Because of the rapidly improving technology of computers, more and more can be done using finite element analysis methods — including the modeling of rotary shaft seals.  

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In the next few blog posts, we are going to delve into the fascinating world of finite element analysis as applied to seals. First, though, let’s begin with some basics.

What Is Finite Element Analysis?

Finite element analysis takes problems that don’t have a simple textbook solution and applies the basic laws of physics and material behavior to approximate how an object would behave under those loadings and constraints. Basically, here’s a very simplified (as in, don’t show this to my old professor) look at what finite element analysis is doing “under the hood:”

• Takes a problem that doesn’t have a straightforward textbook solution, possibly because of of the complexity of the loadings, the complexity of the geometry, or both

• It breaks the component into discrete (finite) elements made up of nodes

• Loadings and boundary conditions are applied to each of these nodes

• A massive system of simultaneous equations is developed based on applying the laws of physics to each node

• That massive system of equations is solved for displacement (for stress / strain analysis) or temperature (for thermal analysis)

• From those results, other values can be obtained (e.g., stress and strain)

The major issue with finite element analysis is this: the results are only as good as the model. 

You can create a model that poorly represents the real-world problem, and while the results may be correct from a mathematical standpoint, they won’t reflect what really happens. It takes skill and experience to learn how to develop accurate models.

Types of Analysis

There are different types of analysis that can be applied to a finite element model. The three major finite element models of interest to those involved in seal design are:

• thermal analysis
• stress / strain analysis
• transient analysis

If thermal loads could significantly affect the behavior of the seal, a thermal analysis needs to be done prior to the stress analysis, and the resulting temperature outcomes used as input for a stress / strain analysis. Loadings include things like heat sources, convection, and conduction. The results of thermal analysis include the temperatures at every node in the model, which is then provided to the stress / strain analysis model.  

Stress / strain analysis predicts the displacement, strains, and stresses that develop under loadings. It is the primary type of analysis performed on seals and reveals much about how the seal will behave under loading.  

There is another interesting wrinkle that can add more complexity to the model, and that is transient, or time-based, phenomena. These may be necessary if multiple loadings are varying with time and their interactions could make or break a seal’s performance. They take much more time than static analyses, but provide a description of the behavior of the seal over time.

Conclusion

Finite element analysis is an excellent way to gain insight into seal behavior under a variety of combined loadings. It is, however, limited by the skill of the analyst. Even though a model may be mathematically correct, it may not be set up to properly reflect the environment that a seal is operating in. In our next blog post, we’ll talk about major considerations when it comes to setting up a model properly.

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