Multi-turn Trimmer Pots - Locking the Adjustment?

JonSea · Mar 23, 2018

Where I used to work, the technician would secure trimmer pots in position with a drop of nail polish. This kept the adjustment in position, yet still allowed it to be adjusted in the future.

In high-vibration applications do you think this is necessary? I'm using DC-DC converters like the one pictured, and a change in voltage will either interfere with operation or damage the connected circuits.

Is there a better choice than nail polish for this? Acrylic paint would probably work as well. A nice dot of red nail polish is a quick way to see the pot has been adjusted!

The "7 transistor" radios of days past always had daps of what looked like red paint securing all the coils and other adjustable components.

dknguyen · Mar 23, 2018

I thought you did not need to lock them at all since they are worm gears which are high friction and can't be back driven? No experience with this though, so educate me.

alec_t · Mar 23, 2018

I agree with the above: but if you're really determined, how about a blob of hot-melt glue?

JonSea · Mar 23, 2018

That's a fair point. I don't know if our technician was being extra cautious or of he'd had a bad experience.

Securing them may keep people from changing them to "see what happens" too.

MikeMl · Mar 23, 2018

The blob of paint-like material is a "tattle-tale" to see if some screwed with the adjustment after it left the factory...
Candle wax would work to keep it from vibrating, if that is a real problem.

unclejed613 · Mar 23, 2018

10 turn pots are usually not going to drift with vibration, at least the small ones, and usually the panel mount ones that can be moved by vibration, usually have a dial lock on the indicator/knob assembly (looks like a combination lock type of knob). sometimes people put 10 or 20 turn panel mount pots in to replace single-turn controls, so that's when you might not have a multi-turn pot locked down in some way (there used to be a bunch of SSB CB radio mods that used a 10 or 20 turn pot on the clarifier knob in place of the original single turn pot). usually, glue or paint on the little trimpots is there to tell whether somebody has tried to adjust it.

crutschow · Mar 23, 2018

At work we used Glyptal to secure anything that could move from vibration.

DerStrom8 · Mar 23, 2018

Anytime I lock down a trimpot at work it's to ensure nobody (customer, technician, etc) just goes back and turns it to see if it changes anything. It's a universal symbol of "this is calibrated and should not be adjusted". When I lock down a trimpot I usually just use red Loctite (271 I believe) though I imagine any number of Loctite variants would work just as well.

dknguyen · Mar 23, 2018

JonSea said:
That's a fair point. I don't know if our technician was being extra cautious or of he'd had a bad experience.

Securing them may keep people from changing them to "see what happens" too.

DerStrom8 said:
Anytime I lock down a trimpot at work it's to ensure nobody (customer, technician, etc) just goes back and turns it to see if it changes anything. It's a universal symbol of "this is calibrated and should not be adjusted". When I lock down a trimpot I usually just use red Loctite (271 I believe) though I imagine any number of Loctite variants would work just as well.

Is that wise? Loctite dissolves plastic. I know that radio controlled helicopters have been destroyed because loctite was used carelessly and made it's way onto plastic parts.

Mikebits · Mar 23, 2018

crutschow said:
At work we used Glyptal to secure anything that could move from vibration.

I remember using that stuff as well. We used it on multi-turn pots.

DerStrom8 · Mar 23, 2018

dknguyen said:
Is that wise? Loctite dissolves plastic. I know that radio controlled helicopters have been destroyed because loctite was used carelessly and made it's way onto plastic parts.

I never had a problem with it, nor have I heard about this....?

dknguyen · Mar 23, 2018

DerStrom8 said:
I never had a problem with it, nor have I heard about this....?

Well, it's a thing. Plastic-safe threadlocker does exist, but it's the exception, not the rule.

DerStrom8 · Mar 23, 2018

dknguyen said:
Well, it's a thing. Plastic-safe threadlocker does exist, but it's the exception, not the rule.

Huh, interesting.... Maybe it's not actually Loctite?

AnalogKid · Mar 23, 2018

Some MIL reliability tests vibrate things at kilohertz frequencies or tumble them for days. *Everything* that isn't bolted, glued, or stapled down moves. Pots of every kind, including planetary ball drives and 25-turn worm gears, move enough to matter. Electrolytics, of course, move from here to way over there.

ak

AnalogKid · Mar 23, 2018

DerStrom8 said:
Huh, interesting.... Maybe it's not actually Loctite?

Loctite has a small pile of products for tacking down things with varying degrees of firmness. And yes, they have plastic-safe thread lock.

ak

dknguyen · Mar 23, 2018

DerStrom8 said:
Huh, interesting.... Maybe it's not actually Loctite?

Well most of the time Loctite just refers to threadlocker kind of like Kleenex vs tissue.

The only product that Loctite recommends for use on plastic is 425 which is basically super glue, rather than threadlocker like the purple, blue or red compounds

JonSea · Mar 23, 2018

DerStrom8 said:
I never had a problem with it, nor have I heard about this....?

I used threadlocker on a phone clamp that had a 1/4-20 insert to go on a tripod and the threadlocker caused the plastic to swell so much that it cracked across the width of the clamp destroying it. But in hindsight, I don't recall if it was Locktite or Permatex threadlocker. The package specifically warned against use on plastic, but I didn't think using a little on the brass would damage the plastic.

Tony Stewart · Mar 23, 2018

better in what way needs to asked instead

gophert · Mar 23, 2018

Threadlock compounds are most damaging to styrene containing plastic (EPS/Styrofoam, polystyrene, ABS, ...) and less so on acrylics but will still make clear plexiglas cloudy (as will Superglue) unless it is 100% solvent free (foam safe and or "plastic safe").

They don't bother polypropylene or HDPE (as used on the Bourne trim pots shown in post 1).

kinarfi · Mar 23, 2018

Multi-turn Trimmer Pots - Locking the Adjustment?

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