THE WORLD OF SCIENCE

Te Awamutu Courier, Volume 59, Issue 4220, 6 December 1939, Page 5

THE WORLD OF SCIENCE

LUBRICATING OILS AND “DOPE.” (By Dr R. S. Burdon.) For some time much attention has been devoted to investigating the physical and chemical properties of lubricating oils, particularly with a view to finding those qualities which would enable an oil to stand up to the higher temperatures and greater speeds used in modern machinery. This work has, given us a fairly clear idea of the cause of friction and of how lubricants reduce this, but there is still much to be learned about the behaviour of oils under actual working conditions. At first sight it may seem strange that there is anything to explain about friction—it is familiar to all—and most would be satisfied to say that friction, or the effort necessary to make one surface slide over another, is simply due to the roughness of the surfaces. Actually, it is not quite so simple as that, or we should be able to eliminate friction by making the surfaces smooth. But no motorist could be persuaded to try his engine without oil, just because the cylinder walls and piston surfaces were very smooth. In fact, the smoother the surface the more effectively they seize or weld together. The present view is that friction in general is due to some such welding process. When one metal surface rests on another, it is only at a few points that the two surfaces come into actual contact. Even when considerable pressure is exerted to force the surfaces together a film of air persists between them over all but a small fraction of their areas. When one surface slides over another the points that really come into contact are heated and weld together. The force to keep the surface sliding is just the force necessary to break these minute welds, which are continually being formed. Among the more recent work on friction is the experimental proof that, during the sliding of one surface over another, intense local heating does occur, giving temperatures high enough to produce this welding of the points that come into real contact. In proper lubrication there is a film of lubricant between the surfaces,

preventing them from coming into contact. If this film of lubricant were always complete the surfaces would never come into contact; there would be no welding and tearing apart of the solid surfaces which consequently should last for ever.

In such a case as that of a wheel rotating on an axle, the layer of lubricant is dragged forward between the bearing surfaces by the motion, provided the oil adheres strongly to the metal. When the motion stops the pressure of the wheel against the axle will squeeze out the layer of oil and allow the metal surfaces to come into contact. When the wheel starts again there is not proper lubrication until the motion has re-established the film between the bearing surfaces. In the case of heavy trucks it is believed that practically all the wear occurs during the brief starting periods. ADHERENCE A FACTOR. The adherence of lubricant to bearing surface then is a most important factor in lubrication. The pure mineral oils are not very satisfactory in this respect, being rather easily squeezed out from between the surfaces. In experimental tests this is shown by what is culled “stick-slip” motion, replacing the smooth sliding of ( lubricated surfaces. A flat horizontal surface is lubricated, and another surface is made to slide over it at a steady rate, while 1 the frictional resistance between the surfaces is measured. With proper lubrication the pull remains quite steady. In the stick-slip motion the force of friction increases steadily for a while, then drops suddenly to the value for good lubrication, the process being repeated over and over again as the lubricating film alternately forms and breaks down. Animal and vegetable oils were known to possess advantages over mineral oils for some purposes, and this is due to the presence in them of active constituents which increased their adherence to the bearing surfaces. It was only natural that attempts should be made to add similar active constituents to mineral oils, and perhaps equally inevitable that the added materials should be called “dope.” Some years ago it was found that approximately 1 per cent of one of the fatty acids, such as stearic acid,

made a suitable “dope” for mineral oils, giving a remarkable increase in adherence and converting the stickslip motion into a steady sliding. More recently metallic compounds of fatty acids have been used, the added substance being in reality a form of soap.

Quite recently F. P. Bowden, a former Tasmanian student, published a report of some experimental work which suggests that, under certain conditions of working, the mineral oil may produce its own “dope.” It was found that oil which gave stickslip motion would give smooth gliding after it had been heated for several hoprs in air to a temperature of 150 degrees Centigrade. If heated to 300 degrees, the improvement occurred within an hour, but if the high temperature were maintained for five or six hours the oil darkened in colour and the stick-slip motion return e’d. No change occurred in oils heated in the complete absence of air. The fairly obvious explanation is that the high temperature caused the oxygen to attack the oil and produce a small percentage of the active substances responsible for the improved adhesion between oil and metal. Such changes, no doubt, occur in oil under working conditions that produce moderate temperatures. Prolonged use under conditions of high temperature in the presence of air produces changes that are' injurious. In order to prove that oxidation was the cause of the initial improvement in lubrication a sample of oil was kept at the temperature of melting ice, and ozone—a powerful oxidising agent—was bubbled through it. Within an hour the oil had gained in lubricating properties, so that the stick-slip motion had given place to the smooth sliding of good lubrication. Bowden’s work is of interest as giving some further insight into what actually happens in lubricating oils under working conditions. He also points out that in a short test on a “doped” oil the improvement may be due to precesses occurring in the oil itself rather than to the effect of tht added “dope.”