If you would, step into the cab of our imaginary 65,000-pound crawler excavator and make a hypothetical lift. The site requires your machine to sit with its tracks parallel to the trench, but it's set back from the edge as far as practical for safety. This means you'll initially be picking an 8-foot length of 30-inch-diameter concrete pipe over the idler end of the machine. The lift radius (the distance between the center of the machine's swing bearing and the center of the load hook) is 15 feet, and the lifting eye on the back of the bucket is 5 feet above the ground when the rigging is pulled tight. At this starting point, the machine has 34,210 pounds of lift capacity, so the 3,600-pound pipe on the hook isn't much of a challenge.
But, as you ease the load 5 feet off the ground, thus raising the bucket's lift point to 10 feet, capacity diminishes to 29,630 pounds. As you swing 90 degrees, placing the pipe over the side of the tracks, capacity drops to 23,790 pounds. Now, as you arm out and boom down to center the pipe over the trench, you increase the lift radius to 20 feet and capacity plummets to 15,670 pounds. Then, as you touch the pipe to the trench floor, with the bucket's lifting eye now at 5 feet below grade, capacity is 14,500 pounds. From start to finish, then, lift capacity has diminished by nearly 60 percent during this simple operation.
As our hypothetical lift demonstrates, lift capacity of an excavator—or backhoe—changes radically as the load position varies.
It makes sense, then, when lifting with either of these machines, to first think through the lift and determine where the machine is least capable, and if capacity at that point is sufficient to handle the load safely. To perform this simple, pre-lift analysis, you need to know the weight of the load, and you need to be looking at the machine's load chart, which should be mounted in the cab and legible from the control position.
The hydraulic excavator's load chart provides lift capacities for a range of lift-height/lift-radius coordinates, for both over-end and over-side lifting. True, rated capacities for hydraulic excavators do not exceed 87 percent of hydraulic lift capacity, which is defined by the Society of Automotive Engineers (SAE) test procedure J/ISO 10567 as the smaller of boom or arm hydraulic lift capacity at specific lift positions. Nor, by the same standard, do these capacities exceed 75 percent of the machine's static tipping load. Still, the best advice is not to invade this safety margin. Use a larger machine if you're bumping up against rated capacities.
If for some reason the chart is not in the cab, make a copy from the operator's manual or from specifications given on the manufacturer's website. Laminate it in plastic and keep it handy.
If knowing machine capacity is one side of the coin for making safe lifts, then knowing the weight of the load is the other. Making a pick without knowing how much weight is hanging on the hook is risky.
Since excavators and backhoes, when used as cranes, frequently are handling concrete pipe and related structures, the American Concrete Pipe Association has done the industry a favor by providing weight charts online. But if you're lifting less familiar objects, structural beams, for instance, a pocket-sized rigging manual that helps you calculate volumes and that has weight charts for various materials would be a good resource to have in your back pocket. One such reference we found was Bob's Rigging & Crane Handbook.
Another possibility for determining load weights is to use a crane scale (sometimes called a dynamometer). These relatively small devices, available as both digital and analog units, are designed for use in the load line for checking weight on the hook. Many give a direct reading of weight, and most retain the reading until reset, a feature that is convenient when getting too close to the suspended load might be dangerous.
Example websites include Itin Scale Co. (click on "crane scales" and "Chatillon Dynamometers"); Rice Lake Weighing Systems (click on "crane scales"); and Hanes Supply (click on "catalog online," "catalog index," "below hook lifting" and "load indicator devices."
The third obvious essential in making safe lifts is rigging—how the load is attached to the excavator or backhoe. Rigging is a study in itself, and you're well advised, if you frequently lift with an excavator or backhoe, to keep a rigging reference in the cab. Several handy-to-use volumes are available, and the one we especially like is Rigging, published by the Crane Institute (click "products" and "handbooks").