Connecting rod stress 1

[dodge8564]

Can someone please explain how load is transmitted through the drivetrain to the connecting rods?
I am in a discussion in another forum about how an engine needs stronger connecting rods for towing, even if the engine never reaches above 3500 rpm.
Another person is arguing that load is not transmitted through the drivetrain to the engine itself, and the only reason to have stronger rods and bearings is for high rpm's.

 

[ElwoodGreen]

And here is my 2 cents worth: In summary, it depends on the additional load created by extended towing and the original design constraints. The rods will experience cycles of compressive and tensile forces as mentioned in the better posts above. ANY rod will break if the combination of load and number of cycles exceeds the fatigue failure threshold of the material. A crack will propagate across the material and it will break. A stronger material, or the same material that has been better prepared, will last longer. What are the goals? Do you care if the engine only lasts 50,000 miles because it was operated closer to maximum load most of the time?

My old F-100 has over 300K miles on stock rods (FE series 360 cid), but I only tow a few times a year, and I don't often exceed 3500 rpm. The engine design seems to have perfectly matched my driving habits, but I plan to build a bit more power and torque in the next one. I have a set of stock rods that will be polished up a bit, but I didn't feel the need to spend big bucks on heavier duty pieces. I definitely don't need race pieces.

I have seen 2 such instances of rod breakage on very large industrial gas engines operating at only 900 rpm. These are fully machined rods, by the way. The main difference between these and most automotive rods is in the cap stud bolting arrangement. The big end is cut at an angle, and the studs are threaded into the rod directly. This facilitates removal. Otherwise, the big end won't fit through the bore. The cracks started at the innermost end of the threads for the upper stud and spread from there. The manufacturer, after 2 rounds of rod change-outs on 3 engines and 2 new shortblock assemblies, says they fixed the manufacturing problem. We are now approaching the operating hour range when the first ones broke, and I am getting very nervous. We reduced average loads by about 5%. Will this be enough to avoid additional fatigue failures? Only time will tell.

Happy holidays.

 

[tony90]

why is a connecting rod shaft tapered?