Overloading: How to Avoid Mobile Crane Disasters

Overloading: How to Avoid Mobile Crane Disasters
11/2/2017 3:11:14 PM

Looking through the headlines, it’s frightening to see reports of mobile crane accidents caused by overloading. When a crane operator attempts to lift a load more than the weight capacity of the crane, the results can be disastrous.

Mobile cranes make modern-day construction possible. They can travel loaded or unloaded, on tires or crawlers. In fixed positions, these self-powered jib cranes can be supported by outriggers or other accessories that increase their stability. Mobile cranes can be equipped either with telescopic jibs, articulated jibs or lattice jibs (or a combination of various jib types).

Overloading Accident

An overloaded crane tips. Courtesy of http://www.vertikal.net

Their flexibility and the very fact that these cranes are so common mean crews can get careless with load calculations. Accidents are almost always due to an error on the crew’s part, and these often occur at the end of a long, hard shift when everyone is tired and not paying attention to detail. With proper planning, however, the factors that contribute to an overloading accident can be minimized or eliminated entirely.

Steps for Safe Operation

First, operators must make sure to program the correct information into the load movement indicator (LMI). The LMI gives the crane operator a continuous readout of the boom length,angle and radius. That said, the LMI is only as good as the information provided.

That’s why a crew should never perform a lift without accurate weight information. For example, when lifting a tank, the lift director should account for the weight of any fluid remaining in the tank. Providing the weight of the tank without accounting for the weight of its contents could translate into a huge problem in the field.

Next, operators must use the correct amount of counterweights. Too few counterweights can cause a crane to topple even lifting a relatively light load, assuming the distance is great enough. Make sure every load is rigged correctly and balanced to prevent these kinds of accidents.

Finally, operators should use load charts to confirm the correct crane configuration for the load site and lift conditions. They need to double-check all calculations, including load and rigging weights and net capacity. Remember: Thorough calculations, the correct equipment and communication between crew members always makes for a safe crane lift.

Safety devices such as the Rated Capacity Limiter (RCL) can help prevent overloading. An RCL measures the load’s weight, then compares it to the manufacturer’s recommended load rating given the current boom angle and length extension, taking into account whether or not the stabilizers are raised or lowered. An override key can be used in emergency situations or if the RCL fails.

If you’re not clear about the difference between an RCL and an LMI, this resource may be of use.

Load-testing

Mobile cranes are load-tested by the manufacturer prior to delivery to the buyer. These tests include intentional overloading under defined conditions and according to relevant standards. Tests are made in several configurations corresponding to situations with lowest safety margins regarding mechanical resistance and rigid body stability of the crane.

It’s important to remember that overloads have a deteriorating effect on a crane, so mobile crane manufacturers generally advise that you avoid overload testing. Because mobile cranes have a defined lifetime based on a number of allowable load cycles, overloading reduces their life expectancy.

Still, you should conduct load-testing each time the crane is modified or when load-bearing parts are overhauled. In fact, some countries require overload testing every time the crane configuration is changed to comply with international standards (EN, ISO and FEM). Note: some facilities (including petrochemical plants) don’t allow overload testing at their site.

To ensure the integrity of the crane, conduct a thorough examination after each overload and before executing a lift. Structural failures may be the result of cracks from overloading and/or fatigue and such defects will not be identified by an overload test.

This includes a visual inspection and functional tests and may also include non-destructive testing (magnetic particle or ultrasonic testing). If you discover any problems during the inspection, a competent person should decide whether the lift can be executed or if repair or modification is needed first.

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