Broken sprocket

[huskyhank]

I've not had this happen before - the sprocket shattered into four pieces.
Fortunately I had just started a cut trimming a small piece of wood so didn't have a full load on the chain. I think that may be what saved my saw from major damage. The chain lost two drivers to broken off tips.

 

[mtngun]

Thanks for the pic.

What saw ? I assume it was a 3/8 rim ? Stihl brand ?

Rims are made of heat treated metal, very hard, but they are not supposed to be brittle as yours obviously was.

If I had to guess, I'd say your shattered rim was not tempered correctly.

I have been pondering the subject of how they are heat treated because I was thinking to anneal the 404 sprockets before turning them down to fit lo-pro, then heat treating them after machining. The trick would be choosing the correct tempering temperature -- temper too much, and the rim will be too soft and wear quickly. Temper too little, and the rim will be brittle and end up like your rim.

 

[stipes]

Wow!!!!

I've not had this happen before - the sprocket shattered into four pieces.
Fortunately I had just started a cut trimming a small piece of wood so didn't have a full load on the chain. I think that may be what saved my saw from major damage. The chain lost two drivers to broken off tips.

I never blow a sprocket out yet....I'm wondering myself what did that.....

 

[Andrew96]

I don't know guys.....Heat treating maybe.... but I don't like the wear marks on the splines. I'd say, the sprocket fit loose (maybe incorrectly made), your saw then wedged the sprocket on the splines causing a load outward. Look at the points circled...it looks like only the outer edges of the splines were touching, that's a wedged effect...tremendous force created, overcoming the strength of the part. With it wedged, it wouldn't be able to float in the spline to stay aligned...generating some heat, accelerating the whole thing. Either that or the splines were worn..but they are harder than the sprocket. Food for thought.

 

[huskyhank]

The sprocket is an old one I had laying around - 3/8, 8 tooth. It seemed in good shape........

I had it on my newer 3120 when it fractured. Looking at the sprocket in person does not show any indication of uneven wear or bearing on the splines. I think it was just old or it may have been that one saw's splines are slightly different from the other's. I ran it with a .050 gage chain on the old saw and then a .063 when I put it on the 3120.

I know I will be careful to consider sprockets as "consumable parts" and not re-use one again - even if they do look good.