Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1005929
4 | July - August 2018 | www . machinerylubrication.com AS I SEE IT is small-particle dominance is compounded further by filtration. Most machines with circulating oil have filters. However, most filters remove parti- cles based on size exclusion. is means they don't remove all particles but rather just certain particles above specific micron sizes (based on the average pore size of the filter media). For simplicity, we can refer to this as the filter's particle size cut-off. ose particles larger than the filter's size cut-off are conveniently disposed of with each filter change. e particles smaller than the filter's size cut-off stay with the oil and in the machine. is results in a growing popula- tion of small particles that are uncontrolled by filtration. Because of their extremely small size, they also are not prone to settling (Stoke's law) but rather embed tightly into the oil, held by viscosity, circulation and Brownian motion (like food dye in water). To what size of particles are we referring? Well, if the filter has a 10-micron cut-off, then all particles smaller than 10 microns are ghost riders. By total weight, most of these particles may be submicron (i.e., less than 1 micron in size). ese include organic matter (soft, insoluble contaminants that can lead to sludge and varnish) as well as inorganic hard particles from environmental dust and wear debris. Remember, the human eye can see particles down to about 45 microns. erefore, we can confidently say that ghost riders are only visible with the aid of microscopes and similar laboratory methods. The Lurking Dangers of Small Particles Don't assume you're completely safe if you have a good filter, even if it is a high-capture-efficiency 3-micron filter. Yes, filters are important, and many deliver exceptional performance by mitigating the exposure and risks of particle contamination. When larger particles are quickly filtered from the oil, they can't damage machine surfaces and can't be crushed into small particles in the size range of our ghost riders. For sure, good filtration is essential to contam- ination control. e concerns and risks associated with small particles will vary depending on the type of lubricant and the type of machine. e following is a brief listing of the dangers and harm imposed on lubricants and machines from the abnormal presence of small ghost-rider particles: Polishing and Increased Mechanical Friction Many machines are exposed to periodic or contin- uous boundary lubrication. is means that, due to slow speed and/or high unit loading, the lubricant is unable to maintain an oil film. is results in surfaces that rub mechanically in sliding frictional zones. Wear is only controlled by the mitigating action of extreme-pressure (EP) and anti-wear additives. However, these additives do little to impede the abrasive damage caused by small particles, leading to polished and honed surfaces under common boundary conditions. e higher the popula- tion of these small particles, the more wear damage that results. Of course, where there is wear there is excessive friction, higher energy consumption and negative envi- ronmental consequences, all of which are not good. Silt Lock Small particles are commonly referred to as silt-sized particles or just silt. ese particles can jam and pack into narrow oil ways, glands and orifices. ey can restrict oil flow, leading to lubricant starvation, and impair mechan- ical movement (e.g., in a servo valve), causing motion impediment. Additive Tie-up A high density of small particles exposes the oil to an extensive amount of surface area (the collec- tive outer-shell surfaces of all particles). Many of the lubricant's additives are polar, meaning they are natu- rally attracted to both machine and particle surfaces. Examples include friction modifiers, dispersants, rust inhibitors, metal deactivators, detergents, anti-wear and extreme-pressure additives. When these additives hitch a ride on particles, they lose their functional value to the oil and machine. e particles occupy (tie up) this role instead. e scrubbing of additives by small particles is a common additive depletion mecha- Figure 2. Many additives are attracted to par- ticle surfaces and will adhere tightly. When the particles later get removed by filters or settle to sump floors the additives are stripped from the oil. Those particles that stay suspended in the oil keep the additives occupied and unable to perform their intended function. dispersant antiwear additive detergent rust inhibitor metal deactivator