is this four bar linkage possible? 1

[bpp88]

hello!
I am want to design a four bar mechanism but I don't know if it is possible.
A motor moves the Input bar with the objective of making the output bar rotate 160º. Initially i wanted to make the output bar rotate 180 degress but i realized that it is impossible to do it without going through a blocking position (obviously), i think this position is call dead centre position in english ("posicion de bloqueo" in spanish)
The main idea is to begin in a position of maximum mechanical advantage (where the input link and the coupler link are aligned) and then activate the motor in order to move the output link 160 degrees. The mechanism has to be able to return to its initial position by chaing the direction of the motor rotation.
therefore, the conditions are the following:
- the mechanism should not pass through dead centre positions
- in the initial position, the mechanism has to be in (or close to) the maximum mechanical advantage position (input and coupler links aligned)
- the output bar has to rotate 160 degrees.
Is that even possible?
I've studied the different possibilites and I think that the only way of making this is with the Grashof "Crank-Rocker" mechanism where the crank is the input link. this way the motor rotates in one direction and there are no dead centre positions. but in this case i think it is impossible to make the rocker rotate 160 º....

thank you very much.

sorry for my english. If you don't understand something, pless ask. thanks again

 

[ornerynorsk]

Check this site out:

http://www.brockeng.com/mechanism/index.htm

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[MiketheEngineer]

When pressed with such problems - I get the jig saw out and make a "model" out of wood/plywood. Usually VERY cheap and verifies "real life"

Try it - it is fun!!

 

[zekeman]

Why not the simple 4 bar mechanism, with the input crank and output link the same length, i.e. parallelogram mechanism.

 

[JohnRBaker]

Cardboard & thumbtacks always worked for me, until I started using CAD ;-)

Speaking of CAD, if you have access to any sort of system which supports parametric 'sketching' (in this case, a sort of modern version of 'cardboard & thumbtacks'), this sort of thing can be solved pretty easily.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

UG/NX Museum:

http://www.plmworld.org/p/cm/ld/fid=209

To an Engineer, the glass is twice as big as it needs to be.

 

[MiketheEngineer]

YEP, cardboard and/or CAD will often prove the theory!!

 

[TheTick]

Look up "relative pole" method for designing 4-bar linkages. This method works really well with 2D sketches (e.g. in SolidWorks).

 

[sreid]

While four bar mechanisms still have their place in mechanical design, it's possible that another approach might be a better solution.. For example, a brushless servo motor with an integrated controller and [posibly] an inline gearbox. A single inpot would allow you to go from position 1 to position 2 with controlled acceleration and velocity.

These things have become surprisingly inexpensive these days.

 

[chicopee]

Agree with M-t-E, cardoard with tacks on a piece of plywood or if you still can get one in your country the memorable Erector Set.

 

[BP846]

zekeman,

the paralelogram begins with the four links aligned, this is not a "maximum mecanical advantage" position. it is a singularity position.

 

[SomptingGuy]

Meccano would be my choice of prototyping environment. I have a relatively new set. It is a close cousin of Erectorset.

- Steve

 

[ornerynorsk]

Still have a workbench top with a gazillion tack holes in it from modeling movements. There's still a place for that, even with a pricey CAD system residing in the hard drive.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[zekeman]

My instinct tells me that you are not going to get a 4 bar to do this for the simple reason that you are asking to get 160 degree output motion from a 180 degree input and asking a big MA. When you have a big MA, it means that the crank angle motion divided by the output link motion has to be much larger than unity at the start and since the average MA (180/160)would be closer to unity.

Why don't you tell us what you really need and maybe some of us could help you with a mechanism,but it WON'T be a 4 bar with your high MA demand.

 

[MotionGirl]

I would do it with a 6-bar linkage myself. You would then be able to get about 220 degrees without any serious transmission angle problems...

 

[MotionGirl]

Have I stopped the discussion...? Shall I try to model and animate what I mean?

 

[zekeman]

Go for it.

 

[MotionGirl]

OK here is a 6 bar linkage with an oscillating input of 180degrees with an output link rotating more than 180 and the transmission angles don't get that bad....that's in my humble opinion. Have I missed the requirements though, as I'm not entirely sure what is being asked for....LEt me now if it is off the mark...

 

[zekeman]

Motiongirl,
Can't seem to open yor .avi file.

 

[zekeman]

I finally opened it. It most certainly meets the OP requirements, except he wanted a 4-bar.
Good solution

 

[KENAT]

Cardboard Aided Design, the original CAD.

Nice illustration motiongirl

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