Dangers of High Speed Pneumatic Gun

[neilpatel92]

I'm looking for advice from veteran teams and mentors on pneumatics since we are a 2nd year team.

Our team was planning on using a high speed pneumatic gun to hit the soccer balls with enough velocity so they would go over the ramp.

However recently we discovered by calling Bimba that after speeds of 20 in./s on a pneumatic cylinder, the cylinder will start to break down.

We've done the physics calculations and we need the speed of the pneumatic piston to be atleast 5 m/s to use it as a gun. Obviously we need more speed for the gun design, but we don't want the gun to break down in the middle of a game either.

Is there any way that we can prevent damage on these pistons, and slow the pistons at certain points during the shot to prevent damage? Is there any way to prevent damage completely?

Or should we just scrap the idea of a pneumatic gun?

_Neil

[EricH]

Originally Posted by neilpatel92

Our team was planning on using a high speed pneumatic gun to hit the soccer balls with enough velocity so they would go over the ramp.

...

Is there any way that we can prevent damage on these pistons, and slow the pistons at certain points during the shot to prevent damage? Is there any way to prevent damage completely?

Or should we just scrap the idea of a pneumatic gun?

_Neil

Don't go fully pneumatic. Use the cylinder to swing a kicker bar.

Let r be a radius, L be a linear speed, and w be a rotational speed (technically it should be lower-case omega, but w is close).

L = r*w. Let's say that you can get a linear speed of 5 units/second out of the cylinder. You attach it close to a pivot of a kicker (let's say 5 units away from the pivot).

w is now L/r, in this case 1 radian/second.

The other end is supposed to be traveling at 20 linear units/second, let's say.

20 = r*1. You need a 20-unit long kicking device.

Do you see what I'm getting at here? Use a swinging arm and design the placement carefully, and you can get the same effect as a pneumatic cylinder going straight in at much higher speeds.

[Tom Line]

Think along different lines. You already got your answer on how fast the cylinders can go (and here's a hint - you don't have the volume or the flow rate to get anywhere near that....).

So now you need to look at how much flow rate you DO have, how much pressure and volume you DO have, and you can turn a shorter stroke into a longer one.

Hint 1: Mechanical Advantage.
Hint 2: Think lever arm.

[StevenB]

Originally Posted by Tom Line

So now you need to look at how much flow rate you DO have, how much pressure and volume you DO have, and you can turn a shorter stroke into a longer one.

I'm not a mechanical engineer, but here's what I've been pondering for the last few days. Perhaps someone who really knows can confirm or refute my hypothesis.

For a given pressure, the force you get from an actuator is proportional to the area of that actuator. The speed is inversely proportional to the volume of the actuator. You can trade force for speed with a lever arm of some sort, but that is exactly what a smaller bore size does. You can quadruple your force by using a 2x larger bore size, but that reduces your speed by 4x. In short, the mechanical power of a pneumatic actuator is limited by the size of the tube and the air pressure. Both of these are dictated by the rules, as far as I understand.

If pneumatics alone can't do it, then a pure lever arm won't help. A carefully crafted kicking device might be able to do a better job than a straight pneumatic puncher, but only because it makes better use of the available power.

[MrForbes]

Somewhere on my web page I have a picture of my hand with a nail in it, the result of slightly careless use of a high speed pneumatic (nail) gun.

As for kicking the ball....there are some threads on pnuematic kickers, you might want to read them

http://www.chiefdelphi.com/forums/sh...ad.php?t=80534

http://www.chiefdelphi.com/forums/sh...ad.php?t=80176

http://www.chiefdelphi.com/forums/sh...ad.php?t=80474

http://www.chiefdelphi.com/forums/sh...ad.php?t=80400

[neilpatel92]

Thanks Everyone!

Seems like there is no way to fix this design from the replies I have gotten so I guess the idea of a high speed pneumatic gun should be thrown in the trash.

I'll keep on checking for replies but I think our team will move on to another concept; maybe, the kicker (pneumatic piston attached to lever arm) as most people here recommended.

Thanks once again,
Neil Patel

[sparks-1784]

A team used a pneumatic 'gun' in competition in 2008. That competition required moving something similar to a medicine ball. I don't remember the team number, but they rigged a parallel arrangement of small pneumatic cylinders (and mechanically connected in series) and used valve timing to accelerate the medicine ball. The cylinders were small to obtain higher speeds from the available flow rate. Pretty interesting design.

I would expect that something like this would work for a kicker without any mechanical advantage (read lever arm).

[boomergeek]

So who thinks the FIRST safety inspectors will evaluate the Bimba guideline of 20 in/second and potentially disqualify robots because of it?

I've see a lot of videos and pictures of pistons tied to kicking legs during kicking motion and I have yet to see one that seemed like the piston was traveling less than 50" per second for a long distance kick. Getting 20:1 or 30:1 foot travel versus piston travel ratio is tricky especially in robots that are only 17" tall. (I think kicking hard over long distance is on the order of 40 ft/sec.)

Disconnecting the piston stroke from a kicking motion is feasible to keep the piston under the speed limit and it could be a valuable undertaking depending on whether this "20 in/second" is going to be perceived as a safety issue.

Personally, I think I am much more concerned about designs that are storing hundreds of pound-feet of energy in unrated/unqualified elastics as that would seem to be much more dangerous than speed violations on a pneumatic piston. But then again, I'm not an actuary.

I'm only a second year guy- any wise sages out there have any suggestions?

[Mike Betts]

Originally Posted by boomergeek

So who thinks the FIRST safety inspectors will evaluate the Bimba guideline of 20 in/second and potentially disqualify robots because of it?...

Dick,

The UL safety inspectors only hand out an award. The robot inspectors are the enforcement arm for this situation.

Most of us read CD regularly...

I'm going to refer you to this post I made.

Put yourself in our shoes. What would you do? Could you look yourself in the mirror if a student gets hurt?

What do you think?

Mike

[Ether]

Originally Posted by neilpatel92

We've done the physics calculations and we need the speed of the pneumatic piston to be atleast 5 m/s to use it as a gun.

Would you be willing to share with other teams what these physics calculations were, so others can learn from them?

Thank you.


~

[Jones571]

Edit: Mike Betts covered it.


Al or Mike Can this even be tested at a regional and how would this be done?

[Ether]

IMHO it is actually rather hard to travel 20in per second we use a 4'' cylinder preloaded we would have to actuate this in quicker then 1/5 of a second...

A preloaded cylinder that is vented to atmosphere on the opposite side and that is restrained and released with a mechanical latch can easily exceed 20 ips by a large margin.

Many youtube videos clearly show speeds exceeding 20ips.


~

[Mike Betts]

Originally Posted by Jones571

... Al or Mike Can this even be tested at a regional and how would this be done?

Corey,

I have to think about it. I'm not sure that it is possible if you use a 1/8" NPT to vent...

Regards,

Mike

[Jones571]

Originally Posted by Ether

Many youtube videos clearly show speeds exceeding 20ips.


~

how exactly are you reaching this conclusion? I find it rather hard to calculate this actual IPS on youtube videos.

[Daniel_LaFleur]

Originally Posted by Mike Betts

Corey,

I have to think about it. I'm not sure that it is possible if you use a 1/8" NPT to vent...

Regards,

Mike

I believe that only valves are limited by the 1/8" NPT port, not cylinders.