THE RACK STRIKE
There are two common ways for a clock to determine how many strikes to produce at each hour. One is with a count wheel; the other is with a rack and snail. The count wheel will be left for another day.
The main mechanical bits of every rack-striking clock are:
•a saw-toothed rack, with a tail
•a nautilus-shaped snail, with 12 steps, that turns with the hour hand.
•a rack hook, to support the rack
•a gathering pallet, to engage the teeth of the rack
These elements look different in different clocks, but they all perform the same functions.
The Basic Story: At rest, the rack is resting on the rack hook, and nothing is happening. As the minute hand nears 12:00, a cam on the minute hand shaft moves the rack hook from under the rack, the rack falls until its tail lands on one of the steps of the snail.
This exposes a number of rack teeth to the gathering pallet. The strike train runs, and turns the gathering pallet (GP). Each revolution of the GP gathers and raises a tooth on the rack. As each tooth is gathered, a strike is produced at the chime. After the last tooth, the rack hook falls under the rack, and the train stops. For example, if the rack tail landed on the "9:00" step of the snail, 9 teeth will be exposed, and the clock will strike 9 times.
That's the basic story. Now for the details.
The lifting cam on the minute hand shaft raises a lever —the lifting lever— which in turn raises the rack hook. The lifting lever has a projection — the "warning stub" or warning detent— that sticks in through a hole in the plate. The rack hook has a similar projection —the "locking stub" or locking detent—that sticks in past the plate.
The function of these two stubs is to govern the start and stop of the strike mechanism, by means of the warning wheel.
The warning wheel is the last wheel in the train before the fly, and sits near the front plate. It has a pin (the warning pin) sticking out toward the plate. At rest, the train is prevented from running by the warning pin resting against the locking stub of the rack hook.
When the center cam raises the lifting lever, the rack hook's locking stub is raised out of the way of the warning pin, allowing the train to run. But it only runs a half-turn, because the pin bumps into the warning stud (on the lifting lever) that has moved up into its path. That's the warning run. It sets up the strike train to begin running at a precise moment.
The instant the lifting lever drops off the lifting cam, the warning stub drops out of the way of the warning pin, and the strike train runs.
So long as the GP is gathering teeth, it hold the rack hook up so the locking stub stays out of the way of the warning pin. After the last tooth is gathered, the rack hook falls beneath the rack, the locking stub falls into the path of the warning pin, and the train stops.
As the GP turns, the gearing in the train operates a "star wheel" whose projections lift the strike hammer and drop it, allowing it to hit the chime.
Rack Strike Chiming Clocks
A three-train chiming clock works a bit differently. The lifting cam on the minute hand shaft doesn't directly operate the strike train. Instead, it operates the chime train. The strike is operated by a "long lever" reaching from the chime side to the strike side.
As the clock is chiming the hour, the long lever raises the rack hook, dropping the rack and putting the strike into warning. When the hour chime ends, the long lever is dropped, and the strike train runs.
Yet another variation occurs with "repeating" clocks, such as some Vienna regulators. A repeating clock, or "blind man's clock" has a lever which, when pulled, releases the strike train to repeat the most recent strike.
In order for that to happen, the snail can't be constantly turning with the hour hand. It has to stay in one place throughout the hour, then advance to the next step.
The snail of a repeating clock is on a separate post, backed up by a 12-point star wheel. A gear on the minute hand shaft drives an intermediate wheel that has a projecting finger. The intermediate wheel turns once and hour. At the end of the hour, the finger trips one point of the star wheel, advancing the snail to the next step.
A three-train repeater, such as a Vienna grand sonnerie, combines both of the above: a star-wheel snail for the repeat, and a "long lever" for the chime / strike. control.
The Gathering Pallet
This simple looking item performs a complex job. As it is lifting a tooth on the rack, it is also holding the rack hook away from the rack, to allow for the lift. As that lift is completed, it lowers the rack hook back against the rack to prevent the rack from falling again. When the last tooth has been gathered, it lowers the rack hook all the way, beneath the rack, putting the locking stub in the path of the warning pin and stopping the train.
The architecture can vary from clock to clock, but a good example is the familiar "bean cam" found on cuckoos, Hermles, and others.
The bean cam is a lopsided oval, with a bulge in one place and a dent in another. Jutting out from the rack hook is a projection resting on the bean cam. As the GP is turning, and its pin is lifting a rack tooth, the bulge on the cam moves the rack hook away from the rack. As that lift is completed, the dent on the cam lowers the rack hook back against the rack. During this operation, the rack hook never drops far enough to put the locking stub in the way of the warning pin.
When the last tooth is gathered, and the dent in the cam lowers the rack hook, it goes all the way under the rack, the locking stub does its job, and the train stops.
If the bean cam gets out of adjustment, the strike can't work properly. Typically, the train will continue to strike after the last tooth is gathered, until the spring runs down or the weight hits the floor.
Things are properly adjusted when...
1. When the rack hook is fully dropped beneath the rack,
2. The warning pin is resting against the locking stub; and
3. The projection on the rack hook is nestled in the dent of the bean cam.
That means the rack hook is synchronized with the warning wheel.
The cam is a pressed fit on its arbor. It can be adjusted either by twisting it on the arbor, or by prying it off and repositioning it with its dent against the rack hook pin. Do this while the warning pin is against the locking stub, and it should be good to go.