Originally posted by lytrytyr But at least we can use the collective wisdom of this forum to try to gather some impressions.
One of those impressions is that disuse worsens the problem, and not just through allowing electrical discharge.
It may be some grease hardening, or it may be some other effect.
I've taken a 16-50 apart to free up the motor, and I doubt it's anything to do with a lubricant: quite the converse, as the SDM, like any piezo motor, uses friction as the means of transferring force and motion to the rotor, so a lubricant could only be needed for bearings, and at that size and duty I'd suspect they're the "dry" type. My best guess, without taking a motor apart, is that the gripping piezo elements are the culprits, but without hard evidence it's only a guess. Other piezo motors doing similar duty don't have this issue, so the only thing clear is that it's either a manufacturing defect that the motor supplier won't admit to, or a poor design selection by Pentax.
We also don't really know what sort of drive the DC motor is, as far as I can gather. I know what DC means in the general engineering world, but whether that applies here is hard to say. Manufacturers have a habit of re-badging technology to assuage bad impressions from previous failures, instead of admitting to fault.