Frequency of a wire/spring system?

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Discussion Overview

The discussion revolves around the frequency of a wire under tension, specifically a wire fixed at one end and connected to a spring at the other. Participants explore discrepancies between theoretical calculations and experimental results, focusing on the implications of boundary conditions and measurement techniques.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant calculated the first mode frequency of a wire under tension to be 73.6 Hz using a fixed/fixed closed form solution, while experimental results showed a frequency of 110 Hz.
  • Another participant suggested that the ratio of the two frequencies is close to the square root of 2, implying a potential error in the calculations, possibly related to a missing factor.
  • The original poster clarified that the theoretical frequency assumes both ends are fixed, while in practice, one end is attached to a spring, which may contribute to the frequency discrepancy.
  • Participants inquired about the method of measuring tension, with one confirming the use of a 2 lbs weight to create tension in the wire.

Areas of Agreement / Disagreement

Participants have not reached a consensus on the source of the frequency discrepancy. There are competing views regarding the effects of boundary conditions and measurement methods.

Contextual Notes

Theoretical calculations are based on assumptions that may not fully align with the experimental setup, particularly regarding the boundary conditions of the wire and the influence of the spring.

Liam B
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Hi, I have a wire that is fixed on one end and is under 2 lbs (approx) of tension on the other end due to a spring. Using a fixed/fixed closed form solution I found the first mode to be 73.6 Hz. In a vibration test, we found the first mode to be 110 Hz. Does this seem reasonable and where do you think my error is coming from? The wire is made of 7x7 braided stainless steel and is .018 inches diameter.

Liam
 
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Welcome to the forum.

Maybe I could help you more effectively if you showed your work. But I do note that the ratio of the two is suspiciously close to the square root of 2. Did you drop a factor of root 2?
 
Thank you, I'm happy to be here.

No, that is a good question though. The 73 Hz. is correct and I have checked it with several different sources. It assumes however that both ends are fixed and in reality one end of the wire is attached to a spring. I just didn't think that I would get 1.5x error.

My original equation was

CodeCogsEqn.gif


And this gives the 73Hz.
My Setup:
IMG_20150612_130508.jpg


This is the end with the spring. The other end is fixed.
 
Last edited:
And how did you measure the tension?
 
nasu said:
And how did you measure the tension?
I found weight that measured 2 lbs, hung it off the end of the wire which compressed the spring on the other end, and then crimped and cut the wire at that length.
 

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