Date: Sat, 03 May 2003 07:46:46 GMT
From: Richard Sherratt <richardsnopsamsley.com.au>
Subject: Broken main shaft in manual gearbox - 1989 900 Aero Turbo


It happened when I changed from 2nd to 3rd. No drive in 3rd, 4th and 5th. I could still use 1st and 2nd. There were no symptoms before the break except for a minor vibration. Nothing that demanded immediate attention, but more of a "I wonder what that is? I'd better check the tyres when at the next service station" sort of vibration. The gearbox main shaft has sheared at the oil hole between the 1st/2nd and 3rd/4th gear positions. There's an oilway running up the centre of the shaft and this hole goes from the surface to the oilway. It's a clean shear with no tearing. Almost like someone cut it with a laser (except for the crystalline surface structure, that is). From looking at the fracture surfaces, it looks like the shaft was being held together by a <0.5mm band around 60% of the circumference plus an irregular blob about 1.5mm diameter. It just goes to show how true it is that 90% of the diameter of a shaft is just there to hold the outer 10% in place :-) What could have caused this? Something in drilling the oil hole that stuffed up the temper of the steel maybe? It's an obvious manufacturing fault, not a wear-and-tear thing. Regards, Richard.

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