Rodri, to answer your query, let us start off with a comparison on the makeup of Engine Oil vs. Gear Oil. Also earlier practice of recommending SAE numbers/ Engine Oils for other applications.
In an Engine, explosions occur during the compression cycle. Traces of Sulphur, Nitrogen, etc., in the fuel oxidises and in the presence of moisture and air, forms weak Sulphuric, Nitric and Hydrochloric (in sea or marine environment). The Engine Oil picks up weak acids and over time, as this, is in a circulating system, there is acid built-up and acid concentration increase. Hence, to counter the increasing acidity, Engine Oils have built-in Reserve Alkalinity referred to as TBN. The Reserve Alkalinity package, is sacrificial in nature and as the additive gets depleted in use, the TBN drops.
Till 2000, Fuels met Euro III category, this permitted (among other things) Sulphur up to 350 ppm. And reduced to 50 ppm under Euro IV. And further progressively reduced to 10 ppm of Sulphur in Euro V. Such Diesel fuels are referred as Ultra Low Sulphur Diesels (ULSD.) This reduction of fuels has its own implications.
http://www.slideshare.net/huss...r-low-sulphur-regimeTill 2005, it was a practice to package Diesel Engine Oils with 15 TBN or thereabouts. However the Fuel (nowadays), is stripped of Sulphur at refinery level, due to environmental implications. The current Diesel Engine Oil have 10 TBN or below to meet Euro V type ULSD. Therefore your startup TBN is the SAE 30 oil would probably have been higher than in vogue today.
Marine Industry, till recently have been patronising poor distillates, for two reasons. 1. Less refined hence cheaper 2. Due to lesser environmental impact on Sea and away from land mass/population. However, under MARPOL, the permitted Sulphur % in 2000 was 4.5% and 2015 onwards reduced to 3.5% for high seas sailing. And even lesser, when sailing into major Ports.
The typical gear oil undergoes severe shear and load stress and thus the Gear Oil gets oxidised. However, the additive package for gear oils contain Sulphur /Phosphorus (SP) packages blended with cylinder oils for Extreme Pressure (EP) performance. Currently, under environmental pressure SP additivated packages are being restricted. However as no explosion take place, as in an IC engine, the sulphur activity is tamed. And no fuel present either, to add to complications. It may be noted that the circulating systems in Marine Gear box may not be a completely closed system and breathers may permit sea atmosphere. All the same, the shear stress results in oxidation and there is a “mild” built-up of acidity which we generally measure by its Acid Number (AN) and could be referred as TAN and not TBN. In use, the AN increases.
Finally, it was a practice by many OEM’s, in the earlier days to recommend an SAE 10, 30 or 40 Engine Oil for many Hydraulic, Transmission and Gear applications. These recommendations were largely to optimize Lubricants in use in a Ship by a closest fit policy. Additionally most Marine Oils were defined by a TBN and an SAE number. It may be noted that the SAE numbers do not qualify the oil for its performance; it just indicates "a band in the viscosity range”. The SAE 30 would indicate a 100 cSt Gear Oil in today’s parlance. Additionally, the make-up of current gear Oil for Marine applications has a high Viscosity Index HVI (150 or more) to meet the exacting demands of a Marine and humid environment.
To conclude, TBN would NOT be the right benchmark for Gear Oils, further it decreases in use. Acid number or TAN is a better measure for non engine oils, and increases in use.
