The abbreviation LMI stands for Load Moment Indicator, which is a common term used to describe a crane safety indicator. This is a commonly misused and misunderstood term. To be precise, an LMI is a crane operator aide that provides the crane operator with a display of the weight being lifted, the capacity at that radius, along with the boom angle and length. It shows the crane operator how much they are lifting and how much they are allowed to lift according to the crane manufacturers' capacity charts.
What makes a load moment indicator (LMI) different from a rated capacity indicator (RCI) is not the crane operator sees but how the system collects that information. The LMI uses sensors that measure the force on the crane boom, and the RCI measures the force on the hoist line. For example, with a telescopic boom crane, a hydraulic cylinder normally lifts the boom to change the boom angle. When a pressure transducer is installed on both sides of the lift cylinder, you can measure the force on the boom. Combined with a calculation of the boom radius and geometry of the boom cylinder, the weight of the load being lifted is determined. Whereas an RCI, uses a different style sensor such as a load pin or load link that measures the force or tension on the hoist line resulting from the weight of the load being lifted.
The cranes lifting capacity will vary depending on the crane's configuration as the boom moves through its range of motion, the radius changes, and with it, the load capacity. In general, a crane can lift more at a shorter radius versus a longer one. Another factor is the crane set up before the lift. For example, if the crane is using outriggers, it can normally lift more than when just on the tires or crawler tracks. Also, some cranes have counter weights that increase their capacity or have quadrants where they can lift more, such as over front versers over the side.
The Crane LMI measures the force on the crane boom and provides the crane operator with essential lift-related information about the crane. It does all this in a dynamic environment where the crane can be in motion and have multiple lift configurations. The operator will see the weight of the load being lifted, the capacity or amount the crane is capable of lifting safely, along with the boom angle, length, radius, and possibly the anti-two-block (ATB) warning.
Crane LMI's are one of the most helpful operator aides you can add to your crane.