Richard J Kinch wrote:
George E. Cawthon writes:


Contrary to what others said, I don't
think that one spring breaking indicates the other
is likely to also. Springs don't all receive the
same temper so some break and some don't with the
same use.


An engineer would disagree. See my explanation and references regarding
cycle lifetimes at:

http://www.truetex.com/garage.htm

Of course an engineer wouldn't agree. Engineers
depend on the fact that a single item is like
every other item. The fact is, that even with
tight controls on manufacture there is usually a
large of amount of difference among individuals.
If the life cycle of 95 percent of the items is
longer than the item will be used, who cares if
some items have a 50 percent longer life time.

Springs from the same batch may be pretty similar,
but who knows how much mixing of batches there is.
One simple example: the spring on the left side
of my oven door broke after 5-6 years and I didn't
fix it. The spring on the other side was
sufficient to keep the door closed but wouldn't
hold the door in the "half-cock" position. As a
result the door was mostly left in the full open
position for cool down. That increased the
tension and yet that spring is still working after
29 years of use. The first spring had a useful
lifetime of 6 years the second had a lifetime 5
times as long.