Bungy and Surgical tubing

[Daniel_LaFleur]

Originally Posted by 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?

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]

Originally Posted by 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]

Originally Posted by Ether

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/sh...90&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/sh...7&postcount=31

[Ether]

Originally Posted by vamfun

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.



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[MrForbes]

Originally Posted by Don Rotolo

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]

Originally Posted by squirrel

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

[Daniel_LaFleur]

Originally Posted by 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

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