jointed trees subject to bending

[Gioan]

Hey, guys.
I wanted to take advantage of it to ask for advice,
I have to joint two shafts diameter 60 full from 2000mm each with a threaded bar m30 l=50mm (see sketch attached).
the tree will be subject exclusively to bending ( negligible cutting).
I performed the calculation of the max sigma considering as a resistance module the threaded bar core section and the maximum mf (f/2x2000).
now I have two dubbi:
1) leaving considerations between ductile and fragile materials (in my case, therefore, in fact, the problem would not arise) according to you should be considered a factor of concentration of tensions (usually considered for sectional variations within the same piece, but in this case that are distinct pieces should however be considered? )
2) the length of the mothervite has influence on the seal of the threads and this is obvious, but according to you it is correct to use the axial force that prompts each of the two screws equal to the max sigma (first calculated) multiplied by the hazelnut section of the threaded bar? (in my opinion it should be a precautionary calculation since I do not consider the trend to "failure" due to the bending).
Thank you for your opinions.

 

[Fulvio Romano]

I think the calculation is incorrect.
When bending work, we imagine compressing the part below, the bottom end of the shaft makes you as lightning, then the upper fiber of the screw works with an arm that is the shaft radius plus the screw radius.
the screw therefore is all in traction, it does not go to butterfly. (No, it depends on the diameters, but you know what I mean)

Besides... do you work bending a threaded bar? I have arms hairs all right!

the length of the mother-in-law does not have much influence on the "tenuta" of the fillets. if we speak of steel, they hold almost only the fillets up to a depth equal to the middle of the diameter. at a depth greater than a diameter the fillets rest quietly. the problem here is another... live the life in the first mother-in-law without getting to the end of the race, then screw the second mother-in-law on the screw. this will continue to screw up in the first motherhood until you arrive at the end of the race. when you tighten, who guarantees the position of the screw inside?

One more thing. When you're done screwing up, you string? How much? did you add this preload as a traction in the vine?

to answer questions:
1) a k for what? the problem is the thread, there you have a nice k
2) I really don't understand what you're asking or what you're proposing. .

 

[Gioan]

thanks fulvio for your considerations,
I am thinking of substituting the mothervite with a pin diameter 35 l=50 with at the ends two threaded parts m30 l=30 (in the trees of course I will create a hole 35 l=25 that continues with mothervite m30, with the coupling with the due tolerances), in this way the coupling should improve.
for the fact that you say that the calculation is not correct, I am not of your opinion (of course I can wrong), it is as if I had a variable section tree (length tending to zero) but the neutral axis always remains that at the center of the section.
the k I meant was that,possibly to consider, in the passage between the aber diameter 60 and the mothervite (however it is correct to consider it since they are two distinct bodies)
point 2) was related to the calculation of resistance of the threads of the screws in the trees considering the threaded bar with an axial force calculated in the way I said.

 

[cacciatorino]

Now I'm in a bit of a hurry, but I think he's right, Roman lightning.

things would improve considerably with a substantial precarious of the life.

 

[Fulvio Romano]

I am thinking of substituting the mothervite with a pin

I don't understand how you replace a hole with a pin, but if you talk about the screw then yes. in practice do work to bending a smooth piece. That would be great.

regarding the neutral axis. I can assure you he can't stay in the middle of the section. It's a bit like reinforced concrete beams. cement works only by compression, iron only by traction, and is calculated where the neutral axis passes. you have that all the traction part of the section of the tree does not exist, because the two pieces cannot exchange traction sigma being physically detached.

 

[Gianp80]

Besides... do you work bending a threaded bar? I have arms hairs all right!

fully agree with you fulvio

 

[gerod]

@fulvio: hole? You're vulgar! it says hole!:biggrin:

 

[Gioan]

I meant non-mothered lives (the rush!!!).
As for your example of reinforced concrete beams, I had not really thought about it, and it is a very significant example.
in this case then could I dimension my inner pin (joining) to simple traction (holding also account of the limited length) given by a force that echoes from the balance of moments compared to the fulcrum? that is:
ra reaction bind to the extreme of the tree
f pin traction force (incognita)
l/2 length of the tree in the middle (where I have the joint)
r+r shaft radius more beam per pin

f x (r+r)=ra x l/2

Is that correct?

 

[cacciatorino]

I propose you a variant, which has the advantage of not working the threaded part in bending, and with in addition a fitting in the transition zone, to decrease the carving effect.

Bye.

 

[Fulvio Romano]

@fulvio: hole? You're vulgar! it says hole!:biggrin:

no, gerod, try not to use the term "hole", because lately it reminds me of lawyers! ! !

 

[Fulvio Romano]

I propose you a variant, which has the advantage of not working the threaded part in bending, and with in addition a fitting in the transition zone, to decrease the carving effect.

Bye.

but so the resistant section becomes that of the throat?

 

[cacciatorino]

but so the resistant section becomes that of the throat?

Yes, but always better than that of the thread, the rest if the bottom diameter of the throat is equal or little less than that of the hazel does not change much, but avoid the concentration of tensions at the bottom of the trinagular profile of the thread.

p.s. I attach you a photo of a biella vine of the duchy 851.

 

[Gioan]

thanks hunter for the advice; However thinking about it I prefer the solution with internal pin d35 (with relative tolerance) extreme threaded m30 (see fast sketch attached) as it allows me to keep aligned the two circles as best possible (they are press cylinders).
about the calculation I had reported earlier:
in this case then could I dimension my inner pin (joining) to simple traction (holding also account of the limited length) given by a force that echoes from the balance of moments compared to the fulcrum? that is:
ra reaction bind to the extreme of the tree
f pin traction force (incognita)
l/2 length of the tree in the middle (where I have the joint)
r+r shaft radius more beam per pin

f x (r+r)=ra x l/2

Do you think that's right?

 

[cacciatorino]

Actually, you should consider the preload of life. If you do not apply any preload, you will definitely go to plasticize the material of the two rounds at the point of contact. better explain: the preload of the screw should be such that even under the maximum effort, the two faces in contact of the two columns exchange compression efforts across the area. If instead they start separating (precarious vine < of the traction effort) you will have the contact going to locate in a small contact area and this will most likely cause you plasticization at that point.

then other thing: dealing with printing machine, do you also have to limit deformation? In this case a calculation should also be made to estimate the latter, not only the resistance.

 

[Gioan]

I think that in the face of all the considerations that have arisen, analytical analysis, which take into account, becomes (at least for me) very complicated.
I do not know exactly how to consider that discontinuity.
I think it's a good thing to do a fem analysis.
I would have liked a paper procedure.
thanks for the suggestions you gave me.

 

[Fulvio Romano]

I think that in the face of all the considerations that have arisen, analytical analysis, which take into account, becomes (at least for me) very complicated.
I do not know exactly how to consider that discontinuity.
I think it's a good thing to do a fem analysis.
I would have liked a paper procedure.
thanks for the suggestions you gave me.

no come, just understand well what happens, the fem not only is not necessary for so simple geometries, but above all it is very deleterious because so complex contact patterns, are very simple to consider on paper, but very complex to make the fem understand.
Everything that says hunter is still very accurate.

decide a geometry. Both proposals are good, provided the durable section is the same, there are no steps that focus efforts and there are no stressful threads in bending.
Once geometry and loads are established, use the lower fiber of the column as the core and replace the pin with a force f. calculates f so that there is no detachment of the columns, and multiplies it by a security factor, for example 1.25.
made this, calculates the distribution (by trapeze) of tensions in the pin, whose integral to the section is f.
dimensional the pin so that under f it has a safety factor, that I know, 1.5 - 2 (this factor is different from the one applied previously, so they must be considered both).
point. I don't think it's very complicated, it's just a matter of getting into the de saint venant, without inconvenience or strutturists, or fem.

 

[luigi.paiano]

In my opinion, the so designed mounting is absolutely dangerous... but I press a bending bolt. I agree with the hunter.
feel you can't connect in another way, making the screws in the same direction as the cut?