Gear oils play a critical role in transferring power between mechanical devices. One crucial function is an oil’s fluidity at low ambient temperatures. This is a key aspect of whether it is fit for purpose. A lack of gear oil fluidity is an impediment to maintaining a lubricant coating on the gear teeth.
When there is a lack of fluidity, a gear will cut a channel in the lubricant, resulting in loss of lubricant coating. This loss of the oil coating leads to metal-to-metal rubbing contact between the gear teeth, generating heat which in turn warms the oil. This rubbing with little or no oil between gear teeth results in increased wear. The amount of additional wear will depend on how quickly the lubricant begins re-coating the teeth.
Metal-to-metal contact under high load is known to lead to galling and in turn the generation of metal particles. Galling results in increased load on the teeth due to the reduced load-carrying surface. The fine metal particles generated by metal-to-metal contact are circulated by the lubricant back on to the surface of the teeth. These metal particles then act as an abrasive, which leads to a further increase in wear.
For most equipment, lowtemperature startup is typically only a fraction of its total run-time. Even though this only lasts for a short period of time, poor lubrication will greatly contribute to wear. Thus in cold weather, gear oil fluidity is a critical aspect of maintaining a gear box’s performance.
Why Fluidity Falters
Insufficient fluidity can occur in a number of ways. One is due to crystallization of the base stock wax. Another is viscosity increase due to oxidation, which is the result of excessive heat and metal debris.
Oxidation and metal debris are minimized by the rust inhibitors, antiwear and anti-oxidant components used in the oil formulation. Wax crystallization is controlled through the use of appropriate low-temperature flow improvers, which are sometimes referred to as pour point depressants. Low-temperature flow improvers disrupt the growth of wax crystals to prevent them from forming a strong matrix in the oil. A strong matrix prevents the oil from flowing onto the moving gears. If the gear oil solidifies due to wax crystallization, the oil can exhibit properties similar to those of a gel while having a viscosity well within specifications. A gelled gear oil will likely need to reach a much higher temperature before it will again flow onto the gears.
To ensure a gear lubricant will have adequate fluidity at low temperature, the SAE J306 gear oil specification has low-temperature viscometric limits (see Table 1). These are determined by ASTM D2983, Standard Test Method for Low Temp - erature Viscosity of Lubricants by Brookfield Viscometer. These limits are expressed as a maximum temperature at a viscosity of 150,000 mPa(s) as measured by ASTM D2983.
Although ASTM D2983 has been in use for close to 40 years, it was not the first test to measure low-temperature gear oil fluidity. Prior to D2983, gear oil fluidity was measured using the “Channel Test.” This test, FTM 792-3426, is still cited in the
The Channel Test is performed by placing 650 milliliters of the gear oil in a special container. After preheating to 46 degrees C , the sample is placed in a thermostatically controlled bath which is at the final test temperature. At the completion of the 16-hour soak, the time to fill a 2 centimeter channel cut in the lubricant is measured. Current specifications require an oil to fill the channel in less than 10 seconds.
Too Much Variability?
ASTM D2983 thermal conditioning is very close to the Channel Test. After the 16-hour soak, the sample’s viscosity is measured. First, the sample tube is placed in a pre-chilled insulated holder before removing from the air chamber so the rotor can be attached to the rotational viscometer. The acceptance of ASTM D2983 to replace the Channel Test was due in part to it being more precise and a less subjective assessment of viscosity. However, there are several steps in the D2983 test procedure where operational inconsistencies between tests can result in a large variation of test results and measurement uncertainty. Some of these issues are actual time and temperature of sample pre-heat; variation in time between removing the sample from the air chamber to finishing the viscosity measurement; and disruption of any wax structure in the sample when connecting the viscometer spindle. There are other aspects of the procedure which can contribute to variation in test results. To address these issues, ASTM D6821, Standard Test Method for Low Temperature Viscosity of Drive Line Lubricants in a Constant Shear Stress Viscometer, was developed as an alternative to ASTM D2983, with the objective to minimize inconsistencies between tests. ASTM D6821 utilizes the same Mini-Rotary Viscometer that is used in measuring engine oil lowtemperature viscosity (ASTM D4684), with a few operational changes. For one, the D6821 rotors and the mass used for yield stress and viscosity measurement are different from D4684. Also, D6821 thermally conditions the sample in accordance with D2983 procedure.
The development of D6821 was accomplished by D.L. Alexander with support from Cannon Instrument Co. and summarized in SAE paper 1999-01-3672. In summary, for D6821 a measured sample is placed in one of the test cells of a Mini-Rotary Viscometer (such as Cannon’s CMRV). After the instrument preparation is complete, the instrument automatically thermally conditions the sample using the same time and temperature criteria as D2983. After the 16-hour soak, the yield stress and viscosity are measured with the data automatically recorded by the instrument.
A Way Forward
ASTM D6821 provides several advantages over D2983:
• A smaller sample size of 10 mL.
• Automatic thermal conditioning of the sample.
• Viscosity measurement without disturbing the sample.
• Since the CMRV is a constant stress viscometer, yield stress also can be determined.
These advantages are why ASTM D6821 is a more precise viscosity measurement technique. A comparison of the two methods’ precision is shown in Table 2 at the SAE specification limit. Note that the precision of ASTM D2983-04 is a power function, so percent repeatability and reproducibility vary with measured viscosity. D6821 precision is constant over the viscosity range of the test method and not affected by the magnitude of the viscosity measured.
D6821’s ability to measure yield stress provides another assessment of structure formation that is only visible in the Channel Test. (Yield stress cannot be measured with D2983.) The presence of yield stress is an indication that the sample would not flow. A positive yield stress indicates that the oil has gelled. This is a different phenomenon than seen when a lubricant has an excessively high viscosity. A large yield stress is an indication the structure is strong enough to prevent lubricant flow onto the gear teeth.
The SAE paper by Alexander, et al, demonstrated the value of the D6821 technique for gear oils. This precision study included a range of commercial samples. The data showed excel- lent precision as well as correlation to ASTM D2983. Cannon Instrument Co. has since conducted measurements on a number of commercial samples over the last several years to again demonstrate D6821 test precision. This data set also includes comparisons with D2983 data.
Weighing the Data
The D6821 data presented here was collected in two MRV instruments — Cannon models CMRV-4500 and CMRV-5000. Commercial gear oil samples were obtained from local resellers. The sample set also includes samples from the ASTM Committee D02 Inter Laboratory Cross-check Program (ILCP) for gear oils.
Comparative D2983 viscosity data for the commercial oil samples was obtained from an accredited commercial test laboratory. For the ASTM ILCP sample, the average D2983 viscosity from the test report was used as the reference value.
Table 3 shows the average of eight D6821 measurements for six 75W multigrade gear oils. These measurements had an average variation of 5.2 percent. This corresponds to a reproducibility of approximately 12 percent, which is well within the precision stated in D6821 and a third of the D2983 precision.
With the exception of sample 957, D6821 viscosity data is essentially identical to that obtained with D2983. Sample 957 had yield stress greater than 34 Pa as compared to a yield stress of 11.3 Pa or less for the other samples. Since there is good agreement between D6821 and D2983 for the other samples, the difference seen for sample 957 is likely due to gel-like structure as indicated by the high yield stress.
Additional gear oils tested included 80W-90 and 80W-140, with an average variance in D6821 viscosity for four measurements of 5.1 percent. This variance is virtually identical to that seen in Table 3 for the 75W multigrade oils. The ASTM ILCP 80W gear oil sample had a variance of 5.3 percent, close to the 8.8 percent found in the ILCP report.
Two 80W multi-grade oils show a significant difference between D6821 and D2983 viscosity measurements but both are within test reproducibility. One sample exhibited a D6821 viscosity that is 27 percent greater than the D2983. However another sample’s D6821 viscosity is 32 percent less than the D2983 viscosity.
Data collected on 85W-140 samples is very similar to that seen in Table 3. D6821 viscosity variance for this group is 5.3 percent for four determinations. The D6821 viscosity for two oils was less than D2983 by 30 percent.
Overall five of the 16 samples exhibited a 30 percent difference between D6821 and D2983 data. For D2983, this 30 percent difference is within the method’s reproducibility. It was only the 85W samples where the difference was in only one direction.
Without a Channel Test to evaluate the slump rate, judging the effects of measured yield stress on the fluids’ ability to slump is difficult. It is known from earlier studies on engine lubricants that yield stress and high viscosity aren’t dependent on each other until you approach the fluids’ solidification temperature. In this set of data, sample 957 exhibited the largest yield stress, greater than 34
Preheating and Wax
It is likely that the large viscosity differences seen in the five samples mentioned are at least partially due to differences in base stock wax type and the choice in low-temperature flow improver. Additive component interaction and additive solubility could also be contributing to these differences.
With respect to wax, the current preheat requirement was added to D2983 in the mid-1980s. An automotive gear oil marketer initiated this activity in ASTM. They found that without preheating, some formulations had an exceptionally high variance in D2983 viscosity. Further study determined that variance was associated with a base stock containing significant amounts of micro-crystalline wax.
By adding a 50 C preheat, the variability in D2983 viscosity measurements on samples containing micro-crystalline wax was reduced, bringing the results to within D2983 precision. This discovery established the importance of ensuring that all of the wax is in solution prior to cooling the sample in preparation for viscosity measurement.
The 50 C preheat is effective because it is warm enough to bring the base stock wax into solution but not high enough to cause a change in the gear lubricant’s additive chemistry. Without a sufficient preheat, there are variable amounts of suspended crystallized base stock wax in the test sample. These wax crystals will act as nucleation sites for the wax in solution as the sample is cooled, and reduce the effectiveness of the low-temperature flow improver. D6821 limits this variability by providing a consistent sample preheat both in terms of temperature and time that is difficult to achieve with D2983.
Poor low-temperature flow of automotive gear oils isn’t likely the sole reason the gears fail in a transmission case or gear box. Poor fluidity that results in poor lubrication will definitely contribute to a shorter useful life, or catastrophic failure in a critical situation. ASTM D6821 can help avoid poor fluidity situations because of its ability to indicate whether a formulation has a tendency to gel.
In summary, ASTM D6821 provides a more precise viscosity measurement than ASTM D2983. This better precision is due in large part to eliminating the numerous sample-handling steps of D2983. Additionally, the use of D6821 not only improves the reliability of the test results, it also reduces the amount of technician time required to complete the test. By improving the precision of the test, formulators will gain additional latitude in choosing components that deliver the required performance for their gear oil blends.
The industry would benefit by the addition of ASTM D6821 to the Committee D02 Inter Laboratory Crosscheck Program for Gear Oils. This would provide the industry with a broader sample base for comparing the capabilities of these two methods.
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