The crane operator is allowed to raise the crane’s boom to a certain height based on the safe operating limits. When sufficiently high, the boom will hit a limit, and the crane will automatically prevent any further upward movement to prevent the crane from being overloaded or tipping.
The part of the crane that prevents the operator from raising the boom too high is the boom angle limiter (also called a boom angle limit switch or part of the load moment indicator system (LMI)).
This system prevents the operator from raising the boom past the manufacturer’s limits, which is generally between 75° and 85°, depending on the crane type.
Why Do Cranes Need to Limit the Boom Angle?
The limiter protects the boom from being raised into the “danger zone.” As the boom angle gets higher, the crane becomes more likely to tip backward.
The boom angle limiter is designed to prevent the boom from lifting too high and causing instability, overload, boom failure, or wire-rope breakage.
How Does the Boom Angle Limiter Work?
A boom angle limiter usually includes a sensor + a controller + an alarm + stop action. The sensor tells the system, “The boom is high enough — stop now.”Then the system alarms and blocks further upward movement.
- Angle sensor (mounted at the boom base) measures the boom angle in real time.
- The sensor sends the angle to the controller.
- A few degrees before the limit (usually 1–3° early), the system triggers an alarm.
- When the limit is hit, the system locks out upward motion (hydraulic lock / electric cutoff).
Different types of angle limiters
| Type | Simple explanation of how it works |
| Mechanical contact limit | Boom touches a switch： switch says ,“Hey, that’s high enough!” → system stops. |
| Electrical limit switch | When the preset angle is reached, the switch opens → boom-up function stops. |
| Electronic angle sensor (most modern cranes) | Sensor reads the angle → controller decides → boom-up is blocked. |
| Hydraulic pressure/angle protection | Boom touches a switch： switch says,“Hey, that’s high enough!” → system stops. |
What Is the Maximum Boom Angle? Can You Adjust It?
Indeed, there is an adjustable max boom angle; however, the operator is not the one in control of the adjustments. Only trained technicians or the manufacturer should make this adjustment, as it has to do with the stability and safety of the crane.
Typical maximum boom angles (with adjustability)
| Crane Type | Usual Max Boom Angle | Adjustable? | How It’s Adjusted |
| Truck crane (8–25 ton) | 74°–78° | Yes | Reposition the mechanical limit block |
| Small knuckle boom / service crane | 68°–72° | Yes | Reposition mechanical limit block |
| Crawler crane (large) | Around 80° | Yes | Adjust in system parameters |
| Articulating/knuckle-boom crane | 60°–75° | Yes | Needs manufacturer software access |
What Happens If the Boom Angle Limiter Fails?
There are serious dangers regarding the boom going too far above the safe angle. Of particular concern are boom collapses, tipping, failure of the wire ropes, and severe injuries.
Failure scenarios and risks
| Fault Type | What You See On Site | Possible Danger |
| Angle not showing / wrong display | Angle jumps around, shows 0°, or freezes | Operator cannot judge boom angle → risk of tipping |
| No alarm, no limit action | The alarm sounds too early | Crane overturning, boom failure |
| Early alarm, can’t lift | Alarm only after entering the danger zone | Work cannot continue (safe but annoying) |
| Late alarm | Cylinder damage, boom bending, and full machine instability | Over-angle operation → structural damage |
| Limit doesn’t stop boom | Boom continues rising past max angle | Cylinder damage, boom bending, full machine instability |
How to Inspect, Calibrate, and Maintain the Boom Angle Limiter
In this section, let’s shed light on the practical limitations about which crane device restricts the maximum angle of the boom in the vertical position. Such users tend to seek hands-on, practical, real-world guidance for troubleshooting alarm or limiter-related issues.
Daily Checks (takes 3 minutes before starting work)
| Item | How to Check | Normal Condition | Abnormal Condition |
| Angle display | Power on and observe | Stable, no jumping | Flickering, drifting, no display |
| Wiring / harness | Visual check | Clean, tight, no damage | Loose plugs, oil, broken wires |
| Mechanical linkage | Watch movement | Smooth | Sticking, bent parts |
| Alarm function | Raise boom near limit | Beeps/flashes normally | No alarm / alarms too early |
Weekly Checks
- Make sure sensor bolts are tight
- Check if sensor position shifted (must be within ±1°)
- Confirm the limit switch triggers 1–3° before max angle
- Inspect wiring for wear
Monthly Calibration Steps
- Park the crane on level ground and retract the boom
- Set the boom to 0° using an external inclinometer
- Enter controller calibration mode
- Set 0° point
- Raise to 60° and set the mid-point
- Raise to the max angle (e.g., 74°) and set the limit point
- Save → restart system → verify error is within ±0.5°
Every 6 Months (500–1000 hours)
- Replace waterproof seals
- Clean dust/grease from the limit mechanisms
- Check sensor’s waterproof housing
- Inspect wiring for age cracks
- Perform maximum-angle safety test (must alarm AND stop the boom)
Summary
Boom Angle Limiter (or angle limit switch / LMI system)
- Keeps the crane from attaining dangerous angles that put it at a greater risk of tipping over or having a failure in the boom structure.
- Most types of cranes will set the maximum boom angle to around a 70-80 degree angle.
- Adjustments to these angles is only permitted by professionals due to the high risk of failure.
- Due to faulty limiters leading to overturned cranes, daily inspections and calibration of angle limiters is a must.
If your crane is showing angle alarms or the boom stops too quickly (or too late), check the limiter first – it is your first line of defense against a serious accident, malfunction, or crane failure that could jeopardize the safety of workers on a job site.
