How to Calculate Pressure Exerted by C in a Lever and Fulcrum Configuration?

[john101]

 

[JBA]

Since this is the piston, on the first design, how are you planning to guide it to keep the piston face horizontal in the box?

Is that what the side plates in this last design are intended to do; or, are you planning on using the side slots for controlling it?

At this point, my main calculations are focused on supporting the horizon pressing face plate so that it will be rigid enough to deal with the high compressing pressure; and, in that respect, my calculations show that the face plate should be at the very minimum 1/2" thick (this can be by adding a 6 x12 x 1/4" thick plate on the bottom pf the existing structure.

On your latest design, since you are using the 2 x 2 sq tubing if you insert a third 2x2 sq tube between your current two side tubes to make that part of the structure into an essentially reinforced box and with the added 6x12 3/8" bottom face plate.that would would a really good rigid assembly for resisting the compressing pressure without deflecting so if you can do that, I think your last above design with that third tube added would make a really strong piston. I was intending to ask you add some side gussets to reinforce the first design but the vertical plates on the last design will serve that function.

 

[john101]

The idea is that all edges are flush with the 'case', the two c points are apart only by the thickness of the case walls and that this provides sufficient guide for the piston.

Adding the strengthenings you advise is no problem.

In this drawing I've added the rubber mat and a plate above that. ?

 

[JBA]

That looks like a really good design, during the pressing process, the links will hold the piston face parallel to the press top plate in the direction of the axle centerline but either the vertical side plates or the axle slots will need to act as a guide to keep the axle centered in the direction perpendicular to the axle centerline, otherwise, any variance in material density from one side to the other can cause the axle end of the assembly to shift sideways during the pressing process. If you were pulling up on the piston this wouldn't be a problem but since you are pushing it, the axle end can want to shift one way or the other under load.

If the side plates are used as the guides then they should extend as far down as possible to give them the best mechanical advantage and minimize their side force friction on the box walls but the plates don't need to contact the walls for their full height. They can have a recess along each edge that leaves only about 1 inch at the bottom end for wall contact.

 

[john101]

Ok, I forgot to mention the slots as guides. (Which (slots) should stop 7.5" from the lid.) Well that seems to be it for the piston. ?

I'll let the piston, when built, decide the exact size of the case and in turn the width of the cross bar.

re the case. 1/4" thick walls? Can you see a reason to reinforce that? My understanding is that soil when compressed is not going to exert much sideways force unless over compressed and shears. Though it is plastic. Most of the compression is a tighter rearrangement of the components of the soil and the displacement of water. Because there will be gaps all around (cut the rubber mat smaller so it doesn't end up acting as a seal) water will readily leak out.

As I write the above I feel I'm talking myself into thicker walls. What do you think?

 

[john101]

Roughly a question to ask a Soil Engineer:

I have a cubic foot of loose soil, composition 25% clay, 10% lime, 10% water, 55% sand, well mixed.

I place it in a cubic foot box with one side open. The walls of the box are permeable to allow water and air to escape.

Over the side that is open is placed a piston that compresses the soil in the box. The base opposite the piston is fixed, rigid, impermeable.

If the piston presses down on the soil with 20Mpa.

How thick steel must the 4 walls of the box be in order to not (permanently) deform?

 

[JBA]

I am having a hard time deciding about the walls as well since I agree that the shearing displacement force of the soil should be relatively low. At the same time, that box is going to be supporting all of mechanism weight and loads plus the operator handle loads. So I am inclined to recommend a thicker wall as well; but, I don't know specifically what the thickness should be or how I could calculate it.

The box will be setting on the ground so a bit more weight shouldn't be a factor unless you intend to transport it around a lot, so I think I would be inclined to increase it to a minimum of 3/8" plate.

 

[JBA]

With regard to your last post, I don't have any background in soil mechanics and that is the science that addresses the issue of shearing in the soil that will load the walls of the box.
I have been doing a bit of searching on the subject and much of the information is very technical and hard to draw conclusions from but the below reference that seems to indicate a general average of 30° for compacted sand and clay. If that is true then for each 100 psi of vertical pressure there will be approx 57 psi of lateral pressure which would mean that the wall pressure will be 1443 psi at a 2500 psi compacting load which, as I see it, would mean you would need reinforcing bands around the region of the rectangular compacting chamber and I haven't seen anything like that on the other machines I have viewed; but, as I stated above, I am really out of my area of knowledge with all of this.

http://www.geotechdata.info/parameter/angle-of-friction.html

I guess I would like to have a couple of days to see where the information I have found takes me before making any kind of suggestion about the box construction and the plate thickness required.

 

[john101]

What about instead of a centre tube I add spacer like indicated here in grey.
spanning between the two upright plates (only one shown here) ?

(I've also added the rubber and spacer plate.)

 

[JBA]

How thick is the piston face plate?

 

[JBA]

Edit: See the below #117 post for an update on this and the above question

This morning I decided it was time to do some more internet searching to see what hints I might find about the thickness of the box walls. From what I have seen most of the machines onlne are designed for bricks with cement in the mix that do not require the type of compression loads needed for your type of brick; however, I did locate a few views that appear to indicate that at least 5/8" or thicker plate has been used for the compression chamber wall region of the press. One example is the below site and its video immediately shows a clear view of the top edge of the chamber and, by using visual size comparison, it seems to confirm what little I saw on other locations.

https://offgridworld.com/open-source-compressed-earth-block-machine

 

[JBA]

Edit:
Update on the piston. and box

I have analyzed the stress on the walls assuming the .57 transverse load factor is correct and I have found that, based upon the 12" long side, a 1" thick mild steel plate material should work; for all sides, but nothing thinner than that and be sure to use heavy welds at the corners. The top 6" of the box in the high pressure area is all that really requires that thick of a plate, so a thinner plate can be used for the lower box section, doing that in the piston travel area will require welding the top and bottom section together with a perfectly flat inside face on the joint; but, below that point there isn't any real joining issue and you may want to do that to make the box lighter and the cutting of the the piston rod slots easier.

Since you have already assembled the piston to this point, adding the third center tube is probably not an option because for that to work the three tubes have to be in full side to side contact with welds along the top and bottom joint lines to provide the reinforcement I was looking for.

I think the spacer alternative you have suggested will work; but, I may want you to add similar spacers between the ends of the two tubes as well. Once you give me the the piston frame top plate thickness then I will have a better idea of what i think is needed.

 

[john101]

That is thick. However, it sounds like it would allay any concerns. Is it ok to layer 4 1/4" plates to get that 1" ?

The two top plates are 3/8".

Ok. 3 spacers.

BTW That image and all the other piston ones are Gimp generated. I haven't welded anything yet.

 

[john101]

Just further to the above. I don't like the idea of layering 1/4" plates. Too difficult to keep clean and rust free. Any surface rust that takes off can make it bulge inwards. I'll have a look around town. See if I can get 1" plate. The thickest I have is 3/8".

 

[JBA]

With regard to stacking plates, this does not work to increase the strength to the degree that a single thicker plate does. Simply welding the plates around the edges creates an extremely high shear stress that can crack those welds. For example, if you hold together a couple of strips of anything and bend them you will see that while bent the bottom one will extend out beyond the ends of the top one. That is the shearing effect that happens with stacked plates, and if you really grip your strips tightly in an attempt to prevent that from happening you will see that it is much harder to bend them or that you cannot keep them from slipping.

On the piston, putting inserts between the tubes will strengthen the piston plate from one side to the other but will not add bending resistance along the length of the piston; and, that is what I am concerned about. Depth of a member is the main thing that makes it resistant to bending, that is why the sq tubing works well. Since I now realize your tubes are only 1 15/16" sq, that means three of them across will only be 5 13/16" wide but using three tubes and having only 3/32" of the face plate extending over the side edges is much better than the alternative.

 

[john101]

Ok, I'll go with the three tubes.

I understand what you say about stacking plates and shear of welds. Ok. So single plate it is.

 

[john101]

I've got a few feelers out to see if I can get the 1" plate.

What do you think of 1/4" plate box with a 4" wide 3/8" wraparound at the top of the box. ie the the top 4" is 5/8"?

I'm well on the way of making the piston now. I've decided to let the width of the three square tubes side by side decide the width of the piston. ie 5 13/16".

ie final area 69.75 in^2

 

[john101]

The Piston so far. Spot welded. I added a couple of spacers that helped in construction but are also useful re strength.

 

[JBA]

Your new design looks really good and the spacers are really critical in holding the two arms aligned and adding transverse rigidity to the piston structure. You just were one step ahead of me with the spacers, the only reason I opened this thread this morning was to suggest to you that, even though we had added the third tube, adding the spacer you had suggested would be a good idea.

Most designs are a progressive process and that is why your sending me your update pictures is really a help in my being able to visualize where revisions and/or additions might be needed.

With respect to my stress analysis on these parts, it is a combination of calculation and judgement, because composite structures like you are building are very hard to fully accurately analyse using basic equations. As a result, as we go along in some cases, I will suggest something that I know will add strength and rigidity to the structure but that I cannot exactly calculate amount of benefit, so if I see something of that type and it is reasonable to add it then I tend to recommend doing so.

For me, a big part of what guides me is that, even though your machine is manual and relatively small it is working with very high loads. To put that in perspective, a 50 ton cargo capacity railway boxcar weighs approximately 250,000 lbs and even at the reduced 2550 psi compressing pressure, the load on your links and axles, etc is 183,000 lbs. So in effect you are making parts that should be able to safely lift 3/4 of that boxcar, including its wheel sets.

 

[john101]

Yea, lots of force. The descriptions help me to visualise what's involved.

One thing that concerns me is c.

The thicker the walls are the further apart the piston arms and the arms pulling the piston up are. This really can be a problem I think. ?

 

[JBA]

According to a calculation I have run with a 1 1/4" dia shaft indicates that anything more than 1/4" space between the piston arm outer face and its adjacent pulling arm face is going to b e a potential problem. One solution to this would be to use 1/4" plate for the body below the pressure area and make its inside flush with the inside of the 1" plates and then make the pulling arm bottom shaft mounting blocks extend the 3/4" into the outer faces of that lower section of the box.

Do you have any alternative thoughts?

PS You have not shown me your design for the shaft connections on the bottom of the pull bars.

 

[john101]

I'm having trouble getting hold of a 1" plate unless I buy a lot, which I can't/won't. No one seems to have any offcuts.

How close to failing is the suggested 5/8" made up of 1/4" and 3/8" plates.

I can easily make the narrower sides stronger but that's not where the weakness is.

One alternative thought that is just thinking outside the box that may jog some other ideas. : A walled insert that sits on top of the piston.

I haven't finished designing the U arm except as far as we've talked about already. I won't start making it until the box is made.

I have thought about thickening the bottom part as you suggest but then it may interfere with the block ejection stroke y.

here's a sketch of that part with some naming.

 

[john101]

another idea I have is a shaft that goes right through from side to side.

 

[john101]

case01.jpg

 

[JBA]

I had already assumed that there would be one shaft would be going all the way across, using shaft stubs as shown is a bad idea because they do not give added strength and rigidity to keep the shaft straight in the bearings.

I don't see how you could add the walls onto the piston without creating a pressing problem or having to add a rather deep spacer between the top plate and the fill chamber and when you lifted the brick it would still be buried deep in that chamber.

I assume you are aware that you must add substantial blocks for the shaft connections to the bottom of your square lifting tubes;and, if the current piston arms are not long enough to accommodate say 1/2" of that block above the shaft and allow fully lifting the brick clear of the top of the pressing chamber then that can be compensated by adding a filler on top of the piston plate.

I understand your problem with getting the 1" material so there are a couple of possible solutions for laminating plates to the get the effective 1" sides. Is there any chance you can get some 1/2" plate; because the thicker the laminate plates and the fewer of them the better.

Since we are assuming you are using A36 mild steel plate, even at 1" thick the stress level is right at the specified min yield point of the material, so by even allowing that plate thickness I am counting on the material being actually stronger than that (which is generally the case) and the possibility that the transverse pressure of the bricks will be less than that indicated by what references I could locate.

It is a shame that we do not have any specifications for your material because 1018 plate is also classified as a mild steel but has a 50% greater strength than A36; but we have to use the lowest expected strength as a basis without any knowledge of thee material alloy. (Note: Even with 1018, 7/8" of thickness is required).

Although we have little information on the strength of the sq tubing, that element on the pull bars worries me the least because if overloaded those bars will just stretch and with a 15% elongation to break their limits will be easily identified before any failure and they are the easiest to reinforce by adding side strips.

On the other hand, I see the body as a more critical element because, while I am not concerned about failure, if the sides of that box bow outward during pressing then there is little that can be done to correct that problem once the box is built, other than to simply reduce your target pressing pressure even more to the the point that the existing box sides will handle the load.

 

[john101]

Ok. 1" it is.

I have a lot of 3/8" and 1/4". I'll keep looking for 1" plate though.

I understand what you say re add blocks. I think that can be done easily re 1/2 inch above shaft.

I have found a shaft long enough to go all the way.

I think it's a good idea to assume weak material.

Ok, apart from getting a hold of some 1" plate, I'm happy.

 

[john101]

Does the added on part introduce some torsional force on the lift arm that should be considered.

Is the (light coloured) part above the shaft necessary as the arm is lifting only.

 

[JBA]

The top does not contribute to the support of the shaft and the support could be a "U' shaped part but the added height and top weld will increase the strength of the parts attachment to the bottom of the arm.

The added width will not increase the torque on the arm if lubricated because friction drag is a function only of force not area (although that may be hard to accept since it is counter intuitive). On the other hand, the added contact area will reduce the pressure on the lubricant.

The greatest problem for the shafts is any bending they experience because that will create high contact loads at edges to the shaft holes.

 

[JBA]

In both cases with and without the spacer we are making a compromise to address the fact that he shaft size is not sufficient to provide its own rigidity. In any normal machine design and fabrication that shaft would be sized to have sufficient strength and rigidity to stay square to the arm.

What is at risk even if the shaft is strong enough not to permanently bend any deflection under load could cause the pulling arms to want to slide outward; as a result, I recommend that you place retaining washers and pins or a retaining collars on the ends of the shaft to prevent that from happening.

 

[john101]

I meant torsion on the long arm in the plane of the c/l of the axel. However, that's probably minimal. Just a thought.

I just spotted your last post which seems to address this. Yes, washers and pins.

Anyway, things are coming together now. I've got a few more places to look for 1" plate. We'll see.

 

[JBA]

Actually you make a good point, I knew that adding the thickness on the insides of the pulling arms I was moving the balance point off of their centerlines' and I now realize that I should have included adding an identical thickness plate on the outside of the arm to move that point back to the center. But the washers should still be used to insure the arms are held in place.

But I agree with you that step by step we are moving forward and I believe all of the major issues seem to have been addressed at this point.

 

[john101]

ok, yes, I can do that.

I lined up parts to get an idea of how things are. Some parts are obviously not there and others not made yet but it gives the idea.

 

[JBA]

From your picture, it appears that once the side plates are added then the supporting stand and pivot for your top plate are about al that remains for the main structure.

Truthfully, in looking at your pictures with nothing else to compare to for size, the machine appears much larger than it really is when I realize the pressing cavity is only 6" by 12". (about the same size as the keyboard section on my laptop computer)

 

[john101]

yeah, I should have put a matchbox or something regular in the pic for visual que. Still, it's got a massive presence in real life. I dub it 'ye ole' nut cracker'.

 

[john101]

What about a 1/4" 3x3" angle iron welded to the top of the lid so that when the lid is closed the sides envelope the side walls of the case?