Hydraulic oils are incredibly important to the majority of modern industrial machinery. Hydraulics allows for incredible force to be transferred via the “compression” of fluids, transferring incredible amounts of force to other parts of the system.
Ensuring that you are using the correct hydraulic oil for your uses primarily comes from understanding the different qualities of the fluid you’re using. So, let’s dive right in and see if we can’t help you understand hydraulic oils from the ground up.
A Brief Word About Hydraulics
Hydraulic systems have been with humanity for a long, long time. There’s evidence of their usage quite early on, but the biggest advancements had to wait until the last century to be brought into play.
Hydraulic systems operate off of Pascal’s Law, essentially since fluids are almost entirely incompressible pressure applied to a system can be used to almost instantly transmit power through a liquid medium. This makes for an incredible way to transmit the maximum amount of force with a minimum amount of input.
It’s best to think of hydraulics as a fluid-based alternative to the standard mechanical methods of moving things and it’s a bit too complex to go into in detail here.
The biggest advance in hydraulics since the beginning, apart from electrical inputs, is undoubtedly the use of oils.
While early hydraulic systems utilized water in order to get the work done, water has a relatively low boiling point and thus needs an extremely large reservoir or tons of different heat sink technologies applied in order to keep it in a liquid form.
Oils, comparatively, have a much higher boiling point. They also tend to be less corrosive than water, and also can provide lubrication to the machinery which they power which makes the use of different oils in machinery a huge advantage over the use of water-based hydraulic fluids.
The problem for many operators, however, is that there’s simply a ton of them available on the market which can make choosing the correct one troublesome even for those with a lot of experience.
An Introduction to Hydraulic Oils
Hydraulic oil has to function differently than most of the oils which exist on the market. While oil is one of the most commonly used lubrication technologies in the world, hydraulic oils have some requirements which simply aren’t there for regular lubricants.
Hydraulic oils have to be able to perform under pressure. Literally, the oil is a power transfer medium within hydraulic machinery.
This means that the oil in question must have a high bulk modulus(meaning it resists changing volume under pressure) and a high viscosity index(meaning it stays the same “thickness” across a wide range of temperatures).
Viscosity is the most important consideration when choosing a hydraulic oil, however, hands down.
However, you’ll also need to take into account the type of system that the oil is contained within, as the conditions which the oil is under are also very important for making sure that you have the best performance possible.
Oils for hydraulic systems are measured differently than those that go into vehicles. Instead of being rated using the SAE scale, they’re measured under the ISO scale.
ISO grades measure viscosity at 104°F(40°C). When you look at, for instance, ISO 32 hydraulic oil it will flow much more quickly than ISO 68 hydraulic oil. Different viscosities are better for different applications in the field.
While it’s usually best to go with the owner’s manual for the equipment in question, keep in mind that the hydraulic oils mentioned there are primarily for “best case” conditions, rather than being hard and fast rules.
While machinery used indoors, like a hydraulic press, is likely to not have any issues there are definitely times when you may need to switch things up.
If you’re operating heavy machinery in temperatures below freezing you may need to go with a thinner oil since it’s unlikely that the machinery is going to reach the 40°C benchmark. For instance, the viscosity of ISO grade 46 hydraulic oil at the optimal temperature is going to be 46cSt.
If significantly below that, then the oil is going to be significantly thicker and you may need to ensure that you’re using a fluid with a lower viscosity to keep things operating properly without adding additional wear to the system.
While most hydraulic oils are measured with the simple ISO grade, there are oils out there with anti-wear properties that are measured separately. For instance, you may see AW 46 hydraulic oil while looking around.
These oils perform at the same grade as their ISO counterparts. An AW 32 hydraulic oil is going to be at 32cSt at 40°C for instance.
The difference here is that there are additives contained within the oil. These are supposed to reduce the corrosion and are designed to ensure that there’s no direct metal-to-metal contact within the machinery.
Most of the additives used for this are zinc-based. While there has been some reactionary movements against them, zinc is a heavy metal and quite unhealthy after all, it also extends the life of machinery by quite a bit.
The reason for using zinc-based additives, like zinc dithiophosphates, is because they have both anti-oxidative qualities and anti-wear ones. When something else is used to replace it, the oil will require two different additives to gain the same advantages earned with the zinc additives.
If you’re using equipment in harsh conditions, or require precise controls, then AW grades are the way to go. Whether you’re using AW 48 hydraulic oil or AW 68 hydraulic oil, these additives help keep wear down on machinery and let it perform for longer.
Issues With Hydraulic Oil
Hydraulic oils, more than any other commonly used oil, face a wide range of different challenges when they’re being used in a system. In addition to the fact that much of the machinery used performs in outdoor conditions with a wide range of temperatures, the oils which are formulated must also be able to handle the following.
Foaming is a serious problem in hydraulic systems, and the primary reason why they must be kept air tight.
Foams occur when air is injected into the fluid of a hydraulic system. It can be reduced by making sure that proper maintenance occurs at regular intervals but since many hydraulic systems operate heavily in uncontrolled conditions, an anti-foaming additive is quite common.
These can help to keep the foam to a minimum. Too much foam and a system can be compromised.
Other causes include the following:
- Water in the system
- Solids in the system
- Depleted defoamant due to overfiltration
- Cross contamination with incompatible fluids
- Too much defoamant
As an issue within hydraulic systems it’s pretty large, and the causes can be myriad. A good hydraulic oil will have the right defoamant to keep things to a minimum however.
Corrosion is another big concern which hydraulic fluids must overcome. If the oil reservoir gets too low, humid air can enter the system and condense within it.
This corrosion can be disastrous if not handled properly. Fortunately, most oils already have anti-corrosive properties and they have a tendency to coat the ferrous parts which are susceptible to oxidation.
Unfortunately, bad engineering of a piece of equipment can make even the best oil useless in this regard.
One of the biggest challenges which face those who need to change the oil in hydraulic equipment is that of compatibility.
In fact, some places actually employ someone specifically to know how to handle this particular issue. For the layman it’s best just not to mix hydraulic oils at all, but never mix zinc and non-zinc oils.
The main problem emerges from the different additives which are used in different oils. Water separation provides a good example of this: while some will cause the water to emulsify within the fluid, others will cause the water to separate. Mixing these can be disastrous.
It’s best to just do a full system flush if you’re planning on switching oils.
The Life Cycle of Hydraulic Oils
Like all oils, hydraulic oils aren’t going to last indefinitely. The life cycle is quite similar to that of most oils.
In the beginning, clean oil is added to the machine in question. This oil functions at top efficiency provided all factors are accounted for.
In theory, hydraulic oil could last pretty much indefinitely within a sealed system. In practice… that just doesn’t happen.
Instead, as time goes on, the oil is going to accumulate debris. Small leaks in the system will allow air and water in. Debris will accumulate from the mechanical parts of the process and the oil will be exposed to spikes in heat during heavy usage.
All of these factors contribute to the “decay” of the oil used.
The Primary Enemy: Oxidation
Oxidation is the main enemy of all oils. While they’ll still function with small amounts of debris contained within them.
The conditions of hydraulic oil lend themselves well to the oxidation process. Microscopic particles will, over time, become oxidized due to temperature, as well as water and air exposure. This can be minimized through the use of additives and adequate filtration but it’s not going to be able to be eliminated.
As the oil becomes oxidized it will begin to change in it’s properties which limits the effective lifespan of hydraulic machinery. The following will begin to occur as time goes on:
- Formation of “sludge”
- Increasing viscosity
- Decreased lubricant properties
- Decreased film strength
- Decreased ability to handle loads
- Damage to system elements and filters
All of these together will eventually render the hydraulic fluid in question unsuitable for continued use.
The correct technology may be able to sort these issues out, and the idea of a circular economy of oil is very real. With this technology applied, a circular economy of oil could be completed. This would allow the oil to be recycled nearly indefinitely.
Types of Wear
Wear is the primary cause of contaminants within oil, and there are a lot of different types of wear which occur over time.
- Abrasive Wear-When two surfaces come into contact, it’s inevitable that they’ll scrape pieces of each other off over time. While the harder surface will win out in most cases, if the materials are of similar hardness then there’s likely to be some “give and take.”
- Aeration Wear-Occurs when air bubbles within the hydraulic fluid burst against a surface, causing tiny amounts of damage to the surface.
- Adhesive Wear-As oil film breaks down on the surface of moving parts, they begin to wear each other down.
- Cavitation Wear-Restricted fluid flow through inlets causes voids which shock surfaces.
- Corrosive Wear-Water contamination can oxidize metallic surfaces, causing fine particles to break away over time.
- Erosive Wear-The high pressure fluid flowing through a system slowly breaks down critical surfaces
With all of these in play, it becomes inevitable that debris and particles are going to make their way into the hydraulic oil within a system.
If you paid close attention, you’ll realize that many of these types of wear will build upon each other. From there, you run the risk of things beginning to get out of hand as the oil gets worse faster and faster.
This means that monitoring the fluid within any hydraulic system is absolutely critical to maintaining the integrity of equipment. If wear is allowed to build up too much, you can permanently damage equipment.
So, keep up on your equipment and the quality of the oil powering it and you’ll be in good hands.
Hydraulic oils are an integral part of the modern world. From construction to manufacturing, hydraulic systems are used in most of the things which make civilization possible. Because of that, hydraulic oils are actually one of the unsung foundations of the society.Whether you’re an operator or just curious, understanding these oils is a fascinating subject and can come in quite handy if you ever find yourself in the position to be replacing the oil in a machine.