What Does a Lumped Mass Do?
A lumped mass is a load that can be applied to a node in a static stress, natural frequency (modal) or modal superposition analysis.
A lumped mass can be used to resist the translation or rotation of a node.
Applying a Lumped Mass
If you have nodes selected, you can right click in the display area and select the "Add" pullout 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.
Analysis Type 
Mass/Weight 
Mass Moment of Inertia  
Static Stress with Linear Materials 
Global Coordinates 

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 

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 

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. 