Torque on a square screw full with water

Gh778

Hi,

I would like to calculate the torque on a color square screw full with water like the drawing show. All grey screws don't move or turn, only color screws can turn (and so move up or down). Color screws are square (see top view for that). There is water between color screw and grey scew (gaskets are not visible ! ;) ), so like that the only faces can put forces are external and internal faces.

The problem for me, I see at each corner a small torque (because the altitude is not the same) but I don't find where this torque is cancel by another part of the screw. Can you help me ?

Maybe you need some others views, don't hesitate to tell.


Thanks :)

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Gh778

This is like I see forces but maybe the weight is not full at bottom ? I'm drawing only a corner like that it's better for see where is another torque.

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Gh778

I add some views of the thread.

I have cut 3D circular thread (first drawing) with 2 rectangular cuboids for obtain square thread (second and third drawings). Like that I'm sure the square thread can turn in the circular thread.

If you look at the square thread you can see a torque when the thread is in water due to external/internal surfaces. This torque is cancel by another torque with up/down surface. But it's easy to cancel this last torque if you put this square thread in a circular thread. So, another torque must cancel all torque but I don't find where. Maybe you can ?

Thanks

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Hassan2

I' not sure I have understood the problem, but if you consider that the forces due to hydraulic pressure is normal to the surface ( which I'm sure you do), the problem is solved.

I think you can simplify the problem to a simple slab in the water. Wouldn't it be the same problem?

Gh778

Hi Hassan2 ;)

Yes, forces are normal to the surface, can you explain more I don't understand how ext and int surfaces give no torque ?

Up and down surfaces can be canceled so we rest with ext and int surfaces. Really, look at the drawings, you'll see a torque due to the corner. I don't think this 3D shape can be resume with a 2D shape.

thanks

Hassan2

The hydraulic force on any differential( infinitesimal)surface in any direction is equal to the force on the projection of the differential surface on the plane perpendicular to that direction.

Any differential surface at any point on the screw has a projection on the plane of the axis and r ( r being the vector from the axis to the point)being the distance from the axis). There is another differential surface on the other side of the plane the same projection on the plane, thus exerting equal forces in opposite directions.

This is true for any arbitrary shape.

Hassan2

About the torque on the screw in the figure, The force on the upper and lower sides are not equal. the torque the net is canceled by the net corner force.

Gh778

I'm ok with #6 message

#7: Yes a torque up/down cancel another ext/int, ok. But like it's a thread we can put it between 2 circular threads (show first drawing) and cancel up and down surfaces (for example with thin thickness of air with gaskets). We have only the external/internal torque. I don't know if you understand me ? Don't forget I cancel the up and down surface pressure easily and without energy.

Hassan2

In one of your figures, I see the words " up" and "down". My up and down surfaces are both sides of the thread separated by the thickness. The are not equal because they are like two parallel surfaces at different depth.

Gh778

The problem is, there are 2 thickness: up thickness and proof thickness. See the drawing below. Imagine air gap for up and down surfaces like that we sure there is no torque from these surfaces and we sure this cost nothing in energy if we turn in circular threads. But external and internal have a torque, for me, sure ;)

Drawing: look at "Z" axe

It's not a problem to cancel the forces on these surfaces if we put small air gap (with gaskets) and put in a "sandwich" in 2 fixed circular threads. This cost nothing (in theory) about energy.

I add a drawing with a square thread in a "sandwich" of circular threads. Air gap at up and down surfaces.

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Hassan2

Thanks for the explanations and the figure. I think I understands the problem now. In this simplified problem, no water touches the up and down and also internal surfaces. The problem becomes interesting and challenging now. Think of the proofs ( gaskets, fillers,..); They experience the same but opposite forces. If the gaskets are free, they transfer the force to the internal surfaces of the screw and cancel the forces on the screw. If there are rind connected like "another" screw, the forces cause a torque which tends to rotate the opposite direction the screw tends to rotate. Again no net torque.


Added: Sorry in case of a rigid proof, my description may be incorrect.

Gh778

Thanks to you for your help ;) it's friendly

It's more difficult to think with the internal face without water because gaskets will be great with important surface. Or maybe I don't understand your explanations. Tell me ?

For my experience (no water at down and up surfaces but water at internal and external surfaces), the air gap could be like 1µm so the surface of the gasket is very small and this can't cancel the torque I see on internal/external surfaces. The weight can't be changed because internal/external surfaces are verticals.

Hassan2

Yes, but with water touching the internal faces too, again there is a net torque on the screw as you said before. But again the gaskets cancel the torque. If the gaskets thickness is much less than the screw thread, much of the forces on the thread is canceled by the forces on other side of the thread the remaining is canceled by the gaskets. If the thickness ratio is not small, again the number of gaskets must be large.

My problem now is with the case of the rigid gaskets fixed to the water container ( pipe). I have no explanation for this yet.

by the way, what software do you use for draw such figures?

Hassan2

You said that the gap is filled with air? it must be pressurized air, the same pressure as the water pressure. If so, it tells you where the cancelling force come from!

Gh778

An air gap with air at pressure 1 bar for example, this air will apply the same force all along up and down surfaces, it's easy to do, and this cancel the pressure of water like that we can forget these surfaces (they have the same surface) and think with only external and internal surfaces.


I use Maxwell (electromagnetic simulator) but I think you can do with Blender (free) but I don't know how.

it's evidence that the external surface apply much torque than internal surface but need some calculations for be net.

When you think about that you imagine a square thread in a sandwich of 2 circular threads ? I don't understand.

I don't understand what case you think. Don't forget the square thread will be turn, and this said it will move up (or down) and if water is not everywhere around, the water can (it depend of the torque) move down and we can lost energy like that.

Could you send a drawing even with pen ? I don't understand.

Hassan2

Read post #14 please. There must be a pressure on the up and down forces otherwise water fills the narrow gaps. This pressure doesn't answer the question? As if the screw was in the water without any gasket.

Gh778

Not like you think, the gasket is only there for prevent water to fill the air gap, you can think with P=0 bar in the air gap. Sure water will put forces on surface of the gasket [because we have X bars in one side due to the water, and this change with altitude, and 0 bar in other side (air gap)] but the surface of the gasket can be so low that we can considered the torque = 0 on the gasket. In this case, I put thr square thread in a sandwich of 2 circular threads, it's very important, the gaskets don't have up and down surfaces, they have only external and internal surfaces but so small...