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Piston Simulation

fphelps
1-Newbie

Piston Simulation

Hey guys. I am new to Creo 3.0 Simulation. I am wanting to run a simple static simulation on a piston. I am wanting to know what would be the best set up for the push pull motion of the piston. I understand where and how to apply the loads, but would it be best to run 2 different analysis? One for the push loads and one for the pull loads? Or is there a way to run the simulation as a "Animation Simulation" figuratively speaking? I know you can animate the simulation. But have it where one cylce is the pull cycle and the other cycle is the push of my simulation. I hope this makes sense. Thanks in advance for the help everyone.


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9 REPLIES 9

You can have two (or more) loadsets within the same analysis, which is a run-time efficient way to do it.

However, if it's a simple linear analysis then reversing the magnitude of the load (reversing the load vector by 180°) will give you the same numbers, just with negative (compressive) stresses where there were previously positive (tensile), and the same for displacements.

Thanks for the input Jonathan. I am understanding your response in two ways. 1) Is that you are pretty much saying reversing the load direction.?.? If this is the case, the piston is not symmetrical in geometry or thickness of materials so the stress on the part would be different depending on which way the load was coming from giving an inaccurate reading. right? 2) The second way I could take it is that you are saying apply one load pushing down on the part in -Z ( pushing the piston down) and then apply a different load going +Z (pushing the piston up)? Would the two loads not just push the top and bottom faces of the part towards each other essentially squishing the part?... Thanks for the input and your help again Jonathan.

What he means is that you apply for instance one load on the top of the piston, and one load on the piston pin. Each load has its own "load set". Creo then calculates linear elastic results. When you show results you can scale each load set independently, and thus create output for many load cases with only one analysis. The loads are usually unit loads, for instance 1000N, so that the scaling is easier. A neat trick is to use this method when there is really only one load in the system, creating a second "dummy" load set, so that the results can be scaled.

What you need to realise though, is that this scaling is nothing more than a scaling of the stress results. So applying a negative factor to a load will simply negate the stresses. In case of a piston that may not be the way to go, because you probably want to use a bearing load on the the piston pin hole. Negating a bearing load will not shift the load to the other side of the hole, it will only negate the pressure, thus giving results that will not be realistic.

You have not really told us yet what it is you want to achieve with your simulation. What are you trying to do? Please be as specific as possible. Is this a combustion engine? Are you applying loads on the top of the piston? Is the piston pull a load case due to high rpm? Etc.

Thanks for that Patrick. Really I am just trying to get my feet wet with the simulation tools in Creo. My main goal is just to simulate the loads on the top and bottom of the piston just to see the stress and deformation on the part. I have a VERY simple piston part and was wanting to apply a load to the top and a load to the bottom, and wanted to see if I could simulate the loads acting on the part independently so I could see the push pull effects on the piston. Hope this helps you realize what I am trying to accomplish .

Hi Frank,

wanted to see if I could simulate the loads acting on the part independently so I could see the push pull effects on the piston

I'm not quite clear what you're trying to do there... however:

For any given condition that you're trying to analyse the part in, you need a constraint set and a load set. (Strictly speaking you can just have a constraint set if you're using enforced displacement, but I don't think that's applicable here.)

Within a single analysis you must choose a single constraint set (IIRC), but you can select multiple load sets. The results for each load set are calculated separately, but you can combine them (and scale them if you like) within the results viewer, as Patrick says.

For a piston (I'm assuming in an internal combustion engine, but much the same would apply in a compressor or hydraulic pump), you might constrain it on the gudgeon pin holes vertically and on the skirt OD in the other two directions (constraint set) and apply a pressure load to all the surfaces on the top of the piston (load set 1). You might also apply a vertical acceleration load to account for the inertia of the piston (load set 2). When you view the results, you could choose to view, for example, just the acceleration with a scale of -1 (representing BDC); both the pressure and the acceleration at a scale of 1 (TDC with combustion pressure), or perhaps just the pressure with a smaller scaling factor (90° after TDC).

For slightly more advanced techniques, note that constraining surfaces directly can create overconstraint; for this reason I often like to apply both load and reaction as forces, and then create a constraint which allows the part to deform fully and should not actually react any load. The 'Review Total Load' tool is really helpful for checking that your loads are in balance. Constraining via a spring and weighted link is also a good technique which has been described elsewhere on these forums, but for now just start by getting a basic analysis running.

Can you share any images, diagrams etc showing what you're trying to analyse?

Jonathan - When a piston is operating your a slight force applied around the gudgeon holes when it is being pushed up and a large force on the top face of the piston when combustion happens and it is being forced back down. These operations do not happen at the same time. That is why I am wanting to know if I can apply two load sets in the same analysis Either have one turned off while the other is running or in the animation have one load run come to conclusion then have the next load run? Thanks for input again everyone.

Frank,

Which bit of my reply was unclear?

Yes, you can apply more than one load set in a single analysis (but only one constraint set).

You can view the results for these load sets separately (in two windows side-by-side if you like), or you can combine the results of more than one loadset, with scaling factors if necessary.

If there is no force on the gudgeon pin while the combustion pressure exists, how does the engine produce torque in the crankshaft?

Jonathan - Thanks for your input. I was simply trying to clear it up for you. You had said in your previous comment. " I'm not quite clear what you are trying to do here." Also yes you are are right there would also be a load set put on the gudgeon pin. Thanks.

gkoch
1-Newbie
(To:fphelps)

Maybe what you are searching for is a dynamic analysis?

Not sure, this will actually address your need, but it hasn't been mentioned here yet (at least not explicitly) and it would allow you to apply a function over time to scale the loads (load sets)

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