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Emerald Group Publishing Limited
Copyright © 2002, MCB UP Limited
World's first NDT machine for determining residual stress on bearing surfaces is 1000x faster than X-ray machines
Keywords: Stress analysis, Bearings
A joint development between NSK's European Technology Centre and AEA Technology has produced BeSt, the world's first NDT Machine capable of measuring residual stresses at any points on the surface of bearing raceways. Compared to X-ray diffraction methods of measunng residual stress, BeSt is up to 1000 times faster (Plate 4).
Plate 4 A joint development between NSK's european technology centre and AEA technology has produced BeSt, the world's first NDT machine for determining residual stress on bearing surfaces is 1000x faster than X-ray machines
The requirement to measure residual stress on the surfaces of bearing raceways is already a common one for aerospace bearings. It is also set to become more common generally, as demands grow for improved bearing fatigue life in other key market sectors such as machine tools and automotive. The objective of the measuring process is to determine exactly what type of stress is involved (i.e. compressive or tensile) and its magnitude. The stresses themselves result from heat treatment processes and grinding and honing operations in the production of bearing rings. Ideally, when the rings are subjected to surface measurement compressive stresses should be revealed. These are instrumental in preventing any surface and near sub-surface cracks from initiating and growing therefore impairing bearing life. It would be unacceptable if are tensile stresses are present since they actually accelerate further growth in surface and near sub-surface cracks, accelerating bearing failure.
The conventional method of measuring for residual stresses is using X-ray diffraction machines. However, these are very expensive (typically £300k) and the process itself is very slow. In addition, the fact that X-ray diffraction is slow limits the amount of data that can be collected. Therefore, data is only collected on small areas of the bearing rings.
In contrast, with BeSt the complete raceway of a bearing can be measured using many thousands of data points and analysed (in both the axial and hoop directions) and the stress results displayed on screen within an hour, rather than the days or weeks it would take with X-ray diffraction methods.
Four axis servo controlled machine
BeSt is a FOUR-axis (X, Y, Z and W), servo controlled machine equipped with specially designed electro-magnetic probes. The probes undertake pre-programmed motion profiles across the raceways, as part of a process that measures stress by analysing the electromagnetic behaviour of a material and identifies changes in its magnetic permeability. This technique is effective because changes in residual stresses also cause very small changes in the materials magnetic permeability. Therefore, by measuring the magnetic permeability, the residual stresses can de determined.
BeSt based upon proven technology
The technology employed by BeSt is based upon an existing product from AEA Technology called MAPS. MAPS has been used by AEA for measuring residual stresses on large structures such as oil refineries, large diameter pipelines and even railway lines. NSK's European Technology Centre (ETC) was quick to spot the relevance of MAPS and the possible application for inspecting bearings. Successive development programmes funded by both NSK's ETC and Aerospace Division developed the MAPS system into BeSt. The developments included small custom made probes, high frequency signal generation, material calibrations for bearing steels, signal detection, and analysis and a suite of propriety computer software to display the data in 2D form.
Complete raceway stress maps within 1 hour
The stress measurements taken by BeSt are carried out at a frequency of 150kHz, giving a measurement penetration of = 18µm, comparable to X-ray diffraction. The operator simply selects the bearing specification from a look-up table together with details of the inspection requirements, allowing the servo controlled robot scanner arms to carry out the measurements. Using this system single measurements take less than 1 second with a spatial resolution of 3mm. This means that a stress map of a complete raceway surface can be collected within around one hour. To accomplish the same task using X-ray diffraction would take weeks, each single surface measurement occupying approximately 30 minutes.
Contact for reader enquiries: David Garner, Corporate Communications Manager NSK Europe Ltd, Mere Way, Ruddington, Notts, NG11 6JZ. Tel: 0115 936 6470.