Knackered lens iris motor

[therustyone]

Extracted from an old Pentax CCTV lens, the tiny 12v motor+gearbox that rotates the aperture ring. The output shaft, 1.8mm dia. is jammed, but it does rotate a few degrees by hand either way. It doesn't rotate if 12v. is connected but it does thump a bit inside and draws 1/2 A, so electrically seems OK. Is it likely to be repairable, given that it's obsolete. What sort of gears would I expect. It's held together by two radial studs at each end of the cylinder housing which could be drilled out at a pinch.

Here is a picture, if it works.

thanks for any ideas.

[newshound]

If you are lucky, you might find it is an "industry standard" size. See if you can spot anything which looks like a product number on it somewhere.

Whatever gears it contains, they are going to be small, and probably a bit of a heroic project to fabricate replacements.

You *may* be able to separate the motor and gearbox, so that at least you chan check out the motor independently.

[therustyone]

That's a good idea. It's a Sayama and searching, they still list similar m otors online, not exactly the same but cost $22, more than the lens is wort h. They seem to make the cheapo Chinese anonymous geared motors sold on eB ay for a couple of quid, so I think I'll get one of them and modify it.

[newshound]

I wanted a small geared motor for a project some time ago and was interested to discover just how much was available. Never used it in the end.

[Brian Gaff]

Are we sure its not a stepper motor? Brian

[The Natural Philosopher]

Not good news. Dealt with a lot with tiny DC motors for model planes. Strongly suspect it has phosphor bronze (not even carbon) brushes and one has worn through and is now jammed on the commutator.

You might be able to ge a similar small motor from a model plane servo,

Or if you enter the part number, you might be able to but a new motor somewhere.

But I estimate chances of a successful repair of that one at the square root of sweet fanny adams

[The Natural Philosopher]

I got 4 model plane sercos for £12 at a model plane show. Those have DC motors and tons of gears.

[therustyone]

Some progress: I got a bit suspicious on finding all 3 motors were in exac tly the same state so a random fault seemed unlikely. Clogged grease insid e sprang to mind so it went into the kitchen fan oven at 75C for half an ho ur and it seems to work fine at that temperature. So old Japanese lubricat ion goop is the problem. The next plan is to put it in a bath of 20-30W motor oil while hot, and let it cool off, hopefully sucking some oil in. I shouldn't think the casing is hermetically sealed so a bit might get in.

[newshound]

Good bit of diagnosis, checking for performance at ~ 75 C

I don't think you would want there to be free oil inside the motor, there will be a small clearance between the rotor and stator and oil there may stop it running altogether.

The motor bearings are probably porous bronze, soaked with oil. No harm adding a few drops.

I agree that degraded grease in the gearbox, or perhaps grease contaminated with dust/dirt/wear debris is a good candidate for the problem.

I would be more inclined to try to dissolve out all the old grease, hopefully removing debris with it.

You only want to partially fill the gearbox with grease (or oil, if it is effectively sealed) for the same reason that you don't pack ball bearings completely full of grease.

[newshound]

Don't exactly disagree, but model plane motors might do more "mileage" than lens motors so there might still be some life in the brushes.

Could be difficult or impossible to fix, but a successful fix is *very* satisfying. I would be looking for replacement motors too, though.

[therustyone]

True the model motors will get more use, but they wouldn't need to work from -10 to +50C. Anyway I got some tiny drops of 10W-30 engine oil through a 1mm screw hole and it works fine now, even after cooling in the fridge. Well worth fixing the lens as it's a serious F1.1 piece and should spot the Pikies by starlight.

[newshound]

Result!