I’ve not blogged about one important clock that I look after – namely the church tower clock at Cobham, Surrey. The clock is a so-called ‘flat-bed’ made by Thwaites and Reed of London in 1896. It uses the tower bells for a ‘ting-tang’ strike on the quarters and it strikes the hour on the tenor (heaviest) bell. The mechanism is shown at the top of this post. It rests on steel I-beams at the level of the clock dial in the photo of the tower.
It used to be wound by the bell-ringers once every week, which meant ascending two vertical ladders, one an ancient timber ladder fixed to the wall, through a trap-door and up another a steel-rung free-standing ladder and trap door; then winding three heavy weights up through the whole height of the tower, a job requiring dedication and effort.
Probably around 1975, when the bells were augmented from six to eight, electric winding was added. No doubt, some of the ringers were starting to feel their age! One motor raises a weight a short distance to keep the pendulum going even during a power failure; two other motors drive the quarter and hour striking chains directly. There is an electronic control box for the striking mechanisms, although I’m not sure what it does. It doesn’t control the number of strikes as this still uses the original mechanical system linked to the position of the clock hands. I suspect that it silences the chimes at night, although I’ve not been there to find out.
However, more than a year ago, just after I became a ringer at Cobham, the clock kept stopping. The clock makers took ages to attend and restart the clock. After a month or so, it stopped again. This happened several times, and the church authorities were getting complaints that the clock was never right.
At first the clockmakers tried to blame various things such as a fault in the church electrical supply, but I pointed out that the clock always stopped at 53 minutes past the hour, although not always the same hour, and that there didn’t appear to be a fault in the drive to the clock hands.
The clockmaker explained the operation of the regulator mechanism fitted to the pendulum. The pendulum is about 7 feet (2.1 metres) long, so it is probably designed to have a period of 3 seconds. The idea is that the pendulum is set to gain very slightly and the position of the hour hand is checked by a tag that should pass a light sensor at 53 minutes past the hour. If the clock is fast, the regulator has a magnetic arm that extends to hold the pendulum stationary for a few seconds and then releases it so that the clock is correct when the hour is struck.
Identifying the problem
After much persuasion, the clockmakers replaced the whole regulator system, but this did not cure the problem. Then I noticed that the catcher plate on the pendulum had twisted slightly so that it was no longer directly aligned with the magnetic arm. The catcher plate is held onto the very heavy cast-iron pendulum bob with plastic cable ties. Since we’d been having unusually hot weather, it is likely that the cable ties were able to stretch slightly, allowing the catcher plate to loosen and slip a tiny amount round the bob. The more off-centre it became, the greater the tendency to slip until the catcher, instead of stopping the pendulum, merely impeded it until the swing was insufficient to maintain the drive.
I discovered that by carefully using a heavy spanner to tap on the catcher plate support, I was able to re-align it exactly and the clock is now keeping time perfectly! I will probably now glue the catcher plate into place with heavy-duty construction adhesive.
The tower bells
I should briefly mention the bells themselves. There are eight bells: two date from the mid 18th century, with the most recent being cast in 1979 when the ring of six was augmented to a ring of eight. The heaviest bell (the Tenor) was cast in 1902. It has a diameter at the mouth of 38 inches (97 cm), weighs half a ton and is tuned to the note of G. There is much more about the bells and ringing on our sister website.
The bells are hung for ‘full circle’ change ringing in a steel and cast iron frame mounted above the clock and they are rung from the ground floor, so they have what is called a ‘long draught’ of about 60 feet.
In the photo above, they are in the ‘down’ position and you can see the blue-painted clapper hanging free in the centre of the bell. There are separate clock hammers which aren’t shown in the photograph.
In this photo of a model bell, the bell is raised or ‘up’ for change-ringing.
The bells are raised and lowered by using the rope to swing them higher and higher. Ringers try to do this whilst ringing the bells in order, a skill that is particularly difficult to acquire.
The clock can only strike the bells when in the down position, so they must be lowered from the ‘up’ position after each ringing session so that the clock can strike. This also requires considerable skill and if a tower doesn’t have a clock, there is quite a temptation to leave the bells ‘up’, but this is generally frowned upon as it would then be very dangerous for anyone go amongst the bells.
