What is a load moment indicator on a crane?

A load moment indicator is an electronic monitoring and warning system integrated into the crane's control system. It uses sensors at multiple points on the crane - typically measuring boom angle, boom length on telescoping cranes, and hoist line pull via a load cell - to calculate the actual load moment in real time. The system compares the calculated load moment against the crane's load chart data stored in its memory for the current configuration and provides the operator with a continuous display of actual capacity utilization as a percentage of rated capacity.

What the LMI Monitors

A typical LMI system monitors and displays the actual load weight, the load radius, the boom angle and length, the percentage of rated capacity being used, and the remaining available capacity. Most systems provide audible and visual warnings when the load approaches a defined threshold, commonly 90% of rated capacity, and will trigger a cutout or warning at or near 100% of rated capacity depending on the system design and configuration. The LMI does not replace the load chart. It augments the operator's situational awareness by providing continuous real-time data that the operator cannot calculate manually during a lift.

OSHA Requirements and Limitations

OSHA 1926.1416(d)(1) requires that cranes designed to accommodate safety devices, including LMI systems, must have those devices installed and functioning. An inoperative LMI is a deficiency that must be documented and corrected before operation. However, OSHA and industry standards also recognize that an LMI is an aid, not a substitute for proper lift planning and load chart consultation. The LMI's accuracy depends on correct configuration programming by the operator: if the operator has programmed the wrong outrigger spread or boom length configuration, the LMI will display incorrect capacity values even while functioning properly.

Additionally, the LMI measures the load on the hoist line, which may differ from the true load weight if the rigging creates a mechanical advantage or if the line is not vertical. Operators must understand these limitations and use the LMI in conjunction with pre-lift load chart verification rather than as the sole capacity authority. ASME B30.5 addresses LMI requirements in its provisions on safety devices for mobile cranes.

LMI Inspection and Calibration

LMI systems require regular calibration and maintenance to maintain accuracy. The pre-shift inspection under OSHA 1926.1412(e) includes a check of safety devices, which encompasses the LMI. A system displaying obviously incorrect values, stuck readings, or error codes must be treated as nonfunctional. On cranes used for critical lifts, the LMI should be independently verified for accurate configuration programming before the lift proceeds. Records of LMI calibration and any detected inaccuracies should be maintained as part of the crane's maintenance records.