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Cleaning variable capacitor bearings

Jul 24th 2011, 10:19

w1rfi

Super Moderator

Joined: Apr 4th 1998, 00:00
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An ARRL member inquired of the technical staff:

How may we help you?: I have a receiving type variable capacitor that appears to be new, but has sat unused for many ( 20+ ? ) years and is now quite sticky when turned. It looks like the grease used on the shaft bearing has dried out. I need to clean and lubricate the shaft bearing and would like a suggestion on cleaners, grease and method. I have only the one capacitor, so common materials that are locally available in small quantities would be ideal. Tnx es 73
Jul 24th 2011, 11:04

w1rfi

Super Moderator

Joined: Apr 4th 1998, 00:00
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To answer the question, I first started with a search of the QST archives. To search the archives, start at the main page, click on the word QST in the menu list near the top and then on the archive. I used the search term "variable capacitors" and seeing a workable number of results, I scanned through the listing to see if anything was a good match.

To my surprise, at least with the search I tried, there doesn't appear to be an article or Hints and Kinks on this subject. So let me offer the way I would handle it.

If the bearing has contamination in it, ie dirt, rust or similar. the bearing will have to be completely flushed out, all old grease and contamination removed and the bearing repacked with fresh grease.

How to do this may depend a lot on the physical construction of the capacitor. If the bearings are in plain site and can be easily reached with a spray material, I'd try brake cleaner to flush out the bearings. Any automotive parts store will have plenty of options for brake cleaner.

Keep spraying and turning the shaft of the capacitor through its full rotation until all the old grease is removed and the rotation feels smooth and not "gritty." Observe all of the cautions on the can of brake cleaner for ventillation and wear safety goggles as the spray can splash back on your eyes.

When the old grease is all removed, you will have to repack the bearings. If you had enough access to spray the bearing, you should be able to use your finger or a cotton swab to force new grease into the bearing. Any automotive bearing or chassis grease should be fine, as would marine grease, etc. A small tube of automotive grease should be available at any auto parts store.

Keep forcing and rotating back and forth unitl the bearing is loaded with grease. A fresh cotton swab, perhaps saturated in brake cleaner, can be used to remove the excess.

If the grease is not contaminated, you can use the above procedure, but it may be possible to use a simpler procedure. What has happened with the old grease is that the less volatile components of the grease have evaporated, leaving a very sticky remainder. (In some cases, others may have tried relubricating the bearings with cooking oil, which tends to turn into a reasonable glue over time. This must be flushed as above.) I would start with a spray can of penetrating oil with a small tube attached to the nozzle. Spray a couple of drops on the bearing and rotate the capacitor. This will almost always free it up. Penetrating oil is usually slowly volatile, so you want to add a drop of motor oil or other light oil to the bearing, and again rotate. If the bearing now turns smoothly, remove the excess oil with a cotton swab and your repair should be complete.

One of the two above should work out pretty well. In some cases, you may be able to disassemble the capacitor and have complete access to the bearings, but variable caps can be tricky to reassemble, so I'd try that only as a last resort.

73,
Ed Hare, W1RFI
ARRL Lab
Technology forums moderator



Jul 27th 2011, 17:09

K8AXW

Joined: Apr 4th 1998, 00:00
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Brake cleaner will work.... WD-40 will work almost as good. What household doesn't have WD-40? After the bearing is cleaned out good, forget it. The WD-40 will have lubricated the bearing as well as cleaned it.

BTW, WD-40 is excellent for removing oil based paint and greasy grim from the hands.
Aug 1st 2011, 15:52

W1ZR

Joined: Apr 4th 1998, 00:00
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There are often two related issues, one the bearing smoothness, the other the rotor connectivity to the frame of the capacitor. In many cases, there are separate grounding springs that bear on rotor plates. Make sure that both surfaces are clean and making good contact.

In some capacitors, the rotor grounding takes place through the bearings. If that is the case, only a conductive grease should be used. There are a number of brands available in the electrical department of good hardware stores, or at electrical supply houses.
Sep 4th 2013, 17:54

WA1MBA

Joined: Apr 4th 1998, 00:00
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Although there are a number of conductive greases, most are really not very conductive. Some are not conductive at all, but are just protective greases that prevent oxygen from corroding or rusting the contact surfaces. The best conductive greases are loaded with metal, preferably with silver. Those can be purchased from Chemtronics, MG Chemicals, Cool Amp, Grainger, and a few others. Look for the words "Silver" and "Conductive Grease". Unfortunately, even small quantities of this material can set you back over $20, but silver isn't cheap, so it makes sense.

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