For 200 years, hydraulic systems have formed the backbone of much of the technology that powers the way we live and work, from powering heavy-duty construction machinery to an aeroplane’s control surfaces. An example of a common application is the braking system in your car.
A hydraulic system is guided by Pascal’s law, which states that pressure applied at any point to an enclosed fluid is transmitted, undiminished, throughout that fluid.
First invented by British mechanic Joseph Bramah, the concept of a hydraulic system has evolved from a simple hydraulic press to the highly sophisticated systems we see at work today. Modern hydraulic systems use smaller components that must work at higher pressures and speeds in a range of environments far exceeding the applications of Bramah’s invention in 1795.
As the expectations on hydraulic systems have increased, so too have the expectations on the hydraulic fluid powering those systems. We often refer to hydraulic fluid as the 'lifeblood' of hydraulic systems because it not only transmits power through the system but also transfers heat, lubricates and cools bearings as well as maintaining pH, removing water, oxygen and other contaminants. All this ensures that hydraulic systems are sufficiently protected and work efficiently.
Ineffective hydraulic fluid can be both costly and harmful. Expensive machinery can be damaged beyond repair. Processes can run inefficiently or at a lower capacity than expected. Unexpected repairs and maintenance can halt production and cause unplanned downtime. In some cases, ineffective hydraulic systems can be a safety hazard for the people who operate them.
Choosing a high-quality hydraulic fluid is an important decision, and one that should not be rushed. In the past, manufacturers have relied on international standards such as ISO6743-4, ISO 15380, ISO12922 and DIN 51524. However, these are increasingly unable to reflect the varied demands on hydraulic systems and, in some cases, only cover the bare minimum requirements needed for effective hydraulic fluid. Furthermore, it is difficult to ascertain from these standards whether a certain fluid is suitable in a particular application.
Companies such as Bosch Rexroth are working to provide a database of independent hydraulic fluid ratings backed by extensive field testing. However, taking a few of the following factors into account will help when making your decision.
What kind of fluid do you need?
The range of hydraulic fluids has significantly expanded in recent years. Most hydraulic fluids are oil-based, the most popular of which is mineral oil. Synthetic, monograde, multigrade and water-based fluids are also available. In addition to their base fluid, hydraulic fluids can possess particular properties depending on how and where they are used. Their properties are determined by the additives in the base oil. For example, hydraulic fluids can be fire-resistant for hot metal-working, or biodegradable for maritime and forestry applications.
Certain fluids will also work best with particular materials within the systems themselves. For example, water-based hydraulic fluids require the hydraulic system to be specifically designed using components and materials. They are also not suitable for high-intensity applications because they offer less protection against corrosion and wear than an oil-based fluid.
With so much choice, it is important that engineers think carefully about the setting, application and component materials of their hydraulics. In order to choose the right fluid for the job, a fluid must be tested in operating conditions that match the “real-life” conditions as closely as possible.
Look beyond the packaging
“It does exactly what it says on the tin” may be a popular maxim for garden paint, but when it comes to hydraulic fluid, it’s not entirely accurate. The information present on hydraulic packaging still largely relies on international standards, but knowing whether or not a fluid meets minimum requirements is no longer enough. For example, does the fluid work adequately with your pump design? Does it meet the necessary viscosity index for your application? Does it meet with the legal requirements for its final environment, whether that is another country or an area of sensitivity, such as a nature preserve or environmentally-protected landscape?
It is important to look beyond the packaging and find concrete data concerning the real-life operation of the fluid. Remember, however, to take the manufacturer’s recommendations into account, as not doing so could affect warranties. If unsure, always consult the manufacturer and a technical specialist before making a decision.
What is your desired end-result?
Most people have three key expectations of their hydraulic fluid. Firstly, the fluid should last for as long as possible. Replacing hydraulic fluid is expensive, therefore it is difficult to achieve significant return on investment if a fluid needs changing every few weeks.
Secondly, they expect the fluid to extend the lifespan of their equipment by reducing the corrosion or build-up of residue in core components of the hydraulic system. Thirdly, they expect hydraulic fluid to optimise the efficiency of the systems by helping them to work smoothly, with increased capability and minimal need for operator intervention.
On top of these three requirements, users may need their hydraulic fluid to reduce static or have as little impact on the environment as possible when being disposed of. Again, users must look at their options and gather as much data as possible in order to match a fluid to its environment and applications.
Maintaining hydraulic fluid – from the inside-out
Once you’ve chosen the right fluid for the right job, it’s important to maintain both the fluid in use and the fluid in storage in order for it to perform correctly. Hydraulic fluid has a limited lifespan and any additives within the fluid will decrease as the fluid is used. This also means that its ability to protect hydraulic systems from wear, excessive temperature, or contaminants will decrease.
Oil that is no longer performing at its best may harbour small particles of contaminant, or a surface layer of water. In these cases, the fluid must be treated with an additional offline filtration or dewatering unit.
Oil only needs to be changed once the additives cease to be effective. This is often indicated by a change in the oil’s colour or odour, which occurs before the oil starts to harm the systems.
Oil analysis is an important part of monitoring any hydraulic system and can increase the longevity of your hydraulic fluid. If you have the right tools and expertise, oil analysis can be done as part of your in-house maintenance programme. If not, it can be undertaken via an external laboratory.
In addition, it is important to store any hydraulic fluid correctly to avoid contamination or loss of quality. Stored fluid should also be sampled regularly to make sure that it is still fit to be used.
Knowledge is power
Picking the right hydraulic fluid shouldn’t be left to chance. After all, you wouldn’t put unleaded petrol in a diesel car. The hydraulic fluid you choose should always exceed the minimum requirements and work as an efficient part of your system that ensures the productivity and longevity of the machinery.
Choosing the right hydraulic fluid may seem daunting, but it doesn’t have to be. Backed by sufficient data, as well as a thorough understanding of your requirements, finding the right fluid for the right job can be straightforward. Expertise and advice is always there when you need it, and databases that analyse a hydraulic fluid beyond its ISO mark are growing in number and reach.
Combine this with effective maintenance of the hydraulic fluid, and the “life-blood” of your systems can keep your hydraulics working for many years to come.
About the author: Kevin Follows is Technical Support Manager, Mobile Applications, at Bosch Rexroth