On 16 Oct 2004 10:44:56 -0700, email@example.com (John) wrote:
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.
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 ?
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.
On 17 Oct 2004 10:27:03 -0700, firstname.lastname@example.org (John) wrote:
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
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
Do you or anyone else have any other suggestions ? I would appreciate
any other feedback.
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? :-)
Brian Whatcott Altus OK
On 18 Oct 2004 10:47:54 -0700, email@example.com (John) wrote:
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
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
In addition it would obvioulsy be helpful if you could define the cam
Otherwise we'll all just thrash around.
Thanks for your message.
When you say define the cam characteristic, what characteristic are
you referring to ? I have the drawings on the webpage
www.cdtd.50megs.com 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
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
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
Spring force: Difference in extension of opposing
2.030 (Cam low point) .37"
0 (dwell at cam mid point) 0.000 (opposing springs are balanced
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.
It's probably easier to visualize if you think
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
firstname.lastname@example.org (Mike Halloran) wrote in message
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