# Bistable spring mechanism with a different transition point?

• Storm_Eagle
In summary, a rotary or linear mechanism that flips over to the "unstable" position closer to the current point of the switch than the midpoint is what is needed for the application.f

#### Storm_Eagle

Hi. I have a application for a bistable spring mechanism. But my problem is that all of the ones i have seen require to go past the "midpoint" before it flips to the other side. Is there any such mechanism that flips over to position 2 before you get to the "midpoint" from position 1?

Example of the mechanisms i have found:

The problem as stated is that you have to go past the "unstable" position in the midpoint before it flips over. What i want is 2 "unstable" positions depending on what side the "switch" is currently on where the "unstable" transition point is closer to the current point of the "switch" than the midpoint.

The mechanism can be rotary or linear, that does not matter for my application.

For my application i can only have one driven part, so that does not work. I am thinking that somehow some joint that moves the "unstable position" must flipp over at the same time as the whole lever flipps over.

My application involves a oscilating part that will change the angle of a plate on the endpoints of the travel, but that looses power before it reaches the normal midpoint of a bistable spring mechanism like that. Therefore it must flipp before the midpoint, and it must do that in both directions because it will move up and down and switch at both the top and the bottom.

I tried to draw what i am trying to achieve:

So the plane (1) is moving up and down the axis (the vertical filled box) and flipping the angle of the plane at each end of the travel. The plane that is flipping is driven with a force up or down when it is angled, but when it is flipping it will loose the driving force just before the plane reaches horizontal angle.

And the bistable spring mechanism is for flipping the plane at each end.

anorlunda
How does that have any relevance to my question?

You are talking to a bunch of physicists plus an occasional engineer. A diagram of what does not meet your needs is not very useful. What we need is a diagram showing what is moving (can be a black box), roughly how far and how fast, the stable points, and the unstable points. If the location of the unstable points differs with direction of movement, that needs to be clearly shown. Does it need to move in a straight line? Horizontal, vertical, at an angle, or all of the above? How stable do the stable points need to be, as in force to move away from the stable point?

We can only help if you clearly show us what you want to do. If it takes you several hours to figure out how to clearly show this, so be it. Sometimes it takes much longer to communicate the problem that needs to be solved than to actually solve the problem. Or, more correctly, properly defining the exact problem to solve IS the problem. Your diagram in Post #3 does not communicate what you are trying to do. You may understand it, but we do not.

Dullard
The mechanical switching points can be moved very close to the centre.
But they cannot pass without loss of the hysteresis needed for bistable operation.
For that reason, early operation of both the on and off points is impossible.

I believe that Baluncore is correct for a single-spring mechanism, but:
A mechanism which 'moves' the fixed end of the spring could change the geometry such that you could get the effect that you seek. The problem is that you'll have to figure out a way to get the energy to do that (it can't come directly from the spring). If you have 'extra' force available during the 'driven' part of the operation, this isn't impossible (I think). I may (as always) be wrong.

I believe that Baluncore is correct for a single-spring mechanism, but:
A mechanism which 'moves' the fixed end of the spring could change the geometry such that you could get the effect that you seek. The problem is that you'll have to figure out a way to get the energy to do that (it can't come directly from the spring). If you have 'extra' force available during the 'driven' part of the operation, this isn't impossible (I think). I may (as always) be wrong.
As i said i have a force driving it, but the problem is that the force diminish to zero right before the midpoint where the switching takes place. If we assume that the plane has sufficient mass and speed the inertia will fix the problem by overcoming the spring force and carrying it past the midpoint, but for my idea here i will assume the inertia is not enough to do this so the switching will have to happen before the midpoint because the driving force will be lost before the midpoint.

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What we need is a diagram showing what is moving (can be a black box),
That is what i have tried in post #3
The bistable mechanism is not in the drawing obviously because that is what i am asking for. But the bistable mechanism will be in the center of the plane that is moving up and down the linear axis, and will change the angle of this plane on each end position as i have tried to draw.

roughly how far and how fast,
That is not relevant for my question. As i do not want to have the inertia of the moving plane as a factor. So assume the inertia is zero.
So no numbers or calculations here just a mechanical mechanism (like you don't need the pressure and temperature values of the steam and the inertia of the flywheel and the dimensions of the cylinders and pistons to illustrate how a steam engine works in principal)

If the location of the unstable points differs with direction of movement, that needs to be clearly shown.
As i have said the bistable mechanism is what i am asking for, the application is what i have drawn. And for that the unstable point will have to move closer to the stable point it is at (switching at each end).

Does it need to move in a straight line? Horizontal, vertical, at an angle, or all of the above?
All the bistable mechanism needs to do is to change the angle of the plane at each end. If it does that in a rotational way, or in a linear way with links then making the rotation of the plane does not matter for the application.

How stable do the stable points need to be, as in force to move away from the stable point?
Well that is determined by the strength of the spring used. That does not have any relevance to what mechanism is used.

We can only help if you clearly show us what you want to do. If it takes you several hours to figure out how to clearly show this, so be it. Sometimes it takes much longer to communicate the problem that needs to be solved than to actually solve the problem. Or, more correctly, properly defining the exact problem to solve IS the problem. Your diagram in Post #3 does not communicate what you are trying to do. You may understand it, but we do not.
• The filled box is the axis the plane is moving on.
• The plane is the unfilled box marked 1
• The dotted line boxes marked 2-5 is to illustrate the angle change of the plane at each end
• The numbers is to illustrate the order of operation of the plane (no i can not make a animation)
• The arrows is to illustrate the direction of movment between the illustrated positions.

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You can show the positions of each element of the assembly in a sequence of diagrams without using animation. The the current figure is to ambiguous to be of help.

There is an activation energy needed to toggle the switch. Reduced movement to reach the energy crest requires a greater force over the shorter movement.

A toggle switch is a state machine with only two states, open and closed.
Transitioning between those states requires crossing one energy crest.

If instead we make a four state machine with (closed, opening, open, closing, closed) fixed direction cycle we can move the thresholds.
A press button on/off switch often has a commutator that rotates by 90° when pushed and 90° when released. The next press and release advances 180° to complete the cycle.
Another example is the PB ballpoint pen.

We design PB switches to require a long gentle activation force.
That is the opposite of the OP requirement for a short heavy activation force.

Hi. I have a application for a bistable spring mechanism. But my problem is that all of the ones i have seen require to go past the "midpoint" before it flips to the other side. Is there any such mechanism that flips over to position 2 before you get to the "midpoint" from position 1?

Example of the mechanisms i have found:
View attachment 254306

The problem as stated is that you have to go past the "unstable" position in the midpoint before it flips over. What i want is 2 "unstable" positions depending on what side the "switch" is currently on where the "unstable" transition point is closer to the current point of the "switch" than the midpoint.

The mechanism can be rotary or linear, that does not matter for my application.
Not sure if this works, I can get my brain only half way around it!

Referring to the approach in the right side of your figure, I'm assuming the curved link is a "too-long" leaf spring that is always curved. How about attaching two coil springs horizontally to the approximate center of the leaf spring, pulling left and right? The coil spring forces will always be unbalanced trying to pull the leaf spring overcenter, but not enough to do so by themselves.

Success of this approach probably depends on the inertia of the leaf spring and the moving load... (haven't really thought it thru in detail.)

Let's see if anyone can build on the idea to make it useful.

Cheers,
Tom

Success of this approach probably depends on the inertia of the leaf spring and the moving load... (haven't really thought it thru in detail.)
Over-centre linkages require the threshold be beyond-the-centre. The indeterminate middle state represents the safe hysteresis of the switch. That is a single path system. It can be modeled as movement in a single dimension, along a line.

When the thresholds cross to the closer side of the centre there are three states. The two end states are stable, the central state is indeterminate. It is high enough to force high, while still being low enough to force low. Those contra-force curves will sum to form the indeterminate stable middle state in which a limited oscillation is possible.

I find the electrical analogies using flip/flops and comparators easier to understand and design. The NE555 timer has two comparators and one F/F. That is a simple two-state system with 33% hysteresis between two switching thresholds.

If the position of the thresholds is swapped, it becomes necessary to have an additional F/F, = bit of storage, to remember the previous state, as that determines the circular path around the square state diagram.

Any mechanism with the same behaviour will require the additional mechanical memory, which might be encoded as two independent rockers, or as a circular motion like the press button ballpoint pen.

Tom.G
I sounds like you are trying to move something back and forth between defined endpoints, with stable positions at each end, and an overcenter force near the current position of the something. The top two push force vs displacement plots in the figure below show what I mean:

When the something is in the left position, it takes a push to start it moving, then after a short distance the push force becomes negative and it moves by itself to the far right position. Similarly, when it is in the right position, it takes a push to start it moving, then after a short distance it moves by itself to the far left position.

A partial mechanism to do this is sketched in the bottom of the figure. Two cam tracks, one behind the other. Each cam track has its own cam follower. Some sort of rocker as alluded to by @Baluncore in Post #14 would switch between cam followers at each end of travel. That switching action would take some energy. That energy would come from the kinetic energy of the something hitting the end stop.

It's a partial idea, maybe somebody can figure out a switchover mechanism.

I sounds like you are trying to move something back and forth between defined endpoints, with stable positions at each end, and an overcenter force near the current position of the something. The top two push force vs displacement plots in the figure below show what I mean:
View attachment 254584
When the something is in the left position, it takes a push to start it moving, then after a short distance the push force becomes negative and it moves by itself to the far right position. Similarly, when it is in the right position, it takes a push to start it moving, then after a short distance it moves by itself to the far left position.

A partial mechanism to do this is sketched in the bottom of the figure. Two cam tracks, one behind the other. Each cam track has its own cam follower. Some sort of rocker as alluded to by @Baluncore in Post #14 would switch between cam followers at each end of travel. That switching action would take some energy. That energy would come from the kinetic energy of the something hitting the end stop.

It's a partial idea, maybe somebody can figure out a switchover mechanism.
That is what i am after.

But i don't particularly like the track and cam idea (but that is the same possible mechanism that was the only one i could think of too). I like a linkage mechanism better (more robust and simple). And the switchover mechanism (for selecting between the two tracks in the case of your drawing) needs to be solved in a simple manner (i don't want a too complex mechanism for my application, as i want to keep it simple to be both robust, inexpensive to make, and have minimal amount of failure points)

The reason i prefer a linkage approach over a track and cam approach is:
• With track and cam there will be side forces on the spring part. While with a linkage mechanism there will be minimal side forces
• With track and cam you have to have a bearing on the cam part for rotation. That bearing can be filled with stuff and fail, while on a linkage system you can use simple plain bushings in the link joints. And a bearing is also more complex and expensive.
• With track and cam loose objects can get wedged between the cam wheel and the track stopping the mechanism from working, or even breaking the mechanism. While on a linkage mechanism this is mostly not a possibility.
• The track can become dirty making the cam run rough or even fail to work.
• The track and cam surfaces will wear more over time i think than the bushings in a linkage mechanism.
My application is in a marine environment so corrosion and dirt and debris is a big concern. That is why i want it simple and robust.

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