Types of Overhead Traveling Cranes

Overhead traveling cranes move large, heavy loads with ease.

In 2007, American companies that manufacture overhead traveling cranes and hoists employed 24,000 workers and produced equipment worth more than $8 billion, according to the U.S. Census Bureau. Overhead traveling cranes move large, heavy loads quickly and conveniently but pose a serious threat to worker safety when misused. A report from the U.S. Bureau of Labor Statistics shows that of 72 crane-related occupational fatalities recorded during 2006, more than 25 percent involved overhead cranes.

Crane Construction

The load-bearing bridge structure of an overhead crane makes up one or more girders, or beams, that span the width of the building that houses the crane. Wheeled end-trucks, attached to each end of the structure, support the bridge. The hoist that raises and lowers the load is located on the crane bridge, usually on a movable trolley that can travel along the bridge. Below the hoist, the load block and hook hang on a chain or a wire rope, using pulleys to increase the mechanical advantage.

Top-Running and Under-Running Cranes

Under-running cranes use the building structure for support.

Top-running overhead cranes ride on steel rails, fixed at a high level down each side of the building. Runway beams support the rails along the length of the bay in which the crane operates. Often the building structure can't withstand the crane load, and additional steel columns reinforce the runway. Under-running, or underslung, overhead cranes use the structure of the building for support and don't require reinforcing columns. These cranes make better use of available space than top-running variants and run along the bottom flange of runway beams that attach directly to the roof framework.

Single-Girder Cranes

The bridge structure of a single-girder overhead traveling crane makes up either a fabricated-steel box-girder or a rolled-steel I-beam. The hoist trolley runs below the girder, traveling along its bottom flange. Single-girder cranes can handle loads up to 15 tons under moderate service conditions. Lift height may be compromised as the hook is positioned under the crane beam, giving a practical height limit of about 50 feet. Single-girder traveling cranes are suitable for top-running or under-running installation and generally cost less than similarly specified double-girder cranes.

Double-Girder Cranes

Double-girder cranes can handle loads up to 100 tons.

Two fabricated-steel box-section beams form the bridge of a double-girder crane. The hoist trolley travels on top of the bridge, running on steel rails welded along the length of each beam. These cranes achieve greater lift heights than single-girder cranes as the hoist can raise the hook between the girders. Rating up to 100 tons, double-girder cranes are suitable for continuous use in severe conditions, and the bridge structure can accommodate cabs, walkways and bulky equipment. Weight considerations mean that top running is the only suitable configuration for double-girder cranes.

References

• U.S. Census Bureau; Industry Statistics Sampler: Overhead Traveling Crane, Hoist and Monorail ...; 2007
• U.S. Bureau of Labor Statistics; Crane-Related Occupational Fatalities; July 2008
• The American Society of Mechanical Engineers; B30.2-2005: Overhead and Gantry Cranes ...; 2005
• PDHonline: Electric Overhead Traveling (EOT) Cranes and Hoists

Resources

• U.S. Department of Labor: Occupational Safety & Health Standards 1910.179: Overhead & Gantry Cranes
• Demag Cranes & Components: Demag Standard Cranes