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  • Brass rod through glass; cracking issue?

    Discussion in 'The main mechanical design forum' started by josh smithenstien, Aug 31, 2016.

    1. josh smithenstien

      josh smithenstien New Member

      Jun 2016
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      Hello engineers,

      I have a fixture designed with 1/4" glass (untempered) and two brass rods going through it, 1/8" diameter. There will be a force downward on the rods, probably max 10lbf total. I'm trying to see if I need to put a bushing in there or rubber, or if I can get away with just the rods going through the hole in the glass. I've never really worked with glass, so this might be a silly question. Any insight greatly appreciated.
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    3. K.I.S.S.

      K.I.S.S. Well-Known Member

      May 2014
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      Hi Josh,
      It's certainly advisable to use a bushing, for two main reasons - Firstly, it appears from your image that the load will be applied vertically downwards and it also appears that you would expect it to be distributed evenly between the two posts (please correct me if my assumption is wrong).
      This won't happen in practice. Glass sheets are not high precision items, so the datum that the operator uses to reference his cutting bit will vary, as will the diameter of the holes relative to the diameter of your Brass rods. This means that most or all of the load will be taken up by only one rod.
      Secondly, because of the previously mentioned difference in diameters between the I.D. of the hole and the O.D. of the rod, when pressure is applied, your rods are going to tilt in the hole. Not by very much perhaps, but certainly to some extent.
      This is going to create two distinct elliptical stress zones, one on either side of the glass, almost like a cone cut in half from top to bottom.
      One will be at the bottom of the rod, where the pressure is applied, and the other will be at the top of the rod, on the other side of the glass.
      These are two very large stress raisers, and it's anyone's guess which will crack first, but under that load, one certainly will crack, primarily due to surface tension.
      A flexible bushing will allow the load to not only be more evenly distributed throughout the depth of your hole, but also possibly between both holes.
    4. Erik van Vliet

      Erik van Vliet New Member

      Dec 2015
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      Hello Josh, a plastic tube and a plastic ring must be used. Tube will be placed over bigger diameter of steel rod. A plastic ring will be placed under the nut.

      Make hole in and through the glass a little bigger than big diameter of the steel rob. Place inside a plastic tube. For small diameter no problem for the glass. Due to weight of glass, glass will hang in corner where biggest diameter gets smaller. Due to tension in the rod due to the nut at smallest rod diameter the rod will not hit/touch the glass. Practical Solution

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