Good lubrication ensures optimum operation. High-performance oil reduces power loss in gears

Increasing requirements on gears in general mean that higher efficiencies and longer lifetimes are also demanded of the single components, including lubricants applied. A German manufacturer has developed a high-performance gear oil adapting to the changing load conditions in the gear teeth. Practical tests and scientific analyses have shown: the oil achieves efficiencies in the gears clearly above the average and power loss is reduced significantly at the same time.

Lubricants are expected to meet a multitude of various requirements while ensuring high performance, highest operational safety as well as flexibility and long-term application at the same time. An additional advantage is the potential saving of energy. Therefore the development of high-performance gear oils is a special challenge for every lubricant manufacturer – as it is necessary to select the optimum and suited base oils and to compose a balanced additive package giving the oil all required characteristics.

Parameter base oil
The selection of a specific base oil and viscosity influences, among other factors, if the lubricant is suited for the relevant operating temperatures. Mineral base oils for example possess an unfavourable pour point and are less suited for temperatures clearly below 0 °C. Therefore synthetic products should be preferred for applications subject to varying temperatures. They cover a wider temperature range and ensure an optimum lubrication both for wind power gears at low outside temperatures and industrial applications with higher temperature loads. 

The viscosity index (VI) of synthetic oils is around 140 usually; depending on the base oil it can be clearly higher. The VI of mineral oil based oils amounts to 90 to 100; generally the higher the oil’s VI, the less energy is required for cold start and low temperatures at equal nominal viscosity.

All lubricants age during their application; the molecules react with oxygen and oxidize. However, usually synthetic oils achieve longer operating times since they have a more stable molecular structure than mineral oils and it is more difficult for oxygen to attack. Due to this specific molecular structure lower friction coefficients of the lubricant can be achieved by selecting the respective synthetic base oils.

It depends on the additives
Increased requirements on gears – more compact construction, significantly higher loading capacity, non-stop operation and higher surface pressures in gear teeth and bearings – cannot be mastered by the base oil alone. Here the influence of additives is of importance; depending on their effect they can be classified into surface-active and fluid-active additives. 

Surface-active additives can be subdivided further:
• Extreme-pressure additives (EP-additives) and anti-wear additives (AW-additives) increase load-bearing capacity and protect against wear. They form very thin protective layers on the metal surfaces of the components to be lubricated and adapt to the surface’s roughness. This way mechanical loading capacity in contact areas is improved. EP- and AW-additives reduce wear caused by material removal and the danger of fretting (local seizing) at heavy operating conditions.
• Anti-corrosion additives protect against corrosion and rust at metallic surfaces.
• PD-additives (plastic deformation) smoothen surface damages and roughness and reduce friction at rolling and sliding contacts.

Fluid-active additives can be grouped into:
• Anti-oxidants prevent premature oil ageing.
• Pour point improver lower the crystallisation point of the oil enhancing flow behaviour at low temperatures.
• Viscosity index improvers (VI improver) increase the oil’s viscosity index.
• Inhibitors prevent foaming and separate air and water from the oil quicker.

ADDINOL Lube Oil GmbH has developed the special substance combination Surftec® which adapts to changing load conditions in gear teeth and bonds to the gear. This special additive package against wear is also contained in the ADDINOL Eco Gear series for industrial gears which is applied for instance in cement, paper and sugar industry as well as gears of ships and wind power plants.

Eco Gear M is based on mineral oils, Eco Gear S on synthetic oils; both prevent the formation of pitting and micro-pitting as well as abrasive wear and provide reliable protection against material removal and deterioration. Load-bearing areas – also of pre-damaged surfaces – are increased by smoothening.

Proven in practice …
Example: The tooth flanks of a wind power gear filled with conventional gear oil type CLP showed plain signs of micro-pitting after some operating time. After the change to ADDINOL Eco Gear the changes of the surfaces were documented by the help of imprints of the tooth flanks which were analysed under a grid electron microscope.
- Three months after changing: Roughness and pitting are still clearly visible on the tooth flank imprint under the grid electron microscope.
- After 25 months: The condition of the surface has improved significantly. Former pittings are smoother; sharp edges and roughness have already disappeared.
- After 36 months: Under the grid electron microscope we see an almost perfectly smoothened surface; new damages did not occur.

Calculations with the CAD software WT plus showed a temperature reduction of approximately 2 °C for industrial gears operated with ADDINOL Eco Gear. In practice even significantly larger temperature decreases could be realised in some cases. In a belt drive in wood-processing industry equipped with a gear of Moventas temperatures were cut by 7 to 10 °C on the average after changing to ADDINOL Eco Gear 320 S. Based on these experiences one can conclude that low operating temperatures prove a lowered friction because of the lubricant. Based on Surftec® technology superior efficiencies and operating lives can be achieved.

… and supported scientifically
In order to confirm these effects also theoretically the "Research Centre for Gears and Gear Unit Design" (FZG) of the Technical University of Munich carried out comprehensive tests for efficiency (heads: Prof. Dr.-Eng. B. Höhn and Dr.-Eng. K. Michaelis). Efficiency is determined in a modified Four Square Gear Oil Tester. Test conditions cover a wide range of operating conditions as relevant in practice. Based on the results of the efficiency tests according to FVA 345 – E-C/0.5:20/5:9/40:120 power loss and efficiency of gears can be calculated by the help of special procedures.

Within a DGMK project (report no. 608) comparative analyses were carried out with various oils and model substances with different additives (P and S) and different base oils (mineral and PAO). Gear oil ADDINOL Eco Gear 100 M displays lowest losses compared to the reference oil under all conditions. The oil’s benefits even increase with decreasing film thickness (higher temperatures or lower peripheral speed). Already at lubricant temperatures of 40 °C losses can be reduced by up to 14 %. At the highest test temperatures (120 °C) the feature is the most distinct and decreases power loss up to 50 % compared to reference lubricant.

Conclusion: the friction-reducing effect of the high-performance gear oil increases with increasingly unfavourable operating conditions. According to FZG the application of ADDINOL Eco Gear enables efficiencies which are usually only achieved with elastohydrodynamic lubrication (EHD).

German article published in antriebstechnik 05/2010 (view German article here)

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