The importance of viscosity index in hydraulic fluids

By Josh Cosford 

As one of the longest, coldest winters on record comes to an end (or doesn’t come to an end, in this case), I consider how all the hydraulic machines must have fared throughout the brutal outdoor conditions we’ve experienced in the past four months. Luckily most hydraulic parts are made from iron or steel, which tend to not care about ambient temperature. However, the concern is when these metal parts either rub against other metal parts, or the metal parts are attempting to push a fluid as thick as ketchup.

Some concern in frigid temperature operation is with seal material, and whether the seal material can avoid becoming brittle. Luckily, seal manufacturers make solutions for arctic conditions, such as low-temp Nitrile or Viton. Regardless, even if the seals can hold up from November to March, they won’t make it any easier to pump that ketchup.

Hydraulic fluid is highly susceptible to viscosity changes based on temperature, especially if the machine has been sitting idle in the cold. The thicker the oil, the harder it is to pump, and if plumbing distance is long with marginal diameter, a lot of energy can be lost to pumping, leaving little left over to perform work. I sometimes see cold hydraulic systems with cylinders that retract slower than they extend because of the extra pressure drop created by flow intensification of cold oil.

The solution to cold weather viscosity problems is to choose appropriate hydraulic fluid. Half of the consideration is with rated viscosity, such as “22 weight” oil, which is considered to be a low viscosity fluid by most. This fluid could be appropriate in most mobile applications, but we should first consider a 22 centistoke oil is rated so at 40° C, not -20° F. So what will the viscosity of a 22 weight oil be at minus twenty? Well, it depends….

Viscosity Index is a dimensionless number to describe an oil’s resistance to a change in viscosity when subjected to a change in temperature. It is often measured by comparing the change in viscosity between 40° C and 100° C. The higher the viscosity index, the higher the resistance to change in viscosity. With hydraulic oil, a viscosity index of 100 is considered excellent (although automotive engine oil VI numbers can approach 200).

A caveat about viscosity index and cold temperatures—often, the oil chemistry specific to the creation of high viscosity index doesn’t always translate into a lower pour point, which is really what we’re concerned with in cold temperatures. A premium 22 cSt oil with a respectable VI of 100 might have a pour point of -27° C. However, you can imagine this oil will be pretty thick and difficult to push at -20° C if at seven degrees colder, it’s barely able to be called a fluid.

For machines to be happy in conditions similar to the winter of 2013-2014, a different kind of oil should be used. These oils often have the words “arctic” or “blue” in their names, but they’re always synthetic oils formulated to flow at extremely low temperatures. If even a quality 22 weight hydraulic oil can barely pour at -27° C, these arctic oils will still exit an upside down beaker at -60° C or colder!

Unfortunately, “arctic” oils are not cheap, and no fleet manager wants to change the oil in their machines every spring because they’re running watery oil inappropriate for the summer. The good news is that these oils have a crazy high viscosity index, a result of the synthetic formula and additive package required for effective low temperature operation.

Arctic oils can have VI’s in the range of 150-200, and because VI is measured from 40-100° C, it is a good indicator of how the oil will perform when summer finally arrives. With a viscosity index of 150, a 22 weight arctic oil will be as effective in the heat as a 32 weight standard oil. With a viscosity index of 200, a 22 weight arctic oil will be as effective in the heat as a 46 weight standard oil.

As you can see, viscosity index is one of the most importantly qualities of hydraulic oil in mobile machines, enabling true all-weather operation with little downside other than cost. However, when you’re faced with record cold temperatures sidelining your fleet while you lose thousands of dollars per hour, the cost of high-quality oil seems like a non-factor in comparison.

Josh Cosford, Certified Fluid Power Hydraulic Specialist, is with www.fluidpowerhouse.com.