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The answer is that it is seldom something truly heavy that gets lifted, it's generally that the crane has to lift something a considerable distance from where it sets up. A 300 ton crane may only be rated to lift something as light as 8 tons, at the limits of it's reach.
Crane capacities are based on the lift distance from the center pin, the amount of boom extension, the amount of counterweights fitted, the direction of the lift, etc. There are charts for every crane that show the limits and they also have the ability to know what the load is and have computers that calculate the limits and prevent the operator from exceeding them. These limits can be overridden in some cases...
Mobile hydraulic cranes like the ones used the other day exist up to something like 1300 tons. They require a train of supporting vehicles, as they won't be fully assembled until they get to the site. The block may be carried separately, the majority of the counterweights will be carried separately, the pads for the outriggers, the rigging (spreader beams, etc.) and other stuff. A second crane will be needed to install these things on site.
The reason for that is simply the weight. A big crane weighs too much for the load limits on the highways. They typically have to get special permits to travel and this limits when and where they can go. When they get to the job site, they often are too heavy for the soil conditions encountered, both when they drive to where they need to make the lift and when the lift occurs, as the soil under the outriggers may not support the load. It's always a balancing act and you want to use the smallest crane that can get the lift done.
These cranes are rented out by the hour. A crane company will buy one if they feel that they can rent it often enough to make a profit on it. Around here, the last time I was involved in that arena, 300 tons was what we had available. Beyond that, you were forced to use a crawler crane, which ups the cost considerably. Maybe there are bigger ones available locally now, I don't know. The alternative is to get one from a larger market.
I don't know what size they used for the 737 lift but they were pretty big.
JR
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