The viscosity grade selection of heavy duty gear oil is directly related to the working temperature of the gearbox. Temperature is one of the core factors that determine the viscosity requirements. The viscosity of gear oil changes with temperature. When the temperature rises, the viscosity decreases, and when the temperature drops, the viscosity increases. In a low temperature environment, if a gear oil with too high a viscosity grade is selected, the starting resistance will increase due to poor fluidity, and the gearbox will not be fully lubricated during the startup phase, which may cause initial wear; while in a high temperature environment, a gear oil with too low a viscosity grade will be over-diluted and unable to form a thick enough oil film on the gear surface, making it difficult to withstand the pressure brought by heavy loads. Therefore, it is necessary to select a gear oil grade that can maintain an appropriate viscosity within the normal working temperature range of the gearbox.
The working temperature range of the gearbox directly determines the lower and upper limits of the viscosity grade. For gearboxes that operate in a low temperature environment for a long time (such as outdoors in winter or refrigeration equipment), it is necessary to select a low viscosity grade gear oil with good low temperature fluidity. This type of oil can flow quickly at low temperatures to ensure that the gear meshing parts are lubricated in time. For gearboxes that continuously work in high-temperature environments (such as near industrial kilns or long-term high-load operation), high-viscosity gear oils should be selected. They have a stronger ability to maintain viscosity at high temperatures and can prevent the oil film from breaking due to excessive temperature. For example, under high-temperature conditions, the molecular structure of high-viscosity gear oils is more stable, and it is not easy to become thin due to thermal oxidation, and it can continue to provide protection for gears.
Load intensity is another key factor affecting the selection of heavy duty gear oil viscosity grade. High loads require oil films of corresponding viscosity to provide support. When the gearbox is running at high load, the contact pressure between the gear tooth surfaces increases. At this time, gear oil with sufficient viscosity is required to form a thicker and tougher oil film to resist the extrusion and friction between the tooth surfaces. If low-viscosity gear oil is selected when the load intensity is high, the oil film is easily destroyed by high pressure, resulting in direct contact between the tooth surfaces, causing scratches, wear and even sintering. When the load intensity is small, gear oil with too high viscosity will increase internal friction resistance, resulting in increased power loss, and may not reach the lubrication point in time due to poor fluidity.
The requirements of different load intensities for oil film thickness further refine the selection criteria for viscosity grades. Under light load conditions, the contact pressure on the gear tooth surface is small, and a thin oil film can meet the lubrication requirements. At this time, choosing a medium viscosity grade gear oil can not only ensure lubrication, but also reduce energy loss. Medium load conditions require gear oils with slightly higher viscosity to form a more stable oil film to cope with tooth surface friction under medium pressure. In extremely heavy load conditions, such as gearboxes for mining machinery and heavy engineering machinery, high viscosity grades of heavy duty gear oil must be selected. The oil film strength is higher and can maintain integrity under extreme pressure to prevent metal contact on the tooth surface.
The synergistic effect of operating temperature and load intensity requires that the selection of viscosity grade must take into account the combined effects of both. Under the dual conditions of high temperature and high load, gear oil faces the greatest challenge. At this time, it is necessary to select products with high viscosity grades and good high temperature stability. Because high temperature will reduce the viscosity of the oil, and high load requires sufficient oil film strength, high viscosity grade gear oil can still maintain a certain viscosity at high temperature to meet the requirements of high load for oil film. On the contrary, under low temperature and light load conditions, low viscosity grade gear oil is more suitable, which can ensure low temperature fluidity and will not increase unnecessary resistance due to excessive viscosity.
The change of gearbox operating conditions will also affect the adaptability of viscosity grade, and the fluctuation range of temperature and load needs to be considered. The operating temperature and load intensity of some gearboxes are not fixed, but there are certain fluctuations. For example, the gearbox of engineering machinery has low temperature and heavy load at startup, and the temperature rises and the load is relatively stable after running for a period of time. In this case, it is necessary to select gear oil with high viscosity index, whose viscosity changes less with temperature, and can maintain relatively stable lubrication performance when temperature and load fluctuate, and can provide reliable protection for gears whether in low temperature startup stage or high temperature operation stage.
In actual selection, it is necessary to refer to the recommended standards of gearbox manufacturers and make adjustments in combination with actual working conditions. Manufacturers usually recommend a suitable viscosity grade range based on factors such as the design parameters, materials, and working environment of the gearbox, which is an important basis for selection. However, in actual use, if the operating temperature of the gearbox is higher than the design temperature, or the load intensity exceeds the rated value, a gear oil with a higher viscosity grade should be selected within the recommended range; conversely, if the temperature is lower or the load is lighter, a slightly lower viscosity grade can be selected. Through this adjustment based on actual working conditions, the viscosity performance of the gear oil can be more closely matched to the operating requirements of the gearbox, maximizing the lubrication and protection effects.
