Digger,
If you've seen a crane up on two outriggers then:
A: That crane was severely overloaded, or the outriggers where not placed properly.
Gee. Do you think so? :cool2
B:According to what I learned when I started on cranes that use outriggers when you set up you are supposed to lift the wheels completely off the ground so that you don't set up pivot points that do cause outriggers to come off the ground.
I'm sure you're correct.
The crane in question (at least in this instance) was an old 15 ton Pettibone. (Picture below--that not the actual crane. I just googled up a picture of one, but it's identical.) It had a different load chart for over the front of the crane than it did for 360 degrees around. That's because it set up longer on its outriggers than it was wide, and I'm sure the engine in the rear added some counterweight as well. Fortunately, it was very forgiving in that respect.
They'd try to reach out as far as they could with it, and more than once I saw the rear stabilizers starting to lift up, and a few times they were actually all the way off the ground, but I never saw them tip it over, (at least not forward.) I had the misfortune of running it sometimes too, and more than once I was out there, exploring the edge of the envelope myself.
Once, I had to set it back on its feet, after they tipped it over sideways. (The tip of the boom hitting/smashing the air compressor they were lifting kept it from going all the way over, but the wheels on the one side were six feet off the ground.)
Once, it was set up on dirt, near the top of a bank. Reaching out, with a bucketful of concrete, the bank gave way under the left front stabilizer, and the concrete bucket fortunately ended up resting/balancing, on top of the wall that was being poured. A nice, real-world demonstation of the concept that the weight on any given outrigger changes with the position of the load.
I was on the recieving end of that one, standing on a scaffold next to the wall. That's why I took an interest in this thread. I just wanted to be sure the future engineers, and everybody else, keep that kind of stuff in mind.