Analytical Test Methods

An assortment of specific analytical techniques and procedures is available to the laboratory chemist for studying lubricants. These include:

• Gas chromatography (GC)

• Scanning electron microscopy (SEM) (for wear surfaces)

• Thermogravimetric analysis (TGA)

• Differential scanning calorimetry (DSC)

• Rotating Pressure Vessel Oxidation Test (RPVOT)

• Remaining Useful Life Evaluation Routine (Ruler™)

• Thin-layer chromatography

A brief description of the last five methods will be given here. These tests are not always standard and depend on the skill of the operator. Validity and usefulness of the results can be enhanced by direct comparison with results on a known material tested in the same way.

TGA is a promising tool for thermally separating certain greases into their component parts, for example:

Highly volatiles 50-150°C (122-302°F)

Medium volatiles 150-650°C (302-1202°F)

Combustibles 650-750°C (1202-1382°F)

Inert & ash Remainder

This ASTM (E 1131) method uses 10 mg of sample in the balance setup shown in Figure 5-7. The procedure is what might be termed a destructive distillation. A heating cycle is programmed over 25 minutes from 50 to 750°C (122-1382°F), first in nitrogen and then in air. The residue number correlates with the gelling agent content of metal-containing greases (most products). It doesn't work with ashless greases such as polyurea-gelled materials.

Tests and Analyses

Figure 5-7

Schematic of TGA Setup

DSC is a thermal analysis technique that measures the heat flow associated with certain physical

and chemical changes in a lubricant. Of most interest is stability to oxidation, which shows up as the time delay to the onset of the oxidative exothermal reaction. The method uses a few

milligrams of the test lubricant in the apparatus shown in Figure 5-8. One copper (catalytic) cell or pan on a sensitive thermocouple contains the lubricant and the other is an empty reference cell on its own thermocouple.

Figure 5-8

Schematic of DSC Apparatus

The apparatus is placed in an oven heated to a set temperature, for example, 200°C (392° F). Oxygen is then passed over the two cells. When the antioxidant in the sample can no longer afford protection, oxidation of the lubricant takes place and is detected by a temperature rise in the cell containing the sample. Results correlate roughly with those from the standard RPVOT (Rotating Pressure Vessel Oxidation Test) method, ASTM D 2272 (see Table 5-5).

HP-DSC is a variant of DSC in which the oxidation takes place under 500 psi pressure. This

reduces volatilization of the test sample during thermal stress. This ASTM Method (D 5483) is more complex than the atmospheric method and is not needed unless volatile components are involved.

RPVOT (ASTM D 2272), formerly RBOT (Rotating Bomb Oxidation Test), measures the

oxidation stability of turbine oil and is a principal way to determine the remaining useful life of the oil. Oils deteriorate through oxidation, which, if allowed to go too far, results in deposit formation and ultimate equipment failure. The traditional test for evaluating turbine oil oxidation stability has been the ASTM D 943 test. This test is unworkable for maintenance evaluations of turbine oils in service. For example, some top-of-the-line turbine oils can go over 20,000 hours to reach the end point; over 8,000 hours is common in this top group. Lesser quality “standard” materials go for 3,000-5,000 hours. These are very long times for research and evaluation. The RPVOT overcomes this difficulty. Top oils reach the end point in only 2,000 minutes; second quality materials go for 400-600 minutes. The RPVOT is run at 302°F (150°C) in a stainless steel vessel with water and a copper metal catalyst present. The vessel is pressurized with oxygen to 90 psi (620.5 kPa) at 77°F (25°C) and the end point is to a drop of 25 psi (172.4 kPa) in the oxygen pressure. Good correlations between TOST (ASTM D-943) and RPVOT have been made. For these results and more details, see Note No. 8, NMAC Lube Notes, October 1999.

RULER™ is a technique to measure the antioxidant levels in lubricants. The small, hand-held

device employs a cyclic voltameter to measure the electrochemically active species in the lubricant (see Figure 5-9).

Tests and Analyses

The choice of solvent in the RULER™ test depends on the oil and additive types. For example, esters are polar materials that are soluble in polar solvents, for example, acetone. Hydrocarbons are not in this category. Also, there are three main types of oxidation inhibitors to consider:

• Aromatic amines

• Hindered phenols

• Metallic dithiophosphates

Each type can require modification in technique and/or solvent.

A varying voltage (0.0-1.0 volt) is applied to the electrodes in the prepared sample. Current flow between the working and other electrodes at certain potentials is a function of type and

concentration of the additives. Current flow changes as electrochemical oxidation of the

additives takes place at the working electrode. The current flow creates mounds (rounded peaks) in the current/applied voltage curve. Figure 5-10 illustrates this for three oxidation inhibitors. Note that all three can be picked up by this method at the same time.

Figure 5-10

Example of Three Additives and Voltammeter Response

The heights of the rounded peaks relate to additive concentrations. Values for the fresh lubricant are used as the 100% standard; the values for the solvent/base oil/electrolyte alone serve as the 0% standard. Various in-between points can be arrived at accurately by testing known

concentrations of the additives in question in base oil. With careful work, repeatability of +/- 5% is claimed for determining the percentage of remaining antioxidant. For more details, see Note No. 5, NMAC Lube Notes, July 1995.

Thin-Layer Chromatography (Herguth Laboratories, Inc.) Chromatography is a technique

for separating a sample into its components for study. This separation involves two mutually immiscible phases, one of which is stationary. The latter is sometimes a solid, as in thin-layer chromatography. The stationary phase is attached to a solid support material, for example, a plate. The sample, when dropped or smeared on the coated plate, moves across or through this stationary phase by capillary action and is separated by the differences in the chemical and physical properties of the components. These differences also govern the rate of movement or migration of the individual components. The components emerge or are eluted from the system in the order of their interaction with the stationary phase. This technique is called radial planer chromatography. Separation of components occurs through adsorption or similar processes. The Blotter Spot Test (Section 5.3.2) is a simple version of this chromatography. However, the blotter test relies only on diffusion around an initial spot on blotter or filter paper. There is no special solid phase. Figure 5-11 illustrates radial planer chromatographs of fresh and used gear oils.

Tests and Analyses

Ideally, a reference oil is tested to establish the baseline of fresh, clear new oil. Used oils from the machine are then spotted on the chromatographic substrate at regular, time-based intervals. Changes in the appearance of the bands/zones are a clear indication that something has changed in the machine or oil. A close look at the zones with the unaided eye or, if needed, with a 10- power magnifying glass can even be correlated with the ISO Particle Code, water contamination, or wear debris. As with most analytical methods, this method is not a predictor of future

performance, but rather is a measurement of the situation at the time of sampling. For more details on this topic, see Note No. 5, NMAC Lube Notes, November 2000.

5.4 Using Test Results

A single test result on a lubricant cannot be considered as definitive, even though it might be on a properly collected sample with a carefully performed procedure. This is because of the inherent variability of any test. The only available statistical appraisal of test precision is for ASTM procedures. That is one of the reasons they are so widely used when standard tests are required. Table 5-5 lists these data for some key procedures. To cite an example, the repeatability of the l/4-scale grease penetration test, D 1403, is listed as 11 points. This means that 95% of the time a result by this procedure will fall within +/- 11 points of the true value. Some 5% of the time the result will fall outside this envelope. So, +/- tolerances are attached to any result.

When a result seems outside the acceptable variability band, begin again with a new sample and a new test. If the second result checks the first, it might truly be showing a problem. The cause of this problem should receive attention. If the two results diverge, additional testing is needed to resolve the disparity.

Non-standard tests (that is, test methods without an ASTM-type statistical matrix) present a different picture. There, test credibility needs to be established. Replicate tests will determine repeatability, but not reproducibility. However, repeatability data should be enough to allow reliable tests. Merely compare results on an unknown or used lubricant with those for fresh material. Use this approach particularly with shortened or special tests conducted at low cost by many commercial laboratories.

5.5 Trending

Enough cannot be said for having in hand samples of the unused lubricant that was

originally charged to a piece of equipment under study. At the least, one should have stored data on tests run on the fresh material when it was introduced. Then, as data are obtained on

used material, they can be compared directly with the stored information. Essentially all test procedures profit from such comparisons.

A lubricant analysis program should feature periodic sampling and testing. The test data can then be plotted on a continuing graph, as in Figure 5-1210, to establish a trend. Place test variable limits above and below the trend line. Thus, minor variations will not affect the equipment operation but results outside these limits will be noticeable. If, on resample and retest, the results

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return to the trend line, all is well. If not, then a problem exists that needs attention. At some point, the trend line will break away and a preset warning limit or flag is up. When this occurs, retest to verify results, then go for changeout, makeup addition, adding inhibitor, etc., and study further if needed. Note the warning limit line in Figure 5-12.

Figure 5-12

Sample Plot of Lubricant Properties

5.6 Warning Limits

Classic warning limits for oils and greases are given in Table 5-711. The following discussions about these limits are for clearer understanding.

• Determine which property test limit of the lubricant is the most critical and trend it. For example, for turbine oils this will be oxidation inhibitor content. The inhibitor is sacrificed in protecting the oil (or grease). When its concentration is reduced sharply, oxidation of the lubricant takes place to form acids and eventually polymers that increase viscosity. But acid formation and viscosity increases occur late; a decrease in inhibitor content is an early warning sign or leading indicator of deterioration.

• Sometimes accelerated performance tests are needed to assess remaining performance properties. For example, DSC or RPVOT (ASTM D 2272) are useful for antioxidation

Tests and Analyses

Table 5-7

Typical Warning Limits1 for Certain Lubricant Services

Service Oils Greases Property Diesel Engine Steam/ Gas Turbine Hydraulic System Gear Air Compressor Bearings, Gears, Actuators Appearance Color/Odor

Unusual Change from Original

Wear Metals Content

By Emission/Absorption Spectroscopy

Unusual Change from Original

Calcium Content By Absorption, ASTM D 4626T, ppm, Max. NA4 20 NA NA NA NA Consistency Viscosity at 40°C (100°F) (ASTM D 445), Change, %. Max. Penetration (ASTM D 217). NLGI Grade Change, Max.

10-25 NA 103 NA 103 NA 103 NA 103 NA NA 1 Grade (30 points) Water Content % Vol. Max. (ASTM D 95)6 0.2 0.05-0.25 0.05 0.03-0.15 0.1 NA

Total Acid Number

mg KOH/g, Max. (ASTM D 664)

NA 0.3 NA NA NA NA

Oxidation Inhibitor2

% of New Lube, Min.

NA 50 50 NA NA NA

EP Additive2

% of New Lube, Min.

NA NA NA 50 50 NA

Rust Test

Oil (ASTM D 665)

NA Fail Test Fail Test NA NA NA

Base No. mg KOH/g, Min. >3 NA NA NA NA NA Fuel Dilution Vol. %, Max. 3 NA NA NA NA NA Notes: 1

These warning limits are derived from past experience. No definitive studies have been conducted to ascertain these points.

2

These points can be determined by Infrared analysis. Also, the atomic absorption procedure can be used for EP additives.

3

This is not a sensitive criterion. Other limits should be used for early warning. If original viscosity is known, apply the 10% increase to it to arrive at the warning limit.

4

NA = Not Applicable.

5

Depending on the application.

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