Viscosity, wading through the numbers
Viscosity, wading through the numbers:
Simply put, viscosity is the measure of resistance to flow, the higher the number the greater the resistance. Seems easy enough until one discovers that there are several ways to describe the same viscosity.
Stationary or mobile?
An easy way to look at it is to consider the application: is the application stationary or mobile?
A 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. Most modern equipment falls into these categories. The AGMA (American Gear Manufacturers Association) is mainly for stationary gear boxes and matches up with the ISO VG viscosity requirements.
However, you may find equipment that was manufactured prior to the ISO VG becoming a standard and here is where it may become confusing. It is not uncommon to find a gear box that requires a SAE 90 gear oil as this was the standard. As you can see the SAE 90 is a wide range and covers the ISO 150 to the ISO 320 viscosity grades. 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.
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. 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.
Most Hydraulic, Industrial Gear, Compressor and Turbine oil viscosities are measured and sold at the ISO VG 40°C scale. Due to the effect of viscosity improvers, multi-grade oils such as engine oils are determined at the ISO VG 100°C scale.
Issues with choosing the wrong viscosity fluid will be discussed at a later date.
Source: SWEPCO®
Viscosity, wading through the numbers indeed. Below mentioned might help you to give you a more clear picture of it.
Example of a Gear oil in different viscosities showing ISO, SAE & AGMA classifications:
SAE Gear Oil Grade …………………. 80W90 ……….. 90 ……………… — ……………….. 140 …………………. — ……………….. 250
ISO Viscosity Grade …………………. 150…………….. 220 ……………. 320 …………….. 460 ……………….. 680 ……………… 1000
AGMA …………………………………….. 4 ……………….. 5 ……………….. 6 ………………… 7 …………………… 8 …………………. 8a
Viscosity, 40°C, cSt …………………. 152……………… 223 …………….. 316 ……………. 470 ……………….. 680 ……………… 1010
Viscosity, 100°C, cSt ………………. 16.38 …………. 17.6 …………….. 26.3 ………….. 35.0 ……………….. 47.0 …………….. 74.0
Example of a Hydraulic Oil in different viscosities , showing ISO & SAE classifications
ISO Viscosity Grade, ASTM 2422 ………………… 22 ………………… 32……………………. 46 …………………. 68………………….. 100
SAE Grade ……………………………………………….. 5 …………………. 10…………………….. 15 ……………….. 20…………………….. 30
cSt @ 40 °C …………………………………………….. 24.5 ……………… 33…………………….. 48 ………………… 70……………………. 102
cSt @ 100 °C …………………………………………… 4.8 ……………….. 5.5 …………………… 7.1 ……………….. 9.1 ………………….. 11.7
Example of an Engine Oil in different viscosities , showing SAE classification only
SAE Grade ……………………………………………….. 5w40 …………15w40
Viscosity, cSt @ 40 o C ……………………………….. 85.48 ……….. 108.39
Viscosity, cSt @ 100 o C ……………………………… 14.85 …………. 15.89