Method for setting constraint conditions in a structure with a rotational axis

In product design, accurate structural analysis that reproduces the behaviour of the actual machine is key to reliable product development. Particularly for structures with rotating shafts, unlike general analysis, careful consideration is required when setting boundary conditions. This article examines the setting of boundary conditions in CAE analysis using models with rotating shafts as an example, and introduces practical know-how.

1. Why is constraint setting for rotational axes important?

Modern products rarely exist as standalone entities; they are typically composed of assemblies comprising multiple components. Consequently, structural analysis using CAE must also evaluate these assemblies. However, for structures with rotational axes, such as air conditioner fins, failure to apply correct boundary conditions can lead to the following issues:

Occurrence of analysis errors (failure to converge)

Unrealistic results due to rigid body movement

Erroneous behaviour lacking structural significance

These issues stem from an inability to correctly model the unique motion inherent to rotating structures.

2. Challenges with conventional fixed conditions

Typically, in CAE, a “fixed” constraint is applied to the end faces or reference planes of a model. This represents a fully constrained condition where “neither translation nor rotation is permitted”, proving effective for stiffness evaluation and deformation measurement.

However, applying this condition to components involving rotational motion causes parts that should rotate to remain stationary, resulting in unexpectedly different stiffness values.

Example: Applying fixed constraints to a fin structure with shafts at both ends causes it to behave like a three-point bending beam supported at both ends. This behaviour, where a component that should rotate acts like a fixed beam, risks leading to erroneous design decisions.

[Analysis Model]

[Analysis Conditions]

[Analysis Results]

3. Effective Constraints: Utilising Cylindrical Support

The Cylindrical Support constraint proves particularly effective here. This offers flexibility by allowing precise definition of constraint presence across three directions (radial, axial, tangential) on a cylindrical contact surface.

Three-Directional Definition of Cylindrical Support

direction of restraint	meaning	The effect of restraint
Radial	Movement from the axis's centre outwards	Prevent deviation from the rotational axis
Axial	Linear motion along an axis	Displacement control via pushing and pulling
Tangential	Movement in a rotational direction	Degrees of freedom of rotation itself

4. Comparison of Behaviour When Each Direction is Free

The analysis compared three patterns where **only one direction** was free, with the other two directions constrained.

Case ①: Free in the radial direction

→ Result: The three-point bending behaviour was identical to that under conventional fixed conditions, with no rotational freedom arising.

Case ②: Axial direction free

Result: Rigid body movement occurs, causing the entire model to slide. This is not the intended rotational behaviour.

Case ③: Free in the tangential direction

→ Result: The fin reproduced rotational motion around its axis. Natural movement closely resembling the actual machine was confirmed.

This result indicates that the constraint method allowing only tangential direction freedom provides the most realistic analysis.

5. Key points for practical application

When performing CAE analysis on rotating structures, the following procedure is recommended:

① Collaboration with Model Designers

Effective communication between designers and analysts is crucial. Model data alone may not accurately convey the designer's intent or anticipated motion. It is vital to consult in advance regarding rotational points and degrees of freedom.

② Preparing the Model Before Setting Constraints

To utilise cylindrical supports, the model must contain cylindrical surfaces where constraints will be applied. Edit the geometry or extract surfaces as necessary.

③ Verifying CAE Software Settings

The definition of ‘cylindrical support’ may differ between analysis software packages. Confirm whether the tangential direction can be explicitly set as free and apply the constraints following the specified procedure.

6. Summary: Correct Constraint Setting Determines Analysis Accuracy

Setting constraints in CAE analysis is one of the most critical factors for reflecting design intent. Particularly for structures with rotational axes, conventional fixed constraints often fail to reproduce behaviour accurately. Consequently, employing **‘cylindrical support with tangential direction free’** as the constraint setting proves highly effective.

Correct constraint configuration enhances the precision of analysis results, significantly contributing to improved design and product reliability. Close collaboration between designers and analysts in mastering CAE represents the technical capability required for future manufacturing.