Gear Failure Analysis

Sorry, didn't know how to post properly.

Low Clutch Gear

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Low Pinion Gear

http://farm2.static.flickr.com/1355/4725213449_dd05e37721.jpg

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Judging by the smearing of metal on your "Low Clutch Gear", it looks like poor heat treating. 

  

The broken tooth on the "Low Pinion Gear" is likely due to overloading when the clutch gear failed.

Hi, You mentioned the smearing is likely due to poor heat treatment. Could it be that it is due to high heat causing the gear teeths to soften overtime? Greasing on the gears might not be performed according to schedule or water could have entered causing poor lubrication.

Could it be that it is due to high heat causing the gear teeths to soften overtime?

No. No amount of friction will raise the temperature of the gear anywhere close to the annealing point.

I would tend to go with the poor or improper heat treating. Use a Rockwell tester to check the hardness at various points, top of tooth, base area where tooth was lost, hub area etc. It may also be that someone substituted a gear of inferior metallurgy to save a few bucks or the manufacturer forgot to heat treat it completely, left out a step or two. It may only have been case hardened if through hardening was required.

Before any temperature during the operation could have gotten hot enough to alter the metal structure I'm pretty sure your lubrication system would have gone up in flames. Cast iron gets annealed up around 1100F so by the time the metal of that gear could get to that temperature you probably already have catastrophic failure.

modern gears are a lot like bearings, they require precision surfaces and close tolerances to operate correctly. So, depending on the gear, and the demanding environment it's in, surface temperatures can absolutely reach high enough temps to cause a physical problem with the gear (it's a cascading effect, not all at once). Especially if the lubrication fails. However, judging from your pictures, you had such a catostrophic failure that any meaningful analysis will be guessing at best. The true cause of failure is most likely masked in that mangled mess of metal. If you've had to replace one gear in the span of a year, your odds of having a problem with the machine (vice the gear) increase dramatically. I would look very closely at your lubrication. Water in the lubrication oil can cause a lot of problems for gears. Ditto for contamination.

From the picture of the broken tooth below, I notice that there are 3 shades - dark, grey and light. Also there is a bump on the top right hand of the gear. Is that a sign of crack appearing before the failure?

I also notice there are pitting all around the gear tooths even on the top land. Is this normal? The size of the gear is around 8" dia and 1" thick.