Automotive manufacturers have suggested that transmission oil should be replaced every 60000 to 100000 km, because the oil may age or degrade over time. Degraded oil has poor lubricity behavior, which adversely affects the service life of transmission components.
This oil degradation is a result of additive depletion, such as Anti-wear (AW), Viscosity Index Improvers (VII's) or Extreme Pressure (EP), as well as an increase in oxidation and decreased resistance to contaminants such as particulate matters or water.
An automotive transmission system’s operating parameters can have an enormous impact on oil aging. For example, transmission oil being repeatedly exposed to high operating temperatures (175 to 220 °C) has the potential to increase oil degradation rates.
In practice, this means that within a transmission system’s service life, oil change intervals will decrease, with the whole transmission system potentially needing to be replaced due to wear. This, of course, increases the overall cost of truck ownership.
A number of experimental methods exist which focus on approaches to delay oil degradation under extreme operating conditions. Lab tests have an important role in any reduction in the costs of this solution development.
Figure 1. Ducom developed dynamic oil aging test protocol in the lab. KRL was used to age the oil and this oil was tested for its friction and wear behavior using FBT-3 and HFRR. Image Credit: Ducom
Lubrication engineers have long relied on lab tests which involved the artificial aging of oils. ASTM D 2893 is one such lab test method where oil present in a clean glass test tube is oxidized via heating in an oil bath at 95 °C or 121 °C, under a controlled flow of dry air.
Changes in test tube color, coupled with an increase in kinematic viscosity are seen as evidence of degraded transmission oil. The total acid number (TAN, percentage change in KOH/g) can also be measured in order to highlight any depletion of AW/EP additives.
However, this is a static aging test, i.e. oil is not sheared mechanically, as observed in a transmission system. There is also no comparison or correlation between lab oxidation and field oxidation results, meaning that, while static aging experiments are repeatable, they may not be wholly reliable.
Image Credit: Ducom
This study attempts to achieve dynamic oil aging within the lab. Because considerable mechanical shearing is required, a Ducom KRL Shear Stability Tester was employed to simulate oil’s viscosity loss in a transmission system. It is anticipated that additives in the KRL aged oils will be extremely depleted.
This hypothesis will be experimentally ascertained through measuring wear in both mixed and boundary lubrication with a Four Ball Tester (FBT-3) and a High Frequency Reciprocating Rig (HFRR) (Figure 1).
Materials and Methods
The transmission oils used in this study were comprised of three types of commercially available oil. Castrol Axle, Shell Spirax and Reference Fluid RL 209 were all bought commercially. Castrol Axle and Shell Spirax share similar viscosity indexes and kinematic viscosity at 100 °C (Table 1).
Table 1. Physical properties of Shell Spirax, Castrol Axle and Reference Fluid (RL 209) used in the KRL aging process. Source: Ducom
The Ducom KRL Shear Stability Tester (Figure 2) is a table-top instrument that is recognized by CEC L-45-99. It is equipped with a precision temperature control system and an automatic pneumatic loading system, making it convenient and user-friendly. It is also fitted with more sensors – such as in situ friction sensors - than any other currently available KRL system, delivering excellent stability throughout long duration tests of over 200 hours.
Figure 2. Image of Ducom KRL Shear Stability Tester used in this study (Table top, Automated Load Control System). Image Credit: Ducom
The Ducom KRL shear stability tester was tested for compliance against CEC L-45-99 by using the reference fluid RL 209 (Figure S1). Following the validation process, the Castrol or Shell oil was sheared for 100 hours according to the protocol outlined in Figure S2. The oil’s viscosity, both before and after KRL aging, was then measured to ascertain the loss of viscosity (Figure 2-1).
Friction and AW Test in FBT-3
Ducom FBT-3 is compliant with DIN, ASTM and IP standards, while possessing a maximum load capacity of 10 kN. Its pre-built test standards, pneumatic loading system, and post-test in vivo wear measurement system has drastically streamlined the user experience, allowing for maximum convenience.
Here, the Ducom FBT-3 was validated against Ducom Reference Fluid in line with ASTM D4172 (Figure S3). Post validation the Shell and Castrol oil – both fresh and KRL aged – was then tested in line with ASTM D4172. Finally, the mean wear scar diameter (MWSD) and friction coefficient (CoF) and were documented and reported.
Friction and AW Test in HFRR
The Ducom HFRR is a ASTM D6079 compliant, ball-on-disk type tribometer which includes a linear reciprocation engine (or vibrator), a temperature control unit and a friction piezo-sensor.
In this experiment, the Ducom HFRR was validated against Reference Fluid A, as outlined above. Following HFRR validation tests, both the Castrol and Shell oil - fresh as well as KRL aged - was tested in line with ASTM D6079. Again, the mean wear scar diameter (MWSD) and friction coefficient (CoF) and were documented and reported.
Interpreting Wear Behavior of Aged Oil
Existing literature often situates the extent of oil aging in relation to the oxidation of additives, resulting in the improved wear resistance of engine oil and gear oil. This is outlined in the articles titled "Characterization of anti-wear films formed from fresh and aged engine oils" (Wear, 2007) and "Influence of Lubricant Aging on Gear Performance" (Borg Warner and Gear Research Center, FZG) respectively.
This earlier work leads to the assumption that lower MWSD or wear in both FBT-3 and HFRR could indicate the severity of oxidation in anti-wear additives in KRL aged oil.
Figure 3 compares all six variables used to differentiate Shell Spirax from Castrol Axle. Comparable analysis was also conducted on RL 209, as a reference (Figure S4). The viscosity loss for Castrol oil was found to be smaller than that of Shell Spirax (Figure S5). This suggests that the Shell oil was severely aged compared to the Castrol oil.
It should be noted that the difference between Shell and Castrol was also verified in oven aged testing (Figure 3, Figure 3-4).
Figure 3. Tribological and physical parameters of fresh, KRL-aged and oven-aged Shell Spirax and Castrol Axle. Each parameter is represented as a percentage of the value shown by the fresh oil for that parameter. The molecular weight (Mn) was determined by Gel Permeation Chromatography (GPC), the degradation temperature (onset temperature) was determined by Thermogravimetric Analysis (TGA). Image Credit: Ducom
Changes in AW and Friction
Under mixed lubrication testing using FBT-3, the KRL aged Shell Spirax exhibited lower wear and higher friction. However, the friction behavior of Castrol was unaffected by aging in either KRL or oven. This revealed that, according to the FBT-3 test of KRL aged oils, Castrol show improved performance when compared to Shell (Figure 3).
Under the boundary lubrication test, using HFRR, the friction behavior of both Shell and Castrol was not impacted upon by KRL aging. Both oils’ wear behavior affected, however, with KRL-aged Shell oil exhibiting lower wear than KRL-aged Castrol. Because lower wear implies additive oxidation, it can be concluded that Castrol performed better than Shell in the HFRR test.
It should be noted that TAN and element concentration analysis (P, ZN, S) should be performed to support the assertion that additives have oxidized due to KRL aging, which is a precursor to severe additive depletion.
Within this study, Shell and Castrol transmission oils were subjected to identical KRL aging protocols, as developed by Ducom. The Castrol oil remained ‘younger’ than the Shell oil, and because of this, friction and anti-wear additives present in Castrol Axle were found to be more stable than additives in Shell Spirax.
This study has confirmed that the Ducom oil aging test protocol, which includes three common tribometers such as KRL, FBT-3 and HFRR, may be employed to extend oil’s service life.
Figure S1. KRL validation test according to CEC L45-99. Real time changes in (A) lubricant temperature, (B) rotating speed and (C) normal load over time for a test run with reference fluid RL 209. The kinematic viscosity measurements are shown in (D). The red dotted lines represent the upper and lower limits set by standard CEC L-45-99. Image Credit: Ducom
Figure S2. Description of operating parameters used for KRL aging. Real time changes in lubricant temperature (A), rotating speed (B) and normal load (C) for a test run according to KRL aging protocol. Image Credit: Ducom
Figure S3. FBT-3 validation test using Ducom Reference Fluid, according to ASTM D4172. Real time changes in (A) lubricant temperature, (B) normal load, (C) frictional torque and optical microscope images of ball wear scar (D). Image Credit: Ducom
Figure S4. Changes in tribological and physical parameters of RL 209 - fresh, KRL-aged and oven-aged. Each parameter is represented as a percentage of the value shown by the fresh oil for that parameter. The molecular weight (Mn) was determined by Gel Permeation Chromatography (GPC), the degradation temperature (onset temperature) was determined by Thermogravimetric Analysis(TGA). Image Credit: Ducom
Figure S5. KRL determined changes in RL 209, Shell Spirax and Castrol Axle in KRL. Real time changes in friction torque for 100 h(A), kinematic viscosity loss of the oils (B) and images of the fresh and KRL-aged oils (C). Image Credit: Ducom
Produced from materials originally authored by the Global Applications Team from DUCOM.
This information has been sourced, reviewed and adapted from materials provided by Ducom.
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