Give your equipment lubricants the respect they deserve. Keep them well-filtered and clean!
By Ken Bannister, Contributing Editor
What often comes to mind when I think of lubricants is comedian Rodney Dangerfield’s signature “No Respect” line: Lubricants rarely get the respect they deserve until they are recognized as an integral part of a machine’s design.
Original equipment manufacturer (OEM) machine designers have long recognized and respected the value of choosing the correct lubricant for the job. They spend considerable time with lubricant-company engineers to choose the types of oils and greases that will adequately perform under given sets of ambient and machine conditions and allow equipment to function per design specifications.
The machine designer also recognizes that a lubricant is a consumable weak link in the design and, as such, is subjected to many stresses, including temperature and contamination that can significantly degrade the product’s ability to protect a machine. To combat temperature excesses, a lubricant with a suitable viscosity and viscosity index (VI) is chosen. To ensure the lubricant has a chance of an extended life cycle, a good designer will incorporate a variety of contamination-control devices in the lubrication-system design. Equally important in the lubricant life-cycle equation is the role of the equipment owner/operator, who also must recognize that an oil or grease is an integral part of the equipment design. Moreover, the owner/operator must be diligent in controlling contamination elements such as dirt and water around the machine—and care for the contamination-control devices that have been built into the machine.
Fig. 1. A typical recirculative oil-filtration system will usually have six to seven of the surface-type filters (numbered 1-7 in this diagram) along with one depth-type filter.
The primary contamination-control device used in an oil lubrication system is a fluid filter, which can be one of two types: 1) a surface filter or, 2) a depth filter. For semi-fluid systems using grease as a lubricant, only surface-type filters (known as strainers and mechanical filters) are used.
Surface-type fluid filters: oil
The most common style of oil filter in use, surface filters come in many configurations. They are primarily designed to work in the direct-flow path of the lubricant and capture any dirt particles (contaminants) held in colloidal suspension as the lubricant flows through or across the filter media. This filter element is typically very porous, the purpose of which is to minimize any differential fluid pressure loss across the filter media. As a result, this style of filter exhibits low capture-efficiency and dirt-holding capacity. In light of their moderate life expectancy, these filters must be inspected, cleaned or replaced on a regular basis.
The actual design and capture capability of the filter will depend on its location within the lubrication system and the desired functional effect of the filter. For example, in a typical re-circulative hydraulic or lubrication system a pump suctions oil from a lubricant reservoir and pumps it under pressure to a moving device such as a valve, cylinder or bearing. Once the lubricant has performed its job, it is allowed to return, under gravity, to the reservoir where it can cool and be cycled again. In a typical lubrication system (Fig. 1), we can generally expect to find six to seven of the following types of surface-filter designs, along with one depth filter.
Filter #1 is a suction filter positioned low in the reservoir and connected to the pump suction inlet via a suction tube. The filter media is typically wire-mesh gauze, paper or felt designed to capture and stop any large debris or metallic wear particles from entering the pump.
Filter #2 is a pressure filter positioned directly on the output-pressure delivery side of the pump—between the pump and the first moving device in the lubrication system. This filter is the primary system-protection filter and uses pleated paper, cellulose or fine porous-metal media designed to withstand the pump system pressure and capture small-micron particulate that has managed to move through the suction filter and pump-gear set.
Filter #3 is positioned on the gravity-return piping side of the lubrication system, just before the reservoir lubricant return inlet. Known as a gravity-return filter, this component generally employs a low-pressure paper medium deigned to capture wear metal and debris washed from the moving parts of the machine by the lubricant.
Filter #4 is a more passive filter designed to attract and hold any ferritic debris and wear metals that might bypass the return filter or that could have been in the reservoir from the start. This filter is magnetic and often serves a second duty as a reservoir drain plug.
Filter #5 is a large-pore metal-mesh strainer-sock positioned in the inlet mouth of the reservoir fill-port. This filter is designed to capture errant large particulate from making its way into the reservoir when filling is taking place or if the fill cap has been left off the reservoir.
Filter #6 is a breather-filter designed to equalize pressure in the reservoir. In its simplest form, this component utilizes a wire-wool media to prevent any particulate of 40 microns and above in the air from entering the reservoir. In a more sophisticated design, breather-filters employ a desiccant-like silica-gel hydrophilic material that allows the reservoir to breathe while preventing outside airborne particulate 3 microns and above from entering the reservoir. These advanced designs are also able to wick and capture moisture from inside the reservoir and prevent outside moisture from entering. Once saturated, the gel turns from blue to pink, indicating visually that the material needs to be changed.
Filter #7 is an optional filter found in larger systems. Employing a filter cart hooked up to the reservoir, it extracts oil from the reservoir using components similar to the previously described #2 pressure filter, or a thick felt-type sock in the form of a “bag” (i.e., a bag filter), to provide fine filtration of the lubricant. The clean lubricant is then returned to the reservoir for further use, thus extending the oil-change frequency.
Depth-type fluid filters: oil
A depth filter differs from a surface filter in that it takes the lubricant through an indirect maze-like flow-path designed to capture and accommodate large amounts of dirt. These filters are highly efficient and capable of withstanding high differential pressure. If the surface filter were to use a single sheet of toilet paper as its media, the depth-filter equivalent would be the entire roll of toilet paper!
Filter #8 is a depth filter that’s typically placed within a bypass circuit on the output-pressure-delivery side of the pump—before the pressure filter. The filter medium can be made from cellulose, fiberglass, felt or diatomaceous earth, designed to deep clean and polish the lubricant.
Fig. 2. Filter efficiency/Beta ratio is calculated as shown here.
Grease doesn’t typically flow the same way oil does, and it is pumped at higher pressures than oil. For these reasons, metal strainers or wedge-wire filters fitted to the pressure side of a pump delivery system are used to trap large debris that’s usually introduced into the system during filling. Although the mesh or wedge-wire (which resembles a tightly coiled spring) mechanically can trap contaminants down to 145 microns, it must be cleaned regularly. LM&T
Ken Bannister is a certified Maintenance and Lubrication Management Consultant for ENGTECH Industries, Inc. He is the author of the Machinery’s Handbook Lubrication chapters, and the Lubrication for Industrytextbook, recognized as part of the ICML and ISO’s Domain of Knowledge. Bannister also conducts formal preparatory training for ICML MLT/MLA certifications and ISO LCAT certifications. For more training information, he can reached at (519) 469-9173 or by email at email@example.com.