The capability of a gear oil to carry a load and wear is dependent on how it has been formulated, rather than the grade of viscosity or the GL classification. In gear systems, metal surfaces are subjected to extreme contact pressures – usually greater than 1 Gpa – in which a minute misalignment, or even shock loading, may result in direct contact of asperities. It is a common belief among many users that increased viscosity or GL-5 rating is a sufficient protection factor, which is not always the case, as far as the whole formulation is concerned.
When not properly formulated, gear oil will fail to retain a protective film when subjected to high load and will thus wear out prematurely. The cumulative coordination of base oil characteristics, chemical composition of the additives, and viscosity performance generates the boundary and elastohydrodynamic films that interpose gear teeth and take up stresses. Formulations that are poorly balanced are terminated abnormally although they may be available in their requirements.
When it comes to high-performance gear oil, which comes with the need to use a high-performance version, formulation is the key when it comes to reliability.
The Fundamental Function of Gear Oil in Load-Bearing Systems
Gear oils have to separate sliding and rolling contacts and deal with shock loads and boundary. During meshing of gears, tooth flanks move in a rolling and sliding motion thus creating heat and pressure that may collapse thin oil films.
The first one is to form and maintain a separating layer of lubricant which separates the contact of metal to metal. Ideally, this is due to hydrodynamic or elastohydrodynamic lubrication, although in practice in gears, there is a transition to mixed or boundary lubrication during startup, shock or overload. In this case, the oil has to absorb impacts and minimise wear of adhesive.
To learn about simple functions, see our guide to gear oil function.
How Load Is Transmitted Through Gear Teeth
Gear teeth are known to have concentrated areas of contact transmitting torque leading to Hertzian stresses that occur at the surface and subsurface. The stresses are loaded in the pitch line and the tooth roots where the velocity of sliding in these areas is greatest.
Starting at these high stress locations micro-pitting should always occur and eventually transition into scuffing should the lubricant film disintegrate. Gears pose an extremely high demand on protection due to extremely high pressure of most gears that ordinary mineral oils cannot suffer without the help of chemicals that enhance their shear strength.
The Role of Extreme Pressure Additives in Load Carrying
Load-carrying capability in gear oils is based on extreme pressure (EP) additives. Modern formulations are dominated by sulfur-phosphorus chemistry which stirs up when heated by friction to form low shear bounderies at the friction boundary.
These additives co-react with the metallic surfaces to form iron sulfide and phosphates layers which inhibit welding and scuffing. The response is regulated- too violent, it causes corrosion and too weak it cannot defend against shock. The motion picture reproduces itself anew, with its self to defend the substrate.
To see the differences in EP chemistry,refer to our comparison of sulfur-phosphorus additives.
Base Oil Quality and Its Impact on Wear Protection
Base oil quality has a direct proportional effect on inherent film strength, thermal stability, and oxidation resistance. Base stocks of high quality, in particular synthetics, have got viscosities that resist temperature and shear, and film integrity that lasts longer with high-grade minerals than low-grade minerals.
Better base oils will withstand high temperatures to minimize varnish and sludge that can affect lubrication. They also aid in increase of good low-temperature flow without loss of high-temperature film thickness. Conversely, low-quality base oils warm quicker, wearing away the coating,
Explore the differences in synthetic base oils for deeper insight.
Viscosity Control and Maintaining a Protective Oil Film
Elastic viscosity control: Elastohydrodynamic lubrication (EHL) has been shown to be governed by high pressure which causes effective viscosity of the oil at the contact surface to dramatically rise and a thin but strong film to develop. The thickness of a film is also determined by speed, load and the pressureviscosity coefficient of the lubricant.
Film thickness reduces under growing load, shifting to mixed lubrication where additives are very important. The stability of shear is also a factor that makes the oil remain in its own grade of viscosity, such that it is not thin and allows asperities to get exposed.
To get to know the manner in which grades affect this, see our explanation of how to maintain proper viscosity film.
Industrial vs Automotive Load Conditions and Formulation Design
Constant heavy loads with sustained heat and regular oxidation service usually pose a problem with industrial gears, and therefore thermal stability and long service are crucial factors. There are intermittent shocks, stop-starts, and variable speed automotive applications, which require a rapid film reformation and shear-resistance EP systems.
The priorities of formulation consequently change: automotive oil focuses its priorities on balancing EP performance with low-temperature fluidity, balance in automotive oil additives, robust base stocks and automotive oil additives, industrial oils are based on the principle of endurance because of the need to focus on robust base stocks in industrial oils.
To have some background on these differences and heavy load protection, heavy load protection, check our post on industrial gear oil vs automotive gear oil.
Common Gear Wear and Failure Modes Caused by Inadequate Formulation
Ineffective formulation results in different patterns of failure. Scuffing is caused by extreme adhesive wear due to discontinuous, worn bright surfaces by collapsing films under high pressure. Pitting is a subsurface fatigue that arises due to craters when the stresses surpass the material stress levels. Micropitting-small pits that produce a frosted look is a result of multiple contacts between asperities in mixed lubrication that are usually exacerbated by low film thickness or low response rates of additives.
These modes lower gear life and efficiency, which underline the requirement of balanced chemistry. Training: Find out about gear wear and failure modes in our dedicated guide.
How Formulation Choices Affect Real-World Gear Life
Formulations that are well developed are known to increase wear life, slow down fatigue initiation, and increase wear rates. There are fewer unscheduled downtimes, longer maintenance intervals, and better system reliability as perceived by the operators. Balanced base oils have recorded significant loss reductions in balancing between micropitting and scuffing in field use accompanied by corresponding decrease on total ownership cost.
Common Misunderstandings About Gear Oil Load Performance
Another common myth is that an increased viscosity is more likely to be successful in terms of load capacity. Whereas, with increased viscosity, the film thickness in EHL is able to develop greater film thickness, excessively high viscosity levels would lead to higher churning losses and heat and, therefore, lead to lower protection.
There is also a myth that GL rating in isolation ensures protection- GL-5 oils have high levels of EP performance but they can be too aggressive in certain applications, whereas GL-4 is used in which it is mild.
Lastly, even with the use of additives, the quality of base oils cannot be fully offset in order to achieve firmness; thermal stability and natural film strength are all backbone qualities.
Conclusion — Load Capacity Is Engineered Into the Formula
Load-carrying capacity of gear oils and wear protection are the results of planned formulation balance in terms of choice of base-oil, EP additive chemistry, and viscosity behavior. It is not any single component that prevails; to be successful engineering the whole system to fit the operating demands is a must.
This approach of formulation first makes improved lubricant selections, and this performance under realistic stresses. To continue reading, further topics on viscosity grades and EP classifications can be discussed to narrow the choice in a certain application.