"G.R. Patterson III" wrote in message
...
Garage door openers drive a sprocket that operates a circular chain.
Some of the older units had a block on the chain that tripped a
switch when the door was raised. This block could be adjusted to
fine-tune the system. My Genie units do not have this. I assume
(but do not know) that they simply pull the chain for a fixed
distance (most doors are 7' tall).

They have essentially the same system, except that the block has been
replaced by an analog inside the opener. A worm drive moves a block back
and forth, that triggers switches at either end of the range of movement.
One end tells the opener when it's done opening, the other tells it when
it's done closing.

As with the older units, the end result is that the opener simply pulls the
chain for a fixed distance. In the case of the older units, the distance is
set by the block, in the newer units this is adjustable (within a small
range) by a setting that affects the exact position of the ends of the
internal mechanism.

So. Measure the distance you have to pull your chain to open the
hangar door. Work out the ratio between that and 7'. Rig a reduction
gear out of two appropriately sized pulleys and a belt and drive
that from a chain driven by a garage door opener.

Some problems I see with that:

* You will want to ensure that the mechanism inside the opener is up to
the task. The garage door opener I've had open has a nylon worm drive to
turn the main sprocket for the chain. It wears out even under normal garage
use, with a counter-spring to help reduce the forces involved. Even if the
hangar door is counter-weighted somehow, there may be more friction with the
larger door. It may be a lot to ask of little plastic pieces.

* The opener has a motor speed sensor that tells it when the thing is
getting bogged down too much. Because its main purpose is to avoid someone
getting crushed by the door, it's very sensitive. The sensitivity can be
adjusted somewhat, but probably not enough to account for a significant
difference in force required.

* To make matters worse, in combination with the above issues, a
reduction gear that increases the effective travel of the opener is
necessarily going to increase the force that the opener is required to
provide.

* Finally, using a belt and pulley system for the reduction gear may
result in the belt slipping. If it slips only at the very beginning of the
door movement, and it slips exactly the same amount when opening as when
closing, this might be okay. But otherwise, it will throw the system out of
adjustment.

I would say that with a large enough (i.e. expensive enough) garage door
opener, the above issues may not be a problem. But then you may start
finding that you'd have been better off just designing a custom-made opener
mechanism from scratch. Electric motors aren't that expensive, and the rest
is just a sprocket, switches and wiring, and possibly a new chain (depending
on what kind of "chain" is currently used with the manual opening system).

Pete