Because of the extremely high demands placed on 4WD transmissions and differentials, they require highly specialized lubrication that differs from the lubrication used in 4×4 vehicle engines in several respects. However, even experienced mechanics often get it wrong- the belief that because a certain oil is “better”, merely because it is “thicker”, is widespread and firmly entrenched in the minds of many so-called “gearbox specialists”, who really should know better. In short, nothing could be further from the truth: viscosity, or the “thickness” of gear oil is of secondary importance in relation to its formulation, and while it is not quite true to say that the viscosity of gear oil is a function of its formulation, it is in the area of formulation that gear oils differ the most from engine oils.
The materials used in gear manufacturing, methods of case hardening, and gear tooth profile are the factors that determine the formulation of gear oils, which are not as affected by temperature, oxidation, and thus sludge formation as engine oils. Although this article is not a beginner’s guide to gear lubrication, it will attempt to explain the differences between the two types of oil, and why using different, but the correct grade of gear oil is crucial for the protection of different gear tooth types.
Gear oil formulation and additives
In contrast with engine oils, which are classified according to their physical properties, gear oils are classified according to the units they are intended to be used in. For this reason the viscosities of gear oils are not relatable to engine oil, for instance, 75W90 gear oil, which at first glance would have a very high viscosity index (resistance to flow), in fact has a similar viscosity index to 10W40 engine oil, according to the API (American Petroleum Institute) grading system.
The 75W90 designation, which is widely used in many modern transmissions, refers to the additive package that is added to the base oil. While there are many additives used in gear oil, not all are present in all oils, nor are they present in the same concentrations in oils of the same designations produced by different manufacturers, since some additives fulfil multiple roles. However, all gear oils of the same designation have additives that are not used in engine oils, some of which are:
• Friction modifiers:
Mostly of a crystalline structure, these materials resemble small plates which enable them to form layers, which slide, or slip over each other easily, preventing direct metal-to-metal contact. This results in reduced fuel consumption, and increased component life.
These friction modifying additives are solids, which are held in uniform suspension in the oil:
- Molybdenum disulfide
- Boron nitride (BN)
- Tungsten disulfide (WS2)
- Polytetrafluoroethylene (PTFE)
• Anti-wear additives:
Anti-wear and friction modifying additives may appear to be same thing; however, anti-wear additives react with the actual metal surfaces, to provide lubrication when the base oil film is squeezed out. The mechanism of operation is an extremely strong bond between the metal surface and the additive, adding to the protection already offered by friction modifying additives, and this bond is what provides protection against scoring and seizure.
Some commonly used anti-wear additives:
- Zinc dithiophosphate (ZDP);
- Zinc dialkyldithiophosphate (ZDDP). Also used in engine oils, albeit at lower concentrations.
- Tricresylphosphate (TCP).
• EP, or Extreme Pressure additives:
Extreme pressure additives are used to further prevent direct metal-to metal contact in extreme pressure conditions, such as those found on hypoid gears. Similar to anti-wear additives, these substances also form a durable layer on metal surfaces, adding further protection against direct metal-to-metal contact.
However, the sulphurous components of these additives are corrosive to yellow metals such as brass and bronze, and should therefore not be used in transmissions and differentials containing components made from these metals such as bushings and synchros.
Extreme Pressure additives include the following:
- Chlorinated paraffins
- Sulphurized fats
- Zinc dialkyldithiophosphate (ZDDP)
- Molybdenum disulfide
• Corrosion inhibitors:
Corrosion inhibitors are to some extent absorbed by metal surfaces to protect against corrosive attack by oxygen, water, and other substances that are chemically active.
Some commonly used rust and corrosion inhibitors:
- Alkaline compounds
- Organic acids
- Amino-acid derivatives
Mineral base oils continually react with atmospheric oxygen, thus forming several organic acids: these reactions are the primary cause of the formation of sludge, varnishes, and foaming. Almost all lubricants contain anti-oxidants in varying concentrations to prevent the oxidation, and thus the degradation process of oils.
The following materials are used as anti-oxidants:
- Zinc dithiophosphate (ZDP)
- Alkyl sulfides
- Aromatic sulfides
- Aromatic amines
- Hindered phenols
• Anti foaming additives:
The extreme agitation of the lubricant that takes place inside a transmission causes the formation of air bubbles in the oil, which is referred to as foaming, and has a severe negative affect on the lubricity of the oil. In addition, the air bubbles in the oil increases, or enhances oxidation, and thus the degradation of the oil.
Most commonly used anti-oxidation additive:
Gear Oil Designations
While the API recognises that some gear lubricants are suitable for a range of conditions, including off-road and 4WD applications, it makes no attempt to classify gear oils based on how well, or not, a particular has performed in tests under laboratory, or any other conditions, and while API designations may be of some assistance in the selection of a gear oil for a particular application, the recommendations of the off-road vehicle must always take precedence. Provided the correct designation is used in any application, most oil products supplied by known and reputable manufacturers will perform satisfactorily, irrespective of the brand.
Some off-road vehicle manufacturers prohibit the use of certain gear oil formulations in their vehicles, and since they do not include a list of the materials used in the manufacture of their 4×4 transmissions, below is a guide on which formulation is the most suited to prevent transmission or differential failure while on an overland off-road expedition through Africa:
This designation is best suited for conditions of very low pressure and sliding velocities, such as those found in axles using worm- and spiral bevel gears. This formulation consists of the base oil with only anti-foaming, corrosion inhibitors, and viscosity index improvers added. Extreme pressure and friction modifiers are not added. This formulation is suitable for some older manual truck transmissions. This designation is obsolete.
Although obsolete, there is no prohibition placed on distributors by the API to use this designation: originally intended for use on worm gears, this formulation only contains fatty acids and sometimes mild extreme pressure and anti-wear additives.
This formulation is best suited for applications where only moderate pressures, loads and speeds are encountered, mainly beyond the requirements of a GL-2 formulation but less that that required by a GL-4 formulation. Not suitable for hypoid gearing due to its low levels of extreme pressure additives, it was originally meant to be used in low load manual transmissions and axles using spiral bevel gearing. This designation is obsolete.
This formulation is widely used in modern off-road and 4WD vehicle transmissions and differentials using hypoid gears under low speed, high torque and high speed, low torque, moderate shock load and speed conditions.
This is arguably the best formulation to use under conditions of high speed, high shock load, and high torque at low speeds, particularly where hypoid gears are used. This designation is especially suitable in all conditions and applications where oil with extreme pressure characteristics is required, such as in extreme off-road conditions.
Originally formulated to reduce gear scuffing and scoring in older muscle cars, this designation is now obsolete.
This formulation is intended primarily for use in bus and truck manual transmissions that are not synchronised.
Gear oils and engines
While some older engines can use gear oils that contain anti-wear or friction modifiers consisting of solid particles, newer engines cannot. The lubrication demands and design features of new engines are such the slightest build-up of these solid particles anywhere in the system will invariably cause engine failure due to insufficient lubrication.
The above apart, the relatively low concentrations of viscosity improvers in gear oil might make gear oil unpumpable in cool or cold weather. Even worse, the high acidity of gear oil is sure to counter, or neutralise the alkaline detergents in engine oil: how long this process will take, or exactly what the interactions between the two types of oil might have on the waste products created by the combustion process might have on for instance, the crank shaft bearing surfaces, or even the ability of the mixture to flow at all, has not been fully researched due to the high number of variables involved.
Nonetheless, the chemical reactions involved are extremely complex and some sort of engine damage is a given, and the middle of anywhere in Africa is not the place to see how long the engine of a 4×4 vehicle will run on gear oil.
ATF, or Automatic Transmission Fluid
There is no room for error in the choice of which fluid to use in automatic transmissions: ATF fluids are not interchangeable and the only formulation that will not destroy an automatic transmission is the one prescribed by the manufacturer.
Incorrect fluids invariably overheat, which causes insufficient lubrication, which causes more overheating, which causes the formation of gums and varnishes that “welds” the internals of automatic transmissions together. In many, if not most cases, the damage is irreparable, and if such a catastrophe should befall a 4×4 vehicle in the depths of Africa, there is virtually no chance of having it repaired.
4×4 Gear Oil, Viscosity and Additives
From the above, it will be clear that viscosity in itself is not a valid criterion for the selection of any particular gear oil for a particular application. Although the slightly higher viscosity (in most cases), of gear oil is a requirement to ensure even distribution of the oil to all parts of a transmission or differential, it is the combination of additives, and their relative concentrations, that determine the suitability of any given oil for a particular purpose.
The use of the correct gear oil is important in any automotive application, but it is vastly more so in the case of off-road or 4WD vehicles. The whole of Africa north of Zimbabwe probably has less than a dozen trustworthy transmission repair shops and it is therefore of vital importance that before leaving on a Cape to Cairo expedition, all 4×4 transmissions and differentials are filled with the correct oil, and that all vehicles carry a supply of replacement oil. While finding the correct oil in South Africa should not present a problem, the chances of finding GL-5 oil, or any Dexron type ATF in the middle of Somalia, or some other backwater in Africa is virtually zero, so make sure that some is always available on each off-road vehicle taking part in the expedition.