Suggested distractor mechanism, any help? 11

[Chaabain]

Dear all mechanical engineers,

I have a problem and I would like to know if you can help me to get a solution:

Our system consists of a fixed plate, a mechanism, and a mobile plate. When the force applied to the mobile plate is equal to Fa =150 N, the mobile plate is stable (Case 1). As this force decreases, the mobile plate must move upward till the applied force is once again equal to Fa (Case 2). The mechanism should be locked after this movement. What type of mechanism could we use to achieve this giving that the initial distance between the plates is 5 mm and the maximum allowed distance is 10 mm?

Illustration:

http://files.engineering.com/getfile.aspx?folder=ebb48677-a214-49b6-ba8e-53c60e5e539b&file=Idea.bmp

Thank you in advance;
Chaabain

 

[alansimpson]

Pneumatic actuator would give constant force with position. However locking would then render that facility inoperative.

A linear motor like SMAC or Linmot can be positioned and detect load on axis. Again not sure if it can hold position and react change in load. Only thing I think would work would be some sort of servo axis with a load sensor.

Getting it to fit into 5 mm and expand to 10 mm would be some engineering feat. Some sort in miniature scissors lift?

Sorry if this is no help

 

[DesignerGuy16]

Custom air bellows with constant pressure and a check valve to dump/equalize pressure during change? 5-10mm is a tiny space to do anything though.

James Spisich
Design Engineer, CSWP

 

[Chaabain]

Thanks Alansimpson and Jspisich for your replies.
I think that the miniature scissor lift is a good solution, but we need to actuate the lift and consequently is it possible to find a miniature actuator that can push the upper mobile plate with the suitable force?
On the other hand, air bellows may also be used, but I dont'k know about these bellows if they need a source of air because I need my system to be completely independent. In other words, I need the source of kinetic power (electrical, pneumatic, ...) to be also embedded within the system.

I would be so grateful for any more information may help.

Thank you in advance,
Chaabain

 

[alansimpson]

If you had room for an accumulator tank you could go the pneumatic way and keep the unit self contained. Hydraulic would also be a solution. There are are nitrogen accumulator tanks used with hydraulics. Pressure from tank should be even if movement isnt too big. A check valve on hydraulic circuit would lock movement since hydraulic fluid is incompressible.
The bellows might be low enough profile to fit into 5 mm space. Something like a bladder or inner tube. Look into metal bellows.

 

[Chaabain]

Ok, I'll add some details to clarify the problem:
The dimensions of the lower and upper plate are:
Width: 32 mm
Length: 37 mm
Height: 2 mm
and as I said before, the initial distance between the two plates is 5 mm and the maximum allowed distance is 10 mm. I need the distractor to fit into this space. For the pneumatic and hydraulic way, I don't think that it works because I don't have enough space for the accumulator tank. A miniature electric actuator may work, but are there linear motors that can fit into this space or even the metal bellows, is it possible to fit overthere.

Thank you in advance for any further information.

Chaabain

 

[MikeHalloran]

So, when the force is reduced, the mechanism expands, and then locks?

After it's locked what happens if the applied force increases? It doesn't move? Ever?

I.e., how many times does this mechanism have to actually work? Example: If the answer is 'one', then the mechanism could be a spring in a balloon filled with wet concrete.

How many can you sell?

How much money have you got?



Mike Halloran
Pembroke Pines, FL, USA

 

[Chaabain]

Non, when the force increases, the upper plateau must move downward until the force is once again equal to Fa (Case 1). Anyway, if the wet concrete becomes dry before the decrease of force, what can we do?

To present, I can't tell you how many I can sell before you tell me how to do with this problem !!!

Thank you Mark for your idea,
Chaabain

 

[MikeHalloran]

If the mechanism expands with decreasing force, and shrinks with increasing force, in what sense is it 'locked', when at the outer position in your original specification?

I.e., so far you've described a spring.



Mike Halloran
Pembroke Pines, FL, USA

 

[Chaabain]

Hi Mike,

It's exactly like a spring but must be locked by user whenever needed. That means I don't need this operation to last forever. I should lock and unlock the mechanism whenever I want and in this case, I don't care about the decrease or increase of force. It's like a spring with controlled lock, does it exist?

If you have any idea about that, Mike, I'll be so grateful.

Chaabain

 

[MikeHalloran]

So, you require a spring, and a manually operated lock... which the user accesses how?

Insert a shim between the platens? There's your lock.

User puts his fingers between the platens? Need guards and/or interlocks to protect the fingers.

Manual operation of electric switch controlling a solenoid that places a shim between the platens? Requires a power source.

Is a shim sufficient, or does the 'lock' need to prevent further outward motion in the event of an extra force applied in an outward direction?



You probably thought your first message carried a complete specification. Not so.


Mike Halloran
Pembroke Pines, FL, USA

 

[DragonTSD]

I dont understand your diagram, specifically the Fb=Fa relation.

I think what you are saying is that you need the mechanism to equalize reflexively by expanding, is that correct? Like a locking spring, like mike said?

This thing is small, is there any more space to be had? what about what's under the fixed plate?

I'm thinking of using a custom stepping motor setup, with holding/breaking voltage acting as the lock

stepping motors are big though...i mean, they arent 37 mm across.

 

[Chaabain]

Hi again Mike and DragonTSD,

The electric switch controlling a solenoid, it's a good idea. But can it fit into the space that I talked about. For the power source, the mechanism may be powered by an exterior induction coil without the need to put a source of power in. Th lock must prevent the movements in all DOF if it's possible.
DragonTSD, for the relation, Fb = Fa, that means: when the force exreted on the upper plate decreases, the upper plate must move until the force Fb becomes again equal to Fb.
Yes, it's like Mike said. The problem is that I've to find a stepping motor that can fit into this space, and I've alread made a research about that, but I couldn't find this motor. The motor I need has a height of 5 mm and can expand to 10 mm and like that it's impossible, especially with the torque required to push the upper plate. Under the fixed plate, there is no space to be used.

Finally, I would like to thank you for your help and I'll be so grateful for any further help.

Yes Mike, you are right, the first message didn't carry a complete specification because I couldn't expect that I'll have such a help. If you are interested, I can describe the whole idea again with the application I need.

Many thanks,
Chaabain

 

[MikeHalloran]

So far, you've described a spring in a cage. Because of the stroke, the cage has to be compound, but it's doable.
Except for the 'lock', the specification for which seems to keep changing.
You can get a solenoid in the specified envelope. Less likely a motor.
Transmission of power by induction requires two coils. Is there space outside of the problem gap for a coil?

Yes, please try to write a complete and coherent specification. Try to design something to meet it yourself; that's one way to improve a spec, by forcing you to ask the questions we would ask of you.



Mike Halloran
Pembroke Pines, FL, USA

 

[FeX32]

What about an elliptical shape on a simple motor. As the motor rotates you can 'lock' and unlock the movement by stopping the motor. And you can move the plates from case 1 to case 2 and vise versa with one rotation of the motor.
Kind of like a cam.


Fe

 

[FeX32]

Or a scissor mechanism. Simple, easy to use. I have seen it called a 'motion' spring before.


Fe

 

[Chaabain]

Hi all,

Dear Mike, I'll try to write a complete specification as you said and back again.

Dear FeX32, for this motor with elliptical shape, can it fit into the small space that I have and is it able to push and expand with the required force?
And for the scissor mechanism,as I said before, it's very good idea but I didn't understand how to actuate it and lock it when needed with taking into account the small space that we have, do you have a full description of that?

Thanks again for all of you,
Chaabain

 

[Chaabain]

Hi again,

I've an idea about "Actuation in induction" but I don't know if it will works or not. Do you have any idea about that?

Thanx

 

[unclesyd]

One possibility is to use a filled sealed metal bellows either with gas or low expansion media that could be controlled by heat or pressure. I have built similar devices using Naphthalene to control movement.

I would also contact Servometer with your problem. They can do many things with their bellows and are capable of making very small bellows.

http://www.servometer.com/

 

[Chaabain]

Hi Unclesyd,

But as I said before, the actuator of this mechanism must be inside the aforementioned available space. In case of metal bellows, I need either heat or pressure to actuate the bellow and that's impossible in my application. Can there bellows be actuated to expand and retract by providing them in an electrical power (from battery for example)?

Thank you in advance for any further information.

Chaabain