[ATM] Re: Second Thoughts on Mirror Support

[mdholm at telerama.com (Mark Holm)]

 > Mike Lindner wrote:

>Jean-Guy Moreau wrote:
>
>>>>>Does anyone know how compute how much compression a 25mm round by 3mm
>>>>>thick RTV pad would experience under a 0.306kg load?
>>>
>
>>From http://www.matweb.com/SpecificMaterial.asp?bassnum=O5200
>Compressive Yield Strength of silicone is 4 MPa or 580 psi.
>
>Using english units (sorry, rest of the world), a 1"x1/8" cylinder holding a 
>2/3 pound mass would be compressed about 0.007". Empirical confirmation of 
>this analysis is left as an exercise to the reader. ;-)
>
You need to use the compressive modulus not the compressive strength. 
 Compressive modulus is not listed at the web site referenced, but 
tensile modulus is.  For small amounts of compression, compressive 
modulus is often pretty close to tensile modulus, so it is probably safe 
to  use that number, which is  89.9 ksi  Which means 89900psi.

Also, don't forget to convert that 2/3 pound mass into pound force 
units.  (Multiply by acceleration due to gravity, but in what units? 
 That is why I usuallly end up converting to metric.  It is a lot easier 
to figure out the units that way, even though one often ends up with a 
lot of zeros to deal with.)

Mark Holm