Bearings are used in many common applications such as wheels,drills, and even toys like the popular fidget spinner. Those applications andothers like them rely on bearings to allow for smooth, efficient motion formillions of rotations.
Researchers from The TimkenCompany, a leading international manufacturer of bearings, are using neutronscattering at the Department of Energy's (DOE's) Oak Ridge NationalLaboratory (ORNL) to extend the lifetime of bearings by gaining a betterunderstanding of how internal residual stresses created during the manufacturingprocess affect their performance.
Bearings are manufactured with precision to have tighttolerances and fit perfectly and are designed to last for many years underextreme loads and extensive use and operation. Performance is especiallyimportant in fields like aerospace and mining where safety is vital. However,residual stresses—which are small internal elastic deformations in thematerial's structure—can have a significant impact in reducing a bearing'slifetime and reliability.
"Residual stresses are generated mainly by themanufacturing process," said Vikram Bedekar, a materials specialistat Timken. "All of the processes they go through—reshaping and theexposure to high heat—create residual stress. If you have a lot ofstresses, the part can distort. It could distort so much that you can't use orrecover the part."
In general, manufacturing bearings begins with steel formed tothe shape of a ring. Next, a lathe is used to get the desired size. At thatpoint, the part is still "green," says Bedekar, which means it isstill considered soft and not ready for use. A heat treatment is then appliedto harden the material. Finally, the part is finished using a lathe or grinderto remove excess material.
Neutrons provide researchers with unique insights into amaterial's atomic structure because of their highly penetrating properties.Previously, the researchers were using laboratory x-rays to look at bearings,but the researchers could only probe up to 200 microns inside of a bearing.Neutrons give them the capability to look at entire sections of bearings atgreater depths.
"Standard x-rays aren't strong enough to go all the waythrough a section," said Bedekar. "Neutrons are the only way to passthrough it and see inside."
Using the Neutron Residual Stress Mapping Facility (NRSF2),HB-2B, at ORNL's High Flux Isotope Reactor (HFIR), the researchers were able tomap out the differing internal stresses from each step in the manufacturingprocess. The neutron data allowed them to observe how a bearing'sstress state changes with each iteration. The researchers say they chose to useNRSF2 because it is uniquely suited for this type of experiment.
"We were looking for what we can do in terms of residualstress mapping," said Rohit Voothaluru, a product development specialistat Timken. "We came to NRSF2 because we felt that we could get the wholegamut of samples characterized and see the residual stresses."
The team says they intend to use the residual stress mappingdata to improve their computational models for improved internal stresspredictions and optimized manufacturing processes.
"Eventually, we can tailor the processing or tailor theresidual stress to the desired performance of the bearing," said Bedekar.
"We have a computational model today that canqualitatively provide direction," said Voothaluru. "But to have amore fundamentally driven quantitative model that's based on the actual physicsof the process, while also capturing the real-time subsurface residual strain,is something that requires extensive empirical validation. We want to validateour model and take it to the next level."