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]


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



tog_minus User Mass Properties 


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]


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.


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.


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