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.
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
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
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
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
RE: Piston pin boss stress
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Piston pin boss stress
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
29MPa is a reasonable gross pressure value.
RE: Piston pin boss stress
RE: Piston pin boss stress
RE: Piston pin boss stress
RE: Piston pin boss stress
"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
RE: Piston pin boss stress
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
RE: Piston pin boss stress
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
RE: Piston pin boss stress
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Piston pin boss stress
RE: Piston pin boss stress
RE: Piston pin boss stress
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
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
RE: Piston pin boss stress
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
RE: Piston pin boss stress