Bungy and Surgical tubing

:cool: My team is trying to hook up our pneumatics system but we are wondering how power it would be? How do you hook up the surgical tubing and the bungy cords to the robot.:cool:

Go to you tube and search 842 kicker and u will several great examples.

Of course, the surgical latex tubing is not intended to be part of the pneumatic system.

The FRC Manual does explain a bit about how to connect the pneumatics. the FIRST Pneumatics Manual goes into greater detail. Both should be read before you do anything more.

As for the latex tubing, we have used it in the past as a spring to retract a mechanism, a cushion for a mechanism that hit the frame too hard, and in place of a rope or wire to kpps game pieces from falling out of the storage hopper.

This year, we tied some into a loop, and use it like a big rubber band to power the kicker. A pneumatic piston pulls the kicker back for another shot.

Yeah, I am with you on this one. The surgical tubing is not recommended or intended for the pneumatics system. It would not be a wise choice to do so, because depending on the amount of pressure that you need for your pneumatic system, the surgical tubing could expand and “pop” off of whatever you attach it to in the system.

Not to mention it’s prohibited by <R72>

I dont understand what you are saying is prohibited? We are currently looking in the manual in the section you said, and we can find notheing that says anything is prohibited.

You may want to re read it Quinn. R72 specifically states what you are allowed to use in your pneumatic system and surgical tubing is not on there as it would be very unsafe to use in a pneumatic system. So Don is 100% correct.

Does your piston move during the shot? Most video examples I’ve seen of various pneumatic/tubing kickers show the piston moving during the shot.
If your piston moves during the shot, does it move faster than 20 inches per second?

Has Bimba (or any other actuator expert) sanctioned any conditions where a piston is “engineered appropriately” to move the piston faster than 20 inches per second?

Rule <R71> is quite clear:

<R71> To satisfy multiple constraints associated with safety, consistency, robot inspection, and constructive innovation, no pneumatic parts other than those explicitly permitted by the Pneumatic System Rules may be used on the ROBOT.

The “Pneumatic System Rules” mentioned in the above rule are those listed in Section 8.3.9 “Pneumatic System”

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Dick,

This “specification” has been floating around these boards for a while now.

I’d love to find out the exact source of the 20"/sec specification. I downloaded the Bimba manual and can find it nowhere.

I calculate a 1" bore cylinder will move at about 70" per second (unloaded, 45 PSI delta with about a 2 CFPM airflow).

This was brought up in another thread. Upon reflection, my thoughts are:

  1. If the cylinder is rated at 125 PSI, I would think it should be built to survive a 110 PSI delta actuation over years of use.
  2. The rules state no more than 60 PSI as a working pressure.
  3. Assuming that the piston is used to deflect the spring assist at a 45 PSI delta, then the effective contribution of the spring is that of a 45 PSI delta.
  4. Added to the 45 PSI delta of the pneumatic system, you have an effective force of 90 PSI delta acting on the piston.

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.

Like I said, I’d love to get to the source…

Regards,

Mike

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!

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.

If you are worried about the piston breaking itself because it hits the end stop to hard to many times then Bimba does sell pistons with adjustable air cushions at one or both ends.
Bruce

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.

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?

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.

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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

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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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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