Grandfather Clock

[TomG]

Plastic doesn't age gracefully, especially when there is oil involved.

Tom

[Dr Stan]

(02-16-2019, 08:13 PM)TomG Wrote: Plastic doesn't age gracefully, especially when there is oil involved.

Tom

That probably also applies to covering the pendulum with the plastic wrap.  Initially I thought it was a good idea, but I recommend having it replated or replace it.

[TomG]

That stuff is designed to hold up on automotive applications and will pretty much last forever. It's been on my truck through five winters and looks as good as the day it went on.

Tom

[Vinny]

Well the new pendulum bob showed up, I put it on and the clock ran slow. REAL SLOW. So I pulled it all off and compared the two bobs and the new one is 240 grams heavier. I tried removing some metal from the back but it's still 100 grams too heavy. So the old one is back on. I'm currently waiting for the wrap, the paint didn't look that great. Also after cleaning and inspecting and testing, there are bushings that will have to be replaced. In the manual it said it was serviced in 1991. I'm thinking it wasn't serviced since then. I have a couple of leads on some used ones that I can tear down, go thru, replace bushings as necessary and reassemble. Then I'll swap it out and go thru this one.

[f350ca]

The weight of the bob shouldn't affect the period, that's supposed to be a function of the length.

[TomG]

Yup, length determines the period. The weight just needs to be adequate to overcome losses in the system.

Tom

[Vinny]

That's true from a physics standpoint, but not for a clock. I read a lengthy article that explained why but got lost about halfway thru. I woulda been better off only reading 1/3 of the article since I understood what they were saying then. But when they went into detail ...Whatever they were saying must have been true since I put the old bob back on and it's keeping time again.

[haroldmulder]

(02-19-2019, 03:37 PM)Vinny Wrote: That's true from a physics standpoint, but not for a clock.  I read a lengthy article that explained why but got lost about halfway thru.  I woulda been better off only reading 1/3 of the article since I understood what they were saying then.  But when they went into detail ...Whatever they were saying must have been true since I put the old bob back on and it's keeping time again.

Interesting you say this because what I have read is that weight should not effect the time period but length will. I know when I built two clocks there were different pendulum designs/weights available for the same mechanism but they were all about the same length. All I had to do was adjust the small nut at the bottom of the pendulum to either shorten or lengthen it. Very little movement could change the timing by quite a bit.
Harold

[TomG]

I would take a close look at how the bobs are mounted. Chances are, the effective length of the pendulum is slightly different between the two.

Tom

[Vinny]

Quote: The art of Clock repair

How does the weight of a pendulum influence timekeeping?

WHAT ABOUT THE IDEA THAT THE WEIGHT HAS NOTHING TO DO WITH THE PERIOD OF A PENDULUM?

Well you can believe what you want. And if you are a physics major you may say that this is true. In theory you are correct. In clock repair not true at all. There are several reasons for this. First, the “center of gravity” of a pendulum determines its effective length as far as the clock is concerned. This has been determined by exaustive empirical study. So if you put a “different” weight on a pendulum the timekeeping will change. In other words if you change the pendulum bob you will probably change the timekeeping characteristics of the clock. It is important to understand here that if you “add” weight to a pendulum, you will change its effective center of gravity. To better understand this concept; think about it this way: How do you determine how long a pendulum is? Is it the pendulum rod, the suspension, the pendulum stick, or the pendulum bob? If you were doing a physics experiment, how would you make the pendulum shorter? You would probably shorten the string holding the weight. To adjust the timekeeping of a clock you move the bob up or down depending on whether or not it is running slow or fast. This changes the effective length of the pendulum, thereby changing the period. If you add weight to a clock pendulum, you will change the period of the pendulum and you will change the characteristic timekeeping of the clock, unless you add the weight in such a manner so as to NOT change the center of gravity, and you add enought power to the mainspring or or weight that supplies power to compensate EXACTLY for the weight you have added. Consider this: Lets say you have a wall clock that has a pendulum that produces one tick per second ( This is typical of many large wall clocks ). How many seconds in a day? 60 * 60 * 24 = 86,400. Now take that times 7 and you have 604,800 seconds in a week. In most repair situations and customer service situations, if a clock is more than 5 minutes a week off, you will get a call from your customer. This error is not acceptable to most people. Lets say you change the length of your pendulum so the period is one tenth of one percent slower that it should be on each swing. That is .001. One thousandth. Doesn’t seem like much? Better think again. 604,800 * .001 = 604 that is 604 seconds in a week. Depending on the gear ratio, the clock could be ten minutes off or more. Ok, lets say the error was only .0001 one ten thousandth. That would still be a minute a week. Annoying, but ok. However, lets say you had a 30 day clock ( there are clocks designed to run for 4 weeks on one winding ). That would be a half hour in a month. Probably not acceptable to most people. A tiny set screw on a small wall clock pendulum will make more difference than the .001. This does, as you can see, illustrate the fact that a heavier pendulum will be more accurate, because adding a small amount of “change of center of gravity” will have less overall effect on a heaver pendulum. Clock timkeeping error is additive in a way most people do not consider, and weight change on a clock pendulum does make a difference; sometimes a large difference. This theory mentioned above is precisely why clocks have been designed with temperature compensating pendulums, and suspension springs. If the temperature changes 10 degrees and the suspension rod expands say .01%, it will change the timekeeping. If the pendulum bob expands, and does so in a non-uniform way, the timekeeping will change, because the center of gravity will be altered by the changing shape of the pendulum bob.

Second: If you put a heavier weight(pendulum bob) on a pendulum the pendulum arc will decrease because you have not changed the amount of power that is pushing the pendulum yet you have added to the friction. With a shorter arc there will be less time between ticks. The escape wheel will move faster and the clock will run faster. Too much weight here and the clock will stop. This problem is worse with spring drive clocks than with weight drive clocks because of isochronal error. That is a fancy way of saying there is less power available from a mainspring when it unwinds. The reduction in power is not linear. Generally speaking the mathematics needed to design a clock was available many years ago. Most clocks have been designed well and the “stock” pendulums and mainsprings are crucial in providing good timekeeping. For this reason, I recommend when ever you can, keep the clock as original as possible. Unless you are a skilled mechanical engineer with years of experience in designing slow moving grear trains, you probably will not be able to improve much on the design of an old clock.

Third: If you put a lighter weight on a clock pendulum ( that is to say if you put a pendulum that is lighter on the clock ) the arc on the swing of the pendulum will increase slightly because you have reduced the amount of friction on the pendulum. Maybe I shouldn’t call it friction. As the weight of the pendulum increases it takes more energy to push it. As the weight of the pendulum decreases, it takes less energy to push it. Then there will be more time between ticks. The escape wheel will move more slowly and the clock will run slower. Remember that this effect is additive here; most clocks will tick hundreds of times in one day.

The strength of the suspension spring will also determine the arc of the pendulum. A thicker (stronger) suspension spring changes the arc , or swing of the pendulum ; it usually makes it shorter. Too thick (strong) and the clock will stop. Too thin (weak) and the clock will stop. A thinner (weaker) suspension spring changes the arc , it usually makes the pendulum arc increase. Remember, when the pendulum arc (swing) increases there is more time between ticks and the escape wheel turns more slowly which makes the clock run slower. All of these factors interact in a very complex way mathematically. Sometimes timekeeping is affected in ways that seem illogical at best. If a clock has an excess of power , sometimes you can get away with putting a stronger suspension spring on it. The key word here is sometimes. As soon as you start changing things you will need to be very careful. I strongly advise against changing a clock suspension spring or pendulum unless you use an exact replacement. If you have a clock that has no suspension or pendulum this may help you devise one by emprical study.