The clock has run for several months, now. And I've made some changes as the result of testing. For one thing, I found that the clock ran much better without the bottom balance bearing -- running in the top bearing alone the balance seems to float, so I've decided to keep it that way.
The clock runs on about a pound of weight, falling only about 11 inches in a weeks run. Unpowered, the balance requires about 4 minutes to decline in motion by one rotation, showing that the "Q" of this balance is quite high.
I had concerns about the balance running in only one little jewel. A bump could cause sideways forces which would either dislocate the jewel from its setting, or crack it. As a remedy, I made a couple of design changes.
To prevent dislocation, I designed a "retainer" which is just a washer which screws down on the top of the jewel in its setting. It fastens with the same screws which secure the setting. The central hole of the retainer is made just slightly smaller than the outside diameter of the jewel, so it it holds the jewel firmly in place.
I made the retainer by first preparing to drill its mounting holes, which must match the mounting holes in the jewel setting. So I put the jewel setting in a chuck mounted on the Sherline rotary table in the little mill. I adjusted the mill to center the drill over a hole in the setting. Thus, the holes in the retainer will mirror the holes in the setting. I don't touch the mill axis wheels after this procedure.
Next, I chucked a piece of 3/8" brass in the Taig lathe, and drilled the center hole. Then, I unscrewed the chuck from the lathe without removing the brass and transferred the chuck to the Sherline rotary table, in a routine which must now be familiar. Then I drilled the first hole and then rotated the table 180 degrees to make the second hole. I returned the chuck and brass to the lathe, where I parted off the retainer.
I replaced the bottom bearing with a simple device which will limit the sideways motion of the balance in case of a bump. It's merely a cylinder with a 1/8" hole drilled through it. The bottom pivot of the balance staff slips through it. When the balance is turning, the balance staff runs clear of the sides of the hole, but bottom of the balance can be displaced sideways no more than 1/16", should the balance be bumped. This will protect the jeweled bearing and also prevent disruption of the clocks running. The cylinder is threaded and the top is flanged so it can be mounted to the bottom horizontal plate.
The motion-limiter is made from a piece of 1/2" hexagonal brass rod. I put the rod in the lathe and cut a 1/4" cylinder on it, which I then threaded with 1/4" fine threads. I drilled a 1/8" hole through it so the balance staff pivot can pass through. I made a nut of the same material, which will secure the motion-limiter to the bottom horizontal plate.
I decided to make a new bottom horizontal bridge, which was simpler than modifying the existing one. It's made of 1/16" brass. The hole that the threaded motion limiter fits through is made oversize, which allows adjustment of the limiter so that it can be exactly centered around the balance staff pivot.
The threads on the motion limiter will serve a dual purpose. I'll also use the threads to screw on a little device which will secure the balance for travel. I'll make this device at a later time.
Today's links:
Retainer and jewel Retainer and jewel together New plate and limiter New plate assembled (top) New plate assembled (bottom) In place on clock (below) In place on clock (above) Plate and limiter GIF drawing Dial drawing, GIF