May 1, 2010
Understanding the benefits of oil analysis can lead to optimum oil use and longer equipment life.
By Steve Waggoner
For several decades, used oil analysis has been a by-word in the realm of lubricated components in aggregate and other stationary equipment. Management and maintenance personnel in the quarrying and construction materials industries will recognize the various operational and other problems that are common to aggregate equipment, and they probably know how important oil monitoring is for the well being of their equipment. What they may not know is how to design, monitor, and benefit from a used oil analysis program appropriate for their specific operational conditions. Optimum monitoring of equipment health will require planning the appropriate array of oil tests, sampling intervals, and interpretation of analysis data.
There are several operational problems, types of contaminants, and other external issues that may be seen during the lifetime of aggregate equipment. These include mechanical problems, oil problems, contaminants, and maintenance issues. Following are some specific types of these issues.
• Misapplication of component (wrong load, wrong speed);
• Worn parts (bearings, bushings, gear teeth);
• Sudden failure (broken gear teeth, etc.); and
• Oil oxidation;
• Wrong oil;
• Under-filled; and
• Dirt (leaking seals or missing breather vents, high-dust atmospheres); and
• Excessive wear metals.
• Missed service interval;
• Wrong or improperly installed parts at rebuild;
• Installation of wrong oil; and
• Temperature control of the lubricated component and of ambient conditions.
While these lists are not all-inclusive, they represent a reasonable sampling of the issues that may be encountered in the world of aggregate equipment.
Oil analysis options
Various oil analysis procedures can be used to monitor the overall health of a lubricated component. Those commonly used in fixed equipment monitoring may include the following:
• Spectrographic (absorption, ICP, or emission) analysis for wear metals and dirt;
• Ferrography for analysis of large-particle metal composition;
• Particle counts for ISO (International Organization for Standardization) Cleanliness Code;
• Infrared (IR) analysis for the presence of moisture contamination; and
• Analysis (usually by IR) for oxidation.
Spectrographic analysis can be performed by one of several different methods, including atomic absorption, ICP (inductively coupled plasma), or emission. The method chosen is by preference of the testing lab, but all three methods provide the same type of information: the concentration of wear metals, additive metals, or silicon (dirt) present in the oil. These methods generally detect normal-sized wear particle, usually at and below the 10µ (micron) to 12µ range. Metals detected by these methods are reported in parts-per-million (PPM), with larger particles being reported by the ferrography method. In addition, most testing laboratories can provide sampling histories for individual components and some can even provide automatic commenting based on previous sample trending.
Particle count testing is often performed to determine the ISO Cleanliness Code for the oil. The particle count test determines the number of particles of various sizes detected in one milliliter of sample. While the test looks for particles 150µ and larger, the code is determined by the count numbers at the 4µ, 6µ, and 14µ sizes. That is, the instrument counts how many particles larger than 4µ, how many larger than 6µ, and how many larger than 14µ are present. The results are then entered into a range table that determines the code (see Figure 1).
ISO has established code guidelines for several types of components, but original equipment manufacturers may have varying requirements. Shown in Figure 2 are some of the ISO guidelines.
Fourier Transform Infrared analysis (FTIR) is a type of infrared analysis that is frequently used in used oil analysis. It can detect diesel fuel, soot, antifreeze, and more specific to fixed equipment, moisture, and oxidation levels. Moisture is usually reported as a percent by weight and assists in controlling water accumulation in components subjected to high ambient humidity or other sources of moisture entry. Oxidation is reported as an absorbance value and is an indicator of excessive operating temperatures or overextended drain intervals. Equipment manufacturers have specific condemning limits for each of these test criteria.
In order to be effective, IR instrumentation must have an unused reference sample of the oil being tested, so it is imperative that each customer provides unused samples of each of the lubricants being used in their equipment. Each instrument develops a library of reference samples, so that the correct reference can be selected for each sample. Analysis results then will consist of a direct comparison of chemical characteristics between the reference sample and the used oil sample.
Frequency of used oil analysis is usually determined by one of three criteria, or a combination of them:
• The equipment manufacturers’ recommended service interval;
• Ambient conditions; and
• Results of previous analysis results.
When changing oil at the OEM’s recommended service interval, it is a good idea to take an oil sample for analysis at that time. The oil change intervals should be as close to the same as possible, so that meaningful historical trending will be developed. Most used oil analysis laboratories provide wear-metal commenting based on trending — when there are enough uncontaminated samples in the database. However, trending is not as effective a tool when drain intervals are not consistent and testing laboratories are only providing a raw number for wear metals, rather than a wear rate.
Wear rates can be easily calculated in those instances (see Figure 3).
Ambient conditions, such as airborne dust, high humidity, or high/low temperatures, may dictate more frequent sampling, or even reduced drain intervals. Mid-drain samples to monitor on-going conditions such as wear and contaminants will contribute to optimum component life. Wear metal levels from mid-drain samples should not be considered when trending, unless the wear rate calculation method is used.
In addition, mid-drain sampling is beneficial after rebuilds, repairs, or after contaminants or high wear have been identified in a previous sample. Consistent monitoring will determine when the component is ready to return to routine operation.
As important as submitting used oil samples to a qualified laboratory is the method and consistency of obtaining the sample. Samples may be obtained by several methods, including the following:
• Through the drain opening at the time of oil change;
• Through the fill plug if not changing the oil, by use of a sampling pump; or
• By use of an inline quick-connect fitting.
With all of these methods, be sure that the area around the sample opening, whether drain, fill, or quick connect, is clean and dry, and that the oil is as close to normal operating temperature as possible. The method used should be the same each time, if possible.
If the sample is taken from the drain plug at an oil change, allow approximately 1/3 of the oil to drain out, and then catch the sample in a clean, dry sample container. If the sample is taken through the fill plug, insert the pickup tube on the sampling pump about half way into the depth of the oil and fill the clean, dry sample container. If using quick-connect fitting, insert the needle into the valve to fill the container.
The final step in the oil analysis process is data interpretation. Oil analysis laboratories provide interpretation and commenting on test results. Trends will be identified, and recommendations will be made as to maintenance actions that should be taken. Further assistance with interpretation can be provided by the testing laboratory.
Not all of the processes associated with used oil monitoring of aggregate equipment are simple, but development of a site-specific used oil analysis regimen will result in data that is meaningful and essential to achieving optimum oil life and extended equipment life. AM
Steve Waggoner is the quality management system manager and in-house technical manager for D-A Lubricant Co. He can be reached at firstname.lastname@example.org.
Determine the ISO Cleanliness Code for the amount of dirt particles in a sample of oil:
Particle count testing detects: 30,000 particles of 4 µ and larger
500 particles of 6 µ and larger
60 particles of 14 µ and larger
30,000 falls into the ISO Code 22 range
500 falls into the ISO Code 16 range
60 falls into the ISO Code 13 range
The ISO Cleanliness Code is 22/16/13.
ISO Code 12/9 14/11 16/13 18/15 20/17 22/19 24/21 26/23
Hydraulic Fluids Very Clean Clean Dirty
Gear Oils Very Clean Clean
Turbine Oils Very Clean Clean Dirty
Iron, PPM Drain, Hours
In this example, the drain intervals and iron levels are very consistent.
This would represent a meaningful trend.
Iron, PPM Drain, Hours
In this example, the drain intervals and iron levels are not consistent.
The trend here will not be meaningful unless the wear rates are calculated.
The wear rate will be the PPM of wear per 100 hours of operation, i.e.:
75 ÷ 5.00 = wear rate of 15 PPM per hour
93 ÷ 6.20 = wear rate of 15 PPM per hour
60 ÷ 4.00 = wear rate of 15 PPM per hour
The wear trend is consistent in this example, even though the drains and wear
numbers are not.