Back-driving screws via vibration

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Ryan Carlyle

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Aug 7, 2019, 9:53:23 PM8/7/19
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Random dumb thought. A screw with a small lead (like Tr8x2) will self-lock and not back-drive because the thread pitch (inclined plane) is too shallow relative to the coefficient of friction. Likewise worm gears can't be back-driven. Happens with basically any sliding-contact transmission element where the drive ratio is >> the reciprocal of the coefficient of friction.

But... could you put a piezo element on it to vibrate the screw and dynamically break friction so it DOES back-drive? Like allowing a Tr8x2 lead screw Z stage to fall under gravity, if you wanted to do that for some reason. Or... more practically... leave one stepper coil energized and run the other coil +/- at a high speed (basically doing +/- half steps) to shake the drivetrain. 

Don't know if there's an application here, maybe loosening a mechanism that gets jammed? Not sure, just a passing thought. 

Wing Wong

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Aug 8, 2019, 10:52:41 AM8/8/19
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Hmm... just a guess: yes, if the shaking can induce sufficient flex in the region of the part where the screw is employ? Like mid-beam where the frequency of the motor or piezo element oscillation corresponds to that point and is able to induce coinciding waves. It might be sufficient to counter or augment the friction factor. 

Would be kind of funny if a particular gcode sewuence could induce that kind of vibration to selectively remove screws from a given printer to have it fall apart by the end of the print. Well.... funny if the only active components were the motors and not the heating elements. :/

Wing



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Ryan Carlyle

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Aug 11, 2019, 2:15:17 PM8/11/19
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The sabotage angle is amusing. Like the Stuxnet worm stealthily over-speeding uranium centrifuges to destroy them... self-destructing printers :-)


On Thursday, August 8, 2019 at 9:52:41 AM UTC-5, Wing Wong wrote:
Hmm... just a guess: yes, if the shaking can induce sufficient flex in the region of the part where the screw is employ? Like mid-beam where the frequency of the motor or piezo element oscillation corresponds to that point and is able to induce coinciding waves. It might be sufficient to counter or augment the friction factor. 

Would be kind of funny if a particular gcode sewuence could induce that kind of vibration to selectively remove screws from a given printer to have it fall apart by the end of the print. Well.... funny if the only active components were the motors and not the heating elements. :/

Wing


On Wed, Aug 7, 2019 at 6:53 PM Ryan Carlyle <temp...@gmail.com> wrote:
Random dumb thought. A screw with a small lead (like Tr8x2) will self-lock and not back-drive because the thread pitch (inclined plane) is too shallow relative to the coefficient of friction. Likewise worm gears can't be back-driven. Happens with basically any sliding-contact transmission element where the drive ratio is >> the reciprocal of the coefficient of friction.

But... could you put a piezo element on it to vibrate the screw and dynamically break friction so it DOES back-drive? Like allowing a Tr8x2 lead screw Z stage to fall under gravity, if you wanted to do that for some reason. Or... more practically... leave one stepper coil energized and run the other coil +/- at a high speed (basically doing +/- half steps) to shake the drivetrain. 

Don't know if there's an application here, maybe loosening a mechanism that gets jammed? Not sure, just a passing thought. 

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Steven Butterfield

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Aug 13, 2019, 9:20:44 PM8/13/19
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I think I may have got that frequency. I had a bunch of screws loosen on my frame. Thanks for the reminder to put thread locker on them.

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