Acoustic emission in monitoring extremely slowly rotating rolling bearing

There are not international standards or universally accepted limit values available, that classify rotating machines as slow or high speed machines.. The old international standard ISO 2372, which has been replaced with the new ISO 10816 standard, gave the vibration velocity severity ranges for different classes of machines. The old standard covered machines with rotational speeds from 600 rpm to 12000 rpm. The new standard does not contain any rotational speed limits. Sometimes the limit for low-speed rotating machines is set to 20 rpm or 30 rpm. In industry, it is easy to find machinery where the rotational speed in continuous running is lower than 2 rpm. In the condition monitoring of rotating machines, it is common practice to measure the vibration velocity or acceleration. At very low frequency, the vibration velocity amplitude becomes weak and therefore displacement measurement can sometimes be a suitable vibration measurement parameter. When the rolling element in the rolling bearing passes the early-stage fault in the case of an extremely low rotational speed, the energy that the collision generated is very low. In that case, the defect is difficult to detect in the frequency domain but can possibly be seen in the time domain The frequency bandwidth of acoustic emission (AE) measurement method is typically in the range 100 kHz to 1 MHz. In that range; vibrations occur in a material by fracture of crystallites, crack nucleation and growth, several mechanisms involving dislocations, phase transformations in materials, boiling and electrical discharges. Each of these mechanisms is characterised by a rapid collective motion of a group of atoms. The present paper describes the use of the acoustic emission method in the monitoring of faults in an extremely slowly rotating rolling bearing. The introduction describes the principle of the measurement method of acoustic emission and the analysis methods used for the acoustic emission signal. The paper contains the results of AE measurements where the rotational speed of the shaft was from 0.5 rpm to 5 rpm. The measurements were carried out using a laboratory test rig with grease lubricated spherical roller bearings of an inner diameter of 130 mm and a load of 70 kN. Prior, to testing the test bearing had been naturally damaged on its outer race during normal use in industry. The results of the acoustic emission measurement have been compared with the results of low-frequency vibration measurements, which have been carried out in the same test arrangement. The paper gives an example where acoustic emission measurements have been used in industry, in the monitoring of slowly rotating machinery.