On Sun, 2 Apr 2006 20:50:50 -0400, "Glenn Ashmore"
wrote:

Little side project. I want to build a vertical stabilizer arm for my
Glidecam camcorder stabilizer. The theory is similar to the Steadicam.
Picture a 10"x3" parallelogram pinned with bearings at the corners. Long
dimension is horizontal. One 3" side is fixed and the other floats and
holds the Glidecam. A spring runs diagonally between the top end of the
fixed side to the bottom of the floating side. The moving parts of the arm
weigh 2 lbs and when level the COG is 7" from the fixed end. The Camcorder
and Glidecam weigh 4.2 lb and the COG is 12.75" from the fixed end.

How do I figure out which spring to use to just maintain the arm slightly
above level? If the spring rate is to high the stabilization effect will be
diminished and if to low it won't recover fast enough. The spring can't be
longer than 6" with no tension and will need about 3.5" of extension to
allow the arm to move from 30 degrees above to 30 degrees below level.
There will also be an adjustment screw to pretension the spring.

I'll assume that the 6" rest length and 3.5" max extension are due to
unmentioned constraints because that's too short for the geometry. The
spring will need a "dead" extension -- a link or piece of wire 2.29"
long.

The load moment at level is 67.55 lbf*in. With the 10" member
horizontal the diagonal spring is at a 16.7 deg angle from
horizontal. It will therefore need to exert 23.5 lbf to balance at
level. Extension at horizontal is 2.15", so spring constant k is
10.93 lbf/inch. To balance "slightly above" level the spring constant
will need to be "slightly greater".