Motorcycle sprocket tooth fractures

[pmrobert]

This was coincidentally (and probably luckily) discovered while a Ducati 1098 was being routinely cleaned. There were no symptoms, noise and reportedly no abuse. Chain and rear sprocket were in good condition, i.e., not damaged at all. It looks like the damage on the 3 involved teeth were perhaps due to fractures. Oh, this was a factory OEM standard part. Would anyone like to comment on this?

 

[Tmoose]

Can you post pictures showing the fracture surfaces?

 

[pmrobert]

I'm working on getting either better pictures or the item itself. A correction on the first post - this was not an OEM part. The OEM part has 15 teeth, this one has 14. A point was raised elsewhere that "less teeth works the chain/sprockets harder". I can find no objective, professional data reference this hypothesis. Anecdotally, many, many owners of this motorcycle operate it with a 14 tooth option as the stock gearing is rather high for most non-Autobahn, non-competition purposes. I can find no similar issues or even increased failures of any sort related to the 14 tooth option. It would seem to me that since the torque multiplication is numerically higher for the smaller sprocket that the countershaft sprocket would actually be transmitting lower torque at a higher RPM in order to generate the same rear wheel speed/torque for a given operating condition. Also, when I obtain the part or better pictures, might this topic be more suited for the Gear & Pulley and this thread red-flagged?

 

[Tmoose]

14 teeth is usually OK. More is better. I think most chain design manuals would warn about 12 0r fewer teeth due to the drive radius (and torque and tension) variation as each tooth receives a roller.

Note there are at least 2 problems here for which "too small sprockets" might be blamed.
file:///C:/Users/Robyn%20Timberlake/Downloads/maintenance%20A4.pdf

This suggests minimum 17 teeth for industrial applications.

http://www.martinsprocket.com/docs/...intenance-troubleshooting-guide.pdf?sfvrsn=14

 

[Maui]

Are these gears through-hardened or are they case carburized? If they are case carburized I would suggest measuring the effective case depth. If they are through-hardened determine the hardness at the surface and in the core. Based on the appearance of the fracture path this looks like the gear has insufficient strength in the tooth root to withstand the loads that it was subjected to in service. In order to improve this you could use a material with a greater core strength (i.e. harder material), increase the effective case depth, or both. You could also lower the applied stresses.

Maui

 

[mrfailure]

It would be ideal if you could perform a destructive evaluation (particularly metallography) on the sprocket, as well as the same evaluation on the OEM part. Besides increased stress per tooth due to lower numbers of teeth, you may have issues concerning microcleanliness, hardness (see Maui's posting), differences in strength of materials used, and heat treatment. I assume fatigue fracture but you should also look visually at the fracture surfaces to confirm and identify which tooth fractured first (rule of thumb: in fatigue the tooth with the greatest fatigue growth before fracture will be #1; the others fail sooner because of increased subsequent stress).

Good luck!

 

[pmrobert]

Thanks to all for the interesting comments and links. Tmoose, the linked pdf you intended to send didn't. If you don't mind I'd like to have a look. Could you retry that? Here are 6 more images of the subject and related items:

http://i1300.photobucket.com/albums/ag90/jimdacey/Sprockets/DSC00617_zps0idxt5f5.jpg