Ken,
The wood dual-beam appears to be your best bet in terms of cost and devise
weight. However, instead of using dimensional lumber, have you considered
laminating strips of plywood to form your beams? Yellow wood glue and wood
screws are all you would need to build. That should provide more than
enough strength (more than ordinary dimensional lumber would) without the
warping issues. Also, buy yourself a large permanent marker or some paint
and label the beam(s) as to their use and cost $$ to replace ... that and
a bit of discipline should discourage you from using them for other
purposes.
On Tue, 12 Jun 2007 08:52:28 -0700, Ken Roberts wrote:
> Hey.
>
> I have a project. I want to be able to lift my UH-18sp while in the
> garage.
>
> I built a gantry crane similar to these:
> http://www.gorbel.com/pdfs/Jib%20Brochure/Gantry.pdf
>
> A is 10', B is 6'2", C is 6'10", D is about 9'4". I have a chain hoist
> on it, everything works fine. The crane is that short so it fits under
> my garage door. I'm just telling you so you can see what the project is
> all about.
>
> The garage door is 6'11" high, and the ceiling is 8' with stuff like
> lights hanging down.
>
> The problem is, by the time I have a chain hoist on it it's all too low
> to tie a strap around the craft and pick it up. I need a sort of
> specially designed harness, and matching hard points on the hull.
>
> Before anyone suggests it, a cherry picker will not work. The only
> place you could get wheels under is from the back, and the wheel plus
> leg can't be higher than about 2". And then I'd need to disassemble the
> entire tail structure and guards in order to get the arm where it would
> do some good. Chances are, I'll need to use this device every time I
> hover, and I'm not interested in a 3-hour process.
>
> The goals of this harness are:
> 1. Pick the hovercraft up with just one man (me), anywhere, to work on
> it. (using jack stands of course)
> 2. Put the hovercraft on the trailer and take it off quickly, without
> starting an engine, disassembling, etc. 3. Avoid annoying the neighbors
> at odd hours with an engine, on nights right before the rally.
>
> I have a couple options I can think of:
>
>
> 2x6 wood dual-beam approach ------------------------------ I can make
> 2x6 beams about 8' long. On the back, they attach to the engine stand.
> On the front, they attach to carriage bolts which go through the hull
> and stick up under the dashboard. There would be a plywood disc or
> rectangle on top and bottom to handle additional stresses. Right at the
> balance point, I attach a cross member (probably steel) to which the
> hoist hook attaches. The cross member would be about at seat level.
> Advantages
> 1. Cheap.
> 2. I could get the hook point about level with the bottom of the padding
> on the seat. This gives me about 24" of vertical travel with the hook.
>
> Disadvantages
> 1. Bulky.
> 2. Wood warps, so what works one time might fail later. 3. Wood boards
> in my garage have a tendency to turn into something else.
>
>
> Steel dual-beam frame approach ------------------------------ This
> would be the same as the wood dual-beam approach, only made of steel.
> Advantages
> 1. Hook point is about the same as above. 2. Steel is much more reliable
> than wood in terms of warping. 3. Easier to make exactly what I need.
>
> Disadvantages
> 1. More expensive than boards.
> 2. Steel square tube in my garage has a tendency to turn into something
> else.
>
> Steel single-beam approach ------------------------------ This would
> be a steel harness that bolts to my engine stand, and another harness
> that bridges the hard points under the dash. Then a single beam would
> run down the middle of the seat to connect the two harnesses.
>
> Advantages
> 1. Use less steel
> 2. Fewer parts to trip over in the garage.
>
> Disadvantages
> 1. Hook point is higher than the dual beam approach.
>
>
> Composite sandwich tunnel approach ------------------------------
> This would be completely different. The standard UH tunnel that runs
> down the center of the cockpit would be modified as follows: 1.
> Through-hull lag bolts would have a plywood disc on the bottom and a
> high-strength sup****t on top which attaches to the tunnel wall. 2. The
> tunnel wall would become a composite sandwich for strength. 3. For an
> attachment point, a hole is drilled on each side of the tunnel and a bar
> goes through.
>
> Advantages
> 1. Integrated into the hovercraft for the most part, so it's easier to
> set up and take down, if that has to be done at all. 2. Lighter weight
> overall.
> 3. Hook point is below the seat, lowest of all approaches I can think
> of.
>
> Disadvantages
> 1. Cost? Not sure on this, might not cost any more than anything else
> I'm talking about.
> 2. I don't know how good this composite structure would have to be. 3. I
> don't know how much of the underside I would have to strengthen in order
> to keep the thing from delaminating.
>
> ------------------------------
>
> I'd like to hear from you engineers out there, for any ideas or
> suggestions.
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