I doubt the surgical tubing manufacturer will get an engineer to stick his/her neck out and put out a blanket statement that say the tubing is safe to use to arm a soccer-ball kicker.

The line has to be drawn somewhere.


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Re: Bungy and Surgical tubing
 

Hi Mike,

If the piston is allowed to smack into the cylinder end-cap, the dynamic impact forces are far higher than one might imagine based on static pressure calculations. This is especially true if the piston shaft has a large mass attached. This is why a jackhammer is able to bust up concrete.

I'd like to see external stops. Then the inspector doesn't have to make a subjective judgment.


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Re: Bungy and Surgical tubing
 

In 5 years of reading FRC rules and Q&A's, I've never seen the GDC make a rule change that prohibited a type of design this late in the season that was legal according to the rules at kickoff. Thus, I'd imagine that the GDC will make an addendum to the safety rules with specific regard to high speed, high tension systems. The requirement may be as simple as requiring a double-solenoid valve on all high tension pneumatically-actuated systems so that the valves are not defaulted to a certain position. That's reasonable to an extent since the KOP valve is a double solenoid.

Regardless of any further GDC arbitration, I do believe that the current rules allow for an inspector's enforcement of high visibility labels on such systems.

Re: Bungy and Surgical tubing
 

Back to the original post:

You know, the quote above is what I was trying to say in my previous posts. However, let me quote the first sentence of <R72>:
OK: Is the latex tubing NOT included in the KOP? Can anyone show me where it is NOT a Pneumatic part?

I completely agree that using it to carry air pressure would not turn out well, and I would never, ever suggest that it should even be attempted.

BUT, considering the exact wording of <R72>, there is some wiggle room here. I cannot imagine the latex tubing being rated at 125 PSI, but I'm unable to find a specification to point at.

To the original poster: Please take the safe route and have your team contact post this to the FRC Q&A forum for an official ruling.


,

Re: Bungy and Surgical tubing
 

Latex tubing will "blow up", like long skinny balloons do, zooming to a dramatically expanded size (8x initial diameter & 2.5x initial length) once a threshold pressure is reached (around 30 lbs for smaller sizes). Then it will only take a limited amount more pressure (or stress to the super taught expanded membrane) before it bursts.

Having used long pieces of it in water fights, with the latex tubing inflated with tap water & wrapped around my neck and body like a boa constrictor, I can tell you that when it does rupture, it does so with dramatic force. I felt like I had been hit by a strong karate chop. Not sure if it was the water or the rubber snapping me, but really stunned (and soaked) me.
-Dick Ledford

Re: Bungy and Surgical tubing
 

My e-mail to Bimba:
We are designing use of Bimba 1.5 in bore and 8" stroke cylinders in a FIRST robotics application.
The application has robust external stops to prevent the piston from reaching both of the end-caps.
The application calls for the piston to reach 80 inches per second (IPS) for a small portion of its non-pneumatically powered return stroke.

Is an unpowered piston speed of 80 IPS allowed by Bimba design rules?

If this high speed does not conform to Bimba design rules, is the non-conformance a safety issue or a warranty/wear issue?

Your prompt attention would be much appreciated....

(within 12 hours(:) ) they sent me the following response...)

80 in/sec is not out of the question as far as speed, but you will not be able to bottom out the cylinder. It needs to hit rigid hard stops to withstand this speed given standard design specs. I am assuming this isn’t a case where it will be oscillating at that speed for long periods of time. If so there would be a worry about heat buildup. Instead I assume this is a load or retract move the cylinder is making. Given that scenario, you should be ok.

C____ W_____
Technical Center Engineer



:) :) :)


So now we have expert opinion that piston speed is primarily an end-cap rule.


I guess we will have to slow down our firing rate on our kicker to less than 10 shots a second so we don't need to worry about heat.;)

Re: Bungy and Surgical tubing
 

Just so we can be clear on the response for us non-pneumatic guys. The response is that the end caps alone cannot take the forces encountered with a cylinder moving at 80 in/sec. Users that design this speed of movement will require external stops that limit the piston from contacting the end caps in operation. Did I read that right?