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Postawiono hipotezę, że proszki PTFE i miedzi lub PTFE i cyny użyte jako napełniacze smaru plastycznego wywołują w procesie tarcia synergizm przeciwzużyciowy i przeciwzatarciowy, dzięki czemu tworzą wysokoefektywną kompozycję smarową dla stalowych węzłów ślizgowych działających w obszarze tarcia mieszanego. Zbudowano hipotetyczny model smarowania węzłów ślizgowych kompozycjami z napełniaczami o właściwościach synergicznych. Hipotezę tę zweryfikowano w badaniach trybologicznych przeprowadzonych na skojarzeniu stal-stal ze stykiem skoncentrowanym (kulka-kulka, aparat czterokulkowy)i stykiem rozłożonym (panewka-czop, stanowisko modelujące pracę łożyska w ruchu wahadłowym), a także za pomocą wyników analiz kompozycji smarowych i powierzchni warstwy wierzchniej współpracujących skojarzeń. Zbadano wpływ stężenia napełniaczy w smarze na właściwości trybologiczne utworzonej kompozycji. Znaleziono - obliczając minimum objętości funkcji zużycia - ich optymalną (według kryterium minimum zużycia) zawartość w smarze. Wyjaśniono mechanizm synergizmu proszków PTFE i miedzi oraz PTFE i cyny w smarze plastycznym w procesie tarcia, wpływający na zwiększenie trwałości ślizgowych węzłów maszyn.
Recent research results have clearly demonstrated that lower friction and wear at machine interfaces lubricated with greases can only be achieved by modifying them with combinations of fillers rather than with single fillers. Such combinations of additives impart better lubricating properties and are also capable of forming metal-based plating films on interacting surfaces. Both experiments with greases filled with PTFE powders and literature data have pointed out to combinations of PTFE powder and copper or PTFE and tin as candidate modifying compounds due to their synergic effect. The available literature has not dealt yet with the problem of synergism in such pairs of additives as PTFE and copper/tin. Based on physical-chemical properties of PTFE, copper and tin and on particular mechanisms of their lubricating action when used as fillers, a hypothesis was put forward by the present author that the above combinations of additives act as synergic pairs when it comes to wear reduction and seizure prevention at interacting surfaces, especially under mixed -lubrication conditions. In order to test the hypothesis, a variety of formulas were investigated involving PTFE, Cu, Sn and PTFE=Cu (or+Sn) compositions. The tests were carried out under local contact conditions using a standard four-ball apparatus and complying with the Polish Standard PN-76/C04147 ('Testing of lubricating properties of oils and greases'). The investigated grease formulas were compounded with 1, 3, 6 and 9 wt% of the adopted fillers. The results showed clearly that the combined action of modifying agents is greater than the sum of their separate, individual actions. An attempt was made to explain mechanisms underlying the observed phenomena. It was assumed that mechanical load combined with sliding velocity act together to cause PTFE's thermal decomposition and thus to produce decomposition products, mainly tetrafluoroethene C2F4 and a metal-plating layer of copper or tin, being plain or complex compounds of fluorine and copper or fluorine and tin. The proposed theory was verified by later tests in which chemical composition of the surface layers from the interacting balls lubricated with single-filler and double-filler formulas was examined. Additional tests involved residual products left by differential thermal analysis of greases compounded with PTFE+Cu and PTFE+Sn. Yet another goal set by the author was to investigate the effect of filler concentration on tribological properties of grease formulas under discussion. The tests were performed according to the experiment planning theory recommendations using three input variables, i.e. average Hertzian pressure p., sliding velocity v and filler concentration u. The variability ranges were assumed to be: p=2600-4600 MPa, v=0.04-0.68 m/s, u=0.5-11.5 wt%. In order to fully exploit the resulting regression functions showing the effect of filler concentration on wear extent, graphical representations of the functions were used as obtained from the commercial Surfer package intended for generating and analysing 3D surfaces. The experimental results for a concentrated (ball-to-ball) load fully confirmed the synergic effects of the fillers but they were further verified under distributed (test bearing) load conditions. The bearing was made up of a sleeve (40mm both in diameter and length) and a journal performing an oscillatory rotating motion. Such a kind of motion creates the most severe operating conditions from a tribological point of view since sliding velocity becomes zero at dead points. The results from these tests were also a firm confirmation of the filler synergism theory. The presented study can be readily implemented in industrial practice. A commercially applicable formula is proposed for lubricating high-load sliding joints operating under mixed-friction conditions. The formula was developed using the author's own method for evaluating the minimum value of the volume contained below the wear function surface. A patent is going to be issued soon ('A grease for maintaining high breaking down-strength lubricating films') covering both the exact formula and the idea of synergic action of PTFE powder and copper. As regards performance the proposed formula exceeds all grades of greases available on the Polish market. The author gives his recommendations regarding practical aspects of the grease application.
Słowa kluczowe
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Tom
Strony
138--138
Opis fizyczny
Bibliogr. 174 poz., tab. 13, rys. 76
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autor
- Instytut Konstrukcji i Eksploatacji Maszyn Politechniki Wrocławskiej , Wybrzeże Wyspiańskiego 27, 50-370 Wrocław
Bibliografia
Typ dokumentu
Bibliografia
Identyfikator YADDA
bwmeta1.element.baztech-article-BPW1-0004-0046
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