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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Google brought me quickly to this web page because I asked the
same questions as Cindy, and I was hoping to verify my suspicion that the
mysterious “self unscrewing bulbs” is a rather common occurrence, which I’m
pleased to say your reply has done.
As a physicist with a lot of electrical experience, I have long
suspected that your well-stated thermal explanation is the correct one. However,
I believe that I can add this one additional insight to your explanation:
The threads and grooves on the bulb and socket have a Spiral
Shape which causes the base of the bulb to move either closer to or further away
from center contact as the bulb is twisted clockwise (cw) or counter cw (ccw).
And the metals used (aluminum, copper, solder, etc.) are, Relatively
Compressible—as evidenced by the fact that a bulb can always be
twisted/tightened substantially beyond the point of initial contact.
However, if there was Zero Friction between the bulb’s base and the socket, the
bulb would, when released, be instantly ejected from the socket—-with a
kinetic energy equal to what was compressively stored in the process of
tightening it.
The friction, as you suggested, is then what Tends to Maintain
the Pressure, and Differential Thermal Expansions is Nature’s Way allowing the
bulb to be pushed out of the socket over time.
I also suspect that the force behind any further
recession of the bulb is apt to be due to either: 1) the bulb’s weight
(i.e, for bulbs with a downword angle), or, 2) an illusion created by some
specialty bulbs that have an unusually shallow/short metal base.
but my experience was that they had been unscrewed and placed on cushions on the dining chairs.
At our house its the porch light. It will be fine for one whole day, but atleast every other day I go outside at night I have to tighten it…after reading this I believe its a cheap wall mount fixture. Looks like an antique Scuba Diving helmet. I was hoping it was some frequency in the air..or the hummin power lines. Something more interesting than my dad hitting his head off the thing everytime he walks out the door. lol. But I can tell when I fix it every other night…if I put a little effort into it I’d prolly start back at thread one, almost could say it will strip the threads, but its more like stretches the socket threads. I’m gonna try a better name bulb. Along with box lot fixture, we got a box of 25 of the cheapest micro florescent bulbs 2 dollars can buy…Prolly a lil motor in the bulb….with the though most people would just un screw it and put in a new one. When light bulbs really never burn out anymore……lightbulb factories must be hurtin. lol.
OH yeah.. BTW. This is like clockwork. every other night. Never works on the third night. I always have to screw it back in.
That’s a more physical way of describing the problem. It’s not a matter of the bulbs becoming loose, but of why some bulbs tend to stay in the socket at all, given the forces that would remove them, such as the effect of gravity on a hanging bulb. The answer to that is, in a word, friction. Then the question becomes one of how the friction mechanism is failing.
There is another explanation which is unsettling. I had what I thought was a burnt out bulb on my back porch. Upon changing the bulb I realized it was not defective at all but unscrewed. I tightened it and all was well for about a week. Early one morning around 4 am I was awakened by noise in my kitchen. The noise I next heard was the sound of a knife being pulled out of my butcher block which sits next to a window. I jumped out of bed and ran to the kitchen and as I switched on the light all I saw was the knife on the counter and a wide open window. The light bulb was again unscrewed by the same person who meant me great harm. Thank God he did not make it through the window.
I was curious as to why I have kept finding all of the light bulbs in my lamps would some how become unscrewed ALL THE TIME. My husband and I were questioning whethere or not it was a ghost, so I said “lets ask Google”. This was the first site on the list and we could not stop laughing at Libby’s post. We were relieved that were not the only ones to have “suicidal” light bulbs!
Glad you enjoyed it.
Grateful to see others wondered the same thing. I don’t usually know a bulb is loose until it flickers due to movement. When I check, all that has been needed was a little tightening. Grateful for an answer. 😃
From Manassas, Virginia August 2, 2018
The light bulb in my table lamp always partially un-screws its self and I periodically must tighten it.
I thought that I was the only one in the world that had this mysterious phenomenon.
I am happy that I am not alone and I appreciate now knowing why this may be occurring. Many thanks
Glad to hear there are rational explanations. I have had a light bulb unscrew and smash this morning. Ten years ago we had bulbs flying out from different light fittings, on one occasion cutting my daughter’s face. Just as it was getting spooky, the phenomenon stopped.
Nevermind light bulbs — the whole freaking light fixture (held together by a spiral) just unscrewed & was left dangling by its wires! The bulb was in tight, but started flashing wildly and then the fixture fell. I screwed it back in, but it wouldn’t turn on. Screwed in the bulb tighter and it went on flickering crazy — then? It would NOT TURN OFF!
Ok, so it is an LED bulb. They’re weird. But really? It took a good 20 seconds to fade. I have to wonder if there is a short somewhere… Something is not right and my instinct is screaming “fire hazard.”
I know that last line was supposed to be a joke, but while reading this and watching my cat shaking the table the lamp is sitting on, I’m pretty sure the cat actually IS to blame for my unscrewed bulb.
The light bulbs on my GE microwave keep coming loose. What I suspect is that the vibration you get when you shut the door makes the bulbs come loose. What I’m wondering is – can you put some aluminum foil on the end of the bulb to make it tighter in the socket?
today – i finally came to the conclusion was my hanging lightbulbs in my ceiling fixture were loosening themselves – so i searched the web and came to this webpage – which does seem to confirm my suspicion
i have 2 ceiling fixtures in my kitchen – and one of them has been working fine – the other one has the flaw – there is a lot of vibration in my apartment building – from the people upstairs (pretty quiet people) – to the laundry room below – but one thing that is in close proximity to the troublesome light fixture is a new refrigerator – it does run a mite strong – i can feel the vibrations in the floor and hear it thrumming at times
thanks to this website – i think i know a possible cause – and i now know the solution – i happen to have an extended grabber tool that should allow me to tighten the bulb without the trouble of dragging a chair around – that feels like a victory
I know from experience, that, light bulbs, which hang from the ceiling, with the bulb and facing the floor, can become unscrewed through vibrations of kids running in the apartment above me! They are so active that I have to screw my bulbs back in about once a month! No mysteries here only common sense and physics.
This has puzzled me for many years. My bulbs variously exploded,shorted or just tumbled out to be found where they landed. I had the rewired and new circuit breakers but this never stopped. I actually observed the bulb UNSCREW ITSELF and hover over the wall sconce, before it dropped back into its receptacle. I am a completely logical person with no clear understanding how this can occur. This event is now firmly relegated to a ghost story…