Pete C. wrote:
I need help in selecting an extension spring to counterbalance most of a
3.5# load over a hair less than 1" of travel. The 3.5# load needs to
rest solidly against the bottom stop at rest, and I need most of the
3.5# load counterbalanced through the 1" stroke, so it seems I need a
spring where the rate will not change significantly over that 1" travel.
I have a good amount of room to mount a fairly long spring, up to about
5" contracted length.

I see plenty of springs on McMaster to choose from, but I'd like to be
able to order the correct spring the first time.

Thanks!

Before you order, you would be better asking the question from a aspring
maker.
He will know the modulous of elasticity of different thicknesses of
steel wire at what diameter coil.
IE spring rate in ozor lbs per 1" of movement.
then when you have this information you can order up a spring to do
the task you specify.