This is a video of my roll on roof.
http://hk.youtube.com/watch?v=lDmHLa...e=channel_page
It uses an electric wheelchair motor/gearbox as is, with a built in dog clutch to release the gearbox for manual pushing.
The 24v DC motor easy to power. In this case 12v 5A was all that was required, and still had enough power to pull the roof right off the walls. At 24v 20A it would pull the OBs out of the ground
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Roll ON is cool, because you can image all night whilst sleeping, and set a timer for autoclose (and in my case rain. One drop and its done.)
Ive used it for 2 years without a problem. Its closed several times on rain correctly, on the 1st drop.
A S/H motor gearbox from a wheelchair dealer or ebay would be cheap, say $50, or buy a junk whole chair for a $100 and you have 2 motors and 2 usable dryfit batteries
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The pull on is centered and I get no skew at all.
To also pull off is trickier, perhaps pullys then to allow 2 ropes, one down iether side along the walls, but still just wind up rope on a capstan.
Basically, I used an electric wheelchair motor/gearbox , because its cheap, vastly underpowered (so its very reliable), ready to go as is and easy to power and speed contol, if required.