FERROUS PARTICLES: THE GREATEST THREAT
Almost all engineers will agree that the mere suggestion of ferrous particles in a fluid power system presents a major issue in terms of system design and reliability. Historically though, many engineers have focused on filtration as an auxiliary component in the design of hydraulic systems. Even further removed from thought is the actual material composition and size of the contaminants. This historical lack of priority and focus on system filtration, coupled with an absence of an effective and efficient technology to address the situation, means little has been done historically to address this long standing problem.
Ferrous particles are the “Smallest”, “Hardest” & “Sharpest” contaminants present in hydraulic systems and they can pass straight through conventional filter elements. This is usually due to two simple reasons: 1) Ferrous particles are very small,
2) Conventional hydraulic filter elements (and screens) are usually higher in their micron rating, than the engineer would like them to be when considering real life and practical hydraulic system designs.
First, it is no surprise that ferrous particles naturally come from ferrous components. The component surfaces of ferrous (steel or iron) materials are very hard. Ferrous is so hard in fact that in Rockwell Hardness Tests steel and iron utilize a completely different scale than aluminium and brass alloys. The inherent hardness of steel means that steel (ferrous) surfaces do not give up pieces of themselves easily. This means that high carbon steel particles are actually torn from these surfaces in only very tiny pieces.
These ferrous particles are typically under 15 microns in size and most are in the 5-10 micron range. The particles are not only very small, but a result of being torn from a hard surface they are also very sharp and angular. This means that by their very nature, ferrous particles are generated in the perfect size, shape and hardness to generate more ferrous particles and wear metals. These wear metals are typically brass and aluminium from other hydraulic system components.
Figures 4, 5 and 6 are electron microscope images of ferrous particles removed from real life hydraulic systems in the field. Figure 6 depicts a scale of 10 microns (noted by the white horizontal bar). Note the quantity of particles in the 15 microns size and less. It is a vast majority, if not all of the particles present. The images were processed by the Department of Science and Engineering at Liverpool John Moors University.
The second reason is a practical result of simple physics, engineering design and cost. Most hydraulic system filter elements, especially in mobile equipment, tend to be 10 micron absolute in their filter rating. The term ‘absolute’ has limited scientific relevancy in this case as the filter could allow particles well above and certainly below 10 microns to pass through without impeding the particles movement. It is one of the reasons that filters are rated on their ability to capture contaminant under ‘multi-pass’ conditions rather than solely ‘single-pass’.
Limitations in hydraulic system design (cost versus performance) arise from the maximum allowable pressure drop across a filter element and the need to maintain certain flow rates. Every engineer would like to use a 1 micron absolute rated filter element in their system, if only the selection didn’t require a large increase in cost, complexity to the overall design, and a large space claim requirement.
Since most ferrous particles are in the 5-10 micron range and most filter elements used today are 10 micron absolute, this means that many of the ferrous particles are left unchecked to freely circulate throughout the hydraulic system. Conventional filter elements can conceivably remove particles of say paint, o-ring, dust and wear metals because these are generally ductile and therefore larger in size than ferrous particles. This leaves almost every piece of high carbon steel under or near 10 microns in size to freely circulate around the system.
As these ferrous particles circulate around the hydraulic system time and time again, they generate more contamination & other particles in the process described previously as the ‘chain reaction of wear’. The particles created are either additional ferrous particulate (only ferrous particles can create other ferrous particles) or they are a combination of ferrous and non-ferrous wear metal particles.
When this condition is considered along with the ‘wash through’ or retention efficiency condition highlighted in the data provided by SSI’s DFE analysis, then the impact that ferrous contamination has on a system is detrimental and obvious. This is why every hydraulic engineer must consider not just the size of the contaminants present in the system, but the inherent threat level posed by each particle type and size.