Transmission and gear oils which are used either in vehicles or in outside locations in cold climates should remain fluid at low temperatures. Low temperature is a relative term and an extreme case is that of aircraft flying at high altitudes which, therefore, encounter temperatures approaching – 70 degree (F). The gear oils used in this case are compounded from synthetic fluids.
Disregarding synthetic lubricating fluids, two general types of mineral oils are available. These are naphthenic and paraffin. The former, as a rule has the best cold test because no wax is present, but as temperatures are reduced such oils may tend to become quite viscous. This tendency will vary both with the viscosity and the source of the base oil. Most paraffin type oils contain wax which crystallizes at low temperatures and may therefore form a solid mass which will not flow or pump and may even channel in a gear case so that the gear teeth would lack a lubricant. By the addition of pour depressants, generally certain types of polymers, the wax crystals are coated so that in place of growing to long needles thy deposit as fine crystals which produce a slurry which moves at a lower temperature than the untreated oil.
A more positive approach to securing low cold test gear oil is to choose lubricating oil which has been refined so as to have a low pour point. Low viscosity oils generally have lower cold tests than high viscosity oils of the same type. Thus, lubricating oils are available with viscosity of 70 to 80 SUS at 100 degree (F) which have pour points of -60 to -70 degree (F). Proper dewaxing will also lower pour points. In extreme cases gear oils are offered which consist of lubricating oils diluted with fractions boiling below the lubricating fractions. Likewise, in the arctic regions Kerosene has been used to dilute gear oils. This poses a hazard to the equipment if the diluted lubricant is not removed from the gear case with the advent of normal temperatures.
High temperature limitations of most gear oils will be due to deterioration of the oil and additives. Oils should be chosen having high flash points if operating temperatures are excessive. The flash point of oil is generally indicative of the temperature at which volatility starts. In most industrial gear applications, the bulk oil temperature seldom exceeds 125 to 150 degree (F). In vehicles such temperatures seldom exceed 300 degree (F). Therefore, for what considered normal gear lubrication, additives are selected with the thought that temperatures of use will not exceed 300 degree(F)
However, Borsoff et al ^8 describes tests run at both 400 degree (F) and 600 degree(F) on a gear test machine in which the gear compartment and the oil were heated while the machine was in operation. Conclusions drawn from this investigation were:
“Due to the thermal instability of the lubricants, the operation of gears for any prolonged period of time at 600 degree (F) or higher is inadvisable; (2) no new or unusual types of failures were observed during the operations of gears at 400degree (F) and 600degree (F). Just as at normal operating temperatures, scoring and abrasion were the two primary destructive failures; (3) the load carrying capacities of oils decreased with the increase in temperature. However, at temperatures over 400degree (F) gum deposits are formed by oils. These deposits serve as a protective coating and improve scoring performance of gears and gear lubricants”. For this reason SAE 20 and SAE 30 mineral oils and “Plexol 201” showed about the same load carrying capacity at 400degree (F) as at 100 degree (F). However, 1010 grade and SAE 60 mineral oils, “Ucons LB-170” and 50-HB-170 and “DC-200” silicone fluid all scored at lower loads at 400degree (F) than at 100 degree (F).
Bowden and Tabor^11 investigated the frictional behavior of oil films on steel surfaces as they were heated and as oxidation took place. At 302 degree (F) after 30 minutes heating the friction became low. At 392 degree (F) the reduction in friction occurred after 15 minutes heating and at 572 degree (F) low friction was observed after less than 2 minutes heating. In the latter case if heating was continued for 20 minutes a thick gum was formed and friction increased to a high value. The above action was no doubt due to the formation of acidic products due to oxidation. Such products, in time, might lead to corrosive wear. Next the oil would increase in viscosity and compounds would from which are insoluble in the oil. When and if acidic products result from heating of gear oils, metal compounds will eventually from and these in turn will act as catalysts to promote further oxidation.
In view of the fact that high temperatures cause rapid deterioration of most gear lubricants, equipment should be so designed that operating gears are subjected to only moderate temperatures if long life of gear oils is expected.
However, since the military desire gear lubricants for jet aircraft and other agencies for space vehicles which will stand very high temperatures, it can be expected that synthetic fluids or solids will be available which will withstand 700 degree (F) or even 1000 degree(F) for a period.
