In this installment:

• FEA plus simulation
• Information exports
• Load analysis

When computer-aided engineering (CAE) methods became available for design work in the 1980s, finite element analysis (FEA) was the first to be widely adopted. Over the years, it's helped design engineers study the structural performance of new products, and replace costly prototype iterations with inexpensive computer simulations run on CAD models.

But now, mechanical products are increasingly complex and competition to bring designs quickly to market is more intense than ever. For these reasons, many engineers are using structural performance modeling with FEA as well as kinematics and dynamics simulation before building physical prototypes.

Tool for every job

Motion simulation and FEA can also work together in mechanism simulation. To understand how, it helps to understand the fundamental assumptions on which each tool is based.

As we reviewed in the first installment of this series (see the March 2008 MSD issue online) FEA is a numerical technique for structural analysis; it analyzes the behavior of firmly supported elastic (in other words, deformable) objects.

For example, applying static load to a fixed bracket induces a new, deformed shape, which then remains motionless. Application of dynamic load causes the bracket to vibrate about the position of equilibrium. FEA can study displacements, strains, stresses, and vibration of the bracket in both situations.

In contrast, a partially supported object, such as a flywheel hinged on the bracket, can rotate without having to deform. It can also move as a rigid body, which classifies the device as a mechanism rather than as a structure. So, to study its motion, we use motion simulation.

But sometimes the difference between a structure and mechanism isn't obvious. Consider a swing arm mounted to an immovable base by a hinge, with a spring also connecting the arm end and base. The only possible continuous arm motion is vibration about the position of equilibrium, and any arm movement deforms the spring, so the device is a structure.

In contrast, the same device without a spring is a mechanism, because the swing arm can rotate freely. Whether it spins about the hinge or oscillates about a position of equilibrium, no part of the device has to deform during the arm movement.

For the version with the spring, FEA can analyze arm vibration, and even calculate strains and stresses in spring and other components treated as elastic bodies. For the version without the spring, motion simulation returns joint reactions and inertial forces that act upon each link of the mechanism. But how can the two tools be used together?

Motion simulation results supply input data — required for accurate FEA structural analysis. It's actually quite easy, because motion simulation always calculates these factors, whether or not it's followed by FEA. Joint reactions and inertial forces are, by definition, in balance, and mechanism components subjected to a balanced set of loads can be submitted to FEA and treated by the analysis program as structures.

Engineers can transfer that data from motion simulation to FEA manually, but the best results are generated if the motion simulation software can export results to FEA automatically.

When used this way, motion simulation and FEA perform coupled simulation: This defines FEA loads automatically, and eliminates the guesswork and possible errors common of manual setup.

To combine motion simulation and FEA: