Calling all mechanical wizards

I'm no expert, go google "constant force springs". There are spiral wound springs a like a tape measure that provide constant force within the limits of their range.

something like this:

Not my thing..but I'll guess..

Motor with torque control?

D from BC myrealaddress(at)comic(dot)com BC, Canada Posted to usenet sci.electronics.design

When I worked in the U.K. "Tensator" produced constant force springs. Nowadays they seem only to produce products that incorporate them.

looks more promising.

-- Bill Sloman, Nijmegen

Those gas cylinders that are used on car rear hatches provide constant force, and are available in many sizes and strengths. The force is, however, greatly dependent on speed, which may or may not be a problem in your particular application.

If you use a spring with very low K and preload it to 20lb, you would get a relatively constant force over your desired range. Not absolutely constant, though. Also, the spring may need to be impractically large.

Flat springs wound round a large diameter mandrel work pretty well. The part of the spring that's already been pulled straight produces side force but no pull--it's just the part that's being uncurled that determines the spring constant.

Cheers,

Phil Hobbs

A looooong bit of elastic shock cord? Parallel up enough cords to get the 20 lb

A brushless 3-phase motor with constant voltage drive will sort of do that (you'll see torque ripple that is periodic with rotation of the armature).

A servo motor with limit sensors and force for feedback.

Bob

Search for constant force spring on Yahoo or Google.

Bob

Or

Not terribly impressive lifespan on those things.

Good catch. They do get a lot of flexing in normal use--the bend gets completely straightened out, over and over and over--so I suppose a short life is understandable.

They're pretty useful for stuff like vacuum chambers, though--you can effectively reduce the weight of some big heavy Conflat thing to near zero, which makes it much easier and safer to work with. Hopefully one wouldn't need to take it apart 2000 times!

Cheers

Phil Hobbs

Depending on the motor you may see a _lot_ of torque ripple. This bit me hard, once (late in the project, when we couldn't do much about it, naturally).

So if you go there, use a motor that's specified for torque ripple.

a pneumatic or hydraulic cylinder with a 1 square inch area and 20 psi air or oil, or any equivalent combination (1/2 square inch & 40 psi, 2 square inch and 10 psi, etc...

Just curious, why is the weight and pulley inconvenient? And would a teeter-totter (aka see-saw, aka lever) be any more convenient for your application?

That looks very promising. 20 lbs may be tough (large?), but at least there's hope.

Thanks!

Bob

The application is a little odd. It involves something that will maintain the tightness of the last string pulled when stringing a tennis racquet. It's a little difficult to explain exactly what I'm doing, but having a small device (2" x 2" x 2" max) would be ideal.

I thought about a level-fulcrum setup, but it doesn't buy me much in terms of total space and placement flexibility.

Thanks.

Bob

Like the air flow meter spring on a 280Z. I am thinking constant pressure air cylinder, or suction, or long spring.

greg

DC motor and small pulley?

...Jim Thompson

Goofy idea: cylinder, piston with o-rings, vacuum inside.

Or a thing like garage door openers use, a long torsional spring twisting a shaft with a cable wound around it. Not quite constant force, unless you taper the shaft.

A cable wrapped around a tapered, maybe grooved, drum can do all sorts of nonlinear compensations.

I once met the guy who invented the Nautalus exercize machines, when I was a kid and he ran an alligator farm out on old Highway 90 near Slidell. He used to land his plane on the highway. Nautalus machines have some nice nonlinear force mapping mechanisms.

John