Bearings in planetary gears, hydraulic pumps and other gears are often exposed to an environment where elastic deformation of the surrounding parts can cause the rings to be severely misaligned with respect to each other. This misalignment can lead to localized pressure peaks and a corresponding reduction in bearing life. Correct raceway crown reduces these effects and is therefore a highly relevant consideration during supplier approval.
The effect of local stress can be considered during the design process, but if the bearing performance is to be verified on a test bench, the effect of local stress also needs to be taken into account during the test process. Here, speeding up test runs complicates things. Typically, the test load is higher than the application load because the years of service life must be expressed in weeks on the test bench. When investigating the effects of misalignment, the selected test conditions must result in a non-uniform stress distribution similar to that expected in the application.
When testing, custom fixtures can be used to control the degree of misalignment. However, it is important to verify that the test conditions are sufficient to mimic the application. FVA Workbench is a powerful tool to calculate local stresses for applications and test cases. This article will demonstrate a general test method for applications where severe bearing misalignment is expected.
Durability testing is a powerful way to verify bearing performance and approve relevant suppliers. However, one of the challenges during testing was to simulate years of service life as weeks of endurance testing on a testbed without changing the conclusions. Generally, speed and load are parameters that control the test time. Higher speeds can achieve the same number of revolutions in less time, but also change lubrication conditions. These changes can be compensated by changing the lubricant viscosity.
Increasing the load also helps reduce test time. However, doing so can lead to problems on the test bench that may be completely unrelated in the field. For applications with a wide range of load characteristics, such as passenger cars, where peak torque is only a small fraction of the operating time, testing can be accelerated by focusing on the maximum normal load, regardless of time-consuming “cruising” conditions.
However, if eccentricity needs to be considered in addition to load and speed, special attention must be paid to local stresses. The dome of the raceway can be optimized for a specific amount of inclination and a specific load. If the test conditions exceed those of the actual application, the amount of shaft bending may also be higher and potentially lead to face edge contact on the test bench, which would not be present in the application. Therefore, a potentially good supplier may not be considered qualified because the test data was generated under inappropriate conditions.
Therefore, it is necessary to be able to calculate the local stress distribution of the application and test setup to ensure that the test data is adequately representative of the actual conditions. FVA Workbench is a powerful tool capable of simulating a large number of real-world applications and their potential test setups.
In the most recent test program, a complete set of rolling bearings will be tested, which are subjected to a large number of misalignments in the application. To select the test conditions, the raceway profile was measured, which was then used in the application to calculate the pressure distribution and bearing life according to ISO 26281.
Before introducing the FVA Workbench simulation, this article briefly introduces the test bench, and finally compares its conclusions with experimental data.
The tests were carried out on an EELPRAAX-130 test bench manufactured by Elgeti Engineering GmbH (Figure 1). The EELPRAAX series test stands can apply axial, radial or combined loads and are available in different sizes to accommodate bearings with an outer diameter of 320 mm. Like all Elgeti Engineering test benches, they feature two fully independent workstations on the same rack.
Install the bearing into the housing using a custom-made clamp. The shaft and motor are connected by a double cardan coupling, which prevents minor misalignments and dampens vibrations. The motor and test head are mounted on a solid base plate, which is connected to the main frame via shock absorbers. These eliminate the effects of external vibration, especially from the opposing test station.
The test rig features a variable flow oil circulating lubrication system. The oil temperature determines the operating temperature of the bearing and is measured at the inlet and outlet of the test head and adjusted with a cooler. Heating is optional, but usually not required. The standard oil filter has a 10µm mesh; a finer mesh is optional. The load is applied via controlled hydraulic cylinders so that not only constant loads but also ramps, steps and cycles can be applied.
Measured quantities include outer ring temperature of all bearings, housing vibration and motor torque. These quantities are used to detect bearing failures and trigger automatic shutdowns. This allows the machine to run continuously without supervision.
Eccentricity is controlled by the design of the shaft; both the diameter of the shaft and the distance between the bearings define the bending line of the shaft. During the design process, it must be remembered that bearings add stiffness to the shaft and proper simulation in the FVA bench is essential to produce the desired results.
Figure 1: EELPRAAX-130 Test Bench
Workbench overview
The FVA Workbench is a powerful tool for designing and analyzing mechanical systems involving bearings and gears at the part and system level. To recreate the test environment outlined above, a CAD model of the test head was imported into the workspace, where the tooling and bearings were modeled (Figure 2). Assign appropriate materials to tools and housings. The selected lubricant type and temperature, load and speed correspond to the initial test parameters. They can be further refined based on simulation results.
Figure 2: Setup in Workbench including test bench housing, tools and bearings
FVA Workbench allows users to select bearings from a library, import XML files from manufacturers, or define them directly. Whichever option is selected, a default user-defined crowning profile can be applied; a default crowning profile can also be applied. The user-defined profile may eg be based on measurements of the bearing in question. In this case, the main geometry is specified from the manufacturer’s drawings, while the cam profile is specified from measurements.
Figure 3: Schematic diagram of a cylindrical roller bearing
Results and Conclusions
Figure 4 shows the local stress distribution under test conditions representative of practical applications. Based on these simulation results, perform a test run with the same parameters. The measured lifetimes are generally higher than the calculated lifetimes (Figure 5). Therefore, the bearings involved meet the requirements and the supplier has been approved.
Figure 4: Hertzian contact stress on the raceways of the inner ring (left) and outer ring (right)
Figure 5: Comparison of experimental results with theoretical lifetime
The approach taken here – using simulation to inform test parameter selection – ensures that test conditions actually represent specific application characteristics and are not exaggerated or underestimated. This is especially important in this application when severe misalignment between bearing rings is expected.