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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
vibration node Q
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<blockquote data-quote="Austin" data-source="post: 30817" data-attributes="member: 1956"><p>A couple comments here: You are correct about a longer bbl "moving" more than a short one, but when it comes to bbl harmonics, what is happening is that as the bullet travels down the bbl, the majority of the movement is in a circular motion due to the spin the rifling is putting on the bullet. These harmonics cause the bbl to move in a circular position at the end and not so much in a "whipping" motion. Due to harmonics, the bullet travels down the bbl and exits at any given point in this rotational movement. Thus, you might have a bullet leaving the bbl as it is passing 12:00, then the next shot at 3:00, and the next at 6:00. This is the cause for triangular patterns on the target. I have found that by putting a small piece of delrin (which has a very low heat expansion factor) in between my stock and the end of the bbl, I can "tune" the harmonics of the bbl for any given load. Also, it depends on other factors such as the composition of the bbl, the surface area of the bbl, and the weight. I do have some printouts using an O-Scope to measure vibration frequency along with all the math showing the Sin waves of one of my bbl's and I think it may have a lot of the info you are looking for. One thing I can recall about V Node Q is that; if the forces acting upon an object vary quickly, as in a rifle bbl, then, the displacement within the node is going to vary relative to time. This being true, then the outside forces are extended by the distribution of the forces of interia and, if I am not mistaken (which I may be), these are directly proportional to the acceleration. Again, if I am not mistaken, you can use the equation: Ku = Q - Mu". Where: M = diagonal mass of bbl, u" = the second derivative of the Q node displacement or rotation. It's been a LONG time since I took Engineering Physics, so that's mostly all I remember. When Q starts to approach 0, you run into Eigen Values and it gets difficult. If I can find all the stuff I did on the oscilloscope I will forward it along to you. I know I didn't answer all your questions, but, hopefully, this can be of some help to you. Good luck!</p><p></p><p>TH</p></blockquote><p></p>
[QUOTE="Austin, post: 30817, member: 1956"] A couple comments here: You are correct about a longer bbl "moving" more than a short one, but when it comes to bbl harmonics, what is happening is that as the bullet travels down the bbl, the majority of the movement is in a circular motion due to the spin the rifling is putting on the bullet. These harmonics cause the bbl to move in a circular position at the end and not so much in a "whipping" motion. Due to harmonics, the bullet travels down the bbl and exits at any given point in this rotational movement. Thus, you might have a bullet leaving the bbl as it is passing 12:00, then the next shot at 3:00, and the next at 6:00. This is the cause for triangular patterns on the target. I have found that by putting a small piece of delrin (which has a very low heat expansion factor) in between my stock and the end of the bbl, I can "tune" the harmonics of the bbl for any given load. Also, it depends on other factors such as the composition of the bbl, the surface area of the bbl, and the weight. I do have some printouts using an O-Scope to measure vibration frequency along with all the math showing the Sin waves of one of my bbl's and I think it may have a lot of the info you are looking for. One thing I can recall about V Node Q is that; if the forces acting upon an object vary quickly, as in a rifle bbl, then, the displacement within the node is going to vary relative to time. This being true, then the outside forces are extended by the distribution of the forces of interia and, if I am not mistaken (which I may be), these are directly proportional to the acceleration. Again, if I am not mistaken, you can use the equation: Ku = Q - Mu". Where: M = diagonal mass of bbl, u" = the second derivative of the Q node displacement or rotation. It's been a LONG time since I took Engineering Physics, so that's mostly all I remember. When Q starts to approach 0, you run into Eigen Values and it gets difficult. If I can find all the stuff I did on the oscilloscope I will forward it along to you. I know I didn't answer all your questions, but, hopefully, this can be of some help to you. Good luck! TH [/QUOTE]
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