Defining duty cycle
24 April 2008
The term “duty cycle crane” can be interpreted several different ways. To some degree, all cranes perform duty cycle work, lifting, moving and putting down heavy objects. In addition, many regular lift cranes are adept at some duty cycle applications. However, in the purist sense of the term, a duty cycle crane is one that is “purpose-built” for heavy duty cycle work.
Wolfgang Herzog at Liebherr put it this way: “True duty cycle machines are design-built for duty cycle applications.” On the other hand, some owners alter or convert their lift cranes to make them duty cycle cranes, he says. “Some duty cycle machines are converted lift cranes that are not design-built for duty cycle applications.”
Herzog contends for a crane to be truly “duty cycle,” it should have a big engine, at least 500 horsepower, to run all movements simultaneously and to power external tools such as large hydraulic hammers, large piling vibratory hammers, large drills, lead systems, casing oscillators and the like. Most duty cycle cranes are more complicated to run, Herzog says, with computer programs for functions such as data logging, dynamic compaction, slurry wall, clamshell operations and vibrators.
But the most distinguishing characteristic of a duty cycle crane is its heavy duty structure, Herzog says. A duty cycle crane's structure is loaded and unloaded, at the very least, once a minute, while a lift crane may only pick a load once an hour, and even then very slowly and carefully, he says. “Because of this, most duty cycle cranes are made out of high-strength steel that has a much shorter fatigue life, thus making them a lot heavier than equivalently-sized lift cranes,” Herzog says.
Duty cycle and conventional crawler cranes often have the same capacity, but lift cranes use a lighter tube boom, while duty cycle cranes use a heavier angle boom so they can take more side loading.
“A duty cycle crane's boom requirement will be a lot heavier than a lift crawler crane or it won't hold up,” Herzog says. “The car bodies are heavier; there are two swing motors because running 100%, a normal lift crane at capacity will work about 40% of its life, and a duty cycle at capacity will work 98% of its life.”
Herzog says true duty cycle cranes are run wide open, and during a 10-hour period, they may not shut down except for lunch. “A duty cycle crane gets more use and more abuse,” he says.
On the other hand, more lift cranes are being used in duty cycle applications, and in some cases, they aren't even crawler cranes.
In Northern Virginia and Maryland, Pulte Homes actually uses truck cranes for what it terms a duty cycle application. The company builds houses using manufactured concrete panels for the foundation and walls. On average each home uses 25 to 38 concrete panels for the foundation, while the floor consists of another 10 to 13 panels. Using a Grove TMS900E truck crane with 95 feet of main boom and 12,900 pounds of counterweight, operators place and set the panels in place. Typical swing time is four to seven minutes per panel. Each panel measures about nine by 36 feet and weighs about 4,000 pounds. An entire foundation can be set in eight to ten hours with a crew of seven, including the crane operator.