On Tue, 09 Mar 2010 15:47:24 -0600, Ignoramus17831
wrote:

This question is of great interest to me. If, say, I has a 1500 lb
rated caster with a urethane wheel, would it deform under 800 lb
load, enough to make it troublesome to move. Thanks

They all flat spot to a degree. How much all depends on the
durometer of the caster, the load, the temperature, and time... I've
seen the tires deform so badly they crack and fall off, and now you
have a cast iron caster.

Several solutions out there, pick one. The easiest would be to use
the urethane casters on a sub-frame, and put crank-down landing jacks
or levelling screw jacks or floor locks or leveling mount "Bun Feet"
(pick one) at the four corners to take most of the load off the
casters when parked.

The casters might flat-spot a bit if they have a little weight left
on them, but it won't be a huge divot.

I'd research air bearings if you've got a big chunk of plate steel -
leave it under the tool permanently and have screw-down leveling feet,
when you need to move it just hook up the air chuck. I know it will
work, but there must be finer points to it.

-- Bruce --