Article 31382 of alt.solar.thermal: Path: news.misty.com!not-for-mail From: nicksanspam@ece.villanova.edu Newsgroups: alt.solar.thermal Subject: Tensioning springs for polycarbonate film glazing Date: 17 Oct 2008 15:48:23 -0400 Organization: Villanova University Lines: 68 Message-ID: NNTP-Posting-Host: acadia.ece.villanova.edu X-Trace: max.inside.misty.com 1224272905 28230 153.104.44.130 (17 Oct 2008 19:48:25 GMT) X-Complaints-To: abuse@misty.com NNTP-Posting-Date: Fri, 17 Oct 2008 19:48:25 +0000 (UTC) Xref: news.misty.com alt.solar.thermal:31382 We are making GE HP92W 0.010"x48" flat Lexan polycarbonate film into 8'x16' double-glazed panels for a sunspace. With an s = 3.75x10^-5 in/in-F thermal expansion coefficient, a piece might expand from 16' to 16+100s = 16.06' with a 100 F winter-to-summer temperature increase. With no tensioning, it might dish in or out by sqrt(8.03^2-8^2) = 0.69' in the center, ie 8.3". We could avoid this with some tension springs to stretch one edge of the glazing frame to take up 0.72" of summer slack. They might have to pull with a 50 pound force in summertime. We might cut Grainger's 1NBX7 spring ($9.35 for 3 12" pieces) into 3 4" springs (* below) with a prestressed length of 2.56" and 1.079" of travel. The 12" spring has an 11.6 lb/in rate, so a 4" piece would have 12/4x11.6 = 34.8, so we need to squeeze it 50/34.8 = 1.44" to make an initial 50 lb force, making the prestressed length 4-1.44 = 2.56". After 3/4" more compression to 1.81", the force would be 76.2 pounds. The solid length with all the coils touching would be (4"x3.25tpi+1)0.106 = 1.484", for a total permissable travel of 2.56-1.484 = 1.08". A stronger spring like Grainger's 1NBX8 (** $20.86 for 3 12" pieces) could also work, with a 5/16" bolt and an 11/16" OD bushing to fit the spring's 0.722" ID, eg a wooden dowl with a hole up the middle. Nick 20 LS=12'stock spring length (") 40 F=50'compressed spring force (lb) 50 DATA 0.964,0.121,3,14,0.844,0.106,3.25,11.6,0.732,0.092,3.75,8.5 55 DATA 0.640,0.080,4.25,6.6 60 FOR NS= 1 TO 4'#stock spring type 70 READ OD,WD,TPI,KS'spring OD, wire diam, tpi, spring constant (lb/") 73 ID=OD-2*WD 75 PRINT 200+5*NS;"'",OD,ID 80 FOR N= 2 TO 4'#springs per stock spring 90 FL=LS/N'free spring length (") 100 SL=(FL*TPI+1)*WD'solid spring length (") 110 K=N*KS'spring constant (lb/") 120 PC=F/K'prestress compression (") 130 CL=FL-PC'prestressed length (") 140 TR=FL-PC-SL'travel (") 150 PRINT 200+5*NS+N;"'";FL,CL,TR 160 NEXT:NEXT .964 OD .722 id 6" 4.214286" 1.915286" 4 2.809524 1.236524 3 2.107143 .8971429** .844 .632 6 3.844828 1.671828 4 2.563219 1.079219* 3 1.922414 .7829138 .732 .548 6 3.058824 .8968236 4 2.039216 .5672156 3 1.529412 .4024118 .64 .48 6 2.212121 9.212136E-02 4 1.474747 3.474748E-02 3 1.106061 6.06072E-03 Nick