Lubricant-Health Monitoring: What We Test For

Keeping contamination and wear-particle levels in check is key to healthy lubricants and lubricated equipment.

Lubricant-health monitoring provides the maintenance department an inexpensive predictive method for understanding the condition of a lubricant. Performing a comparative laboratory analysis of a used-oil sample against its identical virgin-oil sample allows the lab to determine what is in the oil that should not be there, i.e. contamination and wear-particle levels and types (see Table I). This change in the oil’s signature allows maintenance to trend what is happening to the lubricant and to determine a suitable course of action well in advance of bearing failure, and to optimize the oil change/service interval.

Also known as oil and wear-particle analysis, lubricant-health monitoring traces its roots to the 1940s when North America’s railroad industry required an effective and inexpensive way to test engine wear in its new electromotive diesel engines. Using similar spectrographic testing methods as the railroads, the U.S. military performed wear-particle analysis testing on its jet and internal-combustion engines throughout the 1950s and ’60s—which led to the first commercial/ industrial laboratory in the early 1960s.

When subjected to adverse temperature, oxygen (air), contamination and combustion gases, oil will eventually degrade and become less effective in service. Degradation is recognized through viscosity change, additive-package loss, particulate increase (contamination) and oxidation. It’s up to maintenance personnel to collect a representative lube sample, package it and send to the laboratory in a timely manner. Upon receipt of the sample, the laboratory performs a series of tests according to how the lubricant was used in service, and sends a report to the maintenance department.

Elemental Spectrometry

The most popular oil-analysis method, Elemental Spectrometry tests for the concentration levels of wear metal, additive metal and contaminant metal particulate. Trending these levels over time against the known metallurgy of the bearing surfaces and acceptable metal levels inherent in the virgin oil will alert the maintainer of any pending problems. Sharp increases in levels will usually indicate rapid bearing failure or cross-contamination from the introduction of solids contaminants or use of different oil.

Two types of spectrometry-testing instruments (spectrometers) are commonly found in today’s labs: atomic and emission. Of the two, Atomic Spectrometry is the most widely used method. (Spectrometry tests can measure wear particles up to seven microns in size.)

In an Atomic Spectrometry test, a diluted sample of oil is atomized in a 2300 C (4172 F) acetylene flame. This causes metal ions to release photon energy as the metal-particulate atoms absorb light at different wavelengths (change color). Performing a computer comparison against known metal-particulate light-wavelengths, the atomic spectrometer is able to determine what metal wear elements and particulate are present and in what quantity.

In an Emission Spectrometry test, a 15,000-volt (or higher) charge is used to excite the particulate, causing any impurities to emit a characteristic radiation signature that the Emission Spectrometer can measure and analyze.

Analytical Ferrography

Ferrographical Analysis is a visual technique used to determine the size and shape of ferrous (iron-based) wear metals in which wear particles are magnetically separated from the oil on an inclined glass slide. This slide, known as a ferrogram, is then viewed under a bi-chromatic microscope to classify the concentration, shape and size of the different particulate. Knowing the metallurgy of the bearings and the shape and size of the particles, the analyst can determine the wear rate, type of wear that has taken place (rubbing, sliding, rolling, cutting, etc.) and how the particles were formed.

Microscopy

In Microscopy, an oil sample is run through a 0.8-micron filter, and captured particulate is examined under a 100x to 200x optical microscope. This method shows all particulate matter found in the oil, including non-metallic impurities. In addition, other methods are used to monitor the overall state of the oil. The most important are tests for Viscosity, Water Content, Total Acid Number (TAN) and Gauging the Particulate Count.

Viscosity

As defined in the ICML Domain of Knowledge Element 4, “What’s in a Lubricant: Mineral Base Oil and Its Characteristics” (see Lubrication Management & Technology, August 2013), Viscosity is the measure of resistance to flow. Industrial oils are typically measured for viscous flow at a temperature of 40 C (104 F) in a device called a viscometer. A specific amount of oil is poured through this open test-tube-styled device and timed against pre-determined timetables for specific Viscosity ratings: The shorter the time, the less viscous (or thinner) the oil is. The greater the time, the more viscous (or thicker) the oil is.

Lower-than-specification Viscosity can mean water dilution of the oil or the addition of less viscous oil. Higher-than-specification viscosity can mean oxidation (sludge) has taken hold or that more viscous oil has been added.   

Water Content

Water in oil promotes rust and corrosion, and in a dissolved state will accelerate oxidation. Moisture can be introduced as contamination through washdown of the equipment or through leakage.

Testing for Water Content is usually performed using a Karl Fischer titration moisture test that vaporizes the sample that is then carried by oxygen-free nitrogen into a reaction vessel-containing methanol. The trapped water is then titrated to an end point with a reagent to determine the amount of water present in parts per million.

Total Acid Number (TAN)

Oils that have had their antioxidants depleted will quickly oxidize and build up corrosive acids harmful to the lubricant and the lubricated parts. Checking for acid build-up can decrease the oil’s level of serviceability, allowing the oil to be changed before it can cause harm. The TAN is measured using a titration method that dilutes the oil sample with an alkaline solution until a neutral endpoint is achieved. Virgin industrial oils usually have lower TAN values around 0.5, whereas automotive engine oils are much higher, at values close to 1.5.

Gauging the
Particulate Count

Based on ISO 4406: 1999 Solid Contamination Code Suitability, a particulate count can be manually performed using an optical microscope to determine the number of particles in a 100ml sample >5 microns in size and the number >15 microns in size. The final count can then be found in the ISO 4406 Particle Count Chart (shown in Table II) to get a two-number ISO cleanliness rating.

Another technique for gauging particulate counts involves an automatic particle counter. This uses a sensor to detect and count particles based on light-absorption principles. In this method, ISO 4406 calls for three sample counts sized at >4 microns, >6 microns and >14 microns.

Example: If the particle count came to 1030 particles >4 microns, 286 particles >6 microns, and 70 particles >14 microns, using the particle count chart’s corresponding values, a 17/15/13 ISO cleanliness rating would indicate an oil with “light contamination.”