As OEMs build larger wind turbines, bearings manufacturers must follow with larger bearings. Main-shaft bearings can measure 2m on their OD. OEMs are asking bearing manufacturers to ensure that large main shaft or rotor bearings will stand up to the anticipated structural and environmental stresses. Until recently there was no way to run real-world tests on the newer, large-size wind turbine bearings, which would replicate actual conditions for roller bearings in a wind turbine.
“Before even thinking about performing expensive real-world tests on large wind-turbine bearings, we had to first quantify the critical operating conditions to minimize testing time and costs,” says Martin Stief, CAE Integration department engineer with bearing manufacturer Schaeffler Technologies AG. Finite element analysis (FEA) provided the needed insights.
A big test rig
“When planning the large test-rig project, conservative engineering was the watchword for extrapolating bearing lifetime tests from small to big bearings,” says Stief. “FEA helped us more exactly determine the life expectancy for larger bearings. We can now precisely design the size of bearing details required by a particular turbine geometry, which helps keep costs down. We were also able to do kinematic analysis with original parts, from bearings and assemblies provided by customers.
The finished 350 ton test rig measures 16-m long, 6-m wide, and 5.7-m high. As with wind turbines in the field, the rig is on a 5° tilt. It has five main subassemblies: a
drive train, loading frame, auxiliary bearing, test bearing, and tensioning frame. Eight radial and axial cylinders replicate real-world loads. About 500 bolts mount auxiliary and test bearings. The finished rig can accommodate bearings with a maximum diameter of about 3.5m, making it the largest, high performance, large-bearing-test rig in the world. At a cost of about €7 million, the rig represents a significant investment in bearing manufacturer Schaeffler’s future developments in wind power, and other large bearing applications.
The company spent two years designing and constructing the test rig, and of that, two months were spent on mechanical FEA simulations. “We wanted to build the test rig as fast as possible,” says Stief. “A quick and accurate way to simulate, analyze, and verify rolling bearing assemblies let us create a test rig that could accurately measure them in action.”
Schaeffler started by designing a virtual prototype to validate the physical test rig. For that, Stief formed a simulation team experienced with SIMULIA’s Abaqus FEA from Dassault Systèmes’ 3DEXPERIENCE technology (www.3ds.com).
The team divided the test rig analysis into smaller, manageable, and functional FE submodels, which were later reassembled into the overall test rig. To ensure accuracy, the team used its engineering judgment in defining loads, transition regions, and boundary conditions (stiffness, mass, and damping) between FE submodels.
“We used Abaqus to test the strengths of joints and to check connections,” says Stief. “When the overall design of the test rig became clearer, we began using Abaqus on submodels to verify their strength. From those results, we improved the test-rig design using basic mechanical engineering methods, such as strengthening ribs by making them larger. Then we’d run the submodel through Abaqus again for another strength assessment.”
The team also created an additional FE model to quickly evaluate the entire test rig’s modal behavior, as well as confirm their definitions for the boundary conditions between submodels. Modal analysis estimated eigenfrequencies for the test rig.
Large wind turbines typically rotate about 16 rpm, but the engineers wanted their test rig to run up to 60 rpm—the same as a critical excitation frequency of 1 Hz. The modal analysis confirmed that the first natural frequency of the rig was 13 Hz, well beyond this 1 Hz value and thus not an issue.
To further validate the virtual prototype, Stief’s team ran the full FE model of the test rig through a large number of load cases within Abaqus. Even with a coarse mesh of the entire test rig, load-case calculations initially hit the limits of the available computing capacity. Calculating 17 load cases took 48 hours even with 32 GB of RAM. However, that period was later slashed to 10 hours using a new HPC Linux Calculation Cluster with faster CPUs and more RAM.
A few functions in Abaqus helped make model creation efficient and fast. For example, the program includes hundreds of user elements—subroutines that let users define their own FE behavior inside a model. In the bearing-package submodel, rolling elements of the bearings were replaced by customized user elements that represented the precise stiffness of the rolling elements, as well as several degrees of freedom that would have had to be calculated separately. These cut compute times for analysis from about 5 hr to about 5 sec. “The large-bearing test rig has at least 500 of these rolling elements. It would have been impossible to make any FE calculations without user elements,” says Stief.
Other software functions helped facilitate stress analysis and strength verifications. For instance, the screw and bolt-modeling function, says Stief, proved “extremely useful because of the many bolts, we preloaded with values for stress and strain.” This function let the team create bolts, mesh them, and copy them as many times as needed in the 3D models. Subsequent copies were automatically meshed for FE analysis.
Going forward, simulation—validated with test-rig runs—will provide the company with bearing-specific values, such as load distributions, pressure distributions, and contact angles, along with important data about the elastic behavior of bearing components under high preload. “This will lead to even more realistic results in bearing lifetime calculations,” says Stief.
Their current work now helps Schaeffler detect critical operating conditions early in the development of large bearings, and minimize bearing test time on the rig. From this, the company can optimize its bearing products earlier and easily in all the design stages and put added focus on reducing friction in its roller bearings. WPE