Holy Grail Attained – A National HRO NPW Gear Drive and Dial in FB Condition

Firstly, I must apologize for posting so many pictures of these National HRO gear drives and dials. Once I decided that a building a regen with one of these dials and drives as the centerpiece was a goal, it became something of a preoccupation to find both a dial and drive that were just right. Luckily, I have now found that perfect combination, and can promise that this will be the last post on the subject 🙂

An idea that has been brewing in the back of my head was to write a post on the subject of dials and reduction drives. I have purchased and looked at a number of different types over the last year or so, found a few to be slightly lacking, and others to be eminently suitable, for the purpose of tuning a home-built radio receiver. For a number of reasons (that I may go into in a future post one day), I consider the National HRO micrometer dial and gear drive to be one of the best solutions. Luckily, there are still quite a few of them knocking around, for the builder who is willing to spend a little time looking for just the right one.

When I saw this particular drive on eBay, I knew I had to have it, at almost any price (within reason). Firstly, the gear drive was in nice physical condition. Also, it had a feature I had not seen before. Although I have been aware of these drives since my teenage years, I have only been paying close attention to them in the last few months. Perhaps this is not that unusual, but it stood out to me – there was an extra feature on the front of this drive, in the form of a shaft rotation limiter. It is an ingenious mechanical device that will not allow the dial to rotate more than 10 turns in either direction. The rignt-angle drives have end-stops built into the inside of the top cap of the gear box, but the standard straight-through models have no such rotational limits.  The variable capacitor I have been planning to use with this next project does not have end stops, so to have one built-in to the drive would be great. The fact that this drive looked to be in really good condition as well (judging by the photos online) elevated it to “must have” status.

A week or two later, this little beauty was in my hands. These are the “before” pictures, so if you look at these, make sure to stick with me to the end of the post to see how things turned out. Having said that, it looked pretty darned good from the get-go though. Although you can’t really see how the shaft rotation limiter works, you can see it’s component parts that are stacked over each other on the tuning shaft, in between the metal end plate and the cast metal gear box –

On removing the top cap, I was expecting to see a moderate amount of partially dried grease on the various component parts of this drive. What I found instead was a large amount of grease, applied in generous dollops. I wonder if there had been a special directive to apply a lot of grease in order to protect this batch against an extreme climate, ot perhaps it was just down to the whimsy of the worker who had been applying the grease to these gearboxes?

Look at all that grease! You can’t see it. but there were some very large dollops underneath the gears, in the interior of the gearbox –

Before dousing the gearbox with WD-40 and then dish soap, I removed as much of the grease as possible with a toothpick, before partially disassembling it. Then I went to town on the gearbox and all the parts, with generous amounts of WD-40, a toothbrush, then dish soap and the toothbrush, followed by nice long soaks in warm dish-soapy water (and the toothbrush again). Then came a good rinse in non-soapy water, and plenty of attention from a hairdryer on the hot setting to dry it all out.

Look at the squeaky-clean result. It just amazed me, looking at this, to realize that it was something like 70 years old, and it has plenty of life left in it still –

All the parts were now clean, dry, and ready for reassembly. It is not my intent to give detailed how-to instructions here. If you read my previous posts about these drives, as well as the other supporting information online that I have linked to in the previous posts, you’ll know enough to figure things out – especially once you’ve looked at one in real life.  Although these straight-through gearboxes have more parts than the right-angle drives, they are easier to figure out. In the following shot, you can see the slip-on washers of the shaft rotation limiter that fit over the tuning shaft. Each washer has a tab with a small protrusion that prevents it from slipping over the collar next to it. There are 11 of these tabbed washers in all –

The plastic lid under the loose parts once held fruit salad from Genova Delicatessen. If you visit Oakland, CA and like East coast style deli’s, Genova’s on Telegraph and 51st is a must!

Remember to set the anti-backlash setting on the gear that is spring-loaded, before inserting the tuning spindle into the gear box. Do not over-tension it, or there will be too much friction when tuning. All you need is just enough tension to counter the backlash and no more. The sprung gear only needs to be offset by one, or two cog-teeth at the most.

Next came the lubrication stage. There needs to be grease in every place where there is metal moving over metal. The shaft rotation limiter with it’s 11 tabbed washers added a fair bit of drag, even when coated with a thin film of grease, so I squibbed a small amount of turbine oil in between each washer. While doing this, I was wondering about mixing turbine oil in with the synthetic grease. So far, it has seemed to work fine. If there are any problems in the future, it would not be hard to remove just the front plate and the washers, so they can be cleaned and re-lubricated. At this point, I am almost looking for excuses to do things like that!

This is such a good-looking gear drive. I almost can’t believe that it is something like 70 years old.  If you look closely at the 4 screws (of which you can see 3 in the following picture) holding the eccentric hub flange to the front plate, and compare with the very first picture in this blog, you’ll see that I have carefully filed them down. I did this in order to maximize clearance with the back of the micrometer dial. I am planning on using a 4mm front panel in my next regen, and need all the clearance I can get –

The dial that came with this particular drive was in good shape. However, I already had one that was in even nicer condition. This particular combination of dial and drive are the best in my small collection. They have made the cut, and with the 20:1 reduction ratio and nice heavy dial, will make a grand main tuning dial for a homebrew receiver –

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