CSI - Iron Pony
As I mentioned in the last post, what I do with failure analysis is more like a Post-Mortem than anything else; The idea is to go through whatever evidence I have (usually an oily pile of broken parts and shrapnel) and try to figure out what happened. The customer is always hoping that I will find a factory defect, and the factory always hopes that I will find a user error. Either way, somebody is going to pay, and the stakes can get pretty high. In this case, the 35 HP Vanguard engine retails for just under $4k Can$, so we're talking a good chunk of money.
I thought I'd put together a little sample with some pictures of what I was dealing with:
So here's the engine, getting ready to be taken apart to see what the heck happened inside. This is a 35 Horsepower Vangaurd V-Twin, 983 cc engine.
Once I drained the oil out, I split the casing open, and discovered the horror-show inside. A few large chunks of broken metal from the connecting-rods, and a lot of shrapnel all over the inside of the crankcase. The pistons are still up inside the cylinders, but they were easily removed with a slight push, as neither of them had seized.
Here's where all of the trouble started:
the #1 connecting rod, and its end-cap. As you can see, it's been stretched out of its original perfectly round shape into a mal-formed oval. When the stress became too much, it finally broke in two places. Instantly, the rest of the connecting rod shattered as well, sending debris flying.
Here's a closer look at one of the two broken sections. If you look closely (or at the full-sized image) you can clearly see where the head of the bolt hammered against the the end-cap. This is the critical evidence that proves that the bolts had not been tightened correctly. This observation was what allowed me to finally figure out what had happened to this engine.
Finally, here is the crankshaft itself. In this image, the upper half of the crank is where the #1 connecting rod sat, and the lower half is where #2 sat. You can see the holes in the crank where oil would be pumped through to keep everything well-lubricated and running smoothly. You can see abrasions and scuff-marks on #1, where the connecting rod would have been hammering against it, but what you don't see is evidence of "metal transfer."
Compare that to the lower half, where you can see how molten aluminum from the #2 connecting rod has welded itself onto the crank. When the #1 rod shattered, the oil hole for it was left wide open, so oil gushed out through the open hole instead of lubricating the #2 rod. Spinning at over 3000 rpm with barely any lubrication, it didn't take long for the friction and heat to build up enough to cause the aluminum connecting rod to start to melt, until enough molten metal adhered to the steel crankshaft to weld the two peices together. At that point, everything came to a screeching halt and the connecting rod shattered.
Briggs & Stratton has agreed with my findings, and has authorized this engine to be replaced under warranty. As per standard procedure, I have to hold on to all of the parts for 30 days, in case they wish to review them.
I thought I'd put together a little sample with some pictures of what I was dealing with:
So here's the engine, getting ready to be taken apart to see what the heck happened inside. This is a 35 Horsepower Vangaurd V-Twin, 983 cc engine.
Once I drained the oil out, I split the casing open, and discovered the horror-show inside. A few large chunks of broken metal from the connecting-rods, and a lot of shrapnel all over the inside of the crankcase. The pistons are still up inside the cylinders, but they were easily removed with a slight push, as neither of them had seized.
Here's where all of the trouble started:
the #1 connecting rod, and its end-cap. As you can see, it's been stretched out of its original perfectly round shape into a mal-formed oval. When the stress became too much, it finally broke in two places. Instantly, the rest of the connecting rod shattered as well, sending debris flying.
Here's a closer look at one of the two broken sections. If you look closely (or at the full-sized image) you can clearly see where the head of the bolt hammered against the the end-cap. This is the critical evidence that proves that the bolts had not been tightened correctly. This observation was what allowed me to finally figure out what had happened to this engine.
Finally, here is the crankshaft itself. In this image, the upper half of the crank is where the #1 connecting rod sat, and the lower half is where #2 sat. You can see the holes in the crank where oil would be pumped through to keep everything well-lubricated and running smoothly. You can see abrasions and scuff-marks on #1, where the connecting rod would have been hammering against it, but what you don't see is evidence of "metal transfer."
Compare that to the lower half, where you can see how molten aluminum from the #2 connecting rod has welded itself onto the crank. When the #1 rod shattered, the oil hole for it was left wide open, so oil gushed out through the open hole instead of lubricating the #2 rod. Spinning at over 3000 rpm with barely any lubrication, it didn't take long for the friction and heat to build up enough to cause the aluminum connecting rod to start to melt, until enough molten metal adhered to the steel crankshaft to weld the two peices together. At that point, everything came to a screeching halt and the connecting rod shattered.
Briggs & Stratton has agreed with my findings, and has authorized this engine to be replaced under warranty. As per standard procedure, I have to hold on to all of the parts for 30 days, in case they wish to review them.