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

[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?

 

[john101]

 

[JBA]

Using the 1/4" angle on lid to supply some reinforcement at the top of the side plate will not be effective because it will not provide any longitudinal reinforcement support to the lower region of the side plate or along its bottom edge.

Because of the inability to add any longitudinal horizontal stiffening bars across the outside face of the side, because they would interfere with the pulling links movements, the only alternative is to use a sufficient side plate thickness to provide the necessary bending resistance for the side plates and my selected 1" thickness is actually pushing the limits of an A36 plate material and counting on the fact that the what appears to be 1" plate use on the above mentioned observed machine would indicate that the transverse pressure load factor could be closer to .30 rather than the .55 value I found in my study of other forms of soil compaction.

At the same time, since my last post, I have done a bit more investigation of laminated plate structures and their shearing stresses and found an analysis that indicates that, if getting the 1' plate is impossible but you can get some 1/2" plate, then a time consuming alternative to the 1" plate would be to use one full size 6" x 12" piece of the 1/2" plate and then, cut three 12" long strips of that 1/2" plate material at a width that will allow full depth welds between the strips for their full length to both the base plate and to each other. In this manner those longitudinal welds plus the outer edge perimeter welds will provide the shearing strength needed between the base plate and strips and the strips plus the full depth welds will provide an laminated plate equivalent to the solid 1" thick plate. The main risk with this process is the possible (probable?) warping of the plate assembly due to all of the welding.

The 1/2" plate alone will be sufficient for use for the 6" long end plates.

Even with the use of the 1" plate, the one place where adding thee 1/4" angles will be of definite benefit will be as reinforcement on the vertical joints between the side and end plates.

 

[john101]

All right. Thank you for that detailed explanation. I can see there is no way around it. I'll widen my search. That is ok. It'd be no good to be in full swing making bricks and have a failure because I was in a hurry now.

 

[john101]

I got two pieces 1" plate 6x13 in the city this morning. $80 cut. (In the off time I've been building a hand cranked concrete mixer to mix the soil with.)

 

[JBA]

Congratulations on your find.

 

[john101]

It's a good fit. I feel confident about that part now.

I plan to add a rib to the 3/8" end pieces (in blue).

The lift arms are not long enough. I'll upgrade those to the thicker rectangular tubes. They are 2" x 3" od , 3/16" walls. ? with the cross piece on edge.

 

[JBA]

I like the web idea, probably locating that web 4" from the top would be best because that will form a web box around the region where all of high loading will occur.

Obviously, make sure to get a full fillet weld on the ends of the 1" plates.

 

[john101]

It didn't occur to me that only the top plate and the slot the c axle runs in and the case walls are needed to guide the piston in the case. I'm rebuilding the piston as a result. I'm using the same materials but off cutting bevel sections in the upright plates and the square tubes.

Can you confirm that the rectangular tubes for the lifter arms (dimensions given above) are good as is.

I'll fit the cross piece into the arms, then bolt and weld them.

 

[JBA]

I need to see a clearer picture of what you are doing to the piston to make sure I understand all of the changes you describe; and I also don't quite understand what you are showing me in the above illustration, is that the crossbar at the top end of the arms?

Meanwhile I will check the tubes to see if they need any of the same longitudinal bar reinforcement that I specified for the prior arm tubes. I thought the prior sq tubes had 5/16 wall thickness.

 

[JBA]

Your new 2 x 3 x 3/16" tubing is much weaker in tension than the original 1 15/16 Sq x 5/16". The total wall area for the new 2x3 is only 85% of that of the 1 15/16 sq with the 1/8" thicker wall.