What should be the glass thickness at high vacuum chamber viewport. How should I calculate it? I would like to have a view diameter of 20". Thank you.
- posted
18 years ago
What should be the glass thickness at high vacuum chamber viewport. How should I calculate it? I would like to have a view diameter of 20". Thank you.
Uncle Al gave very good advice.
Please consider a much smaller window, or no window at all. A dome will be stronger than a flat window. (Domed to put the glass in compression, NOT IN TENSION.)
You ask a question with serious consequences, as failure of this window could cause serious injury, or even death.
Do not rely blindly on the information you get here. Your design should be reviewed by someone at your institution (a licensed engineer, or a safety review committee, preferably both) before you try it. Also try contacting vendors. You're talking about an expensive window, so they should be willing to provide assistance.
I remember years ago that a manufacturer of optical windows (can't remember name, sorry) had a product brochure with a table showing required window thickness versus diameter and pressure difference. Later versions of this brochure omitted the table -- I suspect for liability reasons.
[Along similar lines, all I'm trying to do here is point you in the right direction. I'm not designing this window for you, and I assume no liability. Your final design should be reviewed and blessed by someone qualified to do so.]Normally, the way you would design something like this is that you would get a formula for the stress in the window from a source like Roark's Formulas for Stress and Strain, and then compare the calculated stress to the allowable stress in the material you're using.
But, for glass, manufacturers seem reluctant to tell you what value to use for a safe working stress.
This may be partially for liability reasons, but also because the strength of glass is a complicated subject:
Reading material:
According to
Doyle and Kahan
The CRC Handbook of Mechanical Engineering (pg. 12-97) states: "For most types of glass a nominal strength of 70 MPa and a design strength of 7 MPa are typical." (70 MPa = 10,000 psi and 7 MPa = 1,000 psi.)
Additional considerations:
MOST IMPORTANT: I apologize if I'm wrong, but your question causes me to think that you've never had any training or experience in stress analysis. Particularly in a situation where failure could cause serious injury or death, I can't think of a worse situation than having someone who doesn't know what they're doing relying on advice from a newsgroup. Get help from a qualified professional. Then have another qualified professional review the design.
Olin Perry Norton
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