Bushing Clock Plates
I must admit, I had trouble starting this article. There are so many ways to approach this subject. When does a bearing hole need rebushing? What tolerance is allowed, -i.e., what can you get away with, without rebushing? I don't want to get bogged down with gearing theory but we have to understand that, any matched pair of clock gears will have a set, centre distance that is correct. When we wind the clock, a source of power is traveling through the train of gears. This power or torque forces the clocks pivots onto one side of it's bearing hole at all times. Over the years the oil can dry up and dust can enter the pivot holes, this causes wear on one side of the bearing wall. This wear has the effect of spreading the pivot centers of the gears apart. When the wear spreads the pivot centre to a certain point, the teeth of the wheels and pinions start to experience more and more friction until the clock stops. This leads to the often heard quote from the customer," I am sure there is nothing much wrong with the clock, it was working fine until I wound it up!!" The amount of wear that a clock will take before coming to a stop, is hard to put into words, its a touchy feely sort of thing and can only be learned from experience. It also varies with different makes of clocks. When bushing your own clocks, remember that it has to have a certain amount of freedom for the gears to turn, it cannot have a running in phase, it will stop! Most amateur clock repairers and especially model engineers make the bearings too tight, resulting in clocks that refuse to work. There is an old (ancient) saying in the trade, if it rattles it will run. I will come back to this later.
At this point I have to make an apology to anyone who has a bushing machine, I have never used one and do all my bushing by hand. Some of the clocks that I have repaired over the years were worth more than my home, down to the humble Smiths striking clock, all bushed by hand!
First, take all power from the clock -i.e. let the mainsprings down or if it is weight driven take the weights off the pulleys, don't forget that a fusee clock still has power until the barrel click is released under controlled conditions.
I never, ever, clean a clock movement until all repairs are done, we are not forensic detectives but sometimes a part may be missing or damaged and can leave a trace of it's outline behind which can be the making of a successful repair or not!
With the movement on the bench, place a thumb on the barrel wheel or great wheel. Force the train of gears in the direction that they would normally go. Next, reverse the pressure on the wheel train and watch for movement or a jumping motion of the pivots. This will show how much wear is present in certain bearings of the clock plate. This is only a trial run but it will give a good idea of how much
the estimate of the clock repair will be. At this point I will strip the clock down and give a rough clean of the pivots (with a clean cloth) and peg out the bearing holes.
Re assemble one train of gears at a time and repeat the earlier test. Any pivots that show a large movement should be circled with a pencil! Do the test again and this time when the pivot moves take a note of where it goes!!! This place is where the original bearing hole used to be. Mark the spot with the pencil and strip the clock down again. File or (broach over) the hole so that the wear is the same
as the other side using the pencil mark as a guide. Use a cutting broach to open the hole for the new bearing. Always work from the inside of the clock plate when broaching and check constantly that the broach is at 90deg to the clock plate. This is easily done by turning the wrist when broaching.
E.g.- turn the broach, is the broach at 90 deg to the plate, rotate the plate 90deg and check again, is the broach at 90deg. Correct
any slight leaning as you go until the hole will just allow the front end of the bearing to enter. I know this sounds complicated but the human eye has a good idea what a 90deg angle is, after all, we are surrounded by them. Turn the plate over and form a shallow but steep counter sink on the front of the pivot hole. Place the clock plate face down onto a flat anvil or block of steel and insert the bush. Using a hammer with a
polished head or use a flat faced punch, rivet the bush into place. The bush has to be longer than the plate thickness so that the metal spreads into the counter sink formed earlier. Open the hole a little with a broach and then form the oil sink on the front plate. Broach the pivot hole using the same technique as earlier, until the pivot enters the hole. Check for binding by eye,
gently place the pivot into its hole and see if it will lean more one way than another. If it shows a reluctance to stand straight in one direction or will not come up to 90deg of the plate, then ease the broach in the correct direction until it does! These situations still surprise me, its amazing how little metal has to be removed to achieve a successful result. The next test is to place the wheel into the clock plates and
fasten all pillar pins or nuts. Spin the wheel and at the same time give the clock a good shake. If the wheel stops suddenly, check which side of the clock plates the arbor is,
e.g. a tight bearing will hold the pivot. This bearing will have to have a small amount removed with the broach. What we are looking for is a wheel that spins and
gradually coasts to a stop. Undo the plates and install the next wheel and check for smooth transfer of power from one gear to the next. Place a finger on the arbor of the (driven) wheel and you should be able to feel the action of the gears. If the gearing has a nice silky feel to it, then proceed to the next bearing. In the unlikely event that the gearing is not as it should be,
e.g., feels like there is grit in it, try to see
if the wheel is meshing too deep or too shallow. Remove the new bush and as before file the hole in the direction needed and broach a small amount to make the hole round again, install a new bush of a slightly larger diameter. Repeat the above procedure. This is the way I do any bushing but, many clock repairers have their own thoughts on the subject. Once you have mastered the basics you can go on to develop a technique that
is your own.
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