On 20/08/2020 12:43, Chris Hogg wrote:
On Thu, 20 Aug 2020 12:23:02 +0100, "NY" wrote:

I have a granddaughter clock and I'm having great difficulty adjusting it so
it keeps good time. The pendulum bob (a metal disc) sits loosely on a
threaded rod which has lugs allowing it to sit on the oscillating part of
the clock mechanism. The bob rests on an adjustable nut on the rod, which
allows the pendulum to be lengthened or shortened.

It used to lose about 5 minutes a day, but I've progressively wound up the
nut 1/2 turn at a time, to shorten the pendulum so it has a shorter period.
I'd got it almost right, but then I went backwards: a further shortening
made it run *slower*. I wound the nut a full turn shorter which was
evidently too far (the clock ran too quickly), but when I backed it off 1/4
turn, it started to run a lot slower than before I originally adjusted it.

Something about the pivot point or pendulum amplitude has changed then.

Part of the problem is that it is necessary to turn the clock round to get
at the back, and then unhook the pendulum from its mount on the mechanism to
be able to get at the adjustment screw. I'm wondering whether it doesn't
always reseat to the same position and/or slight differences in how the
clock sits on the floor are making things non-reproducible.

It should not be too sensitive to absolute level provided that it isn't
obviously cock eyed or rocking on its feet.

The floor is modern hardwood tiles laid on a concrete floor that was
levelled with self-levelling compound when we had building work last year,
so it's not the problem of a sloping floor or a carpet that is thicker near
the walls than a few inches into the room.

Any suggestions?

Time-keeping of the clock in the Elizabeth Tower (aka and wrongly
called 'Big Ben') is done by adding or subtracting weights (old
pennies etc) to the main pendulum weight. In principle, the period of
a pendulum is independent of the mass of the bob, but by adding or
subtracting pennies they shift the C of G slightly, altering the
effective length of the pendulum.
https://www.careline.co.uk/wp-conten...5/p040d4h0.jpg
Perhaps you could try something similar: little pieces of solder wire
wrapped around the rod just above the bob, for example.

That is true for a simple pendulum but if the bob isn't very heavy when
compared to the mass of its support then that rule is only approximately
right. Likewise being independent of amplitude is only approximate too.

I have a couple of tiny brass nuts sat on my torsion pendulum to correct
out slight imperfections of its compensation with ambient temperature.
In theory is is fully temperature compensated by design.

Some pendulum clocks are too with a pair of different metals invar for
the longer length and then brass or iron for a short length so that the
pendulum bob position stays fixed as a function of temperature.

--
Regards,
Martin Brown