Digger Derrick Components, Terminology, and Load Charts
What this domain tests
Expect questions on component names, how the rated capacity chart is read, and how attachments and rigging reduce available capacity.
Components and terms
A digger derrick is a truck-mounted unit with a turret, lower and upper boom, fiberglass insulating section, auger and Kelly bar, pole claw or grabber, winch, jib, outriggers, and controls. “Radius” is the horizontal distance from the center of rotation to the center of the load, not from the boom pivot or boom tip. Capacity depends on radius, boom angle or length, outrigger configuration, and quadrant.
Load chart rules
- Use the manufacturer’s chart for the exact configuration: outriggers fully extended versus stowed, boom length, jib, basket, and quadrant.
- Deduct the weight of the auger, pole claw, jib, basket, slings, and any stowed gear as the chart notes require.
- Do not read between chart lines or extrapolate beyond the printed values.
- The load moment indicator is an aid; the chart and the physical setup govern.
Worked example
Chart capacity at 15 ft radius with outriggers is 4,000 lb. The pole claw weighs 350 lb, so available capacity is 4,000 − 350 = 3,650 lb. The pole and slings weigh 3,400 lb. Total applied = 3,400 lb, which is less than 3,650 lb, so the lift is within the chart. At 18 ft the same chart shows 3,100 lb; available becomes 2,750 lb, and the 3,400 lb load is now over capacity.
Traps
Measuring radius to the boom tip; ignoring attachment deductions; using “on rubber” capacities while set on outriggers, or the reverse; and assuming the chart applies when the outriggers are only partially extended.
Key points
- Radius is measured from the center of rotation to the load center.
- Capacity changes with radius, boom configuration, and quadrant.
- Deduct attachment and rigging weight per chart notes.
- Never extrapolate beyond the printed chart.
- The LMI is an aid, not a substitute for the rated chart.