What Is Hydraulic Fluid Used For?

By Power Tools

March 10, 2021

Hydraulic fluid is a non-compressible liquid that is used within hydraulic machinery. It helps to transfer power between different machine components. It can also be used as a lubricant, a coolant, and a sealant.

What is hydraulic fluid used for

It can also be used to increase the efficiency of your engine’s pump. Hydraulic fluid is often fire and radiation-resistant and should have modifications made to reduce friction levels.

They are commonly biodegradable and should have a low toxicity level when they are new or decomposed. 

How long is hydraulic fluid good for?

There is not an arbitrary time period that hydraulic fluid stays good for. You should change the fluid in your system when the base oil has degraded or the additives are depleted. 

The degradation of the base oil is due to chemical reactions in the presence of water and air.

This is through processes known as hydrolysis and oxidation, both of which occur faster in the presence of heat. In order to prolong the lifespan of the base oil, you need to keep it dry, cool, and clean.

Additives become depleted as they become attached to particles in the oil which are then filtered out. 

Some manufacturers will give a rough estimate of the lifespan of their specific hydraulic fluid. This tends to be somewhere around the 3,000 – 10,000 operational hours mark.

The lifespan will vary according to the initial quality of the hydraulic fluid, the operating conditions it is kept in, and potential contamination.

As a general rule, good quality hydraulic fluid should last for about 6 months. 

Can I use hydraulic oil instead of hydraulic fluid?

These two terms are commonly used interchangeably, but they are not the same product.

They are both liquids, but hydraulic fluid can contain other liquids such as water, water-oil emulsion, and saline solutions. Hydraulic oil is simply oils and additives designed to act as a coolant and lubricant.

While they are fundamentally similar, hydraulic oil is a more specialized type of fluid for hydraulic systems. This means that you cannot simply exchange one for the other, as each liquid has different properties. 

If you are considering this, it is a good idea to check the internet or with the manufacturer’s information to check the suitability of hydraulic oil. You should check the color and viscosity stated of the hydraulic fluid and compare it with the hydraulic oil. 

The viscosity of oils is measured using an SAE scale, whereas for fluids this is ISO. You will need to convert one figure in order to ensure the relative viscosities match. Some common ratings are listed below. 

10W 32
15 or 20W 46
20W 68
30W 100

Can you mix hydraulic fluids?

This is not a recommended path to go down, and where possible you should not mix hydraulic fluids.

This is due to the additives that the fluids contain. Different fluids will have different additives in them which can cause chemical reactions. This can ultimately cause the different fluids to counteract each other instead of working together in your system. 

Mixing fluids can not only stop them from working, but they could also cause damage to the internal components of your system. If you are unsure of the hydraulic fluid used and need to top up the levels, we recommend draining the fluid completely and replacing it with all one type of hydraulic fluid. 

If you know the fluid used but still wish to switch to a different one, you need to check the miscibility.

This means whether or not the two fluids can fully dissolve in one another at any concentration to form one homogeneous fluid. You can contact the supplier of your lubricant for information about this. If they do not already have this information, they are able to perform a miscibility test to know for sure. 

What color is hydraulic fluid?

Hydraulic fluid is commonly a golden hue, but it can change to become a dark brown. The most common reasons that hydraulic fluid becomes darker are oxidation and thermal stress.

Noticing a color change in your hydraulic fluid does not always mean that it needs to be replaced, but it is worth having a look at. If you are concerned about it then you can take a small sample to a dealership and request that they analyze it for you. 

This analysis would look at the number and viscosity of your hydraulic fluid and compare it to the original figures. If they have remained the same then the color change is likely to be caused by thermal degradation.

You can invest in an infrared camera to look inside the system and check for hot spots. This will let you know whether this is the reason for the color change inside the oil.

Some hydraulic fluids are artificially colored. These are often colored to be indicative of a specific use such as hydraulic fluid used in aircraft. These can be a range of colors including purple, gray, green, and yellow. 

It is wise to take note of the initial color of the hydraulic fluid you have used to be able to effectively monitor changes in color. 

What does UDT stand for in hydraulic oil?

UDT stands for Universal Dynamic Tractor Oil. This is a type of dual-purpose transmission and hydraulic lubricating oil that can be used to replace traditional lubricating oil. This was developed by a company called Kubota. 

This oil can be used in all weather conditions and many different systems. These include hydraulic, final drive, transmission, differential, and wet brake systems. 

It is important to have a trans-hydraulic fluid such as UDT in modern tractors. This is because the hydraulics and transmission both pull fluid from the same reservoir, meaning that your fluid needs to be multipurpose. 

Take care to check the specifications of your tractor and its engine to ensure that you purchase and use the correct fluid. UDT fluids are particularly helpful here as they are designed to meet the specifications of almost all tractor manufacturers.