there's something unique about this potentiometer.
it has three large ball bearings in the back, but what for? (read on!)
the three ball bearings form a planetary "gear" reduction, turning this into a 10-turn potentiometer!
the parts.
here are some approximate dimensions. you can't use the regular planetary gear formulas because this setup has no teeth. ball bearings are 7.9mm diameter but this isn't important.
turns of the carrier (which moves the potentiometer wiper) = turns of the "sun" (potentiometer shaft) * S / (R+S) where R is the ring circumference and S is the sun circumference. since there's no teeth, i'm using the circumference as a stand-in.
(diagram is wrong, shaft diameter is actually 1.55mm)
anyway, do the math, and for 1 full turn of the potentiometer wiper, you get about 12 turns of the input shaft.
of course, the potentiometer wiper never does a full turn. more like 300 degrees or so. therefore 300/360*12= 10 turns.
this 1920s clock technology could be accurate to within 1 second per year, and measurements in the 1980s confirmed that at least one such clock was accurate to within a tenth of a second per year.
today's project is to fix this old Seagate ST-412 hard drive. 10MB, full height, and really loud!
here's a problem: the positioner stepper seems to move around randomly on power up. it's supposed to wait until the rpm goes nominal before moving. could be an issue with one of the motor phases.
looks like the stepper is controlled from an MCU. but there's some electronics in between i should check.