Bungy and Surgical tubing

[Mike Betts]

Originally Posted by Daniel_LaFleur

...I'm not sure that that 'spec' is actually a spec, but instead is a 'recommendation' from Bimba...

Daniel,

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

Mike

[Ether]

Originally Posted by Mike Betts

Daniel,

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

Mike

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.


~

[Mike Betts]

Originally Posted by Ether

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.


~

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

[Ether]

Originally Posted by Daniel_LaFleur

Mike,

I'm not sure that that 'spec' is actually a spec, but instead is a 'recommendation' from Bimba.

Additionally, the speed of the cylinder is not an issue. What is an issue is the impact on the endcap of the cylinder. Consider that some teams are using upwards of a 2 pound mass on the end of a 8-10" lever and accellerating that to 50 ips and you have some serious forces trying to unseat the endcap of the cylinder.

My suggestion to all teams that are using this type of system (including elastomer assist) is to have a hard stop on your robot so that the cylinder does not 'bottom out' against the endcap.

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.


~

[Ether]

Originally Posted by boomergeek

Ether-
For the engineering rule that the maximum of 20 inches per second for piston speed, can be ignored if the piston is moving in excess of that only when powered by surgical tubing as opposed to when the movement is caused by pneumatic pressure against the piston?

I am a complete novice at pneumatics and I'm taking an educated guess that it's the seals between the piston and the cylinder that are only rated for a maximum speed of 20 inches per second, and my guess continues that it causes excessive wear is independent of whether pneumatic force or elastic force is used to drive the piston to the excessive speed.

Can anyone with knowledge of pneumatic mechanisms explain?


Thanks!

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.


~

[Ether]

Originally Posted by boomergeek

Actually I'm worried that the 20 inches per second piston speed has been claimed (by multiple persons posting to CD) to have been given from Bimba Customer Support representatives.
As far as I know, no one with any expertise has stated equivocally under which conditions that it is SAFE to ignore 20 IPS guide on piston speed.

I typically choose to considered it likely to be deemed unsafe by thoughtful robot inspectors unless an expert vouches for the SAFETY of the configuration.

If leaving a piston attached to a "bow" when shooting an arrow is an engineering acceptable use of pneumatic piston, will someone with expertise in the failure modes of actuators please confirm?

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.


~

[boomergeek]

Originally Posted by Ether

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.


~

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).

[Al Skierkiewicz]

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.

[Ether]

Originally Posted by boomergeek

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

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.


~

[Ether]

Originally Posted by Mike Betts

Even with pre-charged cylinders at some percentage extension to maximize speed and direct exhaust at .3 cv, I'm having a difficult time getting a dangerous material fatigue scenario for an expected lifetime of a couple of hundred two minute matches.

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.


~

[JesseK]

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.

[DonRotolo]

Originally Posted by Chris is me

Just to be clear because someone sent me an email about it and I didn't see it in the thread, you can use surgical tubing in tandem with pneumatics (i.e. a surgical tubing power kicker pulled back with a piston), but you can't use surgical tubing to store compressed air (eg, it's not pneumatic tubing). Just in case anyone was confused, sorry if it's obvious.

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>:

<R72> In addition to the items included in the KOP, pneumatic system items specifically permitted on 2010 FRC ROBOTS include the following items.

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.

<R73> Items specifically PROHIBITED from use on the ROBOT include:
A. Any pneumatic part or component rated for less than 125psi.

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.


,

[RRLedford]

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

[boomergeek]

Originally Posted by boomergeek

Actually I'm worried that the 20 inches per second piston speed has been claimed (by multiple persons posting to CD) to have been given from Bimba Customer Support representatives.
As far as I know, no one with any expertise has stated equivocally under which conditions that it is SAFE to ignore 20 IPS guide on piston speed.

I typically choose to considered it likely to be deemed unsafe by thoughtful robot inspectors unless an expert vouches for the SAFETY of the configuration.

If leaving a piston attached to a "bow" when shooting an arrow is an engineering acceptable use of pneumatic piston, will someone with expertise in the failure modes of actuators please confirm?

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.

[Al Skierkiewicz]

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?