We are little concerned with the first gears, which were said to consist of wooden wheels with wood pegs for teeth, since speeds and pressures were low and lubrication was not much of a problem at that time. However metal gears of cast iron required a lubricant to reduce both noise and wear. For the purpose, animal fats were used, followed by petroleum fractions when the latter became available. The first mineral gear lubricants were residua which were quite sticky and therefore resisted displacement by tooth pressure. While such products still have some usage, high speeds and closer tolerance led to the use of lower viscosity gear oils.
In factories the transition from steam drives, with line shafts, pulleys, and belts, to the use of electric motors for specific apparatus led to the use of gearing to reduce or change the direction of drive. Further changes in industrial gear sets has been largely due to both increased power and speed of the driven units. This trend has increased to the point where 5500 hp and higher rolling mill drives have been installed in steel mills. On the other hand , gears in watches and , no doubt, in some instruments have decreased in size. Therefore, when we speak of gear lubrication we think in terms of power delivery varying from a fraction of a hp to several hundred hp.
The wide use of automobiles and the development of gearing for all automotive vehicles has been responsible for the greatest changes in gear lubricants over the last thirty or forty years. The Society of Automotive Engineers (SAE) has been a large factor in improvement of automotive gear oils. The SAE fuels and lubricants committee, which consists of technical men from both the motor car manufacturers and the suppliers of lubricants, has been a meeting ground for ironing out differences and arriving at a solution of many technical problems. While people from governmental departments entered the picture a little later than the above two groups, their suggestions and help has aided in standardizing gear and transmission lubricants.
One cannot discount the efforts of the American Gear Manufacturers Association (AGMA) who have suggested and tabulated standard oils for use in industrial gearing under various operation condition . AGMA was founded in 1917 and consists of a group supplying about 75 per cent of the cut gears marketed in the United States and Canada.
Since that time this organization has issued certain engineering standards and such specifications, relative to gear lubricants and gear lubrication, have been an aid to the lubricants industry and, therefore, will receive further reference. One of the first steps of the SAE group was to establish viscosity ranges for transmission and rear axle lubricants so that the consumer would secure a material within the same viscosity range no matter who the supplier might be. The designations were in terms of the approximate viscosity SUS at 210 degree F, thus No.90, No. 110, and No. 160.Naturally , a certain range was permitted in each grade, and other grades have been in use at various time, such as SAE 80,SAE 250, etc. An SAE report , adopted in February 1924, indicated that at that time transmission and rear axle lubricants were made from mineral oil with or without the addition of animal or vegetable oils, soaps, etc. The purpose of the soaps was to decrease the tendency of the lubricant to leak from the housings. Such addition was said to have little or no effect on the load carrying property, nor did it prevent ease of shifting of gears . The introduction of the hypoid differential drive changed the requirements for gear lubricants for automobiles and led to the use of what are called extreme pressure (EP) gear oils. This change started in 1925 when the Gleason Gear Works perfected gear generating machines which would produce gears of the hypoid type with improved standards of accuracy, strength, and quietness of operation. The Packard Motor Car Company adopted these gears for final drives in their 1926 models. Other motor car manufacturers started to consider the use of hypoid gears and to change over to such use until, by 1937, practically the entire U.S. passenger automobile industry had adopted the hypoid rear axle. A number of truck manufacturers in this country likewise converted to this type of differential. The change in the type of gears in the final drives of automobiles abroad was more gradual. Thus, Towle^10 mentions that the first use of hypoid gears in production cars in England was in 1929 and that it was not until 1934 that further models appeared using this type of gear. In the 1951 Motor show in the United Kingdom
Ninety nine models were equipped with the hypoid axles as compared with forty one with spiral bevel gears. On the continent, the change to hypoid gears has been even more gradual.
Since such gears subject two metal surfaces to a sliding action as well as to a rolling one, the problem of lubrication is more severe than with involute gear types and, yet, is as important as the production of the gears. Experience quickly demonstrated that hypoid gears could not be lubricated with straight mineral oil particularly under severe operating conditions. However, as early as 1869 a “plumboleum’’ lubricant consisting of lead soap and sulfur^4 had been found satisfactory in one model of spiral bevel gears where all other lubricants failed. Gear oils containing lead soaps were being used in industrial applications at the time hypoid gears were introduced in automobiles. It also happened that the oils used with such lead soaps contained sulfur compounds which became active at relatively low temperatures. Consequently, such gear lubricants were tried in the differentials of vehicles equipped with hypoid gears and found useful.
This type of gear compound was used for hypoid axles from 1925 to 1932, but all such compositions did not prove satisfactory. At about this time it was found that other compounds might be desirable in hypoid lubricants and Wolf and Mougey^11 listed three general types of gear oils for the purpose, namely:
(a) Sulfur chlorine treated saponifiable oil base with petroleum oil or sulfur petroleum oil;
(b) Sulfur treated saponifiable oil base with mineral oil or sulfur treated petroleum oil;
(c) Lubricants containing lead soap and sulfur.
At this period the motor car manufacturers were appealing to the distributers of lubricants to provide the necessary EP gear compounds. Thus, Wolf and Mougey^11 stated: advances in gear design were urgently awaiting the development of satisfactory extreme pressure lubricants. In1933 Mougey^7 said: EP lubricants are at the cross roads. Many of the refiners are assuming the attitude that EP lubricants are not needed at the present time, and if and when required, they will produce them, while the automotive manufacturers are hesitating to introduce gear designs which require satisfactory performance in service until these lubricants are universally distributed and are available at all filling stations.
During this development period in perfecting satisfactory hypoid gear lubricants the problem was not only availability and composition but also methods of evaluation of EP oils. For this purpose thought was given to testing machines which, by bench tests, would determine the quality of the lubricant quickly. Unfortunate of the value of EP gear oils did not prove simple.
While several EP test machines have been proposed and are still in use, none of these give sufficient information or correlation to permit approval of EP gear formulations based on such tests alone. Initially the Gleason Gear Works set up a testing procedure using hypoid gears, and lubricants were approved on the basis of this “Four –Square Test.’’ Later, any laboratory tests, even if on full scale axles, were supplemented by use in cars on the proving grounds of automobile manufacturers.
Specifications under which hypoid gear lubricants have been manufactured and sold have changed frequently over the period from the introduction of such gears up until the present. Using the experience of motor car manufacturers and of oil companies, the Federal Government set up such specifications in 1942.Since products meeting these requirements did not prove entirely satisfactory for high torque low speed performance of heavily loaded axles, a Coordinating Lubricants Group, under the Coordinating Research Council was formed. Under their direction further standardization of test methods was arrived at and some suggested changes in government specifications for EP gear oil could be produced which would satisfy all automotive vehicle requirements, whether the operating conditions be one of high speed and low torque or low speed and high torque. At the time of writing, formulations are available which satisfy both conditions, but a few consumers are somewhat dubious.
Automatic Transmission Fluids (ATF) have somewhat the same history and resulting solution as in the case of hypoid lubricants at an earlier date. Since the type of fluid used is rather critical for proper performance and there was no wide distribution of a suitable fluid, the motor car manufacturers at first provided the lubricant under a parts number. Within a matter of a couple of years after the introduction of automatic transmissions on various cars, the oil companies were able to offer approved ATF quite generally.
