Not All Multigrades Are Alike
Hydraulic fluids can be made into multigrade products with the inclusion of high-molecular weight polymers. The molecular structure of these polymers resembles tiny springs, which are chemically calibrated to expand and contract depending on the temperature to which they are exposed. The molecular nature of the polymer causes the oil to adhere to its surface, but the adhering qualities vary depending on operating temperature. When it is hot, the coil expands, and in extreme heat it expands dramatically, resulting in an increase of surface area. When the oil molecules adhere to the increased surface of the polymer coil, it causes the oil to thicken just enough to maintain its grade (as opposed to thinning). Conversely, when temperatures drop, the polymer’s coil shape contracts, resulting in less surface area and causing the oil to thin, again maintaining its grade.
While multigrade fluids can provide superior performance, not all multigrade fluids are equal. This is because the majority of fluids use polymers that are essentially plastic in their composition. As plastic, they tend to break down after repeated cycles through a system’s pump, which can fracture and shatter the polymer after extensive use.
By contrast, a multigrade product that is shear stable is engineered to withstand such punishing conditions. Shear-stable products are formulated with polymers that are more like rubber than plastic. Rather than shattering or breaking apart when cycled through a pump, these polymers will instead bend and flex, returning to their normal shape upon exiting the system. The result is a product that does not break down as quickly, providing longer operating life while requiring less maintenance.
The performance benefits of shear-stable fluids really shine when subjected to the three biggest risk factors: heat, pressure and time. When heavy equipment is in operation, conventional fluids get hotter, which causes thinning.
This is when hydraulic system performance begins to suffer, straining to keep up with output. A real-world example of this would be a forklift operator lifting a heavy load. As the equipment begins to strain, the operator is forced to increase rpms by revving the engine to complete the work, consuming more energy in the process. As this goes on, it takes more time to do the job, and more fuel is consumed. However, a multigrade shear-stable fluid is able to stabilize viscosity at a wider range of temperatures, allowing operators to increase performance and do more work without increasing fuel consumption (see Image 1).
The Bottom Line
Superior multigrade hydraulic fluids provide a number of performance benefits while improving efficiency and prolonging equipment life. Industry studies provide documented evidence that users who put their faith in multigrade viscosity hydraulic fluids experience energy savings through reduced fuel and energy usage. Ultimately, the best proof for even the most skeptical user will be seen in their bottom line.