Cantilevered Counterweights Leverage Real-Time Balance
For decades, crane engineers have created new markets with incremental changes to their tried-and-tested lifting systems. However, the ongoing quest to make heavy-lift cranes more powerful and efficient regularly comes down to one problem: counterweights. Essentially giant slabs of metal, counterweights keep the crane from tipping over but are costly to transport, difficult to move, add ground pressure and take up valuable real estate.
"The goal has always been designing lifting-enhancing devices to get you more capacity with your basic crane," explains one veteran crane engineer. "The struggle is to get what you can from mobility." Manufacturers continue to find clever solutions to the counterweight crux.
Manitowoc Co.'s latest innovation, the Variable Position Counterweight (VPC), makes a static part of the crane suddenly dynamic. Introduced in 2008 at the triennial CONEXPO-CON/AGG exhibition for Manitowoc's then-new 2,500-ton model 31000, the concept freed the counterweights from touching the ground completely and allowed them to travel forward and backward automatically in relation to the load.
The idea is simple: A giant rack-and-pinion assembly extends and retracts the counterweights as sensors and computerized logic detect changing load-moment demands. For example, if the boom angle lowers and load radius increases, the counterweights may move farther away from the center line of rotation to increase lifting capacity.
"We're putting all the counterweight right where you need it," says John Kennedy, Manitowoc's senior vice president of crawler cranes. In April, he gave ENR a guided tour of the firm's crawler-crane plant in Manitowoc, Wis. In a gravel lot behind the plant, engineers were finishing testing two of Manitowoc's latest VPC-equipped cranes, the 330-ton MLC300 and 716-ton MLC650 crawlers. Introduced last year at CONEXPO, both use a similar VPC system as the 31000, but the rigging has been refined.
"The MLC300 is revolutionary," Rick Mikut, crawler-crane division lead, and Scott Jerome, senior branch manager for the ALL Family of Cos., say in a joint email to ENR. Last year, Independence, Ohio-based rental firm ALL purchased 10 of the MLC300 cranes and received the first this past April. The crane is working on a MidAmerican Energy wind-farm site in Primghar, Iowa, where ALL rented the crane to Mortenson Construction's renewables division.
"In the very challenging application of larger crawler-crane usage in wind- turbine construction, the MLC300 is already proving itself not only with its extraordinary load chart but also for its unprecedented ability to travel easily and safely from site to site," ALL says.
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Another buyer, Bigge Crane & Rigging Co., weighed the costs and benefits of the new cranes. It purchased two of the larger MLC650 crawlers, the first of which, delivered this spring, is now working on a wind-farm project.
"We are convinced the benefits of the VPC technology will allow customers and owners to change the way their hoisting is planned and executed," says Weston Settlemier, president of Bigge, in an emailed statement. In all, Manitowoc says it has sold more than 50 of the new MLC300 and MLC650 cranes, which, respectively, are similar in size to the existing 300-ton Manitowoc 2250, priced between $2.5 million and $3.5 million, and the 600-ton Manitowoc 18000, priced between $5.5 million and $8 million.
Cantilevered counterweights are not new, but the VPC system updates the concept. It features a roller path on each side of the rotating bed, and a gear rack is underneath. Steel wheels that ride on the roller paths support a counterweight tray that spans the width of the crane. Pinion drives geared into the rack actuate the tray's motion. For more load-moment capacity, users also can purchase a VPC-MAX system that includes a special beam and additional pinion drives that ride on the roller path, allowing a longer range of motion for the counterweight behind the machine.
