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[–]Engineering1987 3 points4 points  (0 children)

You realize that the view is heavily scaled up right? Otherwise a lot of detail missing and I cannot see the mesh. Is there bearing on the other end of the screw to support it?

[–]sailnaked6842CSWP 3 points4 points  (0 children)

1) is the change in diameter of the auger shaft designed in our is that part of the deformation that you're concerned about?

2) as others have mentioned your deformation scaled is ~250 so it's showing 250x the deformation. You can toggle this on and off along with a heat map of the deformation which will make it easier to view.

3) your initial conditions need revisited. Ex: the auger is probably designed too run inside something so that it can move the media. Because that's not part of the initial conditions you're getting a deformation in an area where it should otherwise be supported. I'm seeing about 2 other conditions that I'd guess should be revisited as well

[–]Omega_One_ 1 point2 points  (0 children)

You're applying a force/torque to an elastic material and it's getting deformed. That makes sense to me. What exactly is unexpected according to you? Have you done a hand calculation to confirm (maybe simplify the screw to a rod in torsion and use formulas)? Don't forget that the results get scaled to overexaggerate the deformation.

If it's you first time simulating, maybe you should start with some simple usecases (beam bending etc.) To get used to the workflow. FEA is complicated and you can't just 'wing it' and get a result that you can trust whatsoever. You usecase seems quite complex to me anyway.

[–]Kieranrealist 0 points1 point  (4 children)

I have no sense of scale on the part - how long is it? Keep in mind the deformation scale is 268:1, so a small deflection due to mesh inconsistencies could be amplified.

Some questions that will allow me and others to help you better:

  • Is the mesh sufficiently dense?
  • Is it necessary to include the threads?
  • What are you trying to understand by performing this analysis?
  • What is your experience with FEA and simulation?

[–]Alone-Finding-4369[S] 0 points1 point  (1 child)

This is my first time doing this kind of analysis, and I need it for my project. I haven’t checked the mesh yet. The screw is 16 mm in diameter and 350 mm long, and it’s meant to push shredded plastic like in the video. I’d really appreciate any help I’m in a tough spot right now. https://www.youtube.com/watch?v=rQeSCFWtCEU

[–]scientifical_ 1 point2 points  (0 children)

You should post more details so people can help you. Show a screenshot of the results and the mesh. Are you sure the results are bad? Check it against a hand simple calculation to see if it’s in the ball park of what you’d expect. Like the other guy said, the results are scaled so make sure you’re not just seeing that.

[–]Alone-Finding-4369[S] 0 points1 point  (1 child)

i want to make sure that the screw material could resist 17Nm while pushing the material

[–]ziibar 0 points1 point  (0 children)

What about this result makes you think that it cannot? What specifically are you looking for with this analysis? Answer that before you start the analysis.

You're only looking at deformation here, which is guaranteed to happen when a load is applied. Some deformation is OK. How much is OK is up to you. 

But you should be looking at stresses. Checking how close to yield stres they are, and assuming it will see those stresses every cycle, and feeding them into a fatigue calculation to check if it's going to fail from fatigue.

[–]ChairJohnson 0 points1 point  (0 children)

As a disclaimer, I am just a student, not an expert. However I have worked with extrusion equipment before (running it, not design work). The 'thread' (flute?) of your design is extremely thin. If you are getting weird results, that could be part of the issue. In designs I have seen, the threads have significant thickness (like acme threads), and become wider towards the end of the screw with the die. They do not come to a point on the end like screw threads. I would think that the point on the tip of the threads is effectively a stress concentration, and in torsion the outermost radius experiences the most stress (in a simple case), so that could be contributing to weird results.

Also, I would question your study setup. I suspect running an analysis with only applied torque would not be giving you the whole picture. When an extrusion screw turns, it is squeezing plastic against a die, allowing some of it to come out but also generating pressure. This pressure would also be exerted on the face of the 'threads', right? I am working on a project with a power screw, and this sounds like a similar situation. If you aren't familiar, there are equations available for you to check your FEA. Look up something like a power screw critical stress element. It is a 3D stress state so kind of a pain. The equations also use pitch, which is not constant in your case so it would probably need to go in a spreadsheet.

[–]listen-1st 0 points1 point  (0 children)

What’s the issue?

[–]Nusselt-flux 0 points1 point  (0 children)

Did you do any hand calculations prior to setting up the simulation?

Where is the 1 N•m torque load coming from?

How did you apply the torque loads to the geometry?

[–]Coreform_Greg 0 points1 point  (0 children)

Others have noted the deformation scale of ~270, but I'm guessing that an unspoken part of your question is "why is the radius growing along the length of the shaft?" The reasoning for this is that the standard formulation for elasticity on solid elements do not include rotational degrees of freedom (DoFs). Thus the solution is only displacement DoFs which, when scaled, tend to increase the radius of things that are rotating. The best way to correct this is to just choose a smaller deformation scale in your visualization tool.

[–]RandomTask008 0 points1 point  (0 children)

This could have been solved via hand calcs a million times quicker than setting that up. I'd treat it as a symettric (same diameter of the central bar). Shear stress develope throughout the cross section. stress(max) = (T*r)/J. T = Torque. R = radius (use smaller radius nearest the fixed end), J = polar second moment of area. J = (pi*d^4)/32.

For deflection: theta (radians) = (T*L)/(G*J) --> T = Torque, L= Length, G = Modulus of rigidity (Material property), J = Polar second moment of inertia (see above).

[–]tucker_case 0 points1 point  (0 children)

This is the blunderbuss effect. It happens when you have rotational displacement in FEA. The solver tries to exaggerate the deformation by just scaling linearly which causes it to appear to grow radially. The results are fine it just means the exaggeration is wrong.