After repairing numerous Schatz and Kundo standard size 400 day clocks recently, I have noticed that sometimes the optimum suspension fork location is different than that shown in the “Horolovar 400 Day Clock Repair Guide”.
Specifically, for Kundo units such as 1 and 3C, I often find that the fork should be slightly higher. For Schatz units such as 6, 7, 8 or 9, sometimes the fork should be slightly lower.
Assuming that the escapement is properly set up (and it is best to leave the escapement adjustments alone if they have never been changed), and assuming that the clock is “in beat”, the escape arc of the pendulum (the minimum arc or amount of rotation at which the escapement will escape at both ends of the pendulum swing ) should be 1/2 turn.
To increase the escape arc, raise the fork. To decrease the escape arc, lower the fork. (Note: other adjustments will affect the escape arc. For example, bending the anchor pin back toward the suspension spring will increase the escape arc, and vice versa. But I do not recommend bending the anchor pin unless it is clearly already bent incorrectly, as it is fragile and you might break it.)
Note: take the suspension unit out of the clock to raise or lower the fork!
After I assemble a Schatz or Kundo standard 400 day clock movement I wind the mainspring up 1-1/2 turns for initial testing. Then assemble the suspension unit and install it in the clock. Put it approximately in beat. Then give the pendulum 1 turn of motion and time 8 pendulum swings with a stopwatch (it should take 1 minute). Adjust the regulator and retest until 8 swings takes 1 minute (plus or minus 1 tenth of a second). (If necessary, the spring is thinned or a different one installed.) Using the stopwatch allows you to make sure the regulator has enough range to bring the clock to time and give you a good starting point for final regulation.
Next, put the clock in beat and check the escape arc to see if it is close to 1/2 turn. If not, raise the fork to increase the escape arc, or lower it to decrease escape arc. After each adjustment, check the beat again. Finally, run the clock for several hours to make sure it has sufficient power (the mainspring is wound up 1-1/2 turns at this point).
Finally, wind the mainspring fully, let the clock run for an hour as the pendulum arc increases, then make sure the escapement action is correct. Sometimes an escape wheel tooth will fall onto the impulse surface of a pallet immediately after dropping from the opposite pallet (instead of the proper action of landing on the locking surface). This can happen if the fork is too low on the suspension spring or if the locking of the escapement is too shallow. I recommend that you study the “Adjustment of the 400 Day Clock Escapement” section of the “Horolovar 400 Day Clock Repair Guide” for excellent drawings and explanations of the escapement action.
Notes on my experience with escape arc greater or less than 1/2 turn
Several Kundo standard clocks that I repaired recently had only about a 1/3 turn escape arc with the fork in the standard position. They ran fine with the mainspring wound up only 1-1/2 turns. With the spring fully wound, however, the action of the fork was so “floppy” that it rebounded after each impulse, allowing an escape wheel tooth to land on an impulse face of the pallet instead of the locking surface. This caused the pendulum motion to not be as great as it should be. After raising the fork to make the escape arc 1/2 turn, the clock would run well with the mainspring fully wound, as well as wound up only 1-1/2 turns.
Similarly, several Schatz standard clocks had a large escape arc of about 2/3 turn with the standard fork setting. They would not stay running with the mainspring wound up 1-1/2 turns, and did not have much overswing when fully wound. After lowering the fork to give a 1/2 turn escape arc, they ran well both fully wound and wound only 1-1/2 turns.
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