Basic Clock Design

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I'm struggling a bit, Russell ... but see if this makes sense:

Although I mentioned SNR, I suppose that in the case of re-oiling the escapement we are really talking about a small non-random message [lubrication >> less wasted energy >> more impulse] buried in a larger 'carrier'.

The low amplitude system; like a racehorse or a MotoGP bike, is sensitive to tiny changes in any factor.

The high amplitude system is relatively insensitive. [which is Harrison's key point]

MichaelG.

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... Now add the 'red wine and sunshine' and tell us the answer when you have it.

I hate to see electronic ideas being used to explain something they can't.

A simpler way of looking it that pendulum looses some energy each time it swings. The escapement puts this back in which involves it generating a force. The force is generated by some systems of gears which will have friction as will any bearings. Add oil and there will usually be an increase in force because the friction has changed. Changes in viscosity and amount will also have slight effects. We are talking rather minor errors here not large ones.

As to the different action of Harrison it might be down to all sorts of things. He was heavily into reducing friction and used some unusual bearing arrangements. There is a multipart video on you tube showing a replica regulator being built. It shows one aspect of his low friction bearings. 2 rather large discs forming a V and the pivot rests in it. The disc rotatate much more slowly then the pivot so the bearings that carry these impart less friction. The other aspect was lignum vitae.

John

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Then just look away, John

People sometimes understand things better when they re-think them in analogous terms with which they are comfortable.

MichaelG.

I think we have got away from a basic clock

In fact I don't event think you can see it from here.!!

;0)

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I posted ^^^ on page 6 of this thread.

Would anyone care to discuss that point ?

MichaelG.

Yes, I would like to comment on that Michael.

One assumption

We are talking about any clock that varies by less than say 5 mins per week. Anything else is either not working properly or isn't really a clock. (That would probably be my definition of basic)

Probably the least important thing about any clock we are likely to make is actually telling the time and I don't mean how well it goes or not. If we want to know the right time we check we use anything from digital watches, radio controlled clocks or the television. It's nice when our clock runs well and is in agrees with GMT within a minute or so but we don't get irritated if its 5 mins out. In fact within a quarter of an hour is often good enough to know that its just about tea time.

We make clocks because we like making clocks.

Some of us do it for the challenge of making something that runs well and to understand the mechanism.

Others because it looks nice.

In the age of atomic clocks all the mechanical clocks are to a greater or lesser degree ornaments when they have been completed.

Once you have completed your mechanical clock you set it by your wrist watch, you don't do it by astronomical observation. If you really want to test it you don't build another just like it as a regulator you use quartz stabilized oscillators to measure the variation.

It's a real challenge to make something that is good to a second or two a day which was as good as George Graham got and he was considered one of the best of his day.

So if we do it for something that looks intriguing, because we enjoy the journey or because we are looking for the challenge of precision or intellectual understanding, but lets not pretend it's because we want to know the time.

regards Martin

What is 'good time'. Before TV soaps five minutes either way was good enough most of the time, as the church bells got you to the service on time. Only the monk who rang the bells needed a better clock.

For Model Engineers the making of the mechanism should be the primary enjoyment and if it actually runs it is bit more useful that the steam engine they made but it doesn't need to be a precision timepiece as there are so many other sources for that
For the beginner a nicely structured series from ME showing the stages and explaining methods enhances the modeller's skills too.

Martin and Bazyle

Thanks for responding ... I don't intend this to be a debate with a 'winner' but I would like to see a range of opinions [no pun intended].

Perhaps a good baseline for a "Basic Clock Design' is Benjamin Franklin's.

MichaelG.

I had considered that but "speeding up" the pendulum or, more correctly reducing the slowing down, by reducing the drag surely results in a greater amplitude not a shorter period?

Russell.

Edited By Russell Eberhardt on 03/05/2016 13:34:31

Yes, of course, but why is the "noise" unidirectional? I imagine it must be the result of a non-linear effect somewhere. Stiction perhaps??

Russell.

As I said previously for small angular swings the pendulum amplitude has very little effect on the period. Thats why most clocks are designed with small swings so the pendulum is isochronous. It's motion is a series of accelerations and decellerations. The period is affected by drag because that is the biggest factor in changing the angular velocity at the vertical (max velocity position). In effect for less friction the average velocity goes up. Crudely put the velocity is higher in the central portion of the swing where velocity is high. This effect dominates the effect of slightly increased arc so the period is shorter and the clock speeds up.

regards Martin

Um, sorry to disagree with that Martin. Viscous drag creates a force in phase with the velocity, dissipating energy but not affecting the rate. By contrast a force, from the escapement for example, which not in phase with the velocity (so for example applying an impulse slightly later or earlier than BDC) does affect the rate.

The period of a pendulum swinging with a small arc is almost independant of amplitude. If viscous drag affects amplitude, and I'd imagine a 'sticky' escapement wheel gave rise to less impulse and so a smaller arc, then drag will affect period.

To get technical, the small amplitude argument relys on the approximation sin(X) = X, wheras sin(X) actually is X-X^3/3! + X^5/5! and so on ad infinitum. For small values of X, X^3 and so on are even smaller, but not zero

The fractional loss in rate of of a pendulum is (amplitude squared)/16 compared to the "theoretical" rate due to the cubic term in that expansion - the remaining terms are negligible. As long as the amplitude remains constant you can adjust this out. If the escapement gets "sticky" (not sure what that means) yes, it will reduce the amplitude and potentially cause the clock to gain relative to its adjusted rate. But it may affect other things too especially the timing of the impulse relative to the swing which could have a larger effect, one way or the other.

As I said earlier, that is what I would expect to occur. However Harrison wrote, "In the case of Mr
Graham’s clocks, with a small pendulum amplitude and the other poor characteristics described earlier, most
especially when the oil is foul, a touch of fresh oil will cause the pendulum amplitude to increase and the clock to
thereby go faster." That is the opposite of my understanding of the theory but seems to be borne out by experience.

Russell.

Changing the phase of the impulse cannot itself change the rate. The impulse as a function of time can be broken down using the Fourrier series and will only consist of the fundamental frequency and its harmonics. It contains no unrelated frequencies so it can only change the rate as a result of changing the amplitude of the swing.

Russell.

edited to correct for erratic "d" key

Edited By Russell Eberhardt on 03/05/2016 20:58:00

Oh dear. Do we have to get into Airy's laws here? When you change the impulse phase it changes the fundamental's phase and this changes the rate. This is all in Rawlings or Woodward. If you put a drop of oil on the escapement the amplitude may increase but this could cause the clock to go faster through escapement error which could be larger than circular error. In the case of Harrison's pendulum clocks it seems that the escapement error, circular error, and barometric error are all carefully balanced to optimise stability.

Here's a real treasure:

Harrison's Manuscript version of 'Concerning Such Mechanism ...'

**LINK**

... I doubt if it adds much to our understanding, though.

MichaelG.

Edited By Michael Gilligan on 04/05/2016 00:44:43

"dissipating energy but not affecting the rate"

So you reduce the kinetic energy and you don't affect the velocity John? Surely if the pendulum is moving more slowly through it's arc it will take more time. Conversely if it's retarded less it will take less time. (OK I know I'm ignoring the change in arc but the argument is that factor is negligible). With air resistance the pendulum must travel slower over its entire swing compared with no air resistance. If we take the distance traveled by the bob to be the same the period will be less in the second case. I suggest that this is the dominant factor for small arc pendulums.

I can see that what I might term the 'conserved' energy will not affect the rate. With no losses you just get an energy exchange from gravitational potential to Kinetic energy which is just a fancy way of saying that the period is independent of the mass of the bob.

In the case of the oil on the pallets I would suggest a bigger impulse will increase the velocity slightly causing the same effect. If my analysis is correct then air resistance would be the dominant effect as it acts for a greater part of the arc.

I know this has zero to do with the design for a basic clock but my understanding seems to be improving with this discussion at least.

regards Martin