Request for help with a spring design and/or lever force problem

Move the spring attachment points on the cam to the right of their existing position to reduce the effective force in one direction campared to the other. This sinusoidal force variation would better match the linear(?) force increase from the roller.

Brian W

Hi Brian,

Thanks for your message.

The spring that creates the follower load is linear, however, since the steepness of the cam profile changes, the torque needed to rotate the cam changes as the cam is rotated.

I don't think I have the space to change the location of the spring attachment points on the cam. The bottom of the cam is very close to a rear cover plate on the device.

I am not real clear on how moving the spring attachment points on the cam, would have an effect on the spring forces, but I think I see what you are saying. For example, if I take the spring on the left side of the cam and put it's attachment point right underneath the cam rotation axis, then take the spring on the right side of the cam and put its attachment point about 1" to the right of the cam rotation axis, then, as the cam is rotated clockwise, the spring on the right side of the cam will extend at a faster rate than the spring on the left side of the cam retracts. Is this what you were suggesting ?

I don't think I have the space to do it, but it's and interesting idea. Do you have any other suggestions ?

Thanks again, John

The crux of the suggestion was to move the fixing for the springs on the cam away from the perpendicular to the cam's rotation axis, Rather than move the cam fixing for the springs, you could move the static fixing for the left spring upwards for much the same effect.

Brian W

Hi Brian,

Thanks for your message and feedback.

I am afraid those suggestions don't seem to solve the problem. When the cam is rotated clockwise from neutral 4.632 degrees, I am "required" to have 50 pounds of unbalanced spring force acting on the cam. I also need equal opposing spring forces at neutral, in order to have a "balanced feel" in the activation lever. However, when rotating the cam counter-clockwise 4.632 degrees from neutral, I need significantly less than 50 pounds of unbalanced spring force acting on the cam, to make it easier to rotate the cam counter-clockwise from neutral.

When I did the math, offsetting the distance of the springs relative to the rotation axis of the cam would not meet "all three" conditions given above. I ran into the same problem just trying to use a lower rate spring on the left side of the cam than on the right side of the cam.

Do you or anyone else have any other suggestions ? I would appreciate any other feedback.

Thanks John

I suspect you are having difficulty getting inside this mechanism.

A piece of board, with some light springs fixed with nails and a cam and lever would be helpful to you.

That's my next suggestion. Is there some reason why this suggestion is also in some way inoperative or impractical too? :-)

Good Luck

Brian Whatcott Altus OK

Hi Brian,

Thanks for your message.

I already have a physical prototype I am experimenting with, but the experiments have not yet yielded a suitable solution.

At present, I have run out of ideas to experiment with :-)

I appreciate your input.

I would appreciate any other suggestions from any of the other forum members.

Thanks John

As I have pointed out before in another forum, it would speed things up if you could come up with a definitive graph of the desired force vs deflection curve.

In addition it would obvioulsy be helpful if you could define the cam characteristic.

Otherwise we'll all just thrash around.

Cheers

Greg

John wrote:

Hi Greg,

Thanks for your message.

When you say define the cam characteristic, what characteristic are you referring to ? I have the drawings on the webpage

for reference.

When you say "deflection curve" are you just talking about the linear deflection of the spring. The springs are opposing, so the required load, divided by the "difference" in extension of the opposing springs, gives the rate of the spring.

The amount of force required to rotate the cam varies throughout each degree of cam rotation, as is normal for a cam that does not have a constant velocity curve.

Rather than worry about the non-linear forces for each degree of cam rotation, I focused on the "critical point" on the cam profile, i.e.,

4.632 degrees clockwise cam rotation from neutral, as explained on the webpage. This is the point on the cam that requires the largest spring rate.

Going from the cam low point to the dwell at the cam mid-point, the following are the forces required by the springs. Each increment is

1.033 degrees of cam rotation. Forces in pounds, spring stretch in inches. I also have the difference in stretch of the opposing springs listed.

When rotating the cam counter-clocwise from netural, I don't hardly need any spring force at all, just enough to pull the cam off the high dwell (practically zero). Therefore, I have not incuded figures for rotating the cam counter-clockwise from neutral.

The spring force should simply be as low as possible when rotating the cam counter-clockwise from neutral, but since I need a "balanced feel" in the activation lever at netural, I have put equal spring forces on the left & right side of the cam. However, from a force or load standpoint, the springs on the left side of the cam could be very weak. After rotating the cam off the high dwell, the roller force will bring the cam back to neutral.

Hopefully the information below will stay on the proper line, after I post this.

Thanks John

Spring force: Difference in extension of opposing springs:

2.030 (Cam low point) .37" 6.259 .347" 11.33 .323 17.590 .299 24.612 .276 31.247 .253 36.419 .229 39.867 .205 41.196 .181 40.310 .157 36.953 .133 31.438 .109 23.959 .085 15.039 .060 5.1157 .036 0 (dwell at cam mid point) 0.000 (opposing springs are balanced here)

I suspect that what Greg meant by defining the cam characteristic, is that perhaps it would be better for you to forget what's inside the mechanism now, and make a graph of how much force you _want_, at the human contact end of the actuating lever, at each angular position, and work inwards from there.

As for me, I haven't quite figured out what you mean by 'smooth balanced feel', but I get the impression from the context that you're talking about having the actuating lever preloaded into the neutral position, with different forces required to start displacing it left or right.

Which becomes simple if you forget about using opposed extension springs.

Instead, use opposed compression springs. Put each of them in some kind of simple cage to keep them from buckling, and to guide them. If you want to get fancy, the cage can enclose the spring like a bucket tappet.

The trick is that each of the spring cages/ tappets has a stop, fixed to structure not the cam, that prevents the spring from exerting any force on the cam when the lever is in the neutral position or moves the 'other' way. I.e., when lever moves one way, one spring is compressed, and the other spring remains at the neutral position. And conversely. Then the springs can have different rates and different preloads, and you don't have to go through all that malarkey about subtracting spring forces from each other.

-Mike-

It's probably easier to visualize if you think

Hi Mike,

Thanks for your message and feedback,

I cannot implement opposing compression springs for a few reasons. The space will be too small, and I won't be able to fit a cage around the springs. Also, if the springs use some type of stop, you will lose the balanced feel at neutral.

What I mean by balanced feel is this. After the activation lever is moved from neutral and released, when it springs back to neutral, I don't want it to come to an abrupt "stop" and feel like there is a "detent" at neutral. I would rather have the activation lever spring back to neutral, and rock back and forth a little instead of come to an abrupt stop. The opposing extension springs provide just the right feel, but then the force required to rotate the cam counter-clockwise from neutral gets a little larger than is desired.

Also, the cam is adjustable up and down for a total distance of 5 mm, so this seems to make compression springs hard to implement (they may be angled a little depending on cam height adjustment). I would rather use compression springs, just to eliminate hooks, but I don't think it is viable.

Thanks again, John