[ATM] Re: Second Thoughts on Mirror Support

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

Hmm.  If that truly is the case, I would not expect Don's Discovery 
mirror cell to work--but it does!

In such a case, a 2.7e-4 inch bend would cause a 153lb differential 
between the inner and outer support points which would undoubtedly warp 
the mirror.

There are a couple of possible fixes:

1) use narrower and thicker pads: gives us a factor of 3.

2) make the back plate thicker [factor of 3?]  (and/or reenforce it) 
[factor of ???]

3) alter the placement of the collimation points so as to minimize the 
elevation difference at the mirror support points by placing them on the 
same contour line. [factor of 2?]

All in all, we can get about a factor of 18, by simple means, which 
still leaves us short by about a factor of 13.  We would need to 
seriously shore up the back plate to achieve that!

Hmmm.

Jeff


Stuart Hutchins wrote:

>Jeff,
>
>Right material, still the wrong number, tensile strength is not the modulus
>of elasticity. Check this link for some average values for silicone rubber:
>http://www.matweb.com/search/SpecificMaterial.asp?bassnum=O5200
>
>for modulus E, Cross section A, height H, compression (dh), the force on a
>sample is
>F = EA(dh)/H.  The tensile modulus listed above is 89 ksi or 89 E3psi.
>Further, the squat cylinder pad is glued on top and bottom surfaces, so it's
>expansion in Area is restrained.  This will boost the Force seen for a given
>(dh) to a higher number than predicted by the bulk modulus.  I'm curious to
>see what my test sample says, but it will be slow curing because top and
>bottom are covered by the aluminum plates which sort of have to be there.
>
>Regards
>
>Stuart Hutchins
>
>
>----- Original Message -----
>From: "Jeff Anderson-Lee" <jonah@eecs.berkeley.edu>
>To: "Mark Holm" <mdholm@telerama.com>
>Cc: <atm@atmlist.net>
>Sent: Thursday, February 05, 2004 1:07 AM
>Subject: Re: [ATM] Re: Second Thoughts on Mirror Support
>
>
>  
>
>>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
>>
>>_______________________________________________
>>ATM mailing list http://www.atmlist.net/
>>
>>    
>>
>
>
>  
>