Monday, 13 April 2009

Unexpected find

While looking through my collection of salvaged stepper motors I found a couple of NEMA17s. This one came out of the hard drive in the first PC that I bought, an 80286 AT clone for about £1200 in the 1980's.

All the subsequent hard drives I have owned had voice coil head servos, but this one, which was a full height, 51/4", 20MB MFM drive, was built more like a floppy drive with a stepper motor to move the heads.

The motor had a plastic wheel with an endstop on it preventing it making more than one revolution. On removing it I was surprised to find that it was also a resonance damping device.

It seems to consist of a brass flywheel isolated from the shaft by a ball bearing, but coupled to it with a viscous fluid, probably some type of oil. I think it behaves like an electrical snubber, which is a resistor and a capacitor in series use to dampen voltage transients. I think this will have an analogous effect on velocity transients.

I found a similar motor in a 51/4 floppy drive, but that was uni-polar whereas this one is bi-polar, and it did not have the damper. It looks like they were pushing the performance of steppers as far as they could before moving to voice coil servos.

I don't know if it still works, it is more than 20 years old and I damaged it a bit removing it from the shaft as it was glued on. I don't think I will need it when driving a high friction, low inertial load like an extruder drive.


  1. Both the 4 wire steppers I have with splined shafts were recoverd from 5.25 hard drives 10MB also Shugart SA465 5.25 floppy (1978) from Nascom1/Map80 this had a bipolar 6 wire stepper I'd put them to one side after reading your extrusion torque experiments.
    Are you sugesting in your post that steppers like this could be capable of driving an ABS extruder ?

  2. Well Adrian's V2 extruder uses a NEMA17 to extrude ABS. I expect modern ones might have more torque though due to better magnets.

    I expect these will drive a screw drive extruder, and pinch wheel with some gearing.

    I wanted a Nascom at the time, but my pocket money and paper round wages were not enough!

  3. I'll give splined steppers a try had my ABS for nearly 3 weeks not done anything with it yet.

    The Nascom was in use till I bought an Atari ST in 1987. By then even with 256k Rom drive and 256k Dram drive Pluto graphics and clocked Hitachi 64180 CPU it was getting a bit slow.

  4. >I wanted a Nascom at the time, but my pocket money and paper round wages were not enough!

    lol. That makes you about the same age as me, because I was in the same situation!

    I went from an Acorn System 1 to a ZX80. I still have the Acorn, best computer I ever built.

    PIC24's & PIC32's are now my favorites :o)