Digger Derrick Capacity Comparison

Most utilities and contractors use digger derricks to dig holes and set poles at 0° with the second boom section extended for most of the work day. In fact, research shows that digger derricks spend 75% to 80% of their working time doing just those tasks. 

In spite of that, no standard comparison lets customers easily match the size of a digger derrick to those working conditions. The usual specifications and comparisons tell only part of the story, such as a digger derrick’s capacity at a 10-ft. radius. That can lead to oversight in selecting equipment.

To help customers accurately pick a digger derrick that has the size and power to complete real-world tasks efficiently without being oversized and expensive, Terex is promoting the “work zone capacity” concept as a new industry standard. 

Work zone capacity describes the ability of digger derricks to per- form the tasks they are built to do—digging holes and setting poles. 

For digger derricks to be as efficient as possible, it’s imperative to understand and match the truck’s capacities to the range of work needed to complete the tasks that digger derricks are used for. The new work zone capacity standard is designed to help utilities and con- tractors do exactly that.
 

How digger derricks work

The most important measure of digger derrick performance is called “load moment,” a calculation that considers boom angle, the distance from the derrick’s centerline of rotation to the winch line or auger, and the derrick’s lifting capacity. 

For example, when an operator digs a hole for a pole, he or she must consider how far away from the truck they are going to dig, what kind of soil they’ll be digging in, and how deep and wide they will have to make the hole—all of which affect digging capacity. Operators also have to know the pole specifications, including height, weight, and diameter in order to determine the best hooking point.

Most operators set their derricks within 2- to 3-ft. of the maximum digging radius, no matter what size the hole. The 2- to 3-ft. of boom reserve helps keep the pole hole as straight as possible. For instance, a digger derrick with a sheave height of 47 ft. has a maximum digging radius of about 26 ft. with a boom angle of 0°. During digging, the boom angle will drop to less than 0° as the hole goes deeper. 

Operators need that extra room to extend the boom and keep the auger digging down in a straight line. Taking that into account, the operator would set up the unit to dig the hole at 23- to 24-ft. from the centerline of rotation. 

If, however, digging conditions are less than ideal due to wet, heavy material, and the digger derrick is using a larger auger diameter, such as a 24 in., 30 in., or more, the operator may have to move the digger derrick closer in order to lift the auger, full of material, out of the hole while digging. Repositioning the truck can reduce the overall efficiency of the crew constructing a new power line or changing a pole. 

After digging the hole, the operator will set the pole by putting a sling around it, attaching the sling to the winch-line hook, and up righting the pole so the derrick can grab and lift it into the hole. That should be fairly easy—as long as an operator knows the parameters of the load and can match them to the rig’s lifting capacity at the radius within the work zone.

That’s why it’s vital that operators understand all of their digger derrick’s capacities, not just those readily available on the manufacturers’ specifications sheets.

Current comparison methods

Throughout the evolution of digger derricks, fleet managers and line workers have used various methods to compare the working capacities of models from different manufacturers. Early on, digger derricks were compared by their capacity with the boom fully retracted and fully elevated. But that is not a practical working position, so comparisons had to evolve to better reflect how the equipment is really used.

Traditionally, digger derrick design has emphasized either digging capability or lifting capacity, so most comparison methods have included:

  • Winch capacities—the styles and capacities of winches used;
  • Lifting capacities—accounting for boom strength and for winch style and capacity; and
  • 10-ft. radius lifting capacity—a “working” comparison developed by Terex Tel-Elect in the early 1980s that considers a rig’s ability to lift loads within a 10-ft. radius from the derrick’s centerline of rotation.


But these more recent comparison methods often do not account for all of a digger derrick’s capacities, such as in-the-hole (ITH) dig- ging and out-of-the-hole (OTH) lifting capacities. 

To illustrate, let’s look at capacity comparisons within a 10-ft. load radius from the derrick’s centerline of rotation. This comparison can be misleading because the load may be too close to the truck to handle safely. 

A more accurate measure of a digger derrick’s capability is checking its capacity where it does most of its work: at lower boom angles, say from 15° to -15°, and at longer radii, for example 16- to 42-ft.—not at the commonly compared 10-ft. radius. In many cases, in order to get to the 10-ft. radius, the boom would need to be raised above 60°. That eliminates more than half of its potential work area.

The 10-ft. load radius is an effective measurement for only 5% to 10% of a digger derrick’s work, for example, setting larger pad mount transformers. The 10-ft. load radius capacity standard is also not effective for comparing digging capability—a digger derrick’s core job. As mentioned above, holes are dug and poles are set in the 20- to 40-ft. range, at 0° to -15°, depending on the size of the derrick. 

The new Terex work zone capacity standard reflects not only the boom’s lifting capacity; it also accounts for the digger derrick’s auger digging and lifting capacity. These capacities need to be close in order to make sure the truck is able to lift an auger filled with material out of the hole. For digger derricks designed to maximize working capacity in the digging zone, including the Terex line, the work zone capacity comparison gives utilities and contractors the information to select a rig properly sized to do all of the required work.

Work zone capacity advantage

The right digger derrick for a job should dig the hole and set the pole without being repositioned. That’s why operators need to consider all three requirements: lifting capacity within a 10-ft. load radius, in-hole digging capacity, and out-of-the-hole lifting capacity. The work zone capacity standard matches all three to the job.

Digger derrick operators know the weights of their loads, but often they don’t consider the force their digger derrick needs to lift the dirt-filled auger out of the hole. 

For example, a Class 1 55-ft. wooden pole weighs just less than 2,000 lbs. Most digger derricks can easily pick one from a trailer and set it at a 10-ft. radius. But, digger derricks are also designed to dig holes for the poles. When digger derricks are designed to maximize lifting capacity at a 10-ft. radius, they are normally challenged to provide enough digging capacity to pull a filled auger out of the hole at a normal digging radius, which needs to be done as part of the project. 

Terex has designed its line of digger derricks to excel at digging and OTH lifting, where most work is performed. When a customer really needs increased 10-ft. radius lift capacity, Terex offers the optional multi-control X-Boost system on its Commander 4000 series digger derricks. 

The system boosts hydraulic power to increase load-handling and lifting capacity within a 10-ft. radius. The Terex X-Boost system allows
customers to maintain the performance of their unit in the digging zone, yet also have increased lifting capacity at higher boom angles.

Calculating work zone capacity

To best calculate a digger derrick’s work zone capacity, operators need to take into account the soil conditions and density of the material the rig will work in, as well as the auger sizes they will use, the flighting size, and the number of flights needed.

Operators can then work with Terex and its sales team to put the numbers into a specialized calculator that determines a digger derrick’s work zone capacity.

The Terex work zone capacity comparison standard is designed to help utilities and contractors buy digger derricks that fit their businesses’ overall operation needs and that are properly sized to perform a variety of jobsite tasks.   

In an increasingly competitive bidding environment, it is more important than ever that a digger derrick be specified correctly to complete the jobs it is tasked to perform, on-time and on-budget. Work Zone Capacity considers all of a digger derrick’s capabilities in order to match the rig to the tasks it needs to do.