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A sliding wear tester for overhead wires and current collectors in light rail systems

A sliding wear tester for overhead wires and current collectors in light rail systems
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  Ž . Wear 239 2000 10– r locate r wear A sliding wear tester for overhead wires and current collectors in lightrail systems Da Hai He a , Rafael Manory  a, ) , Harry Sinkis b a  Department of Chemical and Metallurgical Engineering, Royal Melbourne Institute of Technology, GPO Box 2476V, Melbourne,Victoria 3000, Australia b Public Transport Corporation Victoria, Transport House Le Õ el 13, 589 Collins Street, Melbourne, Victoria 3000, Australia Received 15 July 1999; received in revised form 15 November 1999; accepted 15 November 1999 Abstract This paper presents the dedicated experimental equipment developed for a study of the tribological behaviour of current collectorssliding against overhead contact wires under various conditions. A unique wear tester that replicates the operating conditions of actualpantographs in railway power collection systems was developed. The sliding element moves over the wire in a controlled horizontalmotion at homogeneous velocity, and a constant normal load can be applied on the contact materials during this motion. Dynamic frictioncoefficient data, as well as the dynamic contact resistance between the contact couples were monitored using a signal collection device Ž . equipped with data acquisition DAQ software.The wear couples were examined in sliding using under the same normal load, in dry and lubricated conditions. The frictioncoefficient decreases during the test in all cases, but electrical resistance and contact resistance increase. For Cu vs. Cu in dry contact, thedynamic friction coefficient measured after run-in was 0.34 and the electrical contact resistance increased by approximately 5% after 10 6 wear cycles. Application of graphite grease — a commonly used lubricant — increased the contact resistance by about 300%. Thedynamic friction coefficient of Cu vs. Cu lubricated with common grease was constant — about 0.24, and the electrical contact resistancewas 1.97  mV . Ž . Ž For carbon–copper composite materials CCM in dry sliding against Cu, the dynamic friction coefficient reduces by 20% from 0.27 . to 0.22 after 40,000 cycles, while the dynamic electrical resistance increased slightly from 5.32 to 5.35.  q 2000 Elsevier Science S.A.All rights reserved. Ž . Keywords:  Sliding wear tester; Electrical contact; Current collector materials pantograph and pole shoe ; Dynamic friction coefficient; Contact resistance 1. Introduction The wear behaviour between contact materials and con-tact wires in railway power collection system represents acomplex tribological system, which has not been investi-gated much. The requirement in this system is not onlywear reduction in the couple of contact wire–pantographmaterial, but also improvement of power collecting perfor- w x mance 1 . Various wear test methods and devices used in w x previous studies 2–9 were specifically used for theoreti-cal considerations. In the present work, the tribologicalconditions of sliding current collectors on overhead wires ) Corresponding author. Ion Beam Engineering Experimental Labora-tory, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. Tel.:  q 81-75-753-5950; fax:  q 81-75-751-6774. Ž .  E-mail address: R. Manory . were replicated in order to correlate directly between thelaboratory data and the real application.The pin-on-disc wear tester is a commonly used tribo- w x logical instrument 2 , to which many additional features, Ž such as friction force measurement by strain gauge or .  w x load cell 3 , environment control, lubrication, etc., can beadded. The motion of a pin against a rotating disc how-ever, cannot represent applications such as those encoun-tered in railway power collection systems, which are typi-cal sliding wear tribo-systems.Based on an analysis of the sliding system model,Dupont and Dunlap have designed a vertical unidirectional Ž . sliding shaft wear tester with a load cell sensor to w x measure friction 4 . This device moves only along a veryshort distance with slow speed and a contact normal loadis horizontally applied by springs at two positions, oppo-site each other. A computer was used to program various 0043-1648 r 00 r $ - see front matter q  2000 Elsevier Science S.A. All rights reserved. Ž . PII: S0043-1648 99 00365-8  ( ) D.H. He et al. r Wear 239 2000 10–20  11 motion trajectories and to collect the output data consistingof position, friction force, and normal force. This apparatushowever was not suitable for our case due to its verticalmotion. For horizontal sliding, Lacey and Torrance have w x used a screw shaft device for a slip-line field model 5 .The device is restricted from running at reasonably highspeed in horizontal motion due to the screw shaft rotationlimitation. Saka et al. have studied electric contact wear w x using a horizontal sliding device similar to our design 6 ,but it only moves very short distance in a slow motion.Recently, Nagasawa et al. have designed new copperalloy contact wires and studied the wear performance of railway power collection system against the new contact w x wires 7,8 . A rotating wheel wear tester was designed toexamine the contact wires against current collector materi-als. This device is a flywheel of 0.5 m diameter, whichconsists of two contact wires running against a contactmaterial pressed against it by a spring. This mode of operation is similar in principle with the experimental w x setting made by Yang and Torrance 9 . The electrical w x measurement device designed by Nagasawa et al. 7,8 Ž constantly monitors the contact condition dynamic contact . resistance . The flywheel can rotate at very high speed inorder to give a vertical speed close to 350 and 500 km r hat contact spots. This device provides a simplified wearmodel for contact materials and wires for trains running athigh speed. Their work is a significant breakthrough inresearch on railway power collection systems.The present investigation had a slightly different ap- w x proach than that employed in Refs. 7,8 , in that theequipment was designed to directly replicate the motion inan actual railway power collecting system. The tester Ž focuses on a slow speed wear the velocity is about 0.25 . m r s between contact wires and pantograph materials, andthe results refer to both tribological and electrical be-haviour. The wear at slow speed is more critical than thatat fast speed because of lack of airflow effects that reducethe pressure on the wire, and increases the cut-in period Ž . occurring at the soft metal copper surface.This investigation was conducted in order to betterunderstand the tribological behaviour of sliding electricalcontact materials with a view to improve the wear andelectrical behaviour of this tribo-system. Following thisstudy, a new series of materials was developed whichpresent better performance than the materials currently in w x use 10 . 2. The sliding wear tester The specially designed horizontal sliding wear tester isshown schematically in Fig. 1. This design is unique inthat it replicates the conditions of operation in an actualpantograph sliding along contact wires. It provides a con-trolled horizontal motion at a homogeneous velocity of about 0.25 m r s. A constant vertical pressure is applied onthe contact material. This pressure is a self-weight of 1 mof copper contact wire at nominal size of 161 = 10 y 4 m 2 Ž cross-sectional area hard drawn copper trolley contact . wire; BS.23: 1970 , which is the theoretical pressure ap-plied on a pantograph current collector. This size of con-tact wires is commonly used in the train system in Victo-ria, Australia.Despite the short running distance, the sliding speedalong the wire is constant also towards the end of themovement cycle. Six couples of wear groups, which arethe contact materials, can be tested simultaneously. Thecontact wires are fixed into specially designed grippingfixtures, which do not allow the wire to rotate or to movelaterally, maintaining the sliding face and counter-faces inthe same position during the whole test. There is nosupport at the back of the contact wires, and they are fixedon the vices by tension force. The forward and backwardmotion is driven by a single triangle belt transferringelement to a multi-center off set flywheel. Six samples of current collector material move simultaneously in a smoothand constant movement that is driven through a shaft andsimulates the situation of a train moving at constant speed. Ž Fig. 1. The wear test apparatus, showing the location of thermocouples and the special plate for measuring friction force strain gauges are attached to this . steel plate .  ( ) D.H. He et al. r Wear 239 2000 10–20 12 Ž . Ž . Fig. 2. Trolley contact wires wear test machine: a general view, b top fixtures for contact wires.  ( ) D.H. He et al. r Wear 239 2000 10–20  13 The movement range is about 0.25 m. General viewphotographs of this apparatus are shown in Fig. 2.The tester can run at a very low stable velocity, becauseof the heavy flywheel. The sliding speed is modified byvarying the current of the DC motor. The slowest speed atwhich the experiment can be run is 150 rpm, whichtranslates to sliding speed of 0.12 m r s. During the weartest, a 12-V AC high sensitivity ammeter monitors thecontact conditions frequently. AC current is used becauseit is more sensitive to the contact surface condition thanDC current. When a critical current limit is exceeded by anincrease in the air gap, a reduction in contacting asperitiesis detected by a warning light, and the contact conditions Ž are manually adjusted this device was not activated duringmeasurements of dynamic friction coefficient and resis- . tance . 2.1. Friction coefficient measurement  At the top of the specimen clamp, there is a steel alloyplate to which two groups of strain gages are attached.This plate was calibrated with a series of standard weightsand the data was recorded as a calibration file. The frictionforce between the sliding elements causes a deflection of this plate, causing strain gages to be at different resistancevalues and this enables recording of the friction forceduring motion. For measurements of the dynamic frictionforce: the data was collected from the strain gages for only Ž . half of sliding circle one direction to avoid misreading atthe change of direction. The final data is the average of five measurements and is plotted on a chart. In this designit is possible to simultaneously measure different wearcouples in the same machine. Fig. 3. The electrical test apparatus. T1 — transformer 450 V r 10 A to 12 V r 1500 A; D1 — rectifier 100 V r 1000 A; C1 — capacitor 100 V r 2200  m F;R1 — variable resistor 150 k  V ; R2 — variable resistor 50 k  V ; V1 — voltage meter; I1 — Amp meter; V2, V3, V4 — voltage meters for contactresistance measuring; R3 — variable resistor for adjust the contact resistance measuring range; I2 — amp meter for contact resistance measuring.  ( ) D.H. He et al. r Wear 239 2000 10–20 14Fig. 4. The DAQ system.Fig. 5. Diagram for DAQ 6 PC-LPM-16 block diagram.
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