Expert: Martin Meyer - 7/20/2008

Question
I have had 2 pendulum shelf clocks that have had this slow running problem. Reducing the pendulum length helps but the pendulum length gets too short and the bob can not be seen. Reducing the weight of the bob helps, but only if I reduce the weight to less than 1 ounce. I have also used a stiffer suspension spring with only minor improvement.
Your recommendations would be greatly appreciated.
Mel

Answer
Hi Mel,

First, it is essential that you are winding the clock all the way, as far as it will go. Underwinding is the most common cause of erratic timekeeping.

Also, I want to exclude one other uncommon but possible cause of time loss. This is slippage of the clutch on the handshaft. If the minute hand turns extremely easily, almost effortlessly, when you set the time, the clutch may be slipping and need to be tightened or repaired.

These two issues aside, the timekeeping rate of the movement is entirely dependent on the pendulum length. Escapement errors can cause fast running, but not slow running. Bearing wear or gear problems can cause erratic running and stopping, but not consistent slow running.

Using a stronger suspension spring can cause faster running, but only slightly. Changing the pendulum weight has no direct effect on the movement's rate; what is actually happening is that the lighter bob makes the EFFECTIVE length of the pendulum shorter, by raising the center of mass of the pendulum.

The gearing of every movement is designed such that the movement has a specific rate, in beats per hour, and thus, a specific required pendulum length. This rate can be adjusted with the regulating nut or shaft, but that adjustment only serves to make the pendulum swing at the correct beats-per-hour rate. The beat-per-hour rate cannot be changed without changing the internal gearing of the clock.

Therefore, whatever length that you must shorten the pendulum to, to make the clock keep correct time, is the correct length for that movement. There is no remedy that will place the bob in a lower position so that it can be seen better. In short, the movement itself  is of an incorrect pendulum length for the clock case. Of course, I don't know the particular details of your clocks, but the most common, almost universal, cause of this problem is that the original movement has been replaced with one of incorrect pendulum length. Also, in home-made clocks, an incorrect movement is sometimes originally installed in the clock. Unfortunately, the only remedy that will position the pendulum in the correct "window" of observation, is to replace the entire movement with one of correct pendulum length.  http://www.clockmasteronline.com