The paper proposes a method for diagnosis of lubricants wear degrees based on ... ery is to discover the degradation of lubricant within the oil circulation system ...
Journal of the Balkan Tribological Association
Vol. 15, No 2, 263—269 (2009) Lubrication
EXPERIMENTAL RESEARCHES CONCERNING THE LUBRICANTS DURABILITY A. RADULESCUa, I. RADUlESCUb, C. BALANa, T. SAVUa University Politehnica, 313 Splaial Independentei Blvd., 060 042 Bucharest, Romania S. C. ICTCM S.A. Bucharest (Mechanical Engineering and Research Institute), Romania
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ABSTRACT The paper proposes a method for diagnosis of lubricants wear degrees based on the determination of the rheological properties, more precisely the reduction of the viscosity values during the working time. An experimental stand was designed and realised, according to the squeeze-film theory. Experimental results are analysed and conclusions regarding the durability and the reserve of life for the analysed lubricants are presented. Keywords: lubricants, durability, experiment. AIMS AND BACKGROUND The purpose of most methods of early failure detection in oil-lubricated machinery is to discover the degradation of lubricant within the oil circulation system. These methods can be divided into continuously measuring ones and those requiring the taking of an oil sample. A different method is offered by the lubricant itself. The condition of the oil is subject to the influence of many factors which may degrade its two primary functions, namely cooling and lubrication, to a point where severe damage occurs. Accordingly, the lubricant itself is an important source of information in the strategy of defect avoidance comparable to the role of the human blood in the detection and prevention of diseases. Main parameters which have an influence on the condition of the lubricating oil are: elevated temperature, presence of air, water, fuel or other lubricants, solid matter: wear debris, dust, dirt, etc. The lubricant quality detecting is one of the main factors in condition monitoring of the oil lubricated machinery. * For correspondence.
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Contaminants such as dust, wear debris, etc. chemical by-products and physical changes of the used oil normally alter the performance of lubricant. These changes always reflect a change of dielectric constant of the lubricant, therefore, any abnormal condition, such as contaminant ingression, generated wear debris, severely chemical and/or physical oil degradation etc., can be detected in timely manner by monitoring the dielectric constant of the used lubricant1,2. Concerning the new conditions of the actual manufacturing, it is important to develop a new method of fast diagnosis of liquid lubricants (used for cars, sea ships, engines and different systems). It was started from the idea that the oil change moment is imposed to be made when it is practically completely used and not regarding the theoretical products recommendations. For the actual level of the science and technology, the oil change method from a not completely used system is out-of-date, having great economical waste and involving ecologically effects3. The paper proposes the identification and developing of a new and fast method for the lubricant diagnosis, based on the squeeze-film theory. This method is an original one, very fast (under 30 s) and it uses for diagnosis a low quantity of oil (about 5 ml) (Ref. 4). Thus, it could be precise the wear degree, in a short time, from the stand point of its properties in a hydrodynamic flow condition. If the oil is used more than the normal, it could be an admonition of the owner of installation for troubleshoots which may appear; if the oil is less used, it could say how long will still be used5—7. EXPERIMENTAL The experimental stand used for the carrying out of the determinations was a modified Weissenberg rheogoniometer, built up from the main structural elements (Fig. 1): • central working unit; • driving system of the superior disc, corresponding to the friction couple; • electric and comand system for the servomotor; • pressure transducers; • displacement transducer; • data acquisition system. Fig. 1. General view of the experimental stand 264
Fig. 2. General view of the measuring are
Fig. 3. View of the displacement transducer
Figures 2 and 3 present a detail regarding the construction and the location of the pressure and displacement transducers. For the acquisition and the numerical treatment of the experimental data it has been used the LabVIEW software8. The measurement of the signal provided by the three pressure transducers and the displacement transducer was realised using an acquisition board NI USB-6008. The pressure transducers have been calibrated with a 200 g load, finally obtaining conversion relations between electrical signal and pressure, respectively displacement. TESTING METHODOLOGY In order to obtain practical information concerning the durability of the lubricants, two types of oils were tested (engine oil 15W40 and hydraulic oil HLP 46), coming from different working devices with different degradation stages: — engine oil 15W40 from an essence motor vehicle with 62 000 km on road; — hydraulic oil HLP 46 from a high pressure hydraulic cylinder with 6800 running hours. For the engine oil, the mean life-time recommended by the producers is 10 000 km, and for the hydraulic oil is 7000 h (Ref. 7). During this period, two samples of lubricants were collected, corresponding to two degradation stages: first one is of fresh oil (at the beginning of the working period), and the second is of used oil (considered at the moment of changing imposed by the producers). 265
For the testing protocol, four velocities of the superior disc have been imposed, the third one being used for the properly rheological test. During the squeeze-film process, the instant value of the pressure was measured in three measuring points: central — No 1, median — No 2, lateral — No 3. The variation of the pressure in the film is plotted tak- Fig. 4. Loading cycle ing into account the loading cycle presented in Fig. 4. The shape of this curve, for a given velocity of the superior disc, depends on the fluid viscosity and its elasticity, if the fluid has a thixotropy behaviour. By integration of the pressure distribution, the variation of the load carrying capacity of the film with the squeezing time is obtained, which is a very important parameter for establishing the fluid degradation. For the calibration of the experimental stand, the glycerin was used as a working fluid. This fluid is recognised for its remarkable rheological properties — perfect Newtonian fluid, with a constant viscosity, independent of the strain rate. The loading cycle proposed for the experimental tests of fresh and used fluids has five steps: • the descent of the superior disc is attained between 6 mm film thickness to 4 mm film thickness, with a high squeezing velocity d 10 mm/s; • the descent of the superior disc is attained between 4 mm film thickness to 0.6 mm film thickness, with an average squeezing velocity of 1 mm/s; • the descent of the superior disc is attained between 0.6 mm film thickness to 0.1 mm film thickness, with pre-scheduled testing velocity of 0.25, 0.50, 0.75 and 1 mm/s; • maintenance of the superior disc to the minimum film thickness for a period of 5 s; • retract of the superior disc to the initial position (6 mm film thickness), with an average velocity 1 mm/s. During this cycle, the really interesting measuring area is represented by the third zone, which has the minimum velocity; here appear the main particularities of the squeezing process. In this region, the appearance of maximum pressures, with values of 2—3 MPa, is very significant and makes the difference between fresh and used oils. 266
RESULTS Figures 5 and 6 present two specific examples of data acquisition concerning the variation of the pressure in the lubricant film, for the engine oil 15W40 and the hydraulic oil HLP46, in fresh and used state, at a squeeze velocity 1 mm/s and a temperature 20°C. The other parameters of the loading cycle have been maintained at their initial constant values. In order to observe the difference between the behaviour of the fresh fluid versus used fluid, the squeeze-film curves will be plotted (the variation of the pressure corresponding to different values of the film thickness) for all three pressure transducers. For all four data acquisitions presented in Figs 5 and 6, the results have been numerical treated, obtaining the final characteristic curves (Figs 7 and 8). Analysing Figs 7 and 8, it clearly can be observed the differences between fresh and used oils, especially pointing out the values obtained for the maximum pressure in the film lubricant.
Fig. 5. Data acquisition for 15W40 engine oil a — fresh oil; b — used oil
Fig. 6. Data acquisition for HLP46 hydraulic oil a — fresh oil; b — used oil
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Fig. 7. Pressure variation for 15W40 engine oil a — fresh oil; b — used oil
Fig. 8. Pressure variation for HLP46 hydraulic oil a — fresh oil; b — used oil
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Table 1 shows the values obtained for the maximum pressure, at different velocities of the superior disc, for fresh and used lubricants. Table 1. Maximum values of the film pressure (MPa) Lubricant 15W40 HLP46
fresh used fresh used
0.25 1.31 1.05 0.68 0.65
Velocity (mm/s) 0.5 0.75 2.26 2.58 1.80 2.03 0.83 1.15 0.78 0.87
1 3.18 2.48 1.39 1.16
CONCLUSIONS The experimental stand has as principal component an original device, coupled with an acquisition system, which is also capable to treat numerically the experimental data. Using a small quantity of lubricant and two horizontal semi-couples — an inferior one, which is fixed, and a superior one, which has a translation movement — it is possible to measure the variation of the film thickness simultaneously with the pressure distribution. The registered squeeze-film curves represent the ‘fingerprint’ of the lubricant, which depends on a large range of factors. The degradation stage of the lubricant is one of them. Comparative tests for the fresh and used lubricants have been realised, which point out the dependence between the degradation stage and the maximum pressure in the film lubricant. Once with the degradation of the oil, the maximum pressure in the squeezed film decreases. This effect is due to the decrease of the lubricant viscosity. At the same time, the fidelity degree of the dynamical response of the system decreases with increasing the oil wear rate. REFERENCES 1. G. SALVENDY: Handbook of Industrial Engineering, Section 13.6. John Wiley ans Sons, Inc., New York, 1982. 2. K. WALTERS: Rheometry Chapman&Hall, London, 1975. 3. Y. LIU, Z. LIU, Y. XIE, Z. YAO: Research on an On-line Wear Condition Monitoring System for Marine Diesel Engine. Tribology International, 33, 829 (2000). 4. Al. V. RADULESCU: The Grease Squeeze Film between Circular Plates. In: BALKAN TRIB’93, Sofia, Oct. l993, 484—491. 5. J. BRIANT. et al.: Propriétés rhéologiques des lubrifiants. Ed. Technique, Paris, 1985. 6. R. LEFEVRE: Graissage et Tribotechnique. I. Inst. Franc. Petrole, Paris, 1975. 7. The Rheology of Lubricating Greases. ELGI, Amsterdam, 2000. 8. L. ARSENOIU, T. SAVU, A. SZUDER: Basic Programs in LabVIEW. Ed. Printech, Bucuresti, 1999 (in Romanian). 9. Catalogue of Petroleum Products. Bucharest, PECO, 1993 (in Romanian). Received 30 June 2008 Revised 18 September 2008
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