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QUALITATIVE Torque calculation
3

QUALITATIVE Torque calculation

QUALITATIVE Torque calculation

(OP)
Hi everyone. I've a question related to the QUALITATIVE computation of the required torque to make a rotation.

The image shows a cross section. Two rigid bodies (hollow cylinder and elliptic base cylinder) and a deformable element are shown.
Supposing to know all the mechanical properties required, how do I compute (qualitatively) the torque required to make a 90deg rotation of the inner elliptic body in order to deforme the deformable element?

The deformable body and the outer cylinder are linked along their contact surface, and cannot slide

RE: QUALITATIVE Torque calculation

Do you know what the word "qualitatively" means? What the heck would a qualitative answer be? "Half a whole bunch-ish"? "More than I want"?

RE: QUALITATIVE Torque calculation

As alluded, I believe you want to quantify the torque necessary which involves knowing the material properties of each part so you can take into account a combination of deflection and sliding friction. Depending on the accuracy necessary you're also going to want to take into account the manufacturing tolerance of each part.

RE: QUALITATIVE Torque calculation

this would be a really nasty problem to solve. For example, will the insert deform (as obviously it should) or will the insert slip and rotate ?

another day in paradise, or is paradise one day closer ?

RE: QUALITATIVE Torque calculation

And will it deform smoothly and elastically, or will it deform plastically, or will it just rupture into little pieces?

RE: QUALITATIVE Torque calculation

That is very similar to how many rubber bushings in a car work, except you have a free floating inner rather than it being bonded to the rubber. Even in that simpler case it is very hard to calculate this (or model it) to better than about a factor 1.2 or so.

In your case you are also going to have friction.

Cheers

Greg Locock


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RE: QUALITATIVE Torque calculation

From Google:

"What is difference between qualitative and quantitative?

What is the difference between quantitative and qualitative research? In a nutshell, quantitative research generates numerical data or information that can be converted into numbers. Qualitative Research on the other hand generates non-numerical data."

So, you want non-numerical data? How does that work?

RE: QUALITATIVE Torque calculation

Similar to what Greg said, this could be a rubber bushing for an inner control arm joint with some progressive action? Can you make it solid and vary the density of the elastomer in the shape you have shown to eliminate the friction part?

RE: QUALITATIVE Torque calculation

I'd set up the free body diagram for different angles on the cam. I'd then break down the forces between the cam and the rubber into components into the rubber and tangent to the rubber. That's the easy part. Knowing how the rubber reacts those components would probably require some high level elastomeric experience. Maybe the people in the "Rubber Engineering" forum could help at that point.

RE: QUALITATIVE Torque calculation

(OP)
Sorry. I wasn't so clear.
For "qualitative" I meant that I don't need a precise number but a good procedure to understand more or less the amount of required torque. The deformable part is an elastic element (rubber-like) and it deforms just elastically. I'd like to know (in the case I have to choose a servo motor) the required torque under the assumptions:
-elastic deformation of the deformable element;
-no deformation of the rigid bodies;
-no sliding of the deformable element.
-friction between the elliptic inner cylinder and the deformable element.

I hope I've been clearer

RE: QUALITATIVE Torque calculation

The geometry that you have created and the materials that you are suggesting (I wouldn't call it "chosen", yet) are going to behave highly non-linearly. Build a model and test it.

RE: QUALITATIVE Torque calculation

Because of the complexity of this model (unusual shape, elastic deformation of some parts and rigifity of another ones, friction and so on) I think that the best way to solve it is to use Finite Element Analysis. It may also be the only way apart from physical experiment because I doubt you can find any useful formula for that. All this assumptions you've mentioned can be included in FEA (rigid bodies, frictional contact between inner rigid part and deformable part, no sliding contact between outer rigid part and deformable part) but you can also eliminate some and solve even more complex problem. For example there's no need to make these parts rigid. Required torque can be calculated using some parametric study - apply different torque in each analysis and solve it to find out the displacement (rotation). If it doesn't match required value, repeat the analysis with different torque. The only problem can be the time required to perform all these analyses. But with some simplifications it may not take too much time to solve. Also keep in mind that rubber-like material probably should be modeled using hyperelastic formulation but we can assume it's linear elastic.

RE: QUALITATIVE Torque calculation

I'm with Brian. I think this is way too complicated to get real numbers from FEA and a physical test is what you need. Once you've done a bunch of tests you may have learnt enough to create a general (algebraic) solution or to make a model.

another day in paradise, or is paradise one day closer ?

RE: QUALITATIVE Torque calculation

Well, it’s always experiment > FEA > hand calculations. Of course apart from situations when exact analytical solution exists (like for beams).

But I would try with FEA anyway. Good nonlinear solver (like Abaqus) should provide results that will be close enough to reality.

RE: QUALITATIVE Torque calculation

I never said the model had to be a physical one smile A finite-elements model is another way.

RE: QUALITATIVE Torque calculation

No, you didn't say he had too, but you did "recommend" it. personally, I can't even imagine the properties required to run this in FEA …

so I strongly for testing, Brian's either way (?), and FEA_way is "all the way with FEA" …

many ways to skin cats ...

another day in paradise, or is paradise one day closer ?

RE: QUALITATIVE Torque calculation

(OP)
In case I want to perform a FEA before building a model, which software would you advise to be used? (I'm looking for some simple software, because in a first approximation I don't need accurate results).

RE: QUALITATIVE Torque calculation

FEA and "simple software" do not exist in the same space and time.

RE: QUALITATIVE Torque calculation

(OP)
Brian Petersen. Based on your experience. What do you advise?

RE: QUALITATIVE Torque calculation

Build it (out of steel and rubber or whatever you choose for that odd-shaped internal bushing). Test it.

RE: QUALITATIVE Torque calculation

The various solvers built into the 3d modelers today are fairly simple to use and surprisingly accurate within the elastic range of materials, I'd start there. A lot of folks new to simulation get caught up in solving for a single solution then gripe about a lack of model accuracy vs reality. What you should always start with in simulations is to run sweeps and document the impact of variables on the final result so you understand what factors are significant vs not. As a mentor of mine once told me, its not about the number but rather the trend. Once you understand the trend, getting to the correct values is simple.

RE: QUALITATIVE Torque calculation

(OP)
Which STEP should I use to run it on ABAQUS? Dynamic,Explicit?

RE: QUALITATIVE Torque calculation

(OP)
I want to impose a motion to the internal bushing (angular motion 0° to 90°). In a given angle it touches and then deforms the "deformable element" until it reaches the desired 90° rotation. I would like to get the required torque to perform that motion. What kind of analisys should I perform on Abaqus?

RE: QUALITATIVE Torque calculation

and so we come full circle (you've restated the original post).

We've (generally) advised using reale world testing but you persist with trying to model this.

RE: QUALITATIVE Torque calculation

You can use Abaqus/Standard (Static General or Dynamic Implicit analysis) or Abaqus/Explicit. All should give your results. However in this case I recommend Explicit since high deformations are expected and general contact algorithm will be very useful. Explicit won't have convergence issues but keep in mind that it may take a lot of time to solve. Thus try artificially reducing time step.

RE: QUALITATIVE Torque calculation

How did we get by before FEA? Put the CAM in different positions and break down the load on the rubber into axial and tangential (shearing) components. I'd think you could find some resources on the web that would give you typical stiffnesses for rubber in compression and rubber in shear. Granted your constraints aren't so simple but that is where judgment comes into play.

You want to have some idea of the loads so you can validate your FEA results. Having ball park numbers would certainly be helpful before physically building anything.

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