It is a crane owner’s or operator’s nightmare: By accident or carelessness a crane boom topples, causing physical damage, often substantial, but more tragically, injuries or loss of life.
Crane accident causes are many – typically involving overloading (exceeding weight limitations), operating the equipment in high wind, or other conditions that exert additional strains on the unit’s lift and lift management capabilities. Crane warning systems are valuable tools that can help prevent these incidents.
It is important to realize that crane warning systems do not alert operators to dangers such as siting the crane on unstable ground or failure to properly set and extend outriggers. Only training and common sense can prevent accidents of this type.
Many crane accidents were and are preventable by installing and maintaining crane warning systems called Load Moment Indicators (LMI). These are software-based crane warning systems that monitor and provide operators with critical information necessary to manage their lifts. Among other functions they take into account the rated capacity of the crane, warn of impending contact between the crane hook and the boom tip, and factor in external forces such as wind.
This post describes the importance of load moment indicators that signal potentially dangerous operating conditions. It looks at what you as a crane owner or operator must consider when updating or replacing a malfunctioning or obsolete LMI crane warning system.
You will be provided information related to
- Load Moment Indicators and How they Work
- Responsibilities of Crane Owners and Operators
- Governing Authorities on Safety in Crane Operations
- LMI Replacement Considerations: Risk and Reward
- Crane Failure Case Studies
How do Crane Load Moment Indictors Work?
You may be familiar with these instruments but there is always value in a little refresher. This information may be useful when you have to go to your boss to repair or replace your LMI crane warning system.
The Load Moment Indicator (LMI) assists the operator by providing the information they need to have a safe lifting environment on the jobsite.
Using various sensors on the crane the LMI monitors the lift and provides the operator with a continuous reading of the crane’s safe operation within the load chart.
Readings continuously change as the crane moves through the motions needed to make the lift. If potentially unsafe conditions are approached, the LMI crane warning system will alert the operator by sounding an audible alarm and illuminating a warning light. Many crane LMI systems will also activate a lockout feature at this point.
The LMI provides the operator with information concerning certain geometrical data such as
- Load on the main line
- Load on the auxiliary line
- Boom length out
- Boom angle
- Radius of the lift
- Jib angle especially for a luffing jib
- Potential wind speed danger
- Operating mode
There are two means by which an LMI crane warning system can function.
- The Load Moment or Rated Capacity Indicator
This system aids the equipment operator by sensing (directly or indirectly) the overturning moment on the equipment, i.e., load multiplied by radius.
It compares this lifting condition to the equipment’s rated capacity and indicates to the operator the percentage of capacity at which the equipment is working.
Lights and buzzers are usually incorporated as a warning of an approaching overload condition.
In this case it is critical for the operator to halt the lift and take corrective measures.
- The Load Moment (or Rated Capacity) Limiter
This system aids the equipment operator by sensing (directly or indirectly) the overturning moment on the equipment, i.e., load multiplied by radius.
It compares this lifting condition to the equipment’s rated capacity, and when the rated capacity is reached, it shuts off power to those equipment functions which can increase the severity of loading on the equipment, e.g., hoisting, telescoping out, or luffing out.
Typically, those functions which decrease the severity of loading on the equipment remain operational, e.g., lowering, telescoping in, or luffing in.
The system operates on the principle of reference/real comparison.
The real value, resulting from the pressure measurement and geometric sensors, is compared with the reference data stored in the central processor memory and is evaluated in the microprocessor.
An analog indication of percent capacity is displayed via an operator’s console.
When limits are reached, an overload warning signal is generated. At the same time, the crane movements that increase load moment such as hoist down, telescope out, and boom down will be locked out.
The fixed data regarding the crane, such as dimensional data, capacity charts, boom weights, and centers of gravity are stored in memory chips in the central processor unit.
This is the reference information used to calculate the operating conditions. Boom length and boom angle are registered by the length/angle sensor, located inside the cable reel, mounted on the boom.
The boom length is measured by the cable reel cable, which also serves as an electrical conductor for the anti-two- block (A2B) limiting switch. [This is a safety device that prevents contact between the hook block (ball) and the end of the boom.]
Here is a sequence of operations:
- The crane load is measured by pressure transducers attached to the piston and rod side of the lift cylinder.
- Controls on the operator’s console allow the operator to select the operating mode (main boom, jib, etc.) as well as the parts of the line currently in use.
- The system will then limit operation to the capacities on the selected load chart, or the maximum permissible line pull in that configuration.
- When operations begin the LMI powers up and conducts an automatic test of the complete system. After two successful system tests without errors, the system is ready for operation. In case of a system error, both the overload and anti-two-block lights on the central unit will illuminate. The interactive user guidance considerably simplifies the input of operating modes as well as the setting of geometry limit values.
Crane Owner/Operator Responsibilities
As indicated above the LMI is an operational aid that warns a crane operator of approaching overload conditions and over hoist conditions that could cause damage to equipment and personnel.
The device is not, and shall not, be a substitute for good operator judgment, experience and use of accepted safe crane operating procedures. [OSHA 1926.1417(k) see below]
The responsibility for the safe crane operation shall remain with the crane operator who shall ensure that all warnings and instructions supplied are fully understood and observed.
Prior to operating the crane, the operator must carefully and thoroughly read and understand the information in the user’s manual to ensure that he or she knows the operation and limitations of the crane.
Proper functioning depends upon proper daily inspection and observance of the operating instructions set forth in the operator’s manual.
Governing Authorities Regarding Safe Crane Operations
The U.S. Department of Labor, Occupational Health and Safety Administration (OSHA) has promulgated a series of regulations including Cranes and Derricks in Construction also referenced above.
Of particular relevance to this post is 1926.1417(o)(3)(ii) [italics ours]:
“The operator must begin hoisting the load to determine, using a load weighing device, load moment indicator, rated capacity indicator, or rated capacity limiter, if it exceeds 75 percent of the maximum rated capacity at the longest radius that will be used during the lift operation. If it does, the operator must not proceed with the lift until he/she verifies the weight of the load in accordance with paragraph (o)(3)(i) of this section.”
Another reference, older but in our opinion still valid regarding crane warning systems, is an alert published by the CDC’s National Institute for Occupational Safety and Health (NIOSH) covering Mobile Crane Tip-Over, Boom Collapse, and Uncontrolled Hoisted Loads.
It notes on page 5 that “Modern cranes may incorporate computerized load management indicators, which monitor hoisting data and provide the crane operator with a readout of lift conditions.” It further notes “These devices require periodic maintenance, verification and recertification by a knowledgeable person” and that “The LMI alone does not ensure a safe lift.”
Later in this post we will refer back to this document to present a few crane failure case studies.
LMI Crane Warning System Replacement Considerations: Risk and Reward
It is pretty safe to say that everyone has a vested interest in the safety of the lift as both the job site and the crane operator/owner handling the lift want things to go smoothly.
But sometimes cranes working on these jobsites may be old and have either no or dated LMI equipment that may or may not be OSHA compliant.
Older LMI crane warning systems are prone to failure, and in many cases only one component may need replacement.
Challenges and concerns for both owners and operators are to rationalize the long-term benefits of replacing an entire system vs. the lower cost of replacing only the failed component.
Examples of Crane Failures
We offer examples relating to LMI crane warning system issues. Again, italics ours.
A carpenter at a construction site was killed when an overloaded concrete bucket resulted in a crane tip-over. An investigation revealed that the crane’s recommended capacity had been exceeded and that the LMI had been known to indicate false reading in the past and had not been repaired or recalibrated. The LMI may have contributed to the incident by providing the operator with false information. (Source: NIOSH document cited above, Case 2.)
Federal officials say there was poor communication and faulty safety monitors the day a crane toppled when contractors were attempting to pull down a concrete wall.
OSHA found the safe load indicator on the crane had been malfunctioning for several weeks and noted there should be an audible alarm when an object is considered too heavy and the indicator is working properly. On the day the crane tipped over, its operator overrode the indicator, OSHA found. There also were communication problems between the crane operator and a signalman.
Four months later, the Occupational Safety and Health Administration issued citations to three companies involved in this project totaling more than $10,000 in fines.
As another example, OSHA cited a Florida company for eight safety violations with $70,000 in proposed penalties. The agency opened an investigation after an incident in which an overloaded 30-ton crane fell during the removal of a 40-foot tree behind a commercial building.
The two-part fine included allowing the crane to be put back into service before deficiencies from previous audits and inspections were corrected; overloading the crane’s capacity causing it to tip over, and allowing the crane operator to operate the equipment without taking a written examination.
Wind can be a major factor in crane failures, as reported in an article in Heavy Lift News. It noted that “Wind might be invisible, but it’s still one of the most audacious hazards a crane operator can encounter. According to ANSI, ‘Between 2000 and 2010, there were 1,125 tower crane accidents reported worldwide, resulting in over 780 deaths.’ ”
A wind speed indicator is a critical addition to safe crane operation.
If you are of a mind, an excellent compendium can be viewed by logging on to Crane Accidents, listing failures and fines traced to many causes.
In Conclusion
We hope this post provides ample incentives to both crane owners and crane operators to assure themselves that their cranes are equipped with fully functional load moment indicators. For a thorough description of the components involved in crane warning systems please visit our website or contact us for answers to your questions and concerns.
