Strain Research on Rotating Bearings wins Fylde Prize for Best Paper

Research work performed using the UK’s Synchrotron, Diamond Light Source, has won the prestigious Fylde Prize from the British Society for Strain Measurement (BSSM). The prize, sponsored by Fylde Electronic Laboratories Ltd, is awarded to the Best Paper published in 2017 in the Journal Strain.   

     
The paper - “Dynamic contact strain measurement by time‐resolved stroboscopic energy dispersive synchrotron X‐ray diffraction,” was the result of a collaboration between the Universities of Sheffield, Bristol, Oxford and Diamond Light Source. The researchers set themselves the challenge not just of measuring the strain in a bearing, but of capturing the measurement while the bearing was rotating and under load. This involved using a special stroboscopic X-ray diffraction technique to measure the strain in the rotating piece of machinery.
 
The authors will receive their award from the Journal’s Editorial Board and the British Society for Strain Measurement (BSSM) on 30th August 2018 and have been invited to present their paper at the BSSM’s International Conference on Advances in Experimental Mechanics in Southampton at 29 – 31 August 2018. 
Figure 1: Schematic of the test setup. (a) Bearing in loading rig. (b) Detail showing X-ray strain measurement location. (c) Energy-dispersive X-ray detector.
Figure 1: Schematic of the test setup. (a) Bearing in loading rig. (b) Detail showing X-ray strain measurement location. (c) Energy-dispersive X-ray detector.
The researchers chose to look at bearings because they are one of the most common mechanical components in everything from bicycles to wind turbines. A bearing consists of high strength steel balls or rollers held between an inner and an outer ring called a raceway. As a bearing rotates, the strain at each point in the fixed outer raceway goes up and down as each ball or roller passes. This alternating strain, if it is too high, causes a type of metal fatigue which leads to cracking and failure of the bearing.
 
The strain in the bearing can be increased by sudden overloads, for example when a wind turbine experiences a very strong gust. With modern computer models, it is possible to predict what the strains in a bearing are. However, it is very difficult to verify those models through actual measurement of the strains. Mahmoud Mostafavi, Reader in Structural Integrity Department of Mechanical Engineering at University of Bristol; explained that the challenge for the team, was to not only make strain measurements inside the steel raceway of a bearing, but to do so while the bearing was rotating.   “This is where the I12 (JEEP) beamline at Diamond comes in. Using an intense and highly penetrating multi-energy beam of X-rays generated on I12, we were able to use a specially designed energy-dispersive detector to measure the diffraction of X-rays from deep within an object. Diffraction of X-rays from crystalline materials like steels produces a characteristic pattern with peaks in intensity at specific photon energies. The position of the peaks is 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. From this shift it is possible to calculate the strain, giving researchers an ‘atomic strain gauge’.” 
 
Figure 2: Dynamic stress components derived from strain measurements, for increasing applied loads on the bearing.
Figure 2: Dynamic stress components derived from strain measurements, for increasing applied loads on the bearing.

The next part of the challenge was 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. The gate is used so that 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 a ball was in the correct position.

Diamond Principle Beamline Scientist, Thomas Connolley continued; “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 can be analysed is obtained.  This ability to “freeze frame” and sum the data collection over repeated cycles is why it is known as a stroboscopic technique.  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.”
 
The Bearing Experiment
The Bearing Experiment
For the experiment, a bearing test rig that is normally used at the University of Sheffield was brought to the beamline. The test rig is designed to rotate the bearing at a given speed, under different loads to represent different in-service conditions, as shown schematically in  Figure 1. Strain measurements were taken with the rig running under increasing applied loads. The strains were then used to calculate the stresses in the outer raceway, shown in the plot in Figure 2. The red circles in particular show how the vertical component of stress becomes more negative, that is compressive, as the load on the bearing was increased.

  

Related publication:

A full description of the work can be found in the article in the Journal “Strain” [1]. The concept of the stroboscopic technique was previously proved in an experiment to measure stresses inside a running motorbike engine [2].  A paper describing the I12 beamline is also available [3].

[1] M. Mostafavi D. M. Collins  M. J. Peel  C. Reinhard  S. M. Barhli  R. Mills  M. B. Marshall  R. S. Dwyer‐Joyce  T. Connolley (2017), “Dynamic contact strain measurement by time‐resolved stroboscopic energy dispersive synchrotron X‐ray diffraction”. Strain 53: e12221. doi: 10.1111/str.12221.

[2] N. Baimpas, M. Drakopoulos, T. Connolley, X. Song, C. Pandazaras and A. M. Korsunsky (2013), “A feasibility study of dynamic stress analysis inside a running internal combustion engine using synchrotron X-ray beams” J. Synchrotron Rad. 20: 316-323. doi: 10.1107/S0909049513000885

[3] Michael Drakopoulos , Thomas Connolley , Christina Reinhard , Robert Atwood , Oxana Magdysyuk , Nghia Vo , Mike Hart , Leigh Connor , Bob Humphreys , George Howell , Steve Davies , Tim Hill , Guy Wilkin , Ulrik Pedersen , Andrew Foster , Nicoletta De Maio , Mark Basham, Fajin Yuan , Kaz Wanelik (2015) “I12: the Joint Engineering, Environment and Processing (JEEP) beamline at Diamond Light Source” Journal Of Synchrotron Radiation 22: 828-838. doi: 10.1107/S1600577515003513