You remember how I mentioned that the impulse jewel extends downwards from beneath the balance wheel? Well that is true, but it is actually attached to small stepped, cylinder known as the safety roller, which is affixed to the balance staff beneath the balance wheel.
The Roller is a single piece of metal, but has two parts, or ‘areas’. The first is the table, which is a wide, flat bit that presses up against the shelf of the balance staff to ensure its correct placement in the watch, and the other part is the can, which is a narrower, taller piece that would be perfectly cylindrical were it not for a crescent moon shaped recess cut out of one side.
The impulse Jewel is friction fitted (or sometimes glued) to the table and extends downwards in this crescent-shaped recess, which we refer to as the notch OR the passing hollow (I prefer the latter as this avoids confusion with the notch in the horns of the pallet fork).
The passing hollow/notch is very important to the safe and consistent functionality of the escapement. Without it, the escapement could mislock and jam up, stopping the watch completely.
Thanks to its influence, combined with that of the guard pin (a thin, pointed piece of metal attached to the underside of the pallet fork’s horns) the escapement can operate without fear of mislocking.
The purpose of the passing hollow/notch, is to allow the guard pin to pass from one side to the other during the moment of impulse (also known as the lift).
An unwanted phenomenon known as OVER BANKING can be prevented by the guard pin in conjunction with the safety roller during the supplementary arc, and the extremities of the fork (horns) when the guard pin is adjacent to the passing hollow of the safety roller.
Horn shake is the distance travelled when the lever moves accidentally from its position against a banking pin, to the point where the horn comes into contact with the impulse pin. This forms part of the safety action when the impulse jewel is about to enter the notch of the lever. Horn shake should be approximately half of total lock.
If horn shake is excessive then the escapement could be unsafe. The escapement could unlock at the critical time when the impulse jewel is about to enter the notch should the watch receive a shock at this point.
If the horn shake is excessive, the banking pins should be closed on a watch with adjustable banking pins. On a watch with fixed banking, the pallet bridge should be checked and, if necessary, replaced. In exceptional cases, the lever itself may be worn on the sides and might need replacing to correct the problem.
The Horn shake must be greater than the guard pin shake so that the impulse jewel does not hit the fork as it is trying to enter the notch.
Some watches have adjustable banking pins, responsible for restricting the LATERAL MOVEMENT OF THE PALLET LEVER. Most modern watches have fixed banking, which is part of a pallet bridge. These can be adjusted, but it is a risky job as it requires the watchmaker to physically and permanently alter the piece in question. To increase banking the pallet bridge can be filed away, and to reduce it, the underside of the pallet bridge can be peened with a peening hammer so that the metal is stretched and flared to close the gap within which the pallet lever can travel. Of course, this modified pallet bridge will almost certainly need some further finishing to make smooth and even the fixed banking. It’s a tough and tricky process and shouldn’t be attempted by anyone without significant skill or experience and, preferable, an electroplating machine that can be used to cover up the horrible mess you’ll no doubt make of the pallet bridge’s original aesthetics.
These are the steps, listed in the correct order, that one should follow in order to adjust an escapement with banking pins.
1. Division: of escape wheel/pallets and balance/safety roller.
2. Hornshake: can be adjusted by moving banking pins. Should be equal on both sides.
3. Guard pin: can be shortened or lengthened, stoned It must be less than Hornshake.
4. Depth of lock: can be adjusted by moving the pallet stones with consideration to the run to banking. The lock and run to banking should be equal on both sides.
5. Safety action
The draw is the angle of the pallet stones that encourages the escape wheel tooth to slide down the locking face when under impulse of the mainspring until the lever of the pallet makes contact with the banking pin and is held clear of the oscillator/balance so that the fork and notch of the pallet are positioned correctly to receive the impulse jewel. This prevents mislocking. This is achieved by the angle of the locking face of the pallet from the centre line of the escape wheel, normally 14 degrees (13-16 degrees).
The clearance angle the pallet travels after impulse, caused by the angle of the pallet stone locking face, to ensure the correct position of the fork to receive the impulse jewel.