Author John Phillips
The operation of many cranes is a balancing act — one very similar to the childhood experience of using a playground seesaw. When the equipment isn’t properly balanced, the crane may succumb to the tipping force and fall to one side. Such a tipover is more common in mobile cranes rather than fixed-tower cranes.
A Battle of Forces
In some crane tipover incidents, the weight of the lifted load and boom causes the crane to tip over in the direction of the load and boom. To overcome such a scenario, a crane employs a counterweight to offset the tipping force. However, if the counterweight is too heavy compared to the load and boom forces, the crane can tip over in the opposite direction toward the counterweight side. Cranes also have ground supports such as outriggers, wheels or a concrete foundation that must be able to resist the combined weights and tipping forces of the load, boom, counterweight, and crane superstructure.
This simple diagram shows the loads that contribute to the stability of a mobile truck crane.
A Balanced Boom
The tipping force applied through the crane boom depends on the load weight, boom length, and boom angle. The counterweight of most cranes is at a fixed distance from the crane point of balance and has limited capacity for adjustment for a specific load and boom angle. The crane ground support can provide increased resistance to tipping as its width and weight distribution increase beyond being a simple hinged seesaw point, but the effectiveness of the ground support can be limited by the area where the crane is located or must traverse. In many circumstances, a mobile crane equipped with full counterweights can tip over in the direction of the counterweights if the boom is raised to a high angle with an insufficient load applied to the boom.
Achieving Strength and Stability
Crane manufacturers must follow the American Society of Mechanical Engineers B.30 series safety standards as required by the Code of Federal Regulations (CFR) regarding crane stability and strength. These standards require certain design and testing elements as well as the publication of crane load charts that are posted in sight of the operator. The crane load charts generally state the safe crane working loads for certain loads, boom angles, crane configurations, and counterweights.
With the equipment in proper alignment, the crane remains stable and upright, ready to move large, heavy loads. The crane — and people and industries that rely on it — can safely go about their tasks, safely and securely.
John Phillips, senior consulting engineer at Warren, has more than 30 years of crane and heavy equipment experience and more than 17 years of experience in forensic engineering. A licensed professional engineer in South Carolina, North Carolina, Georgia, Louisiana and Ohio, he’s NCEES registered both as a model engineer and with The United States Council for International Engineering Practice, USCIEP. John has designed crane systems, supervised installation, tested and certified lifting equipment even serving as a project engineer for maintenance and certification of nuclear weapon lifting and handling systems. John is a certified fire and explosion investigator and fire and explosion investigator instructor by the National Association of Fire Investigators. John is a member of the American Society of Materials and American Society of Testing and Materials, as well as a voting member of ASTM Ships & Marine Forensic Sciences, Forensic Engineering, and Performance of Buildings committees.
