**IRLF**, `KEY`

**Specifies that inertia relief calculations are to be performed.**

Compatible Products: DesSpc | Pro | Premium | Enterprise | Ent PP | Ent Solver | DYNA

`KEY`

Calculation key:

**0**—

No inertia relief calculations.

**1**—

Counterbalance loads with inertia relief forces.

**-1**—

Precalculate masses for summary printout only (no inertia relief).

The **IRLF** command specifies that the program
is to calculate accelerations to counterbalance the applied loads
(inertia relief). Displacement constraints on the structure should
be only those necessary to prevent rigid-body motions (3 are needed
for a 2-D structure and 6 for a 3-D structure). The sum of the reaction
forces at the constraint points will be zero. Accelerations are calculated
from the element mass matrices and the applied forces. Data needed
to calculate the mass (such as density) must be input. Both translational
and rotational accelerations may be calculated.

This option applies only to the static (**ANTYPE**,STATIC) analysis. Nonlinearities, elements that operate in the nodal coordinate
system, and axisymmetric or generalized plane strain elements
are not allowed. Symmetry models are not valid for inertia relief
analysis. Models with both 2-D and 3-D element types are not recommended.

Loads may be input as usual. Displacements and stresses are calculated as usual.

Use **IRLIST** to print inertia relief calculation
results. The mass and moment of inertia summary printed before the
solution is accurate (because of the additional pre-calculations required
for inertia relief). See Inertia Relief in the *Mechanical APDL Theory Reference* for calculation
details. See also the *Structural Analysis Guide* for procedural details.

If the inertia relief calculation is to be performed in the
second or later load step, you must specify **EMATWRITE**,YES in the initial load step for the element matrices needed to
perform the calculations to be available.

When a superelement (MATRIX50) is present
in the model, any DOF constraints that you need to apply (**D**) on a degree of freedom (DOF) belonging to the superelement
must be applied in the use pass of the MATRIX50 element (*not* in the generation pass). The command has no effect in the generation pass of a substructure.
In the expansion
pass, precalculation of masses for summary printout (**IRLF**,-1) occurs only on elements that are part of the substructure.

This command is also valid in PREP7.