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Lever and Lever Law

Posted Jul 6, 2022, 9:29 a.m. EDT Modeling Tools & Definitions, Structural Mechanics Version 6.0 2 Replies

Jack Hooper
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I am a beginner with Comsol and was trying to simulate a slightly more complicated lever. I have now simplified the problem a bit and in short, it is simply a block with a hole in the middle where a force acts on one side (boundary load) and on the other side the force is measured (surface integral over solid.sl33) when it is fixed (fixed constraint), see attached picture. However, I have the problem that I am not sure which boundary condition I have to specify for the hole. I have now chosen a roller, but I notice that the law of leverage is not fulfilled, so the boundary condition is probably wrong. Hence my question about which boundary condition is best to choose for such a rotation. Thanks for your suggetions and response

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2 Replies Last Post Aug 10, 2022, 11:55 a.m. EDT
Henrik Sönnerlind COMSOL Employee

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Posted: 2 years ago Jul 18, 2022, 8:41 a.m. EDT

Due to the discretization, a standard Roller constraint on a curved surface may cause overconstraints (since not all surface normals are exactly perpendicular to the surface).

You can use the settings in the Roller Constraint section to overcome this problem.

Alternatively, you can use a Rigid Connector for setting up the constraint, but then there will also be a local constraining effect in the 'depth' direction. The advantage with this approach is that you can fix the hole in one direction only, while with a roller, it will be fixed also along the beam.

Either way, you should measure the reaction forces, not the stresses, since the former are exact while stresses have discretization errors. In particular, measuring solid.sl33 seems wrong, whichever coordinate system you are using. The transverse force should correspond to a shear stress.

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Henrik Sönnerlind
COMSOL
Due to the discretization, a standard *Roller* constraint on a curved surface may cause overconstraints (since not all surface normals are exactly perpendicular to the surface). You can use the settings in the *Roller Constraint* section to overcome this problem. Alternatively, you can use a *Rigid Connector* for setting up the constraint, but then there will also be a local constraining effect in the 'depth' direction. The advantage with this approach is that you can fix the hole in one direction only, while with a roller, it will be fixed also along the beam. Either way, you should measure the reaction forces, not the stresses, since the former are exact while stresses have discretization errors. In particular, measuring solid.sl33 seems wrong, whichever coordinate system you are using. The transverse force should correspond to a shear stress.
Jack Hooper
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Posted: 2 years ago Aug 10, 2022, 11:55 a.m. EDT

Thank you for the answer! In my example, there is only a small difference between the reaction force and solid.sl33 (I guess due to the discretization). For the roller, I have set the setting to Cylinder (with appropriate axis and point), but I still get a wrong result here. Is there anything else I need to consider?

Thank you for the answer! In my example, there is only a small difference between the reaction force and solid.sl33 (I guess due to the discretization). For the roller, I have set the setting to Cylinder (with appropriate axis and point), but I still get a wrong result here. Is there anything else I need to consider?

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