- Excavator:
- All
Several sources of information are currently available with guidelines for the wear analysis of the final drives in Excavators. Certain information could be easily misunderstood. Information that is misunderstood can cause unnecessary repairs. A comprehensive study of the final drives in Excavators was performed in order to provide one safe source of information. The result of the study is reflected in the following information. The following guidelines provide an explanation of the wear analysis and the contamination that occurs in final drives. The guidelines provide a plan of action in order to address abnormal conditions. Unnecessary action that is performed beyond the guidelines is not recommended.
Certain compartments can routinely generate widely varying amounts of debris accumulation. The following variables affect wear in a compartment: geographic locations, climate, type of lubricant, maintenance practices and operating techniques. The variables create a broad range of data from the oil analysis. These guidelines were developed in order to better understand the results of the oil analysis by the S·O·S interpreter and the Caterpillar dealership. The correct understanding of the results of the oil analysis will improve the management of the machine. The correct understanding will also decrease the operating costs for the customer.
Differences Between Final Drive Operation And Application
The final drives in Excavators move the machine between locations. The final drives in other Caterpillar machines move the machine and heavy material between locations.
In a typical application, Excavators operate for many hours with very little movement of the tracks. When an Excavator moves, the travel speed is much slower than other Caterpillar machines with final drives. Also, the seals in the final drives on Excavators are located close to the ground. The final drives on other Caterpillar machines are located higher from the ground. The seals in the final drives that are located close to the ground provide an increased opportunity for packed dirt or packed mud. Machine movement can force dirt or mud beyond the seals and into the final drive. The wear characteristics of the final drives in Excavators are unique because of the differences in the operation and the application.
Contamination In Final Drives
The presence of dirt, mud, or water in a final drive is detected by oil analysis. An oil analysis with elevated values of Si and Al indicate dirt contamination. The Si value will always exceed the Al value. The oil analysis will also detect water contamination. An oil sample with large amounts of water will have a milky brown appearance. A test that uses a hot plate can detect small amounts of water. The Karl Fischer method of a titration can also be used in order to detect a small amount of water.
Dirt will initially produce wear on gears, shafts, and carriers. The initial wear will be identified by elevated Fe readings. Eventually, this wear could extend to the rolling element bearings in the final drive. Wear on the bearings is identified by elevated Cr values.
Guidelines For The Analysis Of Final Drive Oil
Special guidelines have been developed in order to address the unique characteristics of the final drives in Excavators.
Oil Sampling
The Operation and Maintenance Manual recommends an interval of 250 service hours for oil sampling. The sampling interval should be decreased to an interval of every 125 service hours, if the machine has a history of contamination in the final drive or if the operating conditions promote contamination. An oil sample should also be analyzed within 125 service hours, if the final drive has been repaired or if the final drive has been cleaned in order to remove contamination. The analysis of an oil sample may also be preferable at an interval of 50 service hours if the Cr value is abnormal.
Wear Element Analysis
Wear element analysis is used in order to detect small wear particles in the oil and contamination from dirt. Table 1 is a guideline for the wear element analysis. Most Caterpillar tables for wear element analysis are only valid for the recommended oil drain. Table 1 is valid for the final drives in Excavators regardless of the service hours of the fluid.
| Wear Element Analysis (1) | |||
|---|---|---|---|
| Element | No Action Required | Monitor | Action Required |
| Cu | 0-15 | 16-20 | >20 |
| Fe | 0-300 | 301-450 | >450 |
| Cr | 0-7 | 8-12 | >12 |
| Al | 0-35 | 36-60 | >60 |
| Pb | 0-2 | 3-4 | >4 |
| Si | 0-150 | 151-250 | >250 |
| Sn | 0-2 | 3-4 | >4 |
| ( 1 ) | Values are listed in parts per million (ppm). |
Particle Count Analysis
An optical instrument for particle count analysis is used in the S·O·S laboratories in order to measure the larger particles in an oil sample. The instrument for particle count analysis measures particles from approximately 4 microns to 50 microns in size. Wear element analysis can only measure particles up to approximately 10 microns in size. The instrument for a particle count analysis should be capable of detecting dirt contamination and wear material in an oil sample from a final drive. Particle count analysis has not been an effective tool for managing the final drives in Excavators. The following two reasons describe the difficulty in the measurement of the particles:
- Satisfactory final drives with low levels of wear and no significant contamination will produce a reading of 23/22 in the ISO code system. The 23/22 value is near the top of the measurable range on most of the instruments for a particle count analysis. If the particle counts increase, the instrument cannot measure the increase and the reading is high off of the scale. The code for the ISO system will report a High Base Line (HBL) if a condition that is off of the scale occurs.
- Approximately 50% of the samples for the final drives in Excavators produce a measurable value on the particle count. The samples that do not produce a particle count have three problems: too much water in the sample, too dark of a sample and too many particles in the sample
Samples that have too many particles will report a reading of High Base Line (HBL). The reading will be off of the scale.
The data from a particle count analysis cannot be effectively used in order to manage the final drives in Excavators because of the problems that were previously listed. The values from the ISO system that are listed in this document should be used for reference purposes only.
Interpretation Of Oil Sample Results
Interpretation of the results for oil samples from final drives is performed in order to determine if the wear has become abnormal. The main indicator of abnormal wear is the level of Cr. Elevated levels of Cr indicate wear of the rolling element bearings. Excessive wear of the bearings will result in the failure of the final drive.
The two most important tests for the oil sample from final drives are wear element analysis and visual inspection.
Visual inspection of the oil sample can indicate a serious problem of water contamination. Oil samples with a milky brown appearance probably contain an excessive amount of water. A quick test on a hot plate in a laboratory will confirm the excessive water. Oil samples with excessive water cannot be run on any other lab tests. The sample should be given a Red Alert status. At a minimum, the final drive oil should be drained and the compartment should be flushed in order to remove the water. The final drive should be sampled again within the next 125 service hours. Final drives that continually contain excessive water may require new seals.
Visual inspection may also reveal visible metal particles in the oil. The results of the oil sample cannot be obtained by a particle count analysis. If oil samples are analyzed on a particle count analysis, the results will be off of the scale high. Oil samples with visible metal can be run on the wear element analysis. The results from a wear element analysis will determine the extent of the wear.
If the oil sample can be tested for wear element analysis, the following guidelines should be used for interpretation of the results. The guidelines are based on the information in Table 1.
- Refer to Table 2. Perform the following action if the values from the wear element analysis of your oil sample equal the values in the table. Make sure that the final drive compartment is flushed at the next scheduled service interval. The sample should be given a Green Alert or a Yellow Alert status. Flush the final drive with diesel fuel, kerosene, or hydraulic oil. If it is feasible, a loop circuit on a portable filter cart could be used in order to clean the compartment. Evaluate another oil sample after 125 service hours.
Show/hide table
Table 2 Results Element Value (1) Fe 301-450 Si 151-250 Cr 0-7 Show/hide table( 1 ) Values are listed in parts per million (ppm).
- Refer to Table 3. Perform the following action if the values from the wear element analysis of your oil sample equal the values in the table. Make sure that the oil is drained and that the final drive compartment is flushed at the earliest convenience. The sample should be given a Yellow Alert status. Evaluate another oil sample after 125 service hours. Continue operation if the oil sample that was obtained at 125 service hours is normal. Continue with a normal sampling interval and a normal oil drain interval.
Show/hide table
Table 3 Results Element Value (1) Fe 301-450 Si 151-250 Cr 8-12 Show/hide table( 1 ) Values are listed in parts per million (ppm).
- Refer to Table 4. Perform the following action if the values from the wear element analysis of your oil sample equal the values in the table. Drain the oil from the final drive compartment. Schedule service for the final drive within the next 50 hours. Flush the final drive compartment in order to remove the contamination and the wear metals. The sample should be given a Yellow Alert status. Evaluate another oil sample after 125 service hours. Continue operation with a normal sampling interval and a normal oil drain interval, if the oil sample from the 125 service hour interval is normal.
Show/hide table
Table 4 Results Element Value (1) Fe >450 Si >250 Cr 8-12 Show/hide table( 1 ) Values are listed in parts per million (ppm).
- Refer to Table 5. Perform the following action if the values from the wear element analysis of your oil sample equal the values in the table. The oil should be drained immediately from the final drive compartment. Flush the final drive compartment in order to remove contamination and wear metals. The sample should be given a Red Alert status. Evaluate another oil sample within 50 service hours. If the final drive continues to produce levels of Cr that exceed 12 ppm, inspect the final drive for abnormal gear and bearing wear. Make the necessary repairs.
Show/hide table
Table 5 Results Element Value (1) Fe >450 Si >250 Cr >12 Show/hide table( 1 ) Values are listed in parts per million (ppm).
- Refer to Table 6. Perform the following action if the values from the wear element analysis of your oil sample equal the values in the table. This applies to either the Fe value or the Cr value. Make sure that the final drive is immediately inspected. The sample should be given a Red Alert status. Relatively low levels of Si indicate that the entry of dirt is not causing the abnormal wear. A component failure could be occurring.
Show/hide table
Table 6 Results Element Value (1) Fe >450 Si 0-250 Cr >12 Show/hide table( 1 ) Values are listed in parts per million (ppm).
The emphasis is caution for the interpretation of these guidelines. Excavator final drives can safely operate with relatively high levels of dirt and Fe wear particles in the oil. The gear wear is usually minimal because the final drives operate at low speeds. If the level of Cr increases above 12 ppm, the bearings could be experiencing abnormal wear. Action should be taken in order to find the cause of the wear when this condition occurs.
Oil Sample Results From The Final Drives In New Excavators
Many dealerships perform a predelivery inspection of oil samples from the final drives of a new machine. Sampling generally occurs within the first 30 service hours of operation. The analysis of the wear elements should be in the range of no action required. Refer to Table 1.
Grease and other compounds may be used during the initial assembly or the rebuild of final drives. One of the most common materials that is used is anti-seize compound. Anti-seize compound is applied to bearing races, dowel pins, and other components with a tight fit in order to make disassembly easier. Residual anti-seize compound mixes with the oil during the early hours of operation. Oil samples from the first oil change will indicate some of the elements from the anti-seize compound.
The chemical composition of the anti-seize compound commonly contains copper, aluminum, molybdenum, and other elements. The amount and the type of element will vary depending on the manufacturer. Certain particles from the anti-seize compound will appear in the oil sample from the final drive compartment. The oil sample will appear to have a gold shine or a silver shine when light is reflected from the suspended particles. The particles may continue to be observed in the next couple of oil changes. The particles will eventually disappear. The visual particles can occur in the following final drives: new, rebuilt and remanufactured
Particle count analysis will be difficult with the presence of the anti-seize compound. Approximately 20% of the initial oil samples will not produce data from a particle count analysis. The data may be high off of the scale, or the oil sample may be too dark for a measurement. If a particle count is obtained, the value for the ISO code may have an approximate range of 22/19 to 23/21. Use the ISO code values in this document for reference purposes only. Refer to "Particle Count Analysis" section of this article.