Apart from the build-up of excessive heat in fluid power systems, failures as a result of contamination are at the very top of any hydraulic engineer’s list of potential causes. More specifically, contaminants as hard or harder than the material makeup of the system components themselves is the root cause of excessive wear. Yet conventional filtration can let them straight through to system components where it can do the most damage,
Sieve/barrier or interception type of filtration, whether it is 5 micron absolute filter or a 200 mesh screen, all work primarily by doing little but sieving contaminants. Particles smaller than the pores (voids) in the filtration medium pass straight through and of course, the pores cannot be small enough, as they interrupt and restrict fluid flow too much.
Engineers rely on standard Beta Ratios and ISO Tests when specifying conventional filters, which have been continuously developed and redeveloped over the years to meet these same standards (test parameters). But the tests are not truly representative of the real life environment in which these filters are applied.
Within the laboratory environment, filter ISO testing is typically based on a percentage of contaminants captured under a steady state (constant) flow rate. The contaminant introduced is a standard ‘dust’ as defined within the ISO standard.
This cannot, of course, represent the real life characteristics in hydraulic and fluid power systems because it assumes that no valves, actuators, variable volume pumps or external loads are ever imparted on the hydraulic system. Any end-user or designer of fluid power equipment knows that a real-life hydraulic system is anything but constant flow.
Dynamic filer efficiency tests undertaken by Scientific Services Inc of Cary, North Carolina showed conventional filtration efficiency as low as 18% under varying pressure and peak flow. This is equivalent to a conventional element emitting large and concentrated clouds of contaminant downstream of the filter and headlong into the valves, cylinders and pumps.
Yet hydraulic engineers and customers remain puzzled as to why seemingly clean (as far as the ISO rating goes) pieces of equipment fail to perform or fail on the job without any apparent change in conditions. Clearly the real villain of the piece is getting past our conventional filters.
Microscopic ferrous contamination
80-90% of the products manufactured world-wide are manufactured from carbon steel. We all know that, during start up and break in, some of the components that are breaking in and being ‘worn in’ will be steel on steel. A large portion of steel (ferrous) contaminant becomes suspended in fluids during this period.
But wear continues, and these ferrous contaminants are going to be present throughout the life cycle of any fluid power system, where they are responsible for a ‘chain reaction’ in terms of wear - particles circulating in the system and generating additional particles, which, along with the original particles, generate even more particles.
These ferrous particles have been torn from a hard surface and are the smallest, hardest and sharpest contaminants in fluid systems. Few engineers would disagree that the mere suggestion of their presence in a fluid power system presents a major issue in terms of system design and reliability. Yet still many do not prioritise on identifying an effective and efficient technology to address the situation because these ferrous particles are typically under 15 microns in size and most are in the 5-10 micron range. Most conventional filter elements used today are 10 microns.
Every engineer would like to use a 1 micron absolute rated filter element in their system, but this is expensive, complex and impractical as there are limitations regarding the maximum allowable pressure drop across a filter element in order to maintain flow rates. This leaves almost every piece of high carbon steel under or near 10 microns in size to freely circulate around the system. Clearly, conventional filtration needs a bit of extra help!
‘Magnetic’ filtration
Most sophisticated hydraulic systems, particularly aviation hydraulics, already use some form of magnetic filter. There are several existing methods that use magnetism to deal with contaminants, including magnetic drain plugs, external filter pads and magnetic rod inserts. The main problem with these is that the contaminant has to be very close to the magnetic source in order to be attracted.
Even worse, as contaminants build up on the magnetic surface, these devices cannot hold on to them and they simply wash off back into the system. But they do not simply drop back into suspension within the hydraulic fluid. Since the particles have been exposed to a magnetic field, they generally remain as a clump, weakly magnetised together. This is why sudden catastrophic failures of single components occur in systems that appear otherwise to be in perfect condition.
An effective solution to this problem is to use magnetic core technology, designed and patented in the UK by Magnom Corporation. Magnom’s core technology was originally developed in partnership with the Formula 1 motor racing industry and in its basic form comprises a permanent magnet that is sandwiched between two carbon steel plates. Figure 1 shows a single element, but in practice several are used together (see product illustrations).
The steel plates intensify the magnetic flux field as steel transmits magnetic flux up to 15 times more efficiently than the magnet itself. They also project and focus this increase in energy directly into the fluid flow and provide a very secure means of trapping contaminant from the fluid flow. These ‘traps’ prevent captured contaminant from being washed back into the hydraulic system.
The fluid flow channels are cut directly within the steel to ensure that the total cross sectional area of the fluid flow channels exceeds the cross sectional flow area of any feed pipe or hose delivering fluid to the core by up to 15 percent. The configuration ensures that the cores deliver ‘minimal’ pressure drop even if they are full of contaminants. This unimpeded flow means, they can be located anywhere in the hydraulic system and in a ‘full fluid flow’ capacity - before a sensitive variable displacement pump for example.
In real life fluid power situations, core units have been observed to remove particles as small as 0.07 microns in size and to remove up to 97 percent of the particles in a single pass through the core. In other configurations and situations cores can approach closer to 100 percent efficiency on a single pass through the filter.
The bottom line
By isolating and removing the most damaging contaminant to a fluid power system, the magnetic core technology delivers dramatic improvements in the reliability and performance of systems, resulting in lower maintenance and warranty costs. Users can also expect longer hydraulic and lubricating fluid life, extended ‘conventional filter’ life, longer equipment service life and increased equipment availability
Jobey Marlowe is vice-president, technology, Magnom Corporation