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How a Balance Staff is Replaced

I have done a few posts on changing out a balance staff before, but I may as well do another one as some of these photos came out pretty well. Elgin staffs are riveted to the balance wheel. The pivots on the staff are hardened steel, and by far the most fragile part of the movement. Replacing the staff is a very common repair.

Here is the staff with the broken pivots. The roller table, shown, has been removed as has the hairspring.
The old staff is removed by cutting away the hub from the lower side using the lathe.
Here the hub has been cut down, leaving the shaft of the staff and a very thin bit left against the arms of the balance wheel.

The steel of the staff is hardened, so a carbide graver is used. The key is that the graver must be sharp - really sharp. The cutting edges are generally re-dressed for every use, but this will have to be another subject another day.
With the old staff's hub very close to cut away, but not quite, the staking tool is used to pop off the remaining bit and free the balance wheel.

The left over part of the hub is like a tiny washer.  Note that the hub that is cut away is the lower side. The upper side, opposite, of the wheel's arms is where the rivet is. Cutting the hub side is the only completely safe way to do this without risk to the balance wheel.

The balance is riveted on, and tested for turning freely in the watch.

Riveting is a two step process, first using a round face hollow stake, and then a flat face for the finish.
The wheel should turn very freely.
The double roller is friction fit again using the staking set.

This roller is a one piece double roller. Single rollers are just a disc (roller table) without the extra guard, visible in this photo situated above the roller table.

Some double rollers are two pieces. These are trickier to install since the upper part must be perfectly aligned with the jewel pin.
With the hairspring installed, this balance assembly is ready to go.

The rotation of the hairspring is important. The stud location needs to cause the roller jewel to free dead natural, in the middle of the pallet fork. When this is off center, the balance will turn more one way than the other. This is wasteful of power, and causes irregular rates and can even stop the watch.


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