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Piston pin boss stress

Piston pin boss stress

Piston pin boss stress

(OP)
Hi everyone,

Attached is an image of an initial piston design (bosses are huge so I can initially see where material can be removed). Also please ignore the irregular shape of the piston, it's not for a normal application!



The FEA is simulating the loading in the upwards direction (piston is upside down in image), which would be produced by the gudgeon pin, reacting to the gas force in the cylinder (80 Bar ). The load is applied as a bearing load in the appropriate direction

As you can see, the major stress area is the edge of the pin bore. This piston is aluminium (modelled as AL 6061). Max stress at this point is 101 MPa. Would this be a concern? Is the mesh too large perhaps?

Any advice appreciated, thanks.

RE: Piston pin boss stress

the mesh looks large, but 101 MPa sounds like a high load for an aluminum pin bore.

what is the average bearing pressure? your oil film will not support as much load out at the very edge.

Your edge loading will be compounded by the bending of the piston pin (gudgeon pin) - are you able to increase the OD of the pin to reduce bending and spread the load?


RE: Piston pin boss stress

(OP)
I have taken the 80 Bar Pmax and turned it into a force by multiplying by the cylinder bore area. This comes to 46400 N.

I have then applied HALF of this to the pin bore as a bearing load acting in the appropriate direction (so 23200 N); thus assuming each boss takes half the load.

Is this the correct way of looking at it?

There is indeed provision to enlarge the pin.

Many thanks

RE: Piston pin boss stress

that sounds fine for a first look. if you take 23200N / projected area of pin bore, what do you get? I would normally want that figure to be <55MPa.

it has been such a long time that I can't remember what a reasonable guideline for pin bending is - but I suspect you'll be able to reduce your edge load a fair bit with only a small change in pin diameter.

RE: Piston pin boss stress

rusty gears turning ... 80um pin bending rings a bell, for pistons about 100mm diameter.

RE: Piston pin boss stress

Evidently either the pin or the piston is flexing to produce the localized stress at the edge. See if you can figure out where the flex is occuring, and take measures to reduce it.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: Piston pin boss stress

(OP)
I have worked the projected area out to be 2349.9 mm^2 (bore length = 23mm and Dia = 22mm)

I then divide this by two, as the force will only be acting on one half of the bore, giving me 1174.95 mm^2.

Force / Area = 23200 N / 1174.95 mm^2 = 19.745 N/mm^2 = 19.745 MPa.

Why then does the FEA suggest values much higher than this - as can be seen in the image ?

RE: Piston pin boss stress

that should be easy - the load is not evenly distributed?
29MPa is a reasonable gross pressure value.

RE: Piston pin boss stress

(OP)
The force applied in the FEA is a 'radial bearing load' across the top half of the surface area of the bore. I assumed this distributed the load evenly. Does the difference between the FEA and hand calculation arise from the fact it is cylindrical ?

RE: Piston pin boss stress

How are you modeling the wrist pin, and the wrist pin to piston boss interface?

RE: Piston pin boss stress

(OP)
The pin isn't being modelled right now. I'm simply applying a load to the upper face of the pin bore

RE: Piston pin boss stress

Hi HarleyE30,
"I'm simply applying a load to the upper face of the pin bore"

So the entire pin boss bearing surface is restrained or grounded?

RE: Piston pin boss stress

(OP)
The model is constrained on the outside faces of the piston (cylindrical constraint).

RE: Piston pin boss stress

I'm not an FEA jockey so take this with a grain of salt, but it seems to me that, contrary to your assumption, your model is NOT applying an evenly distributed load.
In any case, that would be erroneous, due to flexing as mentioned above.
I'd recommend you model the piston pin, and apply the load in some kind of realistic distribution near the centre of the pin, in accordance with the geometry of the connecting rod.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: Piston pin boss stress

Hmm...if there is no pin in the model, then the red bits can't be from pin bending, can they? Sounds like you need to work on your loads and constraints, then come back if a reasonable model still gives troubling results,

RE: Piston pin boss stress

(OP)
"Hmm...if there is no pin in the model, then the red bits can't be from pin bending, can they? Sounds like you need to work on your loads and constraints, then come back if a reasonable model still gives troubling results"

Yes this is what concerns and confuses me. I will look into it some more. Many thanks all for taking the time to advise me

RE: Piston pin boss stress

I'm curious- is the 80 bar gas pressure a real-world typical number?

RE: Piston pin boss stress

80 bar is certainly a real world number; might even be a peak pressure for a normally aspirated Otto engine. Boosted engines will typically see peak pressures higher than this.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: Piston pin boss stress

It is not unheard of to have peak cylinder pressure >200 bar on a diesel, although 170-185 would be more common.

RE: Piston pin boss stress

Ignore that link-not sure what happened there (phone post)

RE: Piston pin boss stress

what/where is the axial restraint? perhspa the op can show the applied load and applied restraint?

if understand the design the contact pressure in the boss should react against the gas pressure on the face of the piston? i might be wrong given it has been stated as not normal design but should the gas pressure be on the circular face at the bottom?

to me the high stress looks like it due to the flexibility of the crown, i base this off of the deformed shape.

when dealing with close fitting pins the lateral contraction of the bore must be considered as the bore can pinch the pin

RE: Piston pin boss stress

slightly offtopic:
is it difficult to get a position like this @car manufacturer if you are a mech engineer (and not for instance a motorsport engineer that some universities offer) with nonlinear mechanics/ strength of materials (FEA) orientation?
or is that kind of work available if you do a phd?

i would kill for a job like that.

RE: Piston pin boss stress

I'd suggest you try getting in at a consultancy first (ricardo, avl, fev, etc) and then see if you can play that into an OEM role. alternatively you may have luck w/smaller engine manufacturers like Cummins, Cat, Deere.

RE: Piston pin boss stress

HarleyE30,

Have you subtracted the force needed to accelerate the piston towards the crank? Some of the combustion force is accelerating the piston so will not go into the pin. cheers, derek

RE: Piston pin boss stress

ivymike, is the same for suspension analysis (i.e. consultants are the best bet)? i am two countries south of germany.

RE: Piston pin boss stress

I really have no idea - but automotive companies do hire engineers from somewhere.

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