A few years back I built my own "roll-off" roof mechanism for a backyard observatory. The roof is free to roll on an off. The mechanism is very simple. There is a long rack (linear gear) permanently attached to the base of the roof on one side. A reversible motor drives a pinion which engages the rack to move it either to the open or closed position. The roof is guided by a set of garage door rails and wheels. Recently, the pinion has been slipping out of the rack when trying to open the roof. If I pull down on the roof near where the pinion engages it does help and eventually the roof will open. However, I getting to old to do this because it takes a lot of force to keep the pinion and rack engaged. I have been trying to come up with a way to maintain the engagement between the driven pinion and rack, maybe a couple of idler pinions mounted below the driven gear and pushing it up against the pinion. The following photos should give you an idea of the setup.
Many thanks, Frank