RBODY

Bulk Data Entry Defines a rigid body as a set of elements, grid points.

Attention: Valid for Implicit and Explicit Analysis

Format

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
RBODY BID REFG              
  TYPE ID1 ID2 ID3 ID4 ID5 ID6 ID7  
    ID8 etc.            
  TYPE ID1 ID2 ID3 ID4 ID5 ID6 ID7  
    ID8 etc.            
  MASS M              
  INERTIA IXX IXY IYY IXZ IYZ IZZ CID  
  COG X Y Z          

Example 1

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
RBODY 3 1245              
  GRDSET 23 21            
  ELMSET 9 59 48          

Example 2

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
RBODY 4 3456              
  GRDSET 10 11 13 15 22 99 88  
    44              
  SURF 12 34            

Definitions

Field Contents SI Unit Example
BID Body identification number.

No default (Integer > 0)

  
REFG Reference grid. 6

Default = <COG> (Integer > 0)

  
TYPE Flag indicating that the following list of IDs refer to entities of this type.
GRDSET
Grid set
ELMSET
Element set
SURF
Surface

No default

 
ID# Identification numbers of entities of the preceding TYPE field.
Integer
Specifies an identification number for this entity.
This is valid for all the entries listed in the TYPE field.
<String>
Specifies a user-defined string label for this entity. 7
This is valid only for grid and element sets.

No default (Integer > 0 or <String> (only for TYPE = GRDSET or ELMSET)

  
MASS Flag to overwrite the finite element mass of the body. Indicates that a mass value is to follow.  
M Mass.

(Real > 0.0)

  
INERTIA Flag to overwrite the finite element inertia of the body. Indicates that the inertia properties are to follow.  
IXX,IYY,IZZ,IXY,IXZ,IYZ Moments of inertia.

For IXX, IYY, IZZ Real > 0.0

For IXY, IXZ, IYZ Real

  
CID Coordinate system identification number to define the orientation of the inertia tensor.

A CID of zero or blank references the basic coordinate system.

(Integer ≥ 0 or blank)

  
COG Flag to overwrite the finite element center of gravity of the body. Indicates that the center of gravity information is to follow.  
X, Y, Z Location of the center of gravity.

(Real)

  

Comments

  1. At least one Grid Set (GRDSET), Element Set (ELMSET), or Surface (SURF) should be specified.
  2. All elements, grid points, and/or surfaces defined on a RBODY Bulk Data Entry represent one rigid body. The mass and inertia properties are defined by the finite element and mass properties unless they are overwritten by the mass (MASS) and inertia (INERTIA) properties specified on the continuation lines. If mass is not defined on RBODY, then the total mass of the model is the same whether the RBODY is utilized or not. If the mass value is defined on the RBODY, and if the RBODY completely references the specified elements (Figure 1), then the mass of those elements will be zeroed out and their mass will then be sourced from the RBODY. The mass from the RBODY is represented using the CONM2 entry. In some cases (TYPE=GRDSET), all the grids corresponding to a particular element may not be included in the RBODY definition (Figure 1). For such situations, the mass of such partially referenced elements are added to the mass defined for the RBODY.

    rbody_elements
    Figure 1. (A): Completely Referenced Elements / (B): Partially Referenced Elements
  3. The mass, inertia and center of gravity input are optional if element/grid/surface information is provided in the RBODY definition.
  4. If just the principal inertia is specified, IXX, IYY, and IZZ must be positive non-zero values and they must satisfy the condition: the sum of two inertia values must be greater than the third (IXX + IYY > IZZ, IYY+IZZ > IXX, IZZ+IXX > IYY).
  5. Multiple RBODY entries with the same BID can be referenced on the RBODY I/O Options Entry. This allows selection of multiple RBODY's which can move independently of one another.
  6. If MASS continuation line is specified, then either REFG or COG should be present. If MASS continuation line is not input, and both REFG and COG are not specified, the respective center of gravity of the element/grid/surface selections is used.
  7. String based labels allow for easier visual identification, including when being referenced by other entries. For more details, refer to String Label Based Input File.
  8. PARAM,RBD2ELEM,YES can be used to activate a faster method of simulating RBODY entries to help improve performance, especially for large RBODYs.
  9. This entry is represented as a Rigid Body in HyperMesh.