Reports of mobile crane accidents that are a result of overloading tend to make national news. When a crane operator tries to lift a load greater than the crane’s weight capacity, the results can be disastrous.
Mobile cranes are the backbone of modern-day construction. They can travel on crawlers or tires, with or without a load. When in fixed positions, outriggers and other additions can sustain these self-powered jib cranes to provide stability. Mobile cranes can be equipped with either lattice jibs, articulated jibs or telescopic jibs—or a mixture of several jib types.
Along with their flexibility, the fact that these cranes are so popular means that crews can get lax with load calculations. The majority of the time, accidents are caused by an error made by the crew at the end of a long, difficult shift while workers are tired and no longer mindful of detail. However, with the right planning, the factors that lead to an overloading accident can be lessened or eliminated altogether.
STEPS FOR SAFE OPERATION
Crane operators need to be sure that they’re programming the right information into the load movement indicator (LMI). The LMI provides the crane operator with a continuous readout of the boom angle, radius and length. The accuracy of the LMI directly correlates to the accuracy of the information inputted, which is why a crew should avoid doing a lift without the correct weight information. When lifting a tank, for example, the lift director needs to account for the weight of any fluid still in the tank. Noting the tank’s weight, but not the weight of its contents, can create a significant problem in the field.
Operators need to use the right amount of counterweights. Assuming the distance is great enough, having too few counterweights can make a crane topple even if it’s only lifting a light load. Be sure to rig and balance each load correctly to avoid accidents like this.
Operators are advised to use load charts to verify the correct crane configuration for the lift conditions and load site. They need to re-check all calculations, such as net capacity, rigging and load weights. Keep in mind that the proper equipment, careful calculations and consistent communication among crew members will results in a safe lift.
Safety products like the Rated Capacity Limiter (RCL) can help prevent overloading. An RCL measures the weight of the load, compares it to the manufacturer’s suggested load rate given the current length extension and boom angle, taking note of whether or not the stabilizers are lowered or raised. In emergencies, or if an RCL fails, an override key can be used.
Understanding difference between an LMI and an RCL is crucial.
LOAD-TESTING
Manufacturers load-test mobile cranes before they are delivered to the buyer. Load tests involve purposely overloading according to relevant standards and under controlled conditions. Tests are constructed in various configurations, corresponding to situations with minimal safety margins involving rigid body stability of the crane and mechanical resistance.
It is critical to keep in mind that overloads can weaken cranes, which is why mobile crane advisors typically advise against overload testing. Overloading shortens the life expectancy of mobile cranes since mobile cranes have a specified lifetime based on the number of load cycles allowed.
Load tests should still be performed when load-bearing parts are overhauled, or every time the crane is modified. As a matter of fact, certain countries mandate overload testing each time the crane configuration is adjusted to fit the international standards (ISO, FEM and EN). Keep in mind that not all facilities allow overloading testing at their site (petrochemical plants being one example).
To protect the integrity of the crane, thoroughly examine it before each lift and after every overload. Structural downfalls might be due to cracks from fatigue and/or overloading, and defects like these cannot be detected in an overload test.
This incorporates functional tests, a visual inspection and sometimes non-destructive testing (ultrasonic or magnetic particle testing). If issues are noted during the inspection, a competent person needs to decipher whether the lift can still be executed, or if modification and repair are necessary.
