Calibrating a typical American spring-driven mechanical clock

In our quest to have our antique mechanical clocks run accurately the immediate response is to regulate the clock, but have you thought about calibrating your mechanical clock? It is not as difficult as it sounds.

This is not the same as regulating your clock. A properly regulated Anerican-made spring-driven clock will show the correct time at the beginning of the week but will run fast through the week and may gain as much as 3 or 4 minutes mid-week and lose time at the end of the week.

Seth Thomas round top
Seth Thomas spring driven round top 8-day clock

Let’s use a spring-driven mantel clock with an 8-day cycle for our example.

Mainsprings release their peak power at the beginning of their cycle. As the mainspring winds down power is gradually released until the spring unwinds completely and the clock stops.

Gilbert spring driven wall clock

On some antique clocks, one might find “stop works” (otherwise called a Geneva stop) which is a clever star-shaped brass add-on to the main wheel that reduces the full release of power initially by flattening the mainspring’s power curve over its rated cycle (8-days) and thus maintain some level of accuracy through the week.

Geneva stops as indicated by the white arrows

But most clocks I have come across don’t have this ingenious device.

Weight-driven clocks are a different kettle of fish because the release of power is constant throughout the week. Once a weight-driven clock is regulated it should not require calibration.

Gustav Becker Vienna Regulator with weights

Calibration makes the assumption that your spring-driven clock will never accurately tell the time at any one given point in its cycle and essentially means setting your clock so that it loses no more than a couple of minutes at any given time through the week.

Sessions mainsprings, one for the time train and the other for the strike train

According to the Canadian Oxford dictionary to calibrate means “to correlate readings of an instrument with a standard”. If the standard is plus or minus two minutes per week, without the use of “stops” or other means to flatten the power curve, setting the clock two minutes slow at the beginning of the week will ensure that it is never off by more than a minute or two through the week.

According to noted horologist Robert H. Croswell, “If the clock is regulated such that it has a zero net gain or loss of time from the start to the end of the week, then take ½ the maximum fast error during the week and set the clock that many minutes “slow” when the clock is wound.” If the maximum is 6 minutes, then, half would be three minutes.

One could use a complex mathematical formula to determine the precise amount of time to set the clock at the beginning of the week but setting it two minutes slow for a clock that loses 4 minutes each week should suffice for most purposes.

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