Let’s start at the beginning with the question; what is viscosity?
Viscosity is how we describe the thickness and thinness of liquids. High viscosity fluids are thick; they do not flow easily, like treacle. Low viscosity fluids are thin and flow or run easily, like water.
When it comes to hydraulic oil, viscosity is very important. It needs to be just right, both for the job and the conditions it is in. Different machines will require a different viscosity of hydraulic oil. Typically the bigger the workload, the higher viscosity (thicker oil) will be required. Older machines will also usually require a higher viscosity.
The conditions your hydraulic oil is working in will also affect the viscosity you need. Like butter melting in a pan verses butter in the fridge, hydraulic oil becomes more or less viscous in hotter or colder temperatures.
Why viscosity matters, when it comes to hydraulic oil, comes down to the purpose of hydraulic oil. So, the second question we ask is…
What Is Hydraulic Oil?
Hydraulic oil is also called hydraulic fluid. It is most commonly used, as its name suggests, in hydraulic systems to transfer power from one part of the hydraulic machinery to another in order to lift or otherwise move loads. Liquid/Oil is the best at this job because hydraulic oil is non-compressable.
As well as being used for the transfer of power, hydraulic oil can also have the following properties in larger or smaller degrees, depending on the specific hydraulic oil both required and used:
- High flash point (does not ignite even at very high temperatures).
- Non-corrosive.
- Water-resistant.
- Oxidation stability.
- Seal compatibility.
- Thermal stability.
Uses Of Hydraulic Oil
Hydraulic oil is used in a huge number of applications and across many industries. Basically, if heavy things need moving, hydraulics move them, and what makes hydraulic machinery work is hydraulic oil.
The oil within the parts of the hydraulic system, the pistons and cylinders, transfers the power to the moveable levers, arms or forks of the machine.
These hydraulic machines are everywhere and are doing everything, from loading and unloading lorries with forklift trucks to jacking up the lorries when they need repairs.
On building sites, hydraulics and hydraulic oil do the digging, pile-driving, lifting and loading. And in agriculture, the hydraulics on tractors and other farming equipment get things moving, and also with hydraulic brakes, get them stopping too.
Viscosity Index
As mentioned above, the viscosity of hydraulic oil is affected by temperature. Not only the ambient temperature but the temperature created by the engine or machine itself.
Because changes in temperature can cause hydraulic fluid to become thicker/have a higher viscosity or become thinner/have a lower viscosity, this can have profound effects on the hydraulic system.
If the hydraulic fluid has a too high viscosity, it will not flow through the system properly – think of butter out of the fridge and how hard it is to spread. This can lead to the hydraulic system running dry causing any number of problems, from poor lubrication to a total loss of power.
If the viscosity is too low, then the hydraulic fluid is liable to leak and does not provide enough cushioning to the working parts. This leads to a loss of power and higher levels of wear and tear in the system.
To help gauge the right level of viscosity of hydraulic oils across all types of machinery around the world, a Viscosity Index was created by the Society of Automotive Engineers among others.
The numbers in the Viscosity Index or VI scale mean that manufacturers can recommend the right viscosity of oil to their customers to ensure that their hydraulic machines are working at their best and have the longest life. Always follow the manufacturer’s recommendations.
| Low Viscosity | Under 35 |
| Medium Viscosity | 35 to 80 |
| High Viscosity | 80 to 110 |
| Very High Viscosity | Over 110 |
Difference Between Viscosity & Viscosity Index
The difference between viscosity and the viscosity index is that viscosity is the scientific term that means the thickness of a liquid.
The viscosity index was created in 1929 so that there would be a universal, unitless measurement for oil viscosity around the world, aiding both engineers and those using and working with hydraulic machinery day-to-day.
Viscosity Monitoring & Trending
Now you know how important the viscosity of hydraulic oil is to any and all hydraulic systems, you will want to know how to ensure that your hydraulic oil is still at the correct viscosity.
Regular monitoring of hydraulic oil viscosity should be part of a regular and ongoing maintenance schedule of any hydraulic system. As with all oils, hydraulic oil can lose its viscosity over time.
Furthermore, changes in viscosity can also be caused by problems within the hydraulic system. It can be a symptom of a bigger problem. This is why spotting trends and changes in hydraulic oil is an essential part of ongoing maintenance that can be achieved with oil analysis as part of regular servicing.
Regular monitoring by a professional hydraulic engineer can ensure that your oil is still at the right level of viscosity for your system and avoid expensive hydraulic oil changes. This ensures your hydraulic systems are performing at their best with minimal wear and tear, as well as nipping larger and more expensive problems in the bud.
For reliable, professional advice or to book a service with one of our professional hydraulic engineers, get in touch with us at MCH Hydraulics today.