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Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/13/2016 2:59 PM

Hi

For my bachelor's thesis in the second semester I have to make an automatic, pneumatic machine to drop egg trays on a regular interval.

I've watched a real machine do the job, but my coordinator suggested something more simple.

I've drawn the machine really simple. It consists of a cubic stacking container and is meant to be positioned above a conveyor belt.

There are only two important parts, the 'stopper' (red) and the 'pusher' (green). (These are on two sides)

The process is simple: The trays are placed in the container and can't fall through when the stopper is pushed out. When a tray needs to be dropped, the pusher pushes out while the stopper is still in the same position. All the trays but one are lifted up a bit and are now resting on the pusher. The stopper will pull back and a tray falls down. Then the stopper pushes out and the pusher goes back in. The trays now rest on the stopper like in the beginning.

My question now: is there a system to push out/pull in the stopper/pusher on both sides with just one pneumatic cylinder on each side? Or do I have to use four pneumatic cylinders?

Thank you

Matthieu

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#1

Re: Bachelors thesis: stop-push system for egg tray denester.

12/13/2016 3:30 PM

I don't know if you've ever seen a denester. There are a number of ways to accomplish this.

If you use one cylinder, you'd have to design a mechanical system to reset the holder after the pusher recycles after the release.

As I see it, the pusher releases and is mechanically tied into the stopper like a toggle Averagejoe has the word I was looking for a rocker not a toggle.

When the pusher is activated and releases, it also trips a valve for a blast of air in between the bottom carton and the carton 'on deck'.

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#2

Re: Bachelors thesis: stop-push system for egg tray denester.

12/13/2016 3:30 PM

You could design and build a rocker mechanism powered by one pn. cylinder for each side. Fairly easy if you get the kinematics right. Have used many of these to "single" out hot steel slugs in a line to a hot forging press.

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#27
In reply to #2

Re: Bachelors thesis: stop-push system for egg tray denester.

12/14/2016 4:57 PM

I don't really understand what you mean with 'Rocker', probably because I don't really master the language. When I type it in Google I don't see how this could be used for the denesting of trays.

Could you send a link or picture on how you think it works.

Thank you

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#3

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/13/2016 4:07 PM

Why does it have to be pneumatic at all? Why not use electric solenoids?

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#7
In reply to #3

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/13/2016 4:35 PM

The electric solenoid seems like a good solution, but the coordinator wants it to be done with pneumatics. (We mainly focus on pneumatics this year, next year we'll see more about electric actuators)

-> Our big problem is that this 'excercise' is the first practical one. It's the first time we have to search solutions for something while we've only seen some basic theory. So most of the good, practical solutions aren't possible for us because we don't understand them or its to difficult to draw,..

Thank you for your input!

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#9
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/13/2016 7:57 PM

You say, "we don't understand them or its to [it's too] difficult to draw".

To be a good designer doesn't necessarily mean that you can draw something on a screen, it means that you have enough of a grasp of the needed function to "see" it in your mind.

Only then can you transform it into a sketch on a bar napkin, envelope, sheet of paper or your CAD screen.

Relying on a computer screen and keyboard to communicate the function of a device is a luxury you may not always have to hand.

Your #5 shows that you have a good start. Think how can I make green go in and red go out with sufficient timing to drop one at a time and not jam or drop the whole lot at once.

You can do this with one cylinder and the proper linkage. Think about it.

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#10
In reply to #7

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/13/2016 8:07 PM

Actually, The hardest step, is starting...

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#28
In reply to #3

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/15/2016 9:20 AM

Pneumatic gives you some simple options on actuation velocity and force. Direct acting solenoids can get large and impractical as force is increased, though the simplicity of having direct action without translating energy sources is highly desirable.

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#4

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/13/2016 4:07 PM

Not my area of expertise, but it seems you could use a T shaped piece (flipped 90 degrees). A pivot point at the center point of the T would allow it to rotate. Attached to the top arm of the T is the "pusher", and attached to the bottom arm would be the "stopper". A cylinder would push up the base part of the T and a spring could return it.

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#6
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/13/2016 4:29 PM

Yes, that seems a good solution. In the meantime I've thought about something similar, but I think yours will be better in practise.

Thank you!

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#8
In reply to #4

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/13/2016 6:04 PM

The similarity to a clock escape mechanism is notable. Of course, the geometry will need to match the spacing between trays in the stack.

In real life, there may need to be a mechanism for actively pushing down trays that might become stuck.

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#16
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 9:06 AM

In practise the the trays get pulled down. Since this is only a project accounting for 1/6 of the year, they want it to be simple. That's why I can assume the trays will actually fall down.

The tray denester is only a part of the project.. IMO it would be better to make something smaller that actually works, but now I have to make a whole construction that will never function.

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#18
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 9:27 AM

Since this is only a project accounting for 1/6 of the year, they want it to be simple.

In packaging, you want it simple in real life also. The mistake is everyone one wants to show how innovated they are, some work, most don't.

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#21
In reply to #8

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 11:54 AM

Yes, it does resemble a clock mechanism for many mechanical clocks.

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#5

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/13/2016 4:22 PM

Thank you for your fast responses.

I don't really know what you mean with 'rocker', but the mechanical link is a good solution indeed.

I've tried to make a quick assembly:

So the upper back would be connected with a pneumatic cylinder, the upper front with the pusher, and the lower front with the stopper. When I take the right dimensions for the stopper and the pusher, it would be possible to achieve the same result (the pusher would have to go fairly deep).

What are your thoughts?

Thank you

Matthieu

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#11

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 3:54 AM

Hi Matthieus,

I would call it an escapement mechanism.

They can be very simple, depending on the product you are handling but I see a whole lot of potential problems with an egg carton not least being separating them. I don't think a "pusher" will raise the stack to release one carton as it will flex not lift. The suggestion of an air jet is good.

I have found from bitter experience that to rely on gravity "it will fall out/into place/etc." is fraught with problems and where possible try and "put" it where you want it to be.

If it can be done with several "standard" cylinders I do not see a problem any mechanism will be of a bespoke design and that makes maintenance a problem.

Several pneumatic component manufacturers make a cylinder mechanism in the form of an escapement mechanism where the cylinders operate in the correct sequence.

The very best of luck with your project,

John

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#15
In reply to #11

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 9:06 AM

I think it will all be in the shape design of the pusher and puller. If he want the green one to lift all trays above the next one to drop, then when it engages, it will need a wedge shape, and be made of something with low sliding friction.

The red one is there to hold the bottom tray until time, so it needs to pull out evenly from 2 or 4 sides, and suddenly. Perhaps a cam, spring loaded lever driven from the single pneumatic actuator. If it can be made to rotate in a way that actually forces the bottom tray to drop, then so much the better.

When the red one re-engages (to catch the subsequent tray), then the green one has to pull out after the red one is back in place, all a matter of shapes, push-rod length, etc.

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#17
In reply to #11

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 9:15 AM

Hi Jesw55

It's not really visible on the Solidworks assembly, but the pusher is shaped in such a way that it pushes down one tray (by fitting in the space between the trays and the shape of the pusher). Also the trays are made of plastic which is quite sturdy (but also flexible) so it is still likely to bend. The good part about the plastic is, that is has borders, so by lifting it up a bit I really meant that it would already rest on the borders and that the pusher would push a little extra, to be sure the stack of trays doesn't fall down.

I still have alot of other good methods, but I can't use most of them because we're not allowed to use servo motors.

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#19
In reply to #17

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 9:29 AM

I wonder if you have to consider static electricity charge on plastic trays, not allowing gravity to work, esp. plastic foam...

Sometimes a simple design takes much more effort than a complex one, other times a complex mechanism is born of a complex problem...

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#20
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 9:35 AM

No, the trays are heavy enough and have holes in them so there isn't much air friction.

The electric charge has no effect, because of the weight of the trays.

Because of the holes in the trays I can also draw the pusher in such a way that it ends up in one of the holes. That way it isn't possible for the others to drop down.

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#12

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 7:22 AM

Hello and welcome.

It may be possible to have a single cylinder where one end of stroke operates the pusher and the other end of stroke activates the puller. (Pusher and puller not hard connected to shaft so they can "stop" where you want them to.)

Imagine each with a spring pushing them into the central position and the cylinder causing them to displace.

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#13
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 8:46 AM

I'm seeing a single cylinder, underneath, with a series of bell crank linkages to each side to move the escapement, if that's what we're calling it, in and out, with the proper timing.

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#14
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 8:59 AM

Yes, that's what I had in mind! In that way I only need a single cylinder standing upwards.

Thank you

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#22

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 3:40 PM

Welcome to the forum. While I don't have anything useful to add, that hasn't already been said, I do want to say "Thank you."

Thank you for telling the forum the whole situation, the work you've done up to this point and maintaining a regular interchange with the posters. Your approach is so much more refreshing than those who simply have a two sentence demand for an answer as soon as possible with precious little information on how to help these hapless (and sometimes downright rude) souls.

My faith in world has been restored.

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#23
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 4:02 PM

My faith in his corner of the world, Belgium, never really faded, they have good beer, good guns, and good dogs.

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#24
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 4:22 PM

I thought you only drank warm Lone Star beer and shot prairie dogs with .22 rifles out of the back of pickup trucks in Texas.

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#25
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 4:49 PM

Not me. Although I have done both. I don't drink beer any more since wife does not allow - God's Paradise is not under the shadow of the sword, it is under her rolling pin.

I am allowed one glass of wine, if and when she requests that I open the one permitted bottle, and then she comes back and warns me not to finish the bottle (after I have already corked it and put it in the 'frig.)

The only dog I will shoot any more is any mad dog that comes to my front door unannounced, with malicious intent present in his eyes - it makes me fearful for my life. If a Scotsman, they all have that look, but are not mad, so I let them in.

If they are screaming something I cannot discern in Arabic or some African language, I will shoot without hesitation, sorry, but is how I am. Actually I don't own a Belgian firearm, since my shotgun was made in Italy. Excellent tactical weapon.

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#26
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/14/2016 4:54 PM

Haha thank you for your comment!

All I can do is thank the forum community for their blazing fast help! I've never experienced this before, a community that comes up with great answers and people so eager to help me.

So, thank you!

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#29

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/17/2016 10:11 PM

I suggest that you avoid a single cylinder solution because it does not allow for independent movement of the pusher and stopper. You want the upper part of the stack to be locked before you release the bottom crate and the release mechanism to be fully back in place before you drop the stack onto it. The stop should be inserted under the lowest crate because the whole stack weight will rest into it at times. The stack should be gripped by compressing the sides so that any tolerances in the crate dimensions can be accommodated. Work out for yourself why I suggest soft compressible jaws on the upper gripper. A single cylinder operating both sides using a scissor mechanism is preferable to two separate cylinders because if one side cylinder fails the stack cants and locks causing "catastrophic failure". A good design would include an interlock of the upper and lower cylinders.

Simple is not the goal here, reliable is the goal. It has to work every time with less than perfect crates because in real life that is what you get. If you are given a brief that is wrong then designing it correctly and questioning the brief will get you better grades than assuming that the guy who set the brief knows what he is doing. Finally don't think that because you have operated the mechanism that you have dispensed a crate. The stack may be empty, at 90o to the correct orientation, up side down, several crates stuck together. You need to know that the space below the dispenser is clear before you dispense another crate. Your customer needs to confirm that a crate is in place before he puts eggs into it.

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#30
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/18/2016 7:55 AM

Hi, thank you for your comment.

A single cylinder isn't the best solution in terms of fail-proofness indeed. As you state it, it seems better to use independent cylinders te be sure that the pusher doesn't retract while the stopper isn't fully out yet. I could check this with a sensor. (I guess that's what you mean with interlock?)

I don't really understand what you mean with 'scissor mechanism'.

Checking if the Egg-tray is in place will happen on the conveyer belt. Checking if there isn't a tray under the apparatus will happen with a sensor.

Other factors like correct orientation, being stuck together are parameters that aren't important this year. They want us to draw a machine that 'could work'. Next year they want us to think about how to make it really work. (Masters Thesis: 20/60 points - Bachelors Thesis: 5/60 points.) Although I'd rather make part of a machine now that's functional. Now I have to design 3 machines (Egg tray denester, conveyer belt, Egg transfer machine) knowing that none of them will ever function if they're ever built.

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#31
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/18/2016 9:54 AM

I agree with you.

Unnecessary complexity invites failure. Each added part/joint/connection is another opportunity and more cost.

The best design is a simple one that performs the needed function.

Good luck!

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#32
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/19/2016 2:58 PM

What about spring-loading on the pusher and stopper? Then add a little bit of slack in the part of the moving mechanism that times the movements. Just a thought.

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#33
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/19/2016 3:12 PM

I don't really understand what you mean with adding slack on the moving mechanism.

Spring-loaded pusher and stopper would indeed be better when loading trays in the device. The stopper and pusher would already be in the correct starting position.

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#34
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/19/2016 3:16 PM

I am not really seeing it clearly, either, just a thought occurred where one item could be engaged and the other not engaged until range of motion finished on the cylinder.

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#36
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/19/2016 3:52 PM

What about a L bracket drilled to accept a rod, springs and stop nuts on either side of the L.

So, nut/spring/L bracket/spring/nut.

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#35

Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/19/2016 3:16 PM

Just for fun:

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#37
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/19/2016 4:11 PM

Lordy, where did you find that?

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#38
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Re: Bachelor's Thesis: Stop-Push System for Egg Tray Denester

12/19/2016 4:29 PM

Oh, I had a dream one night, and this was all I could remember the next morning, so I whipped it out on CAD.

Rube Goldberg

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