I recently put together a way to lift up a camper above my driveway so I could park a car underneath it. It's been working without a hitch for awhile now, and I
believe it's safe and stable. But I know I'm not any kind of genius -- much less an engineer. So assuming anyone out there's got the time or inclination, I thought I'd invite some constructive pointers and/or criticism from the many guys out there who know more than I do.
I'll try to describe here what I've done. The basic structure of this lift is four pillars, 1/8"-thick 2x4 rectangular tubing, sitting on concrete. There is a rectangle at the top made out of steel, on top of the pillars but also big enough for the camper to move up and through the shape vertically. Two sides of the structure are attached to my house's exterior walls (wood frame, stucco, 10" thick) at multiple points with 12"-14" carriage bolts. The one long side that isn't bolted to the house has a large X-shaped brace to make sure those pillars remain parallel. That side is also bolted to the concrete and tied into the other side with the steel rectangle up top. Because the structure is basically one with the house itself on two perpendicular sides, it appears to have very good rigidity, both torsionally and laterally. (Forgive/correct me if I'm using these terms incorrectly.)
So I believe the basic frame of the thing is strong enough for the task of holding up the camper -- which weighs 1,100 pounds. But the important
function of the thing is raising and lowering the camper, which means an 1,100-pound object is put in danger of falling if my system of sheaves, cables and Chinese-made hoist were to suddenly somehow fail. My design for the lifting mechanism is based on what I've seen used in boat lifts. I'll include a crudely-made illustration, which shows how two cradle pieces are raised and lowered by a set of cables and sheaves that generate 8' of lift by moving the hoist-end of the cable a distance of 32'.
The cradles themselves each have a second cable that makes sure they stay level as the lifting force is applied on only one end of each cradle. This cable is anchored at both ends, with one end adjustable so I can make certain the cradle stays level, even if the cable stretches.
And here's the whole crazy idea put to use. It's important to note that nothing is resting on the cables when the camper's been raised up. It's sitting on four 1/2" pins running from both sides of the cradle pieces and through the pillars. (Since this picture was taken, I welded on a rear-end spare tire mount, which will keep the whole thing from sliding forward, and off, the cradle.)
The cable is rated for more than the weight of the camper. Each of the eight sheaves I use is rated for 3000 pounds. The three snatch blocks I use are rated for 2000 pounds. I assume the load of the camper is getting distributed between the sheaves. But you know what they say about assumptions. I try to get my head around where the force is going in this contraption and I don't get very far. But I haven't found any deflection in the axles I put the sheaves on and it seems as though the hoist (rated for 2,000 pounds) isn't breaking a sweat.
The red zone, danger-wise, is whenever the load is moving up or down. Once it's up, it sits on half-inch pins running through each pillar. Once it's down, it's sitting on its own wheels. I came up with a possibly-good-possibly-bad junkyard safety system, which is an inertial-reel seatbelt on each of the four pillars. The idea is that if a cable breaks and the load drops, the seatbelt reels will lock up and stop its descent. I'll admit that I would love to do more testing with this 'system,' and I'll also assure everyone that I would never put anyone or anything under the camper while it's moving. So the worst-case scenario from a drop is a damaged camper.
In an earthquake, the camper would have to jump quite a bit to migrate off of its cradle. It can't simply slide off because of the presence of a wall at one end and the spare tire mount blocking fore-aft movement at the other end. I've also been using a large c-clamp on the spare-tire end to prevent any jumping around if the ground did start moving. (And for what it's worth, in a major earthquake, the least of my concerns would be a damaged camper.)
I'm open to anyone's thoughts on where I might have messed up and I'd be happy to hear any suggestions on how to improve the thing. And of course it might be that you don't feel it needs improving. (My mechanical-engineer father approved of it.) If I disagree with a comment or suggestion, I promise to remain civil. And of course I'll answer any questions I can, since it might not be at all clear how the thing works from my photo and two illustrations.
Your thoughts, Off Topic brain trust?
Oh, and there is a video, where you can see it in action.
Don't judge me by my acting skill:
https://youtu.be/kr5ErGiyylM