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Request for help with spring design

Request for help with spring design

Request for help with spring design

(OP)
Hello,

I would like to ask  if someone could please provide some feedback on a unique spring design problem.

Thanks
John

PROBLEM:

I have a radial plate cam, with an oscillating roller follower. The small cam is oscillated manually by hand, via a 6" long lever. At the neutral or resting position, the roller follower is in contact with the approximate center of the cam curve or profile. From the neutral position, the cam can be rotated clockwise, (by pushing the 6" activation lever down) or counter-clockwise (by pulling the activation lever up). The maximum angular displacements of the lever and cam are 16 degrees clockwise from the neutral position, and 14.5 degrees counter-clockwise from the neutral position.

When the cam is rotated clockwise from the neutral position, the force  the roller follower exerts on the cam curve decreases, but when the cam is rotated counter-clockwise, the force the roller follower exerts on the cam profile increases.

At the neutral position, the roller follower is in contact with a "dwell" portion of the cam profile, and as such, the force the roller exerts on the cam profile does not create any torque on the cam at the neutral position. However, when the cam is rotated from the neutral position in either direction, the roller force then creates a torque on the cam.

When the cam rotates clockwise from the neutral position, the roller moves towards the cam rotation axis. When the cam rotates counter-clockwise from the neutral position, the roller-follower moves away from the cam rotation axis.

When the cam rotates clockwise from the neutral position the cam follower roller force goes down. When the cam rotates counter-clockwise from the neutral position, the roller force goes up.

I have springs connected to the cam in a series parallel array, i.e., 4 springs on each side of the cam, which hold the cam at, and return the cam to, it's neutral position, when the activation lever is pushed down or pulled up from the neutral position, and then released. The opposing springs are pre-stretched at the neutral position with equal forces on each side of the cam. This provides a "balanced feel" in the activation lever, which is important for the design.
  
My problem is, when the cam rotates clockwise from neutral 4.637 degrees, the un-balanced force of the springs needs to be about 50 pounds to overcome the force of the follower roller and return the cam to the neutral position when the activation lever is released. Since the force of the follower roller and the force of the springs are nearly balanced at this point, it is easy to push the lever down from the neutral position. The roller follwer force "helps" to push the activation lever down.  

However, when pulling the lever up from the neutral position, not only does the force of the follower roller increase, but the force of the opposing return springs also increases (i.e., the un-balanced force will also be 50 pounds at 4.637 degrees counter-clockwise).

Also, the roller follower force works against the activation lever when pulling the lever up from neutral.  

I need a system that provides a "balanced feel" in the activation lever at the neutral position, but allows the force to pull the lever up from neutral, to be small, like it is when pushing the lever down.

I tried using different (lower) rate springs on one side, but when I layed it out in my CAD system, and did the math, I could not get everything to work out.

It seems the opposing spring forces must be balanced at neutral, and the unbalanced force of the springs must be 50 pounds at 4.637 degrees of clockwise cam rotation, but somehow smaller when the cam is rotated counter-clockwise 4.637-degrees.

I basically want the force to pull the lever up from neutral to be as small as possible through the entire 14.5 degrees of counter clockwise rotation. It’s desirable to keep the forces necessary to move the lever as low as possible in either direction, either clockwise or counter-clockwise from neutral. The forces look good for pushing the lever down from neutral, but I would like to see lower forces for pulling the lever up from neutral.  

I cannot really increase the length of the activation lever, since I am using a stock lever and want to keep using it. The space I have to work in is very small and constraining. The entire cam is also adjustable up and down by a total travel of 5 mm, which complicates things further.

The springs I am using now have about a .390" OD, a 1.8" free length, and a 115 lb/in rate. It would be difficult to increase the OD of the springs very much do to limited space.

Please let me know if anyone has any suggestions for a type of spring, spring system, or design that my work.

Thanks for your help.

John

RE: Request for help with spring design

Wow. Could you plot, or list, a curve of desired actuation force vs angle? I think you have described it in essence, but we've recently spent a while analysing a problem where one error in the initial descritpion invalidated most of the thread.

FWIW, on cars we use polyurethane foam cylinders as spring aids (ie they are durable and powerful and smaller than coil springs). By fine tuning their profile and the amount of preload we can get virtually any non linear characteristic we want. They can be turned on a lathe for small quantities, or injection moulded for production.

Cheers

Greg Locock

RE: Request for help with spring design

Added to GregLococ questions. Are you looking for a spring design or a systm design? Can you can define the spring properties (forces vs deflection, life cycle etc.) and the space limitations as though as the spring type (Extension, compression, linear, non-linear etc.

http://www.webspawner.com/users/israelkk/

RE: Request for help with spring design

After reading your thread, I think that you will have to furnish a drawing of this design including the geometry of the  cam and the oscillating follower, including all of the forces or loads including inertia, if any. Also, I don't understand your need for such stiff springs to hold the cam in neutral equilibrium since you are so concerned about forces in the lever.
Again, it would be impossible to help you without further detailed calarification.

SS, consulting engineer

RE: Request for help with spring design

(OP)
Hello everyone,

Thanks for your feedback.

I'm going to try to put a drawing up on a free web host, to clarify the problem. I will come back with a link to the drawing.

For Greglocock: can you tell me where I can find more information on the polyurethane foam cylinders ? Are they strictly custom made & if so, where could I have them made?

I am presently using regular extension springs with hook ends. I don't "think" opposing compression springs will work, and I don't think a torsion spring will work. As the cam is rotated, the extension springs on one side of the cam extend by about .012" per each degree of cam rotation, and the springs on the other side of the cam retract by about the same amount. Which side extends or retracts depends on the direction of cam rotation from the neutral center position. The spring hooks connect to the cam at about 0.673" from the cam rotation axis (center of spring is at this distance). All the parts are very small.  

Everything fits into a space that is only about 1.75 inches thick. Since the lever is activated intermittently by hand, the life cycles dont't need to be as high as most industrial automated machinery. I think it would be considered light duty service.

I think I may need a different "spring system" but it must be cost effective, work in a compact space, and provide a "balanced feel" at the neutral position.

Thanks again,
John

RE: Request for help with spring design

Boge Elastofabrik (sp?) of Germany make ours but for one offs we just buy polyurethane foam and turn it on a lathe.

Cheers

Greg Locock

RE: Request for help with spring design

I am having difficulty clearly picturing your requirement in my own mind. As the others have suggested a force displacement diagram might help a lot.

I doubt if spring design alone will fully answer the problem, but there are some tricks with over centre springs and bell cranks and similar kinematic linkages that can produce some wonderful non linear effects with springs.

RE: Request for help with spring design

(OP)
Hello everyone,

I was finally able to put some drawings and descriptons on the internet of the spring design and/or lever force problem I posted in this thread. I think this will make the problem much more clear. I am hoping this will allow additional comments and suggestions.

Here is the link:

http://www.cdtd.50megs.com/

I would sincerely appreciate any additional feedback or suggestions on this problem. I would like to consider as many viewpoints as possible. I hope I can find a suitable  solution to the problem.

Thank you.
John   

RE: Request for help with spring design

Could you use a second CAM.2, coaxial with the CAM.1
and use the spring-loaded roller arm of this CAM.2 to supply the complementing force so that the sum of the two torques would change as required?

Can you define by equation or table the position of CAM.1
and the required torque on the handle fixed to it?

 

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

RE: Request for help with spring design

(OP)
Hi Nick,

Thanks for your reply.

I had thought of using a mirror cam profile located 180 degrees opposite the profile shown in the drawings, and then using a spring loaded "extra roller" to offset the load of the existing roller. I would still need a couple of opposing extension springs connected to the cam solely to provide a balanced feel in the activation lever at neutral. However, the springs could be small.

I don't think I really have the space to do it. The follower arm pivot point, and the cam, are adjustable up and down by a total distance of 5 mm, and this also makes the dual cam curve approach hard to implement.

I had also thought about using a track cam that can both push and pull the follower, and then put a spring on the follower to oppose the follower load. However, space constraints make that hard to implement to. It is also hard to mount the cam follower with this method. I would need to mount the roller on some type of shoulder screw or something, to get the cam track to fit "around the roller". The OD of the roller is only 3/16", and the shaft the roller rotates on is only 1/8" diameter. If I use a track cam, it looks hard to provide a roller with a strong mounting.   

I don't need much spring force at all on the left side of the cam, just enough to pull the cam off the high dwell, which is not hardly anything since the torque required to rotate the cam when the roller is in contact with a dwell, is practically zero.

The main reason for having equal opposing forces on the cam at neutral, is just to provide a "balanced feel" in the activation lever at neutral. Otherwise, the problem would be easy to solve, I could just put springs on the right side of the cam that are completely retracted at neutral, and then put one very small spring  on the left side of the cam, to pull the cam off the "high dwell" after the cam has been rotated to the extreme counter-clockwise position, and the activation lever is released. However, I think I will then lose the balanced feel in the activation lever at neutral, if I do this.

I need some way to have a balanced feel in the activation lever at neutral, but somehow have lower spring forces on the left side of the cam, when rotating the cam counter-clockwise from netural. Or, somehow gain leverage when rotating the cam counter-clockwise from neutral. But as explained on the site, all of the standard or obvious methods of gaining leverage are not really available due to design constraints.

Please let me know if you have any other suggestions.

Thanks
John      

 

   

RE: Request for help with spring design

The "mirror cam" doesn't have to be at 180 degrees- it
may be at any angle -- since it my be mounted on the same
axis as the first cam i.e. on the drawing behind or in front of
the first cam so it may even be at identical angle.

You may use an e.g. rack and pinion to convert the rotary
motion to linear and use twin linear cams, if it helps
in locating it.

What about some electronic solution?

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

RE: Request for help with spring design

(OP)
Hi Nick,

Thanks for your message. I am afraid an electric solution would not be possible. The mechanism must be hand operated, and there is no way to provide some type of servo or electric assist.

The only real place I could put the mirror profile is right underneath the cam rotation axis, where the springs are presently connected to the cam.

The height of the cam is adjustable up and down by a total distance of 5 mm. The cam plate does not have bearings on each side. The cam plate is mounted to a hub and it has a cantilever bearing mounting, sometimes referred to in bearing catalogs as an "overhung shaft" Bearing mounting.

There is only one cam height adjustment slider, located on one side of the cam only, about 1.5 inches from the cam plate. With an extra spring loaded mirror profile, I think it will bind the cam height adjustment mechanism (dowel slider), and make adjustments difficult. I would have to put an adjustment screw on the extra spring loaded follower, and go back and forth between two adjustment screws, hoping they would not bind.

Please let me know if you have any other suggestions.

Thanks
John

RE: Request for help with spring design

Consider a  spring configuration similar to a toggle. The linear spring at neutral would pass through the center of rotation with no spring torque at neutral. If you rotate clockwise it would increase in torque, rotate counterclockwise it would reverse the torque direction, which is what you are looking for.
It would work better if you could use a pneumatic cylinder at constant pressure in lieu of the spring.  

RE: Request for help with spring design

(OP)
Hi Zekeman,

Thanks for your feedback.

I don't think I can use a pneumatic cylinder to assist, since both the motion & the "responsiveness" of motion between the activation lever and the roller, has to be controlled manually by have via a lever.

I am having a hard time visualizing your toggle spring idea, and how it could be implemented. Could you give me a little more feedback ? I don't think I can wrap anything around the camshaft, due to design constraints.

Your idea is interesting to me. If it can reverse the spring torque, and still provide a balanced feel in the activation lever at neutral, that would seem to do what I need, if it can be implemented in a very small space.

Thanks
John    

RE: Request for help with spring design

Hi John:
Would it not be possible to replace the two (red/violet)
spiral springs with a flat spring with a roller in its
middle ?

This is obviously a more complicated assembly -- there
must be a way to accomodate the few components needed
even if it requires some additional transmission-
shaft,rod,cable,belt,chain, etc.

I don't know where do you have space or what kind
of compromisses you can afford to make. Obviously
the best general solution for the force compensation
is the double cam. I don't know the accuracy required
etc...

By the way how do you know my name ?


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

RE: Request for help with spring design

(OP)
Hi Nick,

Thanks for your message.

I know your name because you gave it to me when you emailed me from the site I uploaded the drawings too.

I have now determined that I could probably fit a "track cam" in the available space, that can both push and pull the roller. I could then put a spring on the follower, which opposes the follower load. I would still need opposing extension springs on the cam to provide a balanced feel in the activation lever at neutral, and to hold the cam at the centered dwell point. However, they should be able to be smaller extension springs than I am using now. I think this arrangement will make it easier to rotate the cam.

I would use the added spring on the roller to "almost" pull the roller off the center cam dwell at neutral, (off the inside profile of the track). Then, the outside track profile would move the roller when rotating the cam clockwise from neutral, an the inside track profile would move the roller when rotating the cam counter-clockwise from neutral.

However, my main concern is that the needed clearance between the track and the roller could cause problems. If the clearance is too small, the roller will jam in the track. If the clearance is to large, there may be a shock or "bump" felt in the activation lever at neutral, when the roller changes from one side of the track to the other.

Presently, the cam follower bushing is a stock item from www.peerinc.com (part # 02FB03). These bushings are really made to be pressed into housings, and the engineering department will not give an OD tolerance on the bushing, just a Min. & Max. tolerance on the housing ID.

In order to get a track cam to work, I think I will need a custom close tolerance to roller. I cannot go larger than 3/16" OD on the roller, because the minmum radius of curvature of the cam curve will then get too small.

The roller presently rotates on a standard 1/8" diameter hardened dowel pin.

Could I turn a piece of steel to close tolerance, harden it, and then use that as a bushing ? I tend to think there might be noise and excessive friction. Even at hand operated speeds with intermittent use, I wonder if wear will be a problem. I would prefer to offer a self-lubricated maintenance free bushing.

Maybe I could somehow turn the OD of the Peer bushing to close tolerance, or burnish it to size by pressing it through a die. The bushings are so small (3/16" OD X 3/16" wide)

Another thing that comes to mind is to use a "U" shaped spring at the bottom of the cam, but I am not sure how to implement it. Extension springs are appealing becasue they make everything easy to adjust.

Please let me know if you have any other thoughts.

Thanks
John

RE: Request for help with spring design

(OP)
Hi Nick,

I have another idea that may be viable. I could machine a "rib" in the plate cam and put a roller on each side of the rib. This should eliminate the "binding" problems that you can get with putting a single roller in a cam track.

This will still allow the cam to both push and pull the follower, and I could then use a spring on the roller to offset the cam-follower load. I would put just enough opposing spring force on the roller to "almost" pull the roller off the center cam dwell at neutral. Then the inner rib profile would move the inner roller when rotating the cam clockwise from netural, and the outer rib profile would move the outer roller when rotating the cam conter-clockwise from neutral.

What do you think of this idea ? I will still need opposing extension springs attached to the cam to provide a balanced feel in the activation lever at neutral, but I think they can be smaller. I think it will probably make the activation lever easier to move in each direction. Do you agree ?

The wall thickness tolerance on the bushings is 0.001", so I think it looks promising (i.e., I can keep clearance between the rollers and rib small).

Please let me know what you think. If you or anyone else has  feedback on this or other approaches, I would still like to consider all options,  but this idea seems most promising so far.

Thanks
John

RE: Request for help with spring design

(OP)
After checking into the rib cam idea, it looks like the pressure angle of the inner rib profile may get to high, since the cam has to be a little smaller to allow for the extra roller.

I am not sure If I can get away with using a very thin curved rib, but it may be possible since there is a roller on each side of the rib i.e., the rollers can possibly give support for the thin rib. If I make the rib 1/4" wide, it seems that makes the cam to small.

Please let me know if anyone has any other suggestions.

Thanks
John    

RE: Request for help with spring design

John:
As I see on the drawing there is not too much non-linearity
in the roller_angle vs handle_angle. Couldn't you just
omit the whole thing, move the output directly with the handle and trust in the human_senses -- brain -- hand --
handle feedback loop to position the output correctly ?

Why do you have to ballance the force?

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

RE: Request for help with spring design

(OP)
Hi Nick,

Thanks for your message.

I must use a cam. I need the dwell on the cam to prevent changes in the follower load, from displacing the follower at neutral. I cannot use another non-backdriving mechanism like a worm gear, or lead screw.

The cam must "block" the follower at neutral. At neutral, the springs do not balance against the follower load, because the roller is in contact with a dwell on the cam profile at neutral, and as such, the roller load will not create any torque on the cam, or cause it to rotate.

Therefore, at neutral, the springs only oppose "each other" and this is only necessary to provide a balanced feel in the activation lever.

Thanks
John

RE: Request for help with spring design

After further review, my idea won't work.

Could you impose an opposite spring at the point of application of the linear spring that produces 115 lb? If so, that could be a starting point. Otherwise, a spring solution under the parameters set forth would be very difficult, since you must know that the fundamental energy equation:

T*Dtheta=F*Dx

where:

T= torque on your hand
Dtheta = the incemental angle in radians that  your hand moves on the lever
F is the applied force 115lb at the linkage
Dx= the incremental motion of the force applied to the link
So you see,it is obvious that by using the opposing springs for the neutral "feel" you have added a strong energy absorber that your hand must deliver at increased angle, especiallyfor springs with a high spring rate.

Under the circumstances, I  wonder whether you could just state the basic problem and if whether you are locked into this configuration. If not, you might be better served by this forum if you could convey what you want to do and then kick around ideas that might lead to a satisfactory solution, but that might mean scrapping your cam and linkage.

RE: Request for help with spring design

(OP)
Hi Zekeman,

Thanks for your message.

I think I have found a solution to the problem. I did not want to use a "track cam" that could both push and pull the roller, because I was worried about the roller binding in the track, since close clearances would be necessary between the roller and the curved track.

However, I found that I can use a "rib cam" i.e., a curved rib with a follower-roller on each side, which will make the cam double acting so that it can both push and pull the roller, and eliminate any roller binding problems.

I had previously thought the maximum pressure angle of the inner rib curved profile would be to high, but I found a way to decrease it to acceptable levels.

With this arrangement, I can put a spring on the roller, which opposes the roller load at neutral. The rib cam with dual rollers, will still be able to move the follower in each direction from neutral, even though the counter spring is connected to the roller to offset the roller force onto the cam.

I will still have opposing springs connected to the cam to provide a balanced feel at neutral, but they can be smaller than with the previous arrangement.

I think this will make the activation lever easy to move whether rotating the cam clockwise or counter-clockwise from neutral. Please let me know what you think.

I am still interested in any alternatives if anyone thinks of anything. It's a close fit, but so far, this seems like a very viable option.    

Thanks
John

RE: Request for help with spring design

John,
Your solution looks ok but I don't understand why you couldn't oppose the 115 lb input spring force with an almost equal inline opposite spring force (or torsion spring in the follower axis) such that there is a slight force inbalance assuring contact between the roller follower and the cam. Then, you could have the lower inline "feel" springs, as you point out, with much smaller spring rates. This would be about the same as your proposed solution, but obviously much simpler.

Zeke

RE: Request for help with spring design

Hi john2004

Looking at your drawings it would appear to me although it
depends on how accurate the drawings are, that your largest
resistance to lever movement is governed by your balance springs. Now if you can't increase the length of your lever
how about reducing the 'moment' the balance springs exert on the cam by moving them vertically upwards closer to the centre of cam rotation, this would have the same effect as increasing the lever length.
Alternatively could you turn the balance springs through 90 degrees and have it pulling vertical through the cam rotation centre, if you could do this you may be able to reduce the number of balance springs. Although you would have no 'moment' exerted in the neutral position any movement clockwise or anticlockwise would stretch the balance spring and generate a balancing moment.

regards desertfox

RE: Request for help with spring design

I posted a reply but Isee it did not go through. Repeating:
I think if you would add a soft spring force {or soft torsion spring in the follower pivot) in line with the input 150 lb spring force stretched to slightly smaller magnitude in order to maintain roller contact, you could do exactly as you describe at a fraction of the complexity.

RE: Request for help with spring design

(OP)
Hi Zekeman and desertfox,

Thanks for your messages.

Due to design constraints, I cannot put an opposing spring "above the follower pivot" the spring that opposes the follower load must go below the follower pivot point. I know that may be hard to understand without seeing the whole mechanism, but that is one other design constraint I have. A torsion spring underneath may be viable, but I will have to check into it.

However, what is the main advantage of using a torsion spring to oppose the follower load, as compared to a couple of extension springs? I also need to be able to adjust the spring load, against different follower loads. I can't put the torsion spring right on the pivot point i.e., follower pivot point in center of torsion spring.
 
If I were to reduce the moment that the balance springs exert on the cam, I would then have to also increase there strength and/or rate, and I would not gain anything. This is because the unbalanced spring force needs to be 50 pounds when rotating the cam 4.632 degrees clockwise from neutral with the present return spring moment arm. If the spring moment arm is shortened, then the spring strength must be increased to provide more than 50 pounds of unbalanced spring force at 4.632 degrees clockwise cam rotation from neutral, in order to create the same torque on the cam as with longer spring moment arm.

Putting the springs in line with the cam rotation axis, would require a very high rate stiff spring, since the moment arm of the springs would go from zero to only a very small length, as the cam is rotated from neutral.

Considering the maximum spring force is needed at only 4.632 degrees clockwise cam rotation, the spring rate may need to be over 1,000 lb/in if the extension springs were located right in the center of the cam rotation axis.

With the solution I mention in my previous post, I will just put a couple of extension springs on the "yoke" that holds the roller, and this will oppose the roller load onto the cam. At neutral, the "spring assist" will have a force almost equal to the roller force at neutral.    

Thanks for your feedback.

John

RE: Request for help with spring design

2nd attempt to reply.
John,
I believe that you can accomplish the same thing with the initial am configuration by placing an opposing spring at the point of application of the 150 lb soft spring input load (or follower torsion spring), slightly less in force so that you maintain contact  of the roller with cam.Then you could then add your lower force "feel springs as you describe.

RE: Request for help with spring design

(OP)
Hi zekeman,

Thanks for your message.

If I add a spring to oppose the the follower load onto the cam, (i.e., a spring force almost equal to the follower load at neutral), then I must use a cam that can both push and pull the roller. If I use the original cam, when the cam is rotated clockwise from neutral, the added spring that opposes the follower load will not allow the roller to follow the decreasing radius of the cam curve. This is why I must use a cam that is double acting, i.e., the rib cam with a roller on each side of the rib.

Thanks
John  

RE: Request for help with spring design

John,

Testing!! I've been trying to get thru for some time.

RE: Request for help with spring design

John,
 
If you balance out the follower moment at the extreme clockwise position of the follower (and cam) with a soft spring on the follower link, and then add your balancing springs on the cam for the neutral, I think you can materially reduce the crank arm forces with the existing cam design. It would depend on the nature of the input 115 lb spring force, the equalizing spring  and their spring rates.

In that connection it would be helpful if you could furnish the nature of the input force and some dimensions of the folower linkage and cam,

Zeke

RE: Request for help with spring design

(OP)
Hi Zeke,

Thanks for your message.

I am pretty sure I need a double acting cam that can both push and pull the follower. I am not sure that counter-balancing the follower forces at the extreme clockwise position of the cam and follower, would be sufficient, since the roller forces are fairly low at that point. I think it is probably necessary to counter-balance the follower spring force at neutral.

Please take look at the following information and let me know what you think.

The spring that exerts the 118 pound force at neutral, (which creates the roller follower load onto the cam), has a rate of 1,390.42 lb/in. As the cam rotates clockwise from neutral, the 118 pound linear spring force at neutral (which creates the roller follower load onto the cam) decreases by about 8.34 pounds per each degree of clockwise cam rotation from neutral. The maximum clockwise cam rotation from neutral is 16 degrees, but the first 0.5 degree is a dwell, and therefore the follower load does not decrease at all, for the first 0.5 degrees of cam rotation from neutral.   

When the cam is rotated counter-clockwise from neutral, the 118 pound spring force (which creates the roller follower load on the cam) increases by about 8.34 pounds per each degree of cam rotation. The maximum counter-clockwise cam rotation from neutral is 14.5 degrees, but the first 0.5 degree is a dwell, so the follower does not move for the first 0.5 degree of counter-clockwise cam rotation from neutral. The roller is in contact with the center of a one-degree dwell at neutral, and as such, the roller does not move for the first 0.5 degree of cam rotation either CW or CCW from neutral.

The perpendicular distance from the follower arm pivot point to the center of the roller at neutral "linear dimension" is 0.6". The length of the follower arm (follower pivot to roller center "aligned dimension") is 0.625". The distance from the cam rotation axis to where the opposing "balancing" springs connect to the cam, is 0.672". The diameter of the follower-roller is .1875".

The distance from the cam rotation axis to the dwell at the cam neutral position is .955". The distance from the cam rotation axis to the dwell at the cam low-point is .851". The distance from the cam rotation axis to the dwell at the cam high point is 1.03".

The maximum clockwise follower-arm rotation from neutral is 9.5 degrees. The maximum counter-clockwise follower-arm rotation from neutral is 7 degrees.

If you draw a line from the follower arm pivot point, to the cam rotation axis, and you draw a line from the follower arm pivot point to the roller center at neutral, the angle between the two lines is 64.9264 degrees.   

Thanks
John

RE: Request for help with spring design

(OP)
Hi Zeke,

I wanted to add one more detail to the dimensions given in my last message.

The perpendicular distance from the line of action of force, of the 118 pound linear spring load (the spring force that causes the roller to push into the cam) to the follower arm pivot point, is .419"

Thanks
John

RE: Request for help with spring design

After looking at your final proposed design, I am in agreement with it- double acting cam and balancing the follower link at neutral.
Assuming the follower balancing spring has the same spring rate as the force spring and is located, as you mention, near the roller center,  I get a  maximum force of about 8 lbs  on your 6 inch arm to move the cam manually plus the added amount you need for the balancing springs which should be quite small. I still don't appreciate the need for this since the follower balancing spring should bring the double acting cam almost back to neutral from either direction, although at exactly neutral, of course, there would be a 1 degree dead zone.
As a further note, in reading your posts, you seem too concerned about pressure angle, to the detriment of the rib wall thickness. You can easily get away with a pressure angle of 70 or 80 degrees for a pivoting follower (your case). The problem of high pressure angle and binding is treated in the literature for translating systems where they usually keep the angle less than 60 degrees. If you need a reference  on this I could furnish one.

Good luck on your design. I think you have a good solution.

Zeke

RE: Request for help with spring design

(OP)
Hi Zeke,

Thanks for your message. I really appreciate all your input and help with this problem, and all the input of the other forum members that posted replies.

Most of the books I have on cam design, say to keep the pressure angle of a cam with a translating roller follower at 30 degrees or less for optimum performance. The book "cams for industry" by John Reeve, says that anytime you go over a 45-degree pressure angle, you must consider the consequences of the high pressure angle very carefully.

However, none of the books I have give pressure angle recommendations for oscillating or pivoting roller followers like the system I have with this design. They simply say that oscillating roller followers can tolerate a higher pressure angle than translating roller followers, (which one would expect) but they do not tell how much higher.

The two main references on cam design I use are "Cams for Industry" by John Reeve, and "Cams, design, dynamics, and accuracy" by Harold A. Rothbart.

I would be interested in learning of any other references you can provide on cam design. In addition to being a help with this project, I just have a general interest in cam design and I am always interested in books, articles, or software related to the design of mechanical cams.

One thing that I found out is that I will need to put the inner roller on some type of slider and spring load the slider and/or inner roller into the inner profile, to offset the force of the outer roller onto the outer profile.

I found that only .002" of clearance between the rollers and the rib curve, will cause 1/4" to 1/2" of backlash in the end of the lever at neutral, i.e., the end of the lever will move for 1/4 to 1/2" before the inner curve touches the inner roller, if there is .002" clearance between the inner roller and the inner profile at neutral.

The force of the outer roller pushing into the outer profile causes the cam to rotate clockwise, but the force of the spring loaded inner roller pushing into the inner profile, causes the cam to rotate counter-clockwise. I think this will still take care of the force problems, and also eliminate the roller clearance and lever backlash problems mentioned above.   

The arrangement above is very similar to what Nick suggested with the mirrored profile arrangement. However, It did not occur to me to do it quite like this before.

Otherwise, considering manufacturing and assembly tolerances, I think it would be very hard to hold acceptable clearances between the rollers and the rib, which would not cause a backlash problem in the activation lever.

Now, I just need to come up with some way to mount the inner roller on some type of a slider, which is connected to the same "yoke" that holds the outer roller, and connect a couple of extension springs to the slider that the inner roller is connected to, in order to push or pull the inner roller into the inner profile, and offset the spring force of the outer roller. I need to do this in a "very" confined space.

Please let me know if you have any other comments or suggestions.

Thanks again,
John

RE: Request for help with spring design

John,
The numbers I threw out on pressure angle are for low speed, low friction systems.The lower numbers that are recommended are predicated mostly on a substantial safety factor and for reduced wear in high speed systems.

The limiting pressure angle for oscillating follower is governed by the friction in the system. As you point out the literature doesn't come up with a number. However, Rothbart and others state that it is very difficult to create a binding condition for this configuration.

 I worked out a limiting number based on a solid roller (i.e one that is bound and will not roll for a worst case)

cot(alpha)> mu

where
mu= friction coefficient
alpha= pressure angle

If you take a pretty severe case of static fricton,namely
mu=0.4, I get
alpha < 62 degrees

If you can use needle bearings, this number gets higher,
approaching 90 degrees, although you obviosly wouldn't design to that.

 As far as refernces, Rothbart is good, but the most comprehensive treatment that I have seen is
Mechanics and Design of Cam Mechanisms,F.Y. Chen,Pergamon Press, 1982. It is a little mathematical at points but is very thorough.

Best Regards,

Zeke

RE: Request for help with spring design

(OP)
Hi Zeke,

I wanted to add one more comment, regarding the following comment you made in a previous post:

Your comment:

"I get a  maximum force of about 8 lbs  on your 6 inch arm to move the cam manually plus the added amount you need for the balancing springs which should be quite small. I still don't appreciate the need for this since the follower balancing spring should bring the double acting cam almost back to neutral from either direction, although at exactly neutral, of course, there would be a 1 degree dead zone".

My reply:

Actually, the opposing springs also serve another purpose, which is to hold the cam at the neutral dwell point. I don't know if you have ever rotated a cam by hand when the roller follower is in contact with a dwell, but the force required to rotate the cam at the dwell point is practically zero, even with a high follower force.

With this design, the lever is only 6" long and about 1/4" diameter. However, even with a 200 pound follower roller load pushing into the cam dwell, just the weight of the small lever will rotate the cam off the dwell, after the lever is released(I have done this with prototypes). The force required to rotate the lever and/or cam when the roller is in contact with the dwell, is so small, I doubt you could hardly even measure it.

This is because the force from the rollers, (no matter how great) will not create any torque on the cam, or cause it to rotate in any way, when the rollers are in contact with the cam dwell.

So, something is needed to hold the cam at the neutral dwell point, and the small opposing springs connected on each side of the cam work very nice for this purpose.

Thanks again for all your help and feedback.

John

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