The next step was to make the hardened steel outer races to replace the prototype brass ones. This, strangely enough was when it all started to go wrong again. The dimension given in the magazine article for the inner diameter of the outer race was 11/16” which translates to about 17.2mm. I had made the prototypes to this size. Somehow when making the steel versions, I just skimmed a little more off the diameter than intended, and the inner diameter ended up at 17.6mm. I thought to myself ‘that can’t make too much difference’. I case hardened the races and assembled the clock and found of course that the balance sat a bit lower than it should have, but again it didn’t seem enough to make a difference. In operation however, I found that the balance would swing for a while then the balance pivots would force the balls apart and the balance would drop down and jam on the cross piece below the balance (which is part of the magnetic circuit). At this point I started to feel despondent and put the clock aside for about 12 months until fortuitously, another Eureka came in for repair.
I now took the opportunity to run a micrometer all over this unmolested Eureka and came back to that 0.4mm difference in diameter of the outer races. I couldn’t believe that this would be a problem, but I had run out of ideas and decided to make a new pair, paying attention to getting the size exactly right. With the new races turned up and case hardened, the clock was again assembled and at last ran reliably without the balance dropping down between the balls.
At this point, you’d think I’d be getting close, but there was still some time to go. I still had poor amplitude, and so turned my attention to the winding of the coil. The insulation on the outer layer was constantly flaking off, so I wondered whether there may be the odd shorted turn. Only one remedy – rewind the coil. The appropriate wire gauge was purchased and the coil rewound. Unfortunately this made no difference. Again, a little despondent I took the cowards way out and increased the voltage from 1.5v to 3.0 volts, and achieved reasonable amplitude, convincing myself that somehow this was OK.
I then let the clock run for a few weeks to ensure that the old “drop between the balls” problem didn’t recur, and the amplitude stayed reasonable. After that time, I was confident that the clock was now reliable, had reasonable amplitude, but was gaining about 15 minutes a week. Now, attention was paid to regulating the clock. Eureka clocks have a conventional regulator with movable curb pins applied to the hairspring. Using this regulator seemed to make little difference, so I used the balance screws to regulate the clock. I could get the rate close to reasonable, but it was still a little erratic. So again, I ran it for a few weeks, still not quite satisfied, but convincing myself (yet again) that this was normal, because I have heard that Eureka clocks have a reputation for not being particularly good timekeepers.
And now, for the final piece of the puzzle.
I had the clock running in my TV room, for want of a better place to put it. It was late at night, the TV was off and I was reading a book. I slowly became aware of a regular clicking noise, clearly coming from the Eureka, and a click which shouldn’t occur at the time in the swing of the balance that I could hear it. I got out a torch and put my strong glasses on and carefully peered at the clock, and quickly found the source of the click. As mentioned before, the balance wheel bearing consists of two balls which run in an outer race, with the balance arbor sitting in the ‘V’ formed between the balls. To stop the balls rolling around to the top of the bearing (say when you’re moving the clock), there are two pins, one above each ball, to keep them in position. The click came from the balls hitting these pins. After hitting the pin, the residual amplitude of the balance was happening with the balance arbor sliding across the balls, rather than rolling the balls around. For some reason, the amplitude had increased since the clock was last assembled and was being limited by these bearing locating pins. I can’t explain why, but I was now able to reduce the voltage back down to 1.5v, yielding amplitude of close enough to 360 degrees; just small enough that the pins weren’t bumped by the balls at the peak of the swing. With the amplitude now limited by ‘natural causes’ rather than a frictional limit of the limit pins, the capacity of the regulator built into the clock to set the rate returned, and within a couple of days, I had the rate within 10-20 seconds a day, and reasonably stable.
I can’t explain why the amplitude increased over several weeks (or looking at it differently, the required running voltage reduced), but it did happen. I then made a voltage regulator to reduce the voltage from a surplus Nokia phone charger to 1.5v, with a switch and a battery holder to give the owner the option of running the clock from a ‘D’ cell.
So the moral of the story if you’re bringing a heavily modified back to original, the key things to remember are:
- Make sure you keep to the exact dimensions of parts you have to make
- Be patient
- Make sure your bearings aren’t bumping up against the limit pins in the bearing assembly
- If you’re doing it for someone else, make sure they’re patient as well.