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Suitable Constraints for Crankshaft

Suitable Constraints for Crankshaft

Suitable Constraints for Crankshaft

(OP)
I am trying to do some simple loading on a crankshaft. It is a 2-main bearing, 2 cylinder crankshaft.

I have applied a bearing load to simulate gas force on the crank pin.

The problem I am trying to solve (over and over) is what suitable constraints to put on the two main (plain) bearing journals. When simply selecting the surfaces and allowing some translation, this is no good.

I have tried creating rigid connections between a point in the centre of the journals and the bearing surfaces and allow one end to be free (except for crank axis translation) and the other fixed with free rotation. This allows a huge amount of bending - almost as if you were holding the journals in your hand and bending the crank like a chest expander!

Tried splitting the bearing surface into two halves (axially) and rigidly connecting a point on the centre of the arc of the end of the journal to each half, but again, this doesn't give me any good results to correlate with my hand calcs.

Any one able to point me in a better direction please?

Many thanks in advance,

Jon Reynolds

Regards,

Jon Reynolds

RE: Suitable Constraints for Crankshaft

Jon,

A couple options you can try:

1) Use cylindrical constraints.  Create a cylindrical coordinate system in Mechanica.  After selecting the surfaces to apply your constraints, select the newly created cylindrical coordinate system.  Try constraining just radially to see if that gets you closer.  Remember, you will need some additional constraints somewhere else in your model to prevent rigid body motion.

2) Use ground springs and rigid connections.  Create a point at the center of the shaft where the bearings would be.  Rigidly connect this point to the outer surfaces of the shaft where the bearing would be in contact.  Create a ground spring at each of the two newly created points.  Adjust the stiffnesses in all 6 dof to get the desired motion.

Hope this helps,

Steve

Stephen Seymour, PE
Seymour Engineering & Consulting Group
www.seymourecg.com

RE: Suitable Constraints for Crankshaft

(OP)
Steve,

Thanks for the reply. I am a little confused by the cylindrical CS. I created one, but then I am not sure how the surfaces that I then pick can have translation in R, Theta and Z? I can understand rotation in Theta, but rotation in R? Or translation in Theta?

Not sure how to tie that down correctly.

Using ground springs was something else I dabbled with... will give it a bit more of a go.

Thanks.

Jon

Regards,

Jon Reynolds

RE: Suitable Constraints for Crankshaft

Jon,

Translation in the R direction is radial, translation in the theta direction is tangential (rotational), and translation in the Z direction is axial motion.

The bottom half of the dialog box pertaining to rotations about R, theta, and Z are ignored for solid elements.  Plus, rotations about these coordinate directions don't make sense.

For your model I would constrain the R at the bearing surfaces, constrain the end surface at one of the ends of the cranks shaft from axial (or normal to end surface) motion, and then find a place on the model where you can constrain the rotations.  You could also attach a small weak spring to some portion of the geometry away from the center of the crankshaft to constrain the motion.

Steve

Stephen Seymour, PE
Seymour Engineering & Consulting Group
www.seymourecg.com

RE: Suitable Constraints for Crankshaft

What version of Pro/E are you running? Som eextremely useful "engineering" constraints were added in WF4 and WF5. In particular there is a pin/cylindrical constraint that makes this task very easy...

RE: Suitable Constraints for Crankshaft

(OP)
WF2 unfortunately, although we do have WF4 coming in soon.

Thanks all.

Regards,

Jon Reynolds

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