The Tower Handbook
The main reason is that some are lighter set than others, ie they cannot go so far beyond the balance before being stopped by the stay. To set a bell, you must make it go over the balance and not return. If the bell does not swing high enough, it won't go over the balance. If it does go over unhindered, it will bump the stay and almost certainly rebound back past the balance. So to set it, you must swing it high enough to go up over the balance, but not too far or you may not be able to stop it bouncing back.
You must detect when it passes the balance point and then decelerate it to prevent the rebound. Ideally you should stop it completely before it touches the stay, but in practice, a gentle bump will still allow it to set. Of course, if you pull too hard to decelerate it, you will pull it back over the balance and it will keep ringing. Setting a bell then, means you have a small time in which to apply just the right amount of force. If you miss the time or misjudge the force, the bell will not set.
Now think about a light set bell. It does not move so far before the stay hits the limit of its travel. This has two effects. The time between passing the balance and hitting the stay is reduced. Also the force needed to pull the bell back past the balance is less, because it is more finely balanced. Both these factors make it harder to set the bell.
Other factors can combine to make the task harder still. With a springy rope you can neither feel what the bell is doing nor control the amount of force you apply to it so accurately. See section 14.8o&r. A springy stay will be less forgiving of your imperfections than one that absorbs some energy when it is bumped. A moving bell frame may mean that just as you have your bell nicely poised, the frame moves and tips it over again.
Many bells are easier to set at handstroke than backstroke. It could be because:
If a bell is light set, there is very little clearance between the bell passing the balance and the stay hitting the end stop. If its position in the ring means it has to be rung relatively slowly to fit with the rhythm of the other bells, then you have to take it past the balance on most strokes, and you will have to bring it to rest within this small gap every stroke (or at least every handstroke). This is much harder than if there is a larger gap. If the bell is difficult to ring for any other reason, it will also be hard to control how rapidly it approaches the balance, and hence to bring it to rest in the required period.
Stays are traditionally made of ash. This is a very resilient wood, especially when seasoned. But it is hard to obtain, so stays may of necessity be made from a different wood such as beech which is less durable. The detailed design of the stay (eg how thick it is) and how it is attached to the headstock (eg whether it concentrates the stress at one point) also affect durability. If you have to buy a lot of stays you are more likely to be tempted to economise by buying inferior ones or to be in a position where you cannot get good ones. Either way you will continue breaking them.
Of course, it may not be the bells, but the ringers that are at fault. That may explain why some towers break more stays than others.
Beware of the notion that stays should be made thicker to avoid being broken. The stay is designed to act as a safety valve. If the bell is mishandled, something may break, and the stay should break first. It is far cheaper to replace a stay than to mend a broken gudgeon.
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