How should I model parts that deform when assembled? (springs, gaskets, O rings)

[Berkamin]

This was asked two years ago, and nobody answered it to my satisfaction, so I'm re-surfacing this question.

What is the proper way to model parts which get deformed when part of an assembly? For example, if I have a rod-seal gasket with a certain profile, I would like to model it as it should be manufactured (in its un-deformed state), but when I assemble it into an assembly, it should exhibit deformation since it is a soft part that needs to deform to perform its function (in this case, sealing around a rod).

Another example would be a torsion spring for a cross-bow trigger mechanism. I'd like to model the spring in its un-deformed state, to indicate how it is to be manufactured, but when it is assembled into the mechanism, it needs to be in its torsioned state.

Please advise.

[gabrielcabellos.103]

Hello! 

You need to model using parametric and adaptive concepts.

WATCH THIS VIDEO WITH PRACTICAL AND TUTORIAL EXAMPLE:

https://www.youtube.com/watch?v=iwwy7mdfTHE

Here is an example of an assembly using parameters:

https://www.youtube.com/watch?v=KknqV54lmXY

And here is an example of model using parameters:

https://www.youtube.com/watch?v=R3vYKBuOlys

[Berkamin]

The spring example is for a compression spring. How does one do a torsion spring? The trick in that first video, with the split-body with revolute joints at the splits, doesn't work for torsion springs (the kind of spring that powers a mouse trap or a trigger system).

[Berkamin]

I viewed all three of the videos you linked. Unfortunately, my questions weren't addressed in them. I'm not asking about how to change the form of something and having the changes propagate throughout the design using parameters. I'm asking about things like gaskets which look one way in isolation, but in an assembly, need to deform to carry out their function. For example, a wiper gasket (from McMaster-Carr) with this cross-section:

This cross-section shows the gasket as it is by itself. However, when installed in an assembly, it needs to squeeze around a rod.

This problem also applies to diaphragms such as this:

In an assembly where there are opposed diaphragms in a double-acting arrangement, one or both diaphragms are always deformed in the assembled machine.

How does Fusion 360 deal with parts which look one way when manufactured, but are deformed in assembly? This is a huge aspect of machine design. I would be really surprised if there isn't some way to address this issue in modern CAD.

[TrippyLighting]

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@Berkamin wrote:

How does Fusion 360 deal with parts which look one way when manufactured, but are deformed in assembly? This is a huge aspect of machine design. I would be really surprised if there isn't some way to address this issue in modern CAD.

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Mostly this is not a huge issue and often  not an issue at all in machine design and I've been involved in the design of machinery for 2/3 of a 3 decade career as an engineer.

Machine design is very cost driven and engineering time isn't cheap. If modeling components in their initial and deformed state does not add significant benefits or something does not have to be modeled it won't be. 

Gaskets. O-Rings etc.

Mostly added to models in the original un-deformed state. Manufacturers either provide technical specs on interface geometry and torques etc to guarantee performance when compressed. Compression specs for standard gaskets and groove shapes and dimensions  for O-Ring and Sealing Rings can be found in Machinery Handbooks. No need to model these in the compressed state.

Springs

Some CAD systems allow to define compressed and uncompressed states , but Fusion 360 does not. Unless a spring model can be downloaded, often these aren't modeled as springs if not necessary.

The purpose of CAD in machine design is not to provide geometric detail to the N'th degree but to design a functional machine. that does not always require a lot of model detail.
For torsion springs, leaf springs and other special geometry springs that might be different. A little resourcefulness in Fusion 360 can go a long way.

E.g. the torsion spring you provided as an image can be split in two and then assembled with. a revolute joint to approximate the movement of the spring when loaded.

[MichaelT_123]

Hi Mr Berkamin,

Consider this trivial solution:

- Create the respective part component

- Place inside two bodies

- one representing an undeformed part

- another in a deformed state

- if there is a requirement add intermediaries

- switch visibility of respective bodies addressing the context/stage of an assembly

As Mr TrippyLighting elaborated, the dynamic deformation of solid bodies is a quite expensive process, not only in F360.

Regards

MichaelT