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Load Radius

A radius is just a straight line measurement…from the very center of any circle to the outside edge of that same circle.

Long boom crane lifting materials

With respect to cranes, the load radius is the horizontal distance from the center of rotation of the crane to the center of gravity of the load being lifted. The load radius is a critical consideration when determining what size crane is required for most lifts. A five foot increase in reach required can easily mean a ten thousand pound reduction in capacity.

Horizontal is another term for level in crane talk. Horizontal measurements are made with the tape measure stretched flat and level (along the ground, or a roof top, for instance) and measuring to the nearest foot is usually good.

The center of rotation is a plumb line passing through the exact center of the crane turntable or swing gear. If you hold a plumb bob over the center of the swing gear, the string would be the center of rotation.

It might have to be a long string. If we want to know the load radius to set an air conditioner out in the middle of a roof, we need to know the horizontal measurement. If the building has a 45′ eave height, we’d need to start a good 40′ directly above the exact center of the crane turntable, a 41′ plumb string would be in order! Obviously, in the real world, its a lot easier to measure to something that is already plumb, like the wall, and then go up on the roof and measure from the eave on across the roof to where the unit center will be. Adding these two measurements will furnish the appropriate load radius.

The center of gravity is just a term for the very middle of the weight of the load. This is not always the tape measured middle of the unit.

If the load is a bundle of 12 foot long 2X4s then the center of gravity (c.g.) is indeed pretty close to 6′ from one the end and about half way across the bundle. If the load is a 50′ steel wide flange beam, the c.g. is pretty much exactly 25′ from the end, go ahead and choke it right there. Most rooftop HVAC units, on the other hand, have heavier internal components concentrated toward one end/side or the other. You can’t accurately determine the c.g. of most HVAC units with a tape measure.

The exact location of the c.g. notwithstanding, if you are so close to overloading your crane that one or two feet internal weight distribution makes a difference, it is usually best to move the crane closer to the load destination.


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