A huge construction crane, mounted on flatbed truck, creeps along the highway. "Wide Load" signs are plastered all over it. Who hasn't been stuck behind one of these behemoths? Well, lots of Europeans probably haven't. In Europe, construction cranes tend to move under their own steam with their boom and cab mounted atop a motorized carrier. These "all-terrain cranes" easily reach highway speeds and can run rings around the unimproved terrain of a construction site. This extra mobility on roads and job sites accounts for such cranes growing popularity.
The GMK 7459 is Grove Worldwide's biggest
all-terrain crane yet. A project so big, in fact, that it required
engineering teams on two continents to design it.
In Europe, the all-terrain style already represents more than 90% of construction cranes in the 35 to 450 metric-ton range, according to John Bittner, who directs marketing for Grove Worldwide, which makes both all-terrain and truck-transported cranes in the U.S. and Europe. The relative convenience of all-terrain cranes has more recently raised their profile in North America too—despite the fact that the all-terrain style costs more to set up and operate than its truck-mounted counterparts. Bittner estimates that 20 to 25% of cranes sold today in the U.S. are now all-terrain. "That's up from 5% ten years ago," he says. And all-terrain cranes have not just grown in popularity. They have just plain grown. "All-terrain models have gotten bigger and taken some of the market occupied by truck cranes," says Bittner.
Bigger may be better. But increasing the size of these already-massive cranes does present some engineering difficulties, as Grove engineers found out when they began work on the company's largest all-terrain model two years ago. With its 60m reach, this GMK 7450 crane has a maximum lift of 450 metric tons, while the company's previous all-terrain models topped out at 300 metric tons. Size aside, their new crane also stands out for its global aspirations. Designed from the ground up for worldwide use, it had to appeal to what Bittner calls differing "crane cultures" throughout the world. Even more challenging in terms of engineering constraints, the new crane also had to meet a hodgepodge of transport and emissions regulations.
To come up with such a crane, Grove engineers had to do far more than a simple scale-up job. Though they did borrow plenty of proven technology from smaller models, they also revamped the design of the carrier and parts of the boom superstructure. The resulting crane squeezes lots of lifting capacity onto a carrier with a uniquely low axle count. "It's the first time anyone has put a 450-ton crane on just seven axles," notes Kurt Richter, the engineering manager responsible for the crane's carrier. "Other cranes with this kind of capacity have eight axles."
One world, one crane. The number of axles is no trivial matter for this global crane, given the dissimilar way road-transport regulations are written in Europe and in North America. As Grove engineering manager Gerhard Kaupert explains, the toughest European road regulations require that a crane's gross vehicle weight doesn't exceed 12 metric tons/axle. "If you meet that limit, you can take the crane anywhere in Europe," he says. As cranes get bigger, manufacturers sometimes even add an axle to satisfy the regulators rather than to bear the crane's loads, Richter adds.
In North America, by contrast, various states permit different weight limits. Far more important here from a crane-design standpoint are bridge laws that dictate relatively large spaces between groups of axles. "The European design philosophy favors many tightly spaced axles to meet a specific weight limit, while North America cares less about the number of axles than about their spacing," Richter says. None of these regulatory differences would be too much of a technical dilemma—except for the fact that Grove's engineers working here and in Germany had to come up with a single design that satisfied regulators on both continents.
Part of the solution to these conflicting weight-distribution approaches comes in the form of a removable auxiliary axle that pins onto the back of the crane during transport, taking the place of the hydraulic outriggers that stabilize the crane during lifts. This extra axle—and the 6-ton weight savings from the absent outriggers—helps the crane meet European weight-and-axle standards. "This solution is well-known on smaller cranes," Kaupert points out. For North America, however, there's a new twist: The crane's third and sixth axles retract from the road surface during transport in order to create three, sufficiently spaced axle groups. "By raising two sets of wheels off the ground, we've actually increased the crane's ability to ride the roads," Richter kids.
The choice of engines also plays a role in the crane's global appeal. Powered by a 563-hp, eight-cylinder Mercedes turbo diesel, the crane carrier can reach 90 km/hr in order to meet European regulations and reach U.S. highway speeds. A second 235-hp Mercedes engine runs the super-structure. Finding a set of engines that would run the crane was no problem, but finding one that would meet global emissions standards proved to be a tough task. Richter recalls that the American team at first pulled for a Cummins engine, while the German engineers wanted Mercedes.
Both had their strong suits. Richter says that the Cummins engine would enjoy better technical support in North America and also enable the crane to meet California's tough on-highway emissions standards. Kaupert points out that Mercedes has a better support network in Europe, and its engines meet Euromot emissions standards and all U.S. emissions standards for off-highway use. "With the Mercedes engine you can sell a used crane from Europe into North America and vice versa. That's not true with the Cummins engine," Kaupert says. "So we decided to go with Mercedes and seek an exemption for the limited on-highway use in California."
Metal matters. With the crane's lift capacity going up to 450 metric tons, the design team was also faced with the prospect of beefing up the frame to carry the extra load. But too much frame would have eliminated much of the weight savings from the axle-swapping, wheel-lifting gambit. "We had to increase the frame cross-section to carry the extra load, but we couldn't tolerate all that extra weight," Richter says. So the engineering team instead went with a stronger steel.
Previous frames had successfully relied on frames made from American steels with a 100 ksi yield strength, according to Richter. This new 450-ton model, however, uses a 140 ksi steel (S 960 QL) from Swedish Steel AB. The benefits from the new steel are most noticeable at the rear half of the 19.5 m carrier frame. With the stronger steel, Grove engineers managed to make this part of the frame, which sits beneath the bearing for the crane's boom superstructure, about one-third deeper than frame height at the front of the carrier. And they did so while reducing overall weight. Richter estimates that the new frame material shaved about 400 kg off the crane, improving on a much smaller weight savings from switching to aluminum wheels.
For all the benefits, the high-strength steel also created its share of challenges in the welding department. For one, the material initially proved more susceptible to cracking, Richter reports. And the manufacturing standards Grove had been using didn't apply to the new steel, forcing the company to make significant changes in welding procedures. "AWS specs don't cover steels with such a high yield strength," says Richter. "We had to travel to Europe to develop new procedures and testing regimens for our welding operations."
A comfy cab. Looming above the carrier, the frame's superstructure also features its share of innovations—though these serve ease-of-use goals rather than roading regulations. Kaupert reports that the cab interior has been widened for the sake of user comfort. "The new wide cab will be standard on all our all-terrain cranes," he says.
The computerized crane controls also feature dual display screens, allowing users to monitor every aspect of a lift without toggling between different views, as they did on past models. Finally, the crane uses CANbus throughout, simplifying the wiring harnesses. This distributed control architecture, for example, requires only a single I/O box and four wires to connect the entire boom to the control system. "Other methods would have required much more wiring," Kaupert reports.
Compromises that work. Taken as a whole, the new crane is not without its compromises. The auxiliary axle and removable outrigger add some cost, Kaupert admits. The engine choice requires trade-offs when it comes to emissions certifications. The crane's joystick controls are oriented toward European preferences. "It's very difficult to design one crane that meets standards everywhere," Kaupert says. "And it's impossible to do without making some concessions." Yet with several cranes already sold in advance of the crane's formal introduction at this month's ConExpo Show in Las Vegas, it looks like Grove engineers may have successfully applied the art of acceptable compromise and come up with a European-style crane that's still as American as apple pie.