Pendulum clock runs *fast* in hot weather

We have a very old pendulum grandfather clock - the pendulum is probably about 2 feet long with a slow tick. I've noticed that in the hot weather, when it's probably been warmer inside the house than normal, the clock is running *fast*. I would expect it to run slow, since the pendulum will lengthen slightly in the warmer temperature and
t = 2 pi sqrt (l/g)
as l increases, so does the time period t between ticks
The pendulum is a single rod, not a multi-bar rod with two different metals so as to compensate for thermal expansion.
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Lower viscosity of the lubrication on the lighter / faster moving bits = less drag?
Cheers, T i m
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On 03/08/2018 10:00, T i m wrote:

My thought too. I wonder if it is a "spring" one rather than having traditional weights. Whilst the spring unwinds very slowly, there's a lot of area in contact with a very thin film of often rather thick, sticky oil. If I had a bit more time I'd have a look at what temperature does to the viscosity of air, which is also slowing the pendulum.
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wrote:

Sorry, I should have said. It uses weights rather than a spring. Bloody nuisance having to remember to wind it every morning :-)
Would viscosity of lubrication oil and air around the pendulum affect the timing of the pendulum? I can imagine them causing more energy to be lost as heat.
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On 03/08/2018 10:30, NY wrote:

Both weights and a spring are still providing a "push" to the escapement, so could be affected by lubrication.
Viscosity of air goes up rather than down with temperature. Damn.
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On Fri, 3 Aug 2018 10:38:15 +0100, newshound
;-)
Cheers, T i m
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<snip> >Would viscosity of lubrication oil and air around the pendulum affect the

No, but the viscosity of the oil might impact the running of the escarpment etc?
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And the escapement. ;-)
Cheers, T i m
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Not according to Mr Newton.
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On 03/08/2018 10:12, Graham. wrote:

If the pendulum is hung from a flexure pivot, then lubricant doesn't affect the "free" period of the pendulum, but the escapement does give it a little "push" (after all, it has to supply some energy to overcome air resistance). So less friction in the drive mechanism may give you slightly more "push".
Engineering Toolbox shows both dynamic and kinetic viscosity of air increasing with temperature, which is not what I was initially expecting. On reflection, though, that is right. It's been a long time since I thought about kinetic theory.
https://en.wikipedia.org/wiki/Kinetic_theory_of_gases
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On Fri, 03 Aug 2018 10:00:14 +0100, T i m wrote:

Thinner air ?
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On 03/08/2018 10:00, T i m wrote:

We have a similar pendulum clock and see that it also runs a bit fast in this weather. My guess was also that it was the lower viscosity of the oil, or perhaps lower air drag on the pendulum.
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T i m submitted this idea :

Less drag, usually means the pendulum can swing higher, the higher it swings, the slower it ticks.
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On Fri, 03 Aug 2018 16:14:16 +0100, Harry Bloomfield

But that's not a lighter / faster bit or the issue is it?
(The idea is) It's less drag on the things that regulate the thing that regulates the time. ;-)
Do you have a more plausible reason?
Cheers, T i m
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T i m formulated on Friday :

No, just expressing my thoughts on the subject from knowing how mechanical time-pieces generally behave. My best guess would be that the pendulum does have some compensation built in and the compensation is over compensating when the weather heats up.
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On Fri, 03 Aug 2018 17:17:09 +0100, Harry Bloomfield

Ok?

Ok.

Ok, so that could be an equally plausible reason. ;-)
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On 03/08/2018 16:14, Harry Bloomfield wrote:

Ahem, I think you may temporarily have overlooked that a - if not the - point about using a pendulum for timekeeping is that the period of oscillation is independent of the amplitude.
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There's also the fact that a pendulum only obeys an *approximation* to simple harmonic motion, and this gets progressively worse as the angle of swing increases. I'm not sure whether the period actually alters if the swing is too wide.
But I agree, for normal amounts of swing, a fundamental defining characteristic of a pendulum's usefulness ought to be that it is independent of swing.
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On 03/08/2018 17:34, NY wrote:

thanks - though it'd have been even more helpful if you'd nudged me on that before I posted ;)
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On 03/08/18 17:34, NY wrote:

yes. It does. Imagine if it ALMOST goes full circle and hesitates at the top...and falls back down.

In reality it is given just enough impetus to have a CONNSTANT amplitude of swing.
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