The mechanical clock pendulum length
A pendulum clock without a pendulum is a shame for sure. Getting the correct pendulum does take some diving into the clock world. If you have no idea what pendulum it would take, this is a basic guide. This is a guide to narrow down to the best pendulum for your clock. This will cover most situations, without special equipment to figure out the length.
On back plate of the clock movement usually there are some numbers and or letters for identification. There maybe an indication of how long the pendulum should be in the mix of these numbers. It may say CM or PL and this stands for centimeters or pendulum length. This would be the length where it should keep approximate time. However this is measured in a few different ways usually dependent on country of origin.
German made movements
The mechanical German made clock movements are easy to figure out the pendulum length. The numbers will let us know or the clearly indicated CM or PL number will be stamped. If it is German, keep in mind the CM length is not the actual pendulum length. Usually the Germans measure this length from the top of the clock movement, and this includes the three components of the pendulum all in one length in centimeters. This length will include the pendulum itself, the leader that it hooks to, and the suspension spring on the top that the leader hooks on to, all in one CM measurement. CM stands for centimeters and PL stands for pendulum length.
If the CM stamp is not on the mechanical movement, any numbers can tell us the pendulum length automatically. If the movement was made post WW2 in Germany like most modern floor clocks, the numbers will tell us. Even though it does not say something like 114CM or just 116 on the movement, the numbers that are there can do the trick such as UW32/1. We know its German made by Urgos, we can go to the Urgos page and see what the PL is. It can get confusing to people who do not care about all this and just want the pendulum. If it gets to be too much, please email us at [email protected] and include a pic or the movement numbers. We will have an answer and hopefully a product.
German grandfather movements
Almost all grandfather clock movements are usually going to be German if its made after 1950. The first thing we need to do is get the numbers off of the back plate of the movement. This is the only way, and we cant cheat by looking at the paperwork or clock case. It has to come right off the back of the clockworks. The manual and the sticker on the case is of no use to get the pendulum.
The American mechanical clock movement manufacturers referred the pendulum length as a "drop". The drop is the length of the pendulum from the hand shaft all the way down to the bottom of the pendulum rating nut threads. It is a different way of measuring the pendulum length from the German made units. Both ways are based on the smallest bob diameter. If the bob is larger or heavier, the length would be longer than what is stamped.
If everything fails for one reason or another, the best chance for a pendulum is the wood stick style. This is the only style of pendulum that can be easily modified because you can chop it. These types of clocks are usually antiques, or of Asian origin, and information such as pendulum length is just simply not available. Sometimes it takes a good guess on where the manufacturer intended to have the bob sit.
To do it this way, you only need a pendulum with a stick that is way too long to begin with. Chop it try it, chop it try it. Each time your slow, cut an inch and half off of the sticks length and hang it back on. It will only take a couple of times and if you start way too long you can always go shorter.
Clock makers method
There is another way, its called setting the beat rate. This is a more involved way and usually just done by clock makers. It involves a beat detecting device that counts out the beats per hour, or the beats per minute. To do this you would need to know what the beat rate is supposed to be for that particular movement first, and then keep adjusting the pendulum length till you have it keep time. There are cell phone aps out there these days that will tell you the beat rate and the cell phone will listen and tell you if the clock will keep time with that pendulum. You go longer or shorter until your phone says the beat rate is set. I have never used these so I cant say much about them, except the app is cheap and people seem to like them. We use the machine here so we never got into the cell phone app yet.