A truck-mounted digger derrick is designed to enable utility companies to dig holes and set poles for electric power transmission and distribution systems.
In an ideal scenario, the derrick should be able to perform both functions – digging and lifting – without your crew having to reposition the truck. This way, your team can get more jobs done in less time, improving service to customers and bolstering your bottom line.
“If you select a derrick that’s too small for the job, you might be able to dig a hole from a certain position. But when you want to set the pole, the load capacity at the truck’s current position might not be sufficient to lift the pole,” said Jon Promersberger, engineer, new product development for Terex Utilities (www.terex.com), a global manufacturer of aerial work platforms and digger derricks. “So you have to move the truck to achieve the proper boom angle and lift capacity to do the job, which wastes a lot of time. If you’re going to dig your hole at 20 feet from the truck, you want to spec the derrick to allow you to also set the pole at 20 feet.”
But with a wide range of weight capacities, boom lengths and other factors to consider, how do you determine the right spec for your application?
Begin by gaining a working knowledge of some of the key terms the industry has used to describe a derrick’s capabilities.
This has been the historic metric for classifying derricks, defined as the maximum lifting capacity when the boom is fully retracted at its highest angle.
“Basically, maximum capacity is what’s measured when the boom is straight up in the air. If you had to lift something that’s heavy, and the boom is real close to the truck, it would tell the derrick’s capability in performing that function at that angle – and that’s about all. It doesn’t say anything about the capability of that boom at other angles,” Promersberger said.
Chris Barnes, derrick marketing manager for Altec (www.altec.com), an aerial equipment manufacturer and service provider for the electrical utility market, agrees. “The term is essentially meaningless today because there is virtually nothing you can pick up that close to the derrick. The truck and the outriggers [stabilizer legs] get in the way.”
Capacity at 10-Foot Radius
Realizing the limits of maximum capacity as a useful metric for selecting a derrick, the industry developed a new measure in the 1980s – capacity at 10-foot radius – to provide deeper insight into a derrick’s real-world capabilities. It defines how much weight the derrick can lift when the load is 10 feet from the center of the truck.
“Ten feet became an industry standard because it was likely the minimum load radius that could be useful in real-world applications,” Barnes explained.
But when comparing derricks based on the 10-foot rating, make sure the lifting capacity is the same from all boom positions, Barnes advised. “A derrick may have a higher capacity number when the boom is positioned to the rear of the truck than it would off the side, so you want to confirm with your manufacturer that you’ve accounted for any potential discrepancies.”
A sheave (pronounced “shiv”) is the pulley at the tip of the boom. Therefore, “sheave height” is defined as the maximum height of the boom when it is fully extended and elevated, assuming a 40-inch frame height of the chassis.
“Sheave height impacts what size pole the derrick can lift and the machine’s optimal digging radius,” Barnes said. “This is especially important because the minimum and maximum digging radius is really the envelope in which your derrick will be operating much of the time.”
Although there is no universal rule on matching a derrick’s sheave height to pole height, Promersberger offered this general guideline: Select a sheave height that’s about two-thirds of the length of the pole you intend to lift and set. For example, if the pole is 90 feet high, you would spec a sheave height of about 60 feet.
While terms like “maximum capacity,” “capacity at 10-foot radius” and “sheave height” can help you narrow down the general requirements for your derrick spec, you’ll have to dig deeper into the details of your application to identify the specifications that meet the unique needs of your application.
So, as you evaluate your derrick requirements, keep these factors in mind:
• What size pole will the derrick be setting? Consider height, diameter and weight.
• What soil type will the derrick be digging? Will it be topsoil, sand, clay, limestone or another type?
• Will the soil be wet or dry?
• What equipment and gear will be hauled on the truck? How much will that cargo weigh at maximum load?
• What gross vehicle weight rating chassis will be required to meet derrick and payload requirements?
One online tool to help you dig deeper into the specifics of your application is Terex’s Work Zone Capacity Calculator (www.terex-calculator.com), which takes into account not only the derrick’s boom lifting capacity, but also the auger (drill) digging and lifting capabilities best suited for your needs.
Also, make sure you’ve covered all your bases by working closely with your derrick manufacturer throughout the specification process. They can walk you through the load capacity charts specific to their products to determine the machine’s capabilities at various load angles and sheave heights, to ensure the derrick is right for all aspects of your application.
When you consider that a typical digger derrick has a six-figure price tag, the stakes are high to get the spec right. Be crystal clear about the derrick's job description and lean on your equipment manufacturer to help you align the derrick spec for the application to maximize productivity – and your return on investment.
About the Author: Sean M. Lyden is a nationally recognized journalist and feature writer for a wide range of automotive and trucking trade publications, covering fleet management strategies, light- and medium-duty trucks, truck bodies and equipment, and green fuel technologies. He blogs at Strategy + Writing (www.seanmlyden.com).