Usage:
The Caterpillar Scheduled Oil Sampling Program was developed to help Caterpillar users realize the highest possible value from their equipment by minimizing repair costs and maximizing availability. The S.O.S program is a series of diagnostic tests which analyze the lubricating oils used in the equipment. By analyzing the oils, problems may be identified early, before an extensive component failure occurs. This reduces both repair cost and downtime.
An S.O.S program must be coupled with a wide range of repair options so that when a problem is identified, an appropriately matched repair plan is available. When S.O.S is used to detect a repair indicator in a repair option program, the dealer can offer the user a more complete service to minimize repair cost and downtime. S.O.S can also measure the effectiveness of the user's maintenance program.
The purpose of this article is to review the specific diagnostic tests in the S.O.S program, summarizing the capabilities, limitations, and relationship of each test. Oil analysis tests not included in the S.O.S program will also be discussed.
Wear analysis is the original diagnostic test which began the S.O.S program. It is still the foundation of the program. Wear analysis monitors the component's (i.e., engine, transmission, hydraulic systems, final drives, etc.) wear rates by measuring the wear elements and contaminants found in the used oil. By monitoring the used oil, component wear trends can be determined, and many failures can be identified when wear elements and/or contaminants significantly exceed past trends. In many cases the source of abnormal wear may be pinpointed (i.e. high lead and aluminum readings may indicate a pending bearing failure).
The failures which can be detected are those caused by component wear and gradual dirt contamination. Wear analysis is not able to predict failures due to component fatigue, sudden loss of lubrication, or sudden ingestion of a large amount of dirt because these situations occur too rapidly to predict.
The atomic absorption (AA) spectrophotometer has been the only equipment recommended. This spectrophotometer can accurately measure the microscopic particles (i.e., less than three microns) suspended in the lubricating oil. These particles are small enough to pass through the filter. This spectrophotometer cannot measure particles which are large enough to be visible.
The Inductively Coupled Plasma (ICP) and Direct Coupled Plasma (DCP) also are capable of performing wear analysis. In the past, this type of spectrophotometer has not been recommended for dealer use because the equipment is considerably more expensive and it was uncertain whether the wear limits developed for the atomic absorption analysis would be applicable. Recent development work has proved that the existing atomic absorption wear limits can be used for some atomic emission analysis machines. For high volume labs, the atomic emission spectrophotometer may be the best choice because it is much faster. Before an equipment decision is made, a complete financial analysis is recommended. (ICP and DCP equipment can cost 30-40% more than AA equipment, but also provides a 30-40% reduction in analysis time.)
Chemical and physical tests are used to determine whether the used oil has been contaminated with water, fuel, or antifreeze and whether these contaminants exceed maximum limits. These tests are recommended for any oil-washed compartment where water, fuel or antifreeze contamination is possible.
The presence and approximate amount of water in oil can be detected by placing a drop of the oil sample on a hot plate. If bubbles appear, water is present in the oil (i.e., 0.1% or more; maximum allowable is 0.5%). The presence of fuel in the oil is determined by the Setaflash Tester. This tester is calibrated to identify oil with 4% (maximum allowable) or more fuel dilution. Oil contamination with antifreeze (i.e., ethylene glycol) can be qualitatively identified through a chemical test (any amount of antifreeze is unacceptable).
These tests cannot determine the exact percentage of the contaminant, but they can identify whether the contaminants exceed maximum limits.
Oil condition analysis, performed through infrared analysis, is a diagnostic test that is a recent addition (1979) to the S.O.S program; it is a Caterpillar application for Caterpillar product only. Infrared analysis determines the degree of deterioration in used oil by measuring the amount of sulfur products, oxidation, nitration, and soot present and the extent of the antiwear (zinc dithiophosphate) additive depletion. Also, it detects the presence of oil contamination from water, antifreeze (i.e., ethylene glycol and Dowtherm) and butyl cellosolve (i.e., cleaning solution). Infrared analysis should never be used alone. It should be used in conjunction with wear analysis and the chemical and physical tests to ensure accurate interpretations and recommendations. By monitoring the used oil condition, a more thorough indication of a failure and its cause can be identified. Infrared analysis answers some of the questions raised by wear analysis (i.e., causes for bearing wear, ring sticking, transmission slippage, etc.)
Infrared analysis is also used to regulate oil change intervals (i.e., reduce, maintain, or extend) for the engine, transmission, and hydraulic systems. The Caterpillar Maintenance Guides publish general oil change interval guidelines that do not take into account specific operating conditions and machine applications. Through infrared analysis, oil change intervals, customized for a particular condition and application, can be developed. Customized oil change intervals can prevent premature component failure when the general guidelines are too long for a particular application and/or reduce oil costs when the general guidelines can be extended for lighter applications. When regulating oil change intervals (i.e., particularly when intervals are extended) caution should be taken to insure the user adheres closely to the new interval, the user has a good maintenance program, and the mode of operation and environment does not change. The savings achieved by reducing oil costs can be quickly offset by a premature component failure or a reduction in total equipment life.
If infrared analysis is only used to help identify pending failures and their cause, it should only be utilized when the used oil has shown unacceptable readings from the wear analysis and/or from the chemical and physical tests. It is estimated that only 15% of the samples require infrared analysis. If the other analyses have satisfactory results, the infrared analysis results would also be acceptable. If infrared analysis is used to regulate oil change intervals, more frequent analysis will be required.
Since infrared analysis is still relatively new, a brief explanation of the by-products causing oil deterioration follows:
Sulfur products are found in the used engine oils and are the result of the by-products of diesel fuel combustion that contaminates the oil. These by-products are corrosive and deplete the neutralizing properties of the oil. The correct neutralizing property (i.e., Total Base Number - TBN) of the oil is determined by multiplying the percent fuel sulfur by twenty. High sulfur fuels of greater than 1.5% sulfur require reducing the recommended oil change interval by 50%, as well as using an oil with a TBN twenty times the present fuel sulfur.
Oil oxidation occurs in the same manner as iron oxidation where the oxygen chemically joins the oil molecule. Whenever oil is in contact with air, oxidation occurs at a very slow rate. Oxidizing agents in combustion gases of diesel engines, temperature, and some contaminants (i.e. copper and glycol) significantly accelerate this process. As oil oxidizes to high levels, lubricating properties are lost and resins are formed resulting in oil thickening, plugged filters, and lacquering.
Nitration occurs in all engines but only reaches problem levels in natural gas engines. Nitrogen compounds resulting from the combustion process contaminates the oil. High nitration causes oil thickening, plugged filters with deposits, and lacquering, thus reducing the lubricating properties.
Soot is also a by-product of the combustion process found in used oils of all engines. Soot is an insoluble particulate that can plug oil filters and deplete the dispersant additives in the oil.
A lubricant additive can be defined as a material which imparts a new desirable property not ordinarily present in the oil or reinforces a desirable property already possessed in some degree by the oil. It is particularly important to monitor the antiwear additive (zinc dithiophosphate) in transmissions.
Infrared analysis is performed on an infrared spectrophotometer using the differential scan process. The differential scan compares a new oil sample to a used oil sample. Consequently, a sample of new oil from the same supplier, grade, and batch as the used oil sample is required. The same grade oil from the same supplier can give erroneous results if it is not from the same batch as the used oil sample.
Infrared analysis does not measure an exact Total Base Number (TBN). TBN is determined using the ASTM D-2896 procedure, or insolubles, determined using the ASTM D-893 procedures. Most oil companies recognize only the ASTM D-2896 and ASTM D-893 procedures when evaluating oil conditions. Consequently, if communications with the oil companies are necessary, the oil in question may require analysis using the ASTM procedures.
Additional Testing (Not Part Of The S.O.S Program)
The following tests should be considered when the previously mentioned tests have not been able to determine the cause of early and rapid oil deterioration causing premature engine failures. It is estimated that considerably less than 1% of the samples received will require these tests. Acceptable and unacceptable limits will be provided by Service Engineering when all of the additional test information is provided to Service Engineering.
ASTM D-2896 - A procedure that measures the residual neutralizing properties of an oil expressed in TBN.
ASTM D-893 - A procedure that measures the percentage of pentane insolubles (i.e. soot, dirt, and oxidation products), toluene insolubles (i.e. soot and dirt), and toluene solubles (i.e., oxidation products) in used oil.
ASTM D-445 - A procedure that measures the used oil viscosity at various temperatures to determine whether the viscosity has changed.
Due to the cost of the test equipment, extensive testing time required, and the technical expertise required, these additional tests are not recommended for most Caterpillar dealer labs. For most dealers, it is recommended that they contact a local independent lab or a local college/university when these tests are needed.
To realize the benefits of the diagnostic capabilities of the S.O.S program, the user's equipment must be on a regular and continuous sampling program. The equipment's operating conditions and maintenance and repair history must be known. The quality of the user's maintenance and repair practices is the cornerstone of an effective oil analysis program.
The purpose of this bulletin is not to change the scope of the S.O.S program, but to show the organization of the program and its capabilities clearly. For over 99% of the oil samples analyzed by Caterpillar dealer labs, wear analysis, chemical and physical tests, and infrared analysis will provide a comprehensive full service diagnosis of oil and component conditions.