DESIGN FACTORS
The Design of the Bell Frame

Bells could quite easily be swung between a series of beams spanning the tower, and this has been done.

This arrangement is not ideal, in that the horizontal forces are taken directly into the tower walls at bell bearing level.

The principle to be aimed at is to minimise the effect of the horizontal forces on the tower structure.

This is achieved by the use of trusses, or frame sides, enclosing each bell pit. These trusses are usually 'A' shaped, made of cast iron, steel or timber, and supported from a double foundation of steel or hardwood beams, with one level laid upon the other at 90, and with the beam ends securely anchored to all four walls.

Forces and Bell Frame Layout

The late Mr. E.H. Lewis, M.A., formerly Mechanical Science Exhibitioner of Trinity College, Cambridge, conducted research and experiments to expose the fallacy of the theory that elasticity in a bell frame relieves the strain on the fabric of a bell tower, a theory that was widely held at the time, and one which can still be met today.

A summary of his work was included in a book entitled "Bell Towers and Bell Hanging, an appeal to Architects", by Sir Arthur Heywood, in 1914. Although long out of print, it may be found in some reference library shelves.

He showed, by means of calculations and graphs, the magnitude of the forces generated by a bell swinging through a full circle. For simplicity, these forces may be resolved into their vertical and horizontal components. The vertical force apprxoximates to 4 times the deadweight of the bell and the horizontal force to 2 times the deadweight of the bell. The graph constructed from Mr. Lewis's figures shews these forces plotted against time for one whole pull. The accompanying diagram illustrates the interaction between bell and ringer for the same whole pull. It will be seen that two horizontal maxima occur in each revolution, in opposite directions, and the interval between them is approximately of a second.

It will also be seen that considerable time is spent at the beginning and end of each revolution when rotational movement is very slow, and it is during this time that the ringer takes whatever action is necessary to change his striking position. If he is 'moving out behind', he will be striking one place later at each stroke, so he will let the bell rise a little higher to allow the ringer of the bell he is changing with, to get in front of him. If he is moving down to lead, he will be striking one blow earlier each time and will check his rope to prevent his bell rising so high. When dodging, he will let the bell rise at one stoke, and cut in at the other.

It will be realised that while the bell is mouth upwards, near the point of balance, movement in the frame or tower is likely to have an adverse effect on the bell, because it could be thrown off balance one way or another. This would not only make the bell more difficult to handle because of the extra exertion required to control it, but the correct timing and rhythm would be upset. Mr. Lewis established that tower movement of as little as 1/32" (0.75mm) at the level of the bell bearings, can start to have an adverse effect on the handling of bells. This illustrates the desirability of making the tower and bell frame as rigid as possible.

The ratio given for the horizontal force with respect with the bell weight ONLY holds good where the frame and tower are rigid. If movement occurs, the forces build up very considerably.

The bells should be arranged in the frame in such a way that the ropes fall as close as possible to a circle in a clockwise direction with the minimum drawing of ropes out of plumb. Normally, some bells swing east-west and the remainder north-south. To establish the extent of the horizontal forces in either direction, it is safest to total the weights of all the bells swinging in that direction and multiply by 2.

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