Expert: William J Perkins - 3/31/2010

Question
QUESTION: I would like to use a lyre pendulum with my Jauch 61 Westminster 3-weight movement. I have determined the weight for the chime side and for the strike [hour] side. I have done this by gradually adding weights until the hammers react when I turn the minute hand. Since I never had the original pendulum, I am trying various size pendulums [close to 61 cm] with various 4 to 6 1/2 bobs. The clock worked at a very slow pace with a 29" lyre pendulum and 6" bob. I can modify that pendulum by machining the rods to shorten them but it would be easier if I could have some idea as to what length that pendulum should be while leaving some play for the fast/slow adjustment nut.

ANSWER: Most Jauch movements have the deigned pendulum length stamped upon the clock movements brass backplate.  Copy down and send me all the information stamped upon the backplate.  With that I can most likely give you a pretty good resume' of the clock.  Regards WJP

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QUESTION: The movement has 2 sets of 4 hammers each [1 for chime and 1 for hours]. It also has a chime-stopping lever at the top. The back plate is marked as follows: Jauch, PL 61 cm, West Germany, 9 74, 60-500/Ke/E
In my original question, I forgot to ask about the suggested weight for the center shell which I believe has some bearing on the action of the pendulum. Thnx. Rob

ANSWER: Unfortunately the information provided by your follow-up does not help me.  Since I cannot identify the exact movement concerned I will have to go fishing with you for information.  First lets get the weight problem out of the way.  I believe the weight requirements for this clock are 4.7lbs for the time train, 4.7lbs for the strike train and 6.6lbs for the chime train.  As you face the clock the trains are located thusly, to your left, the strike train, center the time train, right the chime train.  It is essential that you hang the weights exactly as stated.  Now before we can even begin to address the pendulum problem, the clock must be in good running condition.  Also you must identify whether or not you have a proper suspension spring, pendulum crutch and pendulum leader.  Without them you have no place to go.  The usual procedure for determining pendulum length requirements is by counting teeth and applying a formula to the results (a tedious, error-prone procedure).  The method I have chosen for my own use eliminates all those calculations, but is every bit as accurate, but by far, more time consuming.  If you want to take a shot at this let me know and I will do my best to walk you through.  REMEMBER it is essential that the clock be in otherwise operating condition, and that the pendulum leader, crutch and suspension spring are present.  Otherwise, your efforts , and mine, will be futile.  Regards WJP

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QUESTION: Thanks for the information about the weights. It's a starting point. The suspension spring, crutch and leader are the proper ones. I have confirmed that by closely examining these parts and comparing them for size on various suppliers sites. As mentioned in my first post, the clock is in operating condition as it kept running for days with a "clean" even tick sound. Because I was using a heavy 71 cm/28" lyre pendulum with a 6 1/2" bob instead of 61 cm/24" as marked on the back plate, it was obviously very slow, and I stopped the pendulum. I am willing to try either methods to determine pendulum length. You mention 4.7lbs weight for the time train. Is that for a basic wooden pendulum or a lyre one? Also, I have 4" and 6 1/2" brass bobs. I would like to use the 6 1/2" bob. Thnx.

Answer
Okay let's have it.  To begin with the pendulum length is theoretical.  It measures the distance from the axis of rotation to the center of gravity of the pendulum, not to the bottom of the pendulum.  Further, if the clock uses a suspension spring, the thicker the spring, the faster the clock will run, so a longer pendulum is needed.
Here is a non-mathematical approach for creating a proper length pendulum.  Make up a pendulum as long as the case will allow.  Use the bob you intend to use for final installation.  Screw the bob up as high as possible.  Start the clock, it should run slow.  If it does not run slow a whole set of other problems exist that will need addressing before proceeding.  Presuming the clock does run slow, shorten the pendulum , at the top, by the same length as the length of the thread remaining under the bob.  Repeat this procedure until the clock runs a little fast and then adjust the bob down the screw until it keeps time.  You will have established the correct pendulum length for the clock, using the bob desired.  Using the measurements thus developed you can assemble the pendulum to be esthetically pleasing to you.
The only disadvantage with this method is that it may require for the pendulum to be shortened as many as six times or more, rather than once or twice using one or more of the calculated methods.
One warning, be careful in selecting the size of bob you are to end up with.  It must be capable of total free swinging and depending upon the ancillaries of your pendulum a heavier pendulum spring might be required. thus increasing the speed and arc.  Always allow the clock to run 24 hour before adjusting the length of the pendulum.  
This should do you.  Regards WJP