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

 

[unclesyd]

I don't think that the space will matter as you can get small bellows from Servometer, down to 0.020". I mentioned using a low melting temperature material as the motive force as this was my approach to switch problem.
We also used a thermal heating fluid filled bellows that operated around 600F in a condensate trap.

Checkout this page on the Servometer website.

http://www.servometer.com/additionalinfo.php?Id=3036

 

[unclesyd]

You could also look at construction of an automobile thermostat that uses a small can that is full of a wax that causes a rod to to extend to open the thermostat with an increase in temperature.

 

[Chaabain]

Ok, about the size of bellows, it doesn't matter. But I didn't understand exactly how to actuate it by electric power. Anyway, is it possible to actuate these bellows, for examples by filling them with special gaz where there are two electrical poles in contact with this gaz. When varying the voltage, the bellow will expand or collapse as a response?

Do you have any reference that can explain exactly how do these bellows work?

Thank you in advance,
Chaabain

 

[unclesyd]

I would contact Servometer, at one time they had a very good application manual for their bellows.

The thermostat uses a wax mixture with high thermal expansion to move the piston.

Here are some wax driven thermostatic elements from China. There is also a lot of good information on the design of temperature sensative switches and things.

http://www.wax-thermostatic-element.com/index4.htm

 

[ginsoakedboy]

Somehow, I am thinking of electrostrictive or magnetostrictive materials.

Does the mechanism have to be housed between the 2 plates? Why can it not be placed above the upper plate? Fully self-contained?

 

[Chaabain]

Thank you for your replies.

Unclesyd, I will contact servometer to get this application manual and I'll see what I can do with.
Htlyst, im my application, the mechanism must be between the two plates. Furthermore, the change of electrostrictive and magneostrictive materials as a response of the applied force or magnetic field is too small - of the order 10e-6. As I told before, the change must be between 5 and 10 mm.

Thank you a lot for any further suggestions.

Chaabain

 

[unclesyd]

Could you use two connected bellows to balance out the forces. I'm try to visualize what would be the best working fluid.

 

[toastedhead]

How about magnetic fluid? The lock will be the fluid and the fluid viscosity can be changed by coupling it with a force torque sensor.

No?

 

[Chaabain]

Hi Toastedhead,

Sorry but I didn't understand exactly how we can do that under the conditions that I've talked about.

Thank you in advance for any further information.

 

[Chaabain]

Hi again,

Using the metal bellows that you have suggested. I made an illustration of my application to get an answer to the following question:

Taking into account the initial force applied to the upper plate (given that F1 = 150 N), how could we actuate the bellow when F2<F1 untill F2 = 150 N once again? How could I lock the actuator when needed without taking into consideration the change of force exerted on the upper plate?


Thank you in advance,
Chaabain

 

[MikeHalloran]

Did you notice that the mechanism you've illustrated, and improperly dimensioned, could not possibly have 5mm of stroke and a 5mm collapsed height?

You keep saying "How could I lock.." while defining an envelope that apparently allows no penetrations or extensions for remote control or energy storage or observation. You have also not made it clear what "lock" means to you in this context.


You are talking in circles. While it's possible to make a good living doing that, it's an unusual behavior among engineers. When are you hoping to graduate?






Mike Halloran
Pembroke Pines, FL, USA

 

[Chaabain]

Hi Mike,

Sure you are right, it's improperly dimensioned, but it was just to illustrate the idea. Let's say that expanded height is 7 mm (I didn't use a CAD software to illustrate that). For the lock, you can penetrate the envelope by a good idea that I couldn't imagine."Lock" means that the upper plate will never move to balance the force else if the user allows. I'm still a young undergraduated engineer and I have no experience concerning the question that I've asked.

Anyway, thank you MikeHalloran for your reply. I look forward to get your suggestions.

 

[jmw]

I'd separate out the two functions.
Function one is the phsyical control of the gap and the ability to lock it anywhere.
Function two is to measure the force and use that to control the mechanism.
Roller cams along the edges or paired wedges with strain gauge sensors.
Actuators for the cams (rotational) or wedges (linear).
Actuators controlled by micro processor based on feedback from the force sensor and with a programmable algorithm that controls cam/wedge movement anyway you like. Speed of response, smoothing, define and ignore transients, hysteresis, direct acting and reverse acting, fail safe overpressure response, and so on..... anything you like.



JMW

http://www.ViscoAnalyser.com

 

[Chaabain]

Thank you JMW for your reply. It's still the main problem. Is there an actuator that can fit into the available gap and can provide the necessaire movement with considering the force applied?

Thank you in advance for any further suggestion.

 

[jmw]

The only other thing I can think of is to join the fixed and moveable plates with a stainless steel triple ply bellows and make the cavity into a sort of bellows.
Then use hydraulic oil to fill the cavity. If you close the inlet/outlet valve, that will provide an effective lock.
Alan Simpson suggested looking into stainless steel bellows.
Try here:

http://www.sigmanetics.com/pages/bellows_overview.htm?gclid=CPa7nomr9pwCFU0A4wodlXZsaw

or

http://www.htproducts.co.uk/vac/vacbellows.html

for example.
A minature pump or a nitrogen pressurised bladder have already been suggested.


JMW

http://www.ViscoAnalyser.com

 

[MikeHalloran]

I'm not aware of a bellows that has a stroke anywhere near as big as its collapsed length, unless you can tolerate a very limited life, e.g., one cycle.



Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

Uhh, Chaabain, perhaps it would be helpful for you to explain what a 'distractor' is, or does.



Mike Halloran
Pembroke Pines, FL, USA

 

[Chaabain]

According to the information that Mike provided. I have to exclude the use of Metal bellows because really I need a mechansim whose collapsed height is 5 mm and expanded height at least 7 mm (It's better if I can get 10 mm).

 

[jmw]

Well, how about a telescopic jack? (though I'm pretty sure that a bellows can be designed, with some complex geometry maybe, like the bellows from an aneroid barometer, that will expand sufficiently and not simply a straight bellows as shown on these sites.)

JMW

http://www.ViscoAnalyser.com

 

[jmw]

PS the miniature actuators search thread seems to address a similar problem.

JMW

http://www.ViscoAnalyser.com