Effect of conditioners on Cu-Cu and Al-Cu high current density sliding electrical contacts
Badamikar, Anupam Shirish
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This study was aimed at evaluating the performance of contact conditioners in sliding electrical contacts. Cu-Cu and Al-Cu sliding contacts have been tested at current densities up to 933 A/mm2 and 733 A/mm2 respectively. A pin on disc type test setup was developed around a high speed lathe. The setup comprises of a mechanical loading system, test circuit carrying high current, customized electronics and a data acquisition system. The mechanical system consists of a mechanism for loading the pin at a constant pressure independent of wear and a three component piezoelectric force dynamometer. The high current circuit consists of power switch arrangement, a shunt resistance for measuring the current and current control resistances for setting the peak current. The electronics includes circuitry for triggering, and controlling the width of the current pulse. A four terminal contact voltage measurement was performed during sliding. The contact voltage, shunt voltage, normal and frictional force components were recorded using an A/D card continuously at a rate of 5 kHz. The average pin wear rate was obtained by measuring the weight and length of the pin before and after the test. Two different contact conditioners, a solid lubricant interface conditioner (SLIC) and Uniflor8511® a PTFE thickened lubricant from Nye Lubricants were used. Test results for the copper-copper (C110) sliding with SLIC shows tremendous improvement in contact life. Contacts pre-conditioned with SLIC, loaded to 10 MPa normal contact pressure and subjected to 3.6 ms long current pulses of nominal current density 933A/mm2, survived for more than 4000 revolutions or passes (single pulse/rev) prior to the galling and failure. Sliding under current occurred at an average friction coefficient of 0.24. In comparison, testing without any conditioner shows an average friction coefficient above 0.6 and that the pin fails in less than 9 revolutions. Similar tests were carried out for Al 7075-T6 pin sliding on copper (C110) plate. Testing was carried out in three stages, namely, preliminary testing to evaluate performance of SLIC and Uniflor under high speed sliding, interrupted testing to identify test track condition in the initial stage of sliding and further additional circular testing. Preliminary test results showed that Al transfers from the pin to the copper plate under high current. Testing with SLIC it was found that that the conditioner is less effective in this sliding system considering the amount of Al transfer to the copper plate. Further additional testing has been performed using Uniflor8511. Additional circular testing shows that the average wear rate for sliding without current is much smaller as compared to sliding with current. A new mode of wear, namely, cutting of the transfer layers by the pin in subsequent passes, has been observed to increase the wear rate significantly once galling begins. Results also show that the thickness of the lubricant layer has an effect, with a thin layer of lubricant showing considerably increased friction coefficient and increased Al transfer to the test plate. Repeat pass testing with re-lubrication of pin between two passes provides some improvement in the average pin wear rate obtained. SLIC is a promising conditioner for high current density Cu-Cu sliding contacts but needs to be tested at higher sliding speeds. For Al-Cu sliding contacts, Uniflor8511 delayed the onset of galling to about 30-40 passes (up to 3 pulses/rev), but could not reduce transfer of Al from the pin to the Cu plate.
Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Industrial & Manufacturing Engineering