What is the role of viscosity index in hydraulic fluid performance?

If you've worked for long enough with hydraulic systems, you'll quickly discover that the performance of fluids is not constant. Temperatures fluctuate all day long, through seasons, and even between startup and full load. Every single one of these fluctuations can affect how fluids in your hydraulic system flow, and that's the point where viscosity index is among the top crucial and under-appreciated variables in selecting the right fluid.

Understanding viscosity and the reasons it varies

Viscosity describes the resistance of a fluid to flow. When it comes to hydraulic systems, it determines the efficiency with which the fluid transfers energy, provides lubrication to internal parts, and keeps the seal between the tight clearances. If the fluid is too thick, the fluid has difficulty moving through valves and orifices, causing more energy loss and the buildup of heat. Insufficiently thin, it does not maintain the hydrodynamic layer that is required to shield cylinders, pumps, and motors from metal-to-metal contact.

The issue is that viscosity doesn't remain the same. The majority of synthetic and petroleum-based fluids lose their viscosity when temperatures rise and then increase in thickness when temperature decreases. This is an essential physical property of all fluids. The issue isn't how often this occurs—it does always do so—but how much it happens over the operational range that your device operates within.

This is exactly what the viscosity index determines.

What is the viscosity index?

Viscosity Index, often referred to in the form of VI, refers to a non-dimensional number that describes how the viscosity of an oil alters in response to variations in temperature. It was developed by Dean and Davis in the 1920s as a standardized method to compare the temperature-viscosity behavior of different oils.

A high VI signifies that the viscosity of a fluid fluctuates minimally between high and low temperatures. A low VI indicates that the fluid is thinned rapidly upon heating and then thickens substantially when it is cooled. The greater it is, the steadier the performance of the fluid across the entire temperature range.

The standard mineral hydraulic oils usually contain a VI that is in the 90-100 range. 110. Multi-grade hydraulic fluids with enhancer additives for VI can range from 140 to 160. Synthetic fluids with high performance like polyalphaolefins (PAO) typically exceed 150 and, in certain formulations, push over 200.

Why is VI important to the hydraulic system's performance?

The hydraulic systems are constructed around a specific viscosity range. Pump makers have the maximum and minimum viscosity levels. Proportional valves and servo valves have clearances that require that the fluid maintain a uniform viscosity for exact, controlled, repeatable operation. Pressure-compensated pumps and relief valves depend on the predictable behavior of fluids to operate properly.

If the viscosity of fluids fluctuates widely due to temperature fluctuations, there are a variety of issues that arise.

In a cold start-up the low-VI fluid may be too viscous, causing cavitation at the inlet of the pump. The fluid can't flow fast enough so that it can fill chambers, which can lead to the formation of vapor bubble noise and an increase in wear on the internals of the pump. In extreme situations, the pump may be damaged beyond repair within a matter of minutes after it is started.

In higher operating temperatures, the low-VI fluid can become thin enough to where internal leakage through vanes, pistons, and points of meshing gears increases significantly. The efficiency of the volumetric system decreases. The system is powerless as heat generation rises, and the protective film of the fluid that covers bearing surfaces deteriorates. For continuous-duty applications, presses, and injection molding machines and mobile equipment operating in the summer heat, this is an actual and constant issue.

A fluid with a higher VI performs better throughout this range. The pump is able to produce a sufficient film when it is starting cold, and it retains its body at high temperatures to keep the seal and the lubrication. The whole system functions according to its original design over a larger range of operating conditions.

Viscosity index for Mobile as compared to industrial applications

The significance of VI is contingent in large part on the context in which it is used.

Industrial hydraulic systems that operate in climate-controlled environments that have relatively steady temperatures could be able to function with regular VI mineral oils. If the system begins warm and continues to run at a constant load, 100 VI could be sufficient.

Mobile hydraulic systems have to face an entirely new challenge. A telehandler operating in a northern European winter could start at the minus 20 degree Celsius mark and then reach its full operational temperature within a matter of minutes of starting. Agriculture equipment is used in temperatures of summer and spring cold. Deck equipment for marine use can sit still for a long time in frigid air before being called upon to carry heavy loads in a flash.

In these conditions, a high-VI fluid is not a luxury; it's a necessity. Multi-grade hydraulic oil grades with a VI over 150 are usually used for mobile equipment because the temperature range between operating and startup temperatures is too huge for a mineral oil of a single grade to be able to manage effectively.

The function of VI enhancer additives

The majority of high-VI hydraulic fluids attain their performance by using VI improver polymers that are blended into the base oil. These additives alter molecular conformation as temperatures rise. At lower temperatures, the polymer chains are tightly twisted and exert minimal influence on viscosity. When temperatures rise, they break up and expand and counteract the natural thinning of the base oil.

It is efficient; however, it is not without sacrifices. VI polymers are subject to mechanical shear degrading as time passes. Hydraulic systems with high pressure—particularly those equipped with piston pumps and gear pumps operating at pressures greater than 250 bars—expose the fluid to high shear forces, which can cause permanent damage to chain polymers. If this happens, the fluid is unable to retain some of its VI enhancement and the actual viscosity in service at temperatures above what was specified in the original specifications.

This is the reason shear stability is analyzed along with VI when choosing a hydraulic fluid that is suitable for applications that require a lot of pressure. A fluid that has a VI of 160, which shrinks to one of 130 following 2,000 hours of use, could not offer the security it was intended to provide. Fluids made from high-VI base stocks, like mineral oils of Group III+ or PAO synthetics, have their VI derived from the base oil's chemistry rather than the addition of additives, which is why they offer better shear stability.

High VI and natural synthetic liquids

Polyalphaolefin synthetics as well as some ester-based fluids have naturally high VI values, usually between 150 and 200, and without using a lot of polymer additives. They also provide excellent low-temperature fluidity, meaning that their pour points are significantly lower than comparable mineral oils.

For hydraulic systems that have large operating temperatures or that require extended service intervals, synthetic high-VI fluids provide an impressive performance advantage. Cost is the main consideration. Synthetics usually come with a significant cost premium over standard mineral oil grades. In instances in which downtime and replacement costs for components are substantial, the economic argument of synthetics is simple.

The right VI to your app

When deciding on the right hydraulic fluid, keeping VI requirements in mind begins by determining the range of temperatures your system is likely to encounter—not just the optimal operating temperature but also cold soak temperatures at the beginning of operation and the maximum anticipated temperature of the fluid under high loads and in the presence of heat.

Check this range against the limits of viscosity set by your valve and pump manufacturers. Find out if your system is able to take the variance in viscosity an ordinary mineral oil will produce in this range. If not, you should switch to a high-VI multigrade or synthetic alternative.

Take into consideration the duty cycle and operating pressure also. In the event that shear loads are very high as well as service times are lengthy, prioritize fluids that have large VI stock levels over those that rely mostly on polymer-based additives.

Don't assume that a greater VI will always be better in each situation. In industrial environments that are stable, paying for VI performance that is not going to be required adds up to a cost but is not worth it. The objective is to match the characteristics of fluids to the actual requirements for the particular system.

The viscosity index is among the clearest gauges for how fluid performs over the temperature range that a system will experience. It is a way to determine if your pumps swell at the start as well as whether the clearances and seals function properly under load and if your system is able to perform consistently in a variety of conditions. Understanding VI and correctly describing it is one of the most simple steps that an engineering or maintenance team can take to improve the reliability of the hydraulic system and increase the service life of components.