Bob ocassionaly hits case

Question

Hello Martin, I am an electrician, and love dabbling with old mechanical clocks. Have done what you would consider very minor repairs. Very far from any kind of an expert.

About a year ago I replaced a Hermle #451-050H 94cm movement in a grandfathers clock that my father-in law had given us about 30 years ago. It has a 5.5" bob which had always swung very close to the edge of the clocks case which is 8.375" wide where the bob swings. Required the case to be very well leveled.

I only replaced the movement, nothing else. I used the same wooden rod, and bob. The bob ended up being located at almost the exact same location on the threaded rod. All went, and is doing well, and keeps very good time.

My problem is this...This movement seems to require just a little more "angle of swing" Occasionally with nothing changing to affect the logistics of the clock case or position, the bob will just barely start to slightly tap the case. Sometimes on one side and sometimes the other. I have painfully and patiently leveled the case and adjusted the "tick rate" (bad terminology I'm sure) many times. I have even slightly ground and filed the sides of the bob so that it's original diameter of 5.5" is now 5.3". It will run for several days without bumping, then may tap one or the other side. (not always the same side). Driving me nuts.

I see that I can purchase a 4.5" bob from Black Forest. Can I simply replace the 5.5" with a 4.5" ? I would think I may have to adjust the bobs location slightly and/or add a small amount of weight (like taping a small washer to the back of bob) to change the overall physics of the weight and distance. The question I have is relative to the physics of weight, distance, angle, and force required to get enough swing for the clock escapement to work properly.

Any advice,tips, or thoughts you could give this rookie would be greatly appreciated.

Thank-You

Sincerely, Rick Baumer

Answer

Hi Rick,

This problem is common in "pinch-waist" clocks. Every movement has it own normal arc of swing, and this is not adjustable. The most common cause of this problem is the clock being off-level, but if the hitting can occur on either side, this cannot be the problem. What you call the tick rate is probably the beat. The clock must tick evenly like a metronome, that is, "tick-tick.tick". If it sounds like "tick-TOCK-tick-TOCK" it is out of beat. This movement does not have automatic beat setting. Please browse my previous answers for information on setting the beat.

It is very unusual that the bob could bump either side of the case. This seems to eliminate both leveling and beat as the cause, and I can only surmise that the problem has something to do with an occasional more forceful "tick", that is, impulse causing the swing. The infrequency of the problem is also mysterious. You will probably never be able to determine the cause.

First, check the weights. The heaviest weight should be on the right as you look at the clock. The other two should be the same. If you hear any loose lead shot in any weight, it has been added improperly and must be removed. Extra weight can overpower the escapement and cause excess swinging. The center weight powers the pendulum. On your clock, the center weight should be 4.7 pounds if the clock does not have a second hand and has a wood stick pendulum. If it has a lyre pendulum or a second hand, it should be 6.6 lbs.

The smaller pendulum bob should work. You will have to adjust its height on the stick slightly differently to regulate the timekeeping. The weight of the bob is not a factor and you need not worry about it. Hi Rick,

This problem is common in "pinch-waist" clocks. Every movement has it own normal arc of swing, and this is not adjustable. The most common cause of this problem is the clock being off-level, but if the hitting can occur on either side, this cannot be the problem. What you call the tick rate is probably the beat. The clock must tick evenly like a metronome, that is, "tick-tick.tick". If it sounds like "tick-TOCK-tick-TOCK" it is out of beat. This movement does not have automatic beat setting. Please browse my previous answers for information on setting the beat.

It is very unusual that the bob could bump either side of the case. This seems to eliminate both leveling and beat as the cause, and I can only surmise that the problem has something to do with an occasional more forceful "tick", that is, impulse causing the swing. The infrequency of the problem is also mysterious. You will probably never be able to determine the cause.

First, check the weights. The heaviest weight should be on the right as you look at the clock. The other two should be the same. If you hear any loose lead shot in any weight, it has been added improperly and must be removed. Extra weight can overpower the escapement and cause excess swinging. The center weight powers the pendulum. On your clock, the center weight should be 4.7 pounds if the clock does not have a second hand and has a wood stick pendulum. If it has a lyre pendulum or a second hand, it should be 6.6 lbs.

The smaller pendulum bob should work. You will have to adjust its height on the stick slightly differently to regulate the timekeeping. The weight of the bob is not a factor and you need not worry about it. http://www.clockmasteronline.com