30 percent more bucket volume for one of the world’s largest excavators
Simulation-driven design using NX I-deas permits a huge performance improvement with no increase in weight
Excavators for the harshest conditions
Terex Mining was created on April 1, 1998, when O&K Mining GmbH became part of the Terex Group. Now the O&K unit belongs to Terex GmbH, along with Atlas and Schaeff, both German manufacturers of construction machines. The O&K products are assigned to the Materials Processing and Mining Division, which includes stationary and mobile crushing and screening equipment, dump trucks with up to 380 tons of load capacity, surface and sub-surface mining, and large hydraulic excavators. These excavators are used in mines and pits all over the world. Included in this product line is the RH 400 with 1,000 tonnes* of service weight – the world’s largest hydraulic mining excavator.
The RH 400 is powered by two diesel engines having 60 liters displacement each and providing 3280 kW or 4460 horsepower. Its shovel scoops 43.5 cubic meters and loads “monster trucks” such as a CAT 797B with a payload capacity of 345 tonnes or the CAT 793C that holds 218 tonnes with only three filled shovels. The RH 400 is used in areas such as the tar sand deposits of Alberta, Canada, which demand a great deal of high-tech functionality. The excavator features two air conditioners and computerized monitoring of all hydraulic and engine functions, for example, as well as ruggedly constructed material that meets the toughest demands. The sheet metals of the superstructure and the shovel are up to 50 millimeters thick. Without specific curing processes for the massive cast parts, the shovel teeth or the chain links of the track drive would melt away like ice in the abrasive conditions of these mines.
FEA as basis for engineering
To build in these dimensions, each steel piece of the excavator’s superstructure, undercarriage and equipment is simulated and optimized using the NX™ I-deas™ MasterFEM solution. This is a comprehensive finite element analysis (FEA) software suite from Siemens PLM Software. The suite, which consists of a modeler, meshing tool, solver and post-processor, “is one of the most capable, comprehensive and widespread suites in the industry,” says Detlef Beckmann, who is responsible for FEA simulation of all sheet metal components of the large-scale equipment.
A critical aspect of product engineering is the cost per ton of loaded material because it is on this basis that mining companies and excavation and transportation service providers make their purchase decisions. Truck loading capacity is growing in line with advances in tire manufacturing. Tires currently limit the vehicles to about 380 tons of load capacity although the next step to 400 tons is keeping engineering teams busy. Other important engineering aims include shovel capacity, truck height, the required range of stick and boom as well as the dimensions of hydraulic cylinders, which determine the digging forces of the teeth. “There is no single optimum we can reach,” says Beckmann. “We always have to accept compromises while deciding.”
The RH 400, the world’s largest hydraulic excavator, was launched in 1997. There is no prototype for a vehicle such as this, which costs approximately one million dollars (US). “Every first model of a new product line is sold at a discount and with special conditions for performance and warranty,” explains Ralf Schlicht, engineering director at O&K. “Even the prototype has to prove value at the market.” Practical experiences are taken into account as improvements for succeeding vehicles.
For the RH 400, which almost doubled the weight and performance specifications of the then largest O&K excavator, equipment had to be reinforced significantly. Tar sand in Alberta shows no mercy. It will wear away the 50-millimeter thick side sheet of a shovel to half the original thickness within a year. In an extreme situation, the total weight of the excavator can strike one single shovel tooth. All these factors are taken into account during FEA to achieve optimal construction of all sheet metal pieces.
Simulation to reduce weight
One engineering goal is a low operating weight in proportion to shovel content. “The smaller an apparatus can be, the fewer costs it will drive,” says Schlicht. In this respect, Beckmann recently achieved his most important success so far. A new design concept enabled the new model RH 340 to feature a 34-cubic-meter shovel volume instead of the 26 cubic meters of the next smaller RH 200. At the same time, the equipment such as the stick and boom lost weight. In comparison with the predecessor, it has a 30 percent enhanced digging capacity while operating weight was only enhanced by two percent. It was very important to remove all welded seams from the power flow, allowing elevated traction forces. In addition, the metal sheets for the cylinders are no longer welded on, but now integrated into the side sheets. This patented design astonished the market. “For a new engineering concept you need a good idea and the ability to monitor it with accurate FEA calculations,” reports Beckmann.
It was important that the parameters of the simulation corresponded to reality. “In field tests we attached resistance strain gauges to the equipment to be able to prove our FEM grids,” explains Beckmann. “And we made conditions hard for the excavator.” The test results yielded important conclusions regarding durability, which is anticipated to last about ten years because of such extreme loading.
More functionality for better products
Since calculations are based upon solids and are no longer limited by the number of calculable nodes in little space, it is possible to exactly predict at which point a weakness will occur. “Hot spots are then particularly monitored to guarantee continuous operational reliability,” says Beckmann. Enhanced computing potential helps him do this work. “Contact calculations over three million degrees of freedom are finished in four hours using the new NXN server running SuSE Linux Enterprise,” he says. “In the past they took four days and our local computer was blocked during that time.”
Each sheet metal piece of superstructure, undercarriage and supporting structure is modeled with NX I-deas, even components of the older equipment that have been running for 20 years and were not previously built with solids. To handle this additional work, another workstation was recently equipped with NX I-deas. The advanced modeling functions in this software have made possible a new method of operation. Within simulation-driven product engineering, Beckmann not only checks components, he also returns completed, optimized CAD models to the design department. “I needn’t worry about interfaces,” he says. “Using NX I-deas, I can import and export all foreign models via various geometrical interfaces.”
When working with freeform surfaces, Beckmann benefits from the excellent repair functionality in NX I-deas. “In the past I always had trouble with freeform surfaces such as the surfaces of cast parts,” he explains. With NX I-deas, all models can be checked for gaps and easily patched. Another advantage is accurate data exchange via the STL interface. Labor time that was formerly spent on cross-linking has been reduced drastically. “At the push of a button we are now able to lay consistent meshes over all model areas,” Beckmann adds. “If a finer mesh is needed, the grid can easily be made denser at arbitrary local points. This is where NX I-deas Master FEM is very strong. You very quickly come up to any result you wish.”
Beckmann is also impressed by the large NX I-deas user base. He met other I-deas users during an exchange with the University of Aachen, as well as when he visited his most remote client in Alberta, Canada. When he visited the company Syncrude in Fort McMurray to solve a problem with the prototype with the RH 400, he was not surprised to meet I-deas users there, too. Their common software solution fostered collaboration.
* 1 tonne force = 1000 kgf = 9.80665 kN