Difference in Force Direction Between Rigid-Body and Flexible-Body Simulations

tonynguyen_1432

I performed a simulation of lifting a plate using two cylinders. When the plate was modeled as a Rigid Body, the results showed that force appeared only in one cylinder, and its magnitude was nearly twice the force obtained in the flexible-body case. Therefore, I chose the flexible-body model because it provides a more reasonable load distribution between the two cylinders. However, in the flexible-body simulation, although both cylinders carry force, the force directions at the joints are inclined relative to the cylinder axis. Could you please explain why, in the rigid-body case, the forces at the joints remain aligned with the cylinder axis, whereas in the flexible-body case the force directions become inclined?
Many thanks!!!

SouravDas0306

Hello tonynguyen,

In the rigid-body simulation, the plate is assumed to be perfectly stiff, meaning there is no deformation and the relative positions of the two cylinder attachment points remain fixed. Because of this, each cylinder behaves like an ideal two-force member, transmitting force only along its own axis. The joints do not allow moments, so the reaction forces are constrained to align with the cylinder axes. Additionally, since the system is effectively overconstrained and lacks compliance, even a very small numerical or geometric asymmetry can cause one cylinder to carry almost the entire load while the other carries little to none, which is why you observed a single cylinder taking nearly twice the force.

In contrast, when the plate is modeled as a flexible body, it can deform under load, allowing the two attachment points to move and rotate slightly relative to each other. This deformation introduces compatibility through structural response rather than strict kinematic constraints. As a result, the load is distributed more realistically between the two cylinders. However, because the plate can bend and experience local rotations, the connection points no longer enforce a pure axial force condition. The interaction between the cylinders and the deforming plate generates not only axial forces but also transverse components due to bending and shear effects. Consequently, the reaction forces at the joints appear inclined rather than perfectly aligned with the cylinder axes. These inclined force directions are therefore a natural outcome of the flexible-body behavior and indicate a more physically accurate representation of how the load is transferred through a deformable structure.

Regards,

Sourav