When the door is up, the spring is not bearing any of the weight.
But as the door is being lowered, the spring takes more and more
weight, so that the door opener and its associated portions aren't
carrying the total weight on the chain or worm drive. Also, it allows
the opener to be changed, disconnected, etc, and the door to be
raised by hand. Since it should be able to be raised to about 4 feet
and sit there without rasing or falling more on its own. So the springs,
of whatever configuration, are adjusted to cause that to be able to
be the door position at rest. Below that point, effort should be needed
to raise it, and effort to raise it higher. But by making that midpoint the
'quiescent' point, most doors should be able to be opened with a
half-horsepower motor. Including my 16 foot wooden door that weighs
in at over 400 pounds.
Part of what I'm considering is to counterbalance at least part of the
weight of my door, so that I don't have to have 3 extension springs
on each side. Counterweights went away as an accomodation to
convenience and modernization, rather like electric trolleys.

wrote in message
...
Bill Seurer wrote:

The spring(s) should be sized to the door.

Does a garage door with an electric opener need a spring?

What ever happened to counterweights?

Nick