An on-line Hall-effect device for monitoring wear particle in oils
Introduction
Since the failure of machinery is usually a slow progressive process, wear debris analysis allows predictive maintenance before the surface deterioration advances towards failure. According to the development of ferrographic analyzer, the ferrographic methods can be classified as the off-line analyzer, used in the experimental laboratory, and the on-line analyzer, used in the lubrication cycle system of the machinery to continuously monitor wear condition. Since 1972, the off-line ferrographic method has been developed from stationary type [1]to rotary type 2, 3. This magnetic separation technique has been widely introduced to monitor lubricating conditions in a wide variety of industrial fields, including the bearings of nuclear power generating turbines, automotive engines, power transmission devices, and compressors in chemical plants. However, oil sampling is indefinite and laboratory analysis needs the analysis knowledge of wear particle. On the other hand, the on-line detection method is based on real-time cyclic separation of wear debris from a oil system onto a device so that the concentration of wear debris can be evaluated. Hence, it is able to continuously monitor wear condition and to eliminate some of the required laboratory analysis.
Centers [4]determined the wear concentration by the on-line ferrograph for fluids containing well-characterized debris. Holzhauer and Murray [5]showed a good correlation between the wear loss of ball-bearings and the concentration readings of the ferrograph by using an on-line ferrograph and four-ball tester. Chambers et al. [6]used a trapping magnet to pre-concentrate particles in a flowing sample stream. Upon de-energizing the magnet, pre-concentrate particles was swept through the inductance coil of a radio frequency oscillator, causing a frequency transient proportional to debris mass. Liu et al. [7]developed an on-line ferrograph by using six sets of photoelectric sensors. Their results showed that wear debris greater than 5 μm in size was detected efficiently. Kuo et al. [8]used an electromagnet to catch wear debris and detected it with a photoelectric transducer. By using the luminous flux signal through the transparent oil pipe, the wear index of sample oil was calculated. The Hall-effect sensor employed by Lloyd and Hammond [9]to monitor the wear debris is a magnetic/electric conversion device. It is very convenient to convert the wear amount into an electric quantity, so that it can be used to detect the wear conditions. However, the study on the ferrographic analyzer using Hall-effect sensor is still scarce in the literature.
In this study, a new on-line ferrographic analyzer is developed by using electromagnetic flux measuring method. The principle of this device is that when the wear debris is captured on the air gap between the poles of the magnet, the magnetic flux density varied with the amount of wear debris. This change of magnetic flux can be transformed into the Hall output voltage difference through the Hall sensor. Hence, this voltage difference can be used to detect the amount of wear debris. Furthermore, the fundamental characteristics of this on-line ferrographic analyzer are also investigated in this study. These include the effects of the flow rate, wear debris concentration, and operating time on Hall voltage difference.
Section snippets
Apparatus
According to the analysis of a magnetic field [8], a two-legged magnetic core with air gap is shown in Fig. 1. To reduce eddy current loss, the electromagnetic core consists of 40 sheets of silicon steel. An insulating resin is used between the sheets, so that the current paths for eddy currents are limited to very small areas. The width and cross-sectional area of the air gap between the poles of electromagnet are 2 mm and 40 mm2, respectively. The copper coil has 3000 turns on the core with
Deposition observation of wear debris
In order to understand the deposition process of wear debris on the air gap between the two electromagnetic poles, the following experiment was conducted. The flow rate of the oil sample was adjusted to 250 ml/min with the wear debris concentration of 110 ppm. Fig. 4 shows typical depositions of wear debris on the air gap between the poles of electromagnet at two observation periods of 0.5 and 2 min. It is seen from this figure that a small amount of wear debris is uniformly deposited on the
Conclusions
In this study, an on-line ferrographic analyzer is developed by using electromagnetic flux measuring method. The basic principle for this analyzer is based on the change of magnetic flux density with the amount of wear debris which was captured on the air gap between the poles of the magnet. This change of magnetic flux can be measured by the Hall-effect sensor, and it can be transformed into Hall voltage difference. It has been found that the relationship between Hall voltage difference or
Acknowledgements
The authors would like to express their appreciation to the National Science Council (NSC-85-2212-E-110-005) in Taiwan for financial support, and to Dr. W.F. Kuo for his helpful discussions.
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