Stability, Outriggers, and Load Moment/Radius Management
What this domain tests
Questions focus on why the machine tips, how load moment grows with radius, and how outrigger setup and operating technique preserve stability.
Core rules
- Load moment = load x radius. A small increase in radius sharply increases overturning force.
- All outriggers fully extended, pads in contact, machine level within the manufacturer’s limit. Never use outriggers to level the truck on a slope beyond that limit.
- Avoid shock loading, side loading, dragging the load, and rapid boom movement.
- Use tag lines to control rotation; keep personnel clear of the load and the boom’s path.
- Operate within the correct quadrant and configuration shown on the chart.
Worked example
A 3,000 lb load at 20 ft produces 60,000 ft-lb. The chart at 20 ft allows 3,000 lb, so the lift is at the limit. If the operator booms out to 25 ft, the chart drops to 2,100 lb. The same 3,000 lb load now produces 75,000 ft-lb, well beyond the rating, and the machine can tip even though the load never changed. The fix is to reposition the truck or reduce radius, not to “try it slowly.”
Traps
Remembering only the maximum capacity number and ignoring radius; setting only the side outriggers because the lift is over the rear; assuming the LMI allows an overload; and lifting with the outriggers on soft ground that consolidates under load.
Key points
- Load moment equals load multiplied by radius.
- Fully extend and pad all outriggers; keep the machine level.
- Avoid shock, side, and drag loads.
- Reposition the truck instead of exceeding chart radius.
- The LMI warns; it does not authorize an overload.