Another metallurgy question.

[Guest]

I have always wondered if metal fatigue is caused or increased by intermittent use rather than constant use. <img src="http://forums.aaca.org/images/graemlins/confused.gif" alt="" />

The question comes up in my mind because I havn't had a problem with metal fatigue in my car since I have been driving it (47 years daily). Back in 1947 we broke a back axle and it was obviously metal fatigue. My Grandfather had it hammer welded by an 80 year old blacksmith. That axle is still in the car, still running true and has 400,000 more miles on it. I drive without using the clutch except for starting shifting both up and down (accasionaly I miss a shift <img src="http://forums.aaca.org/images/graemlins/blush.gif" alt="" /> <img src="http://forums.aaca.org/images/graemlins/blush.gif" alt="" />) just using the throttle and the transmission has never been opened except to drain a refill every year.

Is it constant use that keeps everything working or just dumb luck. <img src="http://forums.aaca.org/images/graemlins/smile.gif" alt="" /> <img src="http://forums.aaca.org/images/graemlins/smile.gif" alt="" />

[gossjh]

The most recognized characteristic of a fatigue fracture are the beach marks. These marks are the cycles seen by the part. This is probably not what you meant by intermittant rather than constant, but keep in mind that the stresses on teh part force the crack to probagate slowly over time across the part until final fracture. A used car is likey to be well lubricated and run smoother. A car that seldom gets used will lose its oil film, likely run rougher and possibly have corrosion issues that will increase the stress and likelihood of pushing already failing parts to failure. As far as that axle lasting so long, I bet you don't lug the engine, but rather give it the gas and let her pick up and a responsible speed. The guy that shows off how his car will walk in high gear is as likely to break an axle as is one letting the excessive gas and clutch spin the tires. If you use it like it was designed to be used and use it often enough to keep lubrication and lack of corrosion, then your axle, welded or not, should hold up. Keep it between the ditches. Joe

[Guest]

Thanks for your reply Joe. The interesting thing about the axle my grandfather broke. We were going around a sharp corner about 20 mph and the inner rear wheel dropped into a pot hole. About three car lengths later the right rear sagged and the wheel and eight inches of the axle passed us. It broke diagonally just inside the bearing.

[Guest De Soto Frank]

Don't know if this is relevant to the Pontiac, but '29 & '30 Chevys were known for breaking rear axles...

Chevrolet Division began offering a heavy-duty service replacement in late '30 - early '31...

[Bhigdog]

Constant use is not a friend of anything subject to wear, as all things mechanical are. On the other hand complete disuse can be an even bigger enemy. With disuse lubrication and rust prevention films dry, seals take a set, corrosion creeps in, parts freeze in position, etc. etc. I would not say that you were just lucky, though. The fact that you said you drained and refilled the tranny once a year shows excellant preventive maintenance. And that is the key to mechanical longevity.

BTW, I am an amature ( very ) blacksmith. Forge (or hammer) welding is as much art as science and that axle repair has to be a master piece........Bob