Request for help with extension-spring biasing problem (drawing illustration link provided)

Hello everyone,
I would like to ask if anyone can please help me with the following problem.
I have a small radial plate-cam and bearing housing (cam-housing)
that is oscillated manually by hand via a small lever. The cam-housing is oscillated both clockwise and counter-clockwise from it's centered neutral position at rest. The housing is centered by two opposing extension springs connected to the underside of the housing. The springs are basically connected in series with the housing itself being a connection link between one end of each of the two springs. The other spring ends are connected to anchor points, with one anchor point being adjustable.
I have created three small GIF drawings that illustrate the design layout, which I have zipped into a single small 44.8 KB folder. The file was zipped with the free 7-Zip http://www.7-zip.org/ but you should be able to open it with winzip or most anything. The drawings can be downloaded from the following mediafire link...
http://www.mediafire.com/?eggxtcs95wy
This is a dual roller conjugate radial plate-cam having two rollers that work on an inner and outer cam profile. The drawings show the two opposing springs connected to the cam-housing, but they do not show springs which are connected to the follower-roller swing arm, which counter the load that the outer roller exerts onto the outer cam profile or curve. At the centered neutral position shown in figure 1, the cam follower rollers are in contact with the center of a 3-degree dwell on the cam profile. After the lever is activated and released, the springs connected to the roller swing arm bring the cam back to it's home position, just to the point where the rollers start to contact the start of the 3-degree dwell. After this point, the opposing springs connected to the cam-housing bring the cam back to it's neutral position so that the rollers are in contact with the center of the 3-degree dwell on the cam profile, at the approximate mid-point of the cam profile. Another function of the opposing springs connected to the cam-housing is to hold the cam housing at it's centered neutral position, where the rollers are in contact with the dwell at the cam mid-point.
My main goal is to have the cam be returned as accurately as possible to it's centered neutral position after the lever is activated and released. As long as the cam is returned so that the rollers contact either edge of the 3-degree dwell, or anywhere in-between, the cam follower / rollers will be in their exact neutral position. So, I have a 3-degree dead zone or a 3-degree room for error when returning the cam to it's neutral position.
Even with no rollers in contact with the cam & nothing but the housing itself just oscillating on a shaft with no load, when I move the lever clockwise from it's neutral position and release it, it seems to return to the same exact position each time, but when I then move the lever counter-clockwise from it's neutral position, it is about 1.25- degrees off from where it was before. When the rollers are in contact with the cam and a load is applied, this seems to double the problem to 2.5 degrees, so the end of the lever is basically " from where it should be at neutral. Some of the problem is due to some mis-alignment I had between the cam & rollers which I can fix, and I can also reduce roller friction as much as possible, but this still does not explain why I am 1.25 degrees off even when there are no rollers or load in contact with the cam.
Adding some type of physical or mechanical stop at neutral would seem a logical way to bring the housing back to it's exact position each time, but there are some tactile requirements for this design so I don't want to put a physical or mechanical stop at neutral. I want to preserve the balanced & smooth feel in the lever around the neutral position as much as possible. The opposing springs I am using have a rate of 140 lb per inch, but I may be able to use springs having a rate ranging from 30 to 60 pounds per inch.
Here are some things that occurred to me...
1. Presently, I have an adjustment nut at one end of the two springs which are connected in series at the cam housing. This means that I can only lengthen or shorten both springs at once. I use this adjustment to initially align the cam so that the rollers are in contact with the center of the 3-degree dwell at neutral. However, If I could put an adjustment screw at the spring connection point on the cam, so that when an adjustment is made, one spring is lengthened and one spring is shortened at the same time, it seems this might help. Do you think an arrangement like this may solve my problem ?
2. The spring hooks may be repositioning themselves at the cam-housing as the cam-housing is oscillated. Perhaps I could use a flat-head screw through a loop-end instead of a hook, to connect the springs to the housing. This would prevent the spring ends from repositioning themselves on the housing as it's oscillated. I tried to screw the hooks down with a center screw, but it did not seem to help as I could still see them moving. Perhaps loops are necessary if a screw is used. Referring to the drawings, the spring hooks are connected vertically to the cam, but perhaps rotating the springs 90 degrees about their longitudinal axis and connecting the hooks horizontally would be better ? I have not tried this yet as I will have to redesign the spring anchors.
3. Perhaps the problem is coming from manufacturing variances in the spring rates and/or tensions. I could try to go with a precision made extension spring. Compression springs and torsion springs would be harder to implement, but still possible, if they could offer some advantage. I prefer to use extension springs if possible. Perhaps a urethane or non metallic spring would be better ? I've been thinking of something like a thin rectangular piece of rubber or urethane that can be screwed or connected to the underside of the housing. I'm hoping I can find a compact and cost effective solution (preferably a stock or easily made biasing element).
I don't need this thing to be perfect because the cam dwell gives me some room for error, but I do need to be able to reliably return the cam to it's neutral position so that the rollers are in contact with some portion of the 3-degree dwell at neutral. If I can get a repeatability of or even 1-degree in each direction from neutral, that should be OK.
Presently, the thing is darn close to where it needs to be but I just need to tweak it to get a higher degree of repeatability and accuracy when returning the cam to neutral.
I would appreciate any advice or suggestions anyone may have.
Thank you. John
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snipped-for-privacy@yahoo.com says...

snip
If the springs were adjusted to bring the mechanism close to neutral and then go completely slack, you could add a magnet to do the final centering. Suitably adjusted, a magnet could provide a much different "feel" than a ball-detent device.
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On Thu, 20 Sep 2007 22:07:27 -0700, John2005

The double spring and adjustment layout is unfortunate. A single spring with an adjustable anchor in tension might work better. The conjugate cam and roller layout is unfortunate. A single roller on a single cam might work better.
This design is way complicated for the purpose intended, seems to me. A production automobile designer might do much more with much less material altogether, I believe.
Perhaps a flex with two fixed mounts, and a lever fixed at the flex-center might do the job. Total item count, 1 flex spring strip 2 fixing screws 1 lever with clamp to flex spring.
About 4 or 5 parts including standard screws.
Brian W
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Thanks for your replies guys,
Brian, a dual roller cam is necessary for the design. Actually, the design is not very complicated for what it is actually doing.
I have investigated all logical alternatives for other types of mechanisms, i.e., linkages, levers, gears, single roller cams, fluid power, etc..
A single preloaded extension spring would not work because it would rotate the cam right off the dwell point.
I will keep investigating.
Thanks again, John
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On Fri, 21 Sep 2007 19:07:14 -0700, John2005

I think you may be unfamiliar with the flex hinge concept.
Visualize a strip of spring steel fixed at both ends and making a 90 degree peak, which is held by a clamp on both sides. If this clamp has a lever, the lever may be rotated left or right, but it springs back. It has no pivot at all.
You could look it up, I expect.
Brian Whatcott Altus OK
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Hi Brian,
Were you suggesting to attach the underside of the housing to the flex hinge ?
Is the flex hinge you speak of basically like a leaf spring in a "U" shape which is turned upside down and then fixed to a base ?
The cam and springs are not there "for the purpose" of returning the lever to it's home position. I need the cam to move the follower, the lever activates the cam, and the springs are there to return the cam, and hence the lever, to their home position.
It almost seems you are suggesting to connect the lever directly to a spring and eliminate the cam, which is not possible. A lever connected direct to a spring would serve no purpose to me, I need the cam there to move the follower (the drawings did not show what the cam is moving).
I did some searching online for "flex hinge" and "flex spring". I found this picture...
http://www.aerwayparts.com/c-flex_spring.htm
Is that a picture of what you had in mind ?
Thanks John
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On Sat, 22 Sep 2007 11:12:22 -0700, John2005

John, my suggested keywords were not helpful to you.
Even using a better descriptive keyword - flexure - I could not find an elegantly simple illustration that would be helpful. This was as close as I came in reasonable time:
http://www.bryantlabs.net/Images/hinge.gif
As you can see, it shows something of the same hinge style as kids have used to connect airplane rudders, using cloth hinges. I ask you to suppose that steel shim stock provides spring centering, no friction loss and no hysteresis, the last of which was bugging you I believe.
So this shows that several (two or more) flexures can replace a pivot. This is particularly suited to limited rotation angles. Rather than fixing a cam onto such a flexure, it would be better to replace the cam rollers with flexures too.
The classical application of a flexure is to suspend a pendulum, come to think of it.
Hope this helps a little.
Brian Whatcott Altus OK
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Hi Brian,
Thanks for the additional feedback & picture, that helped to clarify what you were talking about.
So a flexure-hinge is basically a hinge made from flat spring strips ?
Here is a "flexure pivot" that caught my eye when searching for "flexure" online, just scroll down to mid page to see the picture...
http://www.aero.org/publications/crosslink/fall2006/03.html
I will have to work out the spring forces and what size strips I need, but something like your picture or possibly the flexure-pivot shown above seems worth checking into.
I'm not quite clear on how I could replace the rollers with flexure's though, you threw me off there. I need the cam to move the follower about the pivot, the rollers allow this to be down with low friction. I also need the dwell on the cam so that the roller force does not backdrive the cam at neutral. I don't see how I can accomplish this with a flexure unless I am missing something.
Thanks again for the suggestion. John
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On Sat, 22 Sep 2007 22:51:26 -0700, John2005

I passed on that picture because a flexure hinge has no need of cylindrical parts, so this depiction might be confusing. I was suggesting that a cam follower can be substituted by a flex-strip that is shorted as the flexure hinge turns, in suitable circumstances.
Brian Whatcott Alktus OK
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