PSN-L Email List Message

Subject: box flexure
From: S-T Morrissey sean@...........
Date: Wed, 12 Jan 2000 17:36:43 -0600 (CST)


 A compact 3-element crossed flexure can be made in a 3/4" square box
 from a 4 3/8" strip of 3/4" wide 0.002" to 0.005" phosphor bronze.
 Thinner material is more advantageous but fragile. I have used
 0.001" stainless steel.

For an initial pattern that is fairly robust:

 Inside flexure yyy is 1/4" wide;  outer flexures zzz are 3/16" wide,
 ("slot B" can be started with a paper punch and widened to 3/8")

 Folds are made at the lines "|"; initially crease x|y and z|x down
 about 45 deg, and y|b and b|z upwards about 90 deg.

 Then "tab A" is carefully twisted and inserted through "slot B".

 Then all the bends are increased to 45 degree inside angles.

	 "tab A"      flexure         base        "slot B"        top

	<  3/4"   >|<   1 1/16"   >|<  3/4"   >|<  1 1/16"    >|<  3/4"   >

	  x x x x x|               |b b b b b b|z z z z z z z z|x x x x x  
	x x x x x x|               |b b b b b b|z z z z z z z z|x x x x x x
	x x x x x x|y y y y y y y y|b b b b b b|               |x x x x x x
	x x x x x x|y y y y y y y y|b b b b b b|               |x x x x x x
	x x x x x x|               |b b b b b b|z z z z z z z z|x x x x x x
	  x x x x x|               |b b b b b b|z z z z z z z z|x x x x x  


   x x x x x x x x x x x x 	< xxx overlies xxx and are soldered together.
     y                 z
       y             z
         y         z
           y     z
             y z            < the center of rotation is where
             z y            < the flexures cross
           z     y
         z         y
       z             y
     z                 y
   b b b b b b b b b b b b
		  
For installation, 3/4" wide strips of 1/16" brass can be soldered  or
epoxied to the insides of xxx and bbb with extensions for mounting holes.
Note that this flexure ONLY works properly in tension, ie side bbbbbb 
pulled away from side xxxxxx.  But it will actually work as a miniature 
horizontal seismometer when made with 0.005" brass, with side bbbbb as 
the base and a small weight, like a quarter, placed on top of xxxxx.

You can print this and then cut out the strip to make a paper demo.

Also, for lighter loads and reduced restoring forces, thinner material
can be used, and the flexure strips made as small as 1/16" wide. This
makes a pretty floppy assembly, so V-shaped inserts (of 0.005" brass)
are fitted inside the flexures and the strips are lightly soldered to
them. NOTE the flattened end of the V where the flexures cross.  This
flat is about 1/16" wide, and the free length of the strips at the 
crossover is about 1/8". The V inserts more precisely control the
center of rotation, but reduce the overall flexibility of the hinge.

At the risk of pushing this diagraming style, a complete flexure would
look like: where BBBBBBB are the mounting bars, and VVVVVV are the
inserts.

   x x x x x x x x x x x x 	< xxx overlies xxx and are soldered together.
   yVBBBBBBBBBBBBBBBBBBBVy
     yV               Vz
       yV           Vz
         yV       Vz
           yVVVVVz
             y z            < the center of rotation is where
             z y            < the flexures cross
           zVVVVVy
         zV       Vy
       zV           Vy
     zV               Vy
   zVBBBBBBBBBBBBBBBBBBBVy
   b b b b b b b b b b b b

   BBBBBBBBBxxxxxxxxBBBBBBBB     < "top view" showing brass mounting bar
   B       Bzz yy zzB      B       with holes HH.
   B  HH   Bzz yy zzB  HH  B
   B  HH   Bzz yy zzB  HH  B
   B       Bzz yy zzB      B
   BBBBBBBBBxxxxxxxxBBBBBBBB

A proper drawing with all the details will be posted on the web site.
...... Sometime .... I have used a pair of these to make a 12" base
horizontal seis with a 10" boom. A S shaped bracket on the end of the
boom has the upper flexure pulling forward toward the mass, and the
lower flexure pulling away. Separate uprights are used for each, with 
the short lower flexure post in front of the tall upper flexure post.

Regards,
Sean-Thomas

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Larry Cochrane <cochrane@..............>