Weight-Driven Clock Drop Time Calculator
Calculate how long a weight-driven clock runs between windings.
Enter cord drop length and gear train ratio to get the run time in hours and days.
Weight-Driven Clock Run Time
A weight-driven clock runs as long as the weight has further to fall. Two things set the run time:
- Drop distance: how far the weight can travel before it lands or the cord runs out
- Descent rate: how fast the going train pays the cord off the barrel
The pendulum controls the descent rate, not the weight. That is worth saying plainly, because it explains something that surprises people: hanging a heavier weight does not make the clock run for a shorter time, and it does not make it run fast. A heavier weight only supplies more driving force to overcome friction. If the clock keeps stopping, more weight can cure it, and the run time per wind stays exactly the same.
The formula: Run hours = Weight drop (inches) × Pulley factor × hours per inch of cord
The pulley multiplies rather than divides. A single movable pulley doubles the cord that has to pay out for a given fall of the weight, so the same case height buys twice the run.
Measured rate beats any table. Wind the clock fully, mark the cord or the weight position, come back in 24 hours and measure how far it dropped. That one measurement tells you more than any lookup, because hours-per-inch depends on the barrel diameter and the going train of your specific movement, not on the duration stamped on the dial.
Typical rates, hours per inch of cord paid out (single-strung):
| Movement | hr per inch of cord | Sanity check |
|---|---|---|
| 30-hour longcase or 1-day cuckoo | 0.4-0.6 | 58 in fall, no pulley, runs ~30 hr |
| 8-day cuckoo (chain, no pulley) | 2.5-3.2 | 65 in fall, no pulley, runs ~8 days |
| 8-day longcase or Vienna (on pulleys) | 1.6-2.0 | 55 in fall, 2× pulley, runs ~8 days |
| Month-going longcase (on pulleys) | 6-7 | 55 in fall, 2× pulley, runs ~30 days |
| Year clock (on pulleys) | 85-95 | 50 in fall, 2× pulley, runs ~12 months |
Notice that the sanity-check column always reproduces the duration the movement is named for. If a rate table ever tells you a 30-hour movement runs for four days, the table is wrong, not the clock.
A worked example. A longcase 8-day, weights on pulleys, with 55 inches of clear fall inside the case:
55 in × 2 × 1.75 hr/in = 192 hours, which is 8.0 days
The weight itself descends 55 ÷ 8 = about 6.9 inches per day, so the cord pays off the barrel at roughly 13.8 inches per day.
Practical points:
- Wind fully to the top before you count the drop, and stop an inch short of the seatboard rather than crashing the pulley into it
- Most movements are set up to stop with an inch or two of cord still on the barrel, so subtract that from the clear fall
- A chain-driven cuckoo is limited by chain length, not case height, and the chain ends are usually stopped with a ring to keep them from running through the sprocket
If your clock stops well short of its expected duration, the run-time arithmetic is rarely the problem. Check that the weight really was wound to the top, that the cord is not fouling the case side or the pendulum, and that the click ratchet is not slipping back a little on each wind.
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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