“The American Petroleum Institute’s (API) “GL-5” designates the type of service characteristics of gears, particularly hypoid in passenger cars and other automotive equipment operated under high speed/shock load, high speed/low torque and low speed/high torque conditions,” he explains. The SAE 70W, 75W, 80W and 85W oils are for low temperatures and the SAE 80, 85, 90, 140 and 250 are for the high-heat applications.īeyond considering temperature, Ford says it’s also important to take into account other operating conditions. When it comes to selecting axle or transmission lubricants, one needs to consider both the lowest and highest service temperatures. A 150,000 cP fluid appears like a semifluid grease, which will barely pour.” “Each low temperature gear lubricant (SAE grade followed by a W) must not exceed a maximum viscosity of 150,000 cP at the temperature indicated. “It gets quite complicated, but essentially dynamic viscosity tends to be measured in one-hundredths of a poise and symbolized by cP,” says Ford. If you’re using a mono-grade engine oil, the truck engine is likely to seize once it exits the mine.”Īs an example of a multi-viscosity oil Ford cites an SAE 80W-90 oil, which conforms to the SAE 80W requirement at low temperatures and is in the range provided for SAE 90 at high temperatures. For example, if you’re operating trucks that transport ore on mines in Siberia, the temperatures they need to function at inside the mine and outside in the cold can differ substantially. Understanding VI is important to understand whether the lubricant in question meets the requirements of your equipment based on the operating temperature range. “The change in an oil’s viscosity due to a temperature change is called the viscosity index (VI). “Multi-viscosity oils flow differently at different temperatures,” explains Callum Ford, National Marketing Manager at Lubrication Engineers (LE) South Africa. While single-viscosity or mono-grade engine oil works well in a narrow temperature band, swings in temperature may require a multi-viscosity oil. As a general rule, the lower the number, the lower the viscosity. Multi-viscosity oils have been developed to cater to a wider range of grades and temperatures.įord explains that the Society of Automotive Engineers (SAE) developed numerical coding system to grade engine oils according to their viscosity. Viscosity can be influenced by several factors, from contamination to temperature. A lubricant’s viscosity is arguably its most important property – its resistance to flow and shear.
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