February 11, 2007
Engineering Department
The Mystery of the Self-Unscrewing Bulb
Libby asks:
How do light bulbs unscrew themselves? This can't be only be happening to me. My lamps are really old and rickety for the most part but I don't think that's the reason that I have to retighten the bulbs every once in while. Sometimes I think the bulb is out and discover it's just not twisted in far enough. And today I changed a bulb that did burn out but it was only one twist away from unscrewing itself and leaping to the floor.
Are my lightbulbs just suicidal or is there a scientific explanation for this do you think?
This is one of those questions that stuck in my head...let me try some guesswork.
Light bulbs are held in place by friction between the threads of the bulb base and the threads of the socket. The friction forces tend to prevent movement of the bulb, but they don't actively force the bulb into the fully-seated position (where the center electrical contacts are touching, allowing the bulb to light). So if some outside force unscrews the bulb out of the seated position for any reason, it stays there unless another outside force pushes it back.
Assume that some random force occasionally nudges the bulb. This nudge will either screw the bulb further into the socket or unscrew it out of the socket. If the bulb is already fully-seated in the socket, it can't screw in any further. (Actually, it can move a little bit by deforming the socket and base a bit, but that takes more force than simply screwing or unscrewing.) On the other hand, the bulb can always move in the unscrewing direction. There's nothing to stop it until it falls out.
If the random forces are not perfectly balanced, they will tend to nudge the bulb more in one direction than the other. If they tend to unscrew it, over time they will eventually loosen it enough to break the electrical contact and the bulb will go out. You'll notice that and discover the loose bulb. But if the random forces tend to screw the bulb in, it will just stay snug in the socket and you will ignore it.
So what are these mysterious random forces that move the bulb?
Two possibilities come to mind. The first is thermal cycling. When the lamp turns on, the parts of the bulb base and socket heat up, causing them to expand. However, not all parts heat at the same rate. In addition, the parts are made from different materials which have different expansion charactistics. Thus the parts change size, but not in perfect synchronization, which creates internal stresses that push on the bulb, loosening it or tightening it.
(This used to happen with old socketed computer chips such as the CPU and RAM. Over hundreds of turn-on/turn-off cycles they would expand and contract and slowly walk themselves out of the socket until the computer died. I remember some old computer equipment where the first line of problem diagnosis was to pull all the cards and press down firmly on every single chip. It often solved the problem. Most modern computers use complex sockets designed to prevent things from creeping loose—this is why CPU modules and memory cards usually have a retainer that snaps into place.)
Once a bulb unscrews itself this way, it goes out, and therefore thermal cycling stops, which means that thermal cycling can't turn a bulb back on again. On the other hand, it can't unscrew a bulb beyond the point where it stops working, so this doesn't explain how Libby found a bulb that had nearly fallen out. Some other forces must be at work.
All I can think of is random vibrations from things like people walking around, passing vehicles, and circulating air currents. Again, if these vibrations are not perfectly balanced—and they never are—then they will tend to nudge the bulb more in one direction than another. If they're pushing in the unscrewing direction, they will nudge the bulb away from the electrical contacts and eventually out of the socket.
That's about all I can think of to explain the self-unscrewing-bulb phenomenom.
Unless you have cats. They do stuff like this all the time 'cause they think you'll never suspect them.
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