Constant Force vs Displacement Spring?

[sethgus]

Besides a Nickel Titanium spring (which is not accurate in saying constant force vs displacement), are there any combinations of springs or designs of springs that would equate to a constant force vs displacement.

I was thinking conical spring but I'm not sure this is what I'm looking for. I understand there are constant force springs, but these are more tape like and I need something that is closer to a compression spring. The other ideas are closer to a progressive spring, but according to my calculations this would only give a combined spring coefficient.

 

[MikeHalloran]

A Belloram rolling diaphragm cylinder connected to a relieving pressure regulator and an air supply would do what you want. The stroke is limited.

At the cost of more friction, a normal air cylinder will do it over a longer stroke.

At the cost of still more friction, a band-sealed cylinder will do it over a still longer stroke. (I forget right now what you call them. They comprise a C-section extrusion. The load attaches to the side of the piston, not the end. The connection between load and cylinder slides through some interlocking metal and rubber bands that seal the open side of the C but allow the connection to move.)



Mike Halloran
Pembroke Pines, FL, USA

 

[C2it]

It's not clear what scale you are working in, but if big, constant force spring units are common in complex pipe support design. They rely on a geometric pair of moment arms cunningly orientated to act on a normal variable coil spring, always in compression. regardless of arm position, the load is constant. You might be able to adapt a unit or the design.

Try

http://www.pipingtech.com/products/ptpcat/content.htm

or any other of many spring support suppliers.

 

[sethgus]

The problem is this is a very small scale. Without going into exactly what it is, the spring housing diameter is around .1" and the forces are really low as well. I would like a constant .4 to .5 lbs, but I really don't think this is possible.

The other idea I had was to come up with a mechanical stop where one spring would stop and another take over at a certain distance, thus keeping the same displacement force throughout the spring distance.

 

[IRstuff]

This is the smallest model that Airpot makes, which is still huge compared to your requirement, but perhaps they've got something else that might fit the bill:

http://www.airpot.com/beta/html/model56.html

TTFN

FAQ731-376

 

[sethgus]

That looks great, except it is dampening, where I need a constant spring force pushing out. Very similar design though.

 

[IRstuff]

If you push on it, it'll resist with the specified 0.5 lb force for the duration of the stroke. However, it doesn't store the energy, so there's no return force.

Perhaps you need to describe what function you want in more detail, rather than what you assume to tbe the solution.

TTFN

FAQ731-376

 

[sethgus]

The function would be a constant force of .4 to .5 lbs with any type of device pushing out. It would basically be the Airpot device with a spring inside the cylinder instead of the graphite plug. It has the capability of moving in and out at around 600000 cycles of approx .25 to .5 inches of displacement. The problem with the normal compression spring is that the force increases as the spring compresses (obviously) so there is increasing and decreasing spring forces constantly.

Would opposing pole magnetics cause a constant force over displacement?

 

[MikeHalloran]

You could capture a long, low-rate spring in a cartridge of modest length, and design it so that a projecting plunger would require 0.4 lbf to start moving, and the required force would rise to no more than 0.5 lbf with the plunger fully pushed into the housing. I'm guesstimating the cartridge would have to be about an inch long, for a 1/2" plunger stroke. The force wouldn't be constant, but entirely within the range you gave. The spring would buckle and chafe the housing, so you'd need a wear- resistant plastic housing and/or grease inside. Would that work for your application, or to you absolutely need a perfectly constant spring force?



Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

So, it would seem that a spring with 0.2 lb/in spring constant precompressed 2 inches would provide 0.4 lb at the start of stroke and no more than 0.5 lb through a 0.5-in stroke.

TTFN

FAQ731-376

 

[sethgus]

Thanks Mike, that is a good solution. It would work actually. Currently on some of the smaller length tubes, we are close to bottoming the spring, giving the higher forces. I have a feeling though that it probably won't change from our current design. I just wanted to see if there was a solution out there for a constant force spring in a smaller package yet.

 

[Rob45]

There are constant-force springs commercially available that consist (roughly) of a long flat strip wound inside a housing. I forget what they were designed for or the manufacturer, but I remember a company giving small ones away as toys at an SAE show many years ago.

 

[SomptingGuy]

This outfits looks like it has a 0.5lb spring:

http://www.smallparts.com/products/descriptions/cfs.cfm

 

[electricpete]

At the springs link here you'll find info on constant force springs that resemble a clock spring. Don't know if it is small enough for you

http://sdp-si.com/D757/Libdownload.htm

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[sethgus]

Thanks guys. I'm looking for a compression spring type device. I knew about the tape-like constant force springs, but it wouldn't work in my application.