Load moment indicator (crane LMI and rated capacity indicator)

01

Match the crane, load charts and sensing

Recen lists its RC-A11-II safe load moment indicator for tower cranes as monitoring hook load in real time against the load chart to prevent overload and stop within the chart's limits. It shows hook height, working radius (jib angle), slewing angle, hook weight, wind speed and travel distance on a 10 inch touch screen with pre-alarm messages; load charts can be uploaded from a USB drive and programmed on a computer; working data is recorded and exported, with optional online supervision. It lists −20 to 60 °C operation, humidity up to 95% at 25 °C, AC 220 V ±25% supply, continuous duty, overall error of ±5% or less, load cell repeatability within ±0.3% and non-linearity within ±3%. [2]

Editorial calculation: load moment is load × radius; an illustrative 2.0 t at 50 m is 100 t·m, the same moment as 4.0 t at 25 m. The system must hold the crane maker's chart for each configuration rather than a single moment value. At ±5% overall error, a 2.0 t reading may be off by up to 0.1 t; ask how the alarm and cut-out thresholds account for this. [2]

A crane maker's product guide shows how the indicator ties to the load chart. Manitowoc's guide for the YB7700 series industrial hydraulic crane (22 ton, 19.9 t, at 2.6 m radius on outriggers) offers three levels: a wireless LSI load indicator with a load-sensing pin at the boom head transmitting hook load to a cab display; a wireless rated capacity limiter that also shows boom angle and length and stops telescope-out and boom-down when a load limit is exceeded; and a hard-wired load moment indicator that displays boom angle, length, radius and capacity with a colour light bar, audible alarm and function cut-out. Its load chart notes say rated loads on outriggers do not exceed 80% of actual tipping and on rubber 75%, that shaded areas are governed by structural strength and do not rely on tipping, that the ratings assume freely suspended loads with the crane level on a firm, uniform surface, and that for a radius not shown the next larger radius's rating applies. [3]

Editorial calculation: a chart load rated at 80% of tipping means the crane would tip at about 1.25 times the rated load (1 ÷ 0.8), and at 1.33 times on rubber (1 ÷ 0.75); an LMI with ±5% overall error, as Recen lists, uses a fifth of that 25% margin on outriggers. [3] [2]

Editorial note: give the supplier the crane make, model and configurations (jib lengths, counterweights, reeving) and the original load charts, and ask who programs and calibrates them.

Cranes with a main and an auxiliary hook need the indicator to handle both. Kobelco Cranes' European patent EP 1 180 490 B1 (Load moment indicator of crane, filed 26 March 2001, granted 7 June 2006) describes a crawler crane LMI built from a boom angle detector, a boom guyline tension detector (whole hoist load) and tension detectors on the main and auxiliary hoist ropes, feeding a load factor calculation (hoist load ÷ rated load) for the whole crane, the main side and the auxiliary side; when a load factor reaches the preset value, an automatic stop valve halts crane operation. It defines the rated load as the maximum hoist load from crane stability and the strength of members such as rope rupture strength, calculated in advance for every work radius and stored. Its worked example takes a main-side rated load of 40 t at a given radius (set by stability) and an auxiliary-side rated load of 10 t (set by rope strength): lifting on one hook alone, 40 t or 10 t can be lifted, but lifting on both at once the main-side capacity falls from 40 t to 20 t as the auxiliary load rises from 0 to 10 t. The patent converts the other hook's load into a load component of its own side and subtracts it, so the displayed rated load follows the actual combined lift, and it can display the remaining hoist load and the remaining work radius to a load factor of 100%. [4]

Editorial calculation: in that example each tonne on the auxiliary hook costs 2 t of main-hook capacity (20 t ÷ 10 t), so an indicator that keeps fixed rated loads per hook would overstate the main-hook margin by up to 20 t during simultaneous lifts. Ask the supplier whether its LMI recalculates each hook's rated load during simultaneous main and auxiliary hoisting, and whether it shows remaining load and remaining radius. [4]

Give the supplier the crane type, make, model and serial number, configurations and load charts, sensors needed (load, radius or boom angle, slewing, height, wind), warning or limiting functions, motions to cut out, display language, power supply, environment, data recording and remote monitoring needs, and installation and calibration support.

02

Meet the requirement and plan for repairs

Under OSHA's construction crane rule, 29 CFR 1926.1416(e)(4), equipment other than derricks and articulating cranes manufactured after 29 March 2003 with a rated capacity over 6,000 lb must have at least one of a load weighing device, a load moment (rated capacity) indicator or a load moment (rated capacity) limiter; articulating cranes manufactured after 8 November 2011 must have one of these or an automatic overload prevention device. These are Category II operational aids, which must be repaired within 30 calendar days (or within 7 days of receiving parts ordered within 7 days); meanwhile the load weight must be determined from a recognised source or calculation and given to the operator before the lift. [1]

The same section requires operations not to begin unless listed operational aids work or the temporary alternative measures are used, and the operator to stop safely if an aid fails during operations until alternatives are in place; it also lists boom angle or radius indicators and, on luffing-jib equipment, jib angle indicators as operational aids. [1]

Editorial note: rules outside the US differ; give the supplier the jurisdiction and any third-party inspection requirements.

03

Recen's tower, mobile and crawler crane indicators

Recen lists load moment indicators for tower, mobile and crawler cranes alongside anti-collision systems and anemometers; ask any supplier for the crane models supported, sensors and accuracy, alarm and cut-out functions, load chart programming, data recording and calibration procedures.

04

Load cell, radius and angle sensors, chart controller, display and limiter outputs

Recen's RC-A11-II supplies every row here; OSHA 1926.1416 adds boom angle or radius indicators and load moment limiters as operational aids.

05

Recen RC-A11-II values set against an LMI order's fields

Recen's RC-A11-II gives the accuracy, screen, supply, temperature and humidity values; power draw, storage temperature, ingress protection and weight are not stated in the sources reviewed.

06

Send the crane, chart and function data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Crane: [type; make; model; serial]
Configurations: [jibs; counterweights; reeving]
Charts: [original load charts]
Sensors: [load; radius/angle; slew; height; wind]
Functions: [indicate; limit; cut-outs]
Recording: [data log; remote]
Jurisdiction: [rules; inspection]

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