Expert: Martin Meyer - 1/28/2010

Question
We have an 18th century English grandfather clock that runs fast (over an hour per day) even with the pendulum bob set at the very bottom.  The screw on the pendulum can't go any further down without coming off.  I tried adding a light magnetic weight to the end of the pendulum above the bob to try to slow it down, but that hasn't helped.

Is there anything I can do short of having a new pendulum built?

Answer
Hi Tex,

The question I always ask myself first is, does this look like it is the original pendulum? If so, it must have kept time at some point, so replacing it with a longer pendulum should not be the answer. Adding weight will not help. The weight of the bob has no direct effect on timekeeping. It just changes the center of balance which therefore changes the effective length of the pendulum. If you added weight above the bob, it effectively made the pendulum shorter, so you would make the clock run (slightly) faster, not slower.

You need to find the real cause of this problem. An hour a day is too much to be a regulating problem. Check where the suspension spring at the top of the pendulum is hooked onto the post, to make sure that it is all the way down on the post. Look closely at the suspension spring for signs that it may not be original. They break easily, and it could have been replaced with a too-short suspension spring. Finally, it is possible that the escapement is out of adjustment, so that every once in a while, the verge lets several escape wheel teeth past it at once, instead of one at a time. Watch the escape wheel movement and listen to the tick. If several teeth are slipping by at once, the escapement needs to be adjusted. This is a job best done by a professional clock repairer.