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Conrod failure modes 1
- Thread starter sellout
- Start date
Lou Scannon
Automotive
Like anything else I guess... yield strength exceeded, low cycle fatigue, high cycle fatigue, over-temperature (not too likely).
A popular failure mode in normally aspirated engines when pushed to the point that the con-rod is the weak link rpm-wise is distortion (stretching to ovality) of the big end. This causes the bearing to lose clearance at the narrow point, leading to bearing seizure and the rod then becoming part of the crank and carving a path for itself through the crankcase after it has broken in two or yanked the piston pin along with itself.
Boosted engines tend to get pushed to the limits of peak cylinder pressure rather than rpm, so a popular conrod failure mode for them is buckling the rod in compression due to detonation. This is usually not catastrophic, as power and cylinder pressure are immediately reduced as a consequence of the reduced compression ratio.
Connecting rods can also contribute to damage elsewhere while remaining intact themselves if elastic stretch is not taken into account when valve to piston clearance is determined. The rods will stretch at the top of the stroke, potentially causing contact between the piston and valves, if static clearance is inadequate.
A popular failure mode in normally aspirated engines when pushed to the point that the con-rod is the weak link rpm-wise is distortion (stretching to ovality) of the big end. This causes the bearing to lose clearance at the narrow point, leading to bearing seizure and the rod then becoming part of the crank and carving a path for itself through the crankcase after it has broken in two or yanked the piston pin along with itself.
Boosted engines tend to get pushed to the limits of peak cylinder pressure rather than rpm, so a popular conrod failure mode for them is buckling the rod in compression due to detonation. This is usually not catastrophic, as power and cylinder pressure are immediately reduced as a consequence of the reduced compression ratio.
Connecting rods can also contribute to damage elsewhere while remaining intact themselves if elastic stretch is not taken into account when valve to piston clearance is determined. The rods will stretch at the top of the stroke, potentially causing contact between the piston and valves, if static clearance is inadequate.
MikeHalloran
Mechanical
In a marine engine, wet exhausts can leak, or a following sea or a 'crash stop' (reversal at full throttle) can push water upstream, right through a turbocharger, and fill a cylinder.
Conrod fails by buckling. It may be an intentional weak link. Crankshafts are much harder to replace in a completed yacht.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
Conrod fails by buckling. It may be an intentional weak link. Crankshafts are much harder to replace in a completed yacht.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
EngJW
Mechanical
- Feb 25, 2003
- 682
If the bolts are not sufficiently tightened, they will see large variations in loading and will fail quickly by fatigue. Once the bolt breaks, the rod and cap will come apart and destroy themselves and several other parts.
Oil starvation will destroy the bearing and journal, generate a lot of heat, and cause the rod to fail. If it siezes on the journal it will bend.
If a piston is destroyed by detonation, the rod is likely to be all twisted and banged up as a result of being flung around and hitting other parts.
I once saw a rod that had a thin section that required shot peening. It got missed on this set, and it resulted in a nice fatigue crack through the section. The rod and cap were in several pieces so it was hard to find the source of failure. Along this line, any machined corners used for bolt head or nut clearance should have a radius, not a sharp corner, and in some cases should be shot peened.
Oil starvation will destroy the bearing and journal, generate a lot of heat, and cause the rod to fail. If it siezes on the journal it will bend.
If a piston is destroyed by detonation, the rod is likely to be all twisted and banged up as a result of being flung around and hitting other parts.
I once saw a rod that had a thin section that required shot peening. It got missed on this set, and it resulted in a nice fatigue crack through the section. The rod and cap were in several pieces so it was hard to find the source of failure. Along this line, any machined corners used for bolt head or nut clearance should have a radius, not a sharp corner, and in some cases should be shot peened.
Having broken more than my share of conrods, all in the past I hope!
In my NA race engines I have noted two main failures, broken bolts(spun bearings) and breaks in the beam near the small end.
Usually the small end breaks are the result of too much time on overstressed (too high revs and/or overly lightened) condition occuring on the overrun in very high compression ratio engines >13.5:1---
Rod bolt failures have universally been the result of a spun bearing in the first place---spinning a bearing at 10,000 rpm is QUICK distruction! To my knowledge, I have never broken an ARP, Cosworth or, SPS rod bolt directly!!!
(I have broken crankshafts and pistons too. These failures often result in broken rods, but not the direct result of the rod itself failing).
Mike, are you sure those photos are not from my son's Dodge? Sure look identical---aftermarket rods and water leak. ;-)
Rod
In my NA race engines I have noted two main failures, broken bolts(spun bearings) and breaks in the beam near the small end.
Usually the small end breaks are the result of too much time on overstressed (too high revs and/or overly lightened) condition occuring on the overrun in very high compression ratio engines >13.5:1---
Rod bolt failures have universally been the result of a spun bearing in the first place---spinning a bearing at 10,000 rpm is QUICK distruction! To my knowledge, I have never broken an ARP, Cosworth or, SPS rod bolt directly!!!
(I have broken crankshafts and pistons too. These failures often result in broken rods, but not the direct result of the rod itself failing).
Mike, are you sure those photos are not from my son's Dodge? Sure look identical---aftermarket rods and water leak. ;-)
Rod
MikeHalloran
Mechanical
Not sure. Not my photos.
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA
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1
- #10
DesignOutsourcing
Automotive
Sellout,
I'm currently working on some improved rods for the Volvo I5 and I6 engines (will also fit the Ford/Jag Duratec V6) since the originals start to break once you boost the engines beyond 300hp. The breaks are mostly near the big end and appear to be from the higher combustion pressures (maybe some detonation) combined with the higher rpms leading to higher cyclic bending moments and stresses (perpendicular to the crankshaft axis.)
cheers, derek
Join us at
I'm currently working on some improved rods for the Volvo I5 and I6 engines (will also fit the Ford/Jag Duratec V6) since the originals start to break once you boost the engines beyond 300hp. The breaks are mostly near the big end and appear to be from the higher combustion pressures (maybe some detonation) combined with the higher rpms leading to higher cyclic bending moments and stresses (perpendicular to the crankshaft axis.)
cheers, derek
Join us at
DesignOutsourcing
Automotive
Here is a link to an MSc. thesis on rod design which I found very useful. Hope this helps:
cheers, derek
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cheers, derek
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