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accuracy of a grandfather clock

Improving the time-keeping.

A grandfather clock has a 'seconds' pendulum, which means that it takes exactly one second to swing from one side to the other, and one second to swing back again.

Because of this, a thirty-tooth 'scape wheel (Image 1) is normally used and that allows 8 day grandfather clocks to have a 'seconds' hand. Almost all 8 day clocks have one but it's rare to seen a seconds hand on a 30 hour version as there are generally three not four wheels in the train and so it would rotate backwards. That said, some 30 hour movements have an extra wheel in the train to correct this (see Image 2 ).

The pendulum is one of the most accurate mechanical timekeeping devices ever invented and even after 300 years, a clock with a seconds pendulum can still be made to run accurately to less than one minute a week.

The seconds pendulum was called a 'Royal' pendulum after King Charles II who was a great clock enthusiast . It measures 39.139921 inches (almost one metre) from just below the bottom of the top block to the centre of the bob at the lower end. But don't immediately reach for your ruler; timekeeping is all about trial and error and patience, not linear measurement.

With most spring driven clocks, accuracy can vary over the course of a week as the spring gradually weakens. But a weight-driven clock provides a consistent power supply so I suggest daily checks on your newly installed grandfather clock. Try checking at the same time every day and adjust it as follows until it's right.

At the bottom of the pendulum rod, just under the heavy bob, you'll find a brass nut - the rating nut (Image 3). If it's the original, it'll probably be square like this one. Screwing this to the right will raise the position of the bob on its rod and so effectively reduce the length of the pendulum's arc of swing.

A shorter pendulum will make the clock run faster so if it's losing time, try tightening the rating nut a little. Start with one full turn of the nut and then check the clock again for accuracy the following day.

A longer pendulum will cause the clock to run slower because this will increase the length of the arc of swing. So if it's gaining time, try undoing the rating nut a little to lower the bob and then leave it a day before checking its accuracy and adjusting it again. Think: LOWER means sLOWER.

Always make sure that when you unscrew the rating nut the pendulum bob does actually drop fractionally. Many bobs are made of lead and sometimes the hole through them can be quite tight on the pendulum rod like this one in Image 4. If it doesn't drop on its own, hold the rod firmly above the bob with one hand and pull the bob downwards gently until it's resting on the rating nut. If you tug it without holding the rod, you risk snapping the fragile steel suspension (Image 5) fitted to the top and the pendulum will crash to the bottom of the case.

If you make your adjustments at about the same time each day (every 24 hours) and keep notes about which way and how much you've turned the rating nut each time, you will soon be able to make fine adjustments to reach an accuracy of one minute a week or better.

You might not get better than that because pendulum rods are made of steel so tend to expand and contract with the temperature. But you can simply move the minute hand with your finger each week while you're winding it, to keep it spot on.

Well, that's the final part in this six-part series on setting up your grandfather clock. If you get stuck, call me or email me. If I've helped, please leave an independent review on Google HERE or just 'Like' me on Facebook below.


TIME KEEPING

  • A 30 tooth escape wheel cut for a recoil escapement
  • A 30hr longcase movement with an extended 'scape wheel arbor to mount a seconds hand
  • The rating nut suppoting the brass pendulum bob
  • This pendulum bob is tight on its rod
  • The suspension found at the top of the pendulum, sometimes called the 'feather' is quite fragile