FEA question (stuck): Stress analysis on this Connecting Rod

[Smushiehippo]

Homework Statement

1) Explain where the first point of failure is likely to originate, and why.
2) An analysis of the studies, do you think that they will be an accurate representation of what will
happen in practice? how would you improve it.

Relevant Equations

none

Hey

Im doing FEA and got stuck on question. The diagram go me stuck, because looking through the workbook it says high stresses are in red and low in blue, whereas they included both different kind of stresses on the same diagram and no data.

Note: High Compressive strains are noted by blue, high tensile strains are noted in red, low strains are noted as light
blue/green/yellow.

What I Put
I think first point of failure would be located at the small end, where there is high tensile stress. the high tensile stress would cause deformation and fatigue fractures leading to failure.

For part 2 of the question - I layed out how FEA works, what the accuracy of the data depended on. i stated different methrod of checking (check with other collugues, past results, rerun simulations).

I believe comparing this result with past results ran the diagrams show a high level of accuracy, but cannot be 100% certain of accuracy. I do believe areas on the diagram may give errors in results, big end due to zero radius omitting tensiles stress. Ways to improve to include data on the stress to see if it exceeds yeild stress and put different stress on different diagrams.
cheers for reading

 

[Baluncore]

Note: High Compressive strains are noted by blue, high tensile strains are noted in red, low strains are noted as light
blue/green/yellow.

I think that is incorrect.
I believe the connecting rod is shown under compression only. The highest pressure is where the gudgeon = piston pin, presses against the inside of the small end, shown as red. The big end has a greater area on the crank, so less stress. The blue is zero or neutral. The outer ends of the rod carry no load when under compression.

As the tapered rod is compressed, the narrower sections have greater stresses. That explains the colours changing along the length of the rod. The rod must be tapered to connect the small piston pin to the crank bearing.

The piston pin bearing has a low surface speed, and only swings through a small angle. It may have only splash lubrication. The big end has a higher bearing surface velocity, but also has forced lubrication. Red is an obvious problem.

 

[Smushiehippo]

I think that is incorrect.
I believe the connecting rod is shown under compression only. The highest pressure is where the gudgeon = piston pin, presses against the inside of the small end, shown as red. The big end has a greater area on the crank, so less stress. The blue is zero or neutral. The outer ends of the rod carry no load when under compression.

As the tapered rod is compressed, the narrower sections have greater stresses. That explains the colours changing along the length of the rod. The rod must be tapered to connect the small piston pin to the crank bearing.

The piston pin bearing has a low surface speed, and only swings through a small angle. It may have only splash lubrication. The big end has a higher bearing surface velocity, but also has forced lubrication. Red is an obvious problem.

That's what I thought about the diagram and notes being wrong. But unicourse only gave me the diagrams with those notes and won't respond to my emails about it being potential wrong so kind of stuck

 

[TonyStewart]

This looks like an old design of an equal-space lattice truss whose strains are not supported to equalize the strain. Especially the outer triangles are the weakest.
More like this

The Red zone has the highest pressure but this needs pressurized oil on the sleeve bearing during the low pressure up stoke to fill the red zone. I added two gray oil holes on the opposite side of the high force bearings which will allow oil to be pumped and forced down to the red zone.