**4. Important lubricant properties**

The development of a lubricant specification requires an examination of what properties are important and what methods will be used to demonstrate that the specification has been met. Many of the common properties, including the method used for the determination of the property, are shown below.

**Viscosity (ASTM D 445)**—Determines the thickness of the lubricant film that coats the bearing. Viscosity is measured at several temperatures and helps determine the maximum operating temperature of the engine. Changes in viscosity are observed as a lubricant degrades and these changes are a measure of lubricant health.

**Pour Point (ASTM D 97)**—The temperature at which a liquid becomes semi-solid or loses its flow characteristics. This property is related to the ability to service or start an engine in cold climates.

**Flash Point (ASTM D 92)**—The lowest temperature at which vapors of a fluid will ignite. Measuring a flash point requires an ignition source. At the flash point, the vapor may cease to burn when the ignition source is removed. This parameter is used as a way to control flam‐ mability, although understanding flammability requires a knowledge of many other param‐ eters.

**Total Acid Number (ASTM D 664)**—A titration-based determination of the amount of acids contained in oil. Acids in the oil may cause corrosion of the bearings and valve guides. In esterbased lubricants, the acid number can indicate exposure of the lubricant to water, resulting in the hydrolysis of the oil. It can also indicate incomplete reaction when the oil was prepared. As the lubricant is used, the acid number of the used oil is also be an indicator of lubricant degradation.

**Lubricant Compatibility (FED-STD-791, Method 3403)**—A determination of hot aging miscibility of the candidate oil with other oils that might be encountered when the engine is in service. This test is necessary due to differences in the additive packages that various manufacturers use to achieve the required performance standard.

**Elastomer Compatibility [Def Staf 05-50 (Part 61) Method 22]**—A test used to determine the swell and deterioration of seal materials when soaked in oil for an extended time. Some highstability basestocks and additive packages can cause the deterioration of certain seal materials over time.

**Oxidation and Corrosion Stability (ASTM D 4636)**—This test examines the bulk oil stability and the impact of breakdown products on certain representative metallic materials.

**Figure 8.** Thermal oxidation of PE ester at high temperature to give either anhydrides or an aldehyde and an acid.

are depleted.

climates.

eters.

**4. Important lubricant properties**

42 Recent Progress in Some Aircraft Technologies

changes are a measure of lubricant health.

property, are shown below.

The production of either an anhydride or an aldehyde is undesirable because they are reactive and have a tendency to polymerize, creating high molecular weight species. These reactions are eliminated in the presence of either BHT or an amine antioxidant until those concentrations

The development of a lubricant specification requires an examination of what properties are important and what methods will be used to demonstrate that the specification has been met. Many of the common properties, including the method used for the determination of the

**Viscosity (ASTM D 445)**—Determines the thickness of the lubricant film that coats the bearing. Viscosity is measured at several temperatures and helps determine the maximum operating temperature of the engine. Changes in viscosity are observed as a lubricant degrades and these

**Pour Point (ASTM D 97)**—The temperature at which a liquid becomes semi-solid or loses its flow characteristics. This property is related to the ability to service or start an engine in cold

**Flash Point (ASTM D 92)**—The lowest temperature at which vapors of a fluid will ignite. Measuring a flash point requires an ignition source. At the flash point, the vapor may cease to burn when the ignition source is removed. This parameter is used as a way to control flam‐ mability, although understanding flammability requires a knowledge of many other param‐

**Thermal Stability and Corrosivity (FED-STD-791, Method 3411)**—A quality control test for identifying contamination from non-aviation ester-based products.

**Deposit Control**—This test involves a full-scale heated bearing test rig. The test is run for 100 h, and the oil condition is evaluated and the cleanliness of the bearing is evaluated for the formation of deposits.

**Load-Carrying Capacity or Boundary Film Lubricating Ability (FED-STD-791, Method 6509)**—This property is measured with a Ryder Gear Test.

**Foaming Tendency**—Can be measured by either a static (ASTM D 892) or a dynamic method (Fed-STD-791, Method 3214). Oil can foam when subjected to the shear forces in the engine. Foaming is frequently due to impurities of various silicone compounds.

**Sediment or Particulate Contamination (FED-STD-791, Method 310 or 3013)**—This test measures the amount of suspended or dissolved solids in the oil. The low maximum ash content of MIL-PRF-23699 precludes the use of metal containing lubricant additives.

**Hydrolytic Stability [Def Stan 05-50 (Part 61) Method 6]**—Evaluates the storage stability and the stability of the oil in capped lubrication systems. Hydrolysis is a degradation reaction of the oil with water to regenerate the original alcohol and acids used to prepare the ester.

**Evaporation Loss (AST D 972)**—This test determines the amount of volatile components in the lubricant. A low evaporation loss means that too much oil will not be consumed during operation.

**Shear Stability (ASTM D 2603)**—Shear stability means that the lubricant will not lose viscosity due to the mechanical forces encountered in the operating engine.

**Corrosion Inhibition (SAE ARP 4240)**—Determined by ball corrosion testing, this attribute is only required for MIL-PRF-23699 oils of C/I class. Oils of the C/I class are primarily used in marine applications.

**Storage Stability Tests**—This test ensures that all of the lubricants remain miscible during extended storage periods of up to 3 years.

**Acid Assay (FED-STD-791, Method 3500)**—In this method, the component acids used in the preparation of the ester are measured. This is needed to ensure that different batches of the lubricant are similar to the original qualified lubricant.

**Trace Metals**—The trace metal content of new oils is controlled to provide a good baseline for the spectrographic oil analysis program.

**Spectrographic Oil Analysis Program (SOAP)**—The concentration of several wear metals (Fe, Ag, Cr, Al, Mg, Ti, Mo, and V) are measured regularly by atomic emission either using a rotating carbon electrode or by inductively coupled plasma (ICP-OES). As an engine is used, the concentrations of wear metals increase. If the normal wear pattern of the engine is known, changes in the metal concentrations that are not consistent with the pattern can be an indicator of impending problems. Excessive wear metals in the oil are an indication that some type of maintenance is needed and the metals present may provide an indication of the area of concern.
