Tales from the Tower
Post date: Aug 16, 2009 12:34:04 PM
1. Where have all the hour chimes gone?
The Church clock is a remarkably reliable piece of equipment. It has been ticking quietly away in the tower for the best part of ninety years, telling the time and chiming the hours and the quarters largely without complaint or grumble. When things go wrong it is rarely the clock, but more usually some peripheral mechanism or outside influence, such as a power cut, or the warping or rotting of woodwork carrying parts of the bell operating mechanism, or on one occasion showers of debris from building work.
When the clock stopped chiming the hours recently while continuing to chime the quarters, t
his seemed a most curious set of circumstances, and thoughts turned to what might have caused it. The most obvious explanation would be an electrical failure, perhaps a switch or a blown fuse, or the electric motor driving the hour chimes. It was a few seconds work to establish that it was none of these things, and thoughts turned to the linkages connecting the clock to the hammer that strikes the bell two floors above. It may help at this juncture to explain that to strike the hour, the clock mechanism, at the requisite time, depresses a micro switch, which starts an electric motor, which is geared to a shaft mounted on the clock frame. On this shaft is mounted a cam wheel; a cast iron wheel of about eighteen inches diameter which has around its periphery 24 cams, or bumps. As the wheel goes slowly round, each bump in turn presses down on the end of a lever the other end of which is linked by wires and bell cranks to raise the hammer of the bell. When the bump passes the end of the lever, it can hold it no longer; the lever falls back, and the hammer crashes down onto the bell. When the bell has been struck in this way the required number of times the clock mechanism releases the microswitch and the motor stops.
All seemed to be in order with the linkage, except that the lever operated by the cam wheel did not seem to move as far as one would expect as the wheel went round. Perhaps it had slipped on its shaft, or was somehow restrained from returning to its normal resting position? A red herring as it turned out.
Breakage of the operating wires and connecting links, from metal fatigue, is fairly common; a less frequent occurrence is the displacement of the brackets holding the bellcrank levers due to warping or rotting of the timbers on which they are mounted. (These bellcrank levers simply turn the ‘pull’ from vertical to horizontal or vice versa, as seen in the photograph.) To strike the hour, a wire connected to the back end of the cam lever rises through the ceiling of the clock chamber and up to the ceiling of the chamber above, where a bellcrank transmits the drive to a horizontal wire heading eastwards to another bellcrank which turns the drive upwards again through the ceiling and into the bell chamber. Here it is attached to another bellcrank mounted on the bell frame, with a horizontal wire to the upstanding part of yet another bellcrank lever on the other end of which is mounted the hammer, a lump of cast iron weighing around 20 pounds, and held just clear of the bell by a piece of flat spring steel. So, the mechanism raises the hammer an inch or two then lets it drop onto the bell. The hammer is then lifted just clear of the bell by the leaf spring, allowing the note to die away naturally.
Inspection of all these linkages showed everything to be correctly connected and apparently in good order, but there remained the puzzle as to how or why the mechanism was unable to lift the hammer high enough for it to overcome the hold-off spring when dropped to strike the bell. The transfer shafts on the bell frame were of square section fitting in square holes in the levers, so no possibility of slippage there. The effect was that of too much wire between the cam lever and the hammer, but there was no means of adjusting this and no obvious way in which it could change. The weather had been quite hot and one wondered if the thermal expansion of twenty odd feet of wire could put enough slack into the system to explain the problem. Probably not, intuitively, but the next job would be to work it out.
And then, Eureka!
The bell operating wires, made of one-eighth inch diameter steel, do occasionally break due to metal fatigue, but it is always a clean break, they never ever deform or bend, do they? And yet here was one that had done just that! The short link in the picture, connecting the end of the bellcrank to a length of wire, had failed by fatigue where the loop joined the straight section. But with the load thereby relaxed, a tiny web of metal had managed to hold the two fragments together, giving the casual appearance of a sound component!
Mystery solved, a new link was soon made and the clock was chiming merrily again!
Alan P Bibby August 2009