Hydraulic Oil Viscosity vs. Temperature Explained

Hydraulic Oil Viscosity vs. Temperature Explained

May 19th 2023

Hydraulic oils are the linchpin of hydraulic systems. Without them, heavy machinery would cease to function. Oil is the only fluid with the lubricity, compressibility, and corrosion resistance necessary to keep hydraulic systems working properly. Due to its critical role, maintaining hydraulic oil is of the utmost importance—fortunately, that is generally easy to do. The problem arises when mechanical failures, mishandling, and external factors disturb the temperature at which the oil maintains its flow and thickness, otherwise known as viscosity. The question is, how exactly does temperature affect hydraulic oil viscosity?

What Is Viscosity?

Before we can begin to understand the importance of viscosity and how temperature affects it, we must first learn what viscosity is. Essentially, viscosity measures a fluid’s resistance to flow; in other words, it measures how quickly and easily a fluid flows. In terms of oil, viscosity still measures the resistance to flow, but at specific temperatures. Hydraulic oil manufacturers must consider how temperature affects viscosity at the very beginning stages of oil formulation in conjunction with how thick or thin the lubricant becomes with continued use.

Why Does Viscosity Matter?

When we look inside a hydraulic system, we know that there are massive amounts of force being applied within the hydraulic cylinder. Imagine if we were to use an extremely high-viscosity oil, almost like grease, within the cylinder. The amount of force necessary for the piston rod to achieve its full stroke would apply too much pressure, either causing the system to break down or combust. Conversely, if the viscosity of the oil is too low, there may not be enough pressure within the system to create energy. Therefore, the viscosity of the oil is incredibly important, and it must be just right.

What’s the Difference in Oils?

The term “hydraulic oil” may cause some confusion for those new to the field—aren’t all oils the same? The answer is yes and no. While all oils have a certain amount of compressibility, lubricity, and corrosion resistance, engine oils do not have the same additives, nor do they need to meet the same requirements as hydraulic oils. This is because the environment each oil type functions in is extremely different. In general, hydraulic oils have a much higher compressibility and viscosity stability compared to engine oils. In terms of temperature, this means that it takes a higher temperature to destabilize a hydraulic oil’s viscosity in comparison to engine oil, but it can still happen.

How Does Temperature Affect Viscosity?

Let’s move on to the million-dollar question—how does temperature affect viscosity? Remember, oil engineers measure the viscosity of hydraulic oil in different temperature ranges. But how does the temperature affect the oil’s flow rate? As temperature increases, the viscosity of the oil will tend to decrease, becoming thin. This is because the molecules in fluid tend to move faster as the temperature increases. On the other hand, the molecules will slow if the temperature becomes too cold, increasing the viscosity and causing the oil to become thicker.

What Happens if the Oil Gets Too Cold?

We know that when oil becomes too cold, the molecules will slow and the fluid will become thicker. But what does this mean for hydraulic systems? If the oil is too thick, it becomes harder for the substance to flow freely through the system, reducing its ability to circulate. In turn, it becomes more difficult for components of the hydraulic system to move properly. If the oil becomes too cold, it will result in cavitation, lack of lubrication, and starved pumps, which will damage the components.

What Happens if the Oil Gets Too Hot?

If the oil gets too hot, the molecules will move at a faster rate, making the oil thinner and rendering it useless. As previously mentioned, if the hydraulic oil doesn’t produce any resistance, there’s less pressure in the system. This, in turn, reduces the hydraulic system’s ability to produce energy. If there is no fluid power, there is no energy for the system to convert and the machine will cease to function.

Which Additives Keep Viscosity Stable?

Oil manufacturers include viscosity index additives that allow the oil to maintain its viscosity despite temperature fluctuations. These additives are typically complex polymers that expand to thicken the lubricant at high temperatures, and anti-freeze additives that prevent the oil from gelling or freezing. It’s important to note that over time, these additives can break down, especially if the oil is repeatedly exposed to extreme temperatures.

What Factors Might Cause a Change in Temperature?

There are many factors that could cause a change in temperature, which would lead to a change in oil viscosity. Most often, temperature changes are due to environmental factors. If a machine simply works too long in the heat or cold for extended periods of time, it can affect the hydraulic oil. Improper storage and handling can also cause temperature fluctuations. For example, overusing a machine can cause the internal temperature to rise beyond its normal range, leading the oil to become too thin. Conversely, if the machine isn’t properly stored and the external temperature drops overnight, the oil will thicken.

How Can I Tell if My Oil’s Viscosity Has Changed?

If your machine is struggling to start or moves very slowly, it’s likely that the oil’s viscosity has increased due to colder temperatures. On the other hand, if the oil viscosity is too low, you might notice a burning smell or excessive amounts of heat coming from the engine. In either scenario, temperature fluctuations can cause leaks. How, you may ask?

If the oil is too thin, it can slip past the rod seal. And if it’s too cold, it’s possible for the rubber seal to crack, allowing the oil to leak out. Either way, it wouldn’t be a stretch to assume there’s something wrong with your oil’s viscosity if you notice a leak.

When it comes to the battle between hydraulic oil viscosity and temperature, using the right oil—as well as the right system maintenance and handling techniques—will allow your machines to perform at their best. If your system is in need of an oil refill, visit Santie Oil Company today. We’re your one-stop shop for all thing oil and grease related, and we offer a wide selection of hydraulic lubricants from tried and true brands.

Hydraulic Oil Viscosity vs. Temperature Explained