Yuang-Cherng Chiou

Tribo-electrification mechanism for self-mated metals in dry severe wear process: Part II: pure soft metals

The tribo-electrification and wear behavior are investigated for self-mated pure hard metal pairs of Pt, Fe, Mo, Ti and W in dry severe wear process. Results show that the tribo-electrification of pin specimen appears to have much higher possibility of positive charge with higher wear loss at pin specimen by using Pt/Pt, Fe/Fe, and Mo/Mo pairs, but it becomes to have much higher possibility of negative charge with higher wear loss at plate specimen by using Ti/Ti and W/W pairs. It is found from the SEM observations on the wear surface and wear particle that the wear for those hard metal pairs is mainly caused by the asperity removal with small wear particle. According to these results, a model for tribo-electrification mechanism is proposed. Generally, the material transfer for the self-mated hard metals mainly causes the tribo-electrification. When the material transfers from pin specimen to plate specimen, the polarity of tribo-electrification for pin specimen becomes positive, and vice versa.

Formation criterion and mechanism of electrical pitting on the lubricated surface under AC electric field

The formation mechanism of electrical pitting on the lubricated surface of steel pair was investigated, and the threshold condition to avoid the occurrence of electrical pitting was also established by using a static electrical pitting tester with high precision under the influence of AC electric field. Experimental results show that when the electrical pitting occurs, the interface voltage, interface impedance, and interface power increases slowly with increasing film thickness at a certain supply current. However, the interface voltage and interface power increases with increasing supply current, and the interface impedance decreases with increasing supply current at a certain film thickness. Furthermore, the pitting area versus the interface power relationship is a cubic function, where the pitting area increases with increasing interface power. It is also found that the supply current is the parameter that influences the pitting area the most. Moreover, three electrical pitting regimes are found under the influences of shaft voltage and oil film thickness, namely, pitting, transition, and no-pitting regimes. The boundary between the transition and no-pitting regimes is called the first threshold voltage, and another boundary between the transition and pitting regimes is called the second threshold voltage. However, the supply current insignificantly influences these two threshold voltages. Correlation formula of threshold voltage and oil film thickness is also derived as the formation criterion of electrical pitting for a wide range of oil film thickness.

An on-line Hall-effect device for monitoring wear particle in oils

A new on-line ferrographic analyzer is developed using electromagnetic flux measuring method. This analyzer used the Hall-effect sensors between two poles of the magnet to detect wear debris. When the wear debris was captured by magnetic attraction at the air gap between the poles of the magnet, the magnetic flux density varies with the quantity of wear debris. Moreover, the output voltage difference of Hall-effect sensor is significantly influenced by the magnetic flux density. Hence, the relationship between the voltage difference or wear index and the quantity of wear debris can be obtained by using the multilinear regression analysis. The correlation formula for the wear debris concentration can be derived for a wide range of flow rate, operative time, and Hall voltage difference. By using this correlation formula in a single chip microprocessor, the wear debris concentration can be evaluated at a certain flow rate and operative time.

A study on lubrication mechanism and wear scar in sliding circular contacts

Abstract The effects of test method, normal load and sliding speed on the lubricating characteristics of a paraffinic base oil were experimentally investigated using a four-ball tester with a contact resistance instrument. Results show that at light load, elastohydrodynamic lubrication can only be found in the specific speed method, but a little wear scar still existed in the standard test method because of the contact initially. The specific speed method has a higher contact resistance than those of the standard test method at light load, but this is not always true at high load. According to the measured data of friction coefficient, contact resistance, and wear scar, and the observation of the wear surface, the lubrication mode can be clearly classified into four regimes under the various normal loads and sliding speeds. The transition curve between boundary lubrication and initial seizure makes no significant difference for both specific speed and standard test methods. This result shows that the initial seizure is significantly influenced by the load and sliding speed rather than by the test method. In the partial elastohydrodynamic lubrication (P-EHL) regime, the relationship between the wear scar diameter D and the contact resistance R c has been quantitatively and qualitatively obtained. The diameter of the wear scar can be predicted by measuring the contact resistance R c using this empirical equation. The transition curve between the P-EHL and elastohydrodynamic lubrication is in very good agreement with the prediction using the minimum film thickness obtained by Hsu and Lcc with the assumption of Λ = 3.

Pitting mechanism on lubricated surface of babbitt alloy/bearing steel pair under ac electric field

The mechanism of electrical pitting on the lubricated surface of babbitt alloy/steel pair is investigated, and the threshold condition to avoid the occurrence of electrical pitting is also established by using a static electrical pitting tester with high precision under the influence of ac electric field. According to the SEM micro-graph and EDS analysis are, the mechanism of electrical pitting is significantly influenced by the interface power and the oil film thickness. At the smaller oil film thickness, the eroded surface of babbitt alloy exhibits a concave crater with a few micro-porosity in the vicinity of center region with a plateau on its surrounding, especially at high supply current. The polished track can be observed at the plateau. A large amount of tin element transfers to the steel ball surface because the molten tin contacts the ball. At the higher oil film thickness, only a little amount of metal element transfers to each other. The major pitting area of the babbitt alloy is caused at the initial stage of the arc discharge. With increasing arc discharge time, the pitting area increases slowly, and finally reaches a saturated value. When the electrical pitting occurs, correlation formula for the electrical pitting area in terms of interface power and melting point of material has been established. It is found that the higher interface power and the lower melting point of material, the higher electrical pitting area. Two electrical pitting regimes are found, namely, pitting and no-pitting regimes. The boundary between the pitting and no-pitting regimes is called the threshold voltage. Correlation formula for the threshold voltage in terms of oil film thickness and melting point of material is derived.

Effects of pre-rolling and metal removal on the fatigue life of lubricated rolling/sliding contact

In this experiment, after the test specimens are preloaded at different first-step rolling cycles, the surface layer of the lower speed roller is ground to different depths by a grinder along the radial direction. With a new higher speed roller, the effects of the ground depth and first-step rolling cycle on the fatigue life of lubricated rolling/sliding contact are investigated using a 0.45% carbon steel pair. Results show that the prerolling cycles with metal removal can improve the rolling fatigue life. It is found that when the surface hardness of the specimen starts to approximate the saturated value at the first-step rolling cycle with the ground depth to the position of its maximum hardness, the rolling fatigue life with this preloading operation is 3.3 times that with the normal operation for the second-step load of 588 N. This increment in the rolling fatigue life can be reasonably explained by the measured plastic flow of the surface layer and the theoretical results of the residual stress below the contact surface.

Study of wear particle deposition by an improved rotary ferrographic analyzer

Abstract An improved rotary ferrographic analyzer and its specific characteristics are presented. The newly developed rotary ferrographic analyzer combines the design advantages of two traditional ferrographic analyzers previously developed and is applicable to a wide range of wear particles. The analyzer is comprised of four concentric magnetic cylinders, such that the magnetic flux density exhibits three grooves on the magnet surface. Tests on lubricating oils used in a motorcycle and an automobile were carried out and substrates were observed using a scanning electron microscope and an optical microscope. The observations showed that most wear particles were deposited around the gap between the magnetic cylinders on the magnet surface according to the maximum magnetic flux field gradient localization. The radii of the rotating magnets were inversely proportional to the sizes of the deposited particles. A simple theory, accounting for the balance between the magnetic, centrifugal and frictional forces on the wear particles, is presented and shows good agreement with experimental results.