Hi Doug, you dont say what application you want the screwjack for - but is this product any good ? here
Modifying a servo could be done, but you would need to remove the potentiometer and mount it remotely somehow, and mechanically link it to the jack unit to provide feedback to the motor. Sounds like a fair bit of faffing about, and depending on the application, I would think a pupose made unit would be better. Alternatively you could also use a "regular motor" operated by c/o relay contacts, and operate the relay with an electronic switch such as the gloNgo unit or whatever, or more simply, use a micro servo to operate a couple of microswitches switching the motor forward and reverse, with limit switches to limit travel.....oh crikey this is getting long
Its for an idea I've been working on for some time; making a set of working Fowler Flaps. They would have two settings, 10 degrees for takeoff and 30 for landing. The devices you suggested look fine although a bit pricey!
At the moment the flaps are little more than scribblings on a beer mat!
Strang e how things come around - I mentioned Fowler flaps in a different thread a couple of days ago. One way would be to use ordinary servos, disabling the pot, and replacing it with a geared pot externally, if you don't want to go to all the bother of limit switches with an override so that the motor can be turned the other way! It is still messy and bulky though. Of course, using limit switches would only give you up or down, no in-between positions unless you can 'step' the motors, and then you have the problem of getting them to both travel the exact same distance when you do that, otherwise you could (would?) end up with assymetrical flap!
The nearest thing to what you are looking for was a yacht sail winch I used to have. This had a motor driving a geared drum, simple enough, but the positional control was achieved by having two microswitches on a sliding track above a helical thread that was driven by the drum gears. On the helix was a lever, so when the sliding microswitches were moved by a regular servo, one or other of the micro's would 'break', driving the winch motor, the drum, and the helix, which screwed the lever along until the thing 'caught up' with the micro, and 'made' it again.
I thought about using sail winch motor am still have it in the back of my mind. Starting and stopping the flaps in one of the three positions: up, half and full was my next problem but there is a UK company that produces "little black boxes" for just about everything you could dream of! That, coupled with a three position switch on the tranny should have that bit sorted too! So now all I have to worry about is making the whole thing work.
denotes 
