Filters are a small but vital part of hydraulic machinery, although serving no mechanical purpose they are integral to the proper functioning of the machine and to prolonging the active life of the equipment and the hydraulic fluid which drives it. The role of a filter is to remove and/or contain particles of contamination and keep the fluid that flows round the machine clear. Any contamination in the oil can cause damage to moving parts further downstream, and this can lead to seizing up of moving parts, corrosion of other parts and costly replacements. When we consider the consequences of not using a filter, the importance of these little items becomes more significant. There are two main styles of hydraulic oil filter – surface and depth filters. The surface filter, as the name suggests, removes contamination from the surface of the oil and may be useful in applications where gravity feeds the oil through a space suitable for such a filter during operation. A depth filter can be submerged in a reservoir or chamber and will remove particles from the entirety of the body of fluid. Depth filters therefore, are more effective and will retain a larger amount of contamination and unwanted particles before they need to be cleaned or replaced. The materials used to make hydraulic filters varies, as does the cost accordingly. Glass filters are more expensive but they are more efficient, especially when glass fibres are used in a depth filter. Glass is also non-reactive, meaning it can be used with any type of hydraulic fluid. Metal filters are also reasonably efficient, but they cannot be used with all types of hydraulic fluid, due to incompatibilities between certain hydraulic oils and some metals. Where it is appropriate to use a metal filter there is the option of a magnetic filter system, which works as a depth filter and uses a magnetic charge to attract metal particles that may have entered the fluid system. If the hydraulic equipment is used in metal working and fabrication then the chance of potential contamination being from metallic particles is high; magnetic filters will deal with this contamination while another type of filter can be used to deal with other types of pollution. Cellulose or paper can also be used to make a hydraulic fluid filter, but these have a short life expectancy and need frequent replacement. They are cheaper than the other types, but could end up costing more in the long run, due to regular downtime for replacement and the potential damage to machinery if they are not replaced often enough. It may seem like glass is the best choice; despite costing a little more it does the best job and will not need replacing too often. The drawback of a glass filter is actually its strength as well; the superior filtering ability can actually lead to a drop in pressure, in the system, which is usually undesirable. It is especially important, therefore, to check how a drop in hydraulic system pressure will affect productivity and performance if you are considering switching to glass filters from another material type. Consider also whether this drop in pressure is likely to lead to unnecessary adjustments of pressure relief valves, leading to potentially dangerous build ups of pressure elsewhere. The options of hydraulic oil filters do not end after material type and construction, as there is still the issue of ratings and specifications to contend with. All filters have an ISO 4406 rating, and the lower the code of any given filter, the better it is at removing contamination. Hydraulic filters may also have a beta ratio, which is the ratio of particles found upstream of the filter divided by the number found downstream. For beta ratios, the higher it is the better, and this can also be used to give a percentage for effectiveness. It is also important to check that the flow rate of the filter is compatible with the flow rate of the machinery it is to be used in; too fast and the filter will not be able to effectively remove contamination, too slow and it could become clogged quickly. The operational pressure of the hydraulic system is also important, as the filter must be able to withstand that force for a prolonged amount of time. The final thing to check is that the filter can be connected to the equipment, so check the port size of the filter and ensure it is compatible with the machinery in the location it is to be fitted. There is a lot to consider here, but choosing the most suitable hydraulic fluid filter is an exercise that it is worth spending some time on, as it can make a big difference to the performance and lifespan of your hydraulic equipment and save on downtime and replacement costs in the long run.