X-rays resolve rotating bearings


Rebecca Pool

Wednesday, August 8, 2018 - 16:30
Image: Bearing rig installed on Diamond beamline.
Researchers have used the UK’s Synchrotron, Diamond Light Source, to measure the strain in a rotating bearing, under load.
As part of their experimental work, Dr Mahmoud Mostafavi from the Structural Integrity Department of Mechanical Engineering at University of Bristol, and colleagues from the Universities of Sheffield, Bristol, Oxford and Diamond Light Source, used a novel stroboscopic X-ray diffraction technique to measure the strain in the rotating piece of machinery.
As Mostafavi explains: “Using an intense and highly penetrating multi-energy beam of X-rays generated on I12 (JEEP) beamline at Diamond, we were able to use a specially designed energy-dispersive detector to measure the diffraction of X-rays from deep within an object."
While computing models can be used to predict what the strains in a bearing are, it is difficult to verify these models through actual measurement of the strains.
But, according to Mostafavi: "Diffraction of X-rays from crystalline materials like steels produces a characteristic pattern with peaks in intensity at specific photon energies, [with] peak position being related to the regular spacing of atomic planes in the material."
"If the spacing of the planes changes because the material is under strain, then the peaks in the pattern shift," he adds. "From this shift it is possible to calculate the strain, giving researchers an ‘atomic strain gauge’."
As part of the research, Mostafavi and colleagues wanted to take diffraction measurements only when a ball in the bearing was in the correct position below the measurement point on the raceway. 
To do this, the detector has a “gate” in the data collection electronics, so the detector only counts X-rays when a signal is received from an external sensor.
To provide the gate signal, the researchers fitted a proximity sensor to the rotating bearing, so that the detector could be set to collect data only when the ball was in the correct position.
As Diamond Principle Beamline Scientist, Thomas Connolley explains: “That sounds simple, but with the ball only in the correct position for about 2 milliseconds, the X-ray counts for one cycle are not enough for acceptable data analysis."
"By adding up the signal collected at the same point in the cycle over many cycles, a dataset that could be analysed was obtained. This ability to “freeze frame” and sum the data collection over repeated cycles is why this is known as a stroboscopic technique," he adds. "Beamline I12 also has a high speed X-ray imaging camera, which enabled the team to confirm visually, during setup, that the ball was indeed in the right place when the gating signal was sent."
For the experiment, a bearing test rig that is normally used at the University of Sheffield was brought to the beamline.
Strain measurements were taken with the rig running under increasing applied loads, with these strains then used to calculate the stresses in the outer raceway.
This research recently won the Fylde Prize from the British Society for Strain Measurement (BSSM), awarded to the Best Paper published in 2017 in the Journal Strain. 
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