Are there any other important bearing considerations?
Misalignment. Consider this: The number of rolling bearing elements subjected to loading depends on 1) the difference between the inner and outer ring position (determined by shaft and housing deflections) and 2) internal operating clearance.
Under misalignment, fewer rollers carry load, so there are higher contact stresses and risk of failure. Operating clearance is related to assembly: When bearings are fitted onto shafts and into housings, they reduce initial manufacturer-recommended clearance. Temperature effects also influence the contacts within bearings — especially in lightweight housings where thermal exansion rates may vary.
Misalignment can originate from almost anywhere in a system, so design software is useful in that it allows the whole driveline to be modelled — from shafts, bearings, and gears to housings and other structural components. Even internal bearing profiles can be input. Here, the software predicts shaft and housing deflections under application load; then the results are used in bearing durability predictions. Results estimate fatigue life and contact stress at the macro and microgeometry level, as well as stress distributions on rolling elements and raceway surfaces. The results can also indicate where an altered design might help extend life: For example, profiling on cylindrical rollers or raceways can minimize edge stresses where that accelerates failure.
Does modeling account for lubrication issues?
Yes, to an extent. Adequate lubrication films prevent damaging boundary contact between asperities on raceway and rolling elements. Software can calculate this film thickness based on oil viscosity and applied load. Modeling of systems can indicate the performance level of the bearings based on the expected oil cleanliness from the filtration methods used.
Besides predicting oil-dependent bearing durability, software can partially show how efficiency depends on lubricant type. For example, in the transportation industry, where reducing fuel consumption is paramount, it's helpful to estimate frictional efficiency losses from bearings. Here, simulation programs can calculate viscous drag and load-dependent friction based on bearing size and type, speed, and lubrication properties. Work is being done to improve the accuracy of efficiency results by including bearing microgeometry and seal components in calculations.
Can I still use traditional bearing analysis to predict life?
Classical fatigue analysis accounts for surface flaking from cracks initiated at sub-surface inclusions within the steel, below the raceway surface. However, bearing steels have improved over the last 50 years, and modern manufacture minimizes inclusions, so this mode of failure is now less of a concern.
Instead, today's bearings often fail from surface damage, which in turn leads to high stress points and surface flaking. Shock load or vibrations on bearings while stationary are often the culprits; they cause plastic-deformation indentations on contact surfaces. Two other causes of modern bearing failure are corrosion on raceways (from inadequate sealing) and problems due to poor lubrication.
This month's handy tips provided by Carlo Bianco, business development manager of Romax Technology Ltd. For more information on RomaxDesigner software, visit www.romaxtech.com or call Romax Technology Inc. at (248) 250 7691.