[ATM] Re: Second Thoughts on Mirror Support

[jonah at eecs.berkeley.edu (Jeff Anderson-Lee)]

Mark Holm wrote:

> > 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. 


Wrong material.  Try one of the following:

http://www.matweb.com/search/SpecificMaterial.asp?bassnum=PDOWC03
Dow Corning 733 Glass & Metal Sealant  (335 psi)
http://www.matweb.com/search/SpecificMaterial.asp?bassnum=PDOWC12
Dow Corning 832 Multi-Surface Adhesive/Sealant  (350 psi)

>
> 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.) 

The 2/3 lb is in pound force units. ;-)

Jeff