Study on the Tribological Performance of Regenerated Gear Oil with Composite Additives

Abstract

In this study, a comprehensive regeneration process was employed to enhance the recycling efficiency and performance of waste gear oil. The process began with the waste gear oil subjected to extraction flocculation, which was then followed by vacuum distillation for solvent removal. Then, catalytic hydrogenation was performed, and HiTEC 3339 additive was incorporated at concentrations that ranged from 0.25% to 1.5%, thus resulting in the regenerated gear oil. The tribological properties of the regenerated gear oil were investigated under various load conditions using a friction and wear testing apparatus. When a load of 10 N was applied, the filtered oil (Oil 2) exhibited an average friction coefficient of 0.092 and a volumetric wear rate of 8.25 × 10⁻⁸ mm³/Nm, which represented reductions of 8.23% and 42.7%, respectively, when compared to the unfiltered oil (Oil 1). As the load was increased to 50 N, Oil 2 demonstrated a wear rate of 23.4 × 10⁻⁸ mm³/Nm, indicating a 20.9% improvement in wear resistance. As the concentration of the additive increased, the following trends were observed: (i) Under a load of 10 N, the friction coefficients demonstrated a gradual decreasing trend, while at 50 N, the friction coefficients were remarkably similar and significantly lower than those at 10 N. (ii) The wear rates initially decreased and then increased. Among the tested lubricants, Oil 4 (containing 0.5% HiTEC 3339) exhibited the shallowest wear scar depth under various loads, which indicated superior anti-wear performance. When Oil 4 was thoroughly evaluated through bench tests, it indicated excellent extreme pressure and anti-wear properties, as well as superior rust and corrosion prevention capabilities and high–low temperature performance. The overall performance indicators of Oil 4 were discovered to be similar to those of fresh oil.

1. Introduction

The rapid development of transportation has ushered in a new era of accelerated growth for the gear lubricant market. In 2024, China’s lubricant consumption is projected to reach 7.56 million tons, with the growth rate expected to maintain over 10%. Lubricating oils typically consist of 80% to 90% base oil, complemented by additives that enhance various performance characteristics. These oils perform multiple functions, including lubrication, cooling, and rust prevention. Under prolonged high-speed and heavy-load operating conditions, the performance of gear lubricants gradually deteriorates. During this process, a portion of the base oil is subjected to oxidative degradation and deterioration, while various additives are consumed and altered, leading to the formation of acidic oxides, gums, and asphaltenes [1]. Failure to replace gear lubricants in a timely manner can severely impact the transmission efficiency and service life of gear systems [2]. Waste lubricating oil is classified under typical waste mineral oil. It contains a variety of toxic and hazardous substances, including heavy metals, benzene compounds, and polycyclic aromatic hydrocarbons. The direct disposal of these oils can result in water and soil contamination [3,4]. As a result, the development of environmentally friendly, high-yield, and high-quality waste lubricating oil regeneration technologies has become a focal point of research.

Only approximately 10%–25% of the hydrocarbons in waste oil undergo oxidative degradation, while the majority of the base oil remains as an effective component of the lubricant. Through the application of appropriate processing methods, it is possible to regenerate waste lubricating oil into products such as base oils, fuel oils, and diesel. Therefore, the regeneration of waste lubricating oil not only mitigates environmental pollution risks but also effectively conserves petroleum resources, bringing considerable economic benefits to enterprises [5]. The traditional sulfuric acid-clay technology for regenerating waste lubricating oil produces poor quality regenerated oil and generates secondary pollution in the form of acid sludge and wastewater. Therefore, this approach has been gradually phased out [6]. When compared to conventional acid treatment processes, hydrogenation technology stands out as it does not produce acid sludge, acidic water, or waste gases, rendering it one of the preferred waste oil regeneration processes currently available. This approach has increasingly become a focus of research in the field.

The primary contaminants in oils are metal wear debris and corrosion products. These solid particles can affect equipment lifespan through abrasive wear, sedimentation, or seizure. Solid pollutants in waste oil can be purified using separation techniques such as sedimentation, centrifugation, and filtration. Yang et al. [7] utilized a carbon nanotube/PDMS/PVDF membrane for filtering waste oil, which resulted in a 13.4% reduction in wear scar diameter and a 15.9% decrease in friction coefficient when compared to unfiltered lubricant. However, filtration membranes are highly susceptible to contamination and clogging, necessitating frequent replacement and increasing processing costs and complexity. In another approach, Zhou et al. [8] used poly (diallyldimethylammonium chloride) for flocculation and adsorption regeneration of waste rolling oil, which led to a reduction in processing time and costs. To reduce moisture, sludge, and wear particles in waste lubricating oils, oil–water separation membranes can be employed [9,10]. Nevertheless, it is important to recognize that filtration alone only removes solid particles without effectively replenishing depleted additives, rendering it challenging to enhance friction reduction and anti-wear properties of the oil [11]. After the purification of waste lubricating oil, targeted addition of additives can be performed to improve its lubrication and extreme pressure anti-wear performance [12,13]. The existing literature primarily focuses on regeneration processes and physicochemical properties of regenerated lubricating oils, with limited reports on their tribological properties and gear bench tests.

This study investigates waste GL-5 heavy-duty vehicle gear oil. The waste lubricating oil is subjected to solvent extraction and flocculation treatment, followed by vacuum distillation to remove the solvent after separating the flocculated material. Hydrogenation is then performed with a catalyst. After this, the process of regeneration is completed by incorporating HiTEC 3339 composite additive. Taking into account the characteristics of gear transmission, ball-on-disk friction experiments are utilized to study how load impacts the tribological properties of the regenerated oil. To explore the mechanisms underlying the tribological performance and gearbox bench performance of the modified lubricant, comparisons are drawn with fresh oil. Through this comprehensive approach, the study aims to offer technical support for the rational utilization of waste lubricating oil.

2. Experimental Section

2.1. Lubricating Oil Regeneration

In this study, synthetic oils of polyalphaolefin (PAO) and ester oil are chosen as the base oil. The phosphorous-type anti-wear agent is employed as the main additive. Additives such as antioxidants, rust and corrosion inhibitors, anti-corrosion inhibitors, anti-foam agents, and anti-emulsifiers are added in accordance with the optimal formula to obtain GL-5 heavy-duty vehicle gear oil. The oil viscosity grade is 75 W-90, and the viscosity index is 143. The GL-5 heavy-duty vehicle gear oil sample oil is taken from the oil filling port of the gearbox, and 5000 mL waste oil is extract from middle part of gearbox for regeneration test.

Table 1. Physicochemical properties of fresh oil and waste oil.

-	Fresh Oil	Waste Oil
Fe content (µg/g)	0	128.10
Cu content (µg/g)	0	16.30
Si content (µg/g)	0	8.5
Ca content (µg/g)	0	1039
Main additive elements (%)
S	3.87	1.01
P	0.14	0.08

shows the physic-chemical properties of fresh oil and waste oil.

Three main steps of lubricating oil regeneration include filtration purification, catalytic hydrogenation, and additive supplementation. The specific treatment process unfolds as follows: To begin with, the waste oil is placed in a water bath at 50 °C. Under conditions of mechanical stirring, n-butanol is employed as a solvent for extraction and flocculation treatment. This treatment continues for a duration of 40 min, with the mass ratio of n-butanol to waste oil being 2:1. Once the flocculation process is complete, the upper mixed liquid is transferred into a distillation flask. Then, vacuum distillation is carried out at a water bath temperature ranging from 75 to 80 °C. This step removes the solvent and derives the pre-treated hydrogenation feedstock oil.

The hydrogenation experiment is carried out on a fixed bed micro-reaction evaluation device. Firstly, 10 mL NiMoLa/Al₂O₃ catalyst is loaded in the middle of the reactor. After the air tightness test of the device is qualified, the temperature is heated to 120 °C, and the catalyst is carried out. The sulfurized oil used for pre-sulfurization is straight-run jet fuel with a carbon disulfide mass fraction of 2%. When the temperature of the catalyst bed rises to 120 °C, the feed pump is started to introduce the sulfurized oil. As the catalyst bed temperature reaches 120 °C, the feed pump is activated, introducing the sulfurization oil. The sulfurization oil is fed at a volume space velocity of 1.5 h⁻¹, under a pressure of 3.0 MPa, with a hydrogen-to-oil volume ratio of 300:1. Upon the appearance of oil in the high-pressure separator, the temperature is further elevated to 230 °C and maintained at this level for 12 h. Following this, the temperature is raised to 290 °C for 8 h, and then to 320 °C for an additional 8 h, thus completing the catalyst pre-sulfurization process. The system is then purged with hydrogen for 8 h. After the purging process, adjustments are made to the heating furnace temperature, feed space velocity, and reaction pressure to meet the required reaction conditions. At this point, the feed is switched to the solvent-extracted and flocculated oil, marking the beginning of the hydrogenation experiment. The reaction conditions are set as follows: a reaction temperature of 290 °C, a mass space velocity of 0.6 h⁻¹, a reaction pressure of 5.0 MPa, and a hydrogen-to-oil volume ratio of 500:1. To conclude the process, fine filtration is employed to remove impurities.

Following catalytic treatment, HiTEC 3339 gear oil additive is integrated into the gearbox (demonstrated in

Table 2. Physicochemical properties of HiTEC 3339 composite additive.

Project	Typical Value
Acid value (mgKOH/g)	39.9
Base value (mgKOH/g)	25.7
Kinematic viscosity (100 °C)/(mm2/s)	2.5
Flash point (open cup)/°C	76
Density (15.6 °C)/(kg/m3)	1005
Sulfur/% (mass fraction)	32.84
Phosphorus/% (mass fraction)	1.22
Nitrogen/% (mass fraction)	0.96

). Various experimental oils are prepared, each with different additive contents (lubricating oil nomenclature in

Table 3. Composition of lubricating oils.

Experimental Oil Number	Experimental Oil Composition
Oil 1	Used oil before filtration
Oil 2	Used oil after catalytic treatment
Oil 3	Oil 2 + 0.25% H 3339
Oil 4	Oil 2 + 0.5% H 3339
Oil 5	Oil 2 + 1.0% H 3339
Oil 6	Oil 2 + 1.5% H 3339
Oil 7	Fresh Oil

). The specific procedure is as follows: Under 50 °C conditions, a magnetic stirrer is used for 4 h, followed by ultrasonic vibration treatment for 1 h. During this process, the dissolution of the HiTEC 3339 gear oil composite additive in the gear oil is observed.

2.2. Friction and Wear Experiments

The tribological properties of regenerated lubricating oils are evaluated, using a ball-on-disk friction and wear tester. The upper specimen is a GCr15 steel ball (diameter Φ9.5 mm with a hardness of 64 HRC, while the lower specimen is a GCr15 disk sample (Φ30 mm × 5 mm). The testing conditions involve loads of 10 N and 50 N, applied with a reciprocating stroke of 5 mm at a frequency of 6 Hz. Each test is conducted for a duration of 1 h at room temperature. Before initiating the experiment, approximately 10 mL lubricating oil is applied to the disk sample surface, ensuring uniform coverage of the friction surface. No additional oil is added during following experiments.

Polishing of the GCr15 disk samples is carried out utilizing 400#, 600#, and 1000# sandpaper. Prior to testing, both the steel ball and disk sample are ultrasonically cleaned with anhydrous ethanol and acetone for 10 min, followed by drying to eliminate surface residues. After the friction experiment, the ball and disk samples are again ultrasonically treated with anhydrous ethanol and acetone, then dried. A roughness measuring instrument is used to measure the wear profile of the disk sample. To comparatively analyze the anti-wear performance of the lubricating oils, the wear rate is calculated based on the wear scar profile. Post-test analysis of the microscopic morphology of the worn surface is conducted utilizing scanning electron microscopy (SEM). In addition, energy-dispersive X-ray spectroscopy (EDS) is employed to analyze the chemical composition of the worn surface to gain insights into the anti-wear mechanism.

2.3. Bench Tests

Gearbox bench testing represents an essential method for validating gear oil performance, while no specialized bench test standard for gear oils currently exists. Referencing TB/T 3134-2013 Drive Gearboxes for Electric Multiple Units,” tests simulate gearbox operation performance. This stand is the railway industry standard of the People’s Republic of China. The bench evaluation includes high-temperature characteristics, low-temperature startup, noise measurement, sealing performance, transmission efficiency, and overload tests with actual gearboxes [14]. By applying this standard, we can test the performance of the regenerated gear oil in the gearbox.

3. Results and Discussion

3.1. Physicochemical Properties

Table 4. Comparison of waste oil properties before and after hydrogenation.

Property Indicator	Results
	Waste Oil	Extracted and Flocculated Oil	Hydrogenated Regenerated Oil
Water content % (v/v)	1.09	0.08	0.01
Kinematic viscosity at 100 °C mm2/s	15.80	6.30	10.30
Ca content (µg/g)	1039	56.6	51.5
Cu content (µg/g)	16.30	5.42	5.42
Fe content (µg/g)	128.10	3.43	3.43
Zn content (µg/g)	415	6.6	6.6
S content % (m/m)	1.01	0.44	0.44
P content % (m/m)	0.18	0.087	0.087
N content (µg/g)	1400	468	22

presents the property parameters of waste lubricating oil before and after solvent extraction, flocculation, and vacuum distillation treatment. The data demonstrate that after extraction and flocculation treatment, the oil shows significant improvements in water content, viscosity, and metal content properties. However, nitrogen content remains high. After hydrogenation treatment, nitrogen content decreases significantly, meeting the specified indicators for reuse.

Figure 1 demonstrates photographs of the gear oil composite additive dissolved in the base oil. It can be observed that the gear oil composite additive forms a uniform solution in the base oil, with no layering or cloudy appearance. This indicates that the HiTEC 3339 gear oil composite additive exhibits good miscibility with the gear oil.

3.2. Tribological Properties of Regenerated Lubricating Oil

3.2.1. Effect of Filtration

Figure 2 illustrates the friction coefficient versus time curves for Oil 1 and Oil 2 under loads of 10 N and 50 N. Under a load of 10 N, the friction coefficient of Oil 1 rapidly increases in the initial stage, peaking (0.098) at 300 s before stabilizing. As it slides for 600 s, it begins to fluctuate and gradually decrease, eventually stabilizing at 0.097. In contrast, Oil 2 begins with an initial friction coefficient of 0.09, from 1200 s to 1900 s, the friction coefficient curve shows a large fluctuation, and the friction coefficient rises to about 0.095, and then the friction coefficient decreases and gradually returns to a stable state. Throughout the entire friction process, Oil 2 exhibits a more stable and lower friction coefficient compared to Oil 1. This observation suggests that filtration effectively removes wear particles from waste oil, thereby reducing surface abrasive wear [15] and facilitating the formation of a stable lubricating film, which thus maintains a lower friction coefficient.

As the load increases to 50 N, Oil 1 exhibits a gradual increase in its friction coefficient, averaging 0.095 throughout the friction process. In contrast, Oil 2 maintains a relatively stable friction coefficient, averaging 0.090, without experiencing sudden increases. Under higher load conditions, numerous surface asperities enter a state of plastic contact. In this case, small protrusions on the object surface begin to deform, leading to closer contact between them. As this contact evolves, the real contact area expands. Therefore, with the increased contact area and constant total load, the actual contact stress decreases correspondingly. This reduction in actual contact stress has a significant positive impact on the lubrication state. Under lower contact stress, the lubricating oil can form a more uniform film between the contact surfaces, reducing direct friction and wear between metal surfaces. This results in improved lubrication performance [16].

Figure 3 displays the wear profiles of the GCr15 disk lubricated with Oil 1 and Oil 2 under loads of 10 N and 50 N. As under a load of 10 N, the wear scar depths and widths for the two oils are 0.22 mm × 0.55 μm and 0.15 mm × 0.41 μm, respectively. As the load increases to 50 N, these measurements for Oil 1 and Oil 2 expand to 0.57 mm × 3.14 μm and 0.33 mm × 2.72 μm, respectively. These results demonstrate the superior anti-wear performance of Oil 2 compared to Oil 1 under both load conditions. Figure 3b presents the wear rates for both oils. At load of 10 N, these rates are 14.4 × 10⁻⁸ mm³/Nm and 8.25 × 10⁻⁸ mm³/Nm for Oil 1 and Oil 2, respectively. The finding indicates a 42.7% improvement in wear resistance for the filtered oil. Under a load of 50 N, the wear rates are 29.6 × 10⁻⁸ mm³/Nm and 23.4 × 10⁻⁸ mm³/Nm, respectively, indicating a 20.9% improvement in wear resistance for the filtered oil. The wear rates for both oils increase with increasing load. These findings demonstrate that filtration treatment enhances the wear resistance of waste oil under both heavy and light load conditions. However, the improvement in anti-wear performance becomes less significant as the load increases. Therefore, it is necessary to introduce composite additives for further regeneration of the filtered waste oil.

3.2.2. Effect of Additive Concentration

Figure 4 presents the friction coefficient curves for lubricating oils with different additive concentrations (Oil 3–Oil 6) and fresh oil (Oil 7). Under a load of 10 N, Oil 3 exhibits a stable friction coefficient, averaging 0.097. In contrast, Oil 4 initiates with a friction coefficient of 0.092, which abruptly rises to 0.095 at 400 s before stabilizing. As the additive content increases to 1.5%, Oil 6 commences with a friction coefficient of 0.095 and remains steady. A slight fluctuation occurs between 2100 s and 2540 s, with the friction coefficient increasing marginally to 0.096, before decreasing to 0.095 after 2450 s and stabilizing thereafter. Oil 7 distinguishes itself with a notably lower friction coefficient, consistently maintaining 0.092 throughout the entire friction process. Under the load of 10 N, the average friction coefficients can be ranked as follows: Oil 7 < Oil 6 < Oil 5 < Oil 4 < Oil 3.

As the load increases to 50 N, Oil 3, Oil 4, and Oil 5 all exhibit an average friction coefficient of 0.09. Oil 6 starts with a friction coefficient of 0.089, which rises to 0.090 between 1400 s and 1500 s, then remains stable. The friction coefficients of the regenerated oils with four different additive concentrations remain highly stable throughout the entire friction test. Oil 7 consistently exhibits a stable friction coefficient, averaging 0.089 throughout the process, which is marginally lower than the regenerated oils with additives. Further analysis reveals that as the load increases, the friction coefficients of the regenerated oils with additives become more stable. Under the effect of frictional heat and mechanical contact stress, active elements (S and P) in the additives undergo tribochemical reactions with the metal surface, forming a chemical reaction film. This boundary protection film exhibits superior friction reduction and anti-wear properties [9].

Figure 5 illustrates the wear scar profiles and wear rates of the GCr15 disk lubricated with Oil 3, Oil 4, Oil 5, Oil 6, and Oil 7. Under a load of 10 N, the wear scar depths and widths for these oils measure 0.25 μm × 155.27 μm, 0.32 μm × 128.4 μm, 0.40 μm × 137.9 μm, 0.48 μm × 138.5 μm, and 0.28 μm × 149.36 μm, respectively. Oil 6 exhibits the deepest wear scar, while Oil 4 demonstrates the shallowest. As the load increases to 50 N, these measurements expand to 1.61 μm × 364.87 μm, 2.00 μm × 302.05 μm, 2.89 μm × 288.70 μm, 2.89 μm × 331.6 μm, and 2.33 μm × 325.39 μm, respectively. Once again, Oil 6 demonstrates the deepest wear scar, with Oil 4 maintaining the shallowest. Further analysis indicates that under both load conditions, the wear rates of the lubricating oils initially decrease and then increase as the concentration of HiTEC 3339 additive rises. At 0.5% additive concentration (Oil 4), the wear rates are 4.58 × 10⁻⁸ mm³/Nm and 11.2 × 10⁻⁸ mm³/Nm, which are 46.8% and 56.4% lower than those of Oil 6 (1.5% HiTEC 3339), respectively. These findings indicate that the addition of the additive significantly improves the anti-wear performance of the lubricating oil. However, excessive additive concentrations can actually reduce the anti-wear properties of oil. This phenomenon may be attributed to the accumulation of excess additives on the friction surface, potentially hindering tribochemical reactions and reducing oil film strength, thus leading to increased wear. Specifically, Oil 4 consistently maintains good anti-wear performance under both heavy and light load conditions.

3.3. Wear Surface Analysis

Figure 6 presents SEM images of the GCr15 steel disk wear surfaces lubricated with Oil 1 and Oil 2. Under a load of 10 N (Figure 6a), the wear region of Oil 1 displays numerous plough grooves, indicative of abrasive wear. This results from the presence of many abrasive particles in the unfiltered lubricating oil. These hard particles, entering the friction surface and causing relative motion, lead to surface ploughing and two-body or three-body abrasion, exacerbating surface wear [17]. In contrast, when lubricated with Oil 2 (Figure 6b), the wear surface morphology indicates highly shallow wear marks with only a few scratches and minor pitting, suggesting minimal wear. As the load increases to 50 N, the wear scar lubricated by Oil 1 expands in width and depth, accompanied by severe ploughing and metal accumulation in edge regions, indicating severe wear (Figure 6c). The unfiltered lubricating oil, containing numerous abrasive particles, struggles to form a stable, lubricating film of sufficient thickness. As the load increases, the lubricating film further ruptures, leading to severe abrasive wear and even adhesive wear on the friction surface. Under the same load, compared to Oil 1, the wear scar surface lubricated by Oil 2 shows fine, shallow grooves, and the width of the wear marks perpendicular to the sliding direction is reduced. However, material accumulation and significant plastic deformation remain evident on the wear surface (Figure 6d). This suggests that while filtration treatment effectively reduces abrasive wear, the lack of timely supplementation of consumed extreme pressure and anti-wear additives in the lubricating oil hinders the formation of physical or chemical adsorption films on the friction surface.

Figure 7 presents optical micrographs of the wear morphology on the steel balls after lubrication with Oil 1 and Oil 2. When lubricated with Oil 1 (Figure 7a,b), the GCr15 steel ball displays a larger wear scar reflected by deeper wear marks and evident ploughing, indicating severe abrasive wear during the friction process. In contrast, Figure 7c,d indicate that under Oil 2 lubrication, the wear scar surface of the GCr15 steel ball becomes smoother and more even, exhibiting only shallow wear marks and reduced surface ploughing. Among the different lubricating oils, Oil 2 results in the smallest wear scar width and wear spot area on the steel ball. These observations demonstrate that filtering out abrasive particles from the lubricating oil can significantly mitigate severe wear of the steel ball and steel disk friction pair, thereby enhancing the anti-wear performance of the waste gear oil.

Figure 8 shows the worn surface of GCr15 steel disc lubricated by lubricating Oil (Oil 3–Oil 6) with different additive concentrations under a load of 50 N, and Figure 9 shows the EDS spectrum of the lubricated wear surface of Oil 3 and Oil 4 under a load of 50 N.

In comparison to Oil 2, the wear surface lubricated by Oil 3 exhibits more uniform and smoother wear marks with significantly reduced width. No obvious ploughing is observed on the wear scar surface; it has a smooth, band-like transfer film. EDS analysis of the transfer film region (Figure 9a) reveals high abundances of Zn, P, and S elements on the wear scar surface. This indicates that under these boundary lubrication test conditions, the anti-wear additive (HiTEC 3339) interacted effectively with the contact surface, significantly reducing wear. As the additive concentration increases, the modification effect on the lubricating oil varies greatly with different mass fractions of additive content. The wear scar width under 0.5% HiTEC 3339 (Oil 4) lubrication is notably smaller than those under 1.0% and 1.5% HiTEC 3339, with only slight plastic deformation on its surface and the shallowest wear marks. EDS analysis of the wear surface lubricated by Oil 4 detects Fe, O, P, Zn, and S elements. Fe and Si elements originate from the steel ball, while P, S, and Zn elements come from HiTEC 3339, indicating that S elements formed a denser friction transfer film on the wear surface. Moreover, the content of active elements (P, S, and Zn) on the wear surface lubricated by 0.5% HiTEC 3339 exceeds that of 0.25% HiTEC 3339. Friction and wear experiments indicate 0.5% as the optimal additive concentration of HiTEC 3339 for the selected waste oil in this study. The results demonstrate that the additive forms a tribochemical reaction film on the friction surface, preventing direct contact between friction surfaces, reducing wear, and significantly improving the anti-wear ability of the gear oil [18]. Moreover, at the optimal concentration, the adsorption and desorption of the additive on the friction surface reach a dynamic equilibrium, deriving a more tightly and densely arranged adsorption film on the friction surface, leading to optimal anti-wear performance. However, exceeding this optimal level causes excessive additive molecules to adsorb on the friction pair surface, potentially leading to aggregation [19,20]. Therefore, the tribochemical reaction products are more likely to detach and form wear debris, exacerbating abrasive wear (as shown in Figure 8d).

4. Bench Verification

4.1. Gearbox No-Load Test

To simulate the lubrication effect of the restored oil under low-load operating conditions, Oil 4 was utilized for both running-in tests and oil level/quantity tests. These tests measured the temperature of the gearbox test oil rise to thermal equilibrium, bearing temperature rise, noise levels, and leakage conditions. These parameters were utilized to assess whether the lubrication performance of the modified oil under low-load conditions meets the required standards.

Table 5. Physical gearbox no-load test evaluation results of Oil 4.

Test Category	Test Item	Test Result	Technical Requirement
Running-in Test	Gearbox temperatures at various points/°C	44.3	≯100
	Bearing temperature rise rate/(°C/min)	0.4	<15.0
	Leakage inspection	No leakage	No leakage
Oil Level/Quantity Test	Gearbox temperatures at various points/°C	47.0	≯100
	Temperature rise rate/(°C/min)	0.5	<15.0
	Leakage inspection	No leakage	No leakage
Noise Test	Noise sound pressure level/dB(A)	82.1	<90.0

presents the results of the gearbox no-load tests. During the test, the gearbox operated normally, with temperatures at various points and bearing temperature rise rates in normal ranges. No abnormal sounds were detected. Post-test inspection indicated no oil leakage, and the magnetic plug demonstrated no unusual particle attraction. Oil 4 demonstrated good compatibility with both the gearbox and sealing components.

4.2. Gearbox Loading Test

The gearbox loading test encompasses several trials, including the overspeed test, rated speed loading test, simulated operation temperature rise balance test, maximum starting torque loading test, transmission efficiency test, overload test, and vibration test.

Table 6. Gearbox loading test evaluation results of Oil 4.

Test Category	Test Item	Test Result	Technical Requirement
Overspeed Test	Gearbox temperatures at various points/°C	40.3	≯100
	Bearing temperature rise rate/(°C/min)	0.7	<15.0
	Leakage inspection	No leakage	No leakage
Rated Speed Loading Test	Gearbox temperatures at various points/°C	62.7	≯100
	Bearing temperature rise rate/(°C/min)	0.8	<15.0
	Leakage inspection	No leakage	No leakage
	Gearbox vibration acceleration RMS value/(mm/s)	4.9	≯15.0
Simulated Operation Temperature Rise Balance Test	Gearbox temperatures at various points/°C	80.9	≯100
	Bearing temperature rise rate/(°C/min)	2.2	<15.0
	Leakage inspection	No leakage	No leakage
	(Gearbox vibration acceleration RMS value/(mm/s)	8.0	≯15.0
Maximum Starting Torque Loading Test	Gearbox temperatures at various points/°C	50.0	≯100
	Bearing temperature rise rate/(°C/min)	0.9	<15.0
	Leakage inspection	No leakage	No leakage
Overload Test	Gearbox temperatures at various points/°C	50.0	≯100
	Bearing temperature rise rate/(°C/min)	0.9	<15.0
	Leakage inspection	No leakage	No leakage
Transmission Efficiency Test	Transmission efficiency/%	98.5	<97.0

presents the results of these tests. These loading tests simulate various extreme operating conditions a gearbox might encounter in actual use, evaluating its performance under different speeds and loads to assess operational reliability. In all loading tests, Oil 4 demonstrated good performance. Temperatures at various points in the gearbox and bearing temperature rise rates remained in normal ranges. No leakage was observed, and transmission efficiency was high, indicating that the oil can meet the lubrication requirements of gears under extreme conditions.

4.3. High and Low Temperature Tests

The high and low temperature tests comprise a low-temperature startup test and a high-temperature characteristics test [21]. For the low-temperature startup test, all gearbox parts are cooled to −25 °C before acceleration according to the traction motor profile, evaluating low-temperature startup performance [22]. This test involves one forward and one reverse rotation without forced air cooling. The requirement is that the gearbox temperature rise rate should not exceed 15 °C/min. The high-temperature characteristics test begins when all parts of the gearbox reach 40 °C. The gearbox is then accelerated from zero to the maximum test speed to assess its lubrication performance and sealing conditions. This test also involves one forward and one reverse rotation without forced air cooling. The requirements are that the gearbox temperature rise rate should not exceed 15 °C/min, and the time for various parts to reach 100 °C should not be less than 20 min.

Table 7. Physical gearbox high and low temperature test evaluation results of Oil 4.

Test Category	Test Item	Test Result	Technical Requirement
Low-Temperature Startup Test	Temperature rise rate/(°C/min)	1.5	<15.0
	Bearing condition	No burning	No burning
High-Temperature Characteristics Test	Temperature conditions of various gearbox parts Time required to reach 95 °C/min	>20	>20
	Gearbox temperatures at various points/°C	75.4	≯100
	Bearing temperature rise rate/(°C/min)	0.9	<15.0
	Leakage inspection	No leakage	No leakage

presents the test results. In the low-temperature startup test with Oil 4, the bearing temperature rise rate was 1.5 °C/min (<15.0 °C/min), meeting the technical requirements for the low-temperature test. In the high-temperature characteristics test, the highest temperature at various points in the gearbox was 75.4 °C (<100 °C), with a bearing temperature rise rate of 0.9 °C/min (<15.0 °C/min). No leakage was observed in the gearbox, satisfying the technical requirements for the high-temperature test.

4.4. Vibration and Noise Test

Vibration acceleration measurements are taken during the rated speed loading test and the simulated operation temperature rise balance test as part of the vibration test [23].

Table 8. Gearbox vibration test evaluation results of Oil 4.

Test Category	Operating Condition	Rotation Direction	Vibration Velocity RMS (mm/s)				Technical Requirement
			GM-Radial	GM-Axial	PM-Radial	PM-Axial
Vibration Test	Rated Speed Loading	Forward	2.6	2.3	2.7	4.0	Gearbox vibration velocity RMS should be below 15.0 mm/s at full motor power
		Reverse	2.2	2.2	3.3	4.9
	Simulated Temperature Rise Balance	Forward	4.7	5.7	4.5	8.0
		Reverse	3.0	4.0	3.3	5.7

presents these results. The findings exhibit that in both the rated speed loading and simulated temperature rise balance vibration tests, vibration velocities in all directions remained below the technical requirement of 15.0 mm/s. This indicates the excellent lubrication performance of Oil 4 for the gearbox which facilitates smooth operation.

5. Conclusions

This study regenerated gear lubricating oil through filtration and lubricating oil additive incorporation. The tribological performance and lubrication mechanism of the gear oil before and after regeneration were evaluated, followed by gearbox bench verification. The results indicate the following:

• The distillate oil, obtained through n-butanol extraction flocculation and vacuum distillation of waste lubricating oil, showcased significant improvements in appearance, water content, viscosity, and metal content properties.
• Oil 2 demonstrated a significantly more stable and lower friction coefficient than the pre-filtered Oil 1. The wear scar depth and width on the disk sample and the counter steel ball lubricated with Oil 2 were significantly reduced. The removal of solid particles from the lubricating oil through filtration can reduce abrasive wear in waste oil and improve its anti-wear performance.
• Under lower load conditions, the lubricating oil with 0.25% HiTEC 3339 exhibited the highest friction coefficient, which decreased as additive concentration increased. However, under high load conditions, the friction coefficients of oils with HiTEC 3339 were highly similar. Regarding anti-wear properties, the wear rates of the lubricating oils initially decrease and then increase as the concentration of HiTEC 3339 additive rises. At 0.5% additive concentration (Oil 4), the wear rates are 4.58 × 10⁻⁸ mm³/Nm and 11.2 × 10⁻⁸ mm³/Nm, which are 46.8% and 56.4% lower than those of Oil 6 (1.5% HiTEC 3339), respectively. The gear oil with 0.5% HiTEC 3339 (Oil 4) exhibited optimal performance.
• Gearbox bench tests revealed that Oil 4 achieved the GL-5 gear oil quality grade level. Physical gearbox bench tests demonstrated an actual and excellent lubrication effect of Oil 4, compatibility with the gearbox sealing system, normal temperature rise maintenance in the gearbox and bearings during testing, and minimal gearbox wear. It can operate normally in cold, hot, and ambient temperature environments.