Measuring impact force on rubber spacers

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Discussion Overview

The discussion revolves around measuring the impact force absorbed by rubber spacers used in a polycarbonate shell. Participants explore methods for estimating impact loads and stress distribution, considering both theoretical calculations and experimental approaches.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant seeks to determine the impact force absorbed by rubber spacers and how to account for their distribution under varying loads, noting that some spacers may experience tension, compression, or shear.
  • Another participant questions the necessity of calculating the impact force, suggesting that knowing the maximum impact the spacers can withstand might suffice, and proposes a testing method using an accelerometer.
  • A participant highlights the complexity of the overall impact profile, noting that multiple spacers may engage during an impact and suggesting a model using damped springs to simulate impacts.
  • One participant expresses curiosity about the configuration of the spacers with respect to intermediate shells, inquiring about their attachment and interaction.
  • A participant acknowledges the difficulty of setting up an experiment and considers approximating calculations based on a single spacer, while also noting the advantages of a spherical surface for force distribution.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and method of calculating impact forces, with no consensus reached on the best approach or the importance of specific calculations versus experimental testing.

Contextual Notes

Participants mention limitations in their ability to conduct experiments and the complexity of modeling the impact behavior of multiple spacers, indicating that assumptions about force distribution and interaction may not be fully resolved.

Hank P
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I need to find the impact force that can be absorbed by these spacers as shown in the image. The spacers are radially distributed over the shell, and they are supposed to dampen any external impact loads to reduce the effective stress inside the inner shell. Since the FEA model is not showing results, I thought of attempting it by hand to get an approximate value. A curve relating the impact load and stress at the inner shell would be enough.

However, I cannot figure out how to account for the distribution of these spacers, since depending on the load some of these will be in tension, compression and/or shear. The shell is polycarbonate and the spacers are silicon rubber.
 

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Why would you need to know the impact force that can be absorbed?
Surely you just need to know that the spacers can take a prescribed maximum impact?
In which case you need only measure for impact with only one.

Note: it is not the sort of thing you calculate: drop it on something hard (or drop something hard on it) with an accelerometer inside.
That sounds drastic I know - to et an idea of what to expect, you set up a test rig to work out how each spacer behaves.

The overall impact profile will be very complicated - the thingy may initially strike 1, or 2, or 3 at once - probably just one at a glancing blow - then would bounce and roll, maybe also deforming, to bring other spacers in contact. There is no what to predict the exact results.

You could model specific impact profiles by treating the device as two spheres separated by damped springs.
 
This doesn't help with the question, but I'm curious: How does it work with the 2 intermediate shells? Do the spacers just go through holes in them, or are they also attached?
 
Thanks Simon. I guess I'll keep trying and see if the simulation works. Can't set up an experiment where I work. I could just do this with a single spacer, but a spherical surface helps distribute the forces all over, so my calculation would just be an approximation for a flat surface.

Danger, The spacers have cones with flats on both ends, which mate with flats on the shells.
 
Hank P said:
anger, The spacers have cones with flats on both ends, which mate with flats on the shells.
Okay, I think that I can sort of see that now. Thanks.
 

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