May 2011 Archives

Just another rotating platform...

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NOT A SPACE PLATFORM :(  pictured above are VARIACS!

Based on a rough sketch, i was charged with rotating an 18" diameter disc of unknown weight, for a continuous period of time, at an indefinite RPM.  Everything was to sit within a base, like a glorified flower pot.  The sketch also indicated that the shaft was load bearing, and, that the disc was to rotate upon plastic.  These problems were solved with a lazy susan.  Oh and it rotates a snake skeleton.

Next was the motor.  I prototyped this by spinning a large spool of 18 gauge copper with a salvaged 24v DC gearmotor (taken from an old vending machine).  There was a certain amount of noise from the planetary gearbox, but nothing which was a show stopper.  Next the drivetrain shaft, couplers and a new continuous-duty motor were ordered.  At 12V, 1/100 HP, and 25 lbs of torque, this seemed like a viable, and very cheap solution for rotational force.  I added a homemade 30v motor controller for PWM speed control, based off of 555 timing circuits, and was ready to go.

Then the disc/cylinder showed up.  It was far larger and heavier than I expected, and completely acrylic.  I fabricated a flange mount by boring out a steel collar and tapping it into a heavy 4"x3/4" disc of acrylic.  This was then afixed by a peer to the disc.  Meanwhile I made a barbaric motor mount from wood and aluminum.  But this whole time, i never knew the shaft length; everything was arriving just in time, and without proper CADS!  So once all the pieces were here, the shaft was trimmed and everything was installed into the base.  I plugged it in and was driven mad by the noise.  absolutely mad.  It didn't matter that it worked.  The whiiizzz-burr sound of the planetary gearbox under load was too nerve-racking.  I stuffed the base with sound dampering foam, but, to no success.

This is when I upgraded from the cheap $40 DC gearmotor, to a $300 single-phase AC gearmotor, 15 RPM @ 1/20 HP.  Yup, a significant upgrade.  Of course by the time the motor was available for pick-up, I had SIX HOURS TO FINISH!  I grabbed the motor, wired it up, re-trimmed the shaft on the horizontal bandsaw, and put together a new fast-frame aluminum motor mount.  The equipment was installed, and, perfect (for now)!  The blue illumination was then installed via these uber nifty customizable RGB 75 LED adhesive strips.  Deadline accomplished.

But though silent, the 15 RPM motor was deemed a bit too fast.  I'd chosen it since it was reverseable, and indicated on the website that the RPMs could be throttled down via electronic speed control.  But, what did I find in the manual:  Do not use an electronic adjustable speed control device with this gearmotor.

Well, i was angry at that point, but temporarily satisfied with at least having gotten most of the job done.  The next day, upon a suggestion, we throttled the voltage down using a Variac.  At 30v, the speed slowed down.  But at 30v the starter capacitor would never charge up to kick the motor into operation.  oh well.  That is the feeling of failing.  And now I have a 90V DC, direct drive motor, with speed controller on the way.


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This page is an archive of entries from May 2011 listed from newest to oldest.

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