Cracked Engine Sprocket

I have never heard of an original sprocket cracking. I do wonder if this is an aftermarket one and perhaps not to the original specification.
I did have an original sprocket break clean in half on my New Holland 276 baler. A replacement from New Holland was £1200 + VAT. The second hand one at £120 seems like a bargain by comparison. And you thought bike spares were expensive...  

Not something I have ever come accross, However with several different and reversing forces acting on the stress raiser that is the woodruff key slot , I am surprised we have not seen more as fatigue and old age increases. Wear pattern suggests alignment worth checking along with taper engagement.

Perhaps the wrong steel spec?  Was the original case-hardened?  Perhaps this sprocket is through-hardened, lower grade steel and objected to being forced up a taper?

If the sprocket teeth are not hard, (don't really need to be), poor steel spec choice?

Many years ago I worked in a university metallurgy department, we were always on the look out for just this type of failure for students to study and determine how and why the failure occurred.  Quite a few failed motorcycle components found their way through the laboratory!

Keyway a bit shallow in this sprocket?

Typical failure I would expect to see if key is too tall. Something has to give if the sprocket is held off the taper by the key. 

Peter

....would be keyway machined without proper radius at the bottom.

Yes, I have seen it before with an aftermarket part. It destroyed the primary drive. 

... would be to use some fine lapping paste to check for good contact/seating of the sprocket to the shaft.  I'm still very suspicious of the quality of the keyway machining process here with the crack running right out of the keyway.

Never mind all that, Dan, I just want to know how you found it?

I don't take the primary cover off from one year to the next, and I wouldn't be looking that closely at the sprocket if I did.

Don Anson
Melbourne 

... I found with my Electra starter motor sprocket that lapping the tapers then omitting the key has worked for many starts. I think that often the key prevents the tapers from mating correctly.