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Tensioning springs for polycarbonate film glazing

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Posted by nicksanspam on October 17, 2008, 7:48 pm
 
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 ($.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 (** $0.86 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'stock spring length (")
40 FP'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

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