Folks,

I am looking for creative ways to shift the center of mass of a model.  The model is built in Mechanical and the solver indicates that its COG is not at the location that the customer needs it to be.  The difference is in the fact that my simplified model does not have all of the wires, cables, sensors, bolts, cotter pins, gadgets, etc. that their model does.  So when I apply acceleration loads in Mechanical, the load is acting at an incorrect centroid.  I already adjusted the steel density to make up for the mass of missing ancillary items.  But that was a global change to all steel in the model.  I'm wondering if I need to further adjust the density of only select portions of the steel to shift the COG.

I'm curious if anyone else has creative ideas to address this.

Matt Ridzon, PE, MSME
Sr. Engineering Analyst

Email    matt@prime-engineer.commailto:matt@prime-engineer.com
Mail      266 Main St, Burlington, VT 05401
Web      www.prime-engineer.comhttp://www.prime-engineer.com/
[A blue hexagon with white letters  Description automatically generated]
PRIME ENGINEERING LLC

This message (including any attachments) may contain confidential, proprietary, privileged and/or private information. The information is intended to be for the use of the individual or entity designated above. If you are not the intended recipient of this message, please notify the sender immediately, and delete the message and any attachments. Any disclosure, reproduction, distribution or other use of this message or any attachments by an individual or entity other than the intended recipient is prohibited.

Hi Matthew,

Why not use a local coordinate system that puts the COG where you want it,
and apply the acceleration load in that system.

Regards,

James Garroni, P. Eng.
Engineering Simulations  Inc.
66 Dockside Way, Winnipeg, MB, Canada
R3X 2E8
Ph. (204) 256-1237, Mobile (204) 794-4385

-----Original Message-----
From: Matthew Ridzon, PE via Xansys xansys-temp@list.xansys.org
Sent: Tuesday, March 11, 2025 10:07 AM
To: xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com
Subject: [Xansys] Shifting Center of Mass

Folks,

I am looking for creative ways to shift the center of mass of a model.  The
model is built in Mechanical and the solver indicates that its COG is not at
the location that the customer needs it to be.  The difference is in the
fact that my simplified model does not have all of the wires, cables,
sensors, bolts, cotter pins, gadgets, etc. that their model does.  So when I
apply acceleration loads in Mechanical, the load is acting at an incorrect
centroid.  I already adjusted the steel density to make up for the mass of
missing ancillary items.  But that was a global change to all steel in the
model.  I'm wondering if I need to further adjust the density of only select
portions of the steel to shift the COG.

I'm curious if anyone else has creative ideas to address this.

Matt Ridzon, PE, MSME
Sr. Engineering Analyst

Email    matt@prime-engineer.commailto:matt@prime-engineer.com
Mail      266 Main St, Burlington, VT 05401
Web      www.prime-engineer.comhttp://www.prime-engineer.com/
[A blue hexagon with white letters  Description automatically generated]
PRIME ENGINEERING LLC

This message (including any attachments) may contain confidential,
proprietary, privileged and/or private information. The information is
intended to be for the use of the individual or entity designated above. If
you are not the intended recipient of this message, please notify the sender
immediately, and delete the message and any attachments. Any disclosure,
reproduction, distribution or other use of this message or any attachments
by an individual or entity other than the intended recipient is prohibited.

With MASS21, I believe, there is no need to use constraint equations or
CERIGs. It could just be associated on a certain node. The location of this
node and the amount of the mass have to be carefully decorated so that COG
comes to the right place.

This makes the model not too computationally huge.

Good luck!
Mohammad

---====
Mohammad A Gharaibeh, Ph.D.
Associate Professor
Department of Mechanical Engineering
The Hashemite University
P.O. Box 330127
Zarqa, 13133, Jordan
Tel: +962 - 5 - 390 3333 Ext. 4771
Fax: +962 - 5 - 382 6348

---====

Hello Matt,

Why don't you use multiple material definition (MAT1, MAT2, MAT3....so on) for steel. You can then change the density as per your need and assign them to the appropriate sections to tune the COG.

But for this to work you must split your model so that different material can be assigned accordingly OR you can directly assign material to selected set of elements (EMODIF) if your model allows that.

Thanks
Anjum

-----Original Message-----
From: Matthew Ridzon, PE via Xansys xansys-temp@list.xansys.org
Sent: 11 March 2025 20:37
To: xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com
Subject: [External] [Xansys] Shifting Center of Mass

This email is from an external source. Please exercise caution in opening attachments or links.

Folks,

I am looking for creative ways to shift the center of mass of a model.  The model is built in Mechanical and the solver indicates that its COG is not at the location that the customer needs it to be.  The difference is in the fact that my simplified model does not have all of the wires, cables, sensors, bolts, cotter pins, gadgets, etc. that their model does.  So when I apply acceleration loads in Mechanical, the load is acting at an incorrect centroid.  I already adjusted the steel density to make up for the mass of missing ancillary items.  But that was a global change to all steel in the model.  I'm wondering if I need to further adjust the density of only select portions of the steel to shift the COG.

I'm curious if anyone else has creative ideas to address this.

Matt Ridzon, PE, MSME
Sr. Engineering Analyst

Email    matt@prime-engineer.commailto:matt@prime-engineer.com
Mail      266 Main St, Burlington, VT 05401
Web      http://www.prime-engineer.com/http://www.prime-engineer.com/
[A blue hexagon with white letters  Description automatically generated] PRIME ENGINEERING LLC

This message (including any attachments) may contain confidential, proprietary, privileged and/or private information. The information is intended to be for the use of the individual or entity designated above. If you are not the intended recipient of this message, please notify the sender immediately, and delete the message and any attachments. Any disclosure, reproduction, distribution or other use of this message or any attachments by an individual or entity other than the intended recipient is prohibited.

Dear Mr. Ridzon,
have you looked into the Distributed Mass feature? You might apply it at the "not-ancillary/ancillary" parts interface. That requires some little hand calculations to get the overall COG in the correct position.
An easier way is to rely on the Point Mass feature, which must be carefully used due to the potential introduction of a large number of constraint equations.

Best regards
Riccardo Testi

Development and Strategies
2 Wheeler Engines Technical Centre
Piaggio & C. S.p.A
Viale Rinaldo Piaggio, 25
56025 Pontedera (Pisa) - ITALY
Phone:  +39 0587 272850
Fax:        +39 0587 272010
Mobile: +39 339 7241918
E-mail:    riccardo.testi@piaggio.com

-----Original Message-----
From: Matthew Ridzon, PE via Xansys xansys-temp@list.xansys.org
Sent: Tuesday, March 11, 2025 4:07 PM
To: xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com
Subject: [Xansys] Shifting Center of Mass

CAUTION:This email originated from outside the Piaggio Group. Do not click links or open attachments unless you recognize the sender and know the content is safe.

Folks,

I am looking for creative ways to shift the center of mass of a model.  The model is built in Mechanical and the solver indicates that its COG is not at the location that the customer needs it to be.  The difference is in the fact that my simplified model does not have all of the wires, cables, sensors, bolts, cotter pins, gadgets, etc. that their model does.  So when I apply acceleration loads in Mechanical, the load is acting at an incorrect centroid.  I already adjusted the steel density to make up for the mass of missing ancillary items.  But that was a global change to all steel in the model.  I'm wondering if I need to further adjust the density of only select portions of the steel to shift the COG.

I'm curious if anyone else has creative ideas to address this.

Matt Ridzon, PE, MSME
Sr. Engineering Analyst

Email    matt@prime-engineer.commailto:matt@prime-engineer.com
Mail      266 Main St, Burlington, VT 05401
Web      https://urlsand.esvalabs.com/?u=http%3A%2F%2Fwww.prime-engineer.com&e=6e97a7e3&h=87561189&f=y&p=y < https://urlsand.esvalabs.com/?u=http%3A%2F%2Fwww.prime-engineer.com%2F&e=6e97a7e3&h=63971262&f=y&p=y >
[A blue hexagon with white letters  Description automatically generated] PRIME ENGINEERING LLC

This message (including any attachments) may contain confidential, proprietary, privileged and/or private information. The information is intended to be for the use of the individual or entity designated above. If you are not the intended recipient of this message, please notify the sender immediately, and delete the message and any attachments. Any disclosure, reproduction, distribution or other use of this message or any attachments by an individual or entity other than the intended recipient is prohibited.

Thanks to everyone that responded!  I now have some great ideas to address the issue.

Cheers!

-Matt

From: Matthew Ridzon, PE
Sent: Tuesday, March 11, 2025 11:07 AM
To: xansys-temp@list.xansys.org
Subject: Shifting Center of Mass

Folks,

I am looking for creative ways to shift the center of mass of a model.  The model is built in Mechanical and the solver indicates that its COG is not at the location that the customer needs it to be.  The difference is in the fact that my simplified model does not have all of the wires, cables, sensors, bolts, cotter pins, gadgets, etc. that their model does.  So when I apply acceleration loads in Mechanical, the load is acting at an incorrect centroid.  I already adjusted the steel density to make up for the mass of missing ancillary items.  But that was a global change to all steel in the model.  I'm wondering if I need to further adjust the density of only select portions of the steel to shift the COG.

I'm curious if anyone else has creative ideas to address this.

Matt Ridzon, PE, MSME
Sr. Engineering Analyst

Email    matt@prime-engineer.commailto:matt@prime-engineer.com
Mail      266 Main St, Burlington, VT 05401
Web      www.prime-engineer.comhttp://www.prime-engineer.com/
[A blue hexagon with white letters  Description automatically generated]
PRIME ENGINEERING LLC

This message (including any attachments) may contain confidential, proprietary, privileged and/or private information. The information is intended to be for the use of the individual or entity designated above. If you are not the intended recipient of this message, please notify the sender immediately, and delete the message and any attachments. Any disclosure, reproduction, distribution or other use of this message or any attachments by an individual or entity other than the intended recipient is prohibited.