Connecting rod loading 1

My comments are based on mag particle testing a bunch of engine parts when working in a specialty machine shop in the last millenium.

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I'd say down at the Crank shaft end, keeping the big end bore round at the exhaust stroke TDC is very difficult, and any geometry anomalies ( rod bolt head clearance notches etc) are likely to be the highest stressed areas ans subject to fatigue failures.

Fatigue failures of the ubiquitous I=beam shank were pretty rare, and likely to originate at a major surface defect, often man made, like filing or sawing notches in the I-beam to number the con rod. The orienation of the I-beam shape is doubtless convenient for forging a production part, but if the section properties needed more beef in the fore and aft direction, then they would have been forged as a different shape I believe.

Up by the wrist pin end failures were pretty rare indeed.

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Fred Carillo was a proponent of H-beam rods, but those may have had an advantage for machining from billets for custom lengths.
Their superb metallurgy and quality may have more do to with Carillo rods reputation for durablity as the choice of beam section choice.

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Under extreme boost ( and perhaps even more extreme detonation) or heavy doses of nitro or hydraulic locking or when valve heads break off and jam between the piston and cylinder head, when compression loading dominates and can be uniform or highly eccentric, almost anything can happen.






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In the days before the availablity of high performance connecting rods, modifying stock con rods by various means was about the only choice.
Carefully done "Boxing" was a means to buy short term reliability.
Mag particle inspection would often reveal problems developing in the welding, as might be expected.

I would expect the flywheel end of the crankshaft to have more uniform rotation - the crankshaft bends and twists due to the forces present and that gets superimposed on top of the normal rotation, but the flywheel is a large mass that resists such fluctuations (due to its inertia). Such fluctuations would of course also be transmitted to the connecting rods, but one suspects that it's splitting hairs.

Pattern failures in particular engines that I've seen aren't correlated to how close they are to the flywheel. They're more correlated to known deficiencies in oil distribution in the bottom end.

In the absence of crankshaft flexing and misalignment the only bending load seen by the connecting rod is in the plane perpendicular to the crank axis. This an inertial load and so increases with rpm.

je suis charlie

Really ? And what about the force as the rod follows the crank pin, of course there is a bit of pressure pushing on it as well during a power stroke. I see a bending force during that. Anyone have a good FEA on it?
As an example say the crankshaft is locked at say 45 degrees after TC on the power stroke, and there is some tons of pressure pushing on the piston. What do we see?

Consider the boundary condition. An externally induced bending can only be via torque in the bearings, which hopefully offer little resistance in that direction. Draw an FBD. There is also a small bending load induced by the acceleration of the mass of the conrod itself.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

I used to work for an engine manufacture that due to some legacy tooling consideration had a V8 where the rods were slightly offset along the crank axis from the cylinder bore center lines. They had to play all kinds of games to keep the big end bearings from edge loading leading to fatigue failures.

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enginesrus said:

Really ? And what about the force as the rod follows the crank pin, of course there is a bit of pressure pushing on it as well during a power stroke. I see a bending force during that.

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The rod has a hinge at both ends. With the exception of hydrodynamic forces in the bearings, which in any healthy engine are going to be very low, there is no way for the piston or crankshaft to induce torque, and thus produce a bending load, in the rods.

Even these guys understand the subject matter a bit more.

Since my simple example was not understood, just think of a hydraulic lock condition, that would be considered an extreme loading condition would it not?
So what side or sides of the rod is loaded the most?

In piston nomenclature there is a major and minor thrust side which recognizes that during the power stroke the rod angle produces the most side loading on the piston.

The connecting rod is a two point load member owing to the swivel action of the bearings on each end so the the acts through those two centers with the exception of the side to side acceleration forces.

Under compression the rod is a column so the Euler column bending formulas can be used to calculate the additional bending forces that add to normal compression loading.

Yes major and minor thrust on the skirt of the piston, all pretty basic stuff. So now hydro lock that piston at crank angle 45 btc and torque the crap into the crankshaft, don't tell me there is no loading on that rod beam, I've seen the results many times. When either instantaneously from high combustion pressure or with the hydro lock (especially the lock) it does not matter about the swivel action of bearings, because there is no swivel when locked only when something bends or breaks.

"Yes major and minor thrust on the skirt of the piston, all pretty basic stuff. So now hydro lock that piston at crank angle 45 btc and torque the crap into the crankshaft, don't tell me there is no loading on that rod beam,"
The only loading on the rod in that situation is pure axial. (Assuming the two bearings are frictionless) Yes the rod will eventually bend but that is a "column buckling" phenomenon.

To answer your original question, the sides of the rod facing away from the crank axis will experience the greater stresses, but that is a dynamic phenomenon - only present at speed.

je suis charlie

Most all rods bend on the side not for and aft, yes some buckle but that likely depends on the angularity.

Loading does not mean there is a bending moment. Of course the rod will bend but that is a result of the compressive load. GregLocock had it right. Do a free body diagram and figure out the loads. As you said, it's all pretty basic stuff.

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enginesrus said:

Even these guys understand the subject matter a bit more.

http://blog.wiseco.com/wisecos-new-2000hp-capable-...

Click to expand...

Be wary of making engineering judgments based on information provided by marketing personnel.. like a press release.

enginesrus said:

Since my simple example was not understood, just think of a hydraulic lock condition, that would be considered an extreme loading condition would it not?
So what side or sides of the rod is loaded the most?

Click to expand...

The rod is loaded in axial compression- there is no induced bending moment.

The bending you see in FEA plots and in failed parts is due to buckling, not a torque load. It's a different phenomenon.

There is an rpm-dependent, dynamic bending moment present which contributes to all connecting rod failures.

je suis charlie

Doubt it has much impact when an engine hydraulics as that usually happens at cranking speeds. At high speeds then yes, dynamic loading is a major factor but that's not what ToysRUs is arguing.

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here is a couple of things to ponder

- is the pressure within the combustion chamber at any given crank angle constant?

- if the centre of gravity of the piston is not coaxial to the rod axis what happens under tensile inertia loads?