A Load Moment Indicator (LMI) is the primary safety computer on a modern crane. It continuously monitors the crane's load, boom angle, boom length, radius, and configuration, then compares those values against the crane's load chart in real time. When the crane approaches its rated capacity, the LMI warns the operator. When the crane exceeds its capacity, the LMI can physically prevent further movement.
In crane accident litigation, the LMI is often one of the most important pieces of evidence. Was it functioning? Was it calibrated? Was it overridden? The answers to these questions can determine liability.
How a Load Moment Indicator Works
The LMI uses sensors throughout the crane to monitor key variables:
- Boom angle sensor: Measures the angle of the boom relative to horizontal
- Boom length sensor: Tracks telescopic boom extension
- Load cell or pressure transducer: Measures the weight on the hook (either directly via a load cell or indirectly by monitoring hydraulic pressure)
- Swing position (on some models): Tracks whether the crane is working over the side vs over the rear, which affects capacity
The LMI calculates the load moment (load weight multiplied by radius) and compares it to the crane's rated capacity for the current configuration. It displays the percentage of capacity being used, typically on a screen in the cab.
Warning and Cutoff Levels
- 85-90% capacity: The LMI activates an audible alarm (steady beep) and visual warning (yellow indicator)
- 100% capacity: The alarm becomes continuous, the display turns red, and the LMI engages function lockout, preventing the operator from extending the boom, lowering the boom (increasing radius), or hoisting additional load
The lockout function is critical. When working properly, the LMI physically prevents the operator from making movements that would exceed the crane's capacity. The operator can still retract the boom, raise the boom (decreasing radius), or lower the load, all movements that reduce the load moment.
When the LMI Fails or Gets Overridden
In my experience investigating crane accidents, LMI issues fall into several categories:
Calibration Problems
An LMI must be calibrated to the specific crane it's installed on. If the crane is modified (a longer boom section added, counterweight changed, or outrigger configuration altered) and the LMI isn't recalibrated, it can display inaccurate readings. The operator may believe they're at 80% capacity when they're actually at 95%.
Deliberate Override or Bypass
Some operators or companies disable the LMI's function lockout to avoid the capacity limits. This is a serious OSHA violation and is one of the clearest indicators of negligence. Common bypass methods include:
- Using a key switch to enter "test mode" or "setup mode" that disables lockout
- Disconnecting sensors or wiring
- Manipulating the boom angle sensor to make the LMI think the radius is shorter than it actually is
When an accident occurs and the LMI was bypassed, the operator, the crane owner, and potentially the employer all face significant liability.
Malfunction and Failure to Repair
Like any electronic system, LMIs can malfunction. Sensors fail, wiring corrodes, displays go blank. The issue isn't that malfunctions happen. It's whether the crane was operated after the malfunction was known, and whether maintenance records show the issue was reported and addressed.
OSHA Requirements for LMI Systems
Under 29 CFR 1926.1416, OSHA requires that safety devices (including LMIs) be functional before each shift. The operator's daily inspection must verify that the LMI is displaying properly and that warning/cutoff functions are working.
Key regulatory points:
- The LMI must not be overridden except in specific emergency or testing conditions outlined in the manufacturer's procedures
- A crane with a non-functional LMI must not be operated until the device is repaired, unless the operator uses alternative measures approved by a qualified person
- The employer must maintain records of LMI inspections, calibrations, and repairs
What the LMI Data Can Reveal in Litigation
Many modern LMI systems include data logging, sometimes called a "black box." These systems can record:
- Load weight, boom angle, and radius for each lift
- Instances where the capacity warning was triggered
- Instances where the lockout engaged
- Whether the system was in normal operating mode or test/override mode
- Error codes and fault history
This data can reconstruct what the crane was doing in the minutes and seconds before an accident. It can prove whether the crane exceeded its capacity, whether the operator was warned, and whether the LMI was functioning.
Critical for attorneys: Preserve the LMI data immediately. Send a spoliation letter to the crane owner specifying that the LMI unit and its data must not be altered, reset, or removed. Some systems overwrite data after a certain number of events.
Questions to Ask in a Crane Accident Investigation
- Was the LMI functional at the time of the accident?
- When was the LMI last calibrated, and by whom?
- Does the LMI have data logging capability, and was the data preserved?
- Were there any prior complaints or work orders related to the LMI?
- Was the LMI in normal operating mode or was it in test/override mode?
- Has the crane been modified in any way since the last LMI calibration?
- What did the operator's pre-shift inspection notes say about the LMI?
Get Expert Analysis for Your Case
Matthew J. Stull brings 27+ years of hands-on crane operations experience to every case. He can inspect LMI systems, interpret data logs, and testify on whether the safety system was functioning, calibrated, and properly used.
Call (570) 903-7119 or send a message to discuss your case.