Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/490438
2 | March - April 2015 | www.machinerylubrication.com AS I SEE IT Jim Fi t ch | Nori a Corpor at ioN o i l A n a l y s i s W take Particle settlIng and oil sample AgItAtIon seriously hen you throw a rock in a lake, it goes down — fast. Wear particles are heavier than rocks of the same size, often four to five times heavier. Of course, the heavier the object, the faster it falls. Oil is viscous, and this resistance can slow down the rate objects fall, but it doesn't come close to stopping them. The rate at which objects fall in viscous fluids is described by Stokes' law. In sum, (1) the larger the object, (2) the heavier the object (density), (3) the thinner the fluid (lower viscosity), (4) the lower the density of the fluid (oil has extremely low density), the faster the object falls. Conversely, small, low-density objects in highly viscous fluids settle more slowly. In oil analysis, this is critical because you want to know about the particles in your oil — all of the particles, including those that can damage machines and those that reveal damage has already occurred and is continuing to occur. Not much in oil anal- ysis is more important than this. This article will address two common oil analysis-related problems that sadly are often dismissed by both users and labora- tories. These problems are sampling oil from machines at rest (oil not circulating) and failing to properly resuspend particles just prior to analysis. Analyzing the Data In reviewing research on particle sedi- mentation, I discovered that all of the data points to the same general conclusion: particles are very instable in lubricating oils, even those that are completely invisible to the naked eye. In Figures 1-4, you can see how Stokes' law governs the speed of particle descent. Figure 1 reveals that 30-micron iron parti- cles can descend at a rate of 2 centimeters per minute. The viscosity is rather low, but then again this is not uncommon for very warm oils and oils that have been diluted by solvents prior to lab analysis. Figure 2 shows that iron particles settle at a rate roughly five times the speed of dirt particles (silica). In Figure 3, the data indi- cates that barely visible 100-micron steel particles can fall 4 inches in viscous 15W40 motor oil in less than one minute. Finally, in Figure 4, you see that extremely small 20-micron Babbitt particles (say, from journal bearings) settle ½ inch in just four minutes in ISO VG 32 turbine oil. Dormant Fluid Causes Unrepresentative Oil Samples Too often oil samples are taken when machines are at rest (not when they are running). Sometimes this is avoidable, but sometimes it is not. Live oil samples are always best. Circulation keeps the fluid homogenous at the time the sample is taken. Lack of fluid circulation causes particle settling and sedimentation (see Figures 1-4). The longer the delay between when a machine is turned off (stopping oil movement) and when the oil is sampled, the greater the number of particles not received in the sample bottle. Particles are like data. This data provides important information that can prescribe a needed corrective action. When particles settle out of the oil, you lose data. This lost data may prevent you from being aware of an abnormally high particle count or advanced machine wear condition. This would produce a false negative in the oil analysis results. This means the oil's condi- tion may be falsely reported to be better than reality. Evidence of particle sedimentation shows up in oil sumps and reservoir bottoms. Sampling the bottom of the sump or reservoir provides little help since the sludge and sediment that accumulate there Iron Particle Size (microns) Time to Settle 1 cm (0.39 inches) at Room Temperature* 0.5 1 day 1 8 hours 3 45 minutes 10 4 minutes 30 30 seconds *Oil viscosity = 12 centistokes (cSt) at 40 degrees C FIgURE 1 Particle Size (microns) Iron Particle (8.0 density), Time to Settle 3 Inches** Silica Particle (2.2 density), Time to Settle 3 Inches** 3 180 minutes 650 minutes 10 12 minutes 75 minutes 20 3.5 minutes 20 minutes 40 1 minute 5 minutes 100 - 1 minute **Oil viscosity = 5 centistokes FIgURE 2