The balance staff is the axle on which the balance wheel is mounted. They come in several designs, but for the standard carriage clock is of the type seen above.
A balance staff whose length is perhaps the same as your little finger-nail, will have on average 12 critical dimensions, require perfect concentricity throughout, and must have perfectly shaped and finished pivots.
The critical dimensions are:
- Overall length
- Lower pivot diameter
- Lower pivot length (one third of which must be parallel)
- Lower pivot to roller seat
- Roller seat diameter (which must be a gentle taper)
- Lower pivot to balance seat
- Balance seat diameter
- Balance seat height
- Balance rivet height and undercut
- Hairspring collet diameter
- Top pivot diameter
- Top pivot length
Between the roller seat and balance seat is the largest diameter, highest at the balance seat and tapering smaller towards the roller seat. This is the thirteenth critical dimension, but is not quite critical enough to make the list. This taper is often highly polished and adds decorative appeal to the staff, however the reason it is there is to provide clearance for the little dollop of shellac on the rear of the roller table which holds the impulse jewel. An obvious detail which somehow wasn’t apparent to me at first.
So why are these dimensions so critical?
The overall length of the staff must be such that it floats with barely perceivable endshake between the top and bottom endstones, but enough that it runs with absolute freedom. The pivot diameter, must of course fit the jewel holes with perfect side shake, and not a hair too much, pivot length allowing the pivot to rest upon the endstone without the cone of the pivot binding in the jewel hole. Still with me? Good.
The lower pivot to roller seat must position the roller high enough that it clears the lever and places the safety roller in line with the guard pin, and low enough that the impulse jewel enters the notch. The diameter of course must allow for a press fit of the roller onto the staff. The roller seat must the taper up to the balance seat.
The balance must sit high enough to clear the lever cock or bridge at all extremes of endshake, but low enough to leave room for the hairspring. It must fit the staff with no freedom and the rivet must protrude just enough, that when hammered lightly, it sits flush with the balance surface and hold it tight.
The hairspring collet diameter is less critical as the collet is split, but too tight a fit will distort the collet and make it hard to adjust beat, too loose will not hold the hairspring at all.
Staffs can now be easily produced to high tolerance using swiss CNC Lathes, and this is how it is done in mass production. However, in the small workshops of the clockmaker, or in independent watchmaking, they must be turned by hand.
Hand turning to these tolerances is incredibly difficult and requires a lot of practice. I am no expert as I often outsource this work due to time restraints, however it takes me roughly four hours of uninterrupted concentration to produce a well finished staff, a more experienced turner would take a third of this time.
For perfect concentricity of the staff, it should be turned between centres on the lathe or turns (using the turns, a primitive bow driven lathe, is still taught by many watchmaking schools) or should be turned in once piece, without ever removing the piece from the lathe collet.
Many prefer to turn the staff in two ‘chuckings’, or, by turning half of the staff, then turning it around in the collet and turning the other half. Whilst there are some concentricity errors with this method, the affect on timekeeping is minimal, and only noticeable in the best of clocks and watches.
They are made of steel, hardened and tempered to blue. Blue steel rod is available from suppliers and is used almost universally for staff turning. This eliminates any possible distortion from hardening after the workpiece is finished.
The pivots need to be burnished to perfect their surface, and their ends must be rounded over. The shape of the pivot end is very important to reduce friction on the endstone. To do this a special tool is used, the ‘jacot tool’. I can be seen using one in my youtube video ‘repivoting a platform escape arbor’.
As this isn’t an instructional, but merely an informative post, let’s move on to the variants of staff available.
In carriage clocks with the lever escapement, the staff is almost certainly going to be as in the above example. The most likely variation you will meet will be a balance wheel which is a press fit, rather than riveted.
Another type of balance staff you will encounter will be of the alarm clock style. These are often much simplified in design, a necessity of economics, which unfortunately is extended to their quality and function. The staff is a simple rod with ground conical pivots, the safety roller is instead a milled slot in the staff, the impulse jewel, a steel pin protruding from the balance wheel. Others may have a roller table instead of the milled groove.
Of course there are special cases, (Chronometers, Duplex escapements) but these are outside of the scope of this post.
However in watchwork especially, many variants exist.The Ronda balance staff book from 1968 lists a total of 22 staff variants. Many variations are due to the changing ways the balance wheel is mounted, roller design, or pivot shape. This too, is outside the scope of a clock based post.
This concludes my introduction to balance staff design and manufacture. I will in the future post an informative ‘how to’ turning article. If you can provide any further insight, please do so in the comments below.