Author Topic: Meter movement sticking.  (Read 2578 times)

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Offline Vgkid

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Meter movement sticking.
« on: November 21, 2017, 10:51:01 pm »
What are some ways to keep a meter from sticking. It generally sticks from the 20-80 range. Meter is in good shape (it is mounted in a product.)
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Offline EPTech

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Re: Meter movement sticking.
« Reply #1 on: November 21, 2017, 11:05:45 pm »
Hi there,

I suppose you mean an analogue needle meter with a turning coil. place a single droplet of very fine oil where the shaft passes. Do not manually operate the spring though. re-assemble the meter and cycle it on and off with a very low current (big series resistor) until it operates smoothly. Do not use compressed air as it may damage the delicate parts.

Happy metering.
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Offline Ian.M

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Re: Meter movement sticking.
« Reply #2 on: November 21, 2017, 11:12:07 pm »
If an analog meter is sticking, it isn't in good shape. If it moves closer to the correct position when you tap it,  assuming there is no obvious foreign object in the mechanism or bent pointer, either there is dirt in the bearings, or they have been damaged by excessive mechanical shock.  Cleaning the bearings is only an option for an experienced watchmaker or precision mechanical instrument technician.   Lubricating the mechanism is absolutely *NOT* recommended as it will attract dirt and further gum it up and may react adversely with glues and lacquers used in its construction.

The only easily rectifiable problem is if the pointer is being attracted by a static charge on the front cover glass or plastic.  Assuming it can be removed, cleaning both sides of it with an antistatic lens cleaner so a trace static dissipative film is left on the surface will remove existing charge and reduce the risk of new charge buildup.
 

Offline Vgkid

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Re: Meter movement sticking.
« Reply #3 on: November 21, 2017, 11:24:11 pm »
It is an analog movement , and it doesn't always stick either.  Generally it wont stick when I'm trying to null it .
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Offline Ian.M

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Re: Meter movement sticking.
« Reply #4 on: November 21, 2017, 11:53:17 pm »
You could try exercising it with a very low frequency (<1Hz) squarewave with DC offset so it doesn't go negative, aqpplied throug a suitable resistor to keep the deflection on-scale.  If you place the movement face down when exercising it for a while, then repeat face up, you *may* get lucky and dislodge some dirt from the bearings.   However the odds aren't good, as unless you see signs of contamination in the housing, a damaged point on the pivot that runs in the bearing cup or a brinneled or fractured cup surface due to mechanical shock is far more likely.
 

Online Shock

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Re: Meter movement sticking.
« Reply #5 on: November 22, 2017, 05:48:40 am »
If the needle is not interfering with the meter panel and the hair spring is free moving and not damaged. Then a tap easily returns the meter to either zero or to the correct reading, then it's likely a contact point at the top or bottom shaft bearing points (the supports for the shaft). Either it's worn or dirty.

To clean these I would apply a dab of contact cleaner or IPA something that will loosen dirt then manually work the needle over it's range and let the hair spring return it, repeating cleaning and moving several times if it still sticks. If you are going to lubricate, it would be the lightest of oils wiped dry to a minuscule amount. You might find though no lubrication is required as there may be enough residual lubrication left after cleaning, that is if it was even done in the first place.
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Offline floobydust

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Re: Meter movement sticking.
« Reply #6 on: November 22, 2017, 06:05:49 am »
I just encountered a panel meter sticking, I backed off the bearing clearance screw and it's better but still sticks sometimes.
While I could continue surgery- desolder the springs, take it apart and clean the bearings... I realized changing spring tension changes the full-scale calibration.
What a hassle to solder the spring back at the right length.  :bullshit: So I ordered a new meter.

Sometimes metal filings get in and stuck to the magnet - I use masking tape to get that out.
But if the bearings are damaged, I'm not fixing that.
 

Online Gyro

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Re: Meter movement sticking.
« Reply #7 on: November 22, 2017, 10:28:30 am »
I took many forces surplus meters apart in my younger years, I even managed to put some of them back together. I recently had to repace the complete pointer counterweight of an edgewise meter that had suffered a severe drop.

A few points:

1. Cleanliness - you need to get it out of the equipment and onto a large sheet of white paper on the bench, clear of all swarf
2. Light - You need to open the casing under bright illumination so you can see what you're doing (and anything that drops out)
3. Magnification - A binocular microscope eg armscope is ideal, a decent magnifier will do, preferably stand mounted.

It's more likely that something has dropped into the magnetic gap and is fouling the movement than it being the bearings - they are normally a loose fit, just retaining the bearing points for minimum stiction - there should be tiny amount of 'wobble'.

Don't try lubricating it - there isn't really an oil fine enough and it will clog or spread, as I said, the bearing points are very loose in the jewels compared with something like a watch. If it gets on the hairsprings it will be a bugger to get off.

Don't use solvents either - quite a lot of the moving coil assembly components are only held together with varnish of one type or another, it's even used for final balancing of the movement.

The reason for the sticking will probably be visible as you move the needle (preferably with a power source and a pot). Check that the needle base isn't catching the hairspring or dial edge, also that it is still parallel with the scale - likewise the couterweights (rear and side ones) - they can become bent out of line due to mechanical shocks.

Check that the turns of the hairsprings haven't become crossed - again, that can also happen with severe shocks.

Look down the magnetic gap as the moving coil rotates - if it's a transparent casining then you should get good contrast from the white paper. It might be a small piece of magnetic swarf or even a flaked bit of varnish. It should be easy to remove using... very fine magnet wire, a very thin sliver of adhesive tape, or if necessary, a fine steel needle (magnetic swarf).

The tinyest interference (fabric fibre) at the end of the needle can cause it to stick, but as you get close in to the coil and bearing axis the torque goes up and the source of interference easier to see.

Static buildup on the window can cause the meter to be attracted to a particular area and even cause it to stick. The easiest way to dissipate it is to wash the front of the casing (inside and out under running water) shake and blow it dry, you can pick up water droplets with the corner of a tissue but never wipe it dry or you will cause more static.

It would be really helpful to have a couple of photos so we can see what we're dealing with.


EDIT: If the behaviour changes depending on whether the meter is horizontal or vertical, that can be a big clue too.
          Check the current sensitivity too - a 50uA meter has a lot less restoring force than a 1mA one.
« Last Edit: November 22, 2017, 10:42:27 am by Gyro »
Chris

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Offline Vgkid

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Re: Meter movement sticking.
« Reply #8 on: November 23, 2017, 01:14:09 am »
I appreciate all of the tips/hints. Unfortunately I won't be able to look into the meter until next week.
I will keep you updated. I might have a chance to play with it in other positions as well.
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Offline tecman

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Re: Meter movement sticking.
« Reply #9 on: November 24, 2017, 03:23:50 am »
The most common reason for a meter sticking is the presence of some fine metal flakes in the magnetic poles and the moving coil.  Removing them can be a challenge.  The "old pro" method is to use a cotton Q-Tip.  Pull the end threads out to make a fine string-like extension.  Get the end in the gap around the coil and try to "sweep up" any flakes you can see.  The cotton is soft enough to not bend anything, but can grab fine metal flakes.

paul
 

Offline Vgkid

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Re: Meter movement sticking.
« Reply #10 on: November 25, 2017, 05:58:05 am »
Had a real quick play with it. With the face vertical: when moving slowly it will stick in a few places, faster, it might stick in one area. When really fast , no sticking.
When horizontal , I couldn't get it to stick.
I wont really work on it , until next week.
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