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Cracked Engine Sprocket

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Is a cracked engine sprocket a common occurance or more likely a faulty part ? it was relatively new. Crack is at the bottom, starting at the keyway
dan 

Sprocket

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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...  

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Should have bought a Massey Ferguson! This was from a supplier I regularly use, I'll contact them on Tuesday before I mention their name, I cant help thinking that the thread on the crank should strip before the sprocket cracks.

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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.

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Robert It was a new sprocket - the alignment is slightly off but the best I can do with the 3 available offsets. I'll double check the taper (and key) when I get a replacement 
Dan 

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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!

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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

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

In reply to by michael_rettie

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As far as I can see from the standards, BS, DIN and ISO, no radius is specified for the key slot in the sprocket or on the key that mates with it, other than a vague reference to "breaking" the sharp corner on the key. 
   

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... 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.

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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 

Don,
I had checked the timing with a disc and was re tightening the nut - I noticed that the chain was sticking on the engine sprocket and it opened up. The new one arrived, I lapped it onto the crank and measured the distance from the end of the crank to the sprocket then did the same with the key, I needed to take a little off the flat side to get it to sit firmly on the taper.  Hopefully that will be it all sorted!
Dan 

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... 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.

 



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