Lumped Mass


What Does a Lumped Mass Do?

Applying a Lumped Mass

If you have nodes selected, you can right click in the display area and select the "Add" pull-out menu. Select the "Nodal Lumped Mass..." command.

 

Select the appropriate radio button in the "Mass Input" section to determine if the lumped mass input values will be defined in units of force or mass (=weight/gravity).

 

If the lumped mass will be equally effective in all translational directions, activate the "Uniform" checkbox and specify the magnitude of the mass in the "X Direction" field of the "Mass/Weight" section. If the lumped mass have different magnitudes along the three translational directions, deactivate the "Uniform" checkbox and specify the appropriate values in the "X Direction", "Y Direction" and "Z Direction" fields in the "Mass/Weight" section.

 

If the lumped mass will be effective in rotational directions, specify the appropriate values in the "X Direction", "Y Direction" and "Z Direction" fields in the "Mass Moment of Inertia" section.

 

How the masses and moments of inertia behave in each analysis type and coordinate system is summarized in Table 1.

 

Table 1: Masses and Coordinate Systems

Analysis Type

Mass/Weight

Mass Moment of Inertia

Static Stress with Linear Materials

Global Coordinates

  • With gravity, masses are converted to forces by Fi=mi x gi, where i is the X, Y, and Z directions and g is the gravity constant times the multiplier.

  • With centrifugal loads, masses are converted to forces by  Fi=mi x ai, where i is the X, Y, and Z directions and a is the acceleration (r x w2).

With centrifugal acceleration, inertias are converted to torques by Ti=Ii x ai, where i is the X, Y, and Z directions and a is the angular acceleration.

Local Coordinates

Mass behaves as if input is in global coordinates, not local coordinates.

With centrifugal acceleration, inertias are converted to torques by Ti=Ii x ai, where i is the appropriate direction and a is the angular acceleration.

Linear Natural Frequency (Modal)

Global Coordinates

Masses follow global coordinate system and affects the vibration in the corresponding direction.

Inertias follow global coordinate system and affects the vibration in the corresponding direction.

Local Coordinates

Masses follow local coordinate system and affects the vibration in the corresponding direction.

Inertias follow local coordinate system and affects the vibration in the corresponding direction.

Linear Natural Frequency (Modal) with Load Stiffening

Global Coordinates

  • Load stiffening effects due to the masses are included when gravity or centrifugal loads are applied to the model.

  • Masses follow global coordinate system and affects the vibration in the corresponding direction.

Inertias follow global coordinate system and affects the vibration in the corresponding direction.

Local Coordinates

Local coordinate systems not supported.

Local coordinate systems not supported.

Critical Buckling Load

Global Coordinates

  • With gravity, lumped masses are not considered.

  • With centrifugal loads, masses are converted to forces by Fi=mi x ai, where i is the X, Y, and Z directions and a is the acceleration (r x w2).

Mass moment of inertia has no effect.

Local Coordinates

Local coordinate systems not supported.

Local coordinate systems not supported.

Transient Stress (Direct Integration)

Global Coordinates

Inertial effects follow the global coordinate system.

Inertias follow global coordinate system and affects the motion in the corresponding direction.

Local Coordinates

Inertial effects follow the local coordinate system.

Inertias follow local coordinate system and affects the vibration in the corresponding direction.

 

Tip

See the Note on the page "Loads and Constraints" for information about how nodal loads are applied at duplicate vertices.