Overview Of Nastran Mass And Weight Checks - Ata-e

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Webinar:Introduction to Nastran Massand Weight ChecksTommy Board, ATA EngineeringJune 3rd, 202013290 Evening Creek Drive S, San Diego CA 92128(858) g

ATA Provides High-Value Engineering Services WithExpertise in Design, Analysis, and Test2ATA Engineering helps to overcome product design challenges across a range of industriesAerospaceDefenseRobotics &ControlsIndustrial s

ATA is a Value-Added Reseller for Siemens DigitalIndustries SoftwareATA offers training, free resources, and hotline support for avariety of Siemens products. Siemens product lines we support include: Simcenter STAR-CCM Simcenter Femap Simcenter Nastran (formerly NX Nastran) Simcenter 3D NX CAD & CAM Teamcenter Solid Edge Contact the hotline at 877-ATA-4CAE orhttp://ata-plmsoftware.com/support Developer of the official Simcenter Nastran training materials Preferred North American provider of Simcenter Nastran training Recognized as Smart Expert Partner with validated expertise inFemap, STAR-CCM , and Simcenter 3D3

Visit Our Website for Product Information and Free Resourceswww.ata-plmsoftware.com4

Why do we check mass/weight? Mass checks: the process of verifying that themass of each component matches a specifiedvalue and adjusting the model as needed For dynamic analysis or load cases whereacceleration loads are applied, correct massrepresentation is needed to accurately predictresults WEIGHTCHECK: the Nastran tool that prints outmass properties for different DOF sets WEIGHTCHECK tool can provide insight into othermodeling errors such as connections betweenparts and can help you check units5

Checking Mass in Simcenter Femap Tools Mass Properties Mesh Properties Select all elements Review the message window6

Checking Mass in Simcenter 3DYou can access this tool from the .fem or the .sim On the Home ribbon, More Solid Properties Check Select all elements Turn on all options Review the message areacontents: Mass, Volume, CG, Inertia,etc. Note: Don’t includeCONM1 elements in preprocessor mass checks.CONM1 mass is directiondependent, so NX willautomatically excludethese elements from theSolid Properties Check.WEIGHTCHECK will correctlycalculate the directionalmass.7

How Much Mass Checking Should I Do? Minimal check: List the mass of the entire model.Verify against expected mass. Best Practice: List mass by group or component.Verify against expected mass and CG of eachcomponent. A FEMAP macro has been created that lists mass, CG,etc. based on groups ap-group-summary-to-excel/ Different ways to do mass matching Adjust density of the material Add non-structural mass to a mesh8

Nastran WEIGHTCHECK: How to Set It Up in FemapIn Femap, weight check is available in the Analysis Set Manager9

How to Set Up WEIGHTCHECK in Simcenter 3DIn Simcenter, weight check is available in the Solution window10

How to Set Up WEIGHTCHECK in Nastran WEIGHTCHECK reports the mass properties of a part,including mass moment of inertia and center of massWEIGHTCHECK(PRINT,SET (ALL),GRID 1481786,CGI NO,WEIGHT) YESPrint weightcheckresults to the .f06 fileCheck at least the G,N, and A sets, andmake sure the mass isthe same across setsRequests CG andinertia data (if NO,this data is onlyprinted for G set)Can chooseresults inweight ormass unitsWeightcheck depends on a reference locationSee next slide for moreinformation about sets If no grid is specified, origin of the Nastran basicCSYS is used If the origin is far from the center of the mesh, specifya different grid ID near the CG of the assembly11

Understanding Nastran Sets Response of a FEM defined in terms of DOF 6 DOF per GRID, 1 DOF per SPOINT/EPOINT All DOF in Nastran placed in sets G-set: All DOF (except EPOINTs) M-set: All dependent DOF (RBE2, RBE3, MPC) N-set: G-set minus M-set (all independent DOF) S-set: All restrained DOF (user and AUTOSPC) F-set: All free DOF (N-set minus S-set) O-set: Interior or “Omitted” DOF A-set: Solution DOF (F-set minus O-set) Q-set: Modal DOF B-set: Physical DOF held fixed in CMS modal solution C-set: Physical DOF free to vibrate in CMS modal solution12

Sample Output from WEIGHTCHECK13Matrices used in thecalculation of thecenter of gravity (CG)with respect to thereference point3x3 matrix that lists themass of the model onthe main diagonalInertia propertiesof the model withrespect to thereference pointRepeat of mass values fromthe main diagonal of the 3x3matrix (unless you have masselements with some atypicalproperties, like CONM1 withdifferent masses in differentdirections, the three valuesshould all be the same)I(S): inertia about the CGI(Q): principal inertiasCoordinates of the CG of themodel calculated about eachaxis. Note that the diagonalentries are 0. With no atypicalproperties you would expect twoof the values in each column tobe identical, providing thecorresponding component of theC.G. location.Q: principal axes (axes that themodel prefers to spin about)13

More About WEIGHTCHECK OutputHow do the CG coordinates get calculated? Blue matrix in top right is equivalent to this matrix: If you divide blue matrix by mass, you will get the CGcoordinates14

What to Look for in WEIGHTCHECK Output Check if the mass matches your expectations This will help you check if all material properties are correct (both valuesand units!) Compare to mass measured by pre-processor Compare to mass estimated based on CAD volume and material density Check whether you are using WTMASS parameter – is WEIGHTCHECKprinting out masses or weights, and does this match your expectations? If your deck contains DMIG mass matrices, the mass will not matchthe pre-processor You should still compare mass to pre-processor to verify it matchesexpectations Check if the mass is the same between sets Changes in mass between sets are not common but typically indicate amodeling error What could cause this? See next slide What to do if the mass changes between sets? Review other standardmodel checks, including: Check what is being restrained Do ground checks Review all AUTOSPCs15

Examples of Masses Changing AcrossDifferent DOF Sets Mass on boundary conditions i.e. lumped mass on an SPC Mass regions that have been AUTOSPCed Model grounding Lumped mass in the O-set i.e. there is a model reduction Example Scenario: One component of a large assembly was replaced with alumped mass Original model had small lumped mass elements that werenot reconnected Nastran AUTOSPCed the disconnected elements When the AUTOSPC’s were added (in between the N setcheck and A set check), the model mass changed16

Check Mass Using Reaction ForcesAn Alternative to Weight Checks If acceleration loads are applied, look at the netreaction forces Compare the sum of the reaction forces to theFEM total mass times the applied acceleration You should get F ma If not, there is an issue with the model setup Nastran WEIGHTCHECK tool is preferred method,but checking the reaction forces can be a goodadditional check and can be used for othersolvers like ANSYS17

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