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07-10-2012, 08:21 PM   #1
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SDM Failures - Do we have any technical analysis?

SDM failures have been the stuff of much discussion for some time now, and I've read much of it on here, as well as undertaken repairs to my own failed DA*16-50.

What I don't recall seeing (and frankly a search of the threads to find anything specific in this area is a little overwhelming) is anyone undertaking a technical analysis of the cause of the failures, which seems to be the manufacture or duty of the SDM drive motors. So, has anyone actually taken apart one of the motors concerned, and properly analysed it for failure cause?

I don't refer here to the disassembly of the lens and "un-sticking" of the drive, which a number of us have done. What I mean is the removal and disassembly of the actual motor itself.

Aside from the fact that it would be of interest technically (for some of us) to know precisely why these motors fail, that knowledge could be put to good effect in providing a lasting technical solution (I emphasise "technical" to sideline smart-aleck comments like "don't buy Pentax SDM lenses") for the affected lenses that our members have.

Obviously, such an exercise would involve some technical expertise and someone willing to sacrifice (potentially) the drive motor from an otherwise prized lens, but the rewards could be greater, if done properly.

It occurred to me recently, when using my occasionally truculent 16-50, that while the actual cause of failure may be known to Pentax and the motor manufacturer, and they've been tight lipped in divulging it, if they do (I find it hard to believe that they don't, by now), it may be simpler than some of us have been giving credit for.

07-10-2012, 11:49 PM   #2
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None of what I have to say is 1st hand, heck I don't even own a SDM lens. But these are a couple of the "apparent" causes that I have read about.

I had read somewhere on the forum that one of the failure modes was a capacitor loosing it's charge and could be rectified by leaving the camera on it to charge it up and that is why you don't see regularly used ones suffering from this particular failure mode.

Another poster some time back said that they only have a motor on one side (Canikon etc apparently have 2 - one on each side of the lens) which causes a misalignment, hence jamming and motor failure.
07-11-2012, 04:44 AM   #3
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The single-drive misalignment theory, in my view, is the less plausible of those two because I've taken my failed lens apart, and the drive structure appeared very rigid to me. While a two-sided drive might be nicely symmetrical, it isn't necessarily mechanically better than a single drive system, unless the drive motor power or torque is restricted by its size, and you actually need two to meet the duty. SDM systems work (if only for a while) and work reasonably quickly, so this latter isn't a consideration, either.

Piezo motors do have relatively complex electronic drive systems, but capacitors tend to work or fail within limits, so I don't give the capacitor charge theory much credence, either, although it's impossible to say without some proper investigation.

What has been observed is that at least some of the motors seize when they fail. The "fix" for a failed SDM drive (documented in these Forums, and also used by me with my 16-50) involves freeing it by applying a torque to the motor shaft. Given that piezo motors work by friction and lock on the rotor during power off, this suggests to me that this source of failure may be just a matter of incorrect tolerancing in manufacture, causing the motor to stay locked on instead of releasing.

What gives this possibility a little more credibility to me is that, recently, I have found that my reluctant 16-50 can be encouraged to work fairly readily by gently working the focus ring while simultaneously pressing the autofocus button. While this observation is consistent with overcoming static friction, it is by no means a conclusive test, and needs some further investigation. There is the obvious question of why the motor simply doesn't lock each time it finishes focussing (which may simply be the result of thermal expansion). I guess that the theory may be supported to some extent if the motor eventually works without this sort of encouragement, so time will tell. If it does, it suggests that the encouragement causes a degree of wear that increases the tolerances in the motor, but it doesn't immediately indicate why the motors worked for a time before failing.

I do wish Pentax would simply come clean and tell us what they know, because the apparently falling failure rates suggest that something's changed in the SDM drive design or manufacture, and that doesn't happen by accident.

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