SYNOPSIS.

The ever-growing tendency towards transmitting greater power through mechanism of smaller unit size throws an increased responsibility on the lubricant to cope with the greater surface loading, and often also greatly increased sliding velocities. Mineral lubricating oils alone cannot in many cases hope to den1 with this problem which can only be met by blending into the lubricant chemical additives. The problem then remains of specifying and testing the compounded lubricant to be used for any particular application. Chemical and physical specipcations brl themselves have long been proved to be useless, and in the case of crankcase oils the answer has been found to lie in a speclfication based on lubricant performance in certain engine tests. This specification is now almost universally adopted by the Armed Forces of the Northern Atlantic Treaty Powers as well as by a considerable bodyofcommercial users and ?nanufacturerS. At first sight it might seem easier to specify and test a gear lubricant than a crankcase lubricant, but experienre of the American oil industry over many years has conclusively shown that gear lubricants can only be tested in their full-scale applications and that there are two distinct conditions to satisfy, i. e. a high-torque, low-speed condition, where gear failure is by surface deformation or rippling, and a high-speed condition where failure is by actual "pick-up" or scufl'zng between the mating surfaces. Each condition rpquires completely different properties from the oil and the two are not necessarily compatible. As a result of this experience. there is in the USA an Ordnance Specification for "Universal" or "Multi-Purpose" gear lubricants, hased on the satisfactory performance of an oil on two full-scale axle tests. Such oils are "satisfactory for the lubrication of automotive gear units, heavy duty industrial type enclosed units, steering gpars etc.". This paper traces the development of a British Specification No. CS. 2758 which uses British components for full-scale tests and which is accepted by the American Army Ordnance as being of equivalent quality level to their own MIL- L-2105 Universal Gear Lubricant Specification. The paper shows how impossible it is to judge from results obtained on many of the most commonly used laboratory small-scale test machines whether or not an oil is likely to prove satisfactory in service. Additive depletion has long been a topic for discussion when considering the use of treated oils and the author shouis the influence of additive depletion with running lime on the load-carrying capacity of a "multi-purpose" sulphur chlorine phosphorus gear lubricant both with respect to new and run-in gears. The conclusion is drawn that the eflective load capacity of the gear and lubricant combination is not

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