Metal fatigue and the grim reaper

Something happened that got me thinking; any time you put your favorite skin in the care of a piece of technology, the possibility exists that in failing, it will fail you. This is actually our situation just about every hour of every day. Sleep in a house with natural gas, or electricity, or near an industrial plant? Drink water? Eat food? Ride to work on some kind of vehicle? Work in an office with an air-conditioning system? It’s easy to forget just how reliable these technologies are, and what can happen when they fail.

Case in point

A friend of mine came walking up the driveway with his bike a couple weeks ago. He’d been riding on one of the busiest streets in town, and there was a snap-clunk sound, and suddenly it was much harder to pedal. His rear axle had broken.

broken bike axle
Multi-view study of broken bicycle axle (Click to embiggen)

At right is a multi-view composite collection of images of the axle. You can see that it broke just to the inside of the freewheel-side cone. I’m still trying to figure out the mechanics of that break, but my guess is strain from a powerful rider applied to a subtle manufacturing defect. And I am thankful the front axle didn’t break instead, which (unless one is a circus acrobat) guarantees a hard landing.

In the close-up of the fracture plane, you can see shiny spots, where the crack propagated in large bits instead of smaller granules.  The last bit of metal to break was the outer thread. It’s almost as if the axle was inconsistently tempered – and perhaps made from poorly-alloyed steel in the first place. Notice that one side of the axle is light gray, the other half dark gray. You can actually see layered striations across the axle. At a guess I would say this axle was made from a cut length of rolled plate steel, which was incorrectly treated at the mill.

Most threads, including these, are made by a cutting device called a die. If you have ever cut glass or any other material by scratching its surface and applying force, you see the potential for weakening the part. A few variables are the sharpness of the die, and how fast (and with what kind of lubrication) the threads are cut. Cutting with a dull die, or too fast, with the wrong or too little lubricant, weakens the steel. Tool chatter results in a poorly finished surface.

Freewheel-side cone
Freewheel-side cone, click to embiggen

Another factor may have been friction on the bearings. Here’s a closeup of the freewheel-side cone, showing pitting. These are like little potholes in the surface of the cone. Such pitting is much more common in a cone than a cup. In this case it could have been worsened by paint flakes; the hub was painted black, including the cone surfaces, and then assembled with grease, bearings, cups and axle.

Last week, we watched a movie about Apollo 13, which at its core was a survival story that began with a failure of a difficult-to-inspect part. The crew barely made it back.

A bicycle axle is somewhat difficult to inspect as well, requiring an hour’s disassembly and a very good eye. So in general, bike axles are only inspected when the bearing is repacked.

Anyway, my friend is OK, because a broken rear axle is survivable. But somewhere around my garage I’ve got a bike part whose breakage made survival much more a question of luck. I’ll post pictures when I find it.

broken axle



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Older technology guy with photography and history background