GEAR OIL VISCOSITY
The purpose of this article is to provide some basic information about gear oil viscosity which can help engineers to specify the right gear oil. The lubricant is used in a gearing system mainly to control friction and wear between metal surfaces and to transfer heat away from the contact área. One important property of the gear oil is the viscosity and it has direct influence in the formation of the lubricant film. The higher the viscosity of the lubricating the thicker the film and this improve the properties against wear, damping and scuffing loading capacity. We know that the viscosity and temperature are inversely proportional and we need to take care about high viscosity because of the losses which take place due the churning and squezzing. The changing of the viscosity with temperature is determined by the viscosity index (VI). The higher the VI of a gear oil, the less viscosity changes with temperature. The degree to which viscosity changes with temperature depends on the base oil type, such as mineral oil with VI between 85 to 100, polyalphaolefin with VI between 130 to 160 and polglycol with VI between 150 to 260. Please refer to the standard ASTM D2270 for calculating viscosity index from kinematic viscosity at 40°C and 100°C and standard ASTM D341 for viscosity-temperature charts for liquid petroleum products
There are three regime of lubrication, boundary, mixed and hydrodinamic, and other regime that is Elastohydrodynamic lubrication (EHL) which is a development of hydrodynamic lubrication taking into account the elastic deformation ot the contact surfaces. The film thickness ratio or lambda ratio (Λ) is the relationship between minimum film thickness in the contact and the compound surface roughness ( Λ = h / σ). There are different guideline to classify the regime regarding lambda; Tribology in Manufacturing Technology by Jackson, Whitfield, Robinson, Morrell and Davim classified it as:
- Boundary Lubrication, Λ < 1
- Mixed Lubrication, 1 < Λ < 3
- Hydrodynamic Lubrication, 3 < Λ < 10
Boundary Lubrication: A boundary lubrication regime exists when occasional metal-to-metal contact takes place between surfaces, when the speed is low combined with high loads; the surfaces are almost entirely dependent on the lubricant’s additives to provide protection. Anti-wear or extreme pressure additives can reduce friction and wear.
Mixed Lubrication: The lubrication film is thicker than in a boundary lubrication, but some metal-to-metal asperity loading is still occurring combined with loading. This is an intermediary condition between boundary and hydrodynamic/elastohydrodynamic lubrication regimes. Friction can be lower than thick film hydrodynamic lubrication, but mixed film lubrication requires anti-wear additives to reduce wear.
Elasto-Hydrodynamic Lubrication: An elasto-hydrodynamic lubrication regime exists when a sudden reduction of the oil film causes a increase in viscosity, and when viscosity increases, the film can become rigid, creating an elastic deformation of the surfaces. The lubricant’s viscosity and additives work together to protect surfaces in an elasto-hydrodynamic regime.
Hydrodynamic Lubrication: A hydrodynamic, or full-film, lubrication regime exists when two surfaces are completely separated by an unbroken lubricant film. The lubricant’s viscosity assumes responsibility for the majority of wear protection; additives have low influence here. Although full-film lubrication does not generally allow metal-to-metal contact, abrasive wear or scratching can still occur if dirt particles penetrate the lubricating film.
Bellow a model of the Stribeck curve showing us the friction coefficient and the film thickness as function of velocity for the three regimes of lubrication.
Determination of the viscosity of mineral lubricating oils DIN 51509-1
The following method is applied for spur and bevel gears, the nominal kinematic viscosity is determined in cSt (mm2/s) at temperature 40°C as a function of the stribeck loading-speed factor (ks/v) by means of the curves shown in figure 1; this curve is for mineral oil and it is based on empirical gear lubrication data. This fator show us that for high pressure we will get higher viscosity and for high speed the viscosity tends to be lower. The indicated viscosities are guide values referring to an ambiente temperature of 20°C and an oil operating temperature of 70 °C. If the ambient temperature is bellow 10°C a reduction in viscosity is about 10% each 3°C; the other hand if the ambient temperature is above 25°C a increase in viscosity of about 10% is recommended to each 10°C
Where:
v = Speed at reference diameter [m/s]
ks = Stribeck pressure [N/mm2, MPa]
Ft = Nominal tangential load [N]
KA = Application factor
b = Tooth width [mm]
d1 = Reference diameter [mm]
u = Gear ratio Z2 / Z1
ZH = Zone factor*
Zε = Contact factor*
* ZH e Zε according to DIN 3990-2, the zone and contact factors can be taken as estimate equal to ZH2.Zε2 ≈ 3
When determining the stribeck loading-speed factor (ks/v) for viscosity calculations, it is important to take into account that in multi-stage gears the individual gears are subject to different operating conditions. In case of two-stage the operating conditions existing at the final stage are important for the calculations. In case of triple-stage or more stage the factor ks/v has to be determined for the penultimate and the last stage. The average of these two values is required to calculate the nominal kinematic viscosity for the entire gear system. Please refer to the DIN 51509-1 and AGMA 9005 for more information. After having determined a viscosity grade suitable for your gear, we recommend to calculate the scuffing resistance, for instance by means of the AGMA 925. In addition, it is important to take into account the viscosity requirements of the other friction points in the gear system which will also be lubricated with the gear oil, e.g. rolling or plain bearings, mechanical or hydraulic couplings, oil pumps and connected hydraulic systems.