Bent conrod / low hardness / connection? 3

[ivymike]

I've got an engine (diesel) with a bent conrod. Somebody measured the hardness of the rod in the bend region, and found it to be considerably less than spec. I don't generally associate hardness with resistance to buckling, but I'm curious to see whether anyone has another view on the subject (one besides mine and that of the guy who told me the rod bent because the hardness was too low). If you agree with the idea that hardness plays an important role in the bending of a rod, please advise as to how I might incorporate this figure into future buckling cover factor calcs (rod in question had a buckling CF over 2, even at zero speed with exaggerated cyl pressure, by my calcs)...

If, alternatively, you believe that that cylinder probably hydraulically locked, I'd appreciate suggestions regarding where to look for corroborating evidence (pin boss? valve seats?)...

 

[franzh]

I dont necessarily agree with the soft rod issue leading to the bent rod. I suggest that the rod was weakened by the bending. What caused it? Hydrostatic lock is my number one suggestion, but as to what caused it is another issue.

Liquid ingestion from water is possible, but that usually happens to more than one cylinder. Head gasket failure is another, but my first inclination is a leaking injector. Tell tale indicators would be the obvious ones. Check head bolts for stretch, heads for warpage, piston pin bores for egg shape. Check wet sleeves (if equipped) for proper sealing.

Repairing the engine is only part of the service, diagnostics is just as critical.

Franz

 

[ivymike]

The engine was running on a test stand (development engine), so I think we can safely rule out water ingestion from the environment.

Thanks for the suggestions about items to inspect.

 

[Metalguy]

What was the difference in hardness? I'd suspect a fatigue failure would be far more likely, unless the rod is REAL soft.

 

[GregLocock]

The buckling strength of stubby columns is definitely affected by the yield strength, so that is one possibility. The actual transition from elastic buckling to plastic failure as L/r is reduced should be covered in any structures book... with the proviso that life is rarely that simple, and behaviour in the elastic/plastic transition is very shape dependent. Incidentally neither Roark nor Timoshenko's Theory of Elasticity cover this, so it may be less well known than I thought. Bruhn's "Analysis and Design of Flight Strucutures" does go into a fair amount of detail, section A18.25 onwards. Since it is the best structures book I've ever used I'm not surprised that it is in there. In particular fig 22 shows the effect of temperature on the critical stress, for a range of L/r for steel.

The other thing that might be worth checking is that heavily loaded members have a reduced stiffness (and hence a reduced elastic buckling strength) but it is 20 years since I worked on that and don't even have a name for the technique. Tangent modulus comes to mind?

So far as the metal properties of the failed rod, I don't see why failure would cause a softening of the material, I can't remember having come across that before.

Cheers

Greg Locock

 

[patprimmer]

I'm not a metallurgist, but I would have thought that compressing or drawing the metal, as would occur during failure might actually harden the surface.

I would have a metallurgist look at the failed rod vs spec, and I would also look for coolant or fuel leaks into that cylinder.

I guess that's stating the obvious

Regards
pat pprimmer@acay.com.au
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