Request for help with extension-spring biasing problem 2

[John2004]

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 1/4" 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 o the 3-degree dwell at neutral. If I can get a repeatability of 1/2 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

 

[desertfox]

Hi John

I had a quick look at your mechanism there are several things I can comment on but I will do that later as I am at work now.
However in the mean time I suggest you do a free body diagram for the mechanism in various positions including the
one where it gets stuck, I think you might be running out of force towards the neutral position and it cannot overcome the friction generated between the cam and the rollers.
Because the rollers are offset from the centre of the device when the cam works in the other direction you might have enough leverage to overcome friction.

Regards

desertfox

 

[zekeman]

To prevent the repositioning of the spring hooks, I would clamp them rather than let them slide. Also, the cam bearing friction could be part of the problem. Incresing the spring rate and/or using a needle bearing might help.

 

[handleman]

In my opinion you're never going to reliably return to center, especially under load, when you rely on those two opposing extension springs to set your neutral position. You need something to positively stop your center so that the springs are preloaded and exert no force either way. Take apart a lever from an RC car or something to see how to do this.

 

[John2004]

Hi everyone,

Thanks for your replies.

The spring connection is just a hook through a hole. I also tried a hook over a dowel pin with a groove machined in it, but it still performed the same. The dowel pin did not rotate, but was stationary.

Perhaps it would help if the spring hooks were connected to a pin that is free to rotate, and/or connected to an anchor that pivots on a pin. Clamping the hooks down as Zekeman suggested is another option, but I may need to use loops instead of hooks for best performance. I'm thinking of a flat head screw through a loop.

If two extension springs are connected to each other in series (hook to hook) at the connection point, and the other spring ends are fixed to stationary anchor points, then the "hook to hook" connection point is manually displaced and released, will the connection point return to it's exact position or will variances in the spring rates cause an inaccuracy upon return to the home position ?

This thing is always used at normal ambient room temperature & each spring is subject to the same normal temperature changes seen in an average house, so I would not think temperature would have anything to do with it.

My first thought was that friction is the problem, but when there are no rollers or load on the cam, the only real friction is the connection point of the springs and the greased bushing at the rotation axis of the housing. Actually, the cantilever mounted housing is just pre-hard 4140 oscillating on a hardened steel dowel pin with no real "bushing" pressed in. I thought the pre-hard 4140 might wear better than a bronze bushing, & save me the trouble of fooling with a bushing, but I may be wrong. In either case, the friction should be low, especially with no loads.

Do you think it's possible I could see significant improvements if I use full complement drawn cup needle rollers at the housing rotation axis?

Since the springs oppose each other and are balanced at neutral, the force at that point is zero, and small displacements from neutral will not cause much of an increase in spring force unless a very high spring rate is used. Therefore, it may not take much friction to cause problems. I think this is along the lines of what desertfox was suggesting. Unfortunately, going with a higher spring rate is the only option I can think of and the rate is higher than I really want at 149 lb per inch.

On the other hand, once the rollers make contact with the edge or start point of the dwell on the cam profile, the force required to rotate the cam at that point should be practically zero even with a high roller load. I'm using 1/4" OD full complement drawn-cup needle rollers as cam followers. The springs that are connected to the cam-follower (not shown in drawings) should return the follower to the point where the rollers just contact the dwell, and then from there, the opposing springs connected to the housing should bring the cam back to it's home position where the rollers are in contact with the center of the 3-degree dwell. My gut keeps telling me it's a friction problem, and I need to reduce friction as much as possible, and find something to overcome the small amount of friction that will always remain.

Perhaps a third spring or biasing element is needed to overcome the very small amount of friction. After the springs attempt to return the device to neutral & fall a little short, the amount of manual hand force to return the lever to it's actual correct home position is so small it can't hardly even be felt or detected, so I don't need much at all.

The opposing springs are pre-stretched and/or preloaded at neutral, but they are preloaded against each other which makes the forces cancel out to zero. If I use a positive stop as handleman suggested, I think I will feel that stop in the lever, and I need to preserve a smooth balanced feel in the lever at and/or around neutral. Perhaps a third spring or biasing element cold provide a stop, but one that has give so that the tactile requirements are preserved.

What about moving the spring adjustment point to the housing and trying to make it so that as the springs are adjusted, one spring lengthens and one spring shortens at the same time ? Would this help or is this the wrong direction ?

I would appreciate any other thoughts or suggestions anyone may have.

Thanks again,
John

 

[desertfox]

Hi John

Firstly your spring adjustment will only work if both springs are identical in spring rate and pre-load force, in practice this will be very unlikely and one spring will be
stronger or at a higher preload than the opposing spring, so if you start to adjust the spring they both increase in load but still you achieve no balance and what will actually happen is that the activation lever will rotate away from the vertical either clockwise or anticlockwise
dependant on which spring is strongest.
Secondly when you release the arm the spring trying to pull
the activation arm back also as to provide the force to strecth the opposing spring which seriously reduces the force available to operate the cam.
From my obsevations of your diagrams it appears to me that you have a better mechanical advantage with the spring on the cam when the activation lever is rotated clockwise compared with it rotated anticlockwise, however I have no information about the forces exerted by the rollers on the cam are they the same for both positions?
My feelings are as earlier I think you are encountering a friction problem assuming of course that any binding of the spring hooks etc are not causing the problem as mentioned in previous posts. Even if your mechanism worked how are you dealing with the oscillations after you let go of the activation lever as I see no damping of the mechanism.
You need to do free body diagrams of the mechanism at various positions and workout the forces on the cam and then look at the friction generated by these forces to clarify that the mechanism can actually achieve what you want it to do.
I doubt at this stage it is worth putting biasing springs,stops or anything else untill you mathmatically show that your opposing spring mechanism works in theory.

regards

desertfox

 

[John2004]

Hi Desertfox,

Thanks for the additional feedback,

Initially, when I install the springs onto the housing, the lever is not vertical, but to one side or the other due to differences in spring forces.

I use the adjustment nut at the end of one spring to initially make the lever vertical at neutral before any loads are applied. As you point out though, just because the lever is vertical does not mean that the springs truly have equal forces or rates since spring initial tensions and rates vary from spring to spring due to manufacturing and material variances.

If the springs start out with unequal forces, and you stretch both springs at once with the adjustment nut, then it seems to me that you are only increasing the force of both springs (and the position of the center connection point), but the unequal force relationship between the two springs remains basically the same. This is why I had suggested to implement an adjusting screw that lengthens one spring and shortens the other at the same time. At least then, you can make the opposing forces equal at neutral.

Here's what I have in mind...

I would like to take a single threaded bolt & run it through a clearance hole in a rectangular post which is integral with, and extends downward from, the underside of the housing. The bolt will have a flat milled and a hole drilled at each end, so that the spring hooks can be connected to the bolt.

The bolt will have two nuts threaded on it (one nut on each side of the rectangular housing-post). When an adjustment is needed, one nut is loosened and the other nut is used to make the spring adjustment. Then, the loosened nut is tightened back down. It seems to me that this arrangement will cause the length of one spring to extend and the other spring to retract at the same time ? This way, I could truly make the spring force equal at neutral.

Does this sound like the direction I should take or something worth trying ?

As far as making a free body diagram, I've done calculations on the design in general and it should work (calculating exact friction forces seems difficult though). Since I already have a prototype I would rather just implement what seems to be the most logical design changes and then give it a try.

Regarding your comment about damping, I found that no damping was needed. The mass of the parts is very small, and the speeds are very slow. Most of the time the users hand force itself acts as a damping mechanism. The lever is just oscillated by hand at slow speeds. It's not as if it's pushed or pulled to it's extreme position and then quickly let go of. However, I tried that, and even then, no damping device is necessary, the cam rocks back and forth a little at neutral and settles down very quickly.

I'm curious if you think my spring adjustment idea is viable.

Thanks again for the additional feedback.

John

 

[desertfox]

Hi John

You need to adjust the springs independently of one another
not increase one and decrease one at the same time.
Regards friction do the rollers roll or do they just slide?
either way you can estimate the friction force by using the
normal force and the appropriate coeff of friction.
If those friction forces can exceed the forces imposed by the springs then your wasting your time doing anything else
with the springs, you will need to change the mechanical advantage of the mechanism first.
Going back to the adjustment what spec are the springs made too? what tolerances on the rate or load at deflection have you got, are the springs wound with coils touching or open and how if you adjust two springs at once will you know which one is weaker or stronger and that by altering the extension of each spring by the same amount that you will achieve balance.
I repeat you need to do calculations with weak and strong springs ie using the info you have for the springs and making sure the weak springs will work the cam.
For instance if you have a spring made to produce the strongest spring and weakset spring and you put these on the cam will it work?
How do you know how far you can adjust the spring before it over stressses and takes a permanent set.
You can play around with adjustments and you might get lucky but how confident will you be if you start making lots of these cams?

regards

desertfox

 

[handleman]

I can see from your posting history that you've been working on this one little mechanism for three years. I'm extremely curious as to what your application is that you would work at it this doggedly.

 

[MikeHalloran]

The rate doesn't matter. At zero deflection you have zero spring force available for centering. Friction will cause hysteresis in such a mechanism every time. "Centering" a mechanism with two extension springs _never_ works. Write that down somewhere.

You need to use compression springs, preloaded independently against your operating lever.

Go to a car showroom and play with a five-speed shifter until you understand how the shift handle returns to the 3-4 gate when you make a 2-3 or 5-4 shift. Buy a junk transmission and take it apart if you need to see how it's done.





Mike Halloran
Pembroke Pines, FL, USA

 

[John2004]

Hi desertfox,

Thanks for the additional feedback.

Making the spring adjustments of each spring truly independent of each other while maintaining a smooth balanced feel in the lever at neutral seems tough to do. It seems I would need some type of stop for the springs in order to make the adjustments independent of each other.

The needle roller cam followers seem to be rolling and not sliding.

>desertfox wrote:
>If those friction forces can exceed the forces imposed by >the springs then your wasting your time doing anything else
>with the springs, you will need to change the mechanical >advantage of the mechanism first.

The thing is, once the roller gets to the dwell (which is accomplished by the springs connected to the cam followers, not shown in the drawings), then the mechanical advantage should be theoretically near infinite. At that point, the opposing springs are mainly overcoming friction at the cam follower rollers, friction of the housing rotation axis, and friction at the spring connection points. I think one problem is that the opposing spring forces cancel each other out at neutral, so a high spring rate is needed so that a small amount of displacement of the opposing springs from neutral, produces a larger spring force to overcome the friction.

Basically, it looks like I need to reduce all friction points as much as possible, use the highest rate opposing springs possible, and try to provide some way to have more adjustability of the springs (preferably completely independent of each other if possible). Other than that, I could try to add a third biasing element solely to overcome friction forces around neutral. Also, the higher degree of accuracy the springs are made to, the better.

I've uploaded another drawing of the adjuster I described in my previous message. You can view and/or download it at the link below...

http://www.mediafire.com/imageview.php?quickkey=1zyu9xdjz2q&thumb=4

Referring to the drawing at the link above, the spring adjustment is now at the cam-housing and not at the spring end. By loosening one adjustment nut and then turning the other nut, it seemed to me I could stretch one spring and shorten the other at the same time. It also seemed I could choose which spring is lengthened and which spring is shortened at any given time, which would allow the spring forces to be equalized at neutral regardless of manufacturing variances in spring initial tension or spring-rate.

I would appreciate any feedback on this arrangement. Would it work / help, or would it just rotate the cam on the dwell and not really do anything to the springs ?

Thanks again guys,
John

 

[John2004]

Thanks for the additional replies everyone.

>Mikehalloran wrote:
>The rate doesn't matter. At zero deflection you have zero >spring force available for centering. Friction will cause >hysteresis in such a mechanism every time. "Centering" a >mechanism with two extension springs _never_ works. Write >that down somewhere.

I was only implying that the spring rate mattered after the housing is displaced from neutral. At neutral, the forces cancel and rate does not apply. After you leave neutral, the spring-rate times the difference in the length of the two springs controls the force the springs exert on the housing.

I can check into using compression springs, but in order to preload them independently against the lever or housing, it seems I will need some type of stop.

I wish I had a sketch or something to help me understand what you are referring to with the transmission. I will try to check into that further.

Thanks again,
John

 

[John2004]

>John wrote:
>I can check into using compression springs, but in order to >preload them independently against the lever or housing, it >seems I will need some type of stop.

That came out wrong Mike, I can preload the compression springs against the housing and/or each other, without a stop, but do I need to make them independently adjustable, or can I just put an adjustment screw at one end ?

The compression springs would not compress or extend in an exact straight line, since the bottom of the housing arcs as the housing oscillates. The springs would bend a little as they are cycled. Will that cause any problems ?

What are the main reasons compression springs work better for centering ? I am guessing they may just be more consistent in general and since there are no hooks you eliminate friction and positioning problems there.

I could install the opposing compression spring ID's over a bolt as shown in the drawing of my previous post, or machine a boss on the housing for the ID of the compression springs to fit over. I could have a similar arrangement at the other spring ends.

I'm going to try to implement the compression springs. Any other pointers or feedback on using the compression springs would be appreciated.

Thanks
John

 

[MikeHalloran]

Compression springs of moderate length will tolerate a little misalignment. You typically need to loosely capture the ends in a shallow cup feature so they don't slide out from under the force applied.



Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi John

If you mount two springs whether they be compression or tension (it doesn't matter)onto your adjuster in your last post and lets say one as a spring rate of 20lb/in and the other 19lb/in each one is stretched or compressed say 2in so one is now giving a preload of 40lb and the other 38lb so if we now adjust the springs say to reduce the 40lb spring to 39lb we need to compress or extend it by 1/20" and consquently the weaker spring will compress or extend by 1/20" of an inch but will only increase its force by 0.95lb therefore you cannot balance the forces by compressing one and extending the other at the same time by the same amount.
Additionally any out of balance in the springs isn't going to help your mechanism as it means one side will have to work harder than the other.
The mechanical advantage I mentioned was in the two extreme positions and not at neutral, if you draw lines of force for the springs and the reaction of the cam rollers on your diagrams around the pivot for your figures 2 & 3 you will see what I mean, however I am assuming that both springs are identical and I have no knowledge of the roller reactions on the cam. I asked earlier whether they were the same.
Finally it isn't the amount of force that needs increasing
into the mechanism it is to do with the angles of the mechanism ie cam, rollers,roller reactions and tension spring forces acting at the point where the cam stops that needs looking at, you can use a elephant to pull the cam but if the the elephant isn't pulling efficiently that mechanism won't budge but it might very well break.
John can you upload your calculations for the mechanism like you have done for the diagram of the mechanism.

Regards

desertfox

 

[John2004]

Hi Desertfox & Mike,

Thanks for your additional input.

Regarding Deserfox's comments on the spring adjuster in my second drawing, if you have a spring with 38 pounds on one side, and 40 pounds on the other side, and you change the force of the springs so they balance, the spring forces at balance do not have to equal 40 or 39 pounds (or any specific force). The spring forces just need to be equal, with each spring having a pre-load or stretch.

Referring to my adjuster drawing, if the 38 pound spring is on the right side of the housing, and the 40 pound spring is on the left side of the housing, you could loosen or "back off" the adjusting nut on the right side of the housing, & then tighten the nut on the left side of the housing. This would slowly stretch the 38 pound spring and retract the 40 pound spring at the same time.

It seems to me that at some point, the forces should equal regardless of spring rates. The forces will not be 40 or 38 pounds, but the important thing is that they will be equal. when the adjustment is complete, the nut which was backed-off, is snugged back down.

If the 40 pound spring has a 20 lb/in rate, and the 38 pound spring has a 19 lb/in rate, and you lengthen the 38 pound spring by .0515" and retract the 40 pound spring by the same amount...

20 X 2" = 40 pounds
19 X 2" = 38 pounds

20 X 1.9485" = 38.97 pounds
19 X 2.0515" = 38.9785 pounds

In the above scenario, the length of each spring was changed by .0515" & a balance point of 38.97 pounds was found. There is a slight force difference using my cursory calculation of .0515", but I think at some point, the forces would be exact.

My concern was that the adjuster shown in my drawing might not do what I intended. Instead of lengthening one spring and shortening the other, it may just rotate the cam along the dwell & not do much to the springs. Can anyone please comment on this ?

Once the spring forces are equalized at neutral, after the springs are displaced from neutral, the spring rate controls the amount of force the springs exert on the housing, which needs to overcome the friction at "or around" neutral.

The extension springs connected to the cam follower (not shown in the drawings) bring the cam back to the point where the rollers start to contact the dwell. There's no problem getting the cam back to the point where the rollers just start to contact the 3-degree dwell. From there, the two opposing springs rotate the cam so the rollers are in contact with the center of the 3-degree dwell at neutral. It's making sure that the rollers always "roll onto" the dwell each time the lever is released, that concerns me.

The opposing springs just need to reliably bring the cam back to the point where the rollers are actually onto the dwell, they don't need to return the cam to it's "exact" mid point neutral position, but I would like to get it within 1/2 to 1 degree if possible. I have a 3 degree room for error due to the cam dwell at neutral.

As far as reaction forces on the rollers and the mechanical advantage, that does not seem important to me because the adjustable extension springs connected to the cam-follower, (not shown in the drawings) have no problem bringing the cam back to the point where the rollers just start to contact the cam dwell.

The extension springs connected to the cam-follower are not opposing. Just imagine extension springs connected to the right side of the cam follower in my drawings. The extension springs counter the roller force onto the cam,& bring the cam back to the point where the rollers just contact the cam dwell. From that point, since the rollers are in contact with a dwell, the roller force (no matter how great)cannot create any torque on the cam. From there, the opposing springs connected to the cam take over and align the cam so that the rollers are in contact with the center of the 3-degree dwell.

Note that the two rollers cannot oscillate about the pivot point independently of each other, they oscillate together and are basically locked at a fixed distance apart from each other. However, the inner roller is located on an adjustable slider so I can zero in on the right clearance.

The outer roller is loading the cam, and the springs connected to the follower/rollers that I mentioned above, completely counter the roller force at neutral. When the cam is rotated clockwise, roller force goes down, when the cam is rotated counter-clockwise, roller force goes up.

It's very close to where it needs to be, and given I have a 3-degree room for error, I may be able to make it work well enough with extension springs. However, I'm still going to look into Mike's suggestion to use compression springs, although after thinking about it again, I will probably need some type of mechanical stop, in order to independently preload and adjust the springs. This is not desirable, but should be acceptable if I cannot find another solution.

If the springs cannot be truly independently preloaded or adjusted, are there still advantages to using compression springs over extension springs ? I've also considered a torsion spring located underneath the housing but I have not worked out the details to implement it.

I'm also considering trying Urethane or rubber springs either in compression or tension.

I'm hoping my explanation above might clear up what I am shooting for with the adjuster shown in my drawing. I'm just not sure if it will equalize the spring forces or just rotate the cam along the dwell without really changing spring length ?

Thanks again guys, I appreciate your input.
John

 

[desertfox]

Hi John2004

Yes the adjuster will rotate the cam unless the spring forces and rates are identical, I stated this in my earlier
post and I assumed that the actuation lever was fixed to the cam from your diagrams which is why I stated:-

"so if you start to adjust the spring they both increase in load but still you achieve no balance and what will actually happen is that the activation lever will rotate away from the vertical either clockwise or anticlockwise
dependant on which spring is strongest."

The above was when you were adjusting both springs by either
extending both of them or shortening both of them.

Now your new adjuster shotens one and lengthens the other but unless the forces are equal the cam will have a small torque on it in one direction or the other.
In addition my springs of 40lb and 38lb were just a example to show you cannot balance the forces by extending and or decreasing the extensions or compressions of the spring and despite doing this calculation for yourself and getting 37.97, 37.9785 your still convinced you can balance these forces by the statement "but I think at some point the forces would be exact".
So then perhaps you can tell us how you intend to adjust 0.0015" on a screw thread or indeed how much
extension or compression of the springs to 4 decimal places
you will need to achieve balance, bearing in mind that your weaker spring now according to your example as the higher preload.

It seems to me that the cam follower springs during operation keep the rollers in contact with the cam and therefore oppose the spring force
given by the tension springs to return the cam to neutral and therefore place an additional burden on the tension spring.

Can you post another diagram with the cam follower springs shown and in addition your calculations to show that this mechanism works?

regards

desertfox

 

[John2004]

Hi desertfox,

>Desertfox wrote:
>So then perhaps you can tell us how you intend to adjust >0.0015" on a screw thread or indeed how much
>extension or compression of the springs to 4 decimal >places...

I didn't intend to measure anything, you just turn the screw until the lever & cam are where you want them, that would be the balance point. There is a 3-degree room for error, so it does not have to be perfect.

If you divide your desired load by your rates, you get the spring lengths to produce perfect balance. In the case of your example, this would be carried out much further than 4 decimal places though. So at least in theory it seems they could balance, but whether or not the adjuster in my drawing would get them to balance is another story.

19 X 2.0513 = 38.9747
20 x 1.9487 = 38.974

Carry out the decimal place of the .0513 figure further and you get more balance accuracy, would it ever be perfect ? I don't know. Even if it was mathematically perfect in the real world it would never be.

Nothing is ever perfect, but with say a 10-32 adjustment screw, you almost have near infinite adjustment, i.e., if you just rotate the nut a very slight amount with a wrench it's not going to move the screw by hardly at all. Perhaps it would be close enough, given I have 3 degrees to play with.

It just seemed to me that if you move the spring connection points at the housing one way or the other, the spring forces will balance at some specific point & load, but I don't know how to predict what the exact load would be, and the load is not important just that the springs are in balance (or close enough).

I understood that having an adjustment nut at the end of one spring only, would rotate the cam, but may not insure equal forces at neutral.

My question / concern was with the second adjuster drawing having two adjuster nuts at the cam connection point. I was not sure if this would offer advantages and/or if it would do what I intended, i.e., lengthen one spring and shorten the other at the same time.

>It seems to me that the cam follower springs during >operation keep the rollers in contact with the cam and >therefore oppose the spring force
>given by the tension springs to return the cam to neutral >and therefore place an additional burden on the tension >spring.

That's not the case, the springs I referred to as "cam-follower" springs are not conventional cam-follower springs. The springs I mentioned help reduce the effort to rotate the cam. The follower load itself actually acts like a spring force. You might think of the follower load as spring. The "cam follower" springs I mentioned oppose the load springs.

>Desertfox
>Can you post another diagram with the cam follower springs >shown and in addition your calculations to show that this >mechanism works?

I appreciate your offer for further help, but I think I will just implement the most logical design changes and see if I can make the prototypes work a little better. I know from my experience with prototypes that it works, I just want to tweak it a little.

I thought of a conical compression spring under the housing, standing straight up like a cone with the apex at the connection point on the housing (perhaps in conjunction with the opposing springs).

It was also suggested to me by someone to use a flexure-hinge or flexure-spring to bias the housing to neutral. A torsion bar came to mind, but displacements are probably too great.

One interesting thing is that even when trying different size and different rate opposing extension springs, the lever always seems to be off by the same basic amount & in the same direction. This also makes me think it's mainly a friction and/or alignment problem.

I'm sure I will come up with something.

Thanks again for your feedback, I appreciate it.

John

 

[desertfox]

Hi John

I wish you luck in finding your solution.

regards

desertfox