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ISO 26422:2014 Shear Stability of Lubricating Oils Tapered Roller Bearing KRL Test Method
Determination of shear stability of polymer-containing lubricating oils using a tapered roller bearing
1. Introduction to ISO 26422 and the KRL Test
ISO 26422:2014 specifies a method for determining the shear stability of polymer-containing lubricating oils using a tapered roller bearing (KRL Kegelrollenlager) test. This second edition cancels and replaces ISO 26422:2011, harmonizing the methodology with CEC L-45-99 to eliminate discrepancies between technically equivalent test methods.
Polymer-containing oils are widely used in automotive and industrial applications where viscosity modifiers (VI improvers) are added to achieve the desired viscosity-temperature behaviour. Under high shear conditions such as those found in gears, roller bearings, and manual transmissions these polymer chains can be mechanically degraded, causing permanent viscosity loss that compromises lubrication performance.
The KRL test subjects fluids to significantly higher shear rates than alternative methods like the diesel injector nozzle test (ISO 20844), making it particularly appropriate for lubricants used in high-shear applications including gearboxes and rolling element bearings.
2. Test Principle and Apparatus
2.1 Principle of Operation
A 40 ml sample of lubricating fluid is tested at 60+/-1 degree C in a tapered roller bearing (SKF 32008 X/Q) driven by a four-ball machine. The bearing operates under a constant load of 5000 N at 1475+/-25 min-1 for 1,740,000 motor revolutions (approximately 20 hours). Kinematic viscosity is measured at 100 degrees C before and after the test, and the percentage viscosity loss (RV) is calculated.
| Test Parameter | Specification | Tolerance |
|---|---|---|
| Drive motor speed | 1475 min-1 | +/-25 min-1 |
| Lubricant temperature | 60 degree C | +/-1 degree C |
| Lubricant quantity | 40.0 ml | +/-0.5 ml |
| Test load | 5000 N | +/-200 N |
| Test duration | 1,740,000 revolutions | ~20 h |
| Bearing type | SKF 32008 X/Q | Matched pair |
2.2 The Percentage Viscosity Loss
The shear stability is quantified by the percentage viscosity loss, RV, calculated as:
RV = ((nu_0 – nu_1) / nu_0) x 100
where nu_0 is the kinematic viscosity before shear and nu_1 is the kinematic viscosity after shear, both measured at 100 degrees C. A small RV value indicates high resistance to shear-induced viscosity loss.
Note that some classification systems specify kinematic viscosity measurements at temperatures other than 100 degrees C (e.g., 40 degrees C). The test can be adapted, but the precision data established in ISO 26422 is based on 100 degrees C measurements.
3. Engineering Design Insights for Implementation
3.1 Bearing Selection and Running-In
The SKF 32008 X/Q tapered roller bearing is the specified test bearing, incorporating optimized contact geometry and surface finish. Precision has been established exclusively with this bearing. Critical aspects of bearing management include:
- Matched pairs: Inner and outer rings should be treated as matched pairs from the supplier and never interchanged between bearing sets.
- Running-in: New bearings must be run-in using RL 209 reference fluid following a defined validation flowchart. If the initial validation fails, further running-in may be attempted.
- In-service monitoring: Bearings are monitored through periodic reference testing with RL 209 and RL 210 reference fluids. Exceeding control limits indicates end of service life.
3.2 Reference Fluids and Quality Control
Two reference fluids are used for quality control:
- RL 209: SAE 75W/80 automotive gear lubricant (viscosity loss 7.5%-10.9%), also used for bearing running-in
- RL 210: API GL5 automotive gear lubricant (viscosity loss 16.5%-26.3%)
The referencing scheme requires that candidate testing can only commence after the reference test at test number X=1 (RL 209) satisfies the control limits. In-service referencing at test numbers 10, 20, 30, 40, 50 ensures ongoing validity.
Interlaboratory precision data from 18 laboratories (390 test results) demonstrates repeatability (r) of 3.91% and reproducibility (R) of 4.67% for RL 209 at a mean viscosity loss of 8.61%. This robust statistical basis gives engineers confidence in result comparability across different laboratories.
4. Test Duration Flexibility and Applications
While the standard 20-hour test is appropriate for most applications, shorter or longer durations may be specified:
- 4-8 hours: Suitable for high-viscosity index hydraulic fluids and shock absorber fluids
- 20 hours (standard): General purpose gear and bearing lubricants
- 200 hours: Correlates with high-severity conditions in manual transmissions and axles
Important limitation: Precision has not been established for test durations other than the standard 1,740,000 revolutions. Engineers using non-standard durations should validate their results through interlaboratory studies.
ISO 26422 was developed by ISO/TC 28 for petroleum products and lubricants. The precision data in the standard was derived from an extensive interlaboratory study involving 18 laboratories across multiple countries, generating 390 individual test results. This collaborative effort ensures that the method repeatability and reproducibility values are statistically robust and representative of real-world laboratory performance. The standard also includes detailed referencing procedures with two reference fluids RL 209 and RL 210 that serve as quality control benchmarks, allowing laboratories to continuously verify their test equipment performance against established control limits before conducting candidate sample evaluations.
ISO 26422 was developed by ISO/TC 28 for petroleum products and lubricants. The precision data in the standard was derived from an extensive interlaboratory study involving 18 laboratories across multiple countries, generating 390 individual test results. This collaborative effort ensures that the method repeatability and reproducibility values are statistically robust and representative of real-world laboratory performance. The standard also includes detailed referencing procedures with two reference fluids RL 209 and RL 210 that serve as quality control benchmarks, allowing laboratories to continuously verify their test equipment performance against established control limits before conducting candidate sample evaluations.
5. Frequently Asked Questions
Q1: How does the KRL test differ from the diesel injector shear test (ISO 20844)?
The KRL test operates at significantly higher shear rates than ISO 20844. The diesel injector test may not generate realistic permanent viscosity loss for high-shear applications like gears and roller bearings, making the KRL test the preferred method for such applications.
The KRL test operates at significantly higher shear rates than ISO 20844. The diesel injector test may not generate realistic permanent viscosity loss for high-shear applications like gears and roller bearings, making the KRL test the preferred method for such applications.
Q2: Can bearings from manufacturers other than SKF be used?
While the standard strongly recommends SKF 32008 X/Q bearings, alternative bearings meeting ISO 355 dimension series 3CD may be used, but only after validation through interlaboratory exercises to establish technical suitability, test precision, and service life.
While the standard strongly recommends SKF 32008 X/Q bearings, alternative bearings meeting ISO 355 dimension series 3CD may be used, but only after validation through interlaboratory exercises to establish technical suitability, test precision, and service life.
Q3: What is the significance of the reference fluid RL 181?
RL 181 was the original reference fluid used for bearing running-in and in-service validation. It was discontinued in August 2012 due to stock exhaustion. Bearings originally validated with RL 181 may continue in service if they satisfy control limits for RL 209 and RL 210.
RL 181 was the original reference fluid used for bearing running-in and in-service validation. It was discontinued in August 2012 due to stock exhaustion. Bearings originally validated with RL 181 may continue in service if they satisfy control limits for RL 209 and RL 210.
Q4: How should the test report document results?
The test report must include: reference to ISO 26422, complete identification of the product tested, the percentage viscosity loss RV rounded to 0.1%, test duration, date of test, and any deviations from the specified procedure.
The test report must include: reference to ISO 26422, complete identification of the product tested, the percentage viscosity loss RV rounded to 0.1%, test duration, date of test, and any deviations from the specified procedure.
