5 Window Support Case 1: Simply Supported Pressure Lateral translation k = 1.24k1 = 0.696Case 2: Rigidly FixedPressurek = 0.75No Lateral translationk1 = 1.71
6 O-ring Groove Window Force Light coating of Apiezon vacuum grease L or M~20%0.139”Cryostat CaseCryostat CaseO-ring(2-240)0.105”Compressed O-ring0.150”
7 Strength of Material Strength S.F. = Stress Rule of Thumb: Safety Factor = 10Use reference book to get strength of material [in PSI]Normally the Modulus of Rupture (MOR) is usedSafety Factor is:StrengthS.F. =StressSolve for Stress
8 Maximum Stress w R2 Sm = k t2 Solve for t Outside Pressure = Atm = 760 Torr = 14.7 PSIInside Pressure = 10E-6 Torr ~ 0 PSIStress of the Window:w R2Sm =kt2k = coefficient k for circular platesw = uniform pressure across window (outside P – inside P)R = radius of Clear Aperture of windowt = thickness of windowSolve for t
9 Maximum Defection ym = k1 E t3 w R4 Solve for t Window ‘bowing’ can affect optical designDeflection causes plano window to have power, thus creating a meniscus lensOptical design will govern amount of deflection (sag) allowableIf window is Simply Supported and O-ring does not compress fully:ym =k1E t3w R4k1 = coefficient k1 for circular platesw = uniform pressure across window (outside P – inside P)R = radius of Clear Aperture of windowE = Young’s modulust = thickness of windowSolve for t
10 Rules of ThumbTypical k value for stress calculation used in practice is 1.00Reasonable (and typical) material choice for NIR waveband is Fused Silica or BK7Fused Silica: Young’s modulus = 1.06E+07 PSIModulus of Rupture = 7600 PSIBK7: Young’s modulus = 1.19E+07 PSIModulus of Rupture = 2400 PSIk1 for deflection calculations vary from to depending on whether window is constrained by Case 1 or 2Typically use 0.43O-ring types:Buna-N has highest permeation and retains waterViton has lowest permeation and minimal water retention
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