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Posted: 1/27/2010 10:34:37 AM EDT
| If the weight of a bolt and recoil spring is known for a blow back action, how do you figure out the change in spring weight when reducing the bolt weight? Does the equation change if the recoil spring is going to be placed in a new position? For example: original design is to have spring behind the bolt, new design is to have spring above and in front. |
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The spring design will be dictated by its geometry and material (which come together as the spring rate) and the stroke, plus the impulse imposed on the bolt.
The weight of the spring has almost nothing to do with the problem, it's a fall out of the other requirements. That's not a very good answer, but it's all that is possible from your question. |
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here is a site i found looking for spring information maybe it will help...
http://home.earthlink.net/~bazillion/intro.html |
| I have an idea to convert an AR 9mm into a pistol with the recoil system above the barrel. I understand that the strength of the spring is going to have to do 2 things: 1 keep the bolt forward for correct timing. 2 absorb the recoil allowing for enough rearward movement to reload and close on the chamber. Can this question generate the answer that I have been looking for going on 2 yrs? |
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Well, I'm not a gun designer but I think I learned some stuff in my ME classes that might point in the general direction.
If the bolt and carrier are one piece together, then the bolt assembly could be modeled as a simple mass-spring-damper system. I have no idea how they would approximate damping, or how significant damping forces would be (damping would be due to stuff like oil viscosity, air resistance, etc..). So, if you solve the differential equation you can get the output in terms of bolt displacement vs time for an input of force vs. time. The force would be the rearward force from the cartridge being fired over time (you'd have to make some approximations for that function, as the pressure is not uniform). Then, you'd just need to adjust your constants of spring coefficient, damping, and mass of bolt and carrier so that your displacement is small enough until the bullet leaves the barrel and the pressure drops.. but still gets enough momentum in that short time that it can eject and feed. If you had a multiple piece bolt and carrier assembly, you'd have to make the equations to account for any independent motion, etc. Here is a link to a pdf that gives a little explanation (if you type "mass spring damper" google will give a lot of hits) : http://www.seas.upenn.edu/~ese112/labs/MSDSystemLecturePart%202.pdf I can't imagine the spring location would matter, unless it caused significant carrier tilt and binding or something. That being said, these guns run with all different kinds of ammo, which will all have different pressure curves and therefore force inputs. So.. the system can not be that finely tuned. But if you are lightening the bolt obviously there is the risk of it opening too fast and getting a ruptured case. Again, I have no experience so don't take my word for it, but I hope this can help at least some. |
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