Expert: Martin Meyer - 10/4/2008

Question
I bought my first wall clock from eBay - but it is missing the pendulum. How do I work out which type of pendulum it needs, I am not sure how to work out the length and weight etc. Thanks, Jim

Answer
Hi Jim,

This is actually a rather complicated question. The length of the pendulum is what determines the rate of the clock and, if you don't have the original pendulum, determining the correct length can be a trial-and-error process.

A few modern clock movements have their pendulum length indicated on the back of the movement. These are primarily Kieninger and Hermle movements. The pendulum length indicated on these clocks is usually the tip-to-tip length of the pendulum, not including the leader. Clock supply houses may stock complete pendulums of the correct length for these clocks. Other modern movements, and nearly all antique movements, have no indication of pendulum length, and you will probably have to make up a pendulum yourself.

If you are working with a wood stick pendulum, the job will be much easier than with a metal rod or lyre pendulum. First, you need to determine what type of top hook will fit the pendulum leader on your clock. Numerous types are available from clock supply houses. If you are making up the pendulum from scratch, you will also need a regulating unit (the threaded rod and nut at the bottom of the pendulum), a bob (disk), and a wood pendulum stick, longer than you think you will need.

The weight of the bob is not critical. It is the diameter which is more important. Every movement has a "swing" dimension. If you use a bob which is too large, it may hit the sides of the case if the swing dimension is too great.

To determine the correct length, start with a stick which will just miss the bottom of the case. Attach the top piece, but not the regulating unit. Then slide the bob onto the stick and attach it temporarily with masking tape. Now test run the clock. If it runs slow, raise the bob or shorten the stick, and test run it again. In this way, you can "zero in" on the correct pendulum length. Once this is determined, you can cut the stick to its final length and attach the regulating unit.  http://www.clockmasteronline.com