Bungy and Surgical tubing

[Jordan2389Diaz]

: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:

[Jones571]

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

[DonRotolo]

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.

[Team 2811]

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.

[DonRotolo]

Not to mention it's prohibited by <R72>

[Taxi Quinn]

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.

[Jones571]

We are currently looking in the manual in the section you said, and we can find notheing that says its 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.

[boomergeek]

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

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.

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?

[Ether]

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.

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"


~

[Mike Betts]

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?

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

[boomergeek]

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"


~
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!

[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.

[Bruceb]

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

[Daniel_LaFleur]

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 tears 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

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.

[boomergeek]

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

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?

[Mike Betts]

...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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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?

[Daniel_LaFleur]

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?

Al,

There are 2 parts to that answer.

The first is that the seals are capable of 80 ips as long as they are not in constant use. This is a good thing considering most of the applications here in FIRST.

The second is the end cap, which is much more troublesome. Most cylinder endcaps are pressed in (usually with a 1 ton press) and then double cinched (or crimped) to retain the endcap. During normal operations (with restricted air flow) the cylinder should not be able to create enough inertial momentum to cause severe problems. However, in many of our assisted acceleration systems (spring assist, locked cylinders, etc) with large inertial masses causes the impact against the endcap to become greater than the manufacturers expectations. This is his reasoning for requiring not bottoming out on the end cap (our cylinders themselves are not generally capable of those speeds on their own). It would be my suggestion that ANY cylinder that is acceleration assisted or has a large inertial load (1 LB or higher) should be required to have a hard stop before bottoming out on the cylinder end cap (<S1> ruling).

Should a cylinder end cap fail, it will most likely be shot with considerable force. The mass of an endcap of a 1 1/2" cylinder is probably about 1/2 the mass of the soccer ball, so it could be shot further than the machines kicking distance.

Hope this helps.

[Mike Betts]

Dick,

Thank you for your help.

I hope that the GDC takes your data and analysis into account and advise teams in their next update.

Regards,

Mike

[Ether]

I believe designs that do not use tubing or bungee or spring assist can exceed 80psi piston speed.

Specifically, mechanically latch the kicker, pre-charge the kicker with 60psi, leave the opposite end of the cylinder vented to atmosphere (no restriction), then release the latch.

I favor a rule requiring all kicker pistons to have hard external stops. That gives the inspectors clear guidance and they don't have to make subjective judgements on-the-spot about whether or not it's required.


http://www.chiefdelphi.com/forums/showpost.php?p=911424

http://www.chiefdelphi.com/forums/showpost.php?p=911299


~

[Chris is me]

I'm kind of confused, though that's probably because I'm under the weather today. Are the pneumatic cylinders not able to withstand the force of their own deployment? We have to put hard stops in the robot because otherwise when I fire a precharged cylinder, it will break itself?

Has this been an issue before?

[Ether]

I believe designs that do not use tubing or bungee or spring assist can exceed 80psi piston speed.


I did a rough calculation to check this.

Assumptions:

- 1.5" bore 8" stroke cylinder latched at 3" stroke; extend side pre-charged to 60psi

- exhaust side (retract side) of cylinder left open to atmosphere*; assume no backpressure buildup during kick

- 2 pound mass equivalent load

- ignore energy lost due to sliding friction;

- assume adiabatic expansion of diatomic ideal gas (alpha=5/2 gamma=7/5)


Results:

I got a piston speed at end of stroke of 270 ips:


Calculations:

1.5 cylinder bore, inches
3 start stroke, inches
8 end stroke, inches
60 start gauge pressure, psig
14.7 atmospheric pressure, psi
2 weight of load, pounds

0.0381 bore, meters
0.0762 start stroke, meters
0.2032 end stroke, meters
515,057 start absolute pressure, pascals
101,357 atmospheric pressure, pascals
0.91 mass of load, kilograms

0.001140092 piston area, square meters

8.6875E-05 start volume, kiloliters (cubic meters)
0.000231667 end volume, kiloliters (cubic meters)

36.30 work done by gas, joules
14.68 work done against atmospheric pressure, joules
21.63 work done on load, joules

6.90 velocity of load at end of kick, meters/sec
272 velocity of load at end of kick, inches/sec



If vamfun or MattH are reading this thread I welcome your criticism.


* http://forums.usfirst.org/showthread.php?t=14218


~

[boomergeek]

Ether and anyone else interested,

I only asked Bimba specifically about the currently planned team 241 configuration that would reach less than 80 IPS.

Bimba responded quickly to my query. If there is interest in knowing about any potential additional concerns for piston speed of 270 IPS versus 80 IPS, I would suggest that someone could e-mail Bimba customer support and post the answer here.

[evilrobotics]

We at team 1245 are using surgical tubing more of like a sling shot. It's just to help not the function of pneumatics, but we have three cylinders on the Bot to control the kick. our expierience is that Just pneumatics alone was way too slow. We have not tryed using the 8020 extrusions yet for air.

[vamfun]

I did a rough calculation to check this.


21.63 work done on load, joules

272 velocity of load at end of kick, inches/sec



If vamfun or MattH are reading this thread I welcome your criticism.

* http://forums.usfirst.org/showthread.php?t=14218


~

I did a quick check against my excel dynamic piston model and I get
23 joules and 231 in/sec so you are in the ball park with me.

I have the valves and a piping modeled and some approx to the dead volumes of the pistons. If I add a piping factor of .5 *Cv these drop to
22 joules and 219 in/sec.


Hmmmm: I must fess up to the 20in/s that I reported in this post:

http://www.chiefdelphi.com/forums/showpost.php?p=904090&postcount=1

Piston hammering was my concern and I'm in the camp that requires hard stops to prevent it for safety concerns. Foot speeds of over 300 in/s are required for long kicks and teams with pistons stopping the foot (even with a 10 to 1 lever arm are still reaching over 30 in/s).

As for rationale: I think LaFleur was spot on here. http://www.chiefdelphi.com/forums/showpost.php?p=912417&postcount=31

[Ether]

I did a quick check against my excel dynamic piston model and I get 23 joules and 231 in/sec so you are in the ball park with me.

I have the valves and a piping modeled and some approx to the dead volumes of the pistons.


Thanks for that info Chris.

You say you have the valves (plural) modeled. Could I ask you to re-run the model with the exhaust (retract) side of the piston vented directly to atmosphere, with no valve or fitting? GDC seems to have ruled this legal:

http://forums.usfirst.org/showthread.php?t=14218

You wouldn't have to change the model, just use some large value for the Cv and diameter on the exhaust valve and tubing.



~

[MrForbes]

I cannot imagine the latex tubing being rated at 125 PSI, but I'm unable to find a specification to point at.

go to http://www.mcmaster.com and search for "latex tubing" and read away...they say it is rated for less than 100 psi.

There might be a more authoritative spec somewhere else?

Hopefully that's internal pressure when used as a tube, and not tensile strength when used as a spring!

[Mike Betts]

go to http://www.mcmaster.com and search for "latex tubing" and read away...they say it is rated for less than 100 psi.

There might be a more authoritative spec somewhere else?

Hopefully that's internal pressure when used as a tube, and not tensile strength when used as a spring!

Don and Jim (et al),

Before going too far down this path, I would ask on the Official Q&A. I think that most inspectors would view this as an illegal accumulator as per <R72> part A.

JMHO,

Mike.

[Bruceb]

O.K. I have to interject here. Again as it seems noone read my previous post. Our primary sponsor is the local Bimba sales source. The sales guy is one of our mentors so I posed the question to him about the pistons hitting the end stop. He talked to his contact at bimba and they said that something like a few thousand hits will probably not damage the cylinder but if you are realy concerned just order a cylinder with an air cushion at the end in question and there will be no problems. That is what we did. We are using a 1.5 inch by 8 inch stroke piston precharged at 3 inches to 60 psi powering a speed increasing lever kicker that is augmented with a bunch of wraps of surgical tubing. When this fires I don't know the speed but you don't want your hand in the way. We get up to 35 ft of in the air kick on the ball. When the cylinder hits the adjustable air cushion on the end there is NO CLINK and you can see it bounce and then slowly settle to the end. Very cool. SOOO, I am for hard external stops only if you do not have an internally cushioned cylinder.
Bruce

[KC1AJT]

check our kicker out it combines pneumatics and surgical tubing go to http://www.youtube.com/watch?v=h_c7AilTRdI to check it out:D

[Daniel_LaFleur]

O.K. I have to interject here. Again as it seems noone read my previous post. Our primary sponsor is the local Bimba sales source. The sales guy is one of our mentors so I posed the question to him about the pistons hitting the end stop. He talked to his contact at bimba and they said that something like a few thousand hits will probably not damage the cylinder but if you are realy concerned just order a cylinder with an air cushion at the end in question and there will be no problems. That is what we did. We are using a 1.5 inch by 8 inch stroke piston precharged at 3 inches to 60 psi powering a speed increasing lever kicker that is augmented with a bunch of wraps of surgical tubing. When this fires I don't know the speed but you don't want your hand in the way. We get up to 35 ft of in the air kick on the ball. When the cylinder hits the adjustable air cushion on the end there is NO CLINK and you can see it bounce and then slowly settle to the end. Very cool. SOOO, I am for hard external stops only if you do not have an internally cushioned cylinder.
Bruce

No one is ignoring your post (well at least I'm not), but the free cylinders from Bimba do not come with air cushions. Teams will not pay $125, or more, for a cylinder they can get for free just to get an air cushion. Additionally, air cushions add weight (~.25-.50 LB) and size (~.375 -.50") to the cylinder.

If teams can afford air cushions in their cylinders then by all means use them (and bring the specification sheets to assure the inspectors that it is safe), and if you cannot then use a hard stop.

[Bruceb]

Our 1.5 inch 8 inch travel double cushioned cylinder cost us $57. Suppose that could be our sponsors cost though. There is no increased length (it fit right in where we had designed and tested with a standard cylinder and the weight difference I could not detect. Didn't weigh it but if you hold the 2 cylinders side by side you cannot tell them appart unless you see the adjustment screw on the cushioned one. Not trying to argue, just relating our experience in the hopes of helping someone. Also, I did not like the talk of requiring a external hard stop. That would really screw up our now completed design.
Got more detail from the Bimba catalog.
The standard 1 1/2 in cylinder (17 -DP)
base weight .73
length 4.38 + stroke
price $39.40 + $2.85 / inch
Air cushioned cylinder (C-17 - DP)
base weight .77
length 4.38 + stroke
price $ 64.80 + $2.85/ in
Hope that helps,
God I love this site.
Bruce

[Mike Betts]

...Also, I did not like the talk of requiring a external hard stop. That would really screw up our now completed design...

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

[Daniel_LaFleur]

Our 1.5 inch 8 inch travel double cushioned cylinder cost us $57. Suppose that could be our sponsors cost though. There is no increased length (it fit right in where we had designed and tested with a standard cylinder and the weight difference I could not detect. Didn't weigh it but if you hold the 2 cylinders side by side you cannot tell them appart unless you see the adjustment screw on the cushioned one. Not trying to argue, just relating our experience in the hopes of helping someone. Also, I did not like the talk of requiring a external hard stop. That would really screw up our now completed design.


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.


Got more detail from the Bimba catalog.
The standard 1 1/2 in cylinder (17 -DP)
base weight .73
length 4.38 + stroke
price $39.40 + $2.85 / inch


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



Air cushioned cylinder (C-17 - DP)
base weight .77
length 4.38 + stroke
price $ 64.80 + $2.85/ in
Hope that helps,
God I love this site.
Bruce

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.

[Mike Betts]

...FIRST adds another x2 safety factor...

Daniel,

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

Mike

[Daniel_LaFleur]

Daniel,

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

Mike

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.

[Bruceb]

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

[Mike Betts]

...Does that make sense?...

Yes it does

[Mike Betts]

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.

Daniel,

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

Mike

[Ether]

If you have finalized your design, then you should have a good number for the kinetic energy that will be absorbed by the shocks...

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


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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[Vikesrock]

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 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

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?

[KC1AJT]

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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very true

[Mike Betts]

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

[vamfun]

Thanks for that info Chris.

You say you have the valves (plural) modeled. Could I ask you to re-run the model with the exhaust (retract) side of the piston vented directly to atmosphere, with no valve or fitting? GDC seems to have ruled this legal:

http://forums.usfirst.org/showthread.php?t=14218

You wouldn't have to change the model, just use some large value for the Cv and diameter on the exhaust valve and 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.

[Ether]

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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[Ether]

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?

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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