By Jim Fitch
Machinery Lubrication Magazine
When 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 analysis is more important than this.
This article will address two common oil analysis-related problems that sadly are often dismissed by both users and laboratories. These problems are sampling oil from machines at rest (oil not circulating) and failing to properly resuspend particles just prior to analysis.