If I understand statics correctly, in order to be stable, a system’s center of mass needs to be above its base; in the case of a construction crane that would be a rectangle defined by the outside edge of the crawlers. Now, pretty much in order to be useful, a crane needs to be able to deliver its load to a point that is outside of that base. So, in order to stay balanced, the rest of the crane must weigh at least as much as the load it is carrying, right? If it lifted more than that it would fall over, no?
I don’t specifically know about a crane but a lot of construction eqipment are capable of lifting/moving more than their own weight. The forklift i use at the hardware weighs 5.5 tons but it can lift 7. However, when that happens the rear tires (for steering) lift off the ground.
No; all you need to do is move the counterweight further back and it produces more moment weight - you can have a ten-ton load extended ten metres from the balance point of the crane, fully compensated by a five-ton counterweight (a little more than) twenty metres out the oopposite side.
Right, that would work in theory, but on a real job site it would get in the way. I’ve never actually seen a moveable counterweight on a crawler crane; do such things exist?
That’s correct.
That’s a reasonable contention for a crane. If it couldn’t do that, you’d have something more like an elevator.
This is the bit that’s wrong.
It doesn’t depend on the counterweight being movable, either.
A particular crane might weigh 3 T, plus a counterweight of 5 T, for a total of 8 T, all of which is located behind the front of the crawlers. Let’s say the machine is 5 m long. The center of mass of the thing would be located about 4 m behind the front of the crawlers.
Let’s say that we’re trying to lift 16 T, which is twice the weight of the machine. Provided that the load mass was less than 2 m in front of the machine, then there’d be no problem.
As long as:
(total mass of crane) x (distance of crane’s center-of-mass from front of crawlers) is less than (total mass of load) x (distance of load’s center-of-mass from front of crawlers)
you’re OK.
The above calcs ignore the weight of the jib, but that can be allowed for as part or the load mass and subtracted from the crane mass.
Sorry; my mistake; I thought you were talking about this kind of crane, which has counterweights that can be repositioned (or I think so).
But in any case, Desmostylus is on the money; as long as there are more moments behind the front supports than there are in front of them, it stays upright.
Also, remember that many of these sorts of vehicles have extendable legs for support, effectively enlaging the foortprint of the vehicle.
This dude can lift about twice its weight.
Of course, the distance between the centre of mass and the relevant outlying balance point of a crane with outriggers in a rectangular pattern varies as the boom moves around, depending on whether the boom is over an outrigger, or part way between them.
This fact cost a party to litigation I was involved in (not my client) over $2m.