In the late 1800's, large gears were made by casting steel into a large sand mold. About 50 years ago, I did a failure analysis on such a large gear. The gear teeth failed by fatigue which happens when the repetitive applied loads exceed the fatigue strength. Cracks formed at the base of the gear teeth from the applied loads and the fatigue cracks grew until they were large enough that the teeth simply fell off. Investigation showed that all of the gear teeth were cracked and many were cracked more than 50% across the width of the gear teeth.
This gear was part of a railroad car dumper in which the cars were simply turned over to dump out the iron ore being used for steel making. When several of the teeth failed at the same time, and the railroad car came crashing down causing a lot of damage to the structure. It took several days to repair the damage.
The gear was made of 0.18 wt pct carbon steel. During casting, much of the carbon was lost (decarburized) at the surface due to reaction with the sand mold material, thus causing low fatigue strength at the surface of the gear teeth where the maximum stress was applied.
The design of the large gear was probably adequate for the operations at the time. However, over time, the size and capacity of the railroad cars were increased by more than a factor of two causing much higher stresses to be applied to the gear teeth. The engineers had simply failed to upgrade the design of the steel gears.
Today, the steel chemical composition could have been upgraded, and special heat treatments could be utilized to provide much higher fatigue strengths. For the initial design at the time, the strength was likely adequate given the early limitations on manufacturing methods.
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Robert Cryderman
Research Associate Professor
Colorado School Of Mines
Erie MI
(734) 735-3093
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Original Message:
Sent: 10-12-2020 10:16
From: Janine Borofka
Subject: Historic Steelmaking
A friend of mine asked an archeometallurgy question, and I knew just where to turn...
"If there is a large gear in a piece of machinery made in the 1860's that is failing from use, what would be the most common cause?
This is hypothetical - it is a fictive element in a book I'm writing, but I would like to have the science right .. . The gear was originally made in England for one of the ironclad blockade running boats, and forge they will be making the gear is in Richmond where they did have a cupola furnace at the time - Is it enough to suggest that the cupola furnace will make stronger steel than the original forge in England because of the superior process? Or is there an additive I could mention that might have been missing in the original mix that made the metal weaker?"
I am thinking different chemistries due to different ore sources, or inclusions/impurities fewer in cupola furnace process, but I don't know anything about steel making in that era.
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Janine Borofka
Senior Chief Engineer - Materials
JLG Hagerstown MD
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