Hydraulic Fluids Viscosity Chart

Written by Camilo H Rueda Sunday, 08 January 2012 16:55

Articles - Education

The viscosity of a fluid is is one of the most important aspects for longer life of the hydraulic system. The operating

temperature affects the Viscosity of the fluid selected for the system. The fluid should have the proper low viscosity to allow filters and lines to handle the flow without excessive resistance. However, a fluid that has too low viscosity will not maintain a good oil film between sliding surfaces, and would end in wear due to a lack of lubrication. Also, low viscosity will generate lower efficiency and a lot of heat generation.

Working a hydraulic system between the minimum and maximum viscosity limits, assures a long and without troubles life. Off course, this is not the only fact to take into account.There are more factors like, proper filtration, running below pressure limits, speeds, etc.

There are 3 bands of colors to indicate the working limits according to the viscosity and temperatures.

The green band shows the ideal oil viscosity range.Viscosity from 28 to 60 Cst are recommended for longest life.

The yellow bands show additional ranges where the system is allowed to work but is not advised. From 12 to 28 Cst  in the lower side, and from 60 to 500 Cst. The last range is  where is better not run under maximum pressure and speeds. It is better to allow the warm up to higher temperatures.

The red bands are areas where the system may get damaged. If the viscosity is higher than 500 Cst cavitation may occur, there is going to be higher pressure drops due to the resistance to flow and excessive power loss. There could be pilot flow paths clogged and controls that are working slower than normal. On the other hand, if the fluid viscosity is lower than 12 Cst excessive wear and seizing may occur  specially in the pump because the oil film is too thin to prevent the contact of the internal moving parts.

Looking at the chart, according to the colored bands we can get the temperature limits for each oil type. Let's make an example. Let's say we are using ISO 32 oil in our system. The lowest recommended temperature in the green band would be 80 degrees Fahrenheit, while the highest is 110.

The lowest and highest temperatures allowed in the yellow band would be: 20 and 140 degrees Fahrenheit.

Note: This chart was made merely as a guide, it gives you average data. You have to consult the manufacturer of the pump you are using to get exact values. Every manufacturer may give different recommendations, depending on the type, pressure settings, and other factors.