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Certain frequencies liable to loosen screws?

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Infraviolet:
Are there certain vibration frequencies particularly likely to cause the loosening of screws?

I've been doing some PWM (for current limitng) driving of BLDC motors, and notice that the screws holding the motors in the mounts (M3, 8mm bolts going through a through hole in a printed PLA plastic mount in a tapped hole in the motor face) keep loosening themselves.

The motors do "sing" somewhat with the type of PWM being applied, so there are clearly plenty of mechanical vibrations present from the PWMing of relatively high current (10A peak, time averaged 1A) signals.

Is it likely I've got the bad luck to be PWMing at exactly the right frequency to loosen screws (I'm at 7.8KHz), or is screw loosening likely to be from much slower vibrations, such as those related to the time period of the motor's mechanical or electrical (7 poles so 7x mech freq)rotation which vary depending on the speed I'm turning it at.

If there are particular frequencies which cause loosening, is there a chart or equation linking the "frequency to avoid" to metric screw size?

Thanks

Stray Electron:
    Absolutely!   I've had both cars and motorcycles that would vibrate the nuts off of the bolts when they were running if you didn't have them tight!  The worst offenders were the poorly made American cars from about 1965 to the early 1980s with the extremely loose fitting nuts and bolts. The nuts on some of those were loose enough fit that you could almost spin them off and when you ran the engine the nuts would vibrate and rotate and you could watch then work their way back and forth up and down the bolt.  My first Honda motorcycle would do the same thing, in particular on the bolts holding the carburetor to the intake manifold. I used epoxy those bolts on and then just take the entire manifold off if i needed to work on the carb.  On that MC the carburetor was hung out on the end of a rather long tube like intake manifold and an old man that was a long time MC rider taught me that you could add some weight to the carburetor or the intake and that would change the resonant frequency and would sometimes stop the bolts from vibrating out.  On both the cars and the MCs you could change the engine speed and watch how that caused the nuts and bolts to rotate faster or slower and sometimes even reverse direction.

   You might want to look up the Class fit of bolts and study  why poorly made, loose fitting nuts and bolts aren't used on aircraft or where reliability if important.

thm_w:
Sure, but the solution would not be to change the frequency, it would be to use loctite or change the fastening method.

Benta:

--- Quote from: thm_w on March 01, 2024, 11:26:23 pm ---Sure, but the solution would not be to change the frequency, it would be to use loctite or change the fastening method.

--- End quote ---

Exactly. Your bolt torque is too low for due to the PLA mount.
Try using bolts with a flanged head (standard in automotive for the same reason).

Infraviolet:
I've done some further tests with the motor, the screw don't easily start loosening, but once they've begun loosening even slightly vibrations take them out very fast. I deliberately loosened one by "that first bit", then put the motor in the worst vibrating state posiblr and watched the screw turning itself outward at a rate of about 0.3 rotations per second.

I've also found the worst loosening is when the motor encounters a "high speed stall", where the current limit set at a particular time gives insufficient current (in open loop mode) to turn the motor at a given high speed. So the rotor stands "still" (vibrating) while the magnetic fields from the stator coils race round at the speed which the rotot can;t keep up with when the current is low. This causes far more vibrations that just the 8KHz of PWM.

I've had a look at locking washers, is there actually a definite answer as to which type is best?

There's effectively 4 types, split ring, serrated faces, internal teeth and external teeth.

I've heard the split ring sprung ones often make loosening worse not better? Is this true, it is one of those things where anecdotes outnumber data. How about the other sorts, how do they compare?

Might a drip of superglue between the bolt head the the PLA also be a good idea? Its not a true threadlocking compound, but I'd rather avoid using one of those here as it would have to go in to the tapped holes (aluminium motor case, not steel) and would risk leaking towards to the motor windings. A tiny bit of adhesive between the screw head and the PLA though might make it just hard enough to start turning that vibrations can't make the initial loosening, while still being weak enough to be unscrewed when twisting the screw with a screwdriver.

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