In this post I am going to go over the steps to replace a balance staff in a Hamilton 992b pocket watch.
The Balance staff is the center shaft on the Balance wheel. It has top and bottom “pivots” very thin points that the balance wheel rides on and are supported by the balance jewels. The staff pivots can be broken from a drop or hard bump and are the most common problem with an old pocket watch. Unfortunately these older watches were not shock resistant at all, so a drop to a hard surface is going to break a staff just about everytime.
Hamilton 992b balance staff and Hairspring in perfect condition.
Pictured below is a broken staff on the left and a good staff on the right.
First step is to remove the hairspring, roller table and then the broken staff.
When you remove the hairspring and roller table, you need to carefully mark their position on the balance wheel so that when you re-install them, they go back in the precise position so the watch will stay in beat.
We will be using a Staking set to replace the staff. First you find a punch that fits the bottom portion of the staff. The pointed punch shown in the picture is used to “zero” the punches so that they line up perfectly for the following steps.
The staff on a 992b is “friction fit” so it is inserted from the bottom. This is a very precise fit, close does not count here!
Now you carefully tap the staff in place so that it is an exact fit, must fit perfectly onto the bottom portion of the balance.
Once the staff is set, you add the roller table.
Next you need to check to see if it is “poised”. Poise is the term used to see if the weight on the balance wheel is distributed evenly over all of he screws so that there is no “heavy spot” on the balance. Think of it like balancing the tires on your car. If you have a heavy spot, your care wont track properly. Same goes on a watch, if there is a heavy spot, then the watch will not run accurately. Being out of poise, only affects the timekeeping in the stem positions. When the balance is lying dial up or dial down, the wheel is only riding on one pivot and the weight is not a factor. However, when it is in the stem positions, the wheel is riding on both pivots and if there is a heavy spot, then the stems will not run the same. You will get stem up fast, then stem down slow. Not a good situation in any watch.
To poise the balance, you use a poising table and balance the wheel on its pivots, while the wheel swings, the heavy screw will move towards the bottom. Now you carefully file off a “tiny” amount of weight from this screw and check it again. This process may take many try’s, not uncommon to spend the better part of an hour on just this task.
Then add the hairspring.
Hopefully you now have a working balance, back in beat and running properly. The next time you say that your antique pocket watch “only” needs a new balance staff, you now have a better idea of the time, and skill needed to accomplish this task.