Showing posts with label damper. Show all posts
Showing posts with label damper. Show all posts

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.