Any spring experts here?

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.

You want the spring with the lowest spring rate (pounds/inch) that will support your load in the space available. Some of the spring mfrs have selection tools online that make it easier to zero in on a spring than McMaster. This will give you enough information to go back to McMaster to see if they carry something close enough.

For example:

Associated Spring and Lee Spring are two other places to check.

Google "constant-force springs".

Good Luck! Rich

Do a Google on springs. There should be some good design info there.

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!

McMaster has the pounds force and pounds/inch in there search online. It should allow you to get close.

A few years ago I designed a zero gravity simulator for the air force. The concept is a hinged cantilever arm with a linear spring attached between the end of the arm and a point equal distant above the arm and directly above the spring. If the arm is horizontal from the mount, the spring, arm and mount form a 45 deg triangle. Without a weight on the arm it will spring vertical. If one adds an appropriate weight on the end of the arm it is balanced by the spring and will behave as gravity is gone. Alternately one can place a pulley on the upper mount and install the spring in back. One way to look at it is that it implements a zero stiffness or constant force spring.

An other option to implement a low stiffness spring is to use a bucked beam. Think of a yardstick pinned at both ends if on loads it above the bucking load the yardstick deforms with a nearly constant force.

Why not call a spring company and speak to somebody who knows springs?

There's one here called Stanley Spring and Stamping Corp.

The best constant force spring is a air cylinder with a pressure regulator suppling the air. Constant pressure and no fatigue failures.

Apart from friction in the seals. Even on an oil hydraulics system. If you'd ever tried it you'd know. A bellows or air spring would be better though.

It's best to be sure of oneself when playing the know-it-all...

Frictionless and low-friction cylinders are also available from SMC.