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

NOTES:

If your charger isn’t working, consider lint

Y’all know what “pocket lint” is, right? Dust and fibers that can be found in your pocket. Where you put your phone. Now suppose you find your phone won’t charge. It could be for any number of reasons (like a worn-out battery) but there could be a buildup of lint in the charging connector. Here’s how much lint I pulled out of a Lightning connector that wasn’t charging.

Something to consider about connectors: the bigger they are, the more fault-tolerant they are. As connectors get smaller*, the less actual metal-to-metal contact area is available. So a given amount of lint, or oxidation, or misalignment, or wear, has a better chance of interrupting the connection.

“Canned air” didn’t removed this compressed lint; I had to tease it out with a sharpened toothpick. That’s a fairly delicate operation, because the actual connecting springs are fragile. Work carefully, in a comfortable seated position with plenty of light, you can do it.

NOTES:

  • Connectors have gotten smaller, though that isn’t the only factor in their reliability. The D-style connector, invented in 1955 by the ITT corporation, has a lot of metal-to-metal connection area and can be very reliable. Ditto the Centronics connector. But consider the move from the round DIN-5 connector (old-style XT keyboards) to the mini-DIN (IBM PS-2) which was significantly less reliable.
  • USB was a step in the right direction, but the Lightning can be thwarted by just a tiny bit of dirt or corrosion.
  • Micro-USB is certainly prone to the same problems as Lightning.
  • DeOxIT is a real help with all kinds of low-power connections (which tend to be very sensitive anyway). These include antenna connections, audio cables, battery terminals in laptops or phones, USB (all sizes), and more. Obviously you need to clean out dirt and lint before applying it though.

Brother Jeb

“Brother Jeb” is a fixture on our campus; he and his crew visit a couple times a year to warn of the dangers of sin. Holding a crucifix on a sturdy hickory pole, he looks every bit the modern prophet, telling onlookers that God knows when they smoke pot, watch porn, or have impure thoughts. Eternal punishment awaits!

Jeb supporter
All right you Vixens, God is onto your game. (Click to embiggen)

The crowd gathers around him, laughing and jeering. Some put on performances of their own; one fellow preached a whole sermon about how leprechauns were going to destroy you unless you have Lucky Charms cereal for breakfast every morning.

If you’ve been reading my stuff for any length of time, you know I have  no problem mocking hypocrites and fakers. But this guy and his followers… I am less inclined to ridicule. I actually feel some sympathy for him.

Something drives him, year after year, to stand face to face with college students while they yell and sneer at him. He seems genuinely to relish the invectives thrown at him. To me it looks more like mental illness than actual piety or prophecy.

There are truly harmful religious people, like Ted Haggard, Franklin Graham, and Josh Duggar. With their powerful connections in the political right, they drive legislation and prejudices that impact the lives of minorities, of people who have different religions, or no religion, or who just want to marry the same-sex person they love. Jeb is not one of them; he doesn’t have much influence.

I wonder what he would do if he came to campus, and people just ignored him and went about their business?

Jeb crowd

NOTES:

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  • Some people in the crowd do get pretty hostile. There are often police nearby, perhaps to remind citizens that it’s all in good fun until someone throws a punch. And that crucifix is mounted on a decorated shovel handle which could, if necessary, be a potent weapon.
  • His immunity to ridicule may be driven by Matthew 5: “11 Blessed are ye, when men shall revile you, and persecute you, and shall say all manner of evil against you falsely, for my sake. 12 Rejoice, and be exceeding glad: for great is your reward in heaven: for so persecuted they the prophets which were before you.”
  • Ted Haggard used to have a weekly phone conference with the President Of The United States. That’s power.

The key issue

This little trick won’t solve global warming or anything, but it is a durable way to color-code keys.

I’ve used those vinyl key wrappers, but they make the key fatter and I carry plenty of keys. Plus, eventually they come off. Of course you can put colored nail polish on the key but it chips off. Here’s a simple mod:

Drill some dimples onto the key surface. Put some colored nail polish into the dimple. When it dries, put a layer of clear nail polish on top of it. The dimple acts like a little reflector. And because it is recessed into the key surface, it is not subject to so much wear, and won’t chip off.

Once upon a time, stuff like this was called “helpful hints” but today we call them “Life Hacks”.