THIS ARTICLE EXPLAINS THE WORKINGS OF A TYPICAL AMERICAN COUNT-WHEEL STRIKE
(click on pictures to enlarge them.)
A count wheel strike needs to do these things:
•Unlock the train at the appropriate time.
•Start the strike at the appropriate time.
•Allow the strike to happen the correct number of times.
•Lock the train at the end of the strike.
The process is triggered by a cam on the center arbor (minute wheel arbor).
The process begins with two 3-legged lever assemblies, which I call the ACTIVATING levers (red) and the REGULATING levers (blue).
The three activating levers are on the same arbor, and operate together. The same holds for the three regulating levers.
The strike train includes four wheels, plus the fan-fly. The first is the COUNT WHEEL, which has deep notches at different distances around its perimeter. It may also have shallow notches between the deep notches.
Next is the PIN WHEEL or STAR WHEEL, with projections to trip the strike hammer.
Next is the MAINTENANCE WHEEL, which carries a round MAINTENANCE CAM with a dip or notch in it.
Last is the WARNING WHEEL, which has a warning pin sticking out from one side near the rim.
It's worth noting that many American count-wheel clocks don't have a separate pin wheel to work the strike hammer. They have their trip pins directly on the Maintenance Wheel
The three levers of the activating assembly are the J-WIRE or ACTIVATING LEVER, the LIFTING LEVER, and the WARNING LEVER.
The three levers of the regulating assembly are the COUNT LEVER, the MAINTENANCE LEVER, and the LOCKING LEVER.
The J-wire is partnered with the center-shaft cam. The lifting lever is partnered with the count lever. The warning lever is partnered with the warning wheel.
The count lever is partnered with the count wheel, the maintenance lever with t he maintenance wheel, and the locking lever with the warning wheel.
When the strike is at rest,
•the J-wire is resting on the center cam.
•The count lever is in one of the deep notches of the count wheel.
•The maintenance lever is bottomed in the notch of the maintenance cam,
•putting the locking lever into the path of the warning pin on the warning wheel, preventing the train from running.
As the center shaft turns, the cam pushes the J-hook away, raising the other two activating levers. The lifting lever raises the count lever out of its notch, and also raises the maintenance lever out of its notch, and moves the locking lever away from the warning pin and putting the strike into "warning" mode The train runs a bit...enough to move the maintenance cam notch away from the maintenance lever.
The locking lever won't lock it again until the maintenance lever falls to the bottom of the notch on the maintenance cam, which won't happen until the count lever falls into a deep notch on the count wheel. Those two go together: count lever in bottom of deep notch and maintenance lever in bottom of cam notch.
As the lifting lever is raising the regulating levers and putting the train into warning, the warning lever moves into the path of the warning pin, and stops the train.
At the moment the minute hand points straight up, the cam drops off the J-hook. The activating levers drop, releasing the warning wheel and allowing the train to run.
When the count lever was released, it dropped into one of the shallow notches, preventing the maintenance lever from bottoming in the maintenance cam, and so keeping the locking lever out of the way of the warning pin.
As the maintenance cam revolves, t he side of the notch raises the maintenance lever to ride on the edge of the cam, and so lifts t he count lever away from the count wheel.
The count wheel turns a bit. The pin wheel works the hammer. Ding!
The maintenance lever dips a ways into its cam notch, dropping the count lever into the next shallow notch. Each shallow notch yields a strike.
The same thing happens again....and again.
Finally, the count lever lands in a deep notch. The maintenance lever bottoms out in the cam notch. The locking lever blocks the warning pin. The strike train stops.
The strike is done.
DEALING WITH A COMMON PROBLEM
Probably the most common problem encountered after a count-wheel movement is back together, is Failure to Stop. Once the clock starts striking, it just keeps on until the mainspring runs down. To understand the cure, let's review the end-of-strike sequence. When the strike ends, three things happen at once: (1) the count lever lands in the bottom of a deep notch; (2) the maintenance lever lands in the bottom of the maintenance-cam notch; (3) the locking lever falls into the path of the warning/locking pin, and stops the wheel from turning. The failure of (3) is the cause of Failure to Stop. When the two levers fall into their notches, the warning pin MISSES the locking lever, so the wheel keeps turning. And with it, the strike train keeps running. And the two levers rise back out of their notches, and another strike happens; and so on.
The cure is to adjust the warning wheel so that the warning pin DOESN'T miss the locking lever. Here is the procedure.
First, when you assemble the clock make sure that the maintenance wheel is adjusted so that the count lever is in a deep notch, and the maintenance lever is in the bottom of the maintenance cam. That takes care of (1) and (2). Then, try to adjust the warning wheel so that the warning pin is about 1/2 turn away from the locking lever. The reason is that the warning wheel is spinning very fast, so it needs some "lead distance" to make sure the locking lever is down before the warning pin gets to it.
This is touchy, and can be difficult to achieve when assembling the movement. Instead, after it's together RESTRAIN THE MAINSPRINGS. Rotate wheels so that the two levers are in their notches and keep them there. Loosen the pillar nuts just enough to allow you to spread the plates enough to pull the warning wheel out of mesh, turn it so the pin is in the proper position, put it back into mesh, and tighten the pillar nuts. If the mainspring isn't restrained, when you pull the wheel out of mesh the train will buzzsaw, and you will have a Situation.
Then tighten the strike mainspring inside the restraint, enough to allow the train to run a bit. Run the train and see if it stops. If it doesn't, let the mainspring back down into the restraint, and do the adjustment again. Sometimes it will take two or three adjustments to get it right.
NO LOCKING LEVER?
Some count wheel movements don't have a locking lever to halt the strike. Instead, locking is done by the maintenance lever and the maintenance cam. When the count lever isn't in a deep notch, the maintenance lever rides high enough not to get captured by the notch in the maintenance cam. When the count lever falls into a deep notch, the maintenance lever falls far enough into the notch in the maintenance cam to halt the maintenance wheel and stopping the train. This simplifies things, since you don't have to synchronize the warning pin with a locking lever.
As long as the count wheel and maintenance wheel are in synch, the train will lock when the count lever hits a deep notch.
Bob Crosswell adds:
"There is one part that is a bit problematic about the depth of the lever in the cam slot. Generally the count lever is a lot longer than the cam lever. If the count lever blade bottoms in the count wheel and the cam lever bottoms, or is deep in the cam slot, the clock will probably not strike at all. The reason being that a "normal" lift height of the count lever blade will cause a much smaller lift of the cam lever which would be insufficient to allow the train to unlock. The specification that I have read for New Haven clocks (I believe in one of Connover's books but I could be mistaken) is that when the count lever drops, the cam lever, being round, should be just a bit more than 1/2 the diameter of the cam follower below the top edge of the slot to ensure positive lock. Too deep and it won't release. New Haven isn't the only maker to use this system. A generic instruction would be that the cam lever should rest deep enough to ensure a positive lock. Some of these can be quite cranky and somewhat critical between being deep enough to lock and shallow enough to unlock. Any rounding off of the edge of the slot where the cam follower locks makes it even more difficult."
Good luck, mate.