Quote:
Originally Posted by Speedy Squirrel
The porsche crank is born balanced. The little counterweights are only to counter balance the crank throws. It is 100% balanced as is, as long as nothing is broken off of it. It is a light, elegant part.
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Yes, the design of the flat six crank maybe balanced, but you are speaking only in design terms that for example a flat six crank is better balanced than an V engine crank.
However, dynamically balancing a rotating part has to do with an actual measurement of how much in-balance there is in that component. If you measured this from one flat six crank to another coming off the assembly line after balancing you would get different measurements.
Some possible variables in why one crank would measure different in-balance compared to another could be:
1. variables in forging process
2. stacking of machining tolerances that locate and machine the journals
3. operator (human) variables such as how close one meets balance parameters vs another operator, etc
4. balance machine variables such condition of machine, machine sensitivity, etc
Speaking from job experience, a person in my group once went to inspect a gearbox (about $100,000 unit). We like to do a residual balance test which means that once the balance machine operator says that the rotor is balanced, we ask them to put a known amount of weight, at a known angle (usually 0 deg) and physically measured distance, then we ask them to spin it up. The software should tell you what you already know, that there is an in-balance of X amount of weight at so many degrees. Guess what, the software said something totally different which meant that their balance machine had an error. They quickly found that one of the four sensors had failed. They replaced the sensor and rebalanced our gear rotor. But there was also an inventory of previously balanced gearboxes ready to ship sitting right in front of that balance machine. I wonder if they ever went back and re-checked balance on all those crated up, ready to ship gearboxes.
The point is that it's a check that does not cost a lot of money. I would recommend to do it on any build where you have no history.