Nuts and screws vibrating loose?

Jiles

Well-known Member
Might be a silly question to some, but my group of friends were having a discussion and this topic came up.
Why is it that in every case, a nut or screw will vibrate OUT, never is?
One simple example would be to remove a chainsaw bar and spin the nuts back on, without bar. Run the motor and nut will vibrate off within seconds.
Why don't they vibrate in?
 
Maybe just never notice the ones that
vibrate in lol. If you do the chainsaw
trick over a storm drain the nut will
vibrate in the drain.
 
(quoted from post at 14:45:40 04/28/16)
My 1st thought would be nut turns opposite of running engine crankshaft.

I only used chainsaw as an example. One person said--like removing air cleaner assembly and replacing wingnut. It will vibrate UP?
 

I would suspect this is not a true statement. Perhaps if you conduct a test with say 10 CW threaded and 10 CCW threaded bolt/nut sets mounted on your chain saw you would come to a different conclusion.

I have seen the same nut turn CW and CCW when tested on a shaker table (vibration test machine) simply by changing the frequency.
 
Interesting you should post this. My next door neighbor bought a used MTD small 5 hp chipper and the air cleaner assembly is held on by 4 screws that go down through a 4" square metal plate screwed into the top of the carburetor. The air intake is a small maybe 5/8 hole in the plate. 2 screws vibrated loose and fell into the carb. We luckily got them out. He noticed this when one jammed itself into the throttle plate and it commenced to run wide open. Luckily they did not get into the engine. We had an interesting time trying to get them out, one had jammed the throttle plate until neither would move. Some Locktite on all 4 screws solved the problem. A poor design, if a screw vibrates loose, it will be ingested into the carb.
 
A poor design, but a very common one. My father, a professional auto mechanic, always replaced the throttle and choke plate retainer screws with extra-long brass ones when he rebuilt carbs. Then he used sidecutters to spoil the threads that protruded from the other side of the throttle shaft. The theory was that the ruined threads would prevent the screws from backing out and getting sucked into the engine; and if they somehow did fall out and get sucked in, brass would be less harmful to the engine internals than steel. He was not a big believer in Loctite, at least for moving parts that are continously exposed to gasoline.

He also believed that if fuel injection ever caught on for gasoline engines, it would cut his engine repair work by 50%- his opinion was that most engine wear resulted from mis-adjusted or worn-out carburetors diluting cylinder/valve lubrication.
 
Do left hand nuts know to vibrate opposite of the right hand nuts? Pa made a hayrack out of Plymouth spindles. 2-LH wheels, 2-RH wheels. We couldn't back it up without a wheel falling off.
 
I'll try a lock nut. If that doesn't
work I'll take a hammer and chisel and
burger up the threads a little. I had a
large bolt on front axle of tractor
that would not stay tight, burgered up
threads on nut and bolt. Problem
solved.
 
Nuts are held in place by friction. When you vibrate a loose fastener, the friction goes down to almost nothing, making it very susceptible to any differences in the forces against it. My guess is the air between the nut and its mating surface acts as a small spring, with sound waves pushing the nut away from its mating surface. There's no opposing force on the opposite side, so the nut slowly backs away from the mating surface.
 
For the same reason my coffee cup always turns clockwise til the handle hits the dash in the cup hopder
on my truck, never counter-clockwise.
 

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