The pallet staff or pallet arbor, has the important role of supporting the pallet assembly and lever in a lever escapement. They run primarily in jewelled holes, except for in the cheapest of clocks and watches, and in the best clocks, often have an end stone supporting the lower pivot.
The presence of an endstone requires that the pivot is shaped differently, adding further complication to the staff, but this is to be ignored for the remainder of this post. Pivot design is covered in this separate post.
Pallet staffs are either press fit; as used in modern clocks and watches due to ease of manufacture. Or they are a screw fit; as is more common in older lever escapements.
Press fit pallet staffs are a simple cylindrical shape with a pivot at each end. The cylinder is often imperceptibly tapered to aid fitting.
A press fit, also called an interference fit, fastens two components by friction. In the case of a lever escapement, the forces involved in the daily running of the clock are low enough, that the fit needn’t be tight. In many cases, replacement parts are not made to strict measurements but are made to fit.
In mass production however, parts must be made to strict tolerances. Calculating the tolerance of an interference fit can be complicated; it is dependant on material, temperature at time of manufacture, operating temperature etc. but for the purposes of this basic example, we are looking at a shaft oversized in the range of about 0.01 – 0.005mm.
The press fit staff is typically fitted using a jewelling press, where micrometer adjustment can specify the depth to which it is pressed in, and therefore the height of the lever in reference to the escape wheel and balance. Some press fit models do have a shoulder for ease of assembly, in this case, the height cannot be adjusted.
The Screw fit type of pallet staff is a far more complicated design, as there needs to be shoulder, up to which the lever is screwed on. The thread must be cut all the way up to the shoulder, and of course be fitted correctly to the thread in the pallet frame. The shoulder determines the height at which the lever sits, and cannot be adjusted simply by pressing it up or down the staff.
As for the thread pitch etc used for this type of pallet staff, it clearly varies from clock to clock, and in these two examples, is very different.
I don’t know how a repairer is supposed to define the thread in order to cut a replacement, at this scale, thread gauges wouldn’t work. It is not a job I have had to do yet, but my approach would be to turn the outer diameter to match the original, then to pass it through the nearest available die. When the soft arbor is then screwed into the hardened pallet frame, the thread will be rubbed into the correct form. I would then proceed to harden and temper the pallet staff.
If you can provide any insight into how a repairer would define such a small thread, it would be greatly appreciated for you to add your thoughts to the comments below.