Crane Safety Devices: LMIs & Anti-Two-Block
Modern cranes have devices that warn the operator, or stop the crane, before it’s overloaded or the hook block is pulled into the boom tip. They only work when they’re set up right and never bypassed.
- Crane operators
- Lift directors
- Mechanics
Photo: crane cab display showing load, radius, boom angle and percentage of rated capacity from the load moment indicator
Key devices
| Device | What it does |
|---|---|
| Anti-two-block (A2B) | Prevents the hook block or headache ball from being pulled into the boom tip (“two-blocking”), which can break the hoist line and drop the load |
| Load moment indicator (LMI) / rated capacity indicator | Measures the load and radius and warns or stops the crane as it nears the chart capacity |
| Boom angle or radius indicator | Shows the boom angle or the load radius |
| Boom length indicator | Shows telescoping boom length |
| Boom hoist limiting device | Stops the boom from being raised too far |
| Crane level indicator | Shows whether the crane is level |
| Outrigger position sensors | Tell the LMI how far outriggers are extended, on cranes that have them |
| Horn | Warns people nearby |
Set it up right
- Enter the actual configuration into the LMI: outrigger position, counterweight, boom or jib, parts of line, and working area. A wrong setting gives a false sense of safety.
- Test the A2B and LMI during the shift inspection, as the maker describes.
- Watch the display during the lift, but don’t rely on it alone. Know the load and the chart.
Never bypass, override or disable a safety device to make a lift. If the LMI says the crane is at capacity, it’s telling you to stop and re-plan.
When a device fails
- Required safety devices (like the level indicator, boom stops and the horn) must work. If one isn’t working, the crane can’t be operated until it’s fixed.
- For operational aids, OSHA allows limited time to repair with temporary alternative measures in place: generally 7 calendar days for aids like the anti-two-block and boom hoist limiting devices, and 30 days for aids like the LMI and angle indicators, unless parts are unavailable and other conditions are met.
- Temporary measures might include a dedicated spotter for two-blocking, or measuring radius and calculating loads before each lift.
Shift check
- LMI configured to match the crane’s actual setup
- A2B works and its weight or switch is in place
- Angle, length and radius readings look right
- Level indicator shows level
- Horn works
- Any fault codes or warnings resolved before lifting
Standards & references
- 29 CFR 1926.1415: Safety devices
- 29 CFR 1926.1416: Operational aids
- See also: Reading a Crane Load Chart · Crane Inspections
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