Servicing A National PW-D Micrometer Dial and PW Gear Drive

A couple of weeks ago, I posted about some reduction drives I had acquired, including a classic National micrometer dial and PW gear drive, in nice condition. The PW drive has an output shaft that runs parallel to the front panel. Conversely, the drive in which the output shaft effectively runs straight through is called the NPW. I believe that PW stands for “Precision Worm” (EDIT – probably not – see Dave W9BRD’s comments underneath). The PW drive is the original design, and simpler than its counterpart, having only 2 gears – a worm gear and a main split gear, split for the purpose of counteracting backlash. This particular unit came with a quality variable capacitor attached to it which my capacitance meter measured as 245pF, but which I think was a nominal 225pF unit, based on a look at the 1947 National Radio Products Catalog. The dial and drive turned smoothly and worked well. On closer inspection, one of the ceramic insulators in the variable capacitor was cracked and while I was hoping this wouldn’t adversely affect it’s operation, the crack went all the way through and did indeed affect the physical stability of the unit. Nevertheless, the dial and drive were in great condition and were worth it for the price I paid. I will keep an eye open for a stator from a similar unit to replace this one with the cracked insulator. In the meantime, even though the dial and drive were operating smoothly, I couldn’t help wondering if they could be coaxed into operating even more smoothly. I had never owned one of these before, so didn’t know exactly what to expect from it. It was smooth, except for a slight jerkiness when attempting to make very small adjustments to the tuning with the dial starting from a stationary position. It’s picky, I know, but I was looking for an excuse to teach myself how to take it apart, lubricate, and reassemble it. Here’s the condition I received it in – On removing the 4 screws that secured the top of the drive to the main body, the top came off very easily, to reveal a main split gear coated in old yellow/orange grease. Unfortunately, I didn’t take a picture before removing the grease, but although there must have been a thin film on the pieces of metal that came into contact with each other (it did, after all, still operate smoothly), much of the grease had been pushed to the side of the gear and was sitting there doing nothing, having hardened somewhat over the years. The gentleman who sold it to me gave me the advice, “If it ain’t broke, don’t fix it!” but I wanted to clean and re-lubricate it, so I could be sure it was good to go for a few more decades. The other reason was that I wanted to understand how it worked – and not in a “book” way, by looking at diagrams, but by actually taking it apart and seeing how all the parts fit, and work, together. Here’s a rather rough sketch of the PW drive. Hopefully, even my poor drawing will give you an idea of how it works –   There is a great deal of information on servicing these gear boxes, and the National micrometer dial, on this page on the Western Historic Radio Museum website. Look a little over halfway down the page, under the heading “Lubrication and Assembly Of The PW Gear Drive” and underneath that, the section titled “PW-D Micrometer Dial”. This whole page is packed with very useful information on National HRO Receivers, and their restoration. Henry WA7YBS runs the Western Historic Radio Museum website. He ran the “bricks and mortar” Western Historic Radio Museum in Virginia City, NV from 1994 to 2012 and although the museum is no longer, the website is a very valuable source of information about vintage radios. It is a treasure trove. Henry kindly gave me permission to use National’s cut-away photo of this gear drive that I found on his website. It’s a lot more helpful than my drawing –

The National PW Gearbox, before about 1945, when the housing was made of cast metal. Photo courtesy of Charles Hentsch and Henry WA7YBS at http://www.radioblvd.com

I won’t go into a lot of written detail on how to disassemble, lubricate, and reassemble the drive and micrometer dial, as I learned it all from the National HRO page on Henry’s site. I would simply be repeating what is written there. With a gearbox and dial that is already operating fairly smoothly, it is possible to simply work a little extra grease into the parts to add to what is already there. I could have easily done that but once I get started with these things, I like to take them to completion.  I began by spraying WD-40 on the main gear and using a toothbrush to remove the old grease, but I ended up completely removing the worm gear, washers, spring and ball race from the housing, and removing every last trace of the old grease with a combination of an old toothbrush, WD-40 and lots of soapy water. Might as well start afresh. Here’s the partially disassembled gear drive, cleaned of all the old grease. The worm gear is no longer engaged with the main split gear. If it were, the 2 sets of gears in this photo would be much more closely aligned. I have also removed the variable capacitor from the insulated output shaft – I can be quite thick about some things. I knew what backlash was, and also knew that (obviously) an anti-backlash gear was some kind of arrangement that virtually eliminated it. However, I didn’t understand how it worked until I opened this gear box up and saw how it all fitted together for myself. This is why I recommend that you take things apart, play with them, and look at them. That way, you’ll understand how they work and will be able to fix them when they go wrong – Here’s the gear housing, elliptic bearing hub, and worm gear with the washers, spring and ball race assembled on it. Oh, and the end plate. I don’t know what it’s called, so I’m going to call it an end plate – A close-up of the worm gear assembly – The elliptic bearing hub is not actually elliptic. It is just that the hole through which the shaft of the worm gear passes, is off-center to the hub, so that the inner number dial on the micrometer dial is driven in an eccentric fashion. You can see that here – In this shot you can see the conical thrust bearing that is part of the casting of the gearbox in pre-1945 models – A couple more gratuitous shots. I am very taken with the types of mechanics and engineering you find in old radios – and this is in such good condition. It’s hard to believe that it’s around 70 years old. In these next 2 shots, you can see one of the springs that tensions the 2 sides of the split gear, and gives it the anti-backlash action – Henry indicates on his site that he uses Lubriplate 130-A as his main light grease. I was having trouble finding anyone who would sell me a single 14oz container (the smallest available). They all wanted to sell me a 6-pack, whether it was a bricks and mortar supplier, or an online one. Various outlets promised to be back in stock of the individual containers in a few weeks, but I wanted my grease sooner than that. A bit of online research indicated that Mobil 1 synthetic grease was a good (and newer) replacement and was in stock at a local auto parts store. Score! I have since found several restorers of vintage radios who use this grease, a fact which gave me extra confidence. A small pipette/syringe of the type used to administer medicines to cats and dogs, as well as a toothpick, and the judicious use of fingers, proved useful in applying it – Grease was applied (sparingly) to every point where there was metal on metal moving contact.  It was applied to the conical thrust bearing, the worm gear, the inside of the elliptic bearing hub, and the outside of the elliptic hub, where the micrometer dial would make contact. With a toothpick and a great deal of care, I also managed to get grease inside the ball race which of course, is very important. After applying it to the main gear, I put the washers, spring and ball race back on the worm gear, and reassembled the unit, setting the anti-backlash gears to about 1 1/2 teeth of offset, rotated the main gear through several revolutions, and cleaned the excess grease from the sides of the main gear. Once that grease has been pushed off to the side – it’s never coming back. Here’s what the main gear looked like after the excess was cleaned off – Next, the micrometer dial was disassembled and fully cleaned, with the usual generous amounts of WD-40 for degreasing, and lots of soapy water – I really enjoy taking photos of these beautiful pieces of engineering.  I hope it shows how taken I am with them. You’ll notice that the 2 springs that hold the (inner) number dial to the (outer) index dial have a slightly longer “loop” at one end than the other.  The springs fit best when the sides with the longer loop end are connected to the (inner) number dial. You’ll find that out by doing it. The 3 machine screws are for fixing the knob to the index dial. Close-up showing the springs that hold the index dial to the number dial – Sparingly grease the inside of the number dial only, where the toothed edge is. There is no need to grease the index dial. When reassembling, you want to line up the 2 dials so that the machine screws are aligned with the corresponding holes in the number dial, so that you can then screw the knob back on. When in this position, the number 250 should be displaying in the top number slot. You should be able to position the number dial so that 250 is showing. You can rock it back and forth and feel it gently click into place. The number 250 needs to be perfectly centered in the top window before proceeding. If it’s not centered or doesn’t look right in any way, try again. I actually had to remove and reattach the springs before I could get the  number dial to gently but firmly click into place and show the number 250 centrally in the top window. This is what the dial will look like from the back. Notice how the 3 machine screws are aligned with the holes. This only happens when the dial is indicating half-scale (250) – Now, rotate the tuning shaft on the gear box so that the main split gear is set halfway. There are end stops on the main gearbox that make contact with 2 lugs cast into the lid to prevent the main gear from turning more than 180 degrees. In the following photo (which also appeared earlier in this post), you can see the 2 end stops on the left-hand side of the main gear wheel. This is the correct position for when the dial indicates half-scale (250) – For comparison, this picture of an un-greased gearbox shows the gear set to one extreme of travel. Notice one of the end stops at the top of the gear wheel (on the right of the gear) – and notice the 2 “lugs” on the underside of the lid that make contact with the end stops – Slip the micrometer dial onto the shaft and the elliptic hub. It should slip on fairly easily. If it doesn’t, something is wrong. Do not proceed until the micrometer dial easily slips onto the elliptic hub. I fastened the dial onto the shaft with the set screw(s) in the knob, turned it back and forth a number of times and then, because I am particular about these things, disassembled it again and removed all excess grease with my finger. This is what it looked like before reassembly – The fully serviced dial and gearbox operate very smoothly, and should continue doing so for many years. Even better, I now know how to do this. I can pick up more dials and gear drives like this in the future, confident in the knowledge that if they are not operating properly, I can get them to work again. They are very elegantly engineered, with a small number of parts working together to make a very smooth tuning mechanism. They really don’t make ’em like they used to. Oh – and another quick plug for Radio Blvd.  It really is a great site for fans of vintage radios and their restoration. If you enjoy, and get use out of the site, I encourage you to make a small donation to help keep it online. There are links on the site.

The cleaned and lubricated National micrometer dial and PW gear drive, waiting for a replacement variable capacitor unit (spot the errant cat hair on the front of the tuning knob!)