A Bluffers Guide to replacing a balance staff and impulse jewel.

Perhaps the trickiest part of replacing a balance staff is getting the original balance assembly to pieces without damaging the hairspring, the balance wheel or the impulse jewel, however to explain that would take a as long as this post so I am starting with that done and with a balance staff available, again that may be a non trivial exercise as some movements have quite a few types of staff for a given movement and the one has to be found (and paid for @ £15 - £30 or sometimes a lot more!) - or made.

Click any image for a larger view.
The balance for this S16 movement is 0.6" / 1.5 cm
in diameter excluding the screws, some balances
are considerably smaller!

The first thing to do it to fit the staff to the balance wheel, that might need to be done in a number of ways but usually it will be a press (friction) fit or riveted.

This South Bend Watch Co balance is a press fit, the wheel is placed onto a stake (anvil) which has a hole in it just large enough for the staff to pass through, too large a hole and there is a risk of deforming the balance arms. Too small and your expensive staff gets stuck in the stake.

The staff is pressed into the hole using a punch with a hole drilled into it just large enough to accommodate most of the staff but small enough to rest against a step on the staff to press it in.

Frequently, as in this case, the hole in the balance it a tad to big for the staff, in this case after closing up the hole as much as possible with a specially designed punch it was still not a tight fit (due to the hard metal) and so it was necessary to use some Loctite retaining compound (not thread locker) which although quite strong can be release if necessary.

The part finished  assembly is then put onto the callipers which have a pair of jewels for the pivots to run in.

An index (bottom left) is moved close too the edge of the wheel and the wheel spun to check that it is square to the staff and flat.

Usually some manipulation is required to true it up, which can be tricky, especially with a cut compensation balance as an adjustment at the fixed end may require the arm to be twisted to ensure that everything lines up. As it does here.

It is best then to try the balance in the movement to ensure that it turns smoothly with minimal resistance. Particularly if a previously repairer has made adjustments to compensate for a wearing staff, adjustment will frequently be required to get the correct "end float", firstly by undoing all of his adjustments and then preferably by adjusting the position of the balance hole and cap jewels. This process can take as long as all of the other steps put together.

It is now time to fit the roller, in this case however the impulse jewel was crumbling and although it would have worked failure would not have been far off so it had to be replaced. This is a very fiddly job and invariably results in the loss of some quite expensive  jewels which are small and difficult handle and also to measure so trial and error is required.

In this case the jewel was cylindrical but with one side flattened. The full diameter was just 0.41mm (they come in steps 0.01mm big). The vice being used is a happy co-incidence of a hobby and business / hobby and is normally used for tying fishing flies, the jaws stay parallel in normal use, it gives a firm grip and  it can be rotated as required. The down side is that one of this quality would be rather too expensive to buy just for fitting impulse jewels!

After getting the jewel into the hole it has to be secured in place ensuring it is at right angles to the roller in both plains, traditionally this was done with heated shellac on the reverse side but modern equivalents are somewhat easier to use.

The roller it then pressed onto the balance staff using the staking set taking great care not to break the jewel or the staff either by getting things misaligned or by having a punch of the wrong size.

The impulse jewel should normally be at right angles to the arms (of a 2 arm) wheel, if not then positional errors may result. The red marks on the wheel were put there to indicate where the impulse jewel (and the hair-spring stud)  originally aligned in the hope that they were then correct and the watch will be in beat on completion.

At this point it is advisable to again put the balance into the movement to check that the impulse jewel engages correctly with the lever pallet and that they do not rub together, and if it is a single roller, that the pallet safety pin engages correctly.
This had a two piece double roller so next to go on was the smaller section.

A critical part of this operation is to ensure that the cut out in the small roller EXACTLY lines up with the impulse jewel, if it does not problems lie ahead!

After fitting the balance is again trial fitted to the movement to check that the pallet safety pin engages correctly with the roller.

Almost there now!

The next operation is to fit the hairspring, the balance is turned over, usually with a smaller stake to support it.

With a double roller this is relatively straight forward (if there is a spacer between the two sections to support the smaller section) but with a single roller the stake must not foul the impulse jewel, this particular staking set has a stake available with a cut out in it for the jewel to fit in, with others it is necessary to find a stake (or punch used as a stake if the set allows that) with a drilled hole large enough to accommodate the staff but an outside diameter small enough to fit inside the jewel. Get this wrong and the jewel breaks, the roller will have to come off and a new impulse jewel found and fitted.

Making sure it is aligned correctly so that the movement will be in-beat, hopefully using marks made earlier, the spring collet is pressed onto the staff.  It is now ready for fitting and testing.