Is your mechanical clock not running as well as it should? Does it stop intermittently or not run at all? There may be a number of issues with the movement and among them pivot wear.
Pivots are the ends of the axles (the horological term is “arbours”) that spin in small holes drilled in the clock plates as the clock runs. They are reduced or turned down end of an arbour. These, along with the holes themselves, can become worn. The pivot hole must be perfectly round and the pivots must have a mirror-like polished surface in order to minimize friction within the train of gears. The two surfaces must be protected and the barrier between the pivot and the pivot hole is clock oil.
A worn pivot or worn pivot hole causes the gear to slowly move away from the pinion and it will eventually stop when the gears no longer mesh properly. A clock that is in need of bushings runs erratically or stops altogether.
If a clock movement is not serviced (cleaned and oiled) routinely there will be wear in the plates of the movement where the pivots come through. If new oil is applied over the old il it will free up abrasive dirt and keep grinding away at the steel and the brass bushing hole. Worn pivots are typically found in clocks that have been oiled over and over again and not properly cleaned. The term “properly cleaned” implies disassembly of the movement, cleaning the parts, addressing wear issues, reassembly, and testing.
Occasionally you will see punch marks made to close the pivot hole. Though not an accepted practice today this was a common method of repair employed by past clock repairers
Pivots must be cleaned and polished periodically to ensure they can turn freely within the hole in the clock movement plate. A worn pivot hole is easy to observe as they are oval-shaped instead of round as you can see in the next photo.
The process of bushing consists of replacing the worn brass around the pivot so that the hole is round again. A new hole is drilled into the plate. A new, properly sized, the bushing is punched or pushed into the plate using a bushing machine such as a Bergeon Bushing Machine.
Some clock-makers prefer to hand bush using reamers and smoothing broaches and the results are entirely acceptable but a machine simplifies the task.
Badly worn pivots which are made of steel must be replaced with new pivot. This is called re-pivoting in clock circles. A watch or clock lathe is used for re-pivoting which involves drilling into the end of the wheel arbour with a high-speed bit and installing a new pivot made from pivot wire.
Minor wear is tolerable and expected over the life of a clock and can be addressed by careful filing, polishing, and burnishing.
Bushing is an integral part of movement servicing. Some clocks that have been well-cared for may have minimal wear and may not require new bushings while others, through neglect or improper servicing, may require many bushings.
Bushing a clock is one of the most fundamental tasks of the clock-maker. Pivot and bushing work performed correctly will extend the life of a clock movement while ensuring that a clock runs reliably for years.