![]() For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. In materials science and engineering, one is often interested in understanding the forces or stresses involved in the deformation of a material. In a general parallel flow, the shear stress is proportional to the gradient of the velocity. A fluid that has zero viscosity is called ideal or inviscid. Zero viscosity (no resistance to shear stress) is observed only at very low temperatures in superfluids otherwise, the second law of thermodynamics requires all fluids to have positive viscosity. For example, the viscosity of a Newtonian fluid does not vary significantly with the rate of deformation. However, the dependence on some of these properties is negligible in certain cases. In general, viscosity depends on a fluid's state, such as its temperature, pressure, and rate of deformation. ![]() For a tube with a constant rate of flow, the strength of the compensating force is proportional to the fluid's viscosity. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion. Experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow. For instance, when a viscous fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls. Viscosity quantifies the internal frictional force between adjacent layers of fluid that are in relative motion. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. 15.The viscosity of a fluid is a measure of its resistance to deformation at a given rate. 11.7 Example of an Engine Oil in different viscosities, showing SAE classification only 74.0 Example of a Hydraulic Oil in different viscosities, showing ISO & SAE classifications Example of a Gear oil in different viscosities showing ISO, SAE & AGMA classifications: Below mentioned might help you to give you a more clear picture of it. Viscosity, wading through the numbers indeed. Issues with choosing the wrong viscosity fluid will be discussed at a later date. Due to the effect of viscosity improvers, multi-grade oils such as engine oils are determined at the ISO VG 100☌ scale. Most Hydraulic, Industrial Gear, Compressor and Turbine oil viscosities are measured and sold at the ISO VG 40☌ scale. ![]() These additions reduce the span of the SAE 90 down to two ISO grade, the 150 and the 220 and the SAE 140 down to the ISO 320 and 460. Since this chart was developed, SAE has added two new viscosity ranges to deal with viscosity issues, the SAE 110 in between the SAE 90 and SAE 140 and the SAE 190 in between the SAE 140 and the SAE 250. ![]() Usually an ISO 220 can safely be used unless the gear box is exposed to low temperatures where an ISO 150 would be a better choice or high temperatures where the ISO 320 would work better. As you can see the SAE 90 is a wide range and covers the ISO 150 to the ISO 320 viscosity grades. It is not uncommon to find a gear box that requires a SAE 90 gear oil as this was the standard. However, you may find equipment that was manufactured prior to the ISO VG becoming a standard and here is where it may become confusing. The AGMA (American Gear Manufacturers Association) is mainly for stationary gear boxes and matches up with the ISO VG viscosity requirements. Most modern equipment falls into these categories. Seems easy enough until one discovers that there are several ways to describe the same viscosity.Īn easy way to look at it is to consider the application: is the application stationary or mobile?Ī general rule of thumb would be if the application is stationary then the ISO viscosity grade (VG) applies and if it is mobile then the SAE grades apply. Simply put, viscosity is the measure of resistance to flow, the higher the number the greater the resistance. ![]()
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