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

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.

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

With respect to cranes, radius is the horizontal (level) distance from the center pin of the crane to the center of gravity of the load being lifted. You measure, with your tape measure stretched flat and level, from the middle of the load to the middle of the crane. Usually, the longest reach or ‘radius’ required is to where the load is to be placed or installed, so you have to make that measurement as if the load were already there- to the center of where its weight is going to be. Just remember, all ‘radius’ is is a horizontal measurement in feet from where the middle of the crane is set up to where the the middle of the load needs to be.

The center pin of the crane is generally about in the middle of the deck of the crane and is usually, nowadays, a stump containing dozens of hydraulic and electrical connections rather than a pin. By definition any mobile construction crane can swing a load, in other words, the whole upper structure including the boom with the load hanging from it can be easily and smoothly made to rotate about a center point on the job site. That center point is directly under the middle of the crane where the center of the big ring gear would be. All future crane operations from that crane set up site are relative to that center point. The decisions about weight and reach are all based on the distance from where the load needs to be placed back to that center point… the 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-it’ll pick 9 times out of 10. 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. Use a good long chain four way and test lift it, then adjust chain links to satisfactory level for your crew.

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.