Quick question about Pneumatic cyl. travel times

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I've got a little project that I'm wanting to start on, but before I do I have to get some supplies and work some things out. So I need to ask the vast AO Army about something. In the design, one of the key mechanisms involves a pneumatic cylinder pushing against a load of about 75 pounds, give or take maybe as much as 25 (a high and low answer would be nice). The load is spring-type, so there's no inertia coming from that part of the thing. Since it's a pneumatic cylinder the pressure is kinda limited to about 250 psi tops, if I could I would rather run it at something like 100 psi or so. This will probably be coming from a paintball CO2 tank, just cause they're easy to come by. If I have to I will use something else. I can't use HPA for this, cause of some restrictions on the design. I was wanting to use something like maybe a 20 oz tank, just cause it's big. My limitations are as follows:
A cylinder needs to push against a spring that's about 75 pounds
The spring will NOT be pushing it against it's return stroke
The stroke needs to be at LEAST 6 inches, but 6.5 or maybe 7 is OK.
Bore size I figure to be a little over an inch or thereabouts.
Pressure is availible to 250 or so, but I'd like to use 100-150.
It needs to be able to cycle at an absolute minimum of once per second, or 60 per minute.
It mus cycle 60 times at least per tank of gas.
I only have the air availible from a 20 oz CO2 tank, so I need to consume the air carefully. The volume of this thing will be pretty big, so the air will go fast.
My questions to you guys are:
Is it even possible to make the thing cycle once per second? If so, how fast will it go, considering the given limitations?
Will I have enough air? Will a single tank be enough? It will be hard to get much more air on there, 20 is pushing it enough already.
Should I go low pressure, high volume, or high pressure, low volume?

Any help at all will be greatly appreciated. If you guys got any formulas for this kind of thing that would be awesome, cause then I could do it myself. A calculator that I could download maybe would be even better.

The main thing I want to know is if a cylinder can cycle that fast and if I will have the needed air to do this. Thanks in advance!
Bert


PS- Anyone know where I could get a cylinder about that size? I tried Clippard but they're kinda pricey for one that dimension, and Ebay is hard to get one that's exactly right. I wanna pay really no more than about 15 for one of these if I can help it. If I cant find somewhere else I will just wait for Ebay, but if one of you guys has one of these...

[sdlm_]

I dunno about most of it but your cycle speed depends on your pressure, so a HIGHER volume of LOWER Pressure will make you cycle speed LOWER with EQUAL force, likewise if you were to lower the pressure and volume you'd cycle slowly and weakly. at least that is what they told us about pascals law in physics class ( I so can't remember that class)

[Redkey]

you need 75 lbs of force. 100 psi in a 1" diam cylinder should produce about 80 lbs of force with a cylinder volume of 4.71 cu in for a 6" cylinder. 200 psi in a 0.75" diam will produce about 88 lbs of force with a volume of 2.65 cu in for a 6" cylinder.

In theory high pressure air will flow faster than low pressure air... so I would go with as high as pressure as possible. To get the speeds you are looking for you'll need to go with at least 3/8 air lines that are as short as possible. Even then you might have problems with 1 cycle per second.

Don't forget that you are going to be using alot of CO2 with something this size... your CO2 tank will cool rapidly and the pressure will start to drop in a hurry. An expansion chamber before your solenoid will probably be necessary.

what are you building?

[Redkey]

cylinder size

you need 75 lbs of force. 100 psi in a 1" diam cylinder should produce about 80 lbs of force with a cylinder volume of 4.71 cu in for a 6" cylinder. 200 psi in a 0.75" diam will produce about 88 lbs of force with a volume of 2.65 cu in for a 6" cylinder.

In theory high pressure air will flow faster than low pressure air... so I would go with as high as pressure as possible. To get the speeds you are looking for you'll need to go with at least 3/8 air lines that are as short as possible. Even then you might have problems with 1 cycle per second.

Don't forget that you are going to be using alot of CO2 with something this size... your CO2 tank will cool rapidly and the pressure will start to drop in a hurry. An expansion chamber before your solenoid will probably be necessary.

what are you building?

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Have you ever seen the movie Van Helsing?


Well my friends think an automatic crossbow is impossible. (btw id you havent seen it he has a fully automatic crossbow). So I think that it is possible. So I wanna attempt it, and I need to use a pneumatic cylinder to do it. What you think

[Redkey]

hmmm...

interesting. I might have to watch the movie.

As the bow cocks the cocking force is going to increase which means the initial cocking speed will be faster than the speed just before latching. Again, leading me to say it will be hard to do 1 per second. Then there is the feed mechanism to worry about.

Do you have a cross bow you can try this out on? Is it safe to assume the cocking force is 75 lbs?

Also... you'll have to figure out a way to get the cocking mechanism out of the way before the shot is firing, if the cylinder has not retracted completely.

Sound like a fun project.

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yes, at least one other person on the planet thinks its possible...


Anyway. The cylinder is gonna be either above or below the string. It will be moving a little block on some dovetails to actuate the string part. The feeding mechanism will be the hardest part, and one that I have been working on for a while (on paper). I am thinking about using one or two really small cylinders to pop up the arrow (bolt). Try to follow me here, this could be kinda rough.
The loader is a drum feed mechanism. It holds the arrows like this: Think of a gear with pits on it the shape of a semicircle a teeny bit bigger than the arrow. There will be 60 of these teeth around the gear. Around the outside of that is a ring that is just big enough to hold on the arrows in the groove. At the very top of the ring is a channel that is big enough for one arrow (bolt) to go through. Some small pneumatics will push the arrow up out of it's slot into the path of the string. There won't be anything to get in the way of the string this way.
Get all that?
In answer to your question about weight, I will be making all this from scratch, so I could make the pull weight be like 10 pounds if I want, but I figure to make it both reasonable to actuate with a cylinder and have some power to it I'm guessing about 75 pounds. I have to keep in mind stuff like air consumption and all if it's any more than probably 75 or so. I could go lower, but I will not go under 50 and even that is kinda weak. Of course I have never even shot a crossbow before but hey there's a first time for everything. I wanna pick up one of those little toy ones for like $20 but they don't have them anywhere around me that I can think of. Oh yeah, one more question. How far should the string need to travel? Do you think 6 inches is enough, or maybe more than I need? I guess I could make the arrows longer than the string travel, I've just never seen them that way before. I was thinking like a 5.5" or so arrow, but I'll have to figure that out with some trial and error.
You don't know if it's possible to make your own cylinder, do you? I have a lathe I got somewhat recently and while I don't know completely how to use it yet I think I could learn. I could make one with like one inch ports or something for a lot of air flow.
The trigger mechanism will be another interesting part; I have to make it without electronics, so there's probably at least one other cylinder involved in there. I have a vague theory on how I should do that but I haven't really sketched it out too good yet. I'm thinking like a switch with sort of a really long thing on the top of it that would actuate at the end and beginning of a long stroke, instead of slowly going through a long throw.
Anyway I have to go now, let me know if you have any good insight into this project. I've never done anything with crossbows, much less pneumatics, so I need all the help I can get. I really want to be able to prove all my doubting friends wrong here, plus it would be plain cool to have!

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Hey, I just realized you joined AO the same time I did. Lotsa reading, little posting eh? Same here, but I do some

[trains are bad]

Remind anyone eles of Final Fantasy III/VI?

Autocrossbow?