Safety Devices & Instruments
Boom angle indicator, LMI, anti-two-block devices, boom hoist limit switch, swing brake, free fall controls, and drift prevention.
- Explain the purpose and function of the boom angle indicator and Load Moment Indicator
- Define two-blocking and describe the anti-two-block device
- Identify the purpose of the boom hoist limit switch and swing brake
- Describe free fall operation, when it is prohibited, and safe alternatives
- Explain crane drift causes and prevention through inspection and maintenance
Lección 1
Boom Angle Indicator & Load Moment Indicator
Boom Angle Indicator
The boom angle indicator displays the angle of the boom from horizontal. This instrument is mounted on the boom or in the cab and provides the operator with a continuous reading of the boom's current angle. The boom angle is essential because it determines the load radius, which in turn determines the crane's capacity at that moment.
The boom angle indicator is a required instrument on all cranes. Without it, the operator cannot accurately determine the load radius and therefore cannot properly reference the load chart.
How to Use the Boom Angle Indicator
The operator reads the boom angle, then uses the load chart to find the corresponding radius and capacity. Some load charts are indexed by boom angle directly, while others use radius. Either way, the boom angle indicator is the starting point for determining safe capacity.
Load Moment Indicator (LMI)
The Load Moment Indicator (LMI) measures the relationship between load weight and radius to prevent overload. It is an electronic system that continuously monitors the crane's operating condition and warns the operator when approaching or exceeding rated capacity.
LMI Monitors
Load weight - from load cells or hydraulic pressure
Boom angle - from angle sensor
Boom length - from extension sensor
Load radius - calculated from angle and length
LMI Provides
Capacity display - current % of rated capacity
Visual warning - approaching capacity limits
Audible alarm - at preset warning threshold
Function lockout - prevents overload motions
The LMI calculates the load moment (load weight multiplied by load radius) and compares it to the crane's rated capacity for the current configuration. When the load moment approaches the rated limit, the LMI provides graduated warnings.
Exam Tip - LMI Function
The LMI measures the relationship between load weight and radius to prevent overload. It is not a wind speed sensor, engine RPM gauge, or hydraulic pressure monitor - it specifically prevents overloading.
The boom angle indicator displays the boom's angle from horizontal, which determines load radius and capacity. The Load Moment Indicator (LMI) measures the relationship between load weight and radius to prevent overload by providing warnings and function lockout.