Crankshaft Web Deflection

[Marshy]

I was reading about someone having a scoring issue on one side of the cylinder on a MS250. It was suggested that the crank needed a whack with a dead blow to get the crank centered more. Then I also thought about a recent catastrophic engine failure at work on a large diesel engine and how we measured crankshaft web deflection. I did some research and the web deflection has little value on small engines however, I thought maybe it would be very useful on a small chainsaw engine where we wack on the crank to get it centered. It's actually quite simple, just measure the distance between the webs before the crank is installed and then after. A true web deflection measurement is done by measuring that distance as the crankshaft makes a full revolution. But since that's not feasible, just checking it once after the crank is installed and comparing it to the uninstalled measurement is going to tell you if the crank is deflected from the install...

 

[Steve]

That's pretty neat. How did your engine at work measure up?

 

[wcorey]

If you've ever assembled crank halves onto the pin, you know it takes a bit of persuasion to clock it into alignment.
It's much less common to have to adjust the axial deflection but if you do, it takes a serious beating to move anything in that direction.
I think there's more tendency to flex rather than to move permanently.

 

[Marshy]

That's pretty neat. How did your engine at work measure up?

Webs measured less than 2 thou during the deflection test. Crank appears undamaged other than the crank pin scoring. Unfortunately impact damage from the rod to the engine frame requires us to replace the entire engine. Should have it pulled by tomorrow.

 

[Marshy]

If you've ever assembled crank halves onto the pin, you know it takes a bit of persuasion to clock it into alignment.
It's much less common to have to adjust the axial deflection but if you do, it takes a serious beating to move anything in that direction.
I think there's more tendency to flex rather than to move permanently.

It's possible any "adjustment" will leave the crank deflected. That will add stress to the bearings and the crank. The proof will be in the measurements.

Obviously theres not a ton of crank issues out there so it could be a solution for an issue that doesnt exist much.