HOW A WESTMINSTER CHIMING CLOCK WORKS
This is intended to provide a general explanation of how a typical 3-train chiming clock does its thing. Minor details differ from clock to clock, but major principles are all the same.
A chiming clock needs the following elements:
(1) A way to produce notes, and a way to determine which notes to produce;
(2) a way to start the process going at the right time;
(3) a way to determine how long it runs;
(4) a way to stop it; and
(5) a way to trigger the strike when the hour chime has finished.
An additional feature, not required but which is nice and which most clocks have, is
(6) a way to get the chime sequence back on track if it gets out of order
PRODUCING THE NOTES OF THE CHIME
On the back of the movement, or sometimes on the side or the bottom, is a set of hammers which rise and drop, striking chime rods and producing the musical notes of the chime. The action of the hammers is driven by a pin drum or pin barrel—a cylinder or series of disks with raised projections or pins along its width. As the pin drum rotates, the pins lift the tails of the hammers and then release them, allowing the hammers to drop. The pin drum is powered by a train of gears on the back plate, starting with the large main gear and ending with the gear on the pin drum. The main gear may drive the pin drum directly, or there may be one or two intermediate "idler" gears.
On each quarter of the hour, the pin drum rotates a certain amount. Fifteen minutes after the hour, the pin drum rotates just enough to produce the four notes of the first-quarter chime, then stops. Fifteen minutes later, half after the hour, the pin drum rotates just enough to produce the eight notes of the half-hour chime, then stops. And so on. To see what governs the starting and the stopping, turn to the front of the clock.
STARTING THE ACTION
If you are familiar with the rack strike, you understand how a cam on the center shaft works, to lift a lever and start things going. On a chiming clock, the center cam has four lobes, corresponding to the four quarters of the hour.
When the chime train is at rest, it is locked either by a detent sticking into the plates from one of the levers, blocking a "locking pin" or "warning pin" on one of the wheels, similar to the strike warning wheel & pin, or by a locking hook detaining the pin on a chime locking cam, which I call the "locking pallet" because of its resemblance to a gathering pallet. Every 15 minutes, a lobe of the center cam raises a lifting lever which disengages the detent or locking hook, and allows the train to start running.
REGULATING THE ACTION
On the front of the clock there is a rotating disk called the locking plate. It is actually a 4-lobed cam. Its circumference is divided by four notches into unequal segments: a short segment for the first quarter, a longer segment for the half, a still longer segment for the third quarter, and a longest segment for the hour. The locking plate arbor also carries the main wheel on the back of the movement that powers the pin drum. When the locking plate turns, so do the gears of the back train that turns the pin drum.
Riding on the edge of the locking plate is a cam-follower projecting out from the chime locking lever or "long lever". At rest, the cam follower sits down in one of the notches of the locking plate. The plate (and the chime train of wheels) is locked from turning by the locking detent or locking hook described above. Every 15 minutes, the center cam raises the lifting lever, which raises the long lever and locking hook to unlock the chime train. The locking plate turns until the long lever drops into the next notch, the locking hook grabs the pin on the locking pallet. and the train locks again.
At the top of the center cam's lift, the chime train is released and does a brief warning run before locking again, similar to the warning run of a strike train. During the warning run, the locking plate turns just enough that the notch is no longer below the locking lever cam follower.
As the center cam continues to turn, the lifting piece falls off the back of the center cam lobe, dropping the locking lever. The cam follower of the locking lever drops onto the edge of the locking plate (not back into the notch), and the locking plate rotates until the lever's cam follower drops into the next notch, stopping the action.
The distance the locking plate rotates determines the distance the pin drum rotates, and how many notes it produces.
STOPPING THE ACTION
Follower-in-notch is not what prevents the locking plate from turning. On some clocks, a locking wheel inside the plates has a locking pin sticking out, that is arrested by a locking detent on the locking lever through the plate —similar to the setup of a rack & snail strike. Another "warning" wheel inside the plates has the warning pin sticking out. When the locking lever is raised, the locking detent lifts out of the way of the locking pin, and the train is allowed to run for a moment until the warning pin is arrested by a second, warning detent, on the lever.
When the locking lever is dropped by the center cam, to ride on the edge of the turning locking plate, the warning detent drops out of the way of the warning pin; the locking detent is held up (by the edge of the locking plate) just out of reach of the locking pin, so the train can continue to run. When the long lever cam follower drops into the notch, the locking detent falls into the path of the locking pin. The train locks.
On Hermles and some others, the locking pin isn't on an internal wheel. It's on a disk on the front of the movement, officially called a chime locking cam, which I have labeled the "locking pallet" because of its resemblance to the strike's gathering pallet. Instead of a detent through the plate, it's halted by a locking hook which is part of long lever, which grabs the locking pallet pin and locks the train.
TRIGGERING THE STRIKE
The long lever with the locking-plate cam follower extends across the front of the movement into the territory of the strike mechanism. Its purpose is, at the proper moment, to unlock the strike train and put it into warning. This happens while the long full-hour chime is playing.
The long, full-hour segment of the locking plate has a "hump", marked H in the pictures. As the locking plate rotates, the hump raises the long lever an extra little bit...which over at the other end, raises the rack hook to unlock the strike train and put it into warning. A detent on the end of the long lever through the plate catches the warning pin, stopping the strike train.
The strike remains in warning mode until the hour chime is completed. At that point the lifting piece falls off the center cam lobe, dropping the long lever, locking the chime train, and at the same time releasing the strike warning pin. The strike takes place.
Sometimes the chime may get out of synch with the time shown by the hands of the clock. For example, it may play the quarter-chime at half-past, the half-chime at a quarter till the hour, and so on. When that happens, an auto-correction feature, which most chiming clocks have, will automatically put things back on track in very short order.
In addition to the chime mechanism described above, the auto-correcting clock has a second locking device, a "correction hook" which comes into play at the end of the 3rd quarter chime. On the back of the locking plate is a correction cam. The correction hook has a cam follower extension that rides on the correction cam. The correction cam has a notch parallel to the locking-plate notch at the end of the 3rd quarter chime segment.
As the 3rd quarter chime ends and the long lever drops and locks the train, the correction hook cam follower drops into the notch in the correction cam and the correction hook grabs the pin on the locking pallet. The chime train will remain locked at the end of the 3rd quarter chime, and won't run to play the full hour chime, until the correction hook is released by an extra-high lift.
On clocks without locking pallet and external correction hook, the correction is done by a correction hook lever between the plates, working with a correction pin on one of the wheels. When the correction cam follower drops into the notch, the correction hook grabs the correction pin. It won't let go until it's released by an extra high lift.
One of the four lobes of the center cam is slightly higher than the other three. This tall lobe corresponds to the pointing vector of the minute hand: the tall lobe points where the minute hand is pointing. Only the tall lobe can release the correction hook to play the hour chime, so the hour chime can only occur on the hour when the minute hand is pointing straight up and the tall lobe is releasing the correction hook. If the chime sequence gets out of whack, it will play the 3rd qtr chime whenever...but it won't play the 4th quarter chime until the minute hand marks the end of the hour.
ADJUSTING THE WESTMINSTER CHIME
The Westminster is in adjustment when the four descending notes of the first quarter chime have been played as the long-lever cam follower drops into the locking plate notch after the short first-quarter segment.
1. Ignoring the hammers for the moment, allow the chime train to run until it has completed the first-quarter segment on the locking plate. The cam follower is in that notch. The locking plate thinks the first-quarter chime has just been played. Leave it there and turn to the back of the clock.
2. Loosen the set-screw of the main wheel on the back of the movement, and pull it out of mesh with the other gears.
3. Manually turn the pin drum until you see the hammers fall in descending sequence, 1-2-3-4. Those have produced the notes of the first-quarter chime...just as the locking plate thinks.
4. Slide the main wheel back into mesh with the other gears, and tighten the set-screw. The Westminster chime is now in adjustment.