From novak@clark.phys.nwu.edu Wed Jul 31 16:01 CDT 1996 Return-Path: novak@clark.phys.nwu.edu Received: from relay.acns.nwu.edu by belmont.astro.nwu.edu. (SMI-8.6/SMI-SVR4) id QAA19973; Wed, 31 Jul 1996 16:01:43 -0500 Received: from clark.phys.nwu.edu by relay.acns.nwu.edu with SMTP (1.37.109.18/20.4) id AA047977307; Wed, 31 Jul 1996 16:08:27 -0500 Received: by clark.phys.nwu.edu (4.1/SMI-4.1) id AA23514; Wed, 31 Jul 96 16:16:06 CDT Date: Wed, 31 Jul 96 16:16:06 CDT From: novak@clark.phys.nwu.edu (Giles Novak) Message-Id: 9607312116.AA23514@clark.phys.nwu.edu To: jdotson@belmont.astro.nwu.edu Subject: memo update Content-Type: text Content-Length: 3074 Status: RO Hi Jessie, Thanks again for the info on the Web page. It is now much more clear to me what is the best approach here. I finally got around to fixing the typo on the necktube strength calcualtion I did. Updated memo follows. _______________________________________________________________ Jessie: I've done some calculations using standard beam theory to determine the stresses and strains that the necktube will be under if we use it as the sole support at the "top" of the dewar. (1) The total gravitational deflection that I calculate is 1.0 mils worst case. This is the amount by which the top of the He-4 can sags when the dewar is moved from vertical to horizontal. It results from the individual sags of the 3 sections of necktube. (2) The maximim stress in the neck tube is 3,000 psi. This stress occurs at two points along the length of the necktube: the solder joint to the piece that is at 300K, and, following the necktube down from this solder joint, at the point where the necktube meets the OVCS copper thermal link. Values for the tesile strength of stainless steel in the Goodfellow catalog range from 65,000 to over 100,000 psi. (3) The following assumptions were used in the above calculations: Each section of necktube is 3 inches long. (3 sections) The wall of the necktube is .040 inches .625 inch OD necktube The insides of the dewar weigh 55 lbs. The center of mass of the dewar insides is half way between the two support points. The 55 lbs. of weight is equally distributed among the OVCS, IVCS and 4 K components. (this last assumption only affects the calculation of the deflection, not the maximum stress, which occurs on the outermost segment of the necktube.) I used room temperature values for the elasticity and strength of stainless steel. Stainless steel (at least 304 stainless steel) gets slightly stronger and stiffer as you cool it. It apparently does not go into a brittle phase like some high carbon steels do. I got this info from Barron (Cryogenic Engineering). I would estimate that my calculations are good to within a factor of 2. The formulae themselves are very simple and given in many texts on mechanical design. I had to think pretty carefully about which formula was applicable to our case: where a simple cantilevered beam supports a load at the far end of the beam and that load is also supported on the opposite end by the Kevlar. But in the end the calculation is straightforward. The maximum stress is inversely proportional to the square of the necktube OD and inversely proportional to the wall thickness, and directly proportional to the length of the necktube segment and the weight. I assumed that the weight is shared equally between the two ends of the support structure, when in reality the ratio of the weights shared by each end is the inverse of the ratio of the distance of the center of mass from each end. The conclusion is that we don't need lateral support of the necktube provided that we don't make major changes in the dimensions of the necktube. Cheers, Giles