Lubricants influence on wear in sharp rail curves

Abstract: Rail lubrication in curves was widely introduced in Sweden during the 1970’s due to the high wear rate of gauge faces in sharp curves. The first tests performed in Sweden showed that lubrication decreased wear rates by roughly 10 times. Based upon these results, the Swedish track system was equipped with wayside lubricators called Clicomatic. Aside from wear, rolling contact fatigue (RCF) is another limiting factor in railway infrastructure productivity. Head checking is one of the fatigue forms that arises in curves of heavily loaded tracks as well as fast train tracks. Head checkings grow at the rail head, close to the gauge corner. High pressure and a sliding motion lead to plastic strain of the material from the rail ball towards the gauge face. There is a coupling between wear rates and RCF damages, high wear rates may lead to reduced RCF damages and vice versa. Hence, frequent application of lubricants to the rail does extend the rail life by several times but may, on the other hand, lead to fatigue problems. Preventive grinding in combination with lubrication has been the most efficient way to avoid fast degrading of curved rail track. This thesis concerns the progress of rail lubrication in terms of how environmentally adapted lubricants function both in wayside lubricators and at the track. Another aspect of the thesis is to find a lubricant able to reduce the wear rate at the gauge face while simultaneously decreasing the crack growth at the rail head. One thought was to use controlled wear to reduce rolling contact fatigue. Controlled wear in this situation means that the usage of a lubricant should give a sufficiently high wear rate to continuously wear off the surface layers. Another idea along the same lines was that of low contact friction. Low friction in the contact gives less plastic strain and subsequently less head checking. Rapeseed oil had shown low contact friction in earlier testing, therefore, synthetic esters were of interest to examine in this application. From both field tests and laboratory tests it was concluded that the wear protection for environmentally adapted lubricants was comparable to that of previously used lubricants if they were applied by trackside lubricators. Although they were known to be more sensitive to low temperatures, testing showed how to overcome this issue. Wear and friction tests were performed in the laboratory to evaluate lubricating blends. The tests evaluated rolling / sliding as well as pure sliding contacts. The research performed in this thesis yielded the following results. Pure synthetic ester (TMP-Oleat and TMP-C8-C10) in a sliding contact resulted in high wear rates and a lot of abrasive marks at the surface. By adding friction modifiers, the friction as well as the wear rate decreased considerably in the sliding contact. In the rolling sliding contact, synthetic ester alone and synthetic ester with the addition of friction modifiers both showed similarly low friction and wear rates. PAO gave higher friction, but at the same time, the surfaces had significantly fewer cracks as compared to synthetic ester formulations. One observation was that surfaces lubricated by synthetic esters became less hard and the hardness interval was wider than those lubricated with PAO. One explanation is that even if the overall friction is low, local spots of high friction between asperities can lead to localized strain and crack initiation. The conclusions are that Rapeseed oil is the recommended base for rail lubricants. The tested synthetic esters are a more expensive alternative useful if excellent low temperature properties are needed. Graphite is not necessary in the lubricant if it is applied continuously by a wayside lubricator. Twin-disc tests showed that low contact friction does not necessary mean that crack initiations are avoided in the surface. In conclusion, the base fluid and the additives influenced not only the wear rate but also the ability for cracks to grow.