Understanding Oil Viscosity: The Thick and Thin of It
If you have a gearbox with a manufacturer’s nameplate instructing you to use the American Gear Manufacturer’s Association (AGMA) #4 viscosity oil at a given operating temperature, or if it specified a 750 SSU viscosity oil, would you know exactly what viscosity oil you need? If your grease application specified a base oil viscosity of 220 cSt for a given operating temperature would you know which of your greases might fit that specification, if any?
Does it really matter? Oil is oil, grease is grease, and more is better, right?
WRONG!
The importance of proper oil viscosity in your large array of equipment and the varying lubrication regimes they present is one of the most important maintenance practices one can focus on in their facilities. Viscosity is the most important physical property of a lubricant, and viscosity is the most important specification for a lubricant. Along with this, viscosity is the easiest thing to mess up!
A simple definition of viscosity is the thickness of an oil. While this is the most common understanding of viscosity, a more technical definition of viscosity is a measurement of the oil’s internal resistance or its resistance to flow by gravity. Viscosity is what carries the load, separating surfaces in relative motion from touching, thus reducing friction and wear, extending equipment life.
Viscosity should always be measured at a given temperature. Normally viscosity is inversely proportional to temperature, meaning as the temperature of an oil increases, its viscosity generally will decrease.
Stating an oil’s viscosity is found in many different formats depending on the application. The International Standards Organization (ISO) is the universally accepted method for stating oil viscosity (ISO VG) through-out industry (ISO 3448). This ranges from an ISO VG 2 to an ISO VG 3200. ISO VG is stated at 40°C.
AGMA specify grades an oil’s viscosity for industrial gear applications, also at 40ºC. The AGMA uses a #1 through #8A designation.
SUS - or - SSU is not in use much anymore, but you may still find it referenced on an older gearbox nameplate or an OEM (original equipment manufacturer) manual. This stands for Saybolt Universal Seconds - or - Saybolt Seconds Universal, you’ll see it stated either way.
Society of Automotive Engineers (SAE) Crankcase and SAE Gear classifications are different yet. 0W, 5W, 10W, etc., and straight weights 30, 40, and 50 are designations for crankcase oils, while 70W to 85W and 80 to 250 are designations for automotive gear oils.
If all of this is making sense, I commend you. You are likely on top of your game and know exactly which oils and viscosities belongs in each application throughout your facility. But if this sounds like a foreign language that you do not understand, it’s okay, as long as you now realize that your equipment may be in jeopardy of shorter life cycles and there is potential for cost saving improvements that will greatly enhance your equipment’s reliability.
