You have to replace the mainspring barrels in Hermle clocks after about 10 or 15 years of running because the bearings are insubstantial. The instance on the right shows a black goo made of metal particles and grease around the arbor.
It used to be cost-effective to replace with new barrels, but nowadays it is more cost-effective to repair them with a bushing, which can be done quickly and easily as shown in this article. While the thin brass of the barrel is often worn, the hole in the steel cover is usually OK.
The first step in bushing the barrel is to ream out the worn hole. I do this quickly and easily with a 3/8" reamer. You can ream with a lathe, but it's quicker to use a drill-press, as shown here. The reamed hole should come out centered, but if it doesn't, don't worry -- centering comes later.
Make the shank about .001" oversize the 3/8" hole reamed in the barrel, and make the shoulder about .062" deep. The button will be press-fit into the 3/8 " hole in the barrel.
I made this one with my Taig lathe, which has a rear-mounted cut-off tool.
Next, I use a small arbor press to press the "button" into the hole in the barrel. The button must fit tightly into the barrel, and an arbor press makes the job easy. Support the inside of the barrel with a block or piece of tubing.
If you don't have an arbor press, you can use a hammer cushioned with a block of wood or a length of dowel.
If the bushing is a little loose, don't be afraid to use some soft solder. These aren't barrels from a classic clock.
Finally, it's a matter of putting a centered hole in the "button" to make it into a bushing. I chuck the barrel in the outside jaws of my Enco 9x20 lathe to center, drill and ream the center hole.
The drill is about .010" or so under the the size of the final hole. This drill has also been prepared to drill brass by properly stoning flats on the cutting edges. Stoning flats keeps the drill from grabbing.
It is important to chuck the barrel by its teeth than by the cylindrical barrel brass, since in some barrels this part is not concentric to the teeth.
Finally, I use a home-made D-reamer to ream the center hole to the proper size. (D-reamers are used in most commercial bushing sets. Check your set if you need an example.)
The reamer is made of drill rod. First, it is cut to a cylinder which is .002" over the size of the Hermle barrel arbor. Then it is tapered with a file to make the cutting edges, and finally the flat is ground to a thickness just half of the diameter of the cylinder.
Finally, the reamer is heated to cherry red, quenched, and tempered to straw.
It's worth the time to make the reamer if you will have repeated need to overhaul Hermle barrels. The reamer makes the whole operation easy. I bushed these three barrels in about 45 minutes, and that included the time needed to take pictures.
Usually, the mainsprings are set at this stage of wear. It's still more time- and cost-effective to bush the barrels and put in new springs than to buy new barrels. The re-bushed, re-sprung barrel is better than a new one, because the brass bearing is so much larger.
It's also wise to file and burnish the barrel arbor pivots as you would any other pivot, since these are often pretty rough and have no doubt contributed to barrel wear.
While the subject here is Hermle barrels, no doubt similar techniques can be used on other makes and even antique barrels.
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