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Try it and see why they don't.
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Kinda like the old spring loaded spear guns.
Except my design is similar to the newer crossbows, I was wondering would it be possible to use a compression spring to shoot crossbow bolts.
I know those springs used in the mold industry are extremely stiff would these be a good candidate to use?
Try it and see why they don't.
I think one large difference is that die springs have a very large spring constant. small displacement=big forces. The means you have only a short amount of time to transfer energy into the arrow. With a crossbow, you have a long gradual application of force with the largest force existing just before the arrow leaves the string.
Matt
Ok next question, I am finding springs that seem like they have the load power 100lbs plus but they are short maybe 2-3 inches long. I was thinking a longer powerful spring would have more throwing power. Ideas?
What you want is energy. Look up the equation for energy stored in a spring. Something to do with force and distance. You want a large force for a long distance. You could of course stack many short springs to get the same effect as one long spring.
Well I did find some air guns springs that look like they would work but I couldn't get any specs on them. The place was closed so I have to wait til tomorrow.
Yeah I guess I could go with some shorter springs and just stack them.
Well I found a 12"x 5/8"mold spring on ebay for 10 bucks delivered. It was blue and the specs I found for the blue springs call for anywhere to 100 to 200 lbs per inch load.
So there are two problems.
1) at 5/8@.625 and my aluminum pipe is .652, this might be ok or when it compresses it will get squeezed in the inside and reduce it power.
2) I am looking for 100 to 150 lb max so there's a 50% chance the spring could be too strong.
And a 3rd problem might be the aluminum pipe I am using if the spring is too strong it could tear out the pipe as the pipe is going to have three slots cut into it. The hole I made in the wood stock was very difficult to use and I hope the aluminum pipe holds up other wise I will have to get a larger diameter pipe.
Heres a pic
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Update, I have opted for a mold injection spring and it is on order.
These are heavy duty designed for thousands and thousands of compressions. It is somewhere in the 100 to 200 lb , I couldn't get the exact spec only know that it is the light duty mold spring(light duty in mold business is heavy duty in our terms). Hopefully under 150lb any stronger and I will have to go purchase(35 miles away) heavy duty steel pipe for the spring housing.
Here's a pic with the rails on it
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I think what you need is a longer spring with a lower spring rate to accelerate the bolt over a long distance. The amount of energy you impart to the bolt depends (approximately) on the average force between the fully cocked spring and the fully extended spring and on the distance over which the spring acts.Originally Posted by ZipSnipe
With a short powerful spring rated at something like 150 lb per inch of compression, compressed about 1-3/8" the force will be about 200 lb. The bolt is initially accelerated with this force of 200 lbs but this rapidly falls to zero when the spring is fully extended. The average force applied to the bolt over a distance of about 1-3/8" is about 100 lbs.
With a longer spring rated at around 50 lb per inch of compression you can pre-compress it to something like 100 lbs. Now the spring can be further compressed over a distance of 2" to give a final force of 200 lb. The bolt in this case is still initially accelerated with a force of 200 lb but this drops to 100 lb so the average force is 150 lb over a distance of 2 inches.
Even so a spring is not going to impart the energy as effectively as a regular bow with a string. On a bow the point on the string which contacts the bolt initially acts with a high force on the bolt and moves at about the same speed as the tips of the bow. This is because the string forms an acute angle and the bow is maximally flexed. When the string approaches its maximum travel the force applied to bolt declines but the string moves faster at the contact points as it 'snaps' straight between the ends of the bow; actually the string overtravels a small amount.
An open mind is a virtue...so long as all the common sense has not leaked out.
the mold spring is 12" . I will probably make up a couple of different lengths firing pins to see what works the best.
I have also decided since I had a vision of the spring busting thru the aluminum pipe and therefor bloodying my face to go ahead and drive out 35 miles and fetch me some good steel pipe.
Here's a a Sketch of the firing pin and the pipe that will house the spring the black represents the slots that need to be milled.
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Well got the mold spring in and it seems like it might be at about 100 to 110 lbs of compression which is exactly what I was looking for.
However I went to mill the slots in the aluminum tube and forgot the coolant and wala, no more aluminum pipe so now I'll get the steel one in the morning.
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