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Guide to Hydraulic Oil

Hydraulic oil is used in many industries and in a wide variety of applications. Read our guide to hydraulic fluids to learn more or alternatively call 0330 123 1444.

Crown Oil

Hydraulic Oil Explained – An Easy Guide

If you’ve ever felt the pressure of choosing the right hydraulic oil for your machinery, you’ll know all too well the minefield of information that’s out there in books or online. Instead of getting lost in the world of hydraulic fluid, hydraulic liquids or hydraulic lubricants, why not take a look at our easy-to-follow guide to hydraulic oil? It’s everything you need to know about hydraulic oils!

Alternatively, give our expert team a call on 0330 123 1444 to place a hydraulic oil order with us. Available for delivery nationwide, within 48 hours of purchase, we’ll have your business well-oiled in no time. 

What is hydraulic oil?

Hydraulic oil is a non-compressible fluid that is used to transfer power within hydraulic machinery and equipment. Otherwise known as hydraulic fluid, hydraulic oil can be synthetic- or mineral-based.

At Crown Oil, as a hydraulic oil supplier, we deal with 99% of mineral-based hydraulic oils.

Although this useful fluid is commonly used in the transfer of power, hydraulic fluid can act as a sealant, coolant and lubricant within machinery and equipment.

The main difference between synthetic and mineral-based hydraulic oil

The majority of oils which are produced are either mineral-based or synthetic. Mineral-based hydraulic oils are derived from crude oil fractions whereas synthetic hydraulic oils are made using chemically produced base fluids.

Synthetic oils can be formulated to impart superior physical properties in comparison to mineral oils, for example, high-temperature performance, biodegradability and oxidation stability.

How do hydraulic systems work?

The key role of hydraulic oil in a hydraulic system is to transfer power from one end of that system to another end through and the various hydraulic components.

When an external force is applied to the non-compressible hydraulic fluid – usually from a piston within a cylinder – the oil is pushed through the hydraulic system and ultimately produces a force on another part of the system. This results in a movement or action.

Usually applying force to material results in compression, so you may be wondering whether hydraulic oil is compressible or not, but a key property of hydraulic fluids is that they must be non-compressible.

‘Non-compressible’ means that the fluid cannot be compressed. Liquids are compressible to some degree, but it is incredibly negligible and not considered for our guide. In contrast, gases are compressible, and so are not used in hydraulics.

Hydraulic Oil Guide

What is hydraulic oil used for?

Hydraulic fluids are used in many applications across every industry. To give you an idea of the vast array of uses for hydraulic fluid and why industrial hydraulic oil is so important, here are 10 examples of equipment and machinery which use hydraulic oil:

  1. Forklift Trucks – The hydraulic system within forklift trucks and stackers is important to help power the incredibly strong forks which need to lift some super-heavy goods.
  2. Log Splitters – The ram mechanism on a hydraulic oil log splitter requires hydraulic fluid inside to give it that immense power which can split logs with ease. Log splitters are also known as wood splitters!
  3. Automotive Lifts – Car lifts (car jacks, automotive lifts, etc.) require hydraulic jack oil to aid their impressive power range! This type of machinery depends heavily on reliable hydraulic oil for safety as well as performance. Hydraulic fluid for a car lift tends to have a higher viscosity grade for high pressure.
  4. Wright Standers – A wright stander is a stand on mower which is usually well-suited to cemeteries and other restrictive grassy areas. The hydraulic part of these machines requires hydraulic oil for power.
  5. Snow Ploughs (Snow Plows) – Hydraulic oil for a snow plough and ploughing equipment is essential in the powerful operation of the hydraulic lift, tilt and angle movements of the snow plough blade. The cold weather conditions associated with the use of a plough means that the hydraulic fluid used in a snow plough will be blended with antifreeze additives.
  6. Skid Steers (Skid-steer Loader and Skidsteer) – Skid steer hydraulic oil is as versatile as the machine it works with. For the many tasks this machine can competently complete, hydraulic oil plays a big role at all times.
  7. Aircraft (aviation) – In the aviation sector, it is essential that aircraft hydraulic oil is reliable as it is used for aviation control systems, aircraft hangar doors, aircraft jacks and aircraft controls.
  8. Air Tools – Air tools and air compressors require high-pressure hydraulic oil which contains anti-wear additives for protection.
  9. Tractors – Tractor hydraulic oil is necessary for the operation of hydraulic brakes and hydraulic systems on agricultural vehicles and machinery. For your hydraulic oil tractor supply, you may want to use a reputable manufacturer to ensure your expensive machinery and vehicles are well looked after and protected.
  10. Cruise Ships and the Marine Industry – If you’ve been lucky enough to go on a cruise ship then you’ll have felt the comforts out at sea. Hydraulic oil is used aboard many marine vessels for the stabilisers. The stabilisers reduce the amount of roll, which can affect the ship’s balance and cause you to experience unfriendly sea-motion sickness. This is just one of the many other applications on marine vessels which require hydraulic oil.

Hydraulic fluid properties

The properties and characteristics of any hydraulic oil are vitally important to the capability of your hydraulic system to work within the operating conditions you need to use it in. This is especially true of industrial or commercial hydraulic oils. So, for a hydraulic oil to be useful it needs to have the below properties:

  • Non-compressible
  • Thermally stable, within a range of operating temperatures
  • Fire resistance
  • Non-corrosive to its system
  • Anti-wearing to its system
  • Low tendency to cavitate
  • Tolerance to water (resistance to water contamination)
  • Total water rejection
  • Constant viscosity, regardless of temperature
  • Long life
  • Cost-effective

Few, if any, fluids perfectly meet the above criteria. However, there is a comprehensive range of hydraulic oils that specialise in meeting the above properties for the set of conditions in which they are required to operate. Those conditions can range from being required to operate in low temperatures (winter hydraulic oil), high temperatures and a variety of others.

The contents of hydraulic oil

Hydraulic oil is made from a variety of different ingredients, with one base fluid. These ingredients can often be mixed depending on the type of oil you require.

Generally speaking, hydraulic liquids are made up of:

  • Mineral oil
  • Esters
  • Glycol
  • Silicone
  • Ethers
  • Esters
  • Some other chemicals which are hard to pronounce!

For the varied applications of hydraulic fluid, blenders will mix the base oil with additives of different types to make the properties of hydraulic oil different.

Hydraulic oil additives

Depending on how you use our hydraulic oil, there will be extra additives which help it perform under different conditions. Different hydraulic fluid additives include:

  • Anti-wear – helps prolong the life of equipment and machinery, you’ll see this on AW-type hydraulic fluids.
  • Cold Flow – additives that enable usage in extreme cold weather conditions
  • Anti-foaming – An anti-foaming agent for hydraulic oil reduces foaming within the fluid which can be caused by detergents. This foaming can reduce the lubricating quality of the product, thus causing damage.
  • Anti-oxidant – Allows for longer periods of use without an oil change while also reducing sludge deposits.
  • Anti-rust – Forms a protective coating which reduces the risk of rust damage from oxygen contact.

These additives are used on their own and together in different blends which are created for different purposes. Hydraulic oil properties can be altered depending on which additives are used, but the typical characteristics tend to be a high viscosity index and be incompressible.

Below is a list of common uses for hydraulic oil and the type of additive/s which may be added to the oil to help it perform at its optimum level.

Hydraulic oil for winter

Hydraulic power is required in some of the coldest places on earth. In these cases, anti-freeze additives are used to prevent the fluid from freezing up or waxing. Low-temperature hydraulic oil is commonly used as a name for fluid which needs to be used in icy conditions.

HGV Hydraulic Oil

Hydraulic oil for high-heat applications

Under high heat, oil becomes less viscous and flows more easily, which means that it can leak or lose its required properties. Additives are used to retain viscosity for fluids used in applications involving exposure to higher temperatures.

Hydraulic oil for heavy-duty use

Heavy-duty hydraulic oil is necessary for high-pressure environments where the fluid needs to be able to cope with large amounts of stress. The hydraulic oil additives used here will typically contain anti-wear properties. Anti-wear hydraulic oil is one of the most common blends used in industry and construction.

Environmentally-friendly hydraulic oil

Biodegradable hydraulic oil is used in applications where an oil spill or leak could potentially contaminate the environment. The typical base oil for biodegradable versions of hydraulic oil includes rapeseed oil and some other vegetable oils.

Environmental hydraulic oil is a strong consideration for those using hydraulic machinery on farms, forests or similar environmentally sensitive sites. This is because the oil is made of a biodegradable base fluid, so it will degrade naturally in the event of a spill.

Hydraulic oil in more detail

Hydraulic oil classification

The classifications of hydraulic oil are a subgroup of different fluids with varying performance levels. Below is a list of common hydraulic oil classifications and their respective descriptions:

  • HL – Refined mineral oils with anti-oxidation and anti-rust properties
  • HM – HL with improved anti-wear properties
  • HR – HL oils with VI improvers

For a detailed list, you can speak with our knowledgeable team by calling us on 0330 123 1444 or alternatively, you can read our explainer of Hydraulic Oil Classifications. Find out Crown Oil’s hydraulic oil product specs.

Hydraulic oil ratings

When an additive company sells an additive pack, it will work alongside a particular manufacturer to create a product which works perfectly with that manufacturer’s application. This will be traceable by a hydraulic oil supplier who has used the additive in the fluid. Hydraulic oil ratings or approvals are stipulated by many end users of oil to ensure they’re using the correct fluid for their equipment.

Hydraulic oil analysis

Commonly known as condition monitoring, a hydraulic oil analysis service is used by people who want the most out of their oil before they have to change it in their hydraulic application.

This works by sending a sample of the hydraulic oil to a lab which analyses the sample and reports back with details as to whether it’s good to use further or if it needs to be changed. This gives the end user reassurance that it is still usable and no damage can be done to expensive equipment due to dirty or worn oil.

Why is hydraulic oil analysis important?

The importance of oil analysis should be high on anybody’s list. Below is a list of just a few reasons why it shouldn’t be ignored and how it pays for itself many times over:

  • Reduces the cost involved in replacing the oil prematurely.
  • Minimise damage to your machinery by identifying issues early.
  • Has the potential to enhance the life and performance of machinery.
  • Reduces the risk of damage to machinery and products.
  • Reduces the risk of injury to people and further costs in claims and damages.

Hydraulic oil temperature range

Depending on the application of hydraulic fluid, it may be subject to cold or hot temperatures. In some cases, hydraulic oil can be subjected to both hot and cold temperatures which can render the oil useless if it hasn’t been blended with the right additives.

Hydraulic fluids have temperature stability which means they’ll retain their properties within a particular temperature range. Anything above or below this will negatively affect the temperature stability and cause the fluid to either wax and freeze under cold conditions or lose viscosity and potentially leak under hotter temperatures. High heat can cause rapid deterioration of hydraulic oil.

Viscosity of hydraulic oil vs temperature

The viscosity of hydraulic oil and temperature is closely linked. As temperature increases, the viscosity of the oil will decrease – a bit like when you put cooking oil in a cold pan it moves slowly but when the pan heats up the oil moves around very quickly and with ease. As the temperature drops hydraulic oil will become more viscous.

Blenders are always trying to make hydraulic oil operate effectively over a wider range of temperatures. This means they’ll go down to low temperatures and still work as effectively as the temperature increases.

Hydraulic oil viscosity index

To measure the change in viscosity of hydraulic oil when the temperature changes, we use the oil viscosity index (VI). If a hydraulic oil has a low viscosity index, a temperature change will alter the viscosity more than if it has a high viscosity index.

Hydraulic oil with a high VI will usually be required in an application which is subjected to a greater range of ambient and/or operating temperatures.

A straight paraffinic mineral base oil will typically produce a fluid with a low viscosity index, whereas a paraffinic mineral base with viscosity improvers will produce a fluid with a high viscosity index.

The SAE (Society of Automotive Engineers) created a classification table (VI Scale) to show low to high viscosity levels depending on temperature °C. Originally the scale only went up to 100°C but with advances in hydraulic oil blends, the scale now goes above this number!

Viscosity IndexClassification
110°C and greaterVery High

Hydraulic oil viscosity explained

Viscosity Machine

When it comes to hydraulic oils, viscosity is the measurement of its resistance to flow and it is an important property of hydraulic fluids. This means the fluid will resist compression at different rates depending on its viscosity and take longer to pass through an orifice as the viscosity increases. Hydraulic fluid with high viscosity will be thicker and harder to compress and move as opposed to low viscosity hydraulic oil which will be thinner and pass through easier.

The viscosity of a hydraulic fluid is measured in Centistokes (cSt) and usually at temperatures of 40°C or 100°C. The value will always have the temperature next to it as without this the value will be meaningless. A fluid’s viscosity is measured in a lab using a viscometer as pictured below!

Hydraulic oil viscosity is important for each different application.

The wrong viscosity could cause damage to equipment or have poor results in function.

Hydraulic oil viscosity conversion chart

Please note:

Viscosity Equivalents - Hydraulic Oil Viscosity Chart
Hydraulic Oil Viscosity Chart

This chart should be read horizontally. It assumes 96 VI single-grade oils. The equivalence is in terms of viscosity at 40°C only. Viscosity limits are just approximate; for precise data then you should consult your supplier, ISO, AGMA and SAE specifications. W grades are represented only in terms of approximate 40°C viscosity. For low-temperature limits, consult SAE specifications.

Hydraulic oil grades

ISO VG – ISO Grade (where ISO is the International Standards Organisation) – the higher the VG number the more viscous the fluid is. The VG number tells you which hydraulic oil is thicker. This is sometimes referred to as the hydraulic oil weight. At the same time, grades with a W next to them indicate the weight (Unlike automobile engine oil which refers to winter oil).

AGMA GradeAmerican Gear Manufacturers Association – Leaders in the standards of gear oils.

SAE – Society of Automotive Engineers

In the UK, ISO VG is used mainly for grading hydraulic oil. Below is a list of common hydraulic oil ISO grades and a general guide to their applications:

  • ISO 100 Hydraulic Oil – ISO VG 100 Hydraulic Fluid tends to be used in industrial machinery with heavy loads.
  • ISO 15 Hydraulic Oil – ISO VG 15 Hydraulic Fluid is typically used in power steering and hydraulic brake systems.
  • ISO 22 Hydraulic Oil – ISO VG 22 Hydraulic Fluid is generally used in airlines for air tools etc.
  • ISO 32 Hydraulic Oil – ISO VG 32 Hydraulic Fluid is ideal for use in high-powered machine tools.
  • ISO 46 Hydraulic Oil – ISO VG 46 Hydraulic Fluid is normally required for industrial plant working under high-pressure etc.
  • ISO 68 Hydraulic Oil – ISO VG 68 Hydraulic Fluid is designed for use in systems which require a large load-carrying ability.

Please note that the above examples are just a rough guide and some grades may cross over. It is always best to check with your supplier or manufacturer!

Hydraulic oil flash point

The hydraulic oil flash point is the lowest temperature at which enough vapours are released from the fluid which can be combustible.

Hydraulic fluids play an ever-important part in commercial use, and it’s vital that you source premium quality hydraulic oil from a supplier you can trust.

So, whether you need high-quality hydraulic oil for tractor supply, construction supply or for use in any other industry, we can supply a wide range of hydraulic fluids.

If you have any further questions about hydraulic oil not answered here, our hydraulic oil experts will be happy to help you. You can contact us by calling us on 0330 123 1444 today. Of course, you can also call to discuss your own requirements and request a quote.

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