CAD-Line | Mass Properties tab

Terminology: Solids, CAD-Line, Kinetostatic Force


To calculate the Forces correctly, it is important that the Mass Properties of each machine part are entered correctly.

MechDesigner has three sources of Mass-Properties:

Mass-Properties that are user defined on the CAD-Line.
Mass Properties imported to a CAD-Line with a SolidWorks document.
Mass Properties derived from Profile/Extrusions on a Part.

Mass Properties may be defined with:

Mass [kg]
Position/location of the Centre-of-Mass [X,Y] [mm]

To define the distribution of the mass around the Centre-of-Mass you can enter:

Moment-of-Inertia [kg.m2]

or

Radius-of-Gyration [mm] of the Mass [kg] about the Centre-of-Mass [XY]

Note: Radius-of-Gyration has no physical meaning. The distribution of mass in real machine elements is function of its real shape. Radius-of-Gyration simply indicates at what radius from the centre-of-mass you could concentrate all of the mass in the machine element to give an equal Moment-of-Inertia [about the Centre-of-Mass] of the real mass distribution in the machine element


Mass Properties tab

Dialog-13.2-CADLine-Masstab

Separators

tog_minus User Mass Properties 

Dialog-13.2-CADLine-Mass-User

Mass : [kg]

Centre-of-Mass: X–axis [mm]

The X-axis is along the CAD-Line, with [0mm] at the start-Point of the Part.

Centre-of-Mass: Y–axis [mm]

The Y-axis is at +90º from the CAD-Line, with [0mm] at the start-Point of the Part.


The mass in a real machine-element is distributed around the Centre-of-Mass. Its mass distribution can be defined by the:

Radius-of-Gyration* [mm] + Mass [kg]

or

Moment-of-Inertia [kg.m2]

Note: Units: kilogram.metres2 NOT kilogram.millimeter2

If you enter Moment-of-Inertia, then MechDesigner calculates the equivalent Radius-of-Gyration

If you enter Radius-of-Gyration, then MechDesigner calculates the Moment-of-Inertia

Radius of Gyratio2


Parallel Axis Theorem

MechDesigner uses the Parallel Axis Theorem to calculate the Moment-of-Inertia, Io , about the Part's rotational axis:

I0 = Ix +M.h2 where h is the distance from the Rotational Axis to the Centre-of-Mass.

tog_minus SolidWorks Mass Properties 

If you import a SolidWorks document, its Mass-Properties are also imported.

This dialog shows the Mass-Properties of the imported SolidWorks document.

You can remove the Mass-Properties completely, or update the Mass Properties to become equal to those of the active SolidWorks document.

Dialog-13.2-CADLine-Mass-SW

1sUpdate Mass Properties to those of the Active SolidWorks Document

MechDesigner imports the Mass Properties of the active document in SolidWorks, as Read-Only values:

Mass : [kg]

Centre-of-Mass : X-axis: [mm]

The distance, from the Origin of the CAD-Line, along the X-axis to the Centre-of-Mass.

Centre-of-Mass Y-axis [mm]

The distance, from the Origin of the CAD-Line, along the Y-axis to the Centre-of-Mass.

Moment-of-Inertia (C.o.G) [kg.m2]

About the Centre-of-Mass


2s Clear/Remove SolidWorks Mass Properties button

Remove the Mass Properties of the imported SolidWorks document, then:

1.Click the button to set the Mass-Properties to zero[0].

tog_minus Total Mass Properties 

Each Added-Part has a CAD-Line from its start-Point to its end-Point.

As a reminder: you may also add CAD-Lines to a Part. All CAD-Lines in a Part may have User Mass Properties and SolidWorks Mass-Properties


The Total Mass Properties include the superposition of:

All Profile/Extrusions that have been added to the Part.
User Mass Properties of all CAD-Lines in the Part
SolidWorks Mass-Properties of all imported SolidWorks documents

These are Read-Only.

Dialog-13.2-CADLine-Mass-Total

Parallel-Axis Theorem

Inertia about Part's Origin = Inertia about Centre-of-Mass  + Mass × (Distance to Part's Origin)2

Relationship between Moment-of-Inertia and Radius of Gyration

Moment-of-Inertia (about Center-of-Mass) = Mass × (Radius-of-Gyration)2

Total Mass of Part =

Mass of all Extrusions + SolidWorks document Mass Properties + User Mass Properties + Total mass of other CAD-Lines in the same Part.

Total Moment-of-Inertia about the Part's Origin =

Inertia of all Extrusions + SolidWorks document Inertia Properties + User Inertia Properties + Total Inertia of other CAD-Lines in the same Part.

Tutorial and Reference Help Files for MechDesigner and MotionDesigner 13.2 + © Machine, Mechanism, Motion and Cam Design Software by PSMotion Ltd