Recoil Spring

   
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Recoil Spring, Real Time Dynamic FEA:
Design a recoil spring for a large caliber semi-auto rifle with limited space.

Special features:
3D virtual compression spring of various cross-sections and other spring parameters.

Output:
Displacement and velocity of an inertial mass (reciprocating bolt-carrier) and reaction forces on same.

Comments:
The semi-auto rifle chambers a large caliber cartridge that would ordinarily result a rifle with an extremely long "pull" (the distance from the trigger to the rifle's butt-stock) making for an impractical design if the designer was forced to install a compression spring wire with a round cross-section. But, if the spring cross-section and other factors could be optimized to handle the large impulse in a much smaller space, this would change an impractical rifle "pull" design to one that was ideal. All of the forces acting on the spring are NOT known beforehand, making spring design by conventional methods problematic. However, the spring and bolt carrier can be modeled as a dynamic, inertial system in the virtual world of finite elements.

By knowing the dynamic chamber pressure curve (pressures as a function of time) and comparing the performance of this system to similar systems, which are well-known and characterized, like the M16, it is possible to adjust the spring parameters using various wire cross-sections to balance the impulse against the available bolt carrier excursion distance to complete one ejection and battery cycle. The spring resulting from this analysis exhibited a rectangular cross-section and fit within the allocated space and yet met other spring criteria for fatigue immunity, material, manufacturability, etc.

Click on the thumbnails above to access some of the data generated during this project.