Plywood generator platform strength

I have an Onan DJE 6kw generator pictured here:
http://igor.chudov.com/tmp/onan/Diesel/home /
This generator weighs approximately 500 lbs.
I am designing a portable platform for it, in the following way:
It will be on four 13", 5/8" axle pneumatic wheels with bearings that are $5.99 at Harbor Freight.
Then there will be a plywood platform over these axles. Size 2x4 feet. Underneath the platform, there will be two 2x6 boards running along the longer edges, with holes drilled for the axles. The axles will not spin, the wheels will spin separately as they are on bearings.
On that platform, there will be two used 2 foot diameter tires whose purpose would be to provide cushioning and vibration dampening.
Above that, there will be another platform to which the generator will be bolted.
The generator legs form a square that roughly 16x16 inches. I plan to put the generator legs on 2x4 pieces, laid in a diamond shape like this
_______________ | / \ | | / \ | | \ / | | \ / | ~~~~~~~~~~~~~~~
The legs of the generator will be bolted to these four edges of the diamond made of 2x4s, each leg being in the middle of each edge. I am sorry if my explanations are not very good, I am a foreigner.
My question is regarding this second platform. I want to make it from two layers of 3/4" plywood, glued together. One 3/4" layer is not strong enough. The question is, is strength of 1.5" plywood (2 3/4" layers) good enough to support a vibrating 500 lbs generator, given that there is plenty of cushioning from the tires underneath.
Someone suggested a torsion box. I do not mind that, as such, however I am concerned that its extra thickness is going to raise the center of gravity of this construction, too much. Hence, I will do a torsion box only if absolutely necessary. I hope to stay under 4 feet total height, in hopes of lowering cost of the generator enclosure.
Thanks!
i
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Yes 1.5" plywood will support the load especially if the legs are positioned over the 2x4 which is under the plywood. I'd put one sheet on top of the 2x4 and one on the bottom. You know that the top platform will vibrate and make noise?
I'd worry about the thing tipping over. Why not lose the tires and top platform and put in some real isolators between the generator and the bottom frame?
-j

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Thanks
That is not possible, since there will be used tires under the top platform, and I want the entire rim of the tire to contact the bottom of the top platform.

Well, if it is also going to be bolted to the tires underneath, I would expect the noise to be dampened by that. Especially if the platform is not hollow. (one more reason not go the torsion box route).

Great question. Let me try to provide some data.
The tires will provide a base that is approximately 2.5 ft wide.
The generator's bottom will be 14-16" from the ground. The height of the whole thing will probably be about 3 feet, the center of gravity, probably 1.5-2 feet off the ground. I am at work now and can provide more detail once I get home. I do not think that it will tip over, but I am very open minded to reasonable suggestions.

Well, I think that the tires would be better vibration isolators. Since this thing will be on wheels (for ease of repairs etc), I particularly want excess dampening so that it does not get an impetus to move about. That's even though, I expect it to be jacked off the ground and put on permanent foundation of some sort. Still, excess dampening cannot be a bad thing.
The standard mounts that this Onan generator came with, are pretty sturdy and make me suspect that the genset was mounted on something sturdy, like a firetruck. It would take a heavy platform to take so much vibration with those mounts. Also, I have only three mounts out of 4.
I asked about using tires for dampening on alt.energy.homepower and rec.crafts.metalworking, and several people said that they have or have seen setups like this, and results were quite good.
i
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I, I have two of the Honda EX5500 - Great Machines! http://www.hondapowerequipment.com/ModelDetail.asp?ModelName=ex5500 One is original and the wheel kit works very well. It make the generator a basic wheel barrow. The axle is placed in such a way as to make it relatively easy to lift and move its 400lbs.
For the second one, I made an angle iron frame and welded 4 large casters to the frame. I use this one in the back of my truck when I go to Mexico.
I would make an angle iron frame, forget the plywood and with only two tires and removable handles, it would be a snap to move it into place. I never cared much for using plywood as a permanent base for a machine. They collect oil, fuel and eventually fall apart.
Dave

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A very nice generator, albeit pretty expensive. Congrats. I am very impressed.

Yes, I will also add some sort of a lifting aide to lift one side or another.

Thanks, maybe I will do that. I may have more vibration to deal with than you. It is a serious diesel engine. I ordered new rubber mounts for this engine, but having tires sandwiched between layers of plywood, will probably protect the base even further.
My plan for protecting this contraption from elements was, first, to build an enclosure around the generator (it is designed so that it is easy to build enclosures around it), and second, apply a protective layer of plain epoxy.
i

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``Why not just replace your plywood idea with a steel frame bolted to the bottom of the generator, and keep your tires and then have a steel frame on the bottom?''
I am kind of open minded about that. Can I simply bolt the frame together from angles and channels? If I was to pay someone to weld it, how much would it set me back?
i
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Sure, Just make sure to use lock washers.
If I was to pay someone to weld

Depends on where you are and who you know.
-j
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I, Bring over to my house, I'll weld it up for you in fifteen minutes. Free.
Seriously, any welding shop, an auto repair shop or even a local neighbor should be able to do this for you easily. In SoCal, I guess it would cost $25.00 or a couple cases of beer!
Dave
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Here are the pictures of the platform that I am building. Its purpose is to provide a cushioned, movable foundation (to be choked in its permanent location) for a 400-500 lbs air cooled diesel generator Onan DJE 6KW.
http://igor.chudov.com/tmp/onan/Diesel/Carriage /
The cushioning is provided by:
- Pneumatic 13" wheels ($6.99 or $5.99 each on sale at harbor freight) - 2 used car tires - Rubber engine mounts.
The platform is 2x4 feet, and with wheels the base is 2.5 feet wide. I estimate that the center of gravity for this beat is about 2 feet off the ground.
The reason for so much cushioning to avoid bending of 5/8" axles, and also to avoid failure of this whole thing after prolonged vibration, and prevent its movement when generator is in operation.
I hope that it will also help reduce noise level a bit.
The top platform is made of two 2x12" by 4' boards, with a thin piece of plywood glued/screwed at the bottom and four boards going across, on top. Also to be glued and screwed on. The boards that support the rubber mounts, will also be bolted to the top platform.
i
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