Making the United States' largest crawler crane easy to transport
Analysis-driven design helps Manitowoc Cranes deliver new, highly mobile product
In the design of its most recent heavy-lift model, Manitowoc Cranes had to make sure that upon completion it could get the huge crane where it was needed.
Engineers at Manitowoc Cranes felt overly constrained by analysis software that was solely geometry based. However, when they switched to Femap™ software, they found that they could easily work on the details of a finite element model – nodes, elements, properties and loads – and when they needed it, Femap also met their toughest geometry and meshing requirements.
Meeting and exceeding standards
Most large cranes are extremely difficult to transport. Even after a crane is dismantled, its individual pieces are still so large that moving them requires special routes, permits and escort vehicles. Manitowoc Cranes wanted to solve this problem in the design of its new Model 21000 lattice-boom crane, the largest crawler crane ever built in the United States. Although this would require a radical departure from the standard two-crawler configuration, the competitive nature of the crane business imposed a tight time frame on this project. Engineers had to come up with a unique design meeting strict weight and size limits while ensuring that the first 21000 that was built complied with all pertinent SAE and ANSI standards.
Modular is better
The design team established a weight limit for each module of the crane (car body, rotating bed, boom, mast, counterweight, hydraulic pumps, hydraulic fluid and fuel tanks, etc.) based on transportation requirements. The modules were modeled in 2D in the company’s CAD program. Then the top, front and side views were transferred to the Femap FEA pre/postprocessor where the geometry was converted into a 3D model. The 3D model was converted to a finite element mesh using a combination of hand meshing and Femap’s automatic meshing capability.
Information from the analysis was used to guide the design of the various modules, both to ensure that they could withstand the required loads, and also to look for areas where material could be removed to reduce weight. Engineers quickly went through six design-analysis iterations to achieve the ideal balance between weight and strength for each of the various modules.
Analysis was also used to validate the design of the 21000 according to SAE and ANSI requirements. Any problems that might cause the design to fail these tests were fixed digitally before the actual crane was built.
Lifting FEA to new heights
By performing analyses prior to physical testing, the company was able to ensure that the first 21000 that was built passed all the required tests. As a result, it was able to progress from a design concept to a finished, certified product in less than 18 months, a significant reduction from previous product development cycle times.
The 21000, which was introduced at Conexpo ‘99, features a new Octa-trac eight-crawler system. Rather than having two huge crawlers, as most large cranes do, the 21000’s eight smaller crawlers are easily transported on tractor-trailer trucks. The 21000’s other components all come under a reduced weight limit as well, giving the 21000 the ease of mobilization features the company wanted.
Fewer steps required
The use of Femap contributed to a faster cycle time by reducing the time between design-analysis iterations. It did this by permitting changes on the finite element model itself rather than requiring engineers to go back and modify the CAD geometry. Femap also made it possible for Manitowoc Cranes to take advantage of legacy data. Engineers were able to import existing Nastran® models, modify nodes and properties and run a new analysis without having to find and modify the original geometry.